proper way of AES128Encryption for a password in ios - ios

Question

I'm facing a problem of password encryption. I'm using AES128 to encrypt the data. Here is the following code:
LoginController.m
#import <CommonCrypto/CommonDigest.h>
#include <CommonCrypto/CommonCryptor.h>
#import <CommonCrypto/CommonKeyDerivation.h>
#import "NSData+AES.h"
- (IBAction)loginButtonClick:(id)sender
{
NSData *toencrypt = [password.text dataUsingEncoding:NSUTF8StringEncoding];
NSString *credentials = [self md5:[NSString stringWithFormat:#"63jhdf83hf73haf3983f"]]; //returns 679fb1ddf7d81bee0aff2ef251fe6bf5
NSString *iv = #"kdf67398DF7383fd";
NSData *data = [toencrypt AES128EncryptWithKey:credentials iv:iv];
NSString *postdata = [data base64Encoding]; //base64Encoding is implemented in NSData+AES.m
NSLog(#"Original Password: %#",password);
NSLog(#"Encrypted Password: %#",postdata);
}
**N.B: md5 conversion and base64Encoding is working fine.
NSData+AES.m
static char encodingTable[64] =
{
'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P',
'Q','R','S','T','U','V','W','X','Y','Z','a','b','c','d','e','f',
'g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v',
'w','x','y','z','0','1','2','3','4','5','6','7','8','9','+','/'
};
#implementation NSData (AES)
- (NSData *)AES128EncryptWithKey:(NSString *)key iv:(NSString *)iv
{
char keyPtr[kCCKeySizeAES128+1];
bzero(keyPtr, sizeof(keyPtr));
[key getCString:keyPtr maxLength:sizeof(keyPtr) encoding:NSUTF8StringEncoding];
char ivPtr[kCCKeySizeAES128+1];
bzero(ivPtr, sizeof(ivPtr));
[iv getCString:ivPtr maxLength:sizeof(ivPtr) encoding:NSUTF8StringEncoding];
NSUInteger dataLength = [self length];
int diff = kCCKeySizeAES128 - (dataLength % kCCKeySizeAES128);
int newSize = 0;
if(diff > 0){
newSize = (int)dataLength + diff;
}
char dataPtr[newSize];
memcpy(dataPtr, [self bytes], [self length]);
for(int i = 0; i < diff; i++){
dataPtr[i + dataLength] = 0x00;
}
size_t bufferSize = newSize + kCCBlockSizeAES128;
void *buffer = malloc(bufferSize);
size_t numBytesEncrypted = 0;
CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt,
kCCAlgorithmAES128,
0x00, //No padding
keyPtr,
kCCKeySizeAES128,
ivPtr,
dataPtr,
sizeof(dataPtr),
buffer,
bufferSize,
&numBytesEncrypted);
if(cryptStatus == kCCSuccess)
{
return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
}
return nil;
}
Now If a send "brad" it should return "ur/bq4Rz". But unfortunately it returning "83+eQZW3eI4UWGdNk4nnUw==". Whats the problem? Please help me out. Thanks in advance.

Answer 1

Note that this code is pretty insecure. It reuses the same Key+IV for every message, which greatly simplifies the job of decrypting the message for an attacker (especially short messages). And it has other security problems (poor keyspace, bad padding scheme)
That said, the piece that is specifically broken is that you're passing a 32-byte (256-bit) "key" but you're only using 128-bits of it. You probably meant to use kCCKeySizeAES256 as the key length. This is unrelated to kCCAlgorithmAES128 which refers to the block size, not the key length. In the diff code, you meant to use kCCBlockSizeAES128 rather than kCCKeySizeAES128, but they are both 16 so it doesn't impact the result.
As #Codo notes, standard CBC mode cannot output cipher text that is not a multiple of 16 bytes in length. Why do you believe "ur/bq4Rz" is the correct encryption? There is a modification of CBC that can do this (it adds what is called "cipher text stealing" or CTS), but this isn't supported by iOS.
Your code is doing zero-padding, which is a very poor way to do it (you're just adding zeros until you get to a 16-byte boundary). The standard (and more secure) solution is PKCS#7 padding, which you're turning off.
If I had to guess, you're copying code that zero-pads the data and then truncates the cipher text to length. The decryption then probably pads the cipher text with zeros and then truncates the garbage at the end (probably based on knowing the length beforehand). I see how this can work, but it's not the normal approach.
If you want an example of how to do this simply, see Properly Encrypting With AES and CommonCrypto. If you want a more secure approach that does all of it for you, see RNCryptor. Neither of these will generate cipher text that is the same length as your plaintext, however.

Answer 2

I found some issues, for example you don't check if getCString:maxLength:endocind failed or not. This is deprecated function and it may fail here, as a result encryption method may use completly different key and IV
Here is my version of this method. This should do the job right:
- (NSData *)AES128EncryptWithKey:(NSString *)key iv:(NSString *)iv
{
int diff = kCCKeySizeAES128 - (self.length % kCCKeySizeAES128);
NSMutableData *adjustedData = [NSMutableData dataWithData:self];
[adjustedData appendData:[[NSMutableData alloc] initWithLength:diff]];
size_t bufferSize = adjustedData.length + kCCBlockSizeAES128;
void *buffer = malloc(bufferSize);
size_t numBytesEncrypted = 0;
CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES128,
0x00, //No padding
[key UTF8String], kCCKeySizeAES128,
[iv UTF8String],
adjustedData.bytes,
adjustedData.length,
buffer,
bufferSize,
&numBytesEncrypted);
if(cryptStatus == kCCSuccess) {
return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
}
return nil;
}