nsdata加密后转化为nsstring为nil

我写了一些字符串加密的的代码，我把字符串转成nadata类型然后用AES256ParmEncryptWithKey进行加密处理，但当我对加密后的data转成nsstring时返回的却是nil
请问下这是什么原因？谢谢。

NSString *testStr  = @"TEST";
    NSData *strData = [testStr dataUsingEncoding:NSUTF8StringEncoding];
    NSData *encryptStrData = [strData AES256ParmEncryptWithKey:@"this is a test"];
    NSString *decryptStr = [[NSString alloc]initWithData:encryptStrData encoding:NSUTF16StringEncoding];
    NSLog(@"%@",decryptStr);

下面是数据加密处理：

- (NSData *)AES256ParmEncryptWithKey:(NSString *)key   //加密
{
    char keyPtr[kCCKeySizeAES256+1];
    bzero(keyPtr, sizeof(keyPtr));
    [key getCString:keyPtr maxLength:sizeof(keyPtr) encoding:NSUTF8StringEncoding];
    NSUInteger dataLength = [self length];
    size_t bufferSize = dataLength + kCCBlockSizeAES128;
    void *buffer = malloc(bufferSize);
    size_t numBytesEncrypted = 0;
    CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES128,
                                          kCCOptionPKCS7Padding | kCCOptionECBMode,
                                          keyPtr, kCCBlockSizeAES128,
                                          NULL,
                                          [self bytes], dataLength,
                                          buffer, bufferSize,
                                          &numBytesEncrypted);
    if (cryptStatus == kCCSuccess) {
        return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
    }
    free(buffer);
    return nil;
}