Signing and Verifying on iOS using RSA

How to sign and verify some data on iOS with an RSA key (preferably using the system own libcommonCrypto)?

Since there hasn't been nearly any knowledge about signing and verifying found on StackOverflow and the Apple docs, I had to manually browse around in the iOS header files and found SecKeyRawSign and SecKeyRawVerify. The following lines of code seem to work.

Signing NSData (using SHA256 with RSA):

NSData* PKCSSignBytesSHA256withRSA(NSData* plainData, SecKeyRef privateKey)
{
    size_t signedHashBytesSize = SecKeyGetBlockSize(privateKey);
    uint8_t* signedHashBytes = malloc(signedHashBytesSize);
    memset(signedHashBytes, 0x0, signedHashBytesSize);

    size_t hashBytesSize = CC_SHA256_DIGEST_LENGTH;
    uint8_t* hashBytes = malloc(hashBytesSize);
    if (!CC_SHA256([plainData bytes], (CC_LONG)[plainData length], hashBytes)) {
        return nil;
    }

    SecKeyRawSign(privateKey,
                  kSecPaddingPKCS1SHA256,
                  hashBytes,
                  hashBytesSize,
                  signedHashBytes,
                  &signedHashBytesSize);

    NSData* signedHash = [NSData dataWithBytes:signedHashBytes
                                        length:(NSUInteger)signedHashBytesSize];

    if (hashBytes)
        free(hashBytes);
    if (signedHashBytes)
        free(signedHashBytes);

    return signedHash;
}

Verification (using SHA256 with RSA):

BOOL PKCSVerifyBytesSHA256withRSA(NSData* plainData, NSData* signature, SecKeyRef publicKey)
{
    size_t signedHashBytesSize = SecKeyGetBlockSize(publicKey);
    const void* signedHashBytes = [signature bytes];

    size_t hashBytesSize = CC_SHA256_DIGEST_LENGTH;
    uint8_t* hashBytes = malloc(hashBytesSize);
    if (!CC_SHA256([plainData bytes], (CC_LONG)[plainData length], hashBytes)) {
        return nil;
    }

    OSStatus status = SecKeyRawVerify(publicKey,
                                      kSecPaddingPKCS1SHA256,
                                      hashBytes,
                                      hashBytesSize,
                                      signedHashBytes,
                                      signedHashBytesSize);

    return status == errSecSuccess;
}

Alternatives (OpenSSL):

There is a very good alternative available which utilizes OpenSSL directly instead of libCommonCrypto. MIHCrypto is a well-designed Objective-C wrapper library for OpenSSL which makes working with cryptography very easy. See the example below.

Generating a key is that simple:

MIHAESKeyFactory *factory = [[MIHAESKeyFactory alloc] init];
id<MIHSymmetricKey> aesKey = [factory generateKey];

Or loading a key from file:

NSData *privateKeyData = [[NSFileManager defaultManager] contentsAtPath:"mykey.pem"];
MIHRSAPrivateKey *privateKey = [[MIHRSAPrivateKey alloc] initWithData:privateKeyData];

Now sign something:

NSError *signingError = nil;
NSData *message = // load something to sign from somewhere
NSData *signature = [privateKey signWithSHA256:message error:&signingError]

For more examples browse the MIHCrypto page.