feat: implement on-demand sharing key derivation

frontend/src/utils/crypto.test.ts

@@ -190,3 +190,44 @@ describe("Sharing Key Decryption", () => {
     expect(decryptedLetter).toBe(letterContent);
   });
 });
+
+describe("extractSharingKey", () => {
+  let masterKey: CryptoKey;
+
+  beforeEach(async () => {
+    utils = new CryptoUtils();
+    await utils.initialize();
+    const bundle = await CryptoUtils.deriveKeyBundle(
+      "password",
+      "test@test.com",
+    );
+    masterKey = bundle.masterKey;
+  });
+
+  it("should return the same key that encryptLetter embedded as sharingKey", async () => {
+    const encrypted = await utils.encryptLetter("any content", masterKey);
+
+    const extracted = await utils.extractSharingKey(
+      encrypted.encrypted_dek,
+      masterKey,
+    );
+
+    expect(extracted).toBe(encrypted.sharingKey);
+  });
+
+  it("extracted key should decrypt the ciphertext produced by encryptLetter", async () => {
+    const plaintext = "hello from the owner";
+    const encrypted = await utils.encryptLetter(plaintext, masterKey);
+
+    const extracted = await utils.extractSharingKey(
+      encrypted.encrypted_dek,
+      masterKey,
+    );
+    const decrypted = await utils.decryptLetterWithSharingKey(
+      encrypted.encrypted_content,
+      extracted,
+    );
+
+    expect(decrypted).toBe(plaintext);
+  });
+});

frontend/src/utils/crypto.ts

@@ -309,4 +309,24 @@ export class CryptoUtils {
     );
     return URL.createObjectURL(new Blob([bytes]));
   }
+
+  //  Re-derives the sharing key (raw DEK) on demand (browser only, not sent to server).
+  public async extractSharingKey(
+    encrypted_dek: string,
+    masterKey: CryptoKey,
+  ): Promise<string> {
+    const [dekIv, wrappedDek] = this.unpackWithIv(encrypted_dek);
+    const rawDek = await crypto.subtle.unwrapKey(
+      "raw",
+      wrappedDek,
+      masterKey,
+      { name: "AES-GCM", iv: dekIv },
+      CryptoUtils.AES_GCM,
+      true,
+      ["decrypt"],
+    );
+    return this.toBase64(
+      new Uint8Array(await crypto.subtle.exportKey("raw", rawDek)),
+    );
+  }
 }