AES128 CTR encryption with iv

Question

I want to implement AES128 CTR with iv and key. I'm looking for any advice how to do that in best way and not reinvent wheel.

I found good lib for this RNCryptor, but looks like this aes is not supported there.

Also I test this approach, but looks like this is not CTR.

EDIT

---

I used zpproach from @zaph

NSData *result = [NSData cryptData:dataStr
                         operation:kCCEncrypt
                              mode:kCCModeCTR
                         algorithm:kCCAlgorithmAES128
                           padding:ccNoPadding
                         keyLength:kCCKeySizeAES128
                                iv:ivHex
                               key:keyHex
                             error:&error];

but receive CCCryptorCreate status: -4305

Just found in sources

@constant kCCUnimplemented Function not implemented for the current algorithm.

Link

Answer 1

You need to use CommonCrypto but not the one-shot CCCrypt version, but instead the full CCCryptorCreateWithMode, CCCryptorUpdate, CCCryptorFinal and CCCryptorRelease with mode kCCModeCTR version.

Here is sample code I have, it may not fit your needs and you will have to decide on the counter (IV) form.

#import <CommonCrypto/CommonCrypto.h>

+ (NSData *)cryptData:(NSData *)dataIn
            operation:(CCOperation)operation  // kCC Encrypt, Decrypt
                 mode:(CCMode)mode            // kCCMode ECB, CBC, CFB, CTR, OFB, RC4, CFB8
            algorithm:(CCAlgorithm)algorithm  // CCAlgorithm AES DES, 3DES, CAST, RC4, RC2, Blowfish
              padding:(CCPadding)padding      // cc NoPadding, PKCS7Padding
            keyLength:(size_t)keyLength       // kCCKeySizeAES 128, 192, 256
                   iv:(NSData *)iv            // CBC, CFB, CFB8, OFB, CTR
                  key:(NSData *)key
                error:(NSError **)error
{
    if (key.length != keyLength) {
        NSLog(@"CCCryptorArgument key.length: %lu != keyLength: %zu", (unsigned long)key.length, keyLength);
        if (error) {
            *error = [NSError errorWithDomain:@"kArgumentError key length" code:key.length userInfo:nil];
        }
        return nil;
    }

    size_t dataOutMoved = 0;
    size_t dataOutMovedTotal = 0;
    CCCryptorStatus ccStatus = 0;
    CCCryptorRef cryptor = NULL;

    ccStatus = CCCryptorCreateWithMode(operation, mode, algorithm,
                                       padding,
                                       iv.bytes, key.bytes,
                                       keyLength,
                                       NULL, 0, 0, // tweak XTS mode, numRounds
                                       kCCModeOptionCTR_BE, // CCModeOptions
                                       &cryptor);

    if (cryptor == 0 || ccStatus != kCCSuccess) {
        NSLog(@"CCCryptorCreate status: %d", ccStatus);
        if (error) {
            *error = [NSError errorWithDomain:@"kCreateError" code:ccStatus userInfo:nil];
        }
        CCCryptorRelease(cryptor);
        return nil;
    }

    size_t dataOutLength = CCCryptorGetOutputLength(cryptor, dataIn.length, true);
    NSMutableData *dataOut = [NSMutableData dataWithLength:dataOutLength];
    char *dataOutPointer = (char *)dataOut.mutableBytes;

    ccStatus = CCCryptorUpdate(cryptor,
                               dataIn.bytes, dataIn.length,
                               dataOutPointer, dataOutLength,
                               &dataOutMoved);
    dataOutMovedTotal += dataOutMoved;

    if (ccStatus != kCCSuccess) {
        NSLog(@"CCCryptorUpdate status: %d", ccStatus);
        if (error) {
            *error = [NSError errorWithDomain:@"kUpdateError" code:ccStatus userInfo:nil];
        }
        CCCryptorRelease(cryptor);
        return nil;
    }

    ccStatus = CCCryptorFinal(cryptor,
                              dataOutPointer + dataOutMoved, dataOutLength - dataOutMoved,
                              &dataOutMoved);
    if (ccStatus != kCCSuccess) {
        NSLog(@"CCCryptorFinal status: %d", ccStatus);
        if (error) {
            *error = [NSError errorWithDomain:@"kFinalError" code:ccStatus userInfo:nil];
        }
        CCCryptorRelease(cryptor);
        return nil;
    }

    CCCryptorRelease(cryptor);

    dataOutMovedTotal += dataOutMoved;
    dataOut.length = dataOutMovedTotal;

    return dataOut;
}

Sample invocation:

NSData *dataIn  = [@"DataInDataInData" dataUsingEncoding: NSUTF8StringEncoding];
NSData *key     = [@"KeyKeyKeyKeyKeyK" dataUsingEncoding: NSUTF8StringEncoding];
NSData *counter = [@"CounterCounterCo" dataUsingEncoding: NSUTF8StringEncoding];
NSError *error;
NSData *encrpted = [Crypto
                    cryptData:dataIn
                    operation:kCCEncrypt
                    mode:kCCModeCTR
                    algorithm:kCCAlgorithmAES
                    padding:ccNoPadding
                    keyLength:kCCKeySizeAES128
                    iv:counter
                    key:key
                    error:&error];
NSLog(@"encrypted: %@", encrpted);

Output: encrypted: 064e8073 76973eba 3192474f 9831db34