CODEWITHSUNDEEP
ioscommoncrypto
Sky
Sky

Reputation: 33

iOS CCCrypt kCCDecrypt using kCCAlgorithmAES128

Here is the code:

- (NSData *) doCipher: (NSData *) plainData key: (NSData *) symmetricKey context:   (CCOperation) encryptOrDecrypt padding: (CCOptions *) pkcs7
{
// Initialization vector; dummy in this case 0's.
uint8_t iv[kChosenCipherBlockSize];
bzero((void *) iv, (size_t) sizeof(iv));

// We don't want to toss padding on if we don't need to
if (encryptOrDecrypt == kCCEncrypt)
{
    if (*pkcs7 != kCCOptionECBMode)
    {
        if ((plainData.length  % kChosenCipherBlockSize) == 0)
            *pkcs7 = 0x0000;
        else
            *pkcs7 = kCCOptionPKCS7Padding;
    }
}
else if (encryptOrDecrypt == kCCDecrypt)
{
    *pkcs7 = 0x0000;
}
else
{
    DLog(@"Invalid CCOperation parameter [%d] for cipher context.", *pkcs7);
    return nil;
}

// Actually perform the encryption or decryption.
NSMutableData *dataOut  = [NSMutableData dataWithLength: plainData.length + kChosenCipherBlockSize];
size_t movedBytes       = 0;

CCCryptorStatus ccStatus = CCCrypt(encryptOrDecrypt,
        kCCAlgorithmAES128,
        *pkcs7,
        symmetricKey.bytes,
        kChosenCipherKeySize,
        iv,
        [plainData bytes],
        [plainData length],
        [dataOut mutableBytes],
        [dataOut length],
        &movedBytes
);

if (ccStatus == noErr)
{
    dataOut.length = movedBytes;
}
else
{
    DLog(@"Problem with encipherment ccStatus == %d", ccStatus);
    return nil;
}

return dataOut;
}

When I using kCCOptionPKCS7Padding on kCCDecrypt sometimes I get error code 4304. I try not using padding when kCCDecrypt as described here Anyone else having trouble with iOS 5 encryption? and I don't get the error. But sometimes the data length after kCCDecrypt is not the same as original data length before kCCEncrypt. I think this is because original data length is not multiplied by encoding block size.

Anyone else have this trouble?

Upvotes: 2

Views: 4738

Answers (1)

Maarten Bodewes
Maarten Bodewes

Reputation: 93968

You cannot just toss out padding. Leave padding on and everything will be alright.

See also: Encrypting 16 bytes of UTF8 with SecKeyWrapper breaks (ccStatus == -4304)

Upvotes: 3

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