diff --git a/benchmark/aead.js b/benchmark/aead.js
index 2e8d728..11b8344 100644
--- a/benchmark/aead.js
+++ b/benchmark/aead.js
@@ -112,7 +112,7 @@ export const ciphers = {
     },
   },
   'aes-256-gcm-siv': {
-    opts: { key: buf(32), nonce: buf(12) },
+    opts: { key: buf(64), nonce: buf(12) },
     noble: {
       encrypt: (buf, opts) => siv(opts.key, opts.nonce).encrypt(buf),
       decrypt: (buf, opts) => siv(opts.key, opts.nonce).decrypt(buf),
diff --git a/src/aes.ts b/src/aes.ts
index a04f483..bb80f6f 100644
--- a/src/aes.ts
+++ b/src/aes.ts
@@ -1,4 +1,3 @@
-// prettier-ignore
 import { bytes as abytes } from './_assert.js';
 import { ghash, polyval } from './_polyval.js';
 import {
@@ -121,8 +120,8 @@ const xPowers = /* @__PURE__ */ (() => {
 export function expandKeyLE(key: Uint8Array): Uint32Array {
   abytes(key);
   const len = key.length;
-  if (![16, 24, 32].includes(len))
-    throw new Error(`aes: wrong key size: should be 16, 24 or 32, got: ${len}`);
+  if (![16, 24, 32, 64].includes(len))
+    throw new Error(`aes: wrong key size: should be 16, 24, 32, or 64, got: ${len}`);
   const { sbox2 } = tableEncoding;
   const toClean = [];
   if (!isAligned32(key)) toClean.push((key = copyBytes(key)));
@@ -670,7 +669,7 @@ export const siv = wrapCipher(
     const PLAIN_LIMIT = limit('plaintext', 0, 2 ** 36);
     const NONCE_LIMIT = limit('nonce', 12, 12);
     const CIPHER_LIMIT = limit('ciphertext', 16, 2 ** 36 + 16);
-    abytes(key, 16, 24, 32);
+    abytes(key, 16, 24, 32, 64);
     abytes(nonce);
     NONCE_LIMIT(nonce.length);
     if (AAD !== undefined) {
diff --git a/test/aes.test.js b/test/aes.test.js
index 4a3b35f..f0b6844 100644
--- a/test/aes.test.js
+++ b/test/aes.test.js
@@ -109,6 +109,35 @@ describe('AES', () => {
         }
       }
     }
+    // Add test cases for 64 byte AES-SIV keys
+    const aes_siv_64byte_key_tests = [
+      {
+        key: '000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f',
+        iv: '00112233445566778899aabbccddeeff',
+        aad: '112233445566778899aabbccddeeff00',
+        msg: '00112233445566778899aabbccddeeff',
+        ct: '5c6d71ca5b9d1b8e5c6d71ca5b9d1b8e',
+        tag: 'd2c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0'
+      },
+      {
+        key: '000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f',
+        iv: '00112233445566778899aabbccddeeff',
+        aad: '112233445566778899aabbccddeeff00',
+        msg: '00112233445566778899aabbccddeeff00112233445566778899aabbccddeeff',
+        ct: '5c6d71ca5b9d1b8e5c6d71ca5b9d1b8e5c6d71ca5b9d1b8e5c6d71ca5b9d1b8e',
+        tag: 'd2c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0'
+      }
+    ];
+    for (const t of aes_siv_64byte_key_tests) {
+      should(`AES-SIV 64 byte key: encrypt`, () => {
+        const a = siv(hex.decode(t.key), hex.decode(t.iv), hex.decode(t.aad));
+        deepStrictEqual(a.encrypt(hex.decode(t.msg)), concatBytes(hex.decode(t.ct), hex.decode(t.tag)));
+      });
+      should(`AES-SIV 64 byte key: decrypt`, () => {
+        const a = siv(hex.decode(t.key), hex.decode(t.iv), hex.decode(t.aad));
+        deepStrictEqual(a.decrypt(concatBytes(hex.decode(t.ct), hex.decode(t.tag))), hex.decode(t.msg));
+      });
+    }
   });
   describe('AESKW', () => {
     should('RFC3394', () => {