How do I base64 encode decode in C

Question

I have binary data in an unsigned char variable  I need to convert them to PEM base64 in c  I looked in openssl library but i could not find any function  Does any body have any idea

User · Answer

glib has functions for base64 encoding  https   developer gnome org glib stable glib-Base64-Encoding html

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In case people need a c   solution  I put this OpenSSL solution together  for both encode and decode   You ll need to link with the  crypto  library  which is OpenSSL   This has been checked for leaks with valgrind  although you could add some additional error checking code to make it a bit better - I know at least the write function should check for return value     include  lt openssl bio h gt   include  lt openssl evp h gt   include  lt stdlib h gt   string base64 encode  const string  amp str         BIO  base64 filter   BIO new  BIO f base64          BIO set flags  base64 filter  BIO FLAGS BASE64 NO NL         BIO  bio   BIO new  BIO s mem          BIO set flags  bio  BIO FLAGS BASE64 NO NL         bio   BIO push  base64 filter  bio         BIO write  bio  str c str    str length           BIO flush  bio         char  new data       long bytes written   BIO get mem data  bio   amp new data         string result  new data  bytes written        BIO free all  bio         return result        string base64 decode  const string  amp str         BIO  bio   base64 filter   bio out      char inbuf 512       int inlen      base64 filter   BIO new  BIO f base64          BIO set flags  base64 filter  BIO FLAGS BASE64 NO NL         bio   BIO new mem buf   void  str c str    str length           bio   BIO push  base64 filter  bio         bio out   BIO new  BIO s mem           while   inlen   BIO read bio  inbuf  512    gt  0            BIO write  bio out  inbuf  inlen               BIO flush  bio out         char  new data      long bytes written   BIO get mem data  bio out   amp new data         string result  new data  bytes written         BIO free all  bio        BIO free all  bio out         return result

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I know this question is quite old  but I was getting confused by the amount of solutions provided - each one of them claiming to be faster and better   I put together a project on github to compare the base64 encoders and decoders  https   github com gaspardpetit base64   At this point  I have not limited myself to C algorithms - if one implementation performs well in C    it can easily be backported to C  Also tests were conducted using Visual Studio 2015  If somebody wants to update this answer with results from clang gcc  be my guest   FASTEST ENCODERS   The two fastest encoder implementations I found were Jouni Malinen s at http   web mit edu freebsd head contrib wpa src utils base64 c and the Apache at https   opensource apple com source QuickTimeStreamingServer QuickTimeStreamingServer-452 CommonUtilitiesLib base64 c    Here is the time  in microseconds  to encode 32K of data using the different algorithms I have tested up to now   jounimalinen                25 1544 apache                      25 5309 NibbleAndAHalf              38 4165 internetsoftwareconsortium  48 2879 polfosol                    48 7955 wikibooks org c             51 9659 gnome                       74 8188 elegantdice                 118 899 libb64                      120 601 manuelmartinez              120 801 arduino                     126 262 daedalusalpha               126 473 CppCodec                    151 866 wikibooks org cpp           343 2 adp gmbh                    381 523 LihO                        406 693 libcurl                     3246 39 user152949                  4828 21    Ren   Nyffenegger s solution  credited in another answer to this question  is listed here as adp gmbh    Here is the one from Jouni Malinen that I slightly modified to return a std  string        Base64 encoding decoding  RFC1341    Copyright  c  2005-2011  Jouni Malinen  lt j w1 fi gt      This software may be distributed under the terms of the BSD license    See README for more details         2016-12-12 - Gaspard Petit   Slightly modified to return a std  string     instead of a buffer allocated with malloc    include  lt string gt   static const unsigned char base64 table 65         ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789            base64 encode - Base64 encode    src  Data to be encoded    len  Length of the data to be encoded    out len  Pointer to output length variable  or  NULL if not used   Returns  Allocated buffer of out len bytes of encoded data    or empty string on failure    std  string base64 encode const unsigned char  src  size t len        unsigned char  out   pos      const unsigned char  end   in       size t olen       olen   4   len   2    3      3-byte blocks to 4-byte         if  olen  lt  len          return std  string       integer overflow         std  string outStr      outStr resize olen       out    unsigned char   amp outStr 0        end   src   len      in   src      pos   out      while  end - in  gt   3             pos     base64 table in 0   gt  gt  2            pos     base64 table   in 0   amp  0x03   lt  lt  4     in 1   gt  gt  4             pos     base64 table   in 1   amp  0x0f   lt  lt  2     in 2   gt  gt  6             pos     base64 table in 2   amp  0x3f           in    3             if  end - in             pos     base64 table in 0   gt  gt  2           if  end - in    1                 pos     base64 table  in 0   amp  0x03   lt  lt  4                pos                            else                pos     base64 table   in 0   amp  0x03   lt  lt  4                     in 1   gt  gt  4                 pos     base64 table  in 1   amp  0x0f   lt  lt  2                      pos                     return outStr      FASTEST DECODERS  Here are the decoding results and I must admit that I am a bit surprised   polfosol                    45 2335 wikibooks org c             74 7347 apache                      77 1438 libb64                      100 332 gnome                       114 511 manuelmartinez              126 579 elegantdice                 138 514 daedalusalpha               151 561 jounimalinen                206 163 arduino                     335 95 wikibooks org cpp           350 437 CppCodec                    526 187 internetsoftwareconsortium  862 833 libcurl                     1280 27 LihO                        1852 4 adp gmbh                    1934 43 user152949                  5332 87   Polfosol s snippet from base64 decode snippet in c   is the fastest by a factor of almost 2x   Here is the code for the sake of completeness   static const int B64index 256      0   0   0   0   0   0   0   0   0   0   0   0  0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0  0   0   0   0   0   0   0   0   0   0   0  62  63  62  62  63  52  53  54  55  56  57  58  59  60  61   0   0   0   0   0   0   0   0   1   2   3   4   5   6  7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25   0  0   0   0  63   0  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51     std  string b64decode const void  data  const size t len        unsigned char  p    unsigned char  data      int pad   len  gt  0  amp  amp   len   4    p len - 1               const size t L     len   3    4 - pad    4      std  string str L   4   3   pad    0         for  size t i   0  j   0  i  lt  L  i    4                int n   B64index p i    lt  lt  18   B64index p i   1    lt  lt  12   B64index p i   2    lt  lt  6   B64index p i   3            str j      n  gt  gt  16          str j      n  gt  gt  8  amp  0xFF          str j      n  amp  0xFF            if  pad                int n   B64index p L    lt  lt  18   B64index p L   1    lt  lt  12          str str size   - 1    n  gt  gt  16           if  len  gt  L   2  amp  amp  p L   2                                n    B64index p L   2    lt  lt  6              str push back n  gt  gt  8  amp  0xFF                       return str

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I fix  ryyst answer s bug and this is a url safe version       include  lt stdio h gt   include  lt stdint h gt   include  lt stdlib h gt   include  lt string h gt    static char encoding table       A    B    C    D    E    F    G    H                                    I    J    K    L    M    N    O    P                                    Q    R    S    T    U    V    W    X                                    Y    Z    a    b    c    d    e    f                                    g    h    i    j    k    l    m    n                                    o    p    q    r    s    t    u    v                                    w    x    y    z    0    1    2    3                                    4    5    6    7    8    9    -         static char  decoding table   NULL  static int mod table      0  2  1     void build decoding table           decoding table   malloc 256         for  int i   0  i  lt  64  i            decoding table  unsigned char  encoding table i     i        void base64 cleanup         free decoding table        char  base64 encode const char  data                      size t input length                      size t  output length           output length   4     input length   2    3         char  encoded data   malloc  output length       if  encoded data    NULL  return NULL        for  int i   0  j   0  i  lt  input length               uint32 t octet a   i  lt  input length    unsigned char data i      0          uint32 t octet b   i  lt  input length    unsigned char data i      0          uint32 t octet c   i  lt  input length    unsigned char data i      0            uint32 t triple    octet a  lt  lt  0x10     octet b  lt  lt  0x08    octet c            encoded data j      encoding table  triple  gt  gt  3   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  2   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  1   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  0   6   amp  0x3F                 int i 0      for  int i   0  i  lt  mod table input length   3   i            encoded data  output length - 1 - i                   output length     output length -2   mod table input length   3       encoded data  output length   0       return encoded data        unsigned char  base64 decode const char  data                               size t input length                               size t  output length          if  decoding table    NULL  build decoding table          if  input length   4    0  return NULL         output length   input length   4   3      if  data input length - 1            output length --      if  data input length - 2            output length --        unsigned char  decoded data   malloc  output length       if  decoded data    NULL  return NULL        for  int i   0  j   0  i  lt  input length               uint32 t sextet a   data i           0  amp  i     decoding table data i              uint32 t sextet b   data i           0  amp  i     decoding table data i              uint32 t sextet c   data i           0  amp  i     decoding table data i              uint32 t sextet d   data i           0  amp  i     decoding table data i                uint32 t triple    sextet a  lt  lt  3   6             sextet b  lt  lt  2   6             sextet c  lt  lt  1   6             sextet d  lt  lt  0   6             if  j  lt   output length  decoded data j       triple  gt  gt  2   8   amp  0xFF          if  j  lt   output length  decoded data j       triple  gt  gt  1   8   amp  0xFF          if  j  lt   output length  decoded data j       triple  gt  gt  0   8   amp  0xFF              return decoded data      int main             const char   data    quot Hello World         quot       size t input size   strlen data       printf  quot Input size   ld  n quot  input size       char   encoded data   base64 encode data  input size   amp input size       printf  quot After size   ld  n quot  input size       printf  quot Encoded Data is   s  n quot  encoded data            size t decode size   strlen encoded data       printf  quot Output size   ld  n quot  decode size       unsigned char   decoded data   base64 decode encoded data  decode size   amp decode size       printf  quot After size   ld  n quot  decode size       printf  quot Decoded Data is   s  n quot  decoded data       return 0

User · Answer

libb64 has both C and C   APIs  It is lightweight and perhaps the fastest publicly available implementation  It s also a dedicated stand-alone base64 encoding library  which can be nice if you don t need all the other stuff that comes from using a larger library such as OpenSSL or glib

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Here s my solution using OpenSSL      A BASE-64 ENCODER AND DECODER USING OPENSSL     include  lt openssl pem h gt   include  lt string h gt    Only needed for strlen     char  base64encode  const void  b64 encode this  int encode this many bytes       BIO  b64 bio   mem bio         Declares two OpenSSL BIOs  a base64 filter and a memory BIO      BUF MEM  mem bio mem ptr       Pointer to a  memory BIO  structure holding our base64 data      b64 bio   BIO new BIO f base64                            Initialize our base64 filter BIO      mem bio   BIO new BIO s mem                                 Initialize our memory sink BIO      BIO push b64 bio  mem bio                Link the BIOs by creating a filter-sink BIO chain      BIO set flags b64 bio  BIO FLAGS BASE64 NO NL      No newlines every 64 characters or less      BIO write b64 bio  b64 encode this  encode this many bytes     Records base64 encoded data      BIO flush b64 bio       Flush data   Necessary for b64 encoding  because of pad characters      BIO get mem ptr mem bio   amp mem bio mem ptr      Store address of mem bio s memory structure      BIO set close mem bio  BIO NOCLOSE       Permit access to mem ptr after BIOs are destroyed      BIO free all b64 bio      Destroys all BIOs in chain  starting with b64  i e  the 1st one       BUF MEM grow mem bio mem ptr    mem bio mem ptr  length   1       Makes space for end null        mem bio mem ptr  data   mem bio mem ptr  length      0      Adds null-terminator to tail      return   mem bio mem ptr  data    Returns base-64 encoded data   See   buf mem st  struct      char  base64decode  const void  b64 decode this  int decode this many bytes       BIO  b64 bio   mem bio         Declares two OpenSSL BIOs  a base64 filter and a memory BIO      char  base64 decoded   calloc   decode this many bytes 3  4 1  sizeof char        1   null      b64 bio   BIO new BIO f base64                            Initialize our base64 filter BIO      mem bio   BIO new BIO s mem                               Initialize our memory source BIO      BIO write mem bio  b64 decode this  decode this many bytes     Base64 data saved in source      BIO push b64 bio  mem bio              Link the BIOs by creating a filter-source BIO chain      BIO set flags b64 bio  BIO FLAGS BASE64 NO NL              Don t require trailing newlines      int decoded byte index   0      Index where the next base64 decoded byte should be written      while   0  lt  BIO read b64 bio  base64 decoded decoded byte index  1       Read byte-by-byte          decoded byte index      Increment the index until read of BIO decoded data is complete          Once we re done reading decoded data  BIO read returns -1 even though there s no error      BIO free all b64 bio      Destroys all BIOs in chain  starting with b64  i e  the 1st one       return base64 decoded           Returns base-64 decoded data with trailing null terminator       Here s one way to base64 encode decode using the base64encode   and base64decode functions    int main void       char data to encode      Base64 encode this string       The string we will base-64 encode       int bytes to encode   strlen data to encode     Number of bytes in string to base64 encode      char  base64 encoded   base64encode data to encode  bytes to encode       Base-64 encoding       int bytes to decode   strlen base64 encoded     Number of bytes in string to base64 decode      char  base64 decoded   base64decode base64 encoded  bytes to decode       Base-64 decoding       printf  Original character string is   s n   data to encode      Prints our initial string      printf  Base-64 encoded string is   s n   base64 encoded      Prints base64 encoded string      printf  Base-64 decoded string is   s n   base64 decoded      Prints base64 decoded string       free base64 encoded                    Frees up the memory holding our base64 encoded data      free base64 decoded                    Frees up the memory holding our base64 decoded data

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This solution is based on schulwitz answer  encoding decoding using OpenSSL   but it is for C    well  original question was about C  but there are already another C   answers here  and it uses error checking  so it s safer to use     include  lt openssl bio h gt   std  string base64 encode const std  string  amp input        BIO  p bio b64   nullptr      BIO  p bio mem   nullptr       try                  make chain  p bio b64  lt -- gt  p bio mem         p bio b64   BIO new BIO f base64             if   p bio b64    throw std  runtime error  BIO new failed              BIO set flags p bio b64  BIO FLAGS BASE64 NO NL     No newlines every 64 characters or less          p bio mem   BIO new BIO s mem             if   p bio mem    throw std  runtime error  BIO new failed              BIO push p bio b64  p bio mem               write input to chain            write sequence  input -- gt  gt  p bio b64 -- gt  gt  p bio mem         if  BIO write p bio b64  input c str    input size     lt   0                throw std  runtime error  BIO write failed               if  BIO flush p bio b64   lt   0                throw std  runtime error  BIO flush failed                  get result         char  p encoded data   nullptr          auto  encoded len      BIO get mem data p bio mem   amp p encoded data           if   p encoded data    throw std  runtime error  BIO get mem data failed               std  string result p encoded data  encoded len               clean         BIO free all p bio b64            return result            catch                     if  p bio b64    BIO free all p bio b64             throw           std  string base64 decode const std  string  amp input        BIO  p bio mem   nullptr      BIO  p bio b64   nullptr       try                  make chain  p bio b64  lt -- gt  p bio mem         p bio b64   BIO new BIO f base64             if   p bio b64    throw std  runtime error  BIO new failed              BIO set flags p bio b64  BIO FLAGS BASE64 NO NL     Don t require trailing newlines          p bio mem   BIO new mem buf  void  input c str    input length             if   p bio mem    throw std  runtime error  BIO new failed              BIO push p bio b64  p bio mem               read result from chain            read sequence  reverse to write   buf  lt  lt -- p bio b64  lt  lt -- p bio mem         std  vector lt char gt  buf  input size   3 4  1           std  string result          for                            auto nread   BIO read p bio b64  buf data    buf size                 if  nread   lt  0    throw std  runtime error  BIO read failed                  if  nread    0    break       eof              result append buf data    nread                         clean         BIO free all p bio b64            return result            catch                     if  p bio b64    BIO free all p bio b64             throw            Note that base64 decode returns empty string  if input is incorrect base64 sequence  openssl works in such way

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GNU coreutils has it in lib base64   It s a little bloated but deals with stuff like EBCDIC   You can also play around on your own  e g    char base64 digit  n  unsigned n      if  n  lt  10  return n -  0     else if  n  lt  10   26  return n -  a     else if  n  lt  10   26   26  return n -  A     else assert 0     return 0     unsigned char base64 decode digit char c      switch  c        case       return 62      case       return 63      default          if  isdigit c   return c -  0         else if  islower c   return c -  a    10        else if  isupper c   return c -  A    10   26        else assert 0         return 0xff     unsigned base64 decode char  s      char  p    unsigned n   0     for  p   s   p  p        n   64   n   base64 decode digit  p      return n      Know ye all persons by these presents that you should not confuse  playing around on your own  with  implementing a standard    Yeesh

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Here s the one I m using    include  lt stdint h gt   include  lt stdlib h gt    static char encoding table       A    B    C    D    E    F    G    H                                    I    J    K    L    M    N    O    P                                    Q    R    S    T    U    V    W    X                                    Y    Z    a    b    c    d    e    f                                    g    h    i    j    k    l    m    n                                    o    p    q    r    s    t    u    v                                    w    x    y    z    0    1    2    3                                    4    5    6    7    8    9              static char  decoding table   NULL  static int mod table      0  2  1     char  base64 encode const unsigned char  data                      size t input length                      size t  output length          output length   4     input length   2    3        char  encoded data   malloc  output length       if  encoded data    NULL  return NULL       for  int i   0  j   0  i  lt  input length              uint32 t octet a   i  lt  input length    unsigned char data i      0          uint32 t octet b   i  lt  input length    unsigned char data i      0          uint32 t octet c   i  lt  input length    unsigned char data i      0           uint32 t triple    octet a  lt  lt  0x10     octet b  lt  lt  0x08    octet c           encoded data j      encoding table  triple  gt  gt  3   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  2   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  1   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  0   6   amp  0x3F              for  int i   0  i  lt  mod table input length   3   i            encoded data  output length - 1 - i              return encoded data      unsigned char  base64 decode const char  data                               size t input length                               size t  output length         if  decoding table    NULL  build decoding table         if  input length   4    0  return NULL        output length   input length   4   3      if  data input length - 1            output length --      if  data input length - 2            output length --       unsigned char  decoded data   malloc  output length       if  decoded data    NULL  return NULL       for  int i   0  j   0  i  lt  input length              uint32 t sextet a   data i           0  amp  i     decoding table data i              uint32 t sextet b   data i           0  amp  i     decoding table data i              uint32 t sextet c   data i           0  amp  i     decoding table data i              uint32 t sextet d   data i           0  amp  i     decoding table data i               uint32 t triple    sextet a  lt  lt  3   6             sextet b  lt  lt  2   6             sextet c  lt  lt  1   6             sextet d  lt  lt  0   6            if  j  lt   output length  decoded data j       triple  gt  gt  2   8   amp  0xFF          if  j  lt   output length  decoded data j       triple  gt  gt  1   8   amp  0xFF          if  j  lt   output length  decoded data j       triple  gt  gt  0   8   amp  0xFF             return decoded data      void build decoding table          decoding table   malloc 256        for  int i   0  i  lt  64  i            decoding table  unsigned char  encoding table i     i      void base64 cleanup         free decoding table       Keep in mind that this doesn t do any error-checking while decoding - non base 64 encoded data will get processed

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Here s the decoder I ve been using for years         static const char  table      ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789         static const int   BASE64 INPUT SIZE   57       BOOL isbase64 char c               return c  amp  amp  strchr table  c     NULL             inline char value char c               const char  p   strchr table  c          if p              return p-table           else             return 0                      int UnBase64 unsigned char  dest  const unsigned char  src  int srclen                dest   0         if  src    0                      return 0                  unsigned char  p   dest         do                     char a   value src 0              char b   value src 1              char c   value src 2              char d   value src 3               p      a  lt  lt  2     b  gt  gt  4              p      b  lt  lt  4     c  gt  gt  2              p      c  lt  lt  6    d            if  isbase64 src 1                              p -  2               break                         else if  isbase64 src 2                              p -  2               break                         else if  isbase64 src 3                              p--               break                        src    4            while  src  amp  amp    src    13     src    10   src                    while srclen-  4           p   0         return p-dest

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glib has functions for base64 encoding  https   developer gnome org glib stable glib-Base64-Encoding html

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The EVP EncodeBlock and EVP DecodeBlock functions make it very easy    include  lt stdio h gt   include  lt stdlib h gt   include  lt openssl evp h gt   char  base64 const unsigned char  input  int length      const int pl   4   length 2  3     char  output   calloc pl 1  1      1 for the terminating null that EVP EncodeBlock adds on   const int ol   EVP EncodeBlock output  input  length     if  ol    pl    fprintf stderr   Whoops  encode predicted  d but we got  d n   pl  ol       return output     unsigned char  decode64 const char  input  int length      const int pl   3 length 4    unsigned char  output   calloc pl 1  1     const int ol   EVP DecodeBlock output  input  length     if  pl    ol    fprintf stderr   Whoops  decode predicted  d but we got  d n   pl  ol       return output

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Here s the one I m using    include  lt stdint h gt   include  lt stdlib h gt    static char encoding table       A    B    C    D    E    F    G    H                                    I    J    K    L    M    N    O    P                                    Q    R    S    T    U    V    W    X                                    Y    Z    a    b    c    d    e    f                                    g    h    i    j    k    l    m    n                                    o    p    q    r    s    t    u    v                                    w    x    y    z    0    1    2    3                                    4    5    6    7    8    9              static char  decoding table   NULL  static int mod table      0  2  1     char  base64 encode const unsigned char  data                      size t input length                      size t  output length          output length   4     input length   2    3        char  encoded data   malloc  output length       if  encoded data    NULL  return NULL       for  int i   0  j   0  i  lt  input length              uint32 t octet a   i  lt  input length    unsigned char data i      0          uint32 t octet b   i  lt  input length    unsigned char data i      0          uint32 t octet c   i  lt  input length    unsigned char data i      0           uint32 t triple    octet a  lt  lt  0x10     octet b  lt  lt  0x08    octet c           encoded data j      encoding table  triple  gt  gt  3   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  2   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  1   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  0   6   amp  0x3F              for  int i   0  i  lt  mod table input length   3   i            encoded data  output length - 1 - i              return encoded data      unsigned char  base64 decode const char  data                               size t input length                               size t  output length         if  decoding table    NULL  build decoding table         if  input length   4    0  return NULL        output length   input length   4   3      if  data input length - 1            output length --      if  data input length - 2            output length --       unsigned char  decoded data   malloc  output length       if  decoded data    NULL  return NULL       for  int i   0  j   0  i  lt  input length              uint32 t sextet a   data i           0  amp  i     decoding table data i              uint32 t sextet b   data i           0  amp  i     decoding table data i              uint32 t sextet c   data i           0  amp  i     decoding table data i              uint32 t sextet d   data i           0  amp  i     decoding table data i               uint32 t triple    sextet a  lt  lt  3   6             sextet b  lt  lt  2   6             sextet c  lt  lt  1   6             sextet d  lt  lt  0   6            if  j  lt   output length  decoded data j       triple  gt  gt  2   8   amp  0xFF          if  j  lt   output length  decoded data j       triple  gt  gt  1   8   amp  0xFF          if  j  lt   output length  decoded data j       triple  gt  gt  0   8   amp  0xFF             return decoded data      void build decoding table          decoding table   malloc 256        for  int i   0  i  lt  64  i            decoding table  unsigned char  encoding table i     i      void base64 cleanup         free decoding table       Keep in mind that this doesn t do any error-checking while decoding - non base 64 encoded data will get processed

User · Answer

Here s the decoder I ve been using for years         static const char  table      ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789         static const int   BASE64 INPUT SIZE   57       BOOL isbase64 char c               return c  amp  amp  strchr table  c     NULL             inline char value char c               const char  p   strchr table  c          if p              return p-table           else             return 0                      int UnBase64 unsigned char  dest  const unsigned char  src  int srclen                dest   0         if  src    0                      return 0                  unsigned char  p   dest         do                     char a   value src 0              char b   value src 1              char c   value src 2              char d   value src 3               p      a  lt  lt  2     b  gt  gt  4              p      b  lt  lt  4     c  gt  gt  2              p      c  lt  lt  6    d            if  isbase64 src 1                              p -  2               break                         else if  isbase64 src 2                              p -  2               break                         else if  isbase64 src 3                              p--               break                        src    4            while  src  amp  amp    src    13     src    10   src                    while srclen-  4           p   0         return p-dest

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In case people need a c   solution  I put this OpenSSL solution together  for both encode and decode   You ll need to link with the  crypto  library  which is OpenSSL   This has been checked for leaks with valgrind  although you could add some additional error checking code to make it a bit better - I know at least the write function should check for return value     include  lt openssl bio h gt   include  lt openssl evp h gt   include  lt stdlib h gt   string base64 encode  const string  amp str         BIO  base64 filter   BIO new  BIO f base64          BIO set flags  base64 filter  BIO FLAGS BASE64 NO NL         BIO  bio   BIO new  BIO s mem          BIO set flags  bio  BIO FLAGS BASE64 NO NL         bio   BIO push  base64 filter  bio         BIO write  bio  str c str    str length           BIO flush  bio         char  new data       long bytes written   BIO get mem data  bio   amp new data         string result  new data  bytes written        BIO free all  bio         return result        string base64 decode  const string  amp str         BIO  bio   base64 filter   bio out      char inbuf 512       int inlen      base64 filter   BIO new  BIO f base64          BIO set flags  base64 filter  BIO FLAGS BASE64 NO NL         bio   BIO new mem buf   void  str c str    str length           bio   BIO push  base64 filter  bio         bio out   BIO new  BIO s mem           while   inlen   BIO read bio  inbuf  512    gt  0            BIO write  bio out  inbuf  inlen               BIO flush  bio out         char  new data      long bytes written   BIO get mem data  bio out   amp new data         string result  new data  bytes written         BIO free all  bio        BIO free all  bio out         return result

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I wrote one for use with C    it s very fast  works with streams  free  and open source   https   tmplusplus svn sourceforge net svnroot tmplusplus trunk src   Feel free to use it if it fits your purpose   Edit  Added code inline by request   The performance boost is acieved by using a lookup table for encoding and decoding   UINT8 is an unsigned char on most OS s       Static Base64 character encoding lookup table    const char CBase64  encodeCharacterTable 65     ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789          Static Base64 character decoding lookup table    const char CBase64  decodeCharacterTable 256          -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1      -1 62 -1 -1 -1 63 52 53 54 55 56 57 58 59 60 61 -1 -1 -1 -1 -1 -1 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21      22 23 24 25 -1 -1 -1 -1 -1 -1 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 -1 -1 -1 -1 -1      -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1      -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1      -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1      -1 -1 -1          brief Encodes binary data to base 64 character data  param in The data to encode  param out The encoded data as characters    void CBase64  Encode std  istream  amp in  std  ostringstream  amp out        char buff1 3       char buff2 4        UINT8 i 0  j      while in readsome  amp buff1 i     1           if  i  3                        out  lt  lt  encodeCharacterTable  buff1 0   amp  0xfc   gt  gt  2               out  lt  lt  encodeCharacterTable   buff1 0   amp  0x03   lt  lt  4      buff1 1   amp  0xf0   gt  gt  4                out  lt  lt  encodeCharacterTable   buff1 1   amp  0x0f   lt  lt  2      buff1 2   amp  0xc0   gt  gt  6                out  lt  lt  encodeCharacterTable buff1 2   amp  0x3f               i 0                 if  --i                for j i j lt 3 j    buff1 j      0            buff2 0     buff1 0   amp  0xfc   gt  gt  2          buff2 1      buff1 0   amp  0x03   lt  lt  4      buff1 1   amp  0xf0   gt  gt  4           buff2 2      buff1 1   amp  0x0f   lt  lt  2      buff1 2   amp  0xc0   gt  gt  6           buff2 3    buff1 2   amp  0x3f           for  j 0 j lt  i 1  j    out  lt  lt  encodeCharacterTable buff2 j             while i   lt 3  out  lt  lt                      brief Decodes base 64 character data to binary data  param in The character data to decode  param out The decoded data    void CBase64  Decode std  istringstream  amp in  std  ostream  amp out        char buff1 4       char buff2 4        UINT8 i 0  j       while in readsome  amp buff2 i   1   amp  amp  buff2 i                        if    i  4                        for  i 0 i  4 i                    buff2 i    decodeCharacterTable buff2 i                 out  lt  lt   char   buff2 0   lt  lt  2      buff2 1   amp  0x30   gt  gt  4                out  lt  lt   char    buff2 1   amp  0xf   lt  lt  4      buff2 2   amp  0x3c   gt  gt  2                out  lt  lt   char    buff2 2   amp  0x3   lt  lt  6    buff2 3                 i 0                       if  i                 for  j i j lt 4 j    buff2 j      0           for  j 0 j lt 4 j    buff2 j    decodeCharacterTable buff2 j             buff1 0     buff2 0   lt  lt  2      buff2 1   amp  0x30   gt  gt  4           buff1 1      buff2 1   amp  0xf   lt  lt  4      buff2 2   amp  0x3c   gt  gt  2           buff1 2      buff2 2   amp  0x3   lt  lt  6    buff2 3            for  j 0 j lt  i-1   j    out  lt  lt   char buff1 j

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This solution is based on schulwitz answer  encoding decoding using OpenSSL   but it is for C    well  original question was about C  but there are already another C   answers here  and it uses error checking  so it s safer to use     include  lt openssl bio h gt   std  string base64 encode const std  string  amp input        BIO  p bio b64   nullptr      BIO  p bio mem   nullptr       try                  make chain  p bio b64  lt -- gt  p bio mem         p bio b64   BIO new BIO f base64             if   p bio b64    throw std  runtime error  BIO new failed              BIO set flags p bio b64  BIO FLAGS BASE64 NO NL     No newlines every 64 characters or less          p bio mem   BIO new BIO s mem             if   p bio mem    throw std  runtime error  BIO new failed              BIO push p bio b64  p bio mem               write input to chain            write sequence  input -- gt  gt  p bio b64 -- gt  gt  p bio mem         if  BIO write p bio b64  input c str    input size     lt   0                throw std  runtime error  BIO write failed               if  BIO flush p bio b64   lt   0                throw std  runtime error  BIO flush failed                  get result         char  p encoded data   nullptr          auto  encoded len      BIO get mem data p bio mem   amp p encoded data           if   p encoded data    throw std  runtime error  BIO get mem data failed               std  string result p encoded data  encoded len               clean         BIO free all p bio b64            return result            catch                     if  p bio b64    BIO free all p bio b64             throw           std  string base64 decode const std  string  amp input        BIO  p bio mem   nullptr      BIO  p bio b64   nullptr       try                  make chain  p bio b64  lt -- gt  p bio mem         p bio b64   BIO new BIO f base64             if   p bio b64    throw std  runtime error  BIO new failed              BIO set flags p bio b64  BIO FLAGS BASE64 NO NL     Don t require trailing newlines          p bio mem   BIO new mem buf  void  input c str    input length             if   p bio mem    throw std  runtime error  BIO new failed              BIO push p bio b64  p bio mem               read result from chain            read sequence  reverse to write   buf  lt  lt -- p bio b64  lt  lt -- p bio mem         std  vector lt char gt  buf  input size   3 4  1           std  string result          for                            auto nread   BIO read p bio b64  buf data    buf size                 if  nread   lt  0    throw std  runtime error  BIO read failed                  if  nread    0    break       eof              result append buf data    nread                         clean         BIO free all p bio b64            return result            catch                     if  p bio b64    BIO free all p bio b64             throw            Note that base64 decode returns empty string  if input is incorrect base64 sequence  openssl works in such way

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glib has functions for base64 encoding  https   developer gnome org glib stable glib-Base64-Encoding html

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The EVP EncodeBlock and EVP DecodeBlock functions make it very easy    include  lt stdio h gt   include  lt stdlib h gt   include  lt openssl evp h gt   char  base64 const unsigned char  input  int length      const int pl   4   length 2  3     char  output   calloc pl 1  1      1 for the terminating null that EVP EncodeBlock adds on   const int ol   EVP EncodeBlock output  input  length     if  ol    pl    fprintf stderr   Whoops  encode predicted  d but we got  d n   pl  ol       return output     unsigned char  decode64 const char  input  int length      const int pl   3 length 4    unsigned char  output   calloc pl 1  1     const int ol   EVP DecodeBlock output  input  length     if  pl    ol    fprintf stderr   Whoops  decode predicted  d but we got  d n   pl  ol       return output

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glib has functions for base64 encoding  https   developer gnome org glib stable glib-Base64-Encoding html

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Here s the decoder I ve been using for years         static const char  table      ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789         static const int   BASE64 INPUT SIZE   57       BOOL isbase64 char c               return c  amp  amp  strchr table  c     NULL             inline char value char c               const char  p   strchr table  c          if p              return p-table           else             return 0                      int UnBase64 unsigned char  dest  const unsigned char  src  int srclen                dest   0         if  src    0                      return 0                  unsigned char  p   dest         do                     char a   value src 0              char b   value src 1              char c   value src 2              char d   value src 3               p      a  lt  lt  2     b  gt  gt  4              p      b  lt  lt  4     c  gt  gt  2              p      c  lt  lt  6    d            if  isbase64 src 1                              p -  2               break                         else if  isbase64 src 2                              p -  2               break                         else if  isbase64 src 3                              p--               break                        src    4            while  src  amp  amp    src    13     src    10   src                    while srclen-  4           p   0         return p-dest

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But you can also do it in openssl  openssl enc command does it       look at the BIO f base64   function

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Here is an optimized version of encoder for the accepted answer  that also supports line-breaking for MIME and other protocols  simlar optimization can be applied to the decoder     char  base64 encode const unsigned char  data                      size t input length                      size t  output length                      bool addLineBreaks        output length   4     input length   2    3       if  addLineBreaks   output length     output length   38      CRLF after each 76 chars      char  encoded data   malloc  output length       if  encoded data    NULL  return NULL       UInt32 octet a      UInt32 octet b      UInt32 octet c      UInt32 triple      int lineCount   0      int sizeMod   size -  size   3      check if there is a partial triplet        adding all octet triplets  before partial last triplet     for    offset  lt  sizeMod                   octet a   data offset             octet b   data offset             octet c   data offset              triple    octet a  lt  lt  0x10     octet b  lt  lt  0x08    octet c           encoded data mBufferPos      encoding table  triple  gt  gt  3   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  2   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  1   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  0   6   amp  0x3F           if  addLineBreaks                        if    lineCount    19                                encoded data mBufferPos      13                  encoded data mBufferPos      10                  lineCount   0                                        last bytes     if  sizeMod  lt  size                octet a   data offset        first octect always added         octet b   offset  lt  size   data offset       UInt32 0     conditional 2nd octet         octet c    UInt32 0     last character is definitely padded          triple    octet a  lt  lt  0x10     octet b  lt  lt  0x08    octet c           encoded data mBufferPos      encoding table  triple  gt  gt  3   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  2   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  1   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  0   6   amp  0x3F               add padding             sizeMod   size   3              last character is definitely padded         encoded data mBufferPos - 1     byte              if  sizeMod    1  encoded data mBufferPos - 2     byte

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If you want to find a workable C solution  I believe you need this  https   github com littlstar b64 c  include  lt stdio h gt   include  lt string h gt   include  lt stdlib h gt   include  quot b64 h quot   int main  void      unsigned char  str    quot brian the monkey and bradley the kinkajou are friends quot     char  enc   b64 encode str  strlen str       printf  quot  s n quot   enc      YnJpYW4gdGhlIG1vbmtleSBhbmQgYnJhZGxleSB0aGUga2lua2Fqb3UgYXJlIGZyaWVuZHM     char  dec   b64 decode enc  strlen enc       printf  quot  s n quot   dec      brian the monkey and bradley the kinkajou are friends   free enc     free dec     return 0

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Small improvement to the code from ryyst  who got the most votes  is to not use dynamically allocated decoding table but rather static const precomputed table  This eliminates the use of pointer and initialization of the table  and also avoids memory leakage if one forgets to clean up the decoding table with base64 cleanup    by the way  in base64 cleanup    after calling free decoding table   one should have decoding table NULL  otherwise accidentally calling base64 decode after base64 cleanup   will crash or cause undetermined behavior   Another solution could be to use std  unique ptr   but I m satisfied with just having const char 256  on the stack and avoid using pointers alltogether - the code looks cleaner and shorter this way   The decoding table is computed as follows   const char encoding table             A    B    C    D    E    F    G    H        I    J    K    L    M    N    O    P        Q    R    S    T    U    V    W    X        Y    Z    a    b    c    d    e    f        g    h    i    j    k    l    m    n        o    p    q    r    s    t    u    v        w    x    y    z    0    1    2    3        4    5    6    7    8    9                unsigned char decoding table 256    for  int i   0  i  lt  256  i        decoding table i      0    for  int i   0  i  lt  64  i        decoding table  unsigned char encoding table i     i   for  int i   0  i  lt  256  i        cout  lt  lt   0x   lt  lt   int decoding table i    lt  16    0         lt  lt  hex  lt  lt  int decoding table i    lt  lt   i    255              lt  lt    i 1    16    0     n      0     cin ignore      and the modified code I am using is           static const char encoding table                     A    B    C    D    E    F    G    H                I    J    K    L    M    N    O    P                Q    R    S    T    U    V    W    X                Y    Z    a    b    c    d    e    f                g    h    i    j    k    l    m    n                o    p    q    r    s    t    u    v                w    x    y    z    0    1    2    3                4    5    6    7    8    9                        static const unsigned char decoding table 256                  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x3e  0x00  0x00  0x00  0x3f              0x34  0x35  0x36  0x37  0x38  0x39  0x3a  0x3b  0x3c  0x3d  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x01  0x02  0x03  0x04  0x05  0x06  0x07  0x08  0x09  0x0a  0x0b  0x0c  0x0d  0x0e              0x0f  0x10  0x11  0x12  0x13  0x14  0x15  0x16  0x17  0x18  0x19  0x00  0x00  0x00  0x00  0x00              0x00  0x1a  0x1b  0x1c  0x1d  0x1e  0x1f  0x20  0x21  0x22  0x23  0x24  0x25  0x26  0x27  0x28              0x29  0x2a  0x2b  0x2c  0x2d  0x2e  0x2f  0x30  0x31  0x32  0x33  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00             char  base64 encode const unsigned char  data  size t input length  size t  amp output length                 const int mod table       0  2  1                 output length   4     input length   2    3                char  encoded data    char  malloc output length                if  encoded data    nullptr                  return nullptr               for  int i   0  j   0  i  lt  input length                      uint32 t octet a   i  lt  input length    unsigned char data i      0                  uint32 t octet b   i  lt  input length    unsigned char data i      0                  uint32 t octet c   i  lt  input length    unsigned char data i      0                   uint32 t triple    octet a  lt  lt  0x10     octet b  lt  lt  0x08    octet c                   encoded data j      encoding table  triple  gt  gt  3   6   amp  0x3F                   encoded data j      encoding table  triple  gt  gt  2   6   amp  0x3F                   encoded data j      encoding table  triple  gt  gt  1   6   amp  0x3F                   encoded data j      encoding table  triple  gt  gt  0   6   amp  0x3F                               for  int i   0  i  lt  mod table input length   3   i                    encoded data output length - 1 - i                      return encoded data                       unsigned char  base64 decode const char  data  size t input length  size t  amp output length                         if  input length   4    0                  return nullptr               output length   input length   4   3               if  data input length - 1           output length --              if  data input length - 2           output length --               unsigned char  decoded data    unsigned char  malloc output length                if  decoded data    nullptr                  return nullptr               for  int i   0  j   0  i  lt  input length                      uint32 t sextet a   data i           0  amp  i     decoding table data i                      uint32 t sextet b   data i           0  amp  i     decoding table data i                      uint32 t sextet c   data i           0  amp  i     decoding table data i                      uint32 t sextet d   data i           0  amp  i     decoding table data i                       uint32 t triple    sextet a  lt  lt  3   6                         sextet b  lt  lt  2   6                         sextet c  lt  lt  1   6                         sextet d  lt  lt  0   6                    if  j  lt  output length  decoded data j       triple  gt  gt  2   8   amp  0xFF                  if  j  lt  output length  decoded data j       triple  gt  gt  1   8   amp  0xFF                  if  j  lt  output length  decoded data j       triple  gt  gt  0   8   amp  0xFF                              return decoded data

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Here s my solution using OpenSSL      A BASE-64 ENCODER AND DECODER USING OPENSSL     include  lt openssl pem h gt   include  lt string h gt    Only needed for strlen     char  base64encode  const void  b64 encode this  int encode this many bytes       BIO  b64 bio   mem bio         Declares two OpenSSL BIOs  a base64 filter and a memory BIO      BUF MEM  mem bio mem ptr       Pointer to a  memory BIO  structure holding our base64 data      b64 bio   BIO new BIO f base64                            Initialize our base64 filter BIO      mem bio   BIO new BIO s mem                                 Initialize our memory sink BIO      BIO push b64 bio  mem bio                Link the BIOs by creating a filter-sink BIO chain      BIO set flags b64 bio  BIO FLAGS BASE64 NO NL      No newlines every 64 characters or less      BIO write b64 bio  b64 encode this  encode this many bytes     Records base64 encoded data      BIO flush b64 bio       Flush data   Necessary for b64 encoding  because of pad characters      BIO get mem ptr mem bio   amp mem bio mem ptr      Store address of mem bio s memory structure      BIO set close mem bio  BIO NOCLOSE       Permit access to mem ptr after BIOs are destroyed      BIO free all b64 bio      Destroys all BIOs in chain  starting with b64  i e  the 1st one       BUF MEM grow mem bio mem ptr    mem bio mem ptr  length   1       Makes space for end null        mem bio mem ptr  data   mem bio mem ptr  length      0      Adds null-terminator to tail      return   mem bio mem ptr  data    Returns base-64 encoded data   See   buf mem st  struct      char  base64decode  const void  b64 decode this  int decode this many bytes       BIO  b64 bio   mem bio         Declares two OpenSSL BIOs  a base64 filter and a memory BIO      char  base64 decoded   calloc   decode this many bytes 3  4 1  sizeof char        1   null      b64 bio   BIO new BIO f base64                            Initialize our base64 filter BIO      mem bio   BIO new BIO s mem                               Initialize our memory source BIO      BIO write mem bio  b64 decode this  decode this many bytes     Base64 data saved in source      BIO push b64 bio  mem bio              Link the BIOs by creating a filter-source BIO chain      BIO set flags b64 bio  BIO FLAGS BASE64 NO NL              Don t require trailing newlines      int decoded byte index   0      Index where the next base64 decoded byte should be written      while   0  lt  BIO read b64 bio  base64 decoded decoded byte index  1       Read byte-by-byte          decoded byte index      Increment the index until read of BIO decoded data is complete          Once we re done reading decoded data  BIO read returns -1 even though there s no error      BIO free all b64 bio      Destroys all BIOs in chain  starting with b64  i e  the 1st one       return base64 decoded           Returns base-64 decoded data with trailing null terminator       Here s one way to base64 encode decode using the base64encode   and base64decode functions    int main void       char data to encode      Base64 encode this string       The string we will base-64 encode       int bytes to encode   strlen data to encode     Number of bytes in string to base64 encode      char  base64 encoded   base64encode data to encode  bytes to encode       Base-64 encoding       int bytes to decode   strlen base64 encoded     Number of bytes in string to base64 decode      char  base64 decoded   base64decode base64 encoded  bytes to decode       Base-64 decoding       printf  Original character string is   s n   data to encode      Prints our initial string      printf  Base-64 encoded string is   s n   base64 encoded      Prints base64 encoded string      printf  Base-64 decoded string is   s n   base64 decoded      Prints base64 decoded string       free base64 encoded                    Frees up the memory holding our base64 encoded data      free base64 decoded                    Frees up the memory holding our base64 decoded data

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This is a decoder that is specifically written to avoid the need for a buffer  by writing directly to a putchar function  This is based on wikibook s implementation https   en wikibooks org wiki Algorithm Implementation Miscellaneous Base64 C  This is not as easy to use as other options above  However  it can be of use in embedded systems  where you want to dump a large file without allocating another large buffer to store the resultant base64 datauri string   It s a pity that datauri does not let you specify the filename    void datauriBase64EncodeBufferless int   putchar fcptr  int   const char  type strptr  const void  data buf  const size t dataLength      const char base64chars      ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789       const uint8 t  data    const uint8 t   data buf    size t x   0    uint32 t n   0    int padCount   dataLength   3    uint8 t n0  n1  n2  n3     size t outcount   0    size t line   0     putchar fcptr  int  d      putchar fcptr  int  a      putchar fcptr  int  t      putchar fcptr  int  a      putchar fcptr  int         outcount    5     while   type strptr      0           putchar fcptr  int  type strptr       type strptr        outcount           putchar fcptr  int         putchar fcptr  int  b      putchar fcptr  int  a      putchar fcptr  int  s      putchar fcptr  int  e      putchar fcptr  int  6      putchar fcptr  int  4      putchar fcptr  int         outcount    8        increment over the length of the string  three characters at a time      for  x   0  x  lt  dataLength  x    3             these three 8-bit  ASCII  characters become one 24-bit number        n     uint32 t data x    lt  lt  16    parenthesis needed  compiler depending on flags can do the shifting before conversion to uint32 t  resulting to 0      if  x 1   lt  dataLength         n      uint32 t data x 1    lt  lt  8   parenthesis needed  compiler depending on flags can do the shifting before conversion to uint32 t  resulting to 0      if  x 2   lt  dataLength         n    data x 2           this 24-bit number gets separated into four 6-bit numbers        n0    uint8 t  n  gt  gt  18   amp  63      n1    uint8 t  n  gt  gt  12   amp  63      n2    uint8 t  n  gt  gt  6   amp  63      n3    uint8 t n  amp  63                 if we have one byte available  then its encoding is spread        out over two characters              putchar fcptr  int base64chars n0        putchar fcptr  int base64chars n1        outcount    2                 if we have only two bytes available  then their encoding is        spread out over three chars             if  x 1   lt  dataLength              putchar fcptr  int base64chars n2          outcount    1                       if we have all three bytes available  then their encoding is spread        out over four characters             if  x 2   lt  dataLength              putchar fcptr  int base64chars n3          outcount    1                Breaking up the line so it s easier to copy and paste        int curr line    outcount 80       if  curr line    line               line   curr line        putchar fcptr  int   r          putchar fcptr  int   n                        create and add padding that is required if we did not have a multiple of 3     number of characters available        if  padCount  gt  0          for    padCount  lt  3  padCount                putchar fcptr  int                    putchar fcptr  int   r      putchar fcptr  int   n        Here is the test   include  lt stdio h gt   include  lt stdint h gt   include  lt string h gt   int main void      char str      test     datauriBase64EncodeBufferless putchar   text plain charset utf-8   str  strlen str      return 0      Expected Output  data text plain charset utf-8 base64 dGVzdA

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But you can also do it in openssl  openssl enc command does it       look at the BIO f base64   function

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I know this question is quite old  but I was getting confused by the amount of solutions provided - each one of them claiming to be faster and better   I put together a project on github to compare the base64 encoders and decoders  https   github com gaspardpetit base64   At this point  I have not limited myself to C algorithms - if one implementation performs well in C    it can easily be backported to C  Also tests were conducted using Visual Studio 2015  If somebody wants to update this answer with results from clang gcc  be my guest   FASTEST ENCODERS   The two fastest encoder implementations I found were Jouni Malinen s at http   web mit edu freebsd head contrib wpa src utils base64 c and the Apache at https   opensource apple com source QuickTimeStreamingServer QuickTimeStreamingServer-452 CommonUtilitiesLib base64 c    Here is the time  in microseconds  to encode 32K of data using the different algorithms I have tested up to now   jounimalinen                25 1544 apache                      25 5309 NibbleAndAHalf              38 4165 internetsoftwareconsortium  48 2879 polfosol                    48 7955 wikibooks org c             51 9659 gnome                       74 8188 elegantdice                 118 899 libb64                      120 601 manuelmartinez              120 801 arduino                     126 262 daedalusalpha               126 473 CppCodec                    151 866 wikibooks org cpp           343 2 adp gmbh                    381 523 LihO                        406 693 libcurl                     3246 39 user152949                  4828 21    Ren   Nyffenegger s solution  credited in another answer to this question  is listed here as adp gmbh    Here is the one from Jouni Malinen that I slightly modified to return a std  string        Base64 encoding decoding  RFC1341    Copyright  c  2005-2011  Jouni Malinen  lt j w1 fi gt      This software may be distributed under the terms of the BSD license    See README for more details         2016-12-12 - Gaspard Petit   Slightly modified to return a std  string     instead of a buffer allocated with malloc    include  lt string gt   static const unsigned char base64 table 65         ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789            base64 encode - Base64 encode    src  Data to be encoded    len  Length of the data to be encoded    out len  Pointer to output length variable  or  NULL if not used   Returns  Allocated buffer of out len bytes of encoded data    or empty string on failure    std  string base64 encode const unsigned char  src  size t len        unsigned char  out   pos      const unsigned char  end   in       size t olen       olen   4   len   2    3      3-byte blocks to 4-byte         if  olen  lt  len          return std  string       integer overflow         std  string outStr      outStr resize olen       out    unsigned char   amp outStr 0        end   src   len      in   src      pos   out      while  end - in  gt   3             pos     base64 table in 0   gt  gt  2            pos     base64 table   in 0   amp  0x03   lt  lt  4     in 1   gt  gt  4             pos     base64 table   in 1   amp  0x0f   lt  lt  2     in 2   gt  gt  6             pos     base64 table in 2   amp  0x3f           in    3             if  end - in             pos     base64 table in 0   gt  gt  2           if  end - in    1                 pos     base64 table  in 0   amp  0x03   lt  lt  4                pos                            else                pos     base64 table   in 0   amp  0x03   lt  lt  4                     in 1   gt  gt  4                 pos     base64 table  in 1   amp  0x0f   lt  lt  2                      pos                     return outStr      FASTEST DECODERS  Here are the decoding results and I must admit that I am a bit surprised   polfosol                    45 2335 wikibooks org c             74 7347 apache                      77 1438 libb64                      100 332 gnome                       114 511 manuelmartinez              126 579 elegantdice                 138 514 daedalusalpha               151 561 jounimalinen                206 163 arduino                     335 95 wikibooks org cpp           350 437 CppCodec                    526 187 internetsoftwareconsortium  862 833 libcurl                     1280 27 LihO                        1852 4 adp gmbh                    1934 43 user152949                  5332 87   Polfosol s snippet from base64 decode snippet in c   is the fastest by a factor of almost 2x   Here is the code for the sake of completeness   static const int B64index 256      0   0   0   0   0   0   0   0   0   0   0   0  0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0  0   0   0   0   0   0   0   0   0   0   0  62  63  62  62  63  52  53  54  55  56  57  58  59  60  61   0   0   0   0   0   0   0   0   1   2   3   4   5   6  7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25   0  0   0   0  63   0  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51     std  string b64decode const void  data  const size t len        unsigned char  p    unsigned char  data      int pad   len  gt  0  amp  amp   len   4    p len - 1               const size t L     len   3    4 - pad    4      std  string str L   4   3   pad    0         for  size t i   0  j   0  i  lt  L  i    4                int n   B64index p i    lt  lt  18   B64index p i   1    lt  lt  12   B64index p i   2    lt  lt  6   B64index p i   3            str j      n  gt  gt  16          str j      n  gt  gt  8  amp  0xFF          str j      n  amp  0xFF            if  pad                int n   B64index p L    lt  lt  18   B64index p L   1    lt  lt  12          str str size   - 1    n  gt  gt  16           if  len  gt  L   2  amp  amp  p L   2                                n    B64index p L   2    lt  lt  6              str push back n  gt  gt  8  amp  0xFF                       return str

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I wrote one for use with C    it s very fast  works with streams  free  and open source   https   tmplusplus svn sourceforge net svnroot tmplusplus trunk src   Feel free to use it if it fits your purpose   Edit  Added code inline by request   The performance boost is acieved by using a lookup table for encoding and decoding   UINT8 is an unsigned char on most OS s       Static Base64 character encoding lookup table    const char CBase64  encodeCharacterTable 65     ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789          Static Base64 character decoding lookup table    const char CBase64  decodeCharacterTable 256          -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1      -1 62 -1 -1 -1 63 52 53 54 55 56 57 58 59 60 61 -1 -1 -1 -1 -1 -1 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21      22 23 24 25 -1 -1 -1 -1 -1 -1 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 -1 -1 -1 -1 -1      -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1      -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1      -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1      -1 -1 -1          brief Encodes binary data to base 64 character data  param in The data to encode  param out The encoded data as characters    void CBase64  Encode std  istream  amp in  std  ostringstream  amp out        char buff1 3       char buff2 4        UINT8 i 0  j      while in readsome  amp buff1 i     1           if  i  3                        out  lt  lt  encodeCharacterTable  buff1 0   amp  0xfc   gt  gt  2               out  lt  lt  encodeCharacterTable   buff1 0   amp  0x03   lt  lt  4      buff1 1   amp  0xf0   gt  gt  4                out  lt  lt  encodeCharacterTable   buff1 1   amp  0x0f   lt  lt  2      buff1 2   amp  0xc0   gt  gt  6                out  lt  lt  encodeCharacterTable buff1 2   amp  0x3f               i 0                 if  --i                for j i j lt 3 j    buff1 j      0            buff2 0     buff1 0   amp  0xfc   gt  gt  2          buff2 1      buff1 0   amp  0x03   lt  lt  4      buff1 1   amp  0xf0   gt  gt  4           buff2 2      buff1 1   amp  0x0f   lt  lt  2      buff1 2   amp  0xc0   gt  gt  6           buff2 3    buff1 2   amp  0x3f           for  j 0 j lt  i 1  j    out  lt  lt  encodeCharacterTable buff2 j             while i   lt 3  out  lt  lt                      brief Decodes base 64 character data to binary data  param in The character data to decode  param out The decoded data    void CBase64  Decode std  istringstream  amp in  std  ostream  amp out        char buff1 4       char buff2 4        UINT8 i 0  j       while in readsome  amp buff2 i   1   amp  amp  buff2 i                        if    i  4                        for  i 0 i  4 i                    buff2 i    decodeCharacterTable buff2 i                 out  lt  lt   char   buff2 0   lt  lt  2      buff2 1   amp  0x30   gt  gt  4                out  lt  lt   char    buff2 1   amp  0xf   lt  lt  4      buff2 2   amp  0x3c   gt  gt  2                out  lt  lt   char    buff2 2   amp  0x3   lt  lt  6    buff2 3                 i 0                       if  i                 for  j i j lt 4 j    buff2 j      0           for  j 0 j lt 4 j    buff2 j    decodeCharacterTable buff2 j             buff1 0     buff2 0   lt  lt  2      buff2 1   amp  0x30   gt  gt  4           buff1 1      buff2 1   amp  0xf   lt  lt  4      buff2 2   amp  0x3c   gt  gt  2           buff1 2      buff2 2   amp  0x3   lt  lt  6    buff2 3            for  j 0 j lt  i-1   j    out  lt  lt   char buff1 j

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I needed C   implementation working on std  string  None of answers satisfied my needs  I needed simple two-function solution for encoding and decoding  but I was too lazy to write my own code  so I found this   http   www adp-gmbh ch cpp common base64 html  Credits for code go to Ren   Nyffenegger   Putting the code below in case the site goes down   base64 cpp         base64 cpp and base64 h     Copyright  C  2004-2008 Ren   Nyffenegger     This source code is provided  as-is   without any express or implied    warranty  In no event will the author be held liable for any damages    arising from the use of this software      Permission is granted to anyone to use this software for any purpose     including commercial applications  and to alter it and redistribute it    freely  subject to the following restrictions      1  The origin of this source code must not be misrepresented  you must not       claim that you wrote the original source code  If you use this source code       in a product  an acknowledgment in the product documentation would be       appreciated but is not required      2  Altered source versions must be plainly marked as such  and must not be       misrepresented as being the original source code      3  This notice may not be removed or altered from any source distribution      Ren   Nyffenegger rene nyffenegger adp-gmbh ch       include  base64 h   include  lt iostream gt   static const std  string base64 chars                  ABCDEFGHIJKLMNOPQRSTUVWXYZ                abcdefghijklmnopqrstuvwxyz                0123456789       static inline bool is base64 unsigned char c      return  isalnum c      c             c              std  string base64 encode unsigned char const  bytes to encode  unsigned int in len      std  string ret    int i   0    int j   0    unsigned char char array 3 3     unsigned char char array 4 4      while  in len--        char array 3 i        bytes to encode         if  i    3          char array 4 0     char array 3 0   amp  0xfc   gt  gt  2        char array 4 1      char array 3 0   amp  0x03   lt  lt  4      char array 3 1   amp  0xf0   gt  gt  4         char array 4 2      char array 3 1   amp  0x0f   lt  lt  2      char array 3 2   amp  0xc0   gt  gt  6         char array 4 3    char array 3 2   amp  0x3f         for i   0   i  lt 4    i            ret    base64 chars char array 4 i          i   0               if  i          for j   i  j  lt  3  j          char array 3 j      0        char array 4 0     char array 3 0   amp  0xfc   gt  gt  2      char array 4 1      char array 3 0   amp  0x03   lt  lt  4      char array 3 1   amp  0xf0   gt  gt  4       char array 4 2      char array 3 1   amp  0x0f   lt  lt  2      char array 3 2   amp  0xc0   gt  gt  6       char array 4 3    char array 3 2   amp  0x3f       for  j   0   j  lt  i   1   j          ret    base64 chars char array 4 j         while  i    lt  3         ret                 return ret      std  string base64 decode std  string const amp  encoded string      int in len   encoded string size      int i   0    int j   0    int in    0    unsigned char char array 4 4   char array 3 3     std  string ret     while  in len--  amp  amp    encoded string in            amp  amp  is base64 encoded string in           char array 4 i      encoded string in    in         if  i   4          for  i   0  i  lt 4  i            char array 4 i    base64 chars find char array 4 i           char array 3 0     char array 4 0   lt  lt  2      char array 4 1   amp  0x30   gt  gt  4         char array 3 1      char array 4 1   amp  0xf   lt  lt  4      char array 4 2   amp  0x3c   gt  gt  2         char array 3 2      char array 4 2   amp  0x3   lt  lt  6    char array 4 3          for  i   0   i  lt  3   i            ret    char array 3 i         i   0               if  i        for  j   i  j  lt 4  j          char array 4 j    0       for  j   0  j  lt 4  j          char array 4 j    base64 chars find char array 4 j         char array 3 0     char array 4 0   lt  lt  2      char array 4 1   amp  0x30   gt  gt  4       char array 3 1      char array 4 1   amp  0xf   lt  lt  4      char array 4 2   amp  0x3c   gt  gt  2       char array 3 2      char array 4 2   amp  0x3   lt  lt  6    char array 4 3        for  j   0   j  lt  i - 1   j    ret    char array 3 j          return ret      base64 h   include  lt string gt   std  string base64 encode unsigned char const    unsigned int len   std  string base64 decode std  string const amp  s     Usage  const std  string s    test   std  string encoded   base64 encode reinterpret cast lt const unsigned char  gt  s c str     s length       std  string decoded   base64 decode encoded

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libb64 has both C and C   APIs  It is lightweight and perhaps the fastest publicly available implementation  It s also a dedicated stand-alone base64 encoding library  which can be nice if you don t need all the other stuff that comes from using a larger library such as OpenSSL or glib

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I needed C   implementation working on std  string  None of answers satisfied my needs  I needed simple two-function solution for encoding and decoding  but I was too lazy to write my own code  so I found this   http   www adp-gmbh ch cpp common base64 html  Credits for code go to Ren   Nyffenegger   Putting the code below in case the site goes down   base64 cpp         base64 cpp and base64 h     Copyright  C  2004-2008 Ren   Nyffenegger     This source code is provided  as-is   without any express or implied    warranty  In no event will the author be held liable for any damages    arising from the use of this software      Permission is granted to anyone to use this software for any purpose     including commercial applications  and to alter it and redistribute it    freely  subject to the following restrictions      1  The origin of this source code must not be misrepresented  you must not       claim that you wrote the original source code  If you use this source code       in a product  an acknowledgment in the product documentation would be       appreciated but is not required      2  Altered source versions must be plainly marked as such  and must not be       misrepresented as being the original source code      3  This notice may not be removed or altered from any source distribution      Ren   Nyffenegger rene nyffenegger adp-gmbh ch       include  base64 h   include  lt iostream gt   static const std  string base64 chars                  ABCDEFGHIJKLMNOPQRSTUVWXYZ                abcdefghijklmnopqrstuvwxyz                0123456789       static inline bool is base64 unsigned char c      return  isalnum c      c             c              std  string base64 encode unsigned char const  bytes to encode  unsigned int in len      std  string ret    int i   0    int j   0    unsigned char char array 3 3     unsigned char char array 4 4      while  in len--        char array 3 i        bytes to encode         if  i    3          char array 4 0     char array 3 0   amp  0xfc   gt  gt  2        char array 4 1      char array 3 0   amp  0x03   lt  lt  4      char array 3 1   amp  0xf0   gt  gt  4         char array 4 2      char array 3 1   amp  0x0f   lt  lt  2      char array 3 2   amp  0xc0   gt  gt  6         char array 4 3    char array 3 2   amp  0x3f         for i   0   i  lt 4    i            ret    base64 chars char array 4 i          i   0               if  i          for j   i  j  lt  3  j          char array 3 j      0        char array 4 0     char array 3 0   amp  0xfc   gt  gt  2      char array 4 1      char array 3 0   amp  0x03   lt  lt  4      char array 3 1   amp  0xf0   gt  gt  4       char array 4 2      char array 3 1   amp  0x0f   lt  lt  2      char array 3 2   amp  0xc0   gt  gt  6       char array 4 3    char array 3 2   amp  0x3f       for  j   0   j  lt  i   1   j          ret    base64 chars char array 4 j         while  i    lt  3         ret                 return ret      std  string base64 decode std  string const amp  encoded string      int in len   encoded string size      int i   0    int j   0    int in    0    unsigned char char array 4 4   char array 3 3     std  string ret     while  in len--  amp  amp    encoded string in            amp  amp  is base64 encoded string in           char array 4 i      encoded string in    in         if  i   4          for  i   0  i  lt 4  i            char array 4 i    base64 chars find char array 4 i           char array 3 0     char array 4 0   lt  lt  2      char array 4 1   amp  0x30   gt  gt  4         char array 3 1      char array 4 1   amp  0xf   lt  lt  4      char array 4 2   amp  0x3c   gt  gt  2         char array 3 2      char array 4 2   amp  0x3   lt  lt  6    char array 4 3          for  i   0   i  lt  3   i            ret    char array 3 i         i   0               if  i        for  j   i  j  lt 4  j          char array 4 j    0       for  j   0  j  lt 4  j          char array 4 j    base64 chars find char array 4 j         char array 3 0     char array 4 0   lt  lt  2      char array 4 1   amp  0x30   gt  gt  4       char array 3 1      char array 4 1   amp  0xf   lt  lt  4      char array 4 2   amp  0x3c   gt  gt  2       char array 3 2      char array 4 2   amp  0x3   lt  lt  6    char array 4 3        for  j   0   j  lt  i - 1   j    ret    char array 3 j          return ret      base64 h   include  lt string gt   std  string base64 encode unsigned char const    unsigned int len   std  string base64 decode std  string const amp  s     Usage  const std  string s    test   std  string encoded   base64 encode reinterpret cast lt const unsigned char  gt  s c str     s length       std  string decoded   base64 decode encoded

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GNU coreutils has it in lib base64   It s a little bloated but deals with stuff like EBCDIC   You can also play around on your own  e g    char base64 digit  n  unsigned n      if  n  lt  10  return n -  0     else if  n  lt  10   26  return n -  a     else if  n  lt  10   26   26  return n -  A     else assert 0     return 0     unsigned char base64 decode digit char c      switch  c        case       return 62      case       return 63      default          if  isdigit c   return c -  0         else if  islower c   return c -  a    10        else if  isupper c   return c -  A    10   26        else assert 0         return 0xff     unsigned base64 decode char  s      char  p    unsigned n   0     for  p   s   p  p        n   64   n   base64 decode digit  p      return n      Know ye all persons by these presents that you should not confuse  playing around on your own  with  implementing a standard    Yeesh

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This is a decoder that is specifically written to avoid the need for a buffer  by writing directly to a putchar function  This is based on wikibook s implementation https   en wikibooks org wiki Algorithm Implementation Miscellaneous Base64 C  This is not as easy to use as other options above  However  it can be of use in embedded systems  where you want to dump a large file without allocating another large buffer to store the resultant base64 datauri string   It s a pity that datauri does not let you specify the filename    void datauriBase64EncodeBufferless int   putchar fcptr  int   const char  type strptr  const void  data buf  const size t dataLength      const char base64chars      ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789       const uint8 t  data    const uint8 t   data buf    size t x   0    uint32 t n   0    int padCount   dataLength   3    uint8 t n0  n1  n2  n3     size t outcount   0    size t line   0     putchar fcptr  int  d      putchar fcptr  int  a      putchar fcptr  int  t      putchar fcptr  int  a      putchar fcptr  int         outcount    5     while   type strptr      0           putchar fcptr  int  type strptr       type strptr        outcount           putchar fcptr  int         putchar fcptr  int  b      putchar fcptr  int  a      putchar fcptr  int  s      putchar fcptr  int  e      putchar fcptr  int  6      putchar fcptr  int  4      putchar fcptr  int         outcount    8        increment over the length of the string  three characters at a time      for  x   0  x  lt  dataLength  x    3             these three 8-bit  ASCII  characters become one 24-bit number        n     uint32 t data x    lt  lt  16    parenthesis needed  compiler depending on flags can do the shifting before conversion to uint32 t  resulting to 0      if  x 1   lt  dataLength         n      uint32 t data x 1    lt  lt  8   parenthesis needed  compiler depending on flags can do the shifting before conversion to uint32 t  resulting to 0      if  x 2   lt  dataLength         n    data x 2           this 24-bit number gets separated into four 6-bit numbers        n0    uint8 t  n  gt  gt  18   amp  63      n1    uint8 t  n  gt  gt  12   amp  63      n2    uint8 t  n  gt  gt  6   amp  63      n3    uint8 t n  amp  63                 if we have one byte available  then its encoding is spread        out over two characters              putchar fcptr  int base64chars n0        putchar fcptr  int base64chars n1        outcount    2                 if we have only two bytes available  then their encoding is        spread out over three chars             if  x 1   lt  dataLength              putchar fcptr  int base64chars n2          outcount    1                       if we have all three bytes available  then their encoding is spread        out over four characters             if  x 2   lt  dataLength              putchar fcptr  int base64chars n3          outcount    1                Breaking up the line so it s easier to copy and paste        int curr line    outcount 80       if  curr line    line               line   curr line        putchar fcptr  int   r          putchar fcptr  int   n                        create and add padding that is required if we did not have a multiple of 3     number of characters available        if  padCount  gt  0          for    padCount  lt  3  padCount                putchar fcptr  int                    putchar fcptr  int   r      putchar fcptr  int   n        Here is the test   include  lt stdio h gt   include  lt stdint h gt   include  lt string h gt   int main void      char str      test     datauriBase64EncodeBufferless putchar   text plain charset utf-8   str  strlen str      return 0      Expected Output  data text plain charset utf-8 base64 dGVzdA

User · Answer

But you can also do it in openssl  openssl enc command does it       look at the BIO f base64   function

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Here s the decoder I ve been using for years         static const char  table      ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789         static const int   BASE64 INPUT SIZE   57       BOOL isbase64 char c               return c  amp  amp  strchr table  c     NULL             inline char value char c               const char  p   strchr table  c          if p              return p-table           else             return 0                      int UnBase64 unsigned char  dest  const unsigned char  src  int srclen                dest   0         if  src    0                      return 0                  unsigned char  p   dest         do                     char a   value src 0              char b   value src 1              char c   value src 2              char d   value src 3               p      a  lt  lt  2     b  gt  gt  4              p      b  lt  lt  4     c  gt  gt  2              p      c  lt  lt  6    d            if  isbase64 src 1                              p -  2               break                         else if  isbase64 src 2                              p -  2               break                         else if  isbase64 src 3                              p--               break                        src    4            while  src  amp  amp    src    13     src    10   src                    while srclen-  4           p   0         return p-dest

User · Answer

GNU coreutils has it in lib base64   It s a little bloated but deals with stuff like EBCDIC   You can also play around on your own  e g    char base64 digit  n  unsigned n      if  n  lt  10  return n -  0     else if  n  lt  10   26  return n -  a     else if  n  lt  10   26   26  return n -  A     else assert 0     return 0     unsigned char base64 decode digit char c      switch  c        case       return 62      case       return 63      default          if  isdigit c   return c -  0         else if  islower c   return c -  a    10        else if  isupper c   return c -  A    10   26        else assert 0         return 0xff     unsigned base64 decode char  s      char  p    unsigned n   0     for  p   s   p  p        n   64   n   base64 decode digit  p      return n      Know ye all persons by these presents that you should not confuse  playing around on your own  with  implementing a standard    Yeesh

User · Answer

But you can also do it in openssl  openssl enc command does it       look at the BIO f base64   function

User · Answer

GNU coreutils has it in lib base64   It s a little bloated but deals with stuff like EBCDIC   You can also play around on your own  e g    char base64 digit  n  unsigned n      if  n  lt  10  return n -  0     else if  n  lt  10   26  return n -  a     else if  n  lt  10   26   26  return n -  A     else assert 0     return 0     unsigned char base64 decode digit char c      switch  c        case       return 62      case       return 63      default          if  isdigit c   return c -  0         else if  islower c   return c -  a    10        else if  isupper c   return c -  A    10   26        else assert 0         return 0xff     unsigned base64 decode char  s      char  p    unsigned n   0     for  p   s   p  p        n   64   n   base64 decode digit  p      return n      Know ye all persons by these presents that you should not confuse  playing around on your own  with  implementing a standard    Yeesh

User · Answer

Here is an optimized version of encoder for the accepted answer  that also supports line-breaking for MIME and other protocols  simlar optimization can be applied to the decoder     char  base64 encode const unsigned char  data                      size t input length                      size t  output length                      bool addLineBreaks        output length   4     input length   2    3       if  addLineBreaks   output length     output length   38      CRLF after each 76 chars      char  encoded data   malloc  output length       if  encoded data    NULL  return NULL       UInt32 octet a      UInt32 octet b      UInt32 octet c      UInt32 triple      int lineCount   0      int sizeMod   size -  size   3      check if there is a partial triplet        adding all octet triplets  before partial last triplet     for    offset  lt  sizeMod                   octet a   data offset             octet b   data offset             octet c   data offset              triple    octet a  lt  lt  0x10     octet b  lt  lt  0x08    octet c           encoded data mBufferPos      encoding table  triple  gt  gt  3   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  2   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  1   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  0   6   amp  0x3F           if  addLineBreaks                        if    lineCount    19                                encoded data mBufferPos      13                  encoded data mBufferPos      10                  lineCount   0                                        last bytes     if  sizeMod  lt  size                octet a   data offset        first octect always added         octet b   offset  lt  size   data offset       UInt32 0     conditional 2nd octet         octet c    UInt32 0     last character is definitely padded          triple    octet a  lt  lt  0x10     octet b  lt  lt  0x08    octet c           encoded data mBufferPos      encoding table  triple  gt  gt  3   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  2   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  1   6   amp  0x3F           encoded data mBufferPos      encoding table  triple  gt  gt  0   6   amp  0x3F               add padding             sizeMod   size   3              last character is definitely padded         encoded data mBufferPos - 1     byte              if  sizeMod    1  encoded data mBufferPos - 2     byte

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If you want to find a workable C solution  I believe you need this  https   github com littlstar b64 c  include  lt stdio h gt   include  lt string h gt   include  lt stdlib h gt   include  quot b64 h quot   int main  void      unsigned char  str    quot brian the monkey and bradley the kinkajou are friends quot     char  enc   b64 encode str  strlen str       printf  quot  s n quot   enc      YnJpYW4gdGhlIG1vbmtleSBhbmQgYnJhZGxleSB0aGUga2lua2Fqb3UgYXJlIGZyaWVuZHM     char  dec   b64 decode enc  strlen enc       printf  quot  s n quot   dec      brian the monkey and bradley the kinkajou are friends   free enc     free dec     return 0

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I fix  ryyst answer s bug and this is a url safe version       include  lt stdio h gt   include  lt stdint h gt   include  lt stdlib h gt   include  lt string h gt    static char encoding table       A    B    C    D    E    F    G    H                                    I    J    K    L    M    N    O    P                                    Q    R    S    T    U    V    W    X                                    Y    Z    a    b    c    d    e    f                                    g    h    i    j    k    l    m    n                                    o    p    q    r    s    t    u    v                                    w    x    y    z    0    1    2    3                                    4    5    6    7    8    9    -         static char  decoding table   NULL  static int mod table      0  2  1     void build decoding table           decoding table   malloc 256         for  int i   0  i  lt  64  i            decoding table  unsigned char  encoding table i     i        void base64 cleanup         free decoding table        char  base64 encode const char  data                      size t input length                      size t  output length           output length   4     input length   2    3         char  encoded data   malloc  output length       if  encoded data    NULL  return NULL        for  int i   0  j   0  i  lt  input length               uint32 t octet a   i  lt  input length    unsigned char data i      0          uint32 t octet b   i  lt  input length    unsigned char data i      0          uint32 t octet c   i  lt  input length    unsigned char data i      0            uint32 t triple    octet a  lt  lt  0x10     octet b  lt  lt  0x08    octet c            encoded data j      encoding table  triple  gt  gt  3   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  2   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  1   6   amp  0x3F           encoded data j      encoding table  triple  gt  gt  0   6   amp  0x3F                 int i 0      for  int i   0  i  lt  mod table input length   3   i            encoded data  output length - 1 - i                   output length     output length -2   mod table input length   3       encoded data  output length   0       return encoded data        unsigned char  base64 decode const char  data                               size t input length                               size t  output length          if  decoding table    NULL  build decoding table          if  input length   4    0  return NULL         output length   input length   4   3      if  data input length - 1            output length --      if  data input length - 2            output length --        unsigned char  decoded data   malloc  output length       if  decoded data    NULL  return NULL        for  int i   0  j   0  i  lt  input length               uint32 t sextet a   data i           0  amp  i     decoding table data i              uint32 t sextet b   data i           0  amp  i     decoding table data i              uint32 t sextet c   data i           0  amp  i     decoding table data i              uint32 t sextet d   data i           0  amp  i     decoding table data i                uint32 t triple    sextet a  lt  lt  3   6             sextet b  lt  lt  2   6             sextet c  lt  lt  1   6             sextet d  lt  lt  0   6             if  j  lt   output length  decoded data j       triple  gt  gt  2   8   amp  0xFF          if  j  lt   output length  decoded data j       triple  gt  gt  1   8   amp  0xFF          if  j  lt   output length  decoded data j       triple  gt  gt  0   8   amp  0xFF              return decoded data      int main             const char   data    quot Hello World         quot       size t input size   strlen data       printf  quot Input size   ld  n quot  input size       char   encoded data   base64 encode data  input size   amp input size       printf  quot After size   ld  n quot  input size       printf  quot Encoded Data is   s  n quot  encoded data            size t decode size   strlen encoded data       printf  quot Output size   ld  n quot  decode size       unsigned char   decoded data   base64 decode encoded data  decode size   amp decode size       printf  quot After size   ld  n quot  decode size       printf  quot Decoded Data is   s  n quot  decoded data       return 0

User · Answer

Small improvement to the code from ryyst  who got the most votes  is to not use dynamically allocated decoding table but rather static const precomputed table  This eliminates the use of pointer and initialization of the table  and also avoids memory leakage if one forgets to clean up the decoding table with base64 cleanup    by the way  in base64 cleanup    after calling free decoding table   one should have decoding table NULL  otherwise accidentally calling base64 decode after base64 cleanup   will crash or cause undetermined behavior   Another solution could be to use std  unique ptr   but I m satisfied with just having const char 256  on the stack and avoid using pointers alltogether - the code looks cleaner and shorter this way   The decoding table is computed as follows   const char encoding table             A    B    C    D    E    F    G    H        I    J    K    L    M    N    O    P        Q    R    S    T    U    V    W    X        Y    Z    a    b    c    d    e    f        g    h    i    j    k    l    m    n        o    p    q    r    s    t    u    v        w    x    y    z    0    1    2    3        4    5    6    7    8    9                unsigned char decoding table 256    for  int i   0  i  lt  256  i        decoding table i      0    for  int i   0  i  lt  64  i        decoding table  unsigned char encoding table i     i   for  int i   0  i  lt  256  i        cout  lt  lt   0x   lt  lt   int decoding table i    lt  16    0         lt  lt  hex  lt  lt  int decoding table i    lt  lt   i    255              lt  lt    i 1    16    0     n      0     cin ignore      and the modified code I am using is           static const char encoding table                     A    B    C    D    E    F    G    H                I    J    K    L    M    N    O    P                Q    R    S    T    U    V    W    X                Y    Z    a    b    c    d    e    f                g    h    i    j    k    l    m    n                o    p    q    r    s    t    u    v                w    x    y    z    0    1    2    3                4    5    6    7    8    9                        static const unsigned char decoding table 256                  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x3e  0x00  0x00  0x00  0x3f              0x34  0x35  0x36  0x37  0x38  0x39  0x3a  0x3b  0x3c  0x3d  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x01  0x02  0x03  0x04  0x05  0x06  0x07  0x08  0x09  0x0a  0x0b  0x0c  0x0d  0x0e              0x0f  0x10  0x11  0x12  0x13  0x14  0x15  0x16  0x17  0x18  0x19  0x00  0x00  0x00  0x00  0x00              0x00  0x1a  0x1b  0x1c  0x1d  0x1e  0x1f  0x20  0x21  0x22  0x23  0x24  0x25  0x26  0x27  0x28              0x29  0x2a  0x2b  0x2c  0x2d  0x2e  0x2f  0x30  0x31  0x32  0x33  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00              0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00             char  base64 encode const unsigned char  data  size t input length  size t  amp output length                 const int mod table       0  2  1                 output length   4     input length   2    3                char  encoded data    char  malloc output length                if  encoded data    nullptr                  return nullptr               for  int i   0  j   0  i  lt  input length                      uint32 t octet a   i  lt  input length    unsigned char data i      0                  uint32 t octet b   i  lt  input length    unsigned char data i      0                  uint32 t octet c   i  lt  input length    unsigned char data i      0                   uint32 t triple    octet a  lt  lt  0x10     octet b  lt  lt  0x08    octet c                   encoded data j      encoding table  triple  gt  gt  3   6   amp  0x3F                   encoded data j      encoding table  triple  gt  gt  2   6   amp  0x3F                   encoded data j      encoding table  triple  gt  gt  1   6   amp  0x3F                   encoded data j      encoding table  triple  gt  gt  0   6   amp  0x3F                               for  int i   0  i  lt  mod table input length   3   i                    encoded data output length - 1 - i                      return encoded data                       unsigned char  base64 decode const char  data  size t input length  size t  amp output length                         if  input length   4    0                  return nullptr               output length   input length   4   3               if  data input length - 1           output length --              if  data input length - 2           output length --               unsigned char  decoded data    unsigned char  malloc output length                if  decoded data    nullptr                  return nullptr               for  int i   0  j   0  i  lt  input length                      uint32 t sextet a   data i           0  amp  i     decoding table data i                      uint32 t sextet b   data i           0  amp  i     decoding table data i                      uint32 t sextet c   data i           0  amp  i     decoding table data i                      uint32 t sextet d   data i           0  amp  i     decoding table data i                       uint32 t triple    sextet a  lt  lt  3   6                         sextet b  lt  lt  2   6                         sextet c  lt  lt  1   6                         sextet d  lt  lt  0   6                    if  j  lt  output length  decoded data j       triple  gt  gt  2   8   amp  0xFF                  if  j  lt  output length  decoded data j       triple  gt  gt  1   8   amp  0xFF                  if  j  lt  output length  decoded data j       triple  gt  gt  0   8   amp  0xFF                              return decoded data

[c] How do I base64 encode (decode) in C?

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