iOS encryption AES128/CBC/nopadding why is not working? - ios

Question

I have an app that needs to encode some data using AES/CBC/no padding. The app is ported also on android. There the encoding is done like this:
byte[] encodedKey = getKey();
SecretKeySpec skeySpec = new SecretKeySpec(encodedKey, "AES");
AlgorithmParameterSpec paramSpec = new IvParameterSpec(initializationVector);
Cipher cipher = Cipher.getInstance("AES/CBC/NoPadding");
cipher.init(Cipher.ENCRYPT_MODE, skeySpec, paramSpec);
int blockSize = cipher.getBlockSize();
int diffSize = decrypted.length % blockSize;
System.out.println("Cipher size: " + blockSize);
System.out.println("Current size: " + decrypted.length);
if (diffSize != 0) {
diffSize = blockSize - diffSize;
byte[] oldDecrypted = decrypted;
decrypted = new byte[decrypted.length + diffSize];
System.arraycopy(oldDecrypted, 0, decrypted, 0, oldDecrypted.length);
for (int i = 0; i < diffSize; i++) {
decrypted[i + oldDecrypted.length] = " ".getBytes()[0];
}
System.out.println("New size: " + decrypted.length);
}
return cipher.doFinal(decrypted);
the initializationVector looks like this:
private byte[] initializationVector = new byte[] { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
on iOS i have something like this for encryption:
- (NSData *)AES128EncryptWithKey:(NSString *)key
{
// 'key' should be 16 bytes for AES128, will be null-padded otherwise
char keyPtr[kCCKeySizeAES128+1]; // room for terminator (unused)
bzero(keyPtr, sizeof(keyPtr)); // fill with zeroes (for padding)
// fetch key data
[key getCString:keyPtr maxLength:sizeof(keyPtr) encoding:NSUTF8StringEncoding];
NSUInteger dataLength = [self length];
//See the doc: For block ciphers, the output size will always be less than or
//equal to the input size plus the size of one block.
//That's why we need to add the size of one block here
size_t bufferSize = dataLength + kCCBlockSizeAES128;
void *buffer = malloc(bufferSize);
size_t numBytesEncrypted = 0;
CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt,
kCCAlgorithmAES128,
0x0000,
keyPtr,
kCCKeySizeAES128,
NULL /* initialization vector (optional) */,
[self bytes], dataLength, /* input */
buffer, bufferSize, /* output */
&numBytesEncrypted);
if (cryptStatus == kCCSuccess) {
//the returned NSData takes ownership of the buffer and will free it on deallocation
return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
}
free(buffer); //free the buffer;
return nil;
}
the method described above is part of a category over NSData.
the method is called like this:
NSData *data = [#"4915200456727" dataUsingEncoding:NSUTF8StringEncoding];
NSData *cipher = [data AES128EncryptWithKey:#"#x#zddXekZerBBw6"];
NSString *ecriptedString = [NSString stringWithFormat:#"%.*s", [cipher length], [cipher bytes]];
the problem that i have is that i don't receive the same encrypted data on iOS and android. On iOS the encrypted data has 0 bytes in length.
Could you give any pointers on how to encrypt a string using AES128 with CBC and no padding and perhaps an example?
Thank you

Answer 1

I found the solution to my problem. In order to make the encryption work without padding i had to add 0x0000 instead of kCCOptionPKCS7Padding or kCCOptionECBMode which are treated.
Also if the data that needs to be encoded doesn't have a length multiple of kCCKeySizeAES128 ( 16 ) then the vector that holds the data must be resized to have the length multiple with kCCKeySizeAES128 and the empty values filled with something. I added spaces.
- (NSData *)AES128EncryptWithKey:(NSString *)key
{
char keyPtr[kCCKeySizeAES128+1];
bzero(keyPtr, sizeof(keyPtr));
[key getCString:keyPtr maxLength:sizeof(keyPtr) encoding:NSUTF8StringEncoding];
int dataLength = [self length];
int diff = kCCKeySizeAES128 - (dataLength % kCCKeySizeAES128);
int newSize = 0;
if(diff > 0)
{
newSize = dataLength + diff;
}
char dataPtr[newSize];
memcpy(dataPtr, [self bytes], [self length]);
for(int i = 0; i < diff; i++)
{
dataPtr[i + dataLength] = 0x20;
}
size_t bufferSize = newSize + kCCBlockSizeAES128;
void *buffer = malloc(bufferSize);
size_t numBytesEncrypted = 0;
CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt,
kCCAlgorithmAES128,
0x0000, //No padding
keyPtr,
kCCKeySizeAES128,
NULL,
dataPtr,
sizeof(dataPtr),
buffer,
bufferSize,
&numBytesEncrypted);
if(cryptStatus == kCCSuccess)
{
return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
}
return nil;
}