I'm trying to decrypt RC2 data with only 64 effective bits.
Since I can only have 64-bits, it's my understanding that before calling CCCrypt I must use a method to reduce the key down to this many bits. Since I couldn't find any such method in Apple's CommonCrypto library, I am using this keyschedule method I found.
These are the method's parameters:
void rc2_keyschedule( unsigned short xkey[64],
const unsigned char *key,
unsigned len,
unsigned bits )
For the actual decryption part, I'm trying to work off of an example that uses AES 256. This is what I have so far:
// setup the key to send to CCCrypt
unsigned char originalKey[16] = /* derived from some other method */;
unsigned short key[64];
unsigned effectiveBits = 64;
rc2_keyschedule(key, originalKey, 16, effectiveBits);
// key is now 128 bytes, and I manually checked it for accuracy
// setup the cipherText to send to CCCrypt
NSData *cipherText = /* derived from some other method */;
// cipherText was manually checked for accuracy
// setup the buffer to send to CCCrypt
size_t bufferSize = [cipherText length] + kCCBlockSizeRC2;
void *buffer = malloc(bufferSize);
size_t numBytesDecrypted = 0;
// call CCCrypt
CCCryptorStatus cryptStatus = CCCrypt(kCCDecrypt,
kCCAlgorithmRC2,
kCCOptionPKCS7Padding,
key, 128,
NULL /* initialization vector (optional) */,
[cipherText bytes], [cipherText length],
buffer, bufferSize, /* output */
&numBytesDecrypted);
if (cryptStatus == kCCSuccess) {
return [NSData dataWithBytesNoCopy:buffer length:numBytesDecrypted];
}
free(buffer); //free the buffer;
return nil;
When I run this, cryptStatus has the value kCCDecodeError which is documented as:
@constant kCCDecodeError Input data did not decode or decrypt properly.
The reason I am sending 128 as the keyLength to CCCrypt is because my key is 64 short ints long, and I believe that 1 short is equal to 2 bytes. Thus, 64 * 2=128.
I have no idea what I need to send for the option. I just used kCCOptionPKCS7Padding which was taken from the AES example. The other options available are kCCOptionECBMode and CBC. When I try the other two options, cryptStatus does become kCCSuccess, but the data is always null. I think that it is erroneously reporting success.
When I say that I "manually checked something for accuracy", I mean that I compared the key and cipher at those points to a JavaScript implementation which works successfully.
How can I use Apple's Libraries (i.e. CommonCrypt) to decrypt RC2 data?