AS3Crypto RSA Signing

Question

Hi All,

I'm having some troubles matching the value returned from RSA signing a Base64 SHA1 hash in the actionscript as3crypto library with the result returned in c#.

I'm passing in a Base64 hash decoded as a byte array to the sign() function provided in as3crypto and base64 encoding the result. However, this result never matches the returned result from a c# function which performs the same task. Does it matter that the function takes in and returns hex even though it works at the byte array level?

Please see my below signing function to check i haven't missed anything!

private function signHash(hashInBase64:String):String
{
       var src:ByteArray = Base64.decodeToByteArray(hashInBase64);
       var key:RSAKey = getRSAKey();
       var dst:ByteArray = new ByteArray();

       key.sign(src, dst, src.length);

       return Base64.encodeByteArray(dst);
}

Anyone had much experience with the AS3Crypto library?

Any help would be great!!!

Thanks,

Jon

Answer 1

I assume that your C# version is using RSA PKCS #1 version 1.5. The standard computes signatures by doing an RSA private key operation over a byte string composed as

0x00 0x01 || 0xff* || 0x00 || OID || hash

Looking at the as3crypto code shows that the RSAKey class does not add any OID during the sign operation. Hence if you don't do it you'll get incorrect results.

Looking at the code also shows that as3crypto is vulnerable to this attack, because it does not verify the padding properly. This attack is more than 3 years old. Hence it seems like a good to use a different library than as3crypto.

Answer 2

Jon. Do You solve the problem ?