- (NSString *)AES256Encrypt:(NSString *)dataString WithKey:(NSString *)key iv:(NSString *)iv {
    
    NSData *keyData = [key dataUsingEncoding:NSUTF8StringEncoding];
    NSData *dataToEncrypt = [dataString dataUsingEncoding:NSUTF8StringEncoding];
    NSData *ivData = [iv dataUsingEncoding:NSUTF8StringEncoding];
    
    NSUInteger dataLength = [dataToEncrypt length];
    
    size_t bufferSize = dataLength + kCCBlockSizeAES128;
    void *buffer = malloc(bufferSize);
    
    size_t numBytesEncrypted = 0;
    CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,
                                          keyData.bytes, kCCKeySizeAES256,
                                          ivData.bytes, // initialisation vector
                                          dataToEncrypt.bytes,
                                          dataToEncrypt.length, /* input */
                                          buffer, bufferSize, /* output */
                                          &numBytesEncrypted);
    if (cryptStatus == kCCSuccess) {
        //the returned NSData takes ownership of the buffer and will free it on deallocation
        NSData *data = [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
        return [[NSString alloc] initWithData:[GTMBase64 encodeData:data] encoding:NSUTF8StringEncoding];
    }
    
    free(buffer); //free the buffer;
    return nil;
}
- (NSString *)AES256Decrypt:(NSString *)originString WithKey:(NSString *)key iv:(NSString *)iv {
    NSData *originData = [originString dataUsingEncoding:NSUTF8StringEncoding];
    NSData *cipherData = [GTMBase64 decodeData:originData];
    
    NSData *keydata = [key dataUsingEncoding:NSUTF8StringEncoding];
    NSData *dataToEncrypt = cipherData;
    NSUInteger dataLength = [dataToEncrypt length];
    size_t bufferSize = dataLength + kCCBlockSizeAES128;
    void *buffer = malloc(bufferSize);
    size_t numBytesDecrypted = 0;
    NSData *ivData = [iv dataUsingEncoding:NSUTF8StringEncoding];
    CCCryptorStatus cryptStatus = CCCrypt(kCCDecrypt,
                                          kCCAlgorithmAES128,
                                          kCCOptionPKCS7Padding,
                                          keydata.bytes,
                                          kCCKeySizeAES256,
                                          ivData.bytes,
                                          [dataToEncrypt bytes],
                                          [dataToEncrypt length],
                                          buffer,
                                          1024,
                                          &numBytesDecrypted);
    if (cryptStatus == kCCSuccess) {
        NSData *data = [NSData dataWithBytesNoCopy:buffer length:numBytesDecrypted];
        return [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
    }
    free(buffer); //free the buffer;
    return nil;
}