1 files changed, 71 insertions, 0 deletions
diff --git a/src/passkey.js b/src/passkey.js
new file mode 100644
index 0000000..4bbd01d
--- /dev/null
+++ b/src/passkey.js
@@ -0,0 +1,71 @@
+// Passkey sign-in: WebAuthn's PRF extension turns a passkey into a
+// deterministic 32-byte secret, which hashes into a nostr key — and the
+// login flow then treats it exactly like a pasted nsec, so the wallet seed
+// derives from it the usual way. No server anywhere: the wallet follows the
+// passkey wherever the platform syncs it (iCloud Keychain, Google Password
+// Manager), and the same passkey always opens the same wallet because the
+// PRF salt is a fixed app-scoped constant.
+import { sha256 } from '@noble/hashes/sha256';
+import { secp256k1 } from '@noble/curves/secp256k1';
+
+const PRF_SALT = sha256(new TextEncoder().encode('coinos-passkey-v1'));
+const DERIVE_TAG = new TextEncoder().encode('coinos-passkey-key:v1');
+
+export const passkeySupported = () =>
+  typeof window !== 'undefined' && !!window.PublicKeyCredential && !!navigator.credentials;
+
+// Hash the PRF output into a valid secp256k1 scalar. An out-of-range hash is
+// a ~2^-128 event, but the loop makes the derivation total rather than lucky.
+function skFromPrf(prf) {
+  let buf = new Uint8Array(DERIVE_TAG.length + prf.length);
+  buf.set(DERIVE_TAG, 0);
+  buf.set(prf, DERIVE_TAG.length);
+  let sk = sha256(buf);
+  while (!secp256k1.utils.isValidPrivateKey(sk)) sk = sha256(sk);
+  return sk;
+}
+
+const NO_PRF = 'this device’s passkeys can’t derive a wallet key (no PRF support)';
+
+async function assertionPrf(allowCredentials) {
+  const cred = await navigator.credentials.get({
+    publicKey: {
+      challenge: crypto.getRandomValues(new Uint8Array(32)),
+      userVerification: 'required',
+      ...(allowCredentials ? { allowCredentials } : {}),
+      extensions: { prf: { eval: { first: PRF_SALT } } },
+    },
+  });
+  const out = cred && cred.getClientExtensionResults().prf?.results?.first;
+  if (!out) throw new Error(NO_PRF);
+  return new Uint8Array(out);
+}
+
+// Sign in with an existing (discoverable) passkey.
+export async function passkeySignIn() {
+  return skFromPrf(await assertionPrf(null));
+}
+
+// Create a passkey, then derive the key from it. Some platforms evaluate PRF
+// during creation; the rest need a follow-up assertion against the new
+// credential — both paths end at the same bytes.
+export async function passkeyCreate() {
+  const cred = await navigator.credentials.create({
+    publicKey: {
+      challenge: crypto.getRandomValues(new Uint8Array(32)),
+      rp: { name: 'Coinos', id: location.hostname },
+      user: {
+        id: crypto.getRandomValues(new Uint8Array(16)),
+        name: 'coinos', displayName: 'Coinos wallet',
+      },
+      pubKeyCredParams: [{ type: 'public-key', alg: -7 }, { type: 'public-key', alg: -257 }],
+      authenticatorSelection: { residentKey: 'required', userVerification: 'required' },
+      extensions: { prf: { eval: { first: PRF_SALT } } },
+    },
+  });
+  const ext = cred.getClientExtensionResults();
+  const direct = ext.prf?.results?.first;
+  if (direct) return skFromPrf(new Uint8Array(direct));
+  if (ext.prf && ext.prf.enabled === false) throw new Error(NO_PRF);
+  return skFromPrf(await assertionPrf([{ type: 'public-key', id: cred.rawId }]));
+}