Decryption using CCCrypt returns kCCSuccess with bad buffer length

Question

I have an encrypted data stream (AES 128, CBC, PKCS7) that I'm trying to decrypt as it arrives. Occasionally I'll get a packet of length 334, which I then try to decrypt. When I do this on an iPhone 5, it returns kCCBufferTooSmall (which is expected for non-mod 16 data). However, when I have the same thing on an iPhone 3GS it returns kCCSuccess and gives me a partially decrypted stream (the last ten bytes or so of the 333 it gives me are bogus - null terminators and random data).

Both devices are iOS 6.1.2. The app is built with base SDK set to latest SDK (6.1) with a deployment target of iOS 5.0.

I created the following test case which also exhibits this problem:

+ (void)decryptionTest {
    NSData *data = [NSMutableData dataWithLength:334]; // 334 % 16 = 14
    NSData *key = [NSMutableData dataWithLength:kCCKeySizeAES128];
    NSData *iv = [NSMutableData dataWithLength:kCCBlockSizeAES128];
    size_t outLength = 0;
    NSMutableData *cipherData = [NSMutableData dataWithLength:data.length];

    CCCryptorStatus result = CCCrypt(kCCDecrypt,
                                     kCCAlgorithmAES128,
                                     kCCOptionPKCS7Padding,
                                     key.bytes,
                                     key.length,
                                     iv.bytes,
                                     data.bytes,
                                     data.length,
                                     cipherData.mutableBytes,
                                     cipherData.length,
                                     &outLength);

    NSLog(@"result = %d", result);
}

Why am I getting kCCSuccess when it should be failing due to not matching the block size?

Decryption using CCCrypt returns kCCSuccess with bad buffer length

0 Answers0