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How to determine if a string is Base64 decoded or not?
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How to determine if a string is Base64 decoded or not?
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4 solutions

Do it like this:

C#
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;
    }
}



The function will return true if the string str was Base64 encoded and false if it was not. There is still is a doubt because a string that contains random letters and numbers might still look Base64 encoded even if it was just a random string. The only good answer in this case is false because that will tell you that the string is definitely not a Base64 encoded string.

Cheers and happy coding!

-MRB
 
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Posted
Updated 6-Apr-11 22:43pm
v2
Comments
Sergey Alexandrovich Kryukov 6-Apr-11 19:22pm    
This seems to be better. My 5. The fact it is not certain is OK: this is so by definition: a format cannot be recognized by the formatted data in principle, as it's not meta-data.
--SA
dan!sh 6-Apr-11 22:42pm    
Very true. One can just create some base64 look alike and send it to this method. There, it will fail. Only way to be 100% sure is ask the creator of string.
Sergey Alexandrovich Kryukov 7-Apr-11 0:04am    
Right. And this is usually possible. In email there is the line with Content-type of the message part, in custom networking one can always add some meta-data before data to the protocol, and so forth. This is a matter of design, of the formulation of the problem. In right design, sufficient meta-data portion always goes before the data.
--SA
dan!sh 7-Apr-11 0:07am    
Yes. Had OP told some more about the kind of application he is working on, content-type would have been #1 contender. :)
the headless nick 13-Oct-11 2:41am    
If str = "HELL"

Then It will not throw any exception so we cannot find whether it is Base64String or a Simple String. So It will decode to a Simple String.

Plz check My Question here http://www.codeproject.com/Questions/267500/Alternate-Convert-FromBase64String-Method
-NS
I think you mean encoded, right?

Use Convert.FromBase64String method. If it throws FormatException, then the original string was not base64. Something like this:

public static bool IsBase64(string base64String){
if(base64String.Replace(" ","").Length % 4 != 0){
return false;
}

try{
Convert.FromBase64String(base64String);
return true;
}
catch(FormatException exception){
// Handle the exception
}
return false;
}


You can add null check etc to this method to make it less exception prone.
 
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Posted
Comments
Sergey Alexandrovich Kryukov 6-Apr-11 19:22pm    
Essentially the same as Manfred's. My 5. See my comment to Manfred's Answer, though.
--SA
Manfred Rudolf Bihy 7-Apr-11 4:45am    
You get my 5 for pointing out that (base64String.Length % 4 == 0) has to be true.
I borrowed that and incorporated it into my solution, but not without giving you proper credit though!

:=)
C#
//java.util.regex.Pattern
private boolean isBase64(String stringBase64){
        String regex =
               "([A-Za-z0-9+/]{4})*"+
               "([A-Za-z0-9+/]{4}|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{2}==)";

        Pattern patron = Pattern.compile(regex);

        if (!patron.matcher(stringBase64).matches()) {
            return false;
        } else {
            return true;
        }
}
 
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Posted
Do you mean encoded?

If so, you can do it like this in C#:

C#
try
{
   byte[] converted = Convert.FromBase64String(base64value);
   return base64value.EndsWith("=");
}
catch
{
   return false;
}
 
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Posted
Comments
Manfred Rudolf Bihy 6-Apr-11 9:52am    
Good one! 5+

The padding character (=) does not have to present though, as it is only used to fill up the number of characters so that str.Length % 4 == 0.

Read here: http://en.wikipedia.org/wiki/Base64
Sergey Alexandrovich Kryukov 6-Apr-11 19:20pm    
Manfred, you did not really vote.
(I did not because I am not sure; you say by yourself that a padding character is not always available by the end (I guess this is the case when all columns are full); I also thing that the problem makes no much sense, if this is e-mail, one could look at Content-type of the part, and so on).
--SA
Manfred Rudolf Bihy 7-Apr-11 4:34am    
Your are right! I'll fix my comment.
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