How can I convert this encryption function from Swift to Objective-C? - ios

Question

I'm writing a helper function for an iOS Objective-C app that is supposed to encrypt a string with the provided key using the AES-128 algorithm.
I already have a working implementation of this function in Swift, but I'm struggling translating it into Objective-C. The function doesn't take into account any initialization vector, which is hard-coded as an empty string: this is intended.
Here is the Swift code that works as expected:
#objc func AES128(_ string: String, withKey: String) -> String {
let data = string.data(using: String.Encoding.utf8)!
let keyData = withKey.data(using: String.Encoding.utf8)!
let ivData = "".data(using: String.Encoding.utf8)!
let cryptLength = size_t(data.count + kCCBlockSizeAES128)
var cryptData = Data(count: cryptLength)
let keyLength = size_t(kCCKeySizeAES128)
let options = CCOptions(kCCOptionPKCS7Padding)
var numBytesEncrypted: size_t = 0
let cryptStatus = cryptData.withUnsafeMutableBytes { cryptBytes in
data.withUnsafeBytes { dataBytes in
ivData.withUnsafeBytes { ivBytes in
keyData.withUnsafeBytes { keyBytes in
CCCrypt(CCOperation(kCCEncrypt), CCAlgorithm(kCCAlgorithmAES), options, keyBytes, keyLength, ivBytes, dataBytes, data.count, cryptBytes, cryptLength, &numBytesEncrypted)
}
}
}
}
if UInt32(cryptStatus) == UInt32(kCCSuccess) {
cryptData.removeSubrange(numBytesEncrypted..<cryptData.count)
}
return cryptData.base64EncodedString();
}
This function works like a charm and encrypting the string Hello World with the key ABCDEFGHIJKLMNOP results in a base64 encoded string of 3G4OU62dM9zXhkbXy8pmuA==.
I translated it into Objective-C:
+(NSString *)AES128:(NSString *)string withKey:(NSString *)key {
NSData *data = [string dataUsingEncoding:NSUTF8StringEncoding];
NSData *keyData = [key dataUsingEncoding:NSUTF8StringEncoding];
NSData *ivData = [#"" dataUsingEncoding:NSUTF8StringEncoding];
size_t cryptLength = data.length + kCCBlockSizeAES128;
NSMutableData *cryptData = [NSMutableData dataWithLength:cryptLength];
size_t keyLength = kCCKeySizeAES128;
CCOptions options = kCCOptionPKCS7Padding;
size_t numBytesEncrypted = 0;
CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES, options, keyData.bytes, keyLength, ivData.bytes, data.bytes, data.length, cryptData.mutableBytes, cryptLength, &numBytesEncrypted);
if (cryptStatus == kCCSuccess) {
[cryptData replaceBytesInRange:NSMakeRange(numBytesEncrypted, cryptData.length - numBytesEncrypted) withBytes:NULL length:0];
}
return [cryptData base64EncodedStringWithOptions:0];
}
The problem is that the Objective-C version provides wrong results, I'd say random results, producing for the same combination of Hello World + ABCDEFGHIJKLMNOP different encrypted values.
7m+HH5NusyA1VAfZ78KYCw== or NY5p8XtYLoAE/4VbCCrPIg== are some of those wrong values.
If you decrypt these strings you get Hello Wold and Hello Worod respectively.
What did I do wrong while translating?
Thank you for your kind help!

Answer 1

This seems to be an issue with your handling of the Initialization Vector. If you notice, the CCCrypt function accepts a pointer to the byte buffer content of your IV, but you aren't providing a length parameter. This means that the length of the buffer is already known.
According to Wikipedia (https://en.wikipedia.org/wiki/Initialization_vector) the length for an IV is "generally the cipher's block size".
So basically, when you are doing
NSData *ivData = [#"" dataUsingEncoding:NSUTF8StringEncoding];
the ivData.bytes property would now be pointing somewhere, where in the best case scenario, it would have 0x00 for the full length, but very likely wouldn't. The CCCrypt() function would be starting at the memory address you are passing (a pointer to an empty array) and continuing to read for the next [IV Length] bytes, using whatever random data it finds there.
To fix this, you need to make sure somehow, that you are passing a pointer to a buffer containing your intended content of all zeroes.
I did this simply with
void* ivBuffer = calloc(1, kCCBlockSizeAES128);
which allocates a single block of 128 bits (16 bytes) and initializes it with all zeroes.
Your call to CCCrypt() then becomes
CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES, options, keyData.bytes, keyLength, ivBuffer, data.bytes, data.length, cryptData.mutableBytes, cryptLength, &numBytesEncrypted);
and you just have to remember to call free(ivBuffer) to free the memory that was allocated for this buffer.
Also, when you are handling the success status, you can just do
if (cryptStatus == kCCSuccess)
{
cryptData.length = numBytesEncrypted;
}
and it will truncate the data to be just the portion you want to use. Up to you, since the result is the same as what you already had.