Signing and Verifying on iOS using RSA

Question

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

Answer 1

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.