SWIFT4中的AES解密产生空值

Question

我正在使用扩展方法来解密由服务器端php脚本使用AES算法加密的字符串。

以下是使用的扩展方法

func aesDecrypt(key:String, iv:String, options:Int = kCCOptionPKCS7Padding) -> String? {
        if let keyData = key.data(using: String.Encoding.utf8),
            let data = NSData(base64Encoded: self, options: .ignoreUnknownCharacters),
            let cryptData    = NSMutableData(length: Int((data.length)) + kCCBlockSizeAES128) {

            let keyLength              = size_t(kCCKeySizeAES128)
            let operation: CCOperation = UInt32(kCCDecrypt)
            let algoritm:  CCAlgorithm = UInt32(kCCAlgorithmAES128)
            let options:   CCOptions   = UInt32(options)

            var numBytesEncrypted :size_t = 0

            let cryptStatus = CCCrypt(operation,
                                      algoritm,
                                      options,
                                      (keyData as NSData).bytes, keyLength,
                                      iv,
                                      data.bytes, data.length,
                                      cryptData.mutableBytes, cryptData.length,
                                      &numBytesEncrypted)

            if UInt32(cryptStatus) == UInt32(kCCSuccess) {
                cryptData.length = Int(numBytesEncrypted)
                let unencryptedMessage = String(data: cryptData as Data, encoding:String.Encoding.utf8)
                return unencryptedMessage
            }
            else {
                return nil
            }
        }
        return nil
    }

这有时会返回空值

unencode = responseString?.aesDecrypt(key: "mykey", iv: "value");

我无法弄清楚发生这种情况的确切原因。当发生这种情况时，responseString不为空。

请咨询