AES256 decrypt

// remember to import in Bridging-Header.h :
//#import <CommonCrypto/CommonCryptor.h>

enum AESError: Error {
        case KeyError((String, Int))
    case IVError((String, Int))
    case CryptorError((String, Int))
}

extension String {

    public func aes256Decrypt(key: String) throws -> Data? {

        guard let data = Data(base64Encoded: self, options: Data.Base64DecodingOptions.ignoreUnknownCharacters) else {
            return nil
        }

        guard let keyData = key.data(using: .utf8) else {
            return nil
        }

        let clearLength = size_t(data.count + kCCBlockSizeAES128)
        var clearData = Data(count:clearLength)

        var numBytesDecrypted: size_t = 0
        let options = CCOptions(kCCOptionECBMode + kCCOptionPKCS7Padding)

        let cryptStatus = clearData.withUnsafeMutableBytes {cryptBytes in
            data.withUnsafeBytes {dataBytes in
                keyData.withUnsafeBytes {keyBytes in
                    CCCrypt(CCOperation(kCCDecrypt), CCAlgorithm(kCCAlgorithmAES), options,
                            keyBytes, kCCKeySizeAES256,
                            nil,
                            dataBytes + kCCBlockSizeAES128, clearLength,
                            cryptBytes, clearLength,
                            &numBytesDecrypted)
                }
            }
        }

        if UInt32(cryptStatus) == UInt32(kCCSuccess) {
            clearData.count = numBytesDecrypted
        }
        else {
            throw AESError.CryptorError(("Decryption failed", Int(cryptStatus)))
        }

        return clearData
    }
}