Triple DES decryption in iOS

Question

I'm currently using the following for Triple DES decryption on iOS:

    NSString* plainText = @"My Text";
    NSString* keyText = @"cf6f1ed3bf0a156e";

    NSData *plainData = [plainText dataUsingEncoding:NSUTF8StringEncoding];
    NSData *keyData = [keyText dataUsingEncoding:NSUTF8StringEncoding];

    size_t bufferSize = plainData.length + kCCBlockSize3DES;
    NSMutableData *cypherData = [NSMutableData dataWithLength:bufferSize];
    size_t movedBytes = 0;

    CCCryptorStatus ccStatus;
    ccStatus = CCCrypt(kCCDecrypt,
            kCCAlgorithm3DES,
            kCCOptionECBMode,
            keyData.bytes,
            kCCBlockSize3DES,
            NULL,
            plainData.bytes,
            plainData.length,
            cypherData.mutableBytes,
            cypherData.length,
            &movedBytes);

    cypherData.length = movedBytes;

    if( ccStatus == kCCSuccess ) {
        NSLog(@"Data: %@",cypherData);
        NSLog(@"Data encoded string: %@",[NSString stringWithUTF8String:[cypherData bytes]]);
        NSLog(@"Data encoded: %@",[[NSString alloc] initWithData:cypherData encoding:NSUTF8StringEncoding]);
    } else {
        NSLog(@"Failed DES decrypt ...");
        return nil;
    }

However, I keep getting the following in the console:

Data: Data encoded string:(null) Data encoded: (null)

Any ideas as to why this is happening? Can anyone see any possible issues with this code?

Answer 1

Your key is 16 bytes long. 3DES takes a key that is 24 bytes long (thanks Zaph for correcting; also noting you're only reading 8 bytes of that). This may not be causing this error, but means the key isn't what you think it is.

A series of hex digits in a string are just UTF-8 values. "00" is not 0x00, 0x00. It's 0x30, 0x30.

The reason you're getting (null) here is this:

NSLog(@"Data encoded string: %@",[NSString stringWithUTF8String:[cypherData bytes]]);

Cryptographic output is very unlikely to be a legal UTF-8 string. If you want to encode random data as a string, you need an encoding like hex encoding or Base64 encoding. Base64 is built in and you can use [NSData base64EncodedStringWithOptions:] to encode it.