98 lines
3.7 KiB
C#
98 lines
3.7 KiB
C#
namespace WpywMail.Native;
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/// <summary>
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/// SMTP DATA 段的线上编码与还原。
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///
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/// 为什么要单独抽出来：
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/// DKIM 是对「签名那一刻的确切字节」做的哈希。任何在签名之后改写报文的行为
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/// （例如把裸 LF 换成 CRLF）都会让接收方算出的正文哈希对不上，签名直接失效。
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/// 因此约定：**先规范化行尾 → 再签名 → 传输阶段除 dot-stuffing 外不得改动任何字节**。
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/// 本类同时被发送侧与自检使用，保证两边行为一致。
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/// </summary>
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internal static class SmtpDataEncoder
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{
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private static readonly byte[] Crlf = [13, 10];
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/// <summary>把报文规范化为统一的 CRLF 行尾形式（不加密、不加终止行）。应在 DKIM 签名之前调用。</summary>
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public static byte[] Normalize(byte[] message)
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{
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using var output = new MemoryStream(message.Length + 16);
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var index = 0;
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while (index < message.Length)
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{
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var current = message[index];
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if (current == (byte)'\r')
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{
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output.Write(Crlf);
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index += index + 1 < message.Length && message[index + 1] == (byte)'\n' ? 2 : 1;
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continue;
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}
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if (current == (byte)'\n')
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{
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output.Write(Crlf);
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index++;
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continue;
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}
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output.WriteByte(current);
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index++;
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}
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var bytes = output.ToArray();
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if (bytes.Length == 0 || !EndsWithCrlf(bytes))
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{
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var padded = new byte[bytes.Length + 2];
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Buffer.BlockCopy(bytes, 0, padded, 0, bytes.Length);
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padded[^2] = 13;
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padded[^1] = 10;
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return padded;
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}
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return bytes;
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}
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/// <summary>
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/// 生成 DATA 段实际要写出的字节：行首的点做 dot-stuffing，结尾补 CRLF 与单独一行的 "."。
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/// 除 dot-stuffing 外不改动任何字节，以保证与 DKIM 签名一致。
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/// </summary>
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public static byte[] Encode(byte[] normalizedMessage)
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{
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using var output = new MemoryStream(normalizedMessage.Length + 16);
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var atLineStart = true;
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foreach (var current in normalizedMessage)
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{
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if (atLineStart && current == (byte)'.') output.WriteByte((byte)'.'); // dot-stuffing
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output.WriteByte(current);
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atLineStart = current == (byte)'\n';
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}
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if (output.Length == 0 || output.GetBuffer()[output.Length - 1] != (byte)'\n') output.Write(Crlf);
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output.Write([(byte)'.', 13, 10]);
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return output.ToArray();
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}
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/// <summary>接收侧还原：去掉终止行并做 dot-unstuffing（自检用于模拟收件端）。</summary>
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public static byte[] Decode(byte[] wire)
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{
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// 去掉结尾的 ".\r\n"
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var end = wire.Length;
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if (end >= 3 && wire[end - 3] == (byte)'.' && wire[end - 2] == 13 && wire[end - 1] == 10)
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end -= 3;
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else if (end >= 2 && wire[end - 2] == (byte)'.' && wire[end - 1] == 10)
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end -= 2;
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using var output = new MemoryStream(end);
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var atLineStart = true;
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for (var index = 0; index < end; index++)
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{
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var current = wire[index];
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if (atLineStart && current == (byte)'.' && index + 1 < end && wire[index + 1] == (byte)'.')
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{
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index++; // 去掉填充的点
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}
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output.WriteByte(wire[index]);
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atLineStart = wire[index] == (byte)'\n';
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}
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return output.ToArray();
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}
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private static bool EndsWithCrlf(byte[] bytes) =>
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bytes.Length >= 2 && bytes[^2] == 13 && bytes[^1] == 10;
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}
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