Encryption and Decryption using Security Framework [closed] - iphone

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Closed 10 years ago.
I am doing file encryption and Decryption using the following code,
- (NSData *) AESEncrypt:(NSString *)key withData:(NSData *)fileData {
// '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];
NSUInteger dataLength = [fileData 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;
CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,keyPtr, kCCKeySizeAES256,
NULL /* initialization vector (optional) */,
[fileData bytes], dataLength, /* input */
buffer, bufferSize, /* output */
&numBytesEncrypted);
if (cryptStatus == kCCSuccess) {
//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;
}
and decryption using
- (NSData *) AESDecrypt:(NSString *)key withData:(NSData *)fileData {
// '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];
NSUInteger dataLength = [fileData 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, kCCKeySizeAES256, NULL /* initialization vector (optional) */,
[fileData 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;
}
Which works fine. But looks like it is using AES256 , but I want to change it to AES128 so I changed the first line from char keyPtr[kCCKeySizeAES256+1]; to char keyPtr[kCCKeySizeAES128+1]; .
Fi I change so, only encryption is working. Decryption is not working fine. Can anybody tell me why ?

Answer 1

One problem is that you confuse a key with a string of characters. A key is not a string, it is binary data. You fill with zero's, but the key may contain zero's as well, so any terminator is out of place.
If you want to create a key from a password, choose a character-encoding, convert your password to that encoding and feed the result together with a salt and iteration count in a function like PBKDF2.