解码Base64字符串返回nil

Question

我一直在尝试在使用 Swift 5 和 Xcode 构建的应用程序中实现 GMAIL API。 我已经成功从用户那里检索到电子邮件，但是我无法从编码的字符串中获取实际文本。每当我尝试将此 Base64 编码的数据转换为普通字符串时，它会返回 nil。

以下是该字符串：

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_RW1haWw9c2lkcmFpYnJhaGltMDM0OUBnbWFpbC5jb20mY29udGludWU9aHR0cHM6Ly9teWFjY291bnQuZ29vZ2xlLmNvbS9hbGVydC9udC8xNTY3ODQ3ODEyMDAwP3JmbiUzRDEyNyUyNnJmbmMlM0QxJTI2ZWlkJTNELTE1NzQzMDEyNjk4Mjg2MTM3OTQlMjZldCUzRDElMjZhbmV4cCUzRGdpdmFiLWZhLS1tZHYyLWZhPg0KWW91IHJlY2VpdmVkIHRoaXMgZW1haWwgdG8gbGV0IHlvdSBrbm93IGFib3V0IGltcG9ydGFudCBjaGFuZ2VzIHRvIHlvdXINCkdvb2dsZSBBY2NvdW50IGFuZCBzZXJ2aWNlcy4NCsKpIDIwMTkgR29vZ2xlIExMQywgMTYwMCBBbXBoaXRoZWF0cmUgUGFya3dheSwgTW91bnRhaW4gVmlldywgQ0EgOTQwNDMsIFVTQQ0K

一些stackoverflow的回答建议给字符串添加填充，但这也没有起作用。

我已经尝试了这里提供的所有解决方案，但都没有起作用。我当前实现的代码如下。

extension String {

    func base64Encoded() -> String? {
        return data(using: .utf8)?.base64EncodedString()
    }

    func base64Decoded() -> String? {
        var st = self;
        print(st.count)
        let remainder = self.count % 4
        if remainder > 0 {
            st = self.padding(toLength: self.count + 4 - remainder,
                                          withPad: "=",
                                          startingAt: 0)
        }

        print(st.count)
        guard let d = Data(base64Encoded: st, options: .ignoreUnknownCharacters) else{
            return nil
        }
        return String(data: d, encoding: .utf8)
    }   
}

但是这并不起作用。 如果有人能找出问题，我将非常感激。

- Mahrukh Salman

Answer 1

你的字符串包含下划线字符（'_' U+005F LOW LINE），这在Base-64中无效。

有一些Base-64的变种，所以你需要找出你的Gmail正在使用哪一个。（维基百科 Base-64）

假设它使用了base64url URL-和文件名安全型，你可能需要编写类似这样的代码：

extension String {
    func urlSafeBase64Decoded() -> String? {
        var st = self
            .replacingOccurrences(of: "_", with: "/")
            .replacingOccurrences(of: "-", with: "+")
        let remainder = self.count % 4
        if remainder > 0 {
            st = self.padding(toLength: self.count + 4 - remainder,
                              withPad: "=",
                              startingAt: 0)
        }
        guard let d = Data(base64Encoded: st, options: .ignoreUnknownCharacters) else{
            return nil
        }
        return String(data: d, encoding: .utf8)
    }
}

例子：

let base64 = "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_RW1haWw9c2lkcmFpYnJhaGltMDM0OUBnbWFpbC5jb20mY29udGludWU9aHR0cHM6Ly9teWFjY291bnQuZ29vZ2xlLmNvbS9hbGVydC9udC8xNTY3ODQ3ODEyMDAwP3JmbiUzRDEyNyUyNnJmbmMlM0QxJTI2ZWlkJTNELTE1NzQzMDEyNjk4Mjg2MTM3OTQlMjZldCUzRDElMjZhbmV4cCUzRGdpdmFiLWZhLS1tZHYyLWZhPg0KWW91IHJlY2VpdmVkIHRoaXMgZW1haWwgdG8gbGV0IHlvdSBrbm93IGFib3V0IGltcG9ydGFudCBjaGFuZ2VzIHRvIHlvdXINCkdvb2dsZSBBY2NvdW50IGFuZCBzZXJ2aWNlcy4NCsKpIDIwMTkgR29vZ2xlIExMQywgMTYwMCBBbXBoaXRoZWF0cmUgUGFya3dheSwgTW91bnRhaW4gVmlldywgQ0EgOTQwNDMsIFVTQQ0K"

print(base64.urlSafeBase64Decoded() ?? "* decoding failed*")

输出：

You received this email to let you know about important changes to your
Google Account and services.
© 2019 Google LLC, 1600 Amphitheatre Parkway, Mountain View, CA 94043, USA

- OOPer

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太棒了，谢谢。我花了4-5个小时来做这个，开始怀疑这是一些其他类型的加密字符串。 - Mahrukh Salman