fix(auth): bound local password encoding

backend/src/auth/password.ts

@@ -59,8 +59,9 @@ function passwordBytes(password: string): Buffer | undefined {
   } else if (/[\uD800-\uDFFF]/.test(password)) {
     return undefined;
   }
-  const bytes = Buffer.from(password, "utf8");
-  return bytes.length >= MINIMUM_PASSWORD_BYTES && bytes.length <= MAXIMUM_PASSWORD_BYTES ? bytes : undefined;
+  const byteLength = Buffer.byteLength(password, "utf8");
+  if (byteLength < MINIMUM_PASSWORD_BYTES || byteLength > MAXIMUM_PASSWORD_BYTES) return undefined;
+  return Buffer.from(password, "utf8");
 }
 
 export function isValidPasswordHash(encoded: string): boolean {

backend/test/auth-password.test.ts

@@ -44,6 +44,28 @@ describe("local Argon2id password verification", () => {
     expect(verifyPassword(loneSurrogatePassword, replacementPasswordHash)).toBe(false);
   });
 
+  test("accepts a multibyte password at exactly the 1024-byte boundary", () => {
+    const password = "é".repeat(512);
+    const passwordHash = "$argon2id$v=19$m=65536,t=3,p=1$AAECAwQFBgcICQoLDA0ODw$LfO3M3JBKeXf1knenCjQ6m9z4B7nedb0As2uc522I1I";
+
+    expect(Buffer.byteLength(password, "utf8")).toBe(1024);
+    expect(verifyPassword(password, passwordHash)).toBe(true);
+    expect(verifyPassword(`${password}é`, passwordHash)).toBe(false);
+  });
+
+  test("rejects an over-limit password before converting it with Buffer.from", () => {
+    const password = "é".repeat(513);
+    const fromSpy = vi.spyOn(Buffer, "from");
+
+    try {
+      expect(Buffer.byteLength(password, "utf8")).toBe(1026);
+      expect(verifyPassword(password, vectors[0].phc)).toBe(false);
+      expect(fromSpy.mock.calls.some(([value, encoding]) => value === password && encoding === "utf8")).toBe(false);
+    } finally {
+      fromSpy.mockRestore();
+    }
+  });
+
   test("rejects malformed and oversized PHC parameters before Argon2 allocation", () => {
     const password = vectors[0].password;
     const digest = vectors[0].phc.split("$")[5];