feat: support FIDO2 security keys through ssh agent

scripts/patch-ssh2-sk.cjs

@@ -0,0 +1,149 @@
+const fs = require("node:fs");
+
+const SK_ED = "sk-ssh-ed25519@openssh.com";
+const SK_EC = "sk-ecdsa-sha2-nistp256@openssh.com";
+
+function replaceOnce(source, from, to, label) {
+  const first = source.indexOf(from);
+  if (first === -1 || source.indexOf(from, first + 1) !== -1) {
+    throw new Error(`ssh2 changed (${label}); review the security key patch`);
+  }
+  return source.replace(from, () => to);
+}
+
+function patchKeyParserSource(source) {
+  if (source.includes("// termix-sk")) return source;
+  let out = replaceOnce(
+    source,
+    `    case 'ecdsa-sha2-nistp521':
+      return true;
+    case 'ssh-ed25519':
+      if (eddsaSupported)
+        return true;`,
+    `    case 'ecdsa-sha2-nistp521':
+      return true;
+    case '${SK_EC}': // termix-sk
+      return true;
+    case '${SK_ED}':
+    case 'ssh-ed25519':
+      if (eddsaSupported)
+        return true;`,
+    "key types",
+  );
+  out = replaceOnce(
+    out,
+    `    default:
+      return new Error(\`Unsupported OpenSSH public key type: \${baseType}\`);
+  }
+
+  return new OpenSSH_Public(`,
+    `    case '${SK_ED}':
+    case '${SK_EC}': {
+      // Public blob only; the security key signs, so there is no private half.
+      const rest = data.subarray(data._pos || 0);
+      const typeLen = Buffer.byteLength(baseType);
+      pubSSH = Buffer.allocUnsafe(4 + typeLen + rest.length);
+      writeUInt32BE(pubSSH, typeLen, 0);
+      pubSSH.utf8Write(baseType, 4, typeLen);
+      pubSSH.set(rest, 4 + typeLen);
+      algo = baseType === '${SK_ED}' ? null : 'sha256';
+      break;
+    }
+    default:
+      return new Error(\`Unsupported OpenSSH public key type: \${baseType}\`);
+  }
+
+  return new OpenSSH_Public(`,
+    "parseDER",
+  );
+  return out;
+}
+
+function patchAgentSource(source) {
+  if (source.includes("skTrailer")) return source;
+  return replaceOnce(
+    source,
+    `              binaryParser.init(signature, 0);
+              binaryParser.readString(true);
+              signature = binaryParser.readString();
+              binaryParser.clear();`,
+    `              binaryParser.init(signature, 0);
+              binaryParser.readString(true);
+              signature = binaryParser.readString();
+              // Security key signatures end with flags and a counter.
+              const skTrailer = signature === undefined
+                ? undefined
+                : binaryParser.readRaw();
+              binaryParser.clear();
+              if (skTrailer && skTrailer.length)
+                signature.skTrailer = Buffer.from(skTrailer);`,
+    "agent sign response",
+  );
+}
+
+function patchProtocolSource(source) {
+  if (source.includes("skTrailer")) return source;
+  const start = source.indexOf("  authPK(username, pubKey, keyAlgo, cbSign) {");
+  const end = source.indexOf("  authHostbased(", start);
+  if (start === -1 || end === -1) {
+    throw new Error("ssh2 changed (authPK); review the security key patch");
+  }
+  let region = source.slice(start, end);
+  region = replaceOnce(
+    region,
+    `      signature = convertSignature(signature, keyType);`,
+    `      const skTrailer = signature && signature.skTrailer;
+      signature = convertSignature(signature, keyType);`,
+    "authPK sign",
+  );
+  region = replaceOnce(
+    region,
+    `          + 4 + algoLen + 4 + sigLen
+      );`,
+    `          + 4 + algoLen + 4 + sigLen + (skTrailer ? skTrailer.length : 0)
+      );`,
+    "authPK alloc",
+  );
+  region = replaceOnce(
+    region,
+    `      writeUInt32BE(packet, 4 + algoLen + 4 + sigLen, p += pubKeyLen);`,
+    `      writeUInt32BE(
+        packet,
+        4 + algoLen + 4 + sigLen + (skTrailer ? skTrailer.length : 0),
+        p += pubKeyLen
+      );`,
+    "authPK length",
+  );
+  region = replaceOnce(
+    region,
+    `      packet.set(signature, p += 4);`,
+    `      packet.set(signature, p += 4);
+      if (skTrailer)
+        packet.set(skTrailer, p + sigLen);`,
+    "authPK copy",
+  );
+  return source.slice(0, start) + region + source.slice(end);
+}
+
+if (require.main === module) {
+  const targets = [
+    ["ssh2/lib/protocol/keyParser.js", patchKeyParserSource],
+    ["ssh2/lib/agent.js", patchAgentSource],
+    ["ssh2/lib/protocol/Protocol.js", patchProtocolSource],
+  ];
+  for (const [id, patch] of targets) {
+    const target = require.resolve(id);
+    const source = fs.readFileSync(target, "utf8");
+    const result = patch(source);
+    if (result !== source) fs.writeFileSync(target, result);
+  }
+  console.log(
+    "[patch-ssh2-sk] Security key public keys and signatures enabled",
+  );
+}
+
+module.exports = {
+  patchKeyParserSource,
+  patchAgentSource,
+  patchProtocolSource,
+};