How to check for a valid Base64 encoded string

Question

Is there a way in C  to see if a string is Base 64 encoded other than just trying to convert it and see if there is an error   I have code code like this      Convert base64-encoded hash value into a byte array  byte   HashBytes   Convert FromBase64String Value     I want to avoid the  Invalid character in a Base-64 string  exception that happens if the value is not valid base 64 string  I want to just check and return false instead of handling an exception because I expect that sometimes this value is not going to be a base 64 string   Is there some way to check before using the Convert FromBase64String function   Thanks     Update  Thanks for all of your answers   Here is an extension method you can all use so far it seems to make sure your string will pass Convert FromBase64String without an exception    NET seems to ignore all trailing and ending spaces when converting to base 64 so  1234  is valid and so is   1234        public static bool IsBase64String this string s        s   s Trim        return  s Length   4    0   amp  amp  Regex IsMatch s      a-zA-Z0-9       0 3     RegexOptions None        For those wondering about performance of testing vs catching and exception  in most cases for this base 64 thing it is faster to check than to catch the exception until you reach a certain length  The smaller the length faster it is  In my very unscientific testing  For 10000 iterations for character length 100 000 - 110000 it was 2 7 times faster to test first     For 1000 iterations for characters length 1 - 16 characters for total of 16 000 tests it was 10 9 times faster   I am sure there is a point where it becomes better to test with the exception based method  I just don t know at what point that is

User · Answer

I will use like this so that I don t need to call the convert method again     public static bool IsBase64 this string base64String out byte   bytes                bytes   null             Credit  oybek http   stackoverflow com users 794764 oybek         if  string IsNullOrEmpty base64String     base64String Length   4    0               base64String Contains         base64String Contains   t      base64String Contains   r      base64String Contains   n                return false           try                        bytes Convert FromBase64String base64String               return true                    catch  Exception                           Handle the exception                    return false

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Why not just catch the exception  and return False   This avoids additional overhead in the common case

User · Answer

Sure   Just make sure each character is within a-z  A-Z  0-9     or    and the string ends with       At least  that s the most common Base64 implementation   You might find some implementations that use characters different from   or   for the last two characters

User · Answer

Yes  since Base64 encodes binary data into ASCII strings using a limited set of characters  you can simply check it with this regular expression      A-Za-z0-9       s n    s  which will assure the string only contains A-Z  a-z  0-9                 and whitespace

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Do decode  re encode and compare the result to original string  public static Boolean IsBase64 this String str        if   str Length   4     0                return false               decode - encode and compare     try               string decoded   System Text Encoding UTF8 GetString System Convert FromBase64String str            string encoded   System Convert ToBase64String System Text Encoding UTF8 GetBytes decoded            if  str Equals encoded  StringComparison InvariantCultureIgnoreCase                         return true                      catch         return false

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I would suggest creating a regex to do the job  You ll have to check for something like this   a-zA-Z0-9     You ll also have to check the length of the string  I m not sure on this one  but i m pretty sure if something gets trimmed  other than the padding      it would blow up   Or better yet check out this stackoverflow question

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I believe the regex should be       Regex IsMatch s      a-zA-Z0-9       0 2       Only matching one or two trailing     signs  not three   s should be the string that will be checked  Regex is part of the System Text RegularExpressions namespace

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I know you said you didn t want to catch an exception   But  because catching an exception is more reliable  I will go ahead and post this answer   public static bool IsBase64 this string base64String            Credit  oybek https   stackoverflow com users 794764 oybek      if  string IsNullOrEmpty base64String     base64String Length   4    0            base64String Contains         base64String Contains   t      base64String Contains   r      base64String Contains   n            return false        try           Convert FromBase64String base64String            return true              catch Exception exception           Handle the exception             return false      Update  I ve updated the condition thanks to oybek to further improve reliability

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Knibb High football rules    This should be relatively fast and accurate but I admit I didn t put it through a thorough test  just a few   It avoids expensive exceptions  regex  and also avoids looping through a character set  instead using ascii ranges for validation   public static bool IsBase64String string s                s   s Trim            int mod4   s Length   4          if mod4  0               return false                    int i 0          bool checkPadding   false          int paddingCount   1   only applies when the first is encountered          for i 0 i lt s Length i                 char c   s i               if  checkPadding                                if  c                                               return false                                    paddingCount                    if  paddingCount  gt  3                                        return false                                    continue                            if c gt   A   amp  amp  c lt   z     c gt   0   amp  amp  c lt   9                    continue                            switch c                  case                    case                       continue                case                        checkPadding   true                   continue                            return false                      if here             length was correct             there were no invalid characters             padding was correct         return true

User · Answer

Just for the sake of completeness I want to provide some implementation  Generally speaking Regex is an expensive approach  especially if the string is large  which happens when transferring large files   The following approach tries the fastest ways of detection first   public static class HelperExtensions          Characters that are used in base64 strings      private static Char   Base64Chars   new      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                        lt summary gt          Extension method to test whether the value is a base64 string          lt  summary gt           lt param name  value  gt Value to test lt  param gt           lt returns gt Boolean value  true if the string is base64  otherwise false lt  returns gt      public static Boolean IsBase64String this String value                The quickest test  If the value is null or is equal to 0 it is not base64            Base64 string s length is always divisible by four  i e  8  16  20 etc              If it is not you can return false  Quite effective            Further  if it meets the above criterias  then test for spaces             If it contains spaces  it is not base64         if  value    null    value Length    0    value Length   4    0                value Contains         value Contains   t      value Contains   r      value Contains   n                return false              98  of all non base64 values are invalidated by this time          var index   value Length - 1              if there is padding step back         if  value index                      index--              if there are two padding chars step back a second time         if  value index                      index--              Now traverse over characters            You should note that I m not creating any copy of the existing strings              assuming that they may be quite large         for  var i   0  i  lt   index  i                    If any of the character is not from the allowed list             if   Base64Chars Contains value i                       return false                 return false              If we got here  then the value is a valid base64 string         return true            EDIT  As suggested by Sam  you can also change the source code slightly  He provides a better performing approach for the last step of tests  The routine       private static Boolean IsInvalid char value            var intValue    Int32 value              1 - 9         if  intValue  gt   48  amp  amp  intValue  lt   57               return false              A - Z         if  intValue  gt   65  amp  amp  intValue  lt   90               return false              a - z         if  intValue  gt   97  amp  amp  intValue  lt   122               return false                or           return intValue    43  amp  amp  intValue    47           can be used to replace if   Base64Chars Contains value i    line with if  IsInvalid value i     The complete source code with enhancements from Sam will look like this  removed comments for clarity   public static class HelperExtensions       public static Boolean IsBase64String this String value            if  value    null    value Length    0    value Length   4    0                value Contains         value Contains   t      value Contains   r      value Contains   n                return false          var index   value Length - 1          if  value index                      index--          if  value index                      index--          for  var i   0  i  lt   index  i                if  IsInvalid value i                    return false          return true               Make it private as there is the name makes no sense for an outside caller     private static Boolean IsInvalid char value            var intValue    Int32 value          if  intValue  gt   48  amp  amp  intValue  lt   57              return false          if  intValue  gt   65  amp  amp  intValue  lt   90              return false          if  intValue  gt   97  amp  amp  intValue  lt   122              return false          return intValue    43  amp  amp  intValue    47

User · Answer

public static bool IsBase64String1 string value                        if  string IsNullOrEmpty value                                 return false                            try                               Convert FromBase64String value                   if  value EndsWith                                             value   value Trim                        int mod4   value Length   4                      if  mod4    0                                                return false                                            return true                                    else                                        return false                                              catch  FormatException                                return false

User · Answer

Imho this is not really possible  All posted solutions fails for strings like  test  and so on  If they can be divided through 4  are not null or empty  and if they are a valid base64 character  they will pass all tests  That can be many strings      So there is no real solution other than knowing that this is a base 64 encoded string  What I ve come up with is this   if  base64DecodedString StartsWith   lt xml gt             This was really a base64 encoded string I was expecting  Yippie    else          This is gibberish      I expect that the decoded string begins with a certain structure  so I check for that

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I have just had a very similar requirement where I am letting the user do some image manipulation in a  lt canvas gt  element and then sending the resulting image retrieved with  toDataURL   to the backend  I wanted to do some server validation before saving the image and have implemented a ValidationAttribute using some of the code from other answers    AttributeUsage AttributeTargets Property  AllowMultiple   false  Inherited   false   public class Bae64PngImageAttribute   ValidationAttribute       public override bool IsValid object value                if  value    null    string IsNullOrWhiteSpace value as string               return true     not concerned with whether or not this field is required         var base64string    value as string  Trim                we are expecting a URL type string         if   base64string StartsWith  data image png base64                 return false           base64string   base64string Substring  data image png base64   Length               match length and regular expression         if  base64string Length   4    0     Regex IsMatch base64string      a-zA-Z0-9       0 3     RegexOptions None               return false              finally  try to convert it to a byte array and catch exceptions         try                       byte   converted   Convert FromBase64String base64string               return true                    catch Exception                        return false                      As you can see I am expecting an image png type string  which is the default returned by  lt canvas gt  when using  toDataURL

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Check Base64 or normal string   public bool IsBase64Encoded String str      try           If no exception is caught  then it is possibly a base64 encoded string     byte   data   Convert FromBase64String str          The part that checks if the string was properly padded to the        correct length was borrowed from d anish s solution     return  str Replace         Length   4    0     catch          If exception is caught  then it is not a base64 encoded string    return false

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The answer must depend on the usage of the string  There are many strings that may be  valid base64  according to the syntax suggested by several posters  but that may  correctly  decode  without exception  to junk  Example  the 8char string Portland is valid Base64  What is the point of stating that this is valid Base64  I guess that at some point you d want to know that this string should or should not be Base64 decoded   In my case  I have Oracle connection strings that may be in plain text like    Data source mydb DBNAME User Id Roland Password          or in base64 like   VXNlciBJZD1sa                                          I just have to check for the presence of a semicolon  because that proves that it is NOT base64  which is of course faster than any above method

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All answers were been digested into 1 function that ensures 100  that its results will be accurate  1  Use function as below      string encoded    quot WW91ckJhc2U2NHN0cmluZw   quot       msgbox  quot Is string base64  quot    IsBase64 encoded     2  Below is the function  public bool IsBase64 string base64String        try               if   base64String Length  lt  1                        if   base64String Equals Convert ToBase64String Encoding UTF8 GetBytes Encoding UTF8 GetString Convert FromBase64String base64String      StringComparison InvariantCultureIgnoreCase   amp   System Text RegularExpressions Regex IsMatch base64String    quot   a-zA-Z0-9       0 2   quot                                  return false                            if   base64String Length   4     0    string IsNullOrEmpty base64String     base64String Length   4    0    base64String Contains  quot   quot      base64String Contains Constants vbTab     base64String Contains Constants vbCr     base64String Contains Constants vbLf                                 return false                                  else                       return false                                         return true            catch  FormatException ex                    return false

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Use Convert TryFromBase64String from C  7 2  public static bool IsBase64String string base64       Span lt byte gt  buffer   new Span lt byte gt  new byte base64 Length       return Convert TryFromBase64String base64  buffer   out int bytesParsed

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It s pretty easy to recognize a Base64 string  as it will only be composed of characters  A    Z    a    z    0    9            and it is often padded at the end with up to three      to make the length a multiple of 4  But instead of comparing these  you d be better off ignoring the exception  if it occurs

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I prefer this usage      public static class StringExtensions                    lt summary gt              Check if string is Base64              lt  summary gt               lt param name  quot base64 quot  gt  lt  param gt               lt returns gt  lt  returns gt          public static bool IsBase64String this string base64                          https   stackoverflow com questions 6309379 how-to-check-for-a-valid-base64-encoded-string             Span lt byte gt  buffer   new Span lt byte gt  new byte base64 Length                return Convert TryFromBase64String base64  buffer  out int                      Then usage if myStr IsBase64String

[c#] How to check for a valid Base64 encoded string

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