243 lines
11 KiB
C#
243 lines
11 KiB
C#
using System.Security.Cryptography;
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using System.Text;
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namespace WpywMail.Native;
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/// <summary>
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/// DKIM 签名（RFC 6376），算法 rsa-sha256，规范化 relaxed/relaxed。
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///
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/// 私钥不存在时会自动生成 2048 位 RSA 并保存为 PEM，同时把需要配置到 DNS 的
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/// TXT 记录打印到日志，方便直接复制到 Cloudflare。
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/// </summary>
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public sealed class DkimSigner
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{
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private const string Crlf = "\r\n";
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private readonly RSA key;
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private readonly string domain;
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private readonly string selector;
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private readonly string[] signHeaders;
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public string PrivateKeyPath { get; }
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/// <summary>SubjectPublicKeyInfo 形式的公钥（自检与 DNS 记录生成使用）。</summary>
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public byte[] PublicKeyBytes => key.ExportSubjectPublicKeyInfo();
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/// <summary>DKIM 公钥所在的 DNS 记录名（不含域后缀）。</summary>
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public string RecordName => $"{selector}._domainkey";
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/// <summary>DKIM 公钥记录值（TXT）。</summary>
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public string RecordValue => "v=DKIM1; k=rsa; p=" + Convert.ToBase64String(PublicKeyBytes);
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private DkimSigner(RSA key, string domain, string selector, string privateKeyPath, string[] headers)
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{
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this.key = key;
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this.domain = domain;
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this.selector = selector;
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PrivateKeyPath = privateKeyPath;
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signHeaders = headers;
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}
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/// <summary>按配置创建签名器；未启用或初始化失败时返回 null（调用方继续正常发信）。</summary>
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public static DkimSigner? Create(AppConfig config)
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{
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if (!config.Dkim.Enabled) return null;
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try
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{
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var domain = string.IsNullOrWhiteSpace(config.Dkim.SigningDomain) ? config.Domain : config.Dkim.SigningDomain;
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var selector = string.IsNullOrWhiteSpace(config.Dkim.Selector) ? "mail" : config.Dkim.Selector.Trim();
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var path = config.Dkim.PrivateKeyPath;
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if (string.IsNullOrWhiteSpace(path)) path = Path.Combine(config.DataDirectory, "dkim", selector + ".private.pem");
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Directory.CreateDirectory(Path.GetDirectoryName(path)!);
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RSA rsa;
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if (File.Exists(path))
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{
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rsa = RSA.Create();
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rsa.ImportFromPem(File.ReadAllText(path));
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AppLog.Info($"[DKIM] 已加载私钥：{path}（{rsa.KeySize} 位，选择器 {selector}，域 {domain}）");
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}
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else
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{
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rsa = RSA.Create(2048);
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File.WriteAllText(path, rsa.ExportPkcs8PrivateKeyPem(), new UTF8Encoding(false));
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AppLog.Info($"[DKIM] 已生成新的 2048 位私钥：{path}");
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}
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var signer = new DkimSigner(rsa, domain, selector, path, config.Dkim.Headers);
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signer.PrintDnsRecord();
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return signer;
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}
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catch (Exception ex)
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{
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AppLog.Error($"[DKIM] 初始化失败，将不签名继续发信：{ex.Message}");
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return null;
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}
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}
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/// <summary>把需要配置到 DNS 的公钥记录打印出来（分段给 TXT 用）。</summary>
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public void PrintDnsRecord()
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{
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var publicKey = Convert.ToBase64String(key.ExportSubjectPublicKeyInfo());
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var name = $"{selector}._domainkey.{domain}";
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var value = "v=DKIM1; k=rsa; p=" + publicKey;
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AppLog.Info($"[DKIM] 请在 DNS 添加 TXT 记录 —— 名称：{name}");
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AppLog.Info($"[DKIM] 值（TXT，可直接整条粘贴）：{value}");
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foreach (var chunk in Chunk(value, 255))
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{
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AppLog.Info($"[DKIM] （分段）\"{chunk}\"");
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}
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}
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public static IEnumerable<string> Chunk(string value, int size)
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{
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for (var index = 0; index < value.Length; index += size)
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{
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yield return value.Substring(index, Math.Min(size, value.Length - index));
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}
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}
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/// <summary>对整封邮件签名，返回带 DKIM-Signature 头的新报文。失败时原样返回。</summary>
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public byte[] Sign(byte[] message)
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{
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try
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{
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var (rawHeaders, body) = SplitRaw(message);
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if (rawHeaders.Count == 0) return message;
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if (!rawHeaders.Any(h => h.Name.Equals("From", StringComparison.OrdinalIgnoreCase)))
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{
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AppLog.Warn("[DKIM] 报文缺少 From 头，跳过签名。");
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return message;
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}
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var bodyHash = Convert.ToBase64String(SHA256.HashData(CanonicalizeBody(body)));
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// 按配置顺序挑出实际存在的头（每个名字只签第一次出现）
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var chosen = new List<(string Name, string Value)>();
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foreach (var name in signHeaders)
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{
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var hit = rawHeaders.FirstOrDefault(h => h.Name.Equals(name, StringComparison.OrdinalIgnoreCase));
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if (hit.Name is not null && !chosen.Any(c => c.Name.Equals(hit.Name, StringComparison.OrdinalIgnoreCase)))
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chosen.Add(hit);
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}
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if (!chosen.Any(c => c.Name.Equals("From", StringComparison.OrdinalIgnoreCase)))
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chosen.Insert(0, rawHeaders.First(h => h.Name.Equals("From", StringComparison.OrdinalIgnoreCase)));
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var headerList = string.Join(":", chosen.Select(c => c.Name.ToLowerInvariant()));
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var baseValue = $"v=1; a=rsa-sha256; c=relaxed/relaxed; d={domain}; s={selector}; " +
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$"t={DateTimeOffset.UtcNow.ToUnixTimeSeconds()}; h={headerList}; bh={bodyHash}; b=";
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// RFC 6376 §3.7「hash step 2」规定的顺序，必须严格照做：
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// 1. 先按 h= 标签里的顺序哈希各被签名头，**每个头后面跟一个 CRLF**；
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// 2. **最后**才哈希 DKIM-Signature 头本身（b= 视为空串），且它**结尾不带 CRLF**。
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// 把 DKIM-Signature 放在最前面、或给它补一个结尾 CRLF，都会让合规的验证器
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// （Gmail / Outlook / OpenDKIM / Mail::DKIM / dkimpy）算出不同的摘要 ——
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// 表现为对方直接判 dkim=fail，而自家自测却可能因为「签名端与验签端犯同一个错」
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// 而假通过。2026-09-13 的真实教训：port25 的验证器就是这样把这个 bug 揪出来的。
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var signingInput = new StringBuilder();
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foreach (var (name, value) in chosen)
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{
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signingInput.Append(CanonicalizeHeader(name, value)).Append(Crlf);
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}
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signingInput.Append(CanonicalizeHeader("DKIM-Signature", baseValue));
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// ⚠ 必须用 Latin1（字节保真）取输入字节，**不能用 ASCII**：
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// ASCII 编码会把非 ASCII 字符替换成 '?'，如果报文头是 8bit 裸 UTF-8（不是 RFC2047
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// 编码字），签名输入就与实际字节不一致 → 对方验签失败。
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// 我们自己发出的信因为头都经过 RFC2047 编码，所以一直是 ASCII（这条缺陷因此潜伏着），
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// 但验签端按真实字节算，一旦遇到 8bit 头就会对不上。自检的 DKIM 用例把它逼了出来。
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var signature = key.SignData(Encoding.Latin1.GetBytes(signingInput.ToString()),
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HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
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var signatureHeader = $"DKIM-Signature: {baseValue}{Convert.ToBase64String(signature)}{Crlf}";
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var output = new MemoryStream(message.Length + signatureHeader.Length + 64);
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output.Write(Encoding.ASCII.GetBytes(signatureHeader));
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output.Write(message);
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return output.ToArray();
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}
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catch (Exception ex)
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{
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AppLog.Error($"[DKIM] 签名失败，将发送未签名报文：{ex.Message}");
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return message;
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}
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}
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// ---------------------------------------------------------------- 规范化
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/// <summary>relaxed 头规范化：小写名、展开折行、多空白折成一个空格、去首尾空白。</summary>
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private static string CanonicalizeHeader(string name, string value)
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{
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var unfolded = value.Replace("\r\n", "").Replace("\n", "");
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var collapsed = CollapseWhitespace(unfolded).Trim();
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return name.Trim().ToLowerInvariant() + ":" + collapsed;
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}
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/// <summary>relaxed 正文规范化：多空白折成一个空格、去行尾空白、去掉末尾空行。</summary>
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private static byte[] CanonicalizeBody(byte[] body)
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{
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var text = Encoding.Latin1.GetString(body).Replace("\r\n", "\n").Replace('\r', '\n');
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var lines = text.Split('\n');
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var normalized = lines.Select(line => CollapseWhitespace(line).TrimEnd(' ', '\t'));
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var joined = string.Join(Crlf, normalized);
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joined = joined.TrimEnd('\r', '\n');
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if (joined.Length > 0) joined += Crlf;
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return Encoding.Latin1.GetBytes(joined);
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}
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private static string CollapseWhitespace(string value)
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{
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var builder = new StringBuilder(value.Length);
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var inWhitespace = false;
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foreach (var c in value)
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{
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if (c is ' ' or '\t')
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{
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if (!inWhitespace) builder.Append(' ');
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inWhitespace = true;
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}
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else
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{
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builder.Append(c);
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inWhitespace = false;
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}
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}
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return builder.ToString();
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}
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/// <summary>把报文拆成「原始头行（未展开）」与「正文字节」，保持字节忠实。</summary>
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private static (List<(string Name, string Value)> Headers, byte[] Body) SplitRaw(byte[] message)
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{
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var headers = new List<(string, string)>();
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var index = 0;
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string? currentName = null;
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var currentValue = new StringBuilder();
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while (index < message.Length)
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{
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var lineEnd = Array.IndexOf(message, (byte)'\n', index);
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if (lineEnd < 0) lineEnd = message.Length;
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var line = Encoding.Latin1.GetString(message, index, lineEnd - index).TrimEnd('\r');
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index = lineEnd + 1;
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if (line.Length == 0) break; // 头结束
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if ((line[0] == ' ' || line[0] == '\t') && currentName is not null)
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{
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currentValue.Append(Crlf).Append(line);
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continue;
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}
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if (currentName is not null) headers.Add((currentName, currentValue.ToString()));
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var colon = line.IndexOf(':');
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if (colon <= 0) { currentName = null; currentValue.Clear(); continue; }
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currentName = line[..colon].Trim();
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currentValue.Clear();
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currentValue.Append(line[(colon + 1)..]);
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}
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if (currentName is not null) headers.Add((currentName, currentValue.ToString()));
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var bodyStart = Math.Min(index, message.Length);
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return (headers, message[bodyStart..]);
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}
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}
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