Question

I hope a smart guy will help me 🙂

I receive a encrypted text from a C# server and i’m unable to decrypt it correctly:
I always have an empty string. Normally the decrypted key should be

1111111111111111

(16time)

I use the AES128 algorithm for the decryption and the settings given by the back end (the guys who encrypted this text) are as follow :

• Padding: PKCS7Padding
• KeySize: 128
• InitVector: null
• Mode: CBC
• text to decrypt (base64 encoded) : 1vycDn3ktoyaUkPlRAIlsA==
• the key: 3a139b187647a66d

Here is the code i use for the decryption

- (NSData *)AES256DecryptWithKey:(NSString *)key {
// 'key' should be 32 bytes for AES256, will be null-padded otherwise
char keyPtr[kCCKeySizeAES2128+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 numBytesDecrypted = 0;
CCCryptorStatus cryptStatus = CCCrypt(kCCDecrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,
                                 keyPtr, kCCKeySizeAES128,
                                 NULL /* initialization vector (optional) */,
                                 [self bytes], dataLength, /* input */
                                 buffer, bufferSize, /* output */
                                 &numBytesDecrypted);

if (cryptStatus == kCCSuccess) {
    //the returned NSData takes ownership of the buffer and will free it on deallocation
    return [NSData dataWithBytesNoCopy:buffer length:numBytesDecrypted];
}

free(buffer); //free the buffer;
return nil;

}

Thanx alot in advance for your help. I’m on this probleme since a long time with no answer in front of me…

Answer 1

The test input you’re using doesn’t appear to be using PKCS7 padding — it’s using no padding. I was able to get a successful decrypt using:

echo 1vycDn3ktoyaUkPlRAIlsA== | openssl enc -aes-128-cbc -d -a -K `echo 3a139b187647a66d | xxd -ps` -iv 0 -nosalt -nopad