diff --git a/.changeset/blue-emus-doubt.md b/.changeset/blue-emus-doubt.md
new file mode 100644
index 0000000..c862633
--- /dev/null
+++ b/.changeset/blue-emus-doubt.md
@@ -0,0 +1,5 @@
+---
+"@9c5s/node-tcnet": patch
+---
+
+fix: Windows BridgeのXTEA暗号文バイトリバースに対応
diff --git a/src/tcnet.ts b/src/tcnet.ts
index d5b40a5..790bd1d 100644
--- a/src/tcnet.ts
+++ b/src/tcnet.ts
@@ -1,5 +1,7 @@
 import { Socket, createSocket, RemoteInfo } from "dgram";
 import { EventEmitter } from "events";
+import { execFile } from "child_process";
+import { platform } from "os";
 import * as nw from "./network";
 import { MultiPacketAssembler } from "./multi-packet";
 import { interfaceAddress, listNetworkAdapters, findIPv4Address, type NetworkAdapterInfo } from "./utils";
@@ -91,6 +93,7 @@ export class TCNetClient extends EventEmitter {
     private _authState: AuthState = "none";
     private sessionToken: number | null = null;
     private authTimeoutId: NodeJS.Timeout | null = null;
+    private bridgeIsWindows: boolean | null = null;
 
     /**
      * TCNetClientを初期化する
@@ -470,6 +473,9 @@ export class TCNetClient extends EventEmitter {
                         }
                     } else if (this.connected) {
                         // 確定後: 従来通りserver更新
+                        if (this.server?.address !== rinfo.address) {
+                            this.bridgeIsWindows = null;
+                        }
                         this.server = rinfo;
                         this.server.port = packet.nodeListenerPort;
                     }
@@ -797,6 +803,67 @@ export class TCNetClient extends EventEmitter {
         return null;
     }
 
+    /**
+     * BridgeのOSがWindowsであるかをpingのTTL値から検出する
+     *
+     * Bridge IPが自分のIPと一致する場合はos.platform()で判定する。
+     * リモートの場合はpingを1回実行しTTL値をパースする (TTL > 64 → Windows)。
+     * WindowsのデフォルトTTLは128、macOS/LinuxのデフォルトTTLは64であるため、
+     * 同一LAN (0-1ホップ) ではこの閾値で正確に判定できる。
+     * 結果はインスタンス変数にキャッシュし、セッション中1回だけ検出する。
+     * @returns Windowsならtrue、それ以外ならfalse
+     */
+    private async detectBridgeIsWindows(): Promise<boolean> {
+        if (this.bridgeIsWindows !== null) return this.bridgeIsWindows;
+
+        const bridgeIp = this.server?.address;
+        if (!bridgeIp) {
+            // serverが未設定の場合はキャッシュせず、次回の呼び出しで再検出を許可する
+            return false;
+        }
+
+        // IPv4フォーマットバリデーション (execFileに渡す前の防御)
+        // 不正IPは確定的でないためキャッシュしない
+        if (!/^\d{1,3}(\.\d{1,3}){3}$/.test(bridgeIp)) {
+            return false;
+        }
+
+        const clientIp = this.getClientIp();
+        if (clientIp && bridgeIp === clientIp) {
+            this.bridgeIsWindows = platform() === "win32";
+            this.log?.debug(`Bridge OS detected (local): ${this.bridgeIsWindows ? "Windows" : "non-Windows"}`);
+            return this.bridgeIsWindows;
+        }
+
+        try {
+            const isWin = platform() === "win32";
+            const args = isWin ? ["-n", "1", "-w", "1000", bridgeIp] : ["-c", "1", "-W", "1", bridgeIp];
+            const output = await new Promise<string>((resolve, reject) => {
+                execFile("ping", args, { timeout: 3000 }, (err, stdout) => {
+                    if (err) reject(err);
+                    else resolve(stdout);
+                });
+            });
+            const match = output.match(/ttl[=:](\d+)/i);
+            if (match) {
+                const ttl = Number.parseInt(match[1], 10);
+                this.bridgeIsWindows = ttl > 64;
+                this.log?.debug(`Bridge OS detected (TTL=${ttl}): ${this.bridgeIsWindows ? "Windows" : "non-Windows"}`);
+            } else {
+                // TTL未検出は確定的でないためキャッシュせず、次回再検出を許可する
+                this.log?.debug("Bridge OS detection: TTL not found in ping output, assuming non-Windows");
+                return false;
+            }
+        } catch (err) {
+            // ping失敗は確定的でないためキャッシュせず、次回再検出を許可する
+            const msg = err instanceof Error ? err.message : String(err);
+            this.log?.debug(`Bridge OS detection: ping failed (${msg}), assuming non-Windows`);
+            return false;
+        }
+
+        return this.bridgeIsWindows;
+    }
+
     /**
      * AppDataパケットを生成する
      * @param cmd - コマンド番号
@@ -831,6 +898,7 @@ export class TCNetClient extends EventEmitter {
     private resetAuthSession(): void {
         this._authState = "none";
         this.sessionToken = null;
+        this.bridgeIsWindows = null;
         if (this.authTimeoutId) {
             clearTimeout(this.authTimeoutId);
             this.authTimeoutId = null;
@@ -874,6 +942,19 @@ export class TCNetClient extends EventEmitter {
         }
 
         const ciphertext = Buffer.from(ct, "hex");
+        const tokenBeforePing = this.sessionToken;
+        // Windows BridgeはXTEA暗号文をバイトリバースして読み取るため、事前にリバースして送信する
+        // 初回呼び出し時のみpingが発生し最大3秒かかる (AUTH_RESPONSE_TIMEOUT=5秒内に収まる想定)
+        if (await this.detectBridgeIsWindows()) {
+            ciphertext.reverse();
+        }
+
+        // detectBridgeIsWindowsのawait中に状態が変わった場合のガード
+        if (!this.server || !this.broadcastSocket || this.sessionToken !== tokenBeforePing) {
+            this.resetAuthSession();
+            return;
+        }
+
         const payload = generateAuthPayload(this.sessionToken, clientIp, ciphertext);
         const auth = this.createAppDataPacket(2, this.sessionToken, payload);
         await this.sendPacket(auth, this.broadcastSocket, TCNET_BROADCAST_PORT, this.config.broadcastAddress);
diff --git a/tests/auth.test.ts b/tests/auth.test.ts
index ad0721b..12056c2 100644
--- a/tests/auth.test.ts
+++ b/tests/auth.test.ts
@@ -460,6 +460,18 @@ class AuthSequenceTestClient extends TCNetClient {
     public setSelectedAdapter(adapter: any): void {
         (this as any)._selectedAdapter = adapter;
     }
+    public callDetectBridgeIsWindows(): Promise<boolean> {
+        return (this as any).detectBridgeIsWindows();
+    }
+    public getBridgeIsWindows(): boolean | null {
+        return (this as any).bridgeIsWindows;
+    }
+    public setBridgeIsWindows(value: boolean | null): void {
+        (this as any).bridgeIsWindows = value;
+    }
+    public setServer(server: any): void {
+        (this as any).server = server;
+    }
 }
 
 describe("sendAuthSequence 状態リセット", () => {
@@ -703,3 +715,92 @@ describe("TCNetApplicationDataPacket.write() payload長検証", () => {
         expect(() => packet.write()).toThrow("ApplicationData payload must be 12 bytes");
     });
 });
+
+describe("sendAuthSequence XTEA暗号文バイトリバース", () => {
+    /**
+     * テスト用アダプタ情報を生成する
+     * @param ip - IPv4アドレス文字列
+     */
+    function createAdapter(ip: string) {
+        return {
+            name: "test0",
+            addresses: [
+                {
+                    address: ip,
+                    netmask: "255.255.255.0",
+                    family: "IPv4" as const,
+                    mac: "00:00:00:00:00:00",
+                    internal: false,
+                    cidr: `${ip}/24`,
+                },
+            ],
+        };
+    }
+
+    it("Windows Bridge判定時に暗号文がバイトリバースされて送信される", async () => {
+        const client = new AuthSequenceTestClient("8ee0dc051b1ddf8b");
+        client.setSessionToken(0xdeec6dfc);
+        client.setAuthState("pending");
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        const sentBuffers: Buffer[] = [];
+        client.setBroadcastSocket({
+            send: vi.fn((buf: Buffer, _port: number, _addr: string, cb: (err: Error | null) => void) => {
+                sentBuffers.push(Buffer.from(buf));
+                cb(null);
+            }),
+        });
+        (client as any).config.broadcastAddress = "255.255.255.255";
+
+        // BridgeをWindowsとして事前設定する
+        client.setBridgeIsWindows(true);
+
+        await (client as any).sendAuthSequence();
+
+        // 2番目のパケット (cmd=2) のpayloadを検査する
+        expect(sentBuffers.length).toBe(2);
+        const authBuf = sentBuffers[1];
+        // payloadはoffset 50から12バイト
+        const payload = authBuf.subarray(50, 62);
+        // auth[4:12]がリバースされた暗号文であることを確認
+        const reversedCiphertext = Buffer.from("8ee0dc051b1ddf8b", "hex").reverse();
+        expect(payload.subarray(4, 12)).toEqual(reversedCiphertext);
+    });
+
+    it("non-Windows Bridge判定時に暗号文がそのまま送信される", async () => {
+        const client = new AuthSequenceTestClient("8ee0dc051b1ddf8b");
+        client.setSessionToken(0xdeec6dfc);
+        client.setAuthState("pending");
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        const sentBuffers: Buffer[] = [];
+        client.setBroadcastSocket({
+            send: vi.fn((buf: Buffer, _port: number, _addr: string, cb: (err: Error | null) => void) => {
+                sentBuffers.push(Buffer.from(buf));
+                cb(null);
+            }),
+        });
+        (client as any).config.broadcastAddress = "255.255.255.255";
+
+        // Bridgeをnon-Windowsとして事前設定する
+        client.setBridgeIsWindows(false);
+
+        await (client as any).sendAuthSequence();
+
+        expect(sentBuffers.length).toBe(2);
+        const authBuf = sentBuffers[1];
+        const payload = authBuf.subarray(50, 62);
+        const originalCiphertext = Buffer.from("8ee0dc051b1ddf8b", "hex");
+        expect(payload.subarray(4, 12)).toEqual(originalCiphertext);
+    });
+
+    it("resetAuthSessionでbridgeIsWindowsキャッシュがクリアされる", () => {
+        const client = new AuthSequenceTestClient();
+        client.setBridgeIsWindows(true);
+        client.setAuthState("pending");
+
+        (client as any).resetAuthSession();
+
+        expect(client.getBridgeIsWindows()).toBeNull();
+    });
+});
diff --git a/tests/bridge-os-detection.test.ts b/tests/bridge-os-detection.test.ts
new file mode 100644
index 0000000..7aeca97
--- /dev/null
+++ b/tests/bridge-os-detection.test.ts
@@ -0,0 +1,232 @@
+import { describe, it, expect, vi, beforeEach, type Mock } from "vitest";
+import { isolateXteaEnv } from "./helpers";
+
+// os.platformとchild_process.execFileをモックする
+vi.mock("os", async (importOriginal) => {
+    const actual = await importOriginal<typeof import("os")>();
+    return { ...actual, platform: vi.fn(() => "win32") };
+});
+
+vi.mock("child_process", async (importOriginal) => {
+    const actual = await importOriginal<typeof import("child_process")>();
+    return { ...actual, execFile: vi.fn() };
+});
+
+import { platform } from "os";
+import { execFile } from "child_process";
+import { TCNetClient } from "../src/tcnet";
+
+const platformMock = platform as unknown as Mock;
+const execFileMock = execFile as unknown as Mock;
+
+isolateXteaEnv();
+
+const MASTER_RINFO = { address: "192.168.0.100", port: 65207, family: "IPv4", size: 0 };
+
+class BridgeOsTestClient extends TCNetClient {
+    constructor() {
+        super();
+        (this as any).config.xteaCiphertext = "8ee0dc051b1ddf8b";
+        (this as any).server = { address: MASTER_RINFO.address, port: MASTER_RINFO.port };
+    }
+    public callDetectBridgeIsWindows(): Promise<boolean> {
+        return (this as any).detectBridgeIsWindows();
+    }
+    public getBridgeIsWindows(): boolean | null {
+        return (this as any).bridgeIsWindows;
+    }
+    public setBridgeIsWindows(value: boolean | null): void {
+        (this as any).bridgeIsWindows = value;
+    }
+    public setServer(server: any): void {
+        (this as any).server = server;
+    }
+    public setSelectedAdapter(adapter: any): void {
+        (this as any)._selectedAdapter = adapter;
+    }
+}
+
+/**
+ * テスト用アダプタ情報を生成する
+ * @param ip - IPv4アドレス文字列
+ */
+function createAdapter(ip: string) {
+    return {
+        name: "test0",
+        addresses: [
+            {
+                address: ip,
+                netmask: "255.255.255.0",
+                family: "IPv4" as const,
+                mac: "00:00:00:00:00:00",
+                internal: false,
+                cidr: `${ip}/24`,
+            },
+        ],
+    };
+}
+
+/**
+ * execFileモックのコールバックを成功として呼び出すヘルパー
+ * @param stdout - 標準出力文字列
+ */
+function mockExecFileSuccess(stdout: string): void {
+    execFileMock.mockImplementation(
+        (_cmd: string, _args: string[], _opts: object, cb: (err: Error | null, stdout: string) => void) => {
+            cb(null, stdout);
+        },
+    );
+}
+
+/**
+ * execFileモックのコールバックをエラーとして呼び出すヘルパー
+ * @param error - エラーオブジェクト
+ */
+function mockExecFileError(error: Error): void {
+    execFileMock.mockImplementation(
+        (_cmd: string, _args: string[], _opts: object, cb: (err: Error | null, stdout: string) => void) => {
+            cb(error, "");
+        },
+    );
+}
+
+describe("detectBridgeIsWindows", () => {
+    beforeEach(() => {
+        vi.clearAllMocks();
+    });
+
+    it("Bridge IPがクライアントIPと一致しos.platform()がwin32ならtrueを返す", async () => {
+        platformMock.mockReturnValue("win32");
+        const client = new BridgeOsTestClient();
+        client.setServer({ address: "192.168.0.10", port: 65207 });
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        expect(await client.callDetectBridgeIsWindows()).toBe(true);
+        expect(client.getBridgeIsWindows()).toBe(true);
+    });
+
+    it("Bridge IPがクライアントIPと一致しos.platform()がdarwinならfalseを返す", async () => {
+        platformMock.mockReturnValue("darwin");
+        const client = new BridgeOsTestClient();
+        client.setServer({ address: "192.168.0.10", port: 65207 });
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        expect(await client.callDetectBridgeIsWindows()).toBe(false);
+        expect(client.getBridgeIsWindows()).toBe(false);
+    });
+
+    it("リモートBridgeでTTL=128ならWindowsと判定する", async () => {
+        platformMock.mockReturnValue("win32");
+        mockExecFileSuccess("Reply from 192.168.0.100: bytes=32 time<1ms TTL=128\n");
+        const client = new BridgeOsTestClient();
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        expect(await client.callDetectBridgeIsWindows()).toBe(true);
+    });
+
+    it("リモートBridgeでTTL=64ならnon-Windowsと判定する", async () => {
+        platformMock.mockReturnValue("darwin");
+        mockExecFileSuccess("64 bytes from 192.168.0.100: icmp_seq=1 ttl=64 time=0.5 ms\n");
+        const client = new BridgeOsTestClient();
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        expect(await client.callDetectBridgeIsWindows()).toBe(false);
+    });
+
+    it("リモートBridgeでTTL=65ならWindowsと判定する (境界値)", async () => {
+        platformMock.mockReturnValue("win32");
+        mockExecFileSuccess("Reply from 192.168.0.100: bytes=32 time<1ms TTL=65\n");
+        const client = new BridgeOsTestClient();
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        expect(await client.callDetectBridgeIsWindows()).toBe(true);
+    });
+
+    it("ping失敗時はfalseを返しキャッシュしない", async () => {
+        platformMock.mockReturnValue("win32");
+        mockExecFileError(new Error("ping failed"));
+        const client = new BridgeOsTestClient();
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        expect(await client.callDetectBridgeIsWindows()).toBe(false);
+        expect(client.getBridgeIsWindows()).toBeNull();
+    });
+
+    it("ping出力にTTLが含まれない場合はfalseを返しキャッシュしない", async () => {
+        platformMock.mockReturnValue("win32");
+        mockExecFileSuccess("Request timed out.\n");
+        const client = new BridgeOsTestClient();
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        expect(await client.callDetectBridgeIsWindows()).toBe(false);
+        expect(client.getBridgeIsWindows()).toBeNull();
+    });
+
+    it("結果がキャッシュされ2回目以降はpingを実行しない", async () => {
+        platformMock.mockReturnValue("win32");
+        mockExecFileSuccess("Reply from 192.168.0.100: bytes=32 time<1ms TTL=128\n");
+        const client = new BridgeOsTestClient();
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        await client.callDetectBridgeIsWindows();
+        await client.callDetectBridgeIsWindows();
+
+        expect(execFileMock).toHaveBeenCalledTimes(1);
+    });
+
+    it("serverがnullの場合はfalseを返しキャッシュしない", async () => {
+        const client = new BridgeOsTestClient();
+        client.setServer(null);
+
+        expect(await client.callDetectBridgeIsWindows()).toBe(false);
+        // キャッシュされていないことを確認 (nullのまま)
+        expect(client.getBridgeIsWindows()).toBeNull();
+
+        // server設定後に再検出が行われることを確認
+        platformMock.mockReturnValue("win32");
+        client.setServer({ address: "192.168.0.10", port: 65207 });
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+        expect(await client.callDetectBridgeIsWindows()).toBe(true);
+    });
+
+    it("不正なIPアドレス形式の場合はfalseを返しキャッシュしない", async () => {
+        const client = new BridgeOsTestClient();
+        client.setServer({ address: "invalid-ip", port: 65207 });
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        expect(await client.callDetectBridgeIsWindows()).toBe(false);
+        expect(client.getBridgeIsWindows()).toBeNull();
+    });
+
+    it("Windowsではping -n 1 -w 1000引数を使用する", async () => {
+        platformMock.mockReturnValue("win32");
+        mockExecFileSuccess("Reply from 192.168.0.100: bytes=32 time<1ms TTL=128\n");
+        const client = new BridgeOsTestClient();
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        await client.callDetectBridgeIsWindows();
+
+        expect(execFileMock).toHaveBeenCalledWith(
+            "ping",
+            ["-n", "1", "-w", "1000", "192.168.0.100"],
+            expect.objectContaining({ timeout: 3000 }),
+            expect.any(Function),
+        );
+    });
+
+    it("macOS/Linuxではping -c 1 -W 1引数を使用する", async () => {
+        platformMock.mockReturnValue("darwin");
+        mockExecFileSuccess("64 bytes from 192.168.0.100: icmp_seq=1 ttl=64 time=0.5 ms\n");
+        const client = new BridgeOsTestClient();
+        client.setSelectedAdapter(createAdapter("192.168.0.10"));
+
+        await client.callDetectBridgeIsWindows();
+
+        expect(execFileMock).toHaveBeenCalledWith(
+            "ping",
+            ["-c", "1", "-W", "1", "192.168.0.100"],
+            expect.objectContaining({ timeout: 3000 }),
+            expect.any(Function),
+        );
+    });
+});