AES 256/CBC/Withoutpadding in objective-c - iphone

Question

I'm trying to achieve AES 256 with zero padding and CBC mode . I tried all methods from cypto but results are coming different what is from the server
I'm using this code
Where I'm passing simple string for check in databstring , key and iv is passed as "iphone".
/**************
- (NSData *)AES256Encrypt:(NSString *)dataString WithKey:(NSString *)key iv:(NSString *)iv {
// 'key' should be 32 bytes for AES256, will be null-padded otherwise
char keyPtr[kCCKeySizeAES256+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];
NSLog(#"keyPtr: '%s'", keyPtr);
NSData *keyData = [key dataUsingEncoding:NSUTF8StringEncoding];
NSLog(#"keyPtr: '%s'", keyData.bytes);
NSData *dataToEncrypt = [dataString dataUsingEncoding:NSUTF8StringEncoding];
NSData *ivData = [iv dataUsingEncoding:NSUTF8StringEncoding];
NSUInteger dataLength = [dataToEncrypt 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;
CCCryptorRef cryptorRef;
CCCryptorStatus rc;
CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES128, 0,
keyData.bytes, kCCKeySizeAES256,
ivData.bytes, // initialisation vector
dataToEncrypt.bytes,
dataToEncrypt.length, /* input */
buffer, bufferSize, /* output */
&numBytesEncrypted);
if (cryptStatus == kCCSuccess) {
// NSString *someDataHexadecimalString = [[NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted] hexadecimalString];
NSLog(#"%#",[NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted]);
//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;
}
**********/
But it prints different result everytime.
please help.

Answer 1

Your IV ("iphone") is too short. CBC mode requires an IV equal to the block size of the cipher algorithm (16 bytes for AES). CCCrypt reads 16 bytes from your provided iv buffer, and since yours is too short, whatever garbage happens to be in memory after the end of the buffer will be used as the rest of the IV.
So essentially you are using a different IV each time, which is why your ciphertext is different each time.
In general, don't use strings for IVs. For security, the IV should be different for each different message, and that's hard to do if you are using hard-coded strings. Just generate 16 random bytes for the IV, and put them at the beginning of the cipher text.