Base64 decode snippet in C

Question

Is there a freely available Base64 decoding code snippet in C

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Using base-n mini lib  you can do the following  some data t in            constexpr int len   sizeof in  sizeof in 0     std  string encoded  bn  encode b64 in  in   len  std  back inserter encoded     some data t out len   bn  decode b64 encoded begin    encoded end    out    The API is generic  iterator-based  Disclosure  I m the author

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Here s my modification of the implementation that was originally written by Ren   Nyffenegger  And why have I modified it  Well  because it didn t seem appropriate to me that I should work with binary data stored within std  string object    base64 h   ifndef  BASE64 H   define  BASE64 H    include  lt vector gt   include  lt string gt  typedef unsigned char BYTE   std  string base64 encode BYTE const  buf  unsigned int bufLen   std  vector lt BYTE gt  base64 decode std  string const amp      endif  base64 cpp   include  quot base64 h quot   include  lt iostream gt   static const std  string base64 chars                 quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot                quot abcdefghijklmnopqrstuvwxyz quot                quot 0123456789   quot     static inline bool is base64 BYTE c      return  isalnum c      c             c              std  string base64 encode BYTE const  buf  unsigned int bufLen      std  string ret    int i   0    int j   0    BYTE char array 3 3     BYTE char array 4 4      while  bufLen--        char array 3 i        buf         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  vector lt BYTE gt  base64 decode std  string const amp  encoded string      int in len   encoded string size      int i   0    int j   0    int in    0    BYTE char array 4 4   char array 3 3     std  vector lt BYTE gt  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 push back 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 push back char array 3 j           return ret     Here s the usage  std  vector lt BYTE gt  myData      std  string encodedData   base64 encode  amp myData 0   myData size     std  vector lt BYTE gt  decodedData   base64 decode encodedData

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I use this  class BinaryVector   public      std  vector lt char gt  bytes       uint64 t bit count   0   public         Add a bit to the end        void push back bool bit           Return false if character is unrecognized        bool pushBase64Char char b64 c       void BinaryVector  push back bool bit        if   bit count    bit count   8    0            bytes push back bit  lt  lt  7             else           uint8 t next bit   8 -  bit count   8  - 1          bytes bit count   8     bit  lt  lt  next bit            bit count          Converts one Base64 character to 6 bits    bool BinaryVector  pushBase64Char char c        uint8 t d          A to Z     if  c  gt  0x40  amp  amp  c  lt  0x5b            d   c - 65      Base64 A is 0              a to z     else if  c  gt  0x60  amp  amp  c  lt  0x7b            d   c - 97   26      Base64 a is 26              0 to 9     else if  c  gt  0x2F  amp  amp  c  lt  0x3a            d   c - 48   52      Base64 0 is 52           else if  c                   d   0b111110            else if  c                   d   0b111111            else if  c                   d   0            else           return false             push back d  amp  0b100000       push back d  amp  0b010000       push back d  amp  0b001000       push back d  amp  0b000100       push back d  amp  0b000010       push back d  amp  0b000001        return true     bool loadBase64 std  vector lt char gt  amp  b64 bin  BinaryVector amp  vec        for  char amp  c   b64 bin            if   vec pushBase64Char c                 return false                      return true      Use vec bytes to access converted data

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A little variation with a more compact lookup table and using C  17 features  std  string base64 decode const std  string view in         table from     to  z    const uint8 t lookup             62   255  62   255  63   52   53  54  55  56  57  58  59  60  61  255        255  0    255  255  255  255  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        255  255  255  255  63   255  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     static assert sizeof lookup      z  -       1      std  string out    int val   0  valb   -8    for  uint8 t c   in        if  c  lt         c  gt   z         break      c -           if  lookup c   gt   64        break      val    val  lt  lt  6    lookup c       valb    6      if  valb  gt   0          out push back char  val  gt  gt  valb   amp  0xFF          valb -  8              return out     If you don t have std  string view  try instead std  experimental  string view

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My variation on DaedalusAlpha s answer  It avoids copying the parameters at the expense of a couple of tests  Uses uint8 t instead of BYTE  Adds some handy functions for dealing with strings  although usually the input data is binary and may have zero bytes inside  so typically should not be manipulated as a string  which often implies null-terminated data   Also adds some casts to fix compiler warnings  at least on GCC  I haven t run it through MSVC yet   Part of file base64 hpp  void base64 encode string  amp  out  const vector lt uint8 t gt  amp  buf   void base64 encode string  amp  out  const uint8 t  buf  size t bufLen   void base64 encode string  amp  out  string const amp  buf    void base64 decode vector lt uint8 t gt   amp  out  string const amp  encoded string       Use this if you know the output should be a valid string void base64 decode string  amp  out  string const amp  encoded string    File base64 cpp  static const uint8 t from base64 128             8 rows of 16   128        Note  only requires 123 entries  as we only lookup for  lt   z   which z 122      255  255  255  255  255  255  255  255  255  255  255  255  255  255  255  255      255  255  255  255  255  255  255  255  255  255  255  255  255  255  255  255      255  255  255  255  255  255  255  255  255  255  255   62  255   62  255   63       52   53   54   55   56   57   58   59   60   61  255  255    0  255  255  255      255    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  255  255  255  255   63      255   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  255  255  255  255  255     static const char to base64 65         quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot       quot abcdefghijklmnopqrstuvwxyz quot       quot 0123456789   quot     void base64 encode string  amp  out  string const amp  buf       if  buf empty          base64 encode out  NULL  0      else       base64 encode out  reinterpret cast lt uint8 t const  gt   amp buf 0    buf size         void base64 encode string  amp  out  std  vector lt uint8 t gt  const amp  buf       if  buf empty          base64 encode out  NULL  0      else       base64 encode out   amp buf 0   buf size        void base64 encode string  amp  ret  uint8 t const  buf  size t bufLen          Calculate how many bytes that needs to be added to get a multiple of 3    size t missing   0     size t ret size   bufLen     while   ret size   3     0               ret size          missing              Expand the return string size to a multiple of 4    ret size   4 ret size 3      ret clear       ret reserve ret size       for  size t i   0  i  lt  ret size 4    i                Read a group of three bytes  avoid buffer overrun by replacing with 0        const size t index   i 3        const uint8 t b3 0    index 0  lt  bufLen    buf index 0    0        const uint8 t b3 1    index 1  lt  bufLen    buf index 1    0        const uint8 t b3 2    index 2  lt  bufLen    buf index 2    0            Transform into four base 64 characters       const uint8 t b4 0                            b3 0  amp  0xfc   gt  gt  2         const uint8 t b4 1     b3 0  amp  0x03   lt  lt  4      b3 1  amp  0xf0   gt  gt  4         const uint8 t b4 2     b3 1  amp  0x0f   lt  lt  2      b3 2  amp  0xc0   gt  gt  6         const uint8 t b4 3     b3 2  amp  0x3f   lt  lt  0             Add the base 64 characters to the return value       ret push back to base64 b4 0          ret push back to base64 b4 1          ret push back to base64 b4 2          ret push back to base64 b4 3                Replace data that is invalid  always as many as there are missing bytes     for  size t i   0  i    missing    i        ret ret size - i - 1             template  lt class Out gt  void base64 decode any  Out  amp  ret  std  string const amp  in       typedef typename Out  value type T         Make sure the  intended  string length is a multiple of 4    size t encoded size   in size        while   encoded size   4     0          encoded size      const size t N   in size       ret clear       ret reserve 3 encoded size 4       for  size t i   0  i  lt  encoded size  i    4                Note   z     122           Get values for each group of four base 64 characters       const uint8 t b4 0                in i 0   lt    z     from base64 static cast lt uint8 t gt  in i 0      0xff        const uint8 t b4 1    i 1  lt  N and in i 1   lt    z     from base64 static cast lt uint8 t gt  in i 1      0xff        const uint8 t b4 2    i 2  lt  N and in i 2   lt    z     from base64 static cast lt uint8 t gt  in i 2      0xff        const uint8 t b4 3    i 3  lt  N and in i 3   lt    z     from base64 static cast lt uint8 t gt  in i 3      0xff            Transform into a group of three bytes       const uint8 t b3 0     b4 0  amp  0x3f   lt  lt  2      b4 1  amp  0x30   gt  gt  4         const uint8 t b3 1     b4 1  amp  0x0f   lt  lt  4      b4 2  amp  0x3c   gt  gt  2         const uint8 t b3 2     b4 2  amp  0x03   lt  lt  6      b4 3  amp  0x3f   gt  gt  0             Add the byte to the return value if it isn t part of an     character  indicated by 0xff        if  b4 1    0xff  ret push back  static cast lt T gt  b3 0           if  b4 2    0xff  ret push back  static cast lt T gt  b3 1           if  b4 3    0xff  ret push back  static cast lt T gt  b3 2             void base64 decode vector lt uint8 t gt   amp  out  string const amp  encoded string       base64 decode any out  encoded string      void base64 decode string  amp  out  string const amp  encoded string       base64 decode any out  encoded string

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My version is a simple fast encoder  decoder  of Base64 for C  Builder     --------------------------------------------------------------------------- UnicodeString   fastcall TExample  Base64Encode void  data  int length        if  length  lt   0          return L quot  quot       static const char set      quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot       unsigned char  in    unsigned char  data      char  pos   out   pos   new char   length - 1    3   1   lt  lt  2       while   length -  3   gt   0                pos 0    set in 0   gt  gt  2           pos 1    set   in 0   amp  0x03   lt  lt  4     in 1   gt  gt  4            pos 2    set   in 1   amp  0x0F   lt  lt  2     in 2   gt  gt  6            pos 3    set in 2   amp  0x3F           pos    4          in    3             if   length  amp  2     0                pos 0    set in 0   gt  gt  2           if   length  amp  1     0                        pos 1    set   in 0   amp  0x03   lt  lt  4     in 1   gt  gt  4                pos 2    set  in 1   amp  0x0F   lt  lt  2                     else                       pos 1    set  in 0   amp  0x03   lt  lt  4               pos 2                            pos 3                 pos    4             UnicodeString code   UnicodeString out  pos - out       delete   out      return code         --------------------------------------------------------------------------- int   fastcall TExample  Base64Decode const UnicodeString  amp code  unsigned char   data        int length      if    length   code Length       0       length  amp  3     0           return 0      wchar t  str   code c str        unsigned char  pos   out   pos   new unsigned char  length  gt  gt  2    3       while   str    0                length   -1          int shift   18  bits   0          do                       wchar t s   str   length               if   s  gt   L A    amp  amp   s  lt   L Z                    bits     s - L A    lt  lt  shift              else if   s  gt   L a    amp  amp   s  lt   L z                       bits     s -  L a  - 26    lt  lt  shift              else if    s  gt   L 0    amp  amp   s  lt   L 9                        bits     s -  L 0  - 52    lt  lt  shift              else if  s    L                        bits    62  lt  lt  shift              else if  s    L                        bits    63  lt  lt  shift              else if  s    L                                   length--                  break                            else                               delete   out                  return 0                                   while   shift -  6   gt   0           pos 0    bits  gt  gt  16          pos 1    bits  gt  gt  8          pos 2    bits          pos    length          str    4              data   out      return pos - out       ---------------------------------------------------------------------------

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My version is a simple fast encoder  decoder  of Base64 for C  Builder     --------------------------------------------------------------------------- UnicodeString   fastcall TExample  Base64Encode void  data  int length        if  length  lt   0          return L quot  quot       static const char set      quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot       unsigned char  in    unsigned char  data      char  pos   out   pos   new char   length - 1    3   1   lt  lt  2       while   length -  3   gt   0                pos 0    set in 0   gt  gt  2           pos 1    set   in 0   amp  0x03   lt  lt  4     in 1   gt  gt  4            pos 2    set   in 1   amp  0x0F   lt  lt  2     in 2   gt  gt  6            pos 3    set in 2   amp  0x3F           pos    4          in    3             if   length  amp  2     0                pos 0    set in 0   gt  gt  2           if   length  amp  1     0                        pos 1    set   in 0   amp  0x03   lt  lt  4     in 1   gt  gt  4                pos 2    set  in 1   amp  0x0F   lt  lt  2                     else                       pos 1    set  in 0   amp  0x03   lt  lt  4               pos 2                            pos 3                 pos    4             UnicodeString code   UnicodeString out  pos - out       delete   out      return code         --------------------------------------------------------------------------- int   fastcall TExample  Base64Decode const UnicodeString  amp code  unsigned char   data        int length      if    length   code Length       0       length  amp  3     0           return 0      wchar t  str   code c str        unsigned char  pos   out   pos   new unsigned char  length  gt  gt  2    3       while   str    0                length   -1          int shift   18  bits   0          do                       wchar t s   str   length               if   s  gt   L A    amp  amp   s  lt   L Z                    bits     s - L A    lt  lt  shift              else if   s  gt   L a    amp  amp   s  lt   L z                       bits     s -  L a  - 26    lt  lt  shift              else if    s  gt   L 0    amp  amp   s  lt   L 9                        bits     s -  L 0  - 52    lt  lt  shift              else if  s    L                        bits    62  lt  lt  shift              else if  s    L                        bits    63  lt  lt  shift              else if  s    L                                   length--                  break                            else                               delete   out                  return 0                                   while   shift -  6   gt   0           pos 0    bits  gt  gt  16          pos 1    bits  gt  gt  8          pos 2    bits          pos    length          str    4              data   out      return pos - out       ---------------------------------------------------------------------------

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I liked this solution on GitHub  It is a single hpp file and it uses the vector lt byte gt  type for raw data unlike the accepted answer   pragma once   include  lt string gt   include  lt vector gt   include  lt stdexcept gt   include  lt cstdint gt   namespace base64       inline static const char kEncodeLookup      quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot       inline static const char kPadCharacter             using byte   std  uint8 t       inline std  string encode const std  vector lt byte gt  amp  input                std  string encoded          encoded reserve   input size     3     input size     3  gt  0     4            std  uint32 t temp            auto it   input begin             for std  size t i   0  i  lt  input size     3    i                        temp      it     lt  lt  16              temp      it     lt  lt  8              temp      it                 encoded append 1  kEncodeLookup  temp  amp  0x00FC0000   gt  gt  18                encoded append 1  kEncodeLookup  temp  amp  0x0003F000   gt  gt  12                encoded append 1  kEncodeLookup  temp  amp  0x00000FC0   gt  gt  6                 encoded append 1  kEncodeLookup  temp  amp  0x0000003F                              switch input size     3                    case 1              temp     it     lt  lt  16              encoded append 1  kEncodeLookup  temp  amp  0x00FC0000   gt  gt  18                encoded append 1  kEncodeLookup  temp  amp  0x0003F000   gt  gt  12                encoded append 2  kPadCharacter               break          case 2              temp      it     lt  lt  16              temp      it     lt  lt  8              encoded append 1  kEncodeLookup  temp  amp  0x00FC0000   gt  gt  18                encoded append 1  kEncodeLookup  temp  amp  0x0003F000   gt  gt  12                encoded append 1  kEncodeLookup  temp  amp  0x00000FC0   gt  gt  6                 encoded append 1  kPadCharacter               break                     return encoded             std  vector lt byte gt  decode const std  string amp  input                if input length     4              throw std  runtime error  quot Invalid base64 length  quot             std  size t padding             if input length                          if input input length   - 1     kPadCharacter  padding                if input input length   - 2     kPadCharacter  padding                       std  vector lt byte gt  decoded          decoded reserve   input length     4    3  - padding            std  uint32 t temp            auto it   input begin             while it  lt  input end                          for std  size t i   0  i  lt  4    i                                temp  lt  lt   6                  if       it  gt   0x41  amp  amp   it  lt   0x5A  temp     it - 0x41                  else if  it  gt   0x61  amp  amp   it  lt   0x7A  temp     it - 0x47                  else if  it  gt   0x30  amp  amp   it  lt   0x39  temp     it   0x04                  else if  it    0x2B                 temp    0x3E                  else if  it    0x2F                 temp    0x3F                  else if  it    kPadCharacter                                        switch input end   - it                                            case 1                          decoded push back  temp  gt  gt  16   amp  0x000000FF                           decoded push back  temp  gt  gt  8    amp  0x000000FF                           return decoded                      case 2                          decoded push back  temp  gt  gt  10   amp  0x000000FF                           return decoded                      default                          throw std  runtime error  quot Invalid padding in base64  quot                                                            else throw std  runtime error  quot Invalid character in base64  quot                       it                             decoded push back  temp  gt  gt  16   amp  0x000000FF               decoded push back  temp  gt  gt  8    amp  0x000000FF               decoded push back  temp         amp  0x000000FF                      return decoded

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Here is one written by me which uses unions and bit fields for maximum efficiency and readibility  const char PADDING CHAR        const char  ALPHABET    quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot   const uint8 t DECODED ALPHBET 128   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 0 0 0 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 0 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 0 0 0 0 0           Given a string  this function will encode it in 64b  with padding      std  string encodeBase64 const std  string amp  binaryText        std  string encoded  binaryText size   3    binaryText size   3  gt  0    lt  lt  2  PADDING CHAR        const char  bytes   binaryText data        union               uint32 t temp   0          struct                       uint32 t first   6  second   6  third   6  fourth   6            tempBytes             std  string  iterator currEncoding   encoded begin         for uint32 t i   0  lim   binaryText size     3  i  lt  lim    i  bytes  3                temp   bytes 0   lt  lt  16   bytes 1   lt  lt  8   bytes 2             currEncoding      ALPHABET tempBytes fourth             currEncoding      ALPHABET tempBytes third             currEncoding      ALPHABET tempBytes second             currEncoding      ALPHABET tempBytes first              switch binaryText size     3            case 1          temp   bytes 0   lt  lt  16            currEncoding      ALPHABET tempBytes fourth             currEncoding      ALPHABET tempBytes third           break      case 2          temp   bytes 0   lt  lt  16   bytes 1   lt  lt  8            currEncoding      ALPHABET tempBytes fourth             currEncoding      ALPHABET tempBytes third             currEncoding      ALPHABET tempBytes second           break             return encoded            Given a 64b padding-encoded string  this function will decode it      std  string decodeBase64 const std  string amp  base64Text        if  base64Text empty             return  quot  quot        assert  base64Text size   amp 3     0  amp  amp   quot The base64 text to be decoded must have a length devisible by 4  quot         uint32 t numPadding      std  prev base64Text end   1     PADDING CHAR      std  prev base64Text end   2     PADDING CHAR        std  string decoded  base64Text size   3 gt  gt 2  - numPadding             union               uint32 t temp          char tempBytes 4              const uint8 t  bytes   reinterpret cast lt const uint8 t  gt  base64Text data          std  string  iterator currDecoding   decoded begin         for uint32 t i   0  lim    base64Text size    gt  gt  2  -  numPadding  0   i  lt  lim    i  bytes  4                temp   DECODED ALPHBET bytes 0    lt  lt  18   DECODED ALPHBET bytes 1    lt  lt  12   DECODED ALPHBET bytes 2    lt  lt  6   DECODED ALPHBET bytes 3              currDecoding      tempBytes 2             currDecoding      tempBytes 1             currDecoding      tempBytes 0              switch  numPadding            case 2          temp   DECODED ALPHBET bytes 0    lt  lt  18   DECODED ALPHBET bytes 1    lt  lt  12            currDecoding      tempBytes 2           break           case 1          temp   DECODED ALPHBET bytes 0    lt  lt  18   DECODED ALPHBET bytes 1    lt  lt  12   DECODED ALPHBET bytes 2    lt  lt  6            currDecoding      tempBytes 2             currDecoding      tempBytes 1           break             return decoded

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I think this one works better   include  lt string gt   static const char  B64chars    quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot    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     const std  string b64encode const void  data  const size t  amp len        std  string result  len   2    3   4            char  p    char   data   str    amp result 0       size t j   0  pad   len   3      const size t last   len - pad       for  size t i   0  i  lt  last  i    3                int n   int p i    lt  lt  16   int p i   1    lt  lt  8   p i   2           str j      B64chars n  gt  gt  18           str j      B64chars n  gt  gt  12  amp  0x3F           str j      B64chars n  gt  gt  6  amp  0x3F           str j      B64chars n  amp  0x3F             if  pad       Set padding               int n   --pad   int p last    lt  lt  8   p last   1    p last           str j      B64chars pad   n  gt  gt  10  amp  0x3F   n  gt  gt  2           str j      B64chars pad   n  gt  gt  4  amp  0x03F   n  lt  lt  4  amp  0x3F           str j      pad   B64chars n  lt  lt  2  amp  0x3F                   return result     const std  string b64decode const void  data  const size t  amp len        if  len    0  return  quot  quot        unsigned char  p    unsigned char   data      size t j   0          pad1   len   4    p len - 1                  pad2   pad1  amp  amp   len   4  gt  2    p len - 2               const size t last    len - pad1    4  lt  lt  2      std  string result last   4   3   pad1   pad2    0        unsigned char  str    unsigned char    amp result 0        for  size t i   0  i  lt  last  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  pad1                int n   B64index p last    lt  lt  18   B64index p last   1    lt  lt  12          str j      n  gt  gt  16          if  pad2                        n    B64index p last   2    lt  lt  6              str j      n  gt  gt  8  amp  0xFF                      return result     std  string b64encode const std  string amp  str        return b64encode str c str    str size        std  string b64decode const std  string amp  str64        return b64decode str64 c str    str64 size        Thanks to Jens Alfke for pointing out a performance issue  I have made some modifications to this old post  This one works way faster than before  Its other advantage is smooth handling of corrupt data as well  Last edition  Although in these kinds of problems  it seems that speed is an overkill  but just for the fun of it I have made some other modifications to make this one the fastest algorithm out there AFAIK  Special thanks goes to GaspardP for his valuable suggestions and nice benchmark

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Here is one written by me which uses unions and bit fields for maximum efficiency and readibility  const char PADDING CHAR        const char  ALPHABET    quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot   const uint8 t DECODED ALPHBET 128   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 0 0 0 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 0 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 0 0 0 0 0           Given a string  this function will encode it in 64b  with padding      std  string encodeBase64 const std  string amp  binaryText        std  string encoded  binaryText size   3    binaryText size   3  gt  0    lt  lt  2  PADDING CHAR        const char  bytes   binaryText data        union               uint32 t temp   0          struct                       uint32 t first   6  second   6  third   6  fourth   6            tempBytes             std  string  iterator currEncoding   encoded begin         for uint32 t i   0  lim   binaryText size     3  i  lt  lim    i  bytes  3                temp   bytes 0   lt  lt  16   bytes 1   lt  lt  8   bytes 2             currEncoding      ALPHABET tempBytes fourth             currEncoding      ALPHABET tempBytes third             currEncoding      ALPHABET tempBytes second             currEncoding      ALPHABET tempBytes first              switch binaryText size     3            case 1          temp   bytes 0   lt  lt  16            currEncoding      ALPHABET tempBytes fourth             currEncoding      ALPHABET tempBytes third           break      case 2          temp   bytes 0   lt  lt  16   bytes 1   lt  lt  8            currEncoding      ALPHABET tempBytes fourth             currEncoding      ALPHABET tempBytes third             currEncoding      ALPHABET tempBytes second           break             return encoded            Given a 64b padding-encoded string  this function will decode it      std  string decodeBase64 const std  string amp  base64Text        if  base64Text empty             return  quot  quot        assert  base64Text size   amp 3     0  amp  amp   quot The base64 text to be decoded must have a length devisible by 4  quot         uint32 t numPadding      std  prev base64Text end   1     PADDING CHAR      std  prev base64Text end   2     PADDING CHAR        std  string decoded  base64Text size   3 gt  gt 2  - numPadding             union               uint32 t temp          char tempBytes 4              const uint8 t  bytes   reinterpret cast lt const uint8 t  gt  base64Text data          std  string  iterator currDecoding   decoded begin         for uint32 t i   0  lim    base64Text size    gt  gt  2  -  numPadding  0   i  lt  lim    i  bytes  4                temp   DECODED ALPHBET bytes 0    lt  lt  18   DECODED ALPHBET bytes 1    lt  lt  12   DECODED ALPHBET bytes 2    lt  lt  6   DECODED ALPHBET bytes 3              currDecoding      tempBytes 2             currDecoding      tempBytes 1             currDecoding      tempBytes 0              switch  numPadding            case 2          temp   DECODED ALPHBET bytes 0    lt  lt  18   DECODED ALPHBET bytes 1    lt  lt  12            currDecoding      tempBytes 2           break           case 1          temp   DECODED ALPHBET bytes 0    lt  lt  18   DECODED ALPHBET bytes 1    lt  lt  12   DECODED ALPHBET bytes 2    lt  lt  6            currDecoding      tempBytes 2             currDecoding      tempBytes 1           break             return decoded

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See Encoding and decoding base 64 with C    Here is the implementation from that page        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      static const std  string base64 chars                 quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot                quot abcdefghijklmnopqrstuvwxyz quot                quot 0123456789   quot     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

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There are several snippets here  However  this one is compact  efficient  and C  11 friendly  static std  string base64 encode const std  string  amp in         std  string out       int val   0  valb   -6      for  uchar c   in            val    val  lt  lt  8    c          valb    8          while  valb  gt   0                out push back  quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot   val gt  gt valb  amp 0x3F                valb -  6                      if  valb gt -6  out push back  quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot    val lt  lt 8  gt  gt  valb 8   amp 0x3F        while  out size   4  out push back           return out     static std  string base64 decode const std  string  amp in         std  string out       std  vector lt int gt  T 256 -1       for  int i 0  i lt 64  i    T  quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot  i     i       int val 0  valb -8      for  uchar c   in            if  T c     -1  break          val    val  lt  lt  6    T c           valb    6          if  valb  gt   0                out push back char  val gt  gt valb  amp 0xFF                valb -  8                      return out

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A little variation with a more compact lookup table and using C  17 features  std  string base64 decode const std  string view in         table from     to  z    const uint8 t lookup             62   255  62   255  63   52   53  54  55  56  57  58  59  60  61  255        255  0    255  255  255  255  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        255  255  255  255  63   255  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     static assert sizeof lookup      z  -       1      std  string out    int val   0  valb   -8    for  uint8 t c   in        if  c  lt         c  gt   z         break      c -           if  lookup c   gt   64        break      val    val  lt  lt  6    lookup c       valb    6      if  valb  gt   0          out push back char  val  gt  gt  valb   amp  0xFF          valb -  8              return out     If you don t have std  string view  try instead std  experimental  string view

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I liked this solution on GitHub  It is a single hpp file and it uses the vector lt byte gt  type for raw data unlike the accepted answer   pragma once   include  lt string gt   include  lt vector gt   include  lt stdexcept gt   include  lt cstdint gt   namespace base64       inline static const char kEncodeLookup      quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot       inline static const char kPadCharacter             using byte   std  uint8 t       inline std  string encode const std  vector lt byte gt  amp  input                std  string encoded          encoded reserve   input size     3     input size     3  gt  0     4            std  uint32 t temp            auto it   input begin             for std  size t i   0  i  lt  input size     3    i                        temp      it     lt  lt  16              temp      it     lt  lt  8              temp      it                 encoded append 1  kEncodeLookup  temp  amp  0x00FC0000   gt  gt  18                encoded append 1  kEncodeLookup  temp  amp  0x0003F000   gt  gt  12                encoded append 1  kEncodeLookup  temp  amp  0x00000FC0   gt  gt  6                 encoded append 1  kEncodeLookup  temp  amp  0x0000003F                              switch input size     3                    case 1              temp     it     lt  lt  16              encoded append 1  kEncodeLookup  temp  amp  0x00FC0000   gt  gt  18                encoded append 1  kEncodeLookup  temp  amp  0x0003F000   gt  gt  12                encoded append 2  kPadCharacter               break          case 2              temp      it     lt  lt  16              temp      it     lt  lt  8              encoded append 1  kEncodeLookup  temp  amp  0x00FC0000   gt  gt  18                encoded append 1  kEncodeLookup  temp  amp  0x0003F000   gt  gt  12                encoded append 1  kEncodeLookup  temp  amp  0x00000FC0   gt  gt  6                 encoded append 1  kPadCharacter               break                     return encoded             std  vector lt byte gt  decode const std  string amp  input                if input length     4              throw std  runtime error  quot Invalid base64 length  quot             std  size t padding             if input length                          if input input length   - 1     kPadCharacter  padding                if input input length   - 2     kPadCharacter  padding                       std  vector lt byte gt  decoded          decoded reserve   input length     4    3  - padding            std  uint32 t temp            auto it   input begin             while it  lt  input end                          for std  size t i   0  i  lt  4    i                                temp  lt  lt   6                  if       it  gt   0x41  amp  amp   it  lt   0x5A  temp     it - 0x41                  else if  it  gt   0x61  amp  amp   it  lt   0x7A  temp     it - 0x47                  else if  it  gt   0x30  amp  amp   it  lt   0x39  temp     it   0x04                  else if  it    0x2B                 temp    0x3E                  else if  it    0x2F                 temp    0x3F                  else if  it    kPadCharacter                                        switch input end   - it                                            case 1                          decoded push back  temp  gt  gt  16   amp  0x000000FF                           decoded push back  temp  gt  gt  8    amp  0x000000FF                           return decoded                      case 2                          decoded push back  temp  gt  gt  10   amp  0x000000FF                           return decoded                      default                          throw std  runtime error  quot Invalid padding in base64  quot                                                            else throw std  runtime error  quot Invalid character in base64  quot                       it                             decoded push back  temp  gt  gt  16   amp  0x000000FF               decoded push back  temp  gt  gt  8    amp  0x000000FF               decoded push back  temp         amp  0x000000FF                      return decoded

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See Encoding and decoding base 64 with C    Here is the implementation from that page        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      static const std  string base64 chars                 quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot                quot abcdefghijklmnopqrstuvwxyz quot                quot 0123456789   quot     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

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I firstly made my own version and then found this topic  Why does my version look simpler than others presented here  Am I doing something wrong  I didn t test it for speed  inline char const  b64units    quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot    inline char  b64encode void const  a  int64 t b        ASSERT a    nullptr       if  b  gt  0            uint8 t const  aa   static cast lt uint8 t const  gt  a           uint8 t v   0          int64 t bp   0          int64 t sb   0          int8 t off   0          int64 t nt     b   2    3    4          int64 t nd    b   8    6          int64 t tl     b   8    6    1   0          int64 t nf   nt - nd - tl          int64 t ri   0          char  r   new char nt   1             for  int64 t i   0  i  lt  nd  i                  v    aa sb   lt  lt  off     aa sb   1   gt  gt   8 - off                v  gt  gt   2              r ri    b64units v               ri    1              bp    6              sb    bp   8               off    bp   8                     if  tl  gt  0                v    aa sb   lt  lt  off               v  gt  gt   2              r ri    b64units v               ri    1                    for  int64 t i   0  i  lt  nf  i                  r ri                     ri    1                    return r        else return nullptr     P S   My method works well  I tested it with Node js  let data    stackabuse com   let buff   new Buffer data   let base64data   buff toString  base64

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See Encoding and decoding base 64 with C    Here is the implementation from that page        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      static const std  string base64 chars                 quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot                quot abcdefghijklmnopqrstuvwxyz quot                quot 0123456789   quot     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

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I firstly made my own version and then found this topic  Why does my version look simpler than others presented here  Am I doing something wrong  I didn t test it for speed  inline char const  b64units    quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot    inline char  b64encode void const  a  int64 t b        ASSERT a    nullptr       if  b  gt  0            uint8 t const  aa   static cast lt uint8 t const  gt  a           uint8 t v   0          int64 t bp   0          int64 t sb   0          int8 t off   0          int64 t nt     b   2    3    4          int64 t nd    b   8    6          int64 t tl     b   8    6    1   0          int64 t nf   nt - nd - tl          int64 t ri   0          char  r   new char nt   1             for  int64 t i   0  i  lt  nd  i                  v    aa sb   lt  lt  off     aa sb   1   gt  gt   8 - off                v  gt  gt   2              r ri    b64units v               ri    1              bp    6              sb    bp   8               off    bp   8                     if  tl  gt  0                v    aa sb   lt  lt  off               v  gt  gt   2              r ri    b64units v               ri    1                    for  int64 t i   0  i  lt  nf  i                  r ri                     ri    1                    return r        else return nullptr     P S   My method works well  I tested it with Node js  let data    stackabuse com   let buff   new Buffer data   let base64data   buff toString  base64

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See Encoding and decoding base 64 with C    Here is the implementation from that page        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      static const std  string base64 chars                 quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot                quot abcdefghijklmnopqrstuvwxyz quot                quot 0123456789   quot     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

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According to this excellent comparison made by GaspardP I would not choose this solution  It s not the worst  but it s not the best either  The only thing it got going for it is that it s possibly easier to understand  I found the other two answers to be pretty hard to understand  They also produce some warnings in my compiler and the use of a find function in the decode part should result in a pretty bad efficiency  So I decided to roll my own  Header   ifndef  BASE64 H   define  BASE64 H    include  lt vector gt   include  lt string gt  typedef unsigned char BYTE   class Base64   public      static std  string encode const std  vector lt BYTE gt  amp  buf       static std  string encode const BYTE  buf  unsigned int bufLen       static std  vector lt BYTE gt  decode std  string encoded string        endif  Body  static const BYTE from base64          255  255  255  255  255  255  255  255  255  255  255  255  255  255  255  255                                      255  255  255  255  255  255  255  255  255  255  255  255  255  255  255  255                                      255  255  255  255  255  255  255  255  255  255  255   62  255   62  255   63                                       52   53   54   55   56   57   58   59   60   61  255  255  255  255  255  255                                      255    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  255  255  255  255   63                                      255   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  255  255  255  255  255    static const char to base64                   quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot                quot abcdefghijklmnopqrstuvwxyz quot                quot 0123456789   quot     std  string Base64  encode const std  vector lt BYTE gt  amp  buf        if  buf empty            return  quot  quot      Avoid dereferencing buf if it s empty     return encode  amp buf 0    unsigned int buf size        std  string Base64  encode const BYTE  buf  unsigned int bufLen           Calculate how many bytes that needs to be added to get a multiple of 3     size t missing   0      size t ret size   bufLen      while   ret size   3     0                  ret size            missing                Expand the return string size to a multiple of 4     ret size   4 ret size 3       std  string ret      ret reserve ret size        for  unsigned int i 0  i lt ret size 4    i                   Read a group of three bytes  avoid buffer overrun by replacing with 0          size t index   i 3          BYTE b3 3           b3 0     index 0  lt  bufLen    buf index 0    0          b3 1     index 1  lt  bufLen    buf index 1    0          b3 2     index 2  lt  bufLen    buf index 2    0              Transform into four base 64 characters         BYTE b4 4           b4 0                                 b3 0   amp  0xfc   gt  gt  2           b4 1      b3 0   amp  0x03   lt  lt  4         b3 1   amp  0xf0   gt  gt  4           b4 2      b3 1   amp  0x0f   lt  lt  2         b3 2   amp  0xc0   gt  gt  6           b4 3      b3 2   amp  0x3f   lt  lt  0               Add the base 64 characters to the return value         ret push back to base64 b4 0             ret push back to base64 b4 1             ret push back to base64 b4 2             ret push back to base64 b4 3                   Replace data that is invalid  always as many as there are missing bytes      for  size t i 0  i lt missing    i          ret ret size - i - 1              return ret     std  vector lt BYTE gt  Base64  decode std  string encoded string           Make sure string length is a multiple of 4     while   encoded string size     4     0          encoded string push back            size t encoded size   encoded string size        std  vector lt BYTE gt  ret      ret reserve 3 encoded size 4        for  size t i 0  i lt encoded size  i    4                   Get values for each group of four base 64 characters         BYTE b4 4           b4 0     encoded string i 0   lt    z     from base64 encoded string i 0     0xff          b4 1     encoded string i 1   lt    z     from base64 encoded string i 1     0xff          b4 2     encoded string i 2   lt    z     from base64 encoded string i 2     0xff          b4 3     encoded string i 3   lt    z     from base64 encoded string i 3     0xff              Transform into a group of three bytes         BYTE b3 3           b3 0      b4 0   amp  0x3f   lt  lt  2      b4 1   amp  0x30   gt  gt  4           b3 1      b4 1   amp  0x0f   lt  lt  4      b4 2   amp  0x3c   gt  gt  2           b3 2      b4 2   amp  0x03   lt  lt  6      b4 3   amp  0x3f   gt  gt  0               Add the byte to the return value if it isn t part of an     character  indicated by 0xff          if  b4 1     0xff  ret push back b3 0            if  b4 2     0xff  ret push back b3 1            if  b4 3     0xff  ret push back b3 2               return ret     Usage  BYTE buf      quot ABCD quot   std  string encoded   Base64  encode buf  4      encoded    quot QUJDRA   quot  std  vector lt BYTE gt  decoded   Base64  decode encoded    A bonus here is that the decode function can also decode the URL variant of Base64 encoding

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I use this  class BinaryVector   public      std  vector lt char gt  bytes       uint64 t bit count   0   public         Add a bit to the end        void push back bool bit           Return false if character is unrecognized        bool pushBase64Char char b64 c       void BinaryVector  push back bool bit        if   bit count    bit count   8    0            bytes push back bit  lt  lt  7             else           uint8 t next bit   8 -  bit count   8  - 1          bytes bit count   8     bit  lt  lt  next bit            bit count          Converts one Base64 character to 6 bits    bool BinaryVector  pushBase64Char char c        uint8 t d          A to Z     if  c  gt  0x40  amp  amp  c  lt  0x5b            d   c - 65      Base64 A is 0              a to z     else if  c  gt  0x60  amp  amp  c  lt  0x7b            d   c - 97   26      Base64 a is 26              0 to 9     else if  c  gt  0x2F  amp  amp  c  lt  0x3a            d   c - 48   52      Base64 0 is 52           else if  c                   d   0b111110            else if  c                   d   0b111111            else if  c                   d   0            else           return false             push back d  amp  0b100000       push back d  amp  0b010000       push back d  amp  0b001000       push back d  amp  0b000100       push back d  amp  0b000010       push back d  amp  0b000001        return true     bool loadBase64 std  vector lt char gt  amp  b64 bin  BinaryVector amp  vec        for  char amp  c   b64 bin            if   vec pushBase64Char c                 return false                      return true      Use vec bytes to access converted data

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Here s my modification of the implementation that was originally written by Ren   Nyffenegger  And why have I modified it  Well  because it didn t seem appropriate to me that I should work with binary data stored within std  string object    base64 h   ifndef  BASE64 H   define  BASE64 H    include  lt vector gt   include  lt string gt  typedef unsigned char BYTE   std  string base64 encode BYTE const  buf  unsigned int bufLen   std  vector lt BYTE gt  base64 decode std  string const amp      endif  base64 cpp   include  quot base64 h quot   include  lt iostream gt   static const std  string base64 chars                 quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot                quot abcdefghijklmnopqrstuvwxyz quot                quot 0123456789   quot     static inline bool is base64 BYTE c      return  isalnum c      c             c              std  string base64 encode BYTE const  buf  unsigned int bufLen      std  string ret    int i   0    int j   0    BYTE char array 3 3     BYTE char array 4 4      while  bufLen--        char array 3 i        buf         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  vector lt BYTE gt  base64 decode std  string const amp  encoded string      int in len   encoded string size      int i   0    int j   0    int in    0    BYTE char array 4 4   char array 3 3     std  vector lt BYTE gt  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 push back 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 push back char array 3 j           return ret     Here s the usage  std  vector lt BYTE gt  myData      std  string encodedData   base64 encode  amp myData 0   myData size     std  vector lt BYTE gt  decodedData   base64 decode encodedData

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According to this excellent comparison made by GaspardP I would not choose this solution  It s not the worst  but it s not the best either  The only thing it got going for it is that it s possibly easier to understand  I found the other two answers to be pretty hard to understand  They also produce some warnings in my compiler and the use of a find function in the decode part should result in a pretty bad efficiency  So I decided to roll my own  Header   ifndef  BASE64 H   define  BASE64 H    include  lt vector gt   include  lt string gt  typedef unsigned char BYTE   class Base64   public      static std  string encode const std  vector lt BYTE gt  amp  buf       static std  string encode const BYTE  buf  unsigned int bufLen       static std  vector lt BYTE gt  decode std  string encoded string        endif  Body  static const BYTE from base64          255  255  255  255  255  255  255  255  255  255  255  255  255  255  255  255                                      255  255  255  255  255  255  255  255  255  255  255  255  255  255  255  255                                      255  255  255  255  255  255  255  255  255  255  255   62  255   62  255   63                                       52   53   54   55   56   57   58   59   60   61  255  255  255  255  255  255                                      255    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  255  255  255  255   63                                      255   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  255  255  255  255  255    static const char to base64                   quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot                quot abcdefghijklmnopqrstuvwxyz quot                quot 0123456789   quot     std  string Base64  encode const std  vector lt BYTE gt  amp  buf        if  buf empty            return  quot  quot      Avoid dereferencing buf if it s empty     return encode  amp buf 0    unsigned int buf size        std  string Base64  encode const BYTE  buf  unsigned int bufLen           Calculate how many bytes that needs to be added to get a multiple of 3     size t missing   0      size t ret size   bufLen      while   ret size   3     0                  ret size            missing                Expand the return string size to a multiple of 4     ret size   4 ret size 3       std  string ret      ret reserve ret size        for  unsigned int i 0  i lt ret size 4    i                   Read a group of three bytes  avoid buffer overrun by replacing with 0          size t index   i 3          BYTE b3 3           b3 0     index 0  lt  bufLen    buf index 0    0          b3 1     index 1  lt  bufLen    buf index 1    0          b3 2     index 2  lt  bufLen    buf index 2    0              Transform into four base 64 characters         BYTE b4 4           b4 0                                 b3 0   amp  0xfc   gt  gt  2           b4 1      b3 0   amp  0x03   lt  lt  4         b3 1   amp  0xf0   gt  gt  4           b4 2      b3 1   amp  0x0f   lt  lt  2         b3 2   amp  0xc0   gt  gt  6           b4 3      b3 2   amp  0x3f   lt  lt  0               Add the base 64 characters to the return value         ret push back to base64 b4 0             ret push back to base64 b4 1             ret push back to base64 b4 2             ret push back to base64 b4 3                   Replace data that is invalid  always as many as there are missing bytes      for  size t i 0  i lt missing    i          ret ret size - i - 1              return ret     std  vector lt BYTE gt  Base64  decode std  string encoded string           Make sure string length is a multiple of 4     while   encoded string size     4     0          encoded string push back            size t encoded size   encoded string size        std  vector lt BYTE gt  ret      ret reserve 3 encoded size 4        for  size t i 0  i lt encoded size  i    4                   Get values for each group of four base 64 characters         BYTE b4 4           b4 0     encoded string i 0   lt    z     from base64 encoded string i 0     0xff          b4 1     encoded string i 1   lt    z     from base64 encoded string i 1     0xff          b4 2     encoded string i 2   lt    z     from base64 encoded string i 2     0xff          b4 3     encoded string i 3   lt    z     from base64 encoded string i 3     0xff              Transform into a group of three bytes         BYTE b3 3           b3 0      b4 0   amp  0x3f   lt  lt  2      b4 1   amp  0x30   gt  gt  4           b3 1      b4 1   amp  0x0f   lt  lt  4      b4 2   amp  0x3c   gt  gt  2           b3 2      b4 2   amp  0x03   lt  lt  6      b4 3   amp  0x3f   gt  gt  0               Add the byte to the return value if it isn t part of an     character  indicated by 0xff          if  b4 1     0xff  ret push back b3 0            if  b4 2     0xff  ret push back b3 1            if  b4 3     0xff  ret push back b3 2               return ret     Usage  BYTE buf      quot ABCD quot   std  string encoded   Base64  encode buf  4      encoded    quot QUJDRA   quot  std  vector lt BYTE gt  decoded   Base64  decode encoded    A bonus here is that the decode function can also decode the URL variant of Base64 encoding

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My variation on DaedalusAlpha s answer  It avoids copying the parameters at the expense of a couple of tests  Uses uint8 t instead of BYTE  Adds some handy functions for dealing with strings  although usually the input data is binary and may have zero bytes inside  so typically should not be manipulated as a string  which often implies null-terminated data   Also adds some casts to fix compiler warnings  at least on GCC  I haven t run it through MSVC yet   Part of file base64 hpp  void base64 encode string  amp  out  const vector lt uint8 t gt  amp  buf   void base64 encode string  amp  out  const uint8 t  buf  size t bufLen   void base64 encode string  amp  out  string const amp  buf    void base64 decode vector lt uint8 t gt   amp  out  string const amp  encoded string       Use this if you know the output should be a valid string void base64 decode string  amp  out  string const amp  encoded string    File base64 cpp  static const uint8 t from base64 128             8 rows of 16   128        Note  only requires 123 entries  as we only lookup for  lt   z   which z 122      255  255  255  255  255  255  255  255  255  255  255  255  255  255  255  255      255  255  255  255  255  255  255  255  255  255  255  255  255  255  255  255      255  255  255  255  255  255  255  255  255  255  255   62  255   62  255   63       52   53   54   55   56   57   58   59   60   61  255  255    0  255  255  255      255    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  255  255  255  255   63      255   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  255  255  255  255  255     static const char to base64 65         quot ABCDEFGHIJKLMNOPQRSTUVWXYZ quot       quot abcdefghijklmnopqrstuvwxyz quot       quot 0123456789   quot     void base64 encode string  amp  out  string const amp  buf       if  buf empty          base64 encode out  NULL  0      else       base64 encode out  reinterpret cast lt uint8 t const  gt   amp buf 0    buf size         void base64 encode string  amp  out  std  vector lt uint8 t gt  const amp  buf       if  buf empty          base64 encode out  NULL  0      else       base64 encode out   amp buf 0   buf size        void base64 encode string  amp  ret  uint8 t const  buf  size t bufLen          Calculate how many bytes that needs to be added to get a multiple of 3    size t missing   0     size t ret size   bufLen     while   ret size   3     0               ret size          missing              Expand the return string size to a multiple of 4    ret size   4 ret size 3      ret clear       ret reserve ret size       for  size t i   0  i  lt  ret size 4    i                Read a group of three bytes  avoid buffer overrun by replacing with 0        const size t index   i 3        const uint8 t b3 0    index 0  lt  bufLen    buf index 0    0        const uint8 t b3 1    index 1  lt  bufLen    buf index 1    0        const uint8 t b3 2    index 2  lt  bufLen    buf index 2    0            Transform into four base 64 characters       const uint8 t b4 0                            b3 0  amp  0xfc   gt  gt  2         const uint8 t b4 1     b3 0  amp  0x03   lt  lt  4      b3 1  amp  0xf0   gt  gt  4         const uint8 t b4 2     b3 1  amp  0x0f   lt  lt  2      b3 2  amp  0xc0   gt  gt  6         const uint8 t b4 3     b3 2  amp  0x3f   lt  lt  0             Add the base 64 characters to the return value       ret push back to base64 b4 0          ret push back to base64 b4 1          ret push back to base64 b4 2          ret push back to base64 b4 3                Replace data that is invalid  always as many as there are missing bytes     for  size t i   0  i    missing    i        ret ret size - i - 1             template  lt class Out gt  void base64 decode any  Out  amp  ret  std  string const amp  in       typedef typename Out  value type T         Make sure the  intended  string length is a multiple of 4    size t encoded size   in size        while   encoded size   4     0          encoded size      const size t N   in size       ret clear       ret reserve 3 encoded size 4       for  size t i   0  i  lt  encoded size  i    4                Note   z     122           Get values for each group of four base 64 characters       const uint8 t b4 0                in i 0   lt    z     from base64 static cast lt uint8 t gt  in i 0      0xff        const uint8 t b4 1    i 1  lt  N and in i 1   lt    z     from base64 static cast lt uint8 t gt  in i 1      0xff        const uint8 t b4 2    i 2  lt  N and in i 2   lt    z     from base64 static cast lt uint8 t gt  in i 2      0xff        const uint8 t b4 3    i 3  lt  N and in i 3   lt    z     from base64 static cast lt uint8 t gt  in i 3      0xff            Transform into a group of three bytes       const uint8 t b3 0     b4 0  amp  0x3f   lt  lt  2      b4 1  amp  0x30   gt  gt  4         const uint8 t b3 1     b4 1  amp  0x0f   lt  lt  4      b4 2  amp  0x3c   gt  gt  2         const uint8 t b3 2     b4 2  amp  0x03   lt  lt  6      b4 3  amp  0x3f   gt  gt  0             Add the byte to the return value if it isn t part of an     character  indicated by 0xff        if  b4 1    0xff  ret push back  static cast lt T gt  b3 0           if  b4 2    0xff  ret push back  static cast lt T gt  b3 1           if  b4 3    0xff  ret push back  static cast lt T gt  b3 2             void base64 decode vector lt uint8 t gt   amp  out  string const amp  encoded string       base64 decode any out  encoded string      void base64 decode string  amp  out  string const amp  encoded string       base64 decode any out  encoded string

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There are several snippets here  However  this one is compact  efficient  and C  11 friendly  static std  string base64 encode const std  string  amp in         std  string out       int val   0  valb   -6      for  uchar c   in            val    val  lt  lt  8    c          valb    8          while  valb  gt   0                out push back  quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot   val gt  gt valb  amp 0x3F                valb -  6                      if  valb gt -6  out push back  quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot    val lt  lt 8  gt  gt  valb 8   amp 0x3F        while  out size   4  out push back           return out     static std  string base64 decode const std  string  amp in         std  string out       std  vector lt int gt  T 256 -1       for  int i 0  i lt 64  i    T  quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot  i     i       int val 0  valb -8      for  uchar c   in            if  T c     -1  break          val    val  lt  lt  6    T c           valb    6          if  valb  gt   0                out push back char  val gt  gt valb  amp 0xFF                valb -  8                      return out

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Using base-n mini lib  you can do the following  some data t in            constexpr int len   sizeof in  sizeof in 0     std  string encoded  bn  encode b64 in  in   len  std  back inserter encoded     some data t out len   bn  decode b64 encoded begin    encoded end    out    The API is generic  iterator-based  Disclosure  I m the author

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I think this one works better   include  lt string gt   static const char  B64chars    quot ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789   quot    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     const std  string b64encode const void  data  const size t  amp len        std  string result  len   2    3   4            char  p    char   data   str    amp result 0       size t j   0  pad   len   3      const size t last   len - pad       for  size t i   0  i  lt  last  i    3                int n   int p i    lt  lt  16   int p i   1    lt  lt  8   p i   2           str j      B64chars n  gt  gt  18           str j      B64chars n  gt  gt  12  amp  0x3F           str j      B64chars n  gt  gt  6  amp  0x3F           str j      B64chars n  amp  0x3F             if  pad       Set padding               int n   --pad   int p last    lt  lt  8   p last   1    p last           str j      B64chars pad   n  gt  gt  10  amp  0x3F   n  gt  gt  2           str j      B64chars pad   n  gt  gt  4  amp  0x03F   n  lt  lt  4  amp  0x3F           str j      pad   B64chars n  lt  lt  2  amp  0x3F                   return result     const std  string b64decode const void  data  const size t  amp len        if  len    0  return  quot  quot        unsigned char  p    unsigned char   data      size t j   0          pad1   len   4    p len - 1                  pad2   pad1  amp  amp   len   4  gt  2    p len - 2               const size t last    len - pad1    4  lt  lt  2      std  string result last   4   3   pad1   pad2    0        unsigned char  str    unsigned char    amp result 0        for  size t i   0  i  lt  last  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  pad1                int n   B64index p last    lt  lt  18   B64index p last   1    lt  lt  12          str j      n  gt  gt  16          if  pad2                        n    B64index p last   2    lt  lt  6              str j      n  gt  gt  8  amp  0xFF                      return result     std  string b64encode const std  string amp  str        return b64encode str c str    str size        std  string b64decode const std  string amp  str64        return b64decode str64 c str    str64 size        Thanks to Jens Alfke for pointing out a performance issue  I have made some modifications to this old post  This one works way faster than before  Its other advantage is smooth handling of corrupt data as well  Last edition  Although in these kinds of problems  it seems that speed is an overkill  but just for the fun of it I have made some other modifications to make this one the fastest algorithm out there AFAIK  Special thanks goes to GaspardP for his valuable suggestions and nice benchmark

[c++] Base64 decode snippet in C++

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