Objective-C decrypt AES 128 cbc hex string - iphone

Question

I am developing an application for iPhone on Snow Leopard with Xcode 3.1 that receives from a restful web service an encrypted text in hexadecimal format with the algorithm AES 128-bit (CBC). The algorithm uses an initialization vector + secret key. How do I decrypt this text? Thanks to everyone for the tips that I will succeed in giving.
EDIT:
I get response from REST Server in hex AND crypted format,I try with this code but i receive always bad param error. Can you help me to find the error?
Is it possible that i firstly convert the string response into binary format?
NSString *response = [request responseString];
NSData *encryptedData = [response dataUsingEncoding: NSASCIIStringEncoding];
NSString *key = #"32charlength";
NSString *iv = #"16charlength";
NSData *unencryptedData = NULL;
size_t unencryptedLength = [unencryptedData length];
CCCryptorStatus ccStatus = CCCrypt(kCCDecrypt, kCCAlgorithmAES128, 0, key, kCCKeySizeAES128, iv, [encryptedData bytes], [encryptedData length], unencryptedData, [encryptedData length], &unencryptedLength);
NSString *output = [[NSString alloc] initWithBytes:unencryptedData length:unencryptedLength encoding:NSUTF8StringEncoding];
if (ccStatus == kCCSuccess) risultato.text = #"SUCCESS";
else if (ccStatus == kCCParamError) risultato.text = #"BAD PARAM";
else if (ccStatus == kCCBufferTooSmall) risultato.text = #"BUFFER TOO SMALL";
else if (ccStatus == kCCMemoryFailure) risultato.text = #"MEMORY FAILURE";
else if (ccStatus == kCCAlignmentError) risultato.text = #"ALIGNMENT";
else if (ccStatus == kCCDecodeError) risultato.text = #"DECODE ERROR";
else if (ccStatus == kCCUnimplemented) risultato.text = #"UNIMPLEMENTED";
EDIT2:
this function return BAD PARAM because haven't the right buffer size where allocate decrypted data. I edit the function in this working way:
NSData *encryptedData = [response dataUsingEncoding: NSASCIIStringEncoding];
const void *key = #"32charlength;
uint8_t *iv = #"16charlength";
char buffer[4*4096];
memset(buffer, '\0', sizeof(buffer));
size_t size = sizeof(buffer);
CCCryptorStatus ccStatus = CCCrypt(kCCDecrypt,
kCCAlgorithmAES128,
0,
key,
kCCKeySizeAES128,
iv,
[encryptedData bytes],
[encryptedData length],
buffer,
sizeof(buffer),
&size);
This function working for me.. thanks so much.
EDIT 23 MARCH------
Now the system work form me with 16 byte key size. Now i have a question, what i can do to implement 32 byte key size ? thanks so much.

Answer 1

This is possible using the CCCryptor functions included in the <CommonCrypto/CommonCryptor.h> header. Check man CCCryptor for the gory details, in your case it sounds like you can use a single call to CCCrypt() to decode the received data:
CCCryptorStatus
CCCrypt(CCOperation op, CCAlgorithm alg, CCOptions options, const void *key, size_t keyLength,
const void *iv, const void *dataIn, size_t dataInLength, void *dataOut, size_t dataOutAvailable,
size_t *dataOutMoved);
Assuming you have the data to be decrypted in NSData *encryptedData you could try something like:
char * key = "shouldbe16chars.";
NSUInteger dataLength = [encryptedData length];
uint8_t unencryptedData[dataLength + kCCKeySizeAES128];
size_t unencryptedLength;
CCCrypt(kCCDecrypt, kCCAlgorithmAES128, 0, key, kCCKeySizeAES128, NULL, [encryptedData bytes], dataLength, unencryptedData, dataLength, &unencryptedLength);
NSString *output = [[NSString alloc] initWithBytes:unencryptedData length:unencryptedLength encoding:NSUTF8StringEncoding];
This is untested, make sure you check the return value of CCCrypt for errors. Check the header file for details, it is well documented.