Send hotlink access token hash on upload; keep secret client-side only.

frontend/src/types/upload/CreateHotlinkUploadRequest.ts

@@ -4,4 +4,5 @@ export type CreateHotlinkUploadRequest = UploadOptionsRequest & {
     iv: string;
     ciphertext: string;
     mimeType: string;
+    accessTokenHash: string;
 };

frontend/src/utils/crypto/hashHotlinkAccessToken.spec.ts

@@ -0,0 +1,50 @@
+import { createHash } from 'node:crypto';
+import { describe, expect, it } from 'vitest';
+import { hashHotlinkAccessToken } from '@/utils/crypto/hashHotlinkAccessToken';
+
+const hashAccessTokenLikeBackend = (accessToken: string): string =>
+    createHash('sha256').update(accessToken, 'utf8').digest('hex');
+
+const hotlinkAccessTokenHashVectors = [
+    {
+        accessToken: 'test-hotlink-access-token_base64url',
+        accessTokenHash: '8defaf078afdd0ddabb77c73af4aa76ac78be24b6a6deffba2c6cfde43e20187'
+    },
+    {
+        accessToken: 'ZWc6ttO-r2qssA_9gdWe6u14X2W0_j7xhRBSVQvfMKI',
+        accessTokenHash: 'cdb772980ea3e958cf74b62b70a2f310eafbb9bf3d2302356b602b4830e89bc6'
+    }
+] as const;
+
+describe('hashHotlinkAccessToken', () => {
+    it.each(hotlinkAccessTokenHashVectors)(
+        'matches fixed SHA-256 hex for $accessToken',
+        async ({ accessToken, accessTokenHash }) => {
+            await expect(hashHotlinkAccessToken(accessToken)).resolves.toBe(accessTokenHash);
+        }
+    );
+
+    it('matches backend hashHotlinkAccessToken (Node crypto) for arbitrary tokens', async () => {
+        const accessToken = 'another_base64url-style_secret-ABC123';
+
+        await expect(hashHotlinkAccessToken(accessToken)).resolves.toBe(hashAccessTokenLikeBackend(accessToken));
+    });
+
+    it('returns 64 lowercase hex characters (SHA-256 digest)', async () => {
+        const hash = await hashHotlinkAccessToken(hotlinkAccessTokenHashVectors[0].accessToken);
+
+        expect(hash).toMatch(/^[a-f0-9]{64}$/);
+    });
+
+    it('changes when the access token string changes', async () => {
+        const [first, second] = hotlinkAccessTokenHashVectors;
+
+        expect(first.accessTokenHash).not.toBe(second.accessTokenHash);
+    });
+
+    it('hashes the UTF-8 string, not base64url-decoded bytes', async () => {
+        const accessToken = 'AQ';
+
+        await expect(hashHotlinkAccessToken(accessToken)).resolves.toBe(hashAccessTokenLikeBackend(accessToken));
+    });
+});

frontend/src/utils/crypto/hashHotlinkAccessToken.ts

@@ -0,0 +1,9 @@
+export const hashHotlinkAccessToken = async (accessToken: string): Promise<string> => {
+    const encoder = new TextEncoder();
+
+    const data = encoder.encode(accessToken);
+
+    const digest = await crypto.subtle.digest('SHA-256', data);
+
+    return [...new Uint8Array(digest)].map(byte => byte.toString(16).padStart(2, '0')).join('');
+};

frontend/src/utils/upload/prepareHotlinkUploadRequest.spec.ts

@@ -4,6 +4,7 @@ import { decryptAesGcm } from '@/utils/crypto/decryptAesGcm';
 import { importAesGcmKey } from '@/utils/crypto/importAesGcmKey';
 import { base64ToBytes } from '@/utils/encoding/base64ToBytes';
 import { base64UrlToBytes } from '@/utils/encoding/base64UrlToBytes';
+import { hashHotlinkAccessToken } from '@/utils/crypto/hashHotlinkAccessToken';
 import { buildHotlinkImagePath } from '@/utils/share/buildHotlinkImagePath';
 import { prepareHotlinkUploadRequest } from '@/utils/upload/prepareHotlinkUploadRequest';
 
@@ -22,10 +23,30 @@ describe('prepareHotlinkUploadRequest', () => {
             ...options,
             mimeType: 'image/png',
             iv: request.iv,
-            ciphertext: request.ciphertext
+            ciphertext: request.ciphertext,
+            accessTokenHash: request.accessTokenHash
         });
+        expect(request.accessTokenHash).toMatch(/^[a-f0-9]{64}$/);
         expect(request).not.toHaveProperty('secret');
-        expect(Object.keys(request).sort()).toEqual(['ciphertext', 'expiresAt', 'iv', 'maxViews', 'mimeType']);
+        expect(Object.keys(request).sort()).toEqual([
+            'accessTokenHash',
+            'ciphertext',
+            'expiresAt',
+            'iv',
+            'maxViews',
+            'mimeType'
+        ]);
+    });
+
+    it('includes accessTokenHash derived from secret without sending secret in the request', async () => {
+        const { request, secret } = await prepareHotlinkUploadRequest(
+            new Uint8Array([9]),
+            'image/png',
+            options
+        );
+
+        await expect(hashHotlinkAccessToken(secret)).resolves.toBe(request.accessTokenHash);
+        expect(JSON.stringify(request)).not.toContain(secret);
     });
 
     it('roundtrips plaintext using request ciphertext and path secret', async () => {

frontend/src/utils/upload/prepareHotlinkUploadRequest.ts

@@ -1,5 +1,6 @@
 import type { PreparedHotlinkUploadRequest } from '@/types/upload/PreparedHotlinkUploadRequest';
 import type { UploadOptionsRequest } from '@/types/upload/UploadOptionsRequest';
+import { hashHotlinkAccessToken } from '@/utils/crypto/hashHotlinkAccessToken';
 import { bytesToBase64 } from '@/utils/encoding/bytesToBase64';
 import { encryptHotlinkUpload } from '@/utils/upload/encryptHotlinkUpload';
 
@@ -9,6 +10,7 @@ export const prepareHotlinkUploadRequest = async (
     options: UploadOptionsRequest
 ): Promise<PreparedHotlinkUploadRequest> => {
     const encrypted = await encryptHotlinkUpload(plaintext);
+    const accessTokenHash = await hashHotlinkAccessToken(encrypted.secret);
 
     return {
         secret: encrypted.secret,
@@ -16,7 +18,8 @@ export const prepareHotlinkUploadRequest = async (
             ...options,
             iv: bytesToBase64(encrypted.iv),
             ciphertext: bytesToBase64(encrypted.ciphertext),
-            mimeType
+            mimeType,
+            accessTokenHash
         }
     };
 };