Skey: a FIDO2 security key on the Raspberry Pi Pico

Firmware for the Pico / Pico 2 that acts as a USB FIDO2 authenticator:
CTAP2 and U2F, passkeys, BOOTSEL as the touch button, and clientPIN with
PIN/UV auth protocols 1 and 2. A host simulator runs the same core so the
python-fido2 test suite exercises it without hardware.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/gen_attestation.py

@@ -0,0 +1,103 @@
+#!/usr/bin/env python3
+"""Creates the batch attestation key and certificate and emits them as C source.
+
+The key and certificate are kept in --key-dir and reused, so rebuilding keeps
+the same attestation identity. Delete that directory to start over.
+"""
+
+import argparse
+import datetime
+import pathlib
+import uuid
+
+from cryptography import x509
+from cryptography.hazmat.primitives import hashes, serialization
+from cryptography.hazmat.primitives.asymmetric import ec
+from cryptography.x509.oid import NameOID
+
+AAGUID = uuid.UUID("225e7977-54f1-4d15-8e35-295b11658d5c")
+OID_FIDO_AAGUID = x509.ObjectIdentifier("1.3.6.1.4.1.45724.1.1.4")
+
+
+def create(key_path: pathlib.Path, cert_path: pathlib.Path) -> None:
+    key = ec.generate_private_key(ec.SECP256R1())
+    # Subject fields required for packed attestation certificates (WebAuthn 8.2.1).
+    name = x509.Name(
+        [
+            x509.NameAttribute(NameOID.COUNTRY_NAME, "US"),
+            x509.NameAttribute(NameOID.ORGANIZATION_NAME, "Skey DIY"),
+            x509.NameAttribute(NameOID.ORGANIZATIONAL_UNIT_NAME, "Authenticator Attestation"),
+            x509.NameAttribute(NameOID.COMMON_NAME, "Skey Pico Attestation"),
+        ]
+    )
+    now = datetime.datetime.now(datetime.timezone.utc)
+    cert = (
+        x509.CertificateBuilder()
+        .subject_name(name)
+        .issuer_name(name)
+        .public_key(key.public_key())
+        .serial_number(x509.random_serial_number())
+        .not_valid_before(now - datetime.timedelta(days=1))
+        .not_valid_after(datetime.datetime(2049, 12, 31, tzinfo=datetime.timezone.utc))
+        .add_extension(x509.BasicConstraints(ca=False, path_length=None), critical=True)
+        .add_extension(x509.UnrecognizedExtension(OID_FIDO_AAGUID, b"\x04\x10" + AAGUID.bytes), critical=False)
+        .sign(key, hashes.SHA256())
+    )
+    key_path.parent.mkdir(parents=True, exist_ok=True)
+    key_path.write_bytes(
+        key.private_bytes(
+            serialization.Encoding.PEM,
+            serialization.PrivateFormat.PKCS8,
+            serialization.NoEncryption(),
+        )
+    )
+    key_path.chmod(0o600)
+    cert_path.write_bytes(cert.public_bytes(serialization.Encoding.DER))
+
+
+def cert_aaguid(cert_der: bytes) -> bytes:
+    cert = x509.load_der_x509_certificate(cert_der)
+    try:
+        return cert.extensions.get_extension_for_oid(OID_FIDO_AAGUID).value.value[2:]
+    except x509.ExtensionNotFound:
+        return b""
+
+
+def c_bytes(data: bytes) -> str:
+    rows = [", ".join(f"0x{b:02x}" for b in data[i : i + 12]) for i in range(0, len(data), 12)]
+    return "{\n    " + ",\n    ".join(rows) + ",\n}"
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--key-dir", type=pathlib.Path, required=True)
+    parser.add_argument("--out", type=pathlib.Path, required=True)
+    args = parser.parse_args()
+
+    key_path = args.key_dir / "attestation_key.pem"
+    cert_path = args.key_dir / "attestation_cert.der"
+    if not key_path.exists() or not cert_path.exists() or cert_aaguid(cert_path.read_bytes()) != AAGUID.bytes:
+        create(key_path, cert_path)
+
+    key = serialization.load_pem_private_key(key_path.read_bytes(), password=None)
+    private_value = key.private_numbers().private_value.to_bytes(32, "big")
+    cert_der = cert_path.read_bytes()
+
+    args.out.parent.mkdir(parents=True, exist_ok=True)
+    args.out.write_text(
+        f"""// Generated by tools/gen_attestation.py. Do not edit.
+#include "attestation.h"
+
+const uint8_t skey_aaguid[16] = {c_bytes(AAGUID.bytes)};
+
+const uint8_t attestation_key[32] = {c_bytes(private_value)};
+
+const uint8_t attestation_cert[] = {c_bytes(cert_der)};
+
+const size_t attestation_cert_len = sizeof(attestation_cert);
+"""
+    )
+
+
+if __name__ == "__main__":
+    main()