diff --git a/Cargo.lock b/Cargo.lock
index 6b7ac8a..4086b01 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1337,8 +1337,10 @@ dependencies = [
 name = "kv-model"
 version = "0.1.0"
 dependencies = [
+ "ciborium",
  "data-encoding",
  "hmac",
+ "p256",
  "psl",
  "rand",
  "serde",
diff --git a/crates/kv-model/Cargo.toml b/crates/kv-model/Cargo.toml
index 9ff4980..0b21acb 100644
--- a/crates/kv-model/Cargo.toml
+++ b/crates/kv-model/Cargo.toml
@@ -19,3 +19,7 @@ sha2.workspace = true
 data-encoding.workspace = true
 url.workspace = true
 psl.workspace = true
+p256.workspace = true
+
+[dev-dependencies]
+ciborium.workspace = true
diff --git a/crates/kv-model/src/lib.rs b/crates/kv-model/src/lib.rs
index 9635351..4141bb0 100644
--- a/crates/kv-model/src/lib.rs
+++ b/crates/kv-model/src/lib.rs
@@ -4,6 +4,7 @@
 pub mod generator;
 pub mod item;
 pub mod matching;
+pub mod passkey;
 pub mod totp;
 
 pub use item::{Item, ItemKind, Passkey};
diff --git a/crates/kv-model/src/passkey.rs b/crates/kv-model/src/passkey.rs
new file mode 100644
index 0000000..910b1b3
--- /dev/null
+++ b/crates/kv-model/src/passkey.rs
@@ -0,0 +1,271 @@
+//! WebAuthn platform-authenticator operations for stored passkeys (kestrove-spec/docs/05 "Passkeys").
+//!
+//! Kestrove acts as a synced (multi-device) authenticator: ES256 only (COSE alg -7), attestation
+//! format `none`, flags BE|BS always set and a signature counter that stays 0, as WebAuthn Level 3
+//! §6.1.1 allows for credentials that may exist on several devices.
+//!
+//! Everything here is pure: the caller decides where the [`Passkey`] is stored. Private keys only
+//! leave this module inside a `Passkey`, which the owning [`crate::Item`] zeroizes on drop.
+
+use p256::{
+    ecdsa::{signature::Signer, Signature, SigningKey, VerifyingKey},
+    pkcs8::{DecodePrivateKey, EncodePrivateKey, EncodePublicKey},
+};
+use rand::{rngs::OsRng, RngCore};
+use sha2::{Digest, Sha256};
+use thiserror::Error;
+use uuid::Uuid;
+
+use crate::Passkey;
+
+/// The Kestrove AAGUID, `e63b1a47-0244-453d-aef9-981639609295` (a random v4 UUID fixed forever).
+/// Relying parties use it to show "Kestrove" next to the passkey.
+pub const KESTROVE_AAGUID: Uuid =
+    Uuid::from_bytes([0xe6, 0x3b, 0x1a, 0x47, 0x02, 0x44, 0x45, 0x3d, 0xae, 0xf9, 0x98, 0x16, 0x39, 0x60, 0x92, 0x95]);
+
+/// COSE algorithm identifier of ES256 (ECDSA P-256 with SHA-256).
+pub const COSE_ALG_ES256: i64 = -7;
+
+/// Length of new credential ids, in bytes.
+pub const CREDENTIAL_ID_LEN: usize = 16;
+
+/// authenticatorData flag bits (WebAuthn §6.1).
+pub mod flags {
+    /// User present.
+    pub const UP: u8 = 0x01;
+    /// User verified.
+    pub const UV: u8 = 0x04;
+    /// Backup eligible.
+    pub const BE: u8 = 0x08;
+    /// Backed up.
+    pub const BS: u8 = 0x10;
+    /// Attested credential data included.
+    pub const AT: u8 = 0x40;
+    /// Extension data included (never set by Kestrove).
+    pub const ED: u8 = 0x80;
+}
+
+/// Flags of a registration response.
+pub const REGISTRATION_FLAGS: u8 = flags::UP | flags::UV | flags::BE | flags::BS | flags::AT;
+/// Flags of an assertion response.
+pub const ASSERTION_FLAGS: u8 = flags::UP | flags::UV | flags::BE | flags::BS;
+
+#[derive(Debug, Error, PartialEq, Eq)]
+pub enum PasskeyError {
+    #[error("invalid relying party id")]
+    InvalidRpId,
+    #[error("user handle must be 1 to 64 bytes")]
+    InvalidUserHandle,
+    #[error("client data hash must be 32 bytes")]
+    InvalidClientDataHash,
+    /// The credential belongs to another relying party.
+    #[error("relying party mismatch")]
+    RpMismatch,
+    #[error("stored passkey key is corrupt")]
+    BadKey,
+}
+
+/// What a registration returns to the platform (no private material).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Registration {
+    pub credential_id: Vec<u8>,
+    /// COSE_Key (EC2, P-256, ES256), CTAP2 canonical CBOR.
+    pub cose_public_key: Vec<u8>,
+    /// SubjectPublicKeyInfo DER (Android's `response.publicKey`).
+    pub public_key_spki: Vec<u8>,
+    pub authenticator_data: Vec<u8>,
+    /// `{"fmt": "none", "attStmt": {}, "authData": …}`, CTAP2 canonical CBOR.
+    pub attestation_object: Vec<u8>,
+}
+
+/// What an assertion returns to the platform.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Assertion {
+    pub authenticator_data: Vec<u8>,
+    /// ES256 over `authenticatorData || clientDataHash`, ASN.1 DER (WebAuthn §6.5.6).
+    pub signature: Vec<u8>,
+}
+
+/// An RP ID is a registrable domain or host as the platform already validated it: lowercase ASCII
+/// (IDNs in punycode), no scheme, port, path or trailing dot. Matching is always exact.
+pub fn validate_rp_id(rp_id: &str) -> Result<(), PasskeyError> {
+    let ok = !rp_id.is_empty()
+        && rp_id.len() <= 253
+        && !rp_id.starts_with(['.', '-'])
+        && !rp_id.ends_with(['.', '-'])
+        && !rp_id.contains("..")
+        && rp_id.bytes().all(|b| b.is_ascii_lowercase() || b.is_ascii_digit() || b == b'.' || b == b'-');
+    if ok {
+        Ok(())
+    } else {
+        Err(PasskeyError::InvalidRpId)
+    }
+}
+
+fn check_client_data_hash(client_data_hash: &[u8]) -> Result<(), PasskeyError> {
+    if client_data_hash.len() == 32 {
+        Ok(())
+    } else {
+        Err(PasskeyError::InvalidClientDataHash)
+    }
+}
+
+/// Generates a new ES256 passkey with a random credential id.
+pub fn generate(rp_id: &str, user_handle: &[u8], user_name: &str, now: i64) -> Result<Passkey, PasskeyError> {
+    validate_rp_id(rp_id)?;
+    if user_handle.is_empty() || user_handle.len() > 64 {
+        return Err(PasskeyError::InvalidUserHandle);
+    }
+    let mut credential_id = vec![0u8; CREDENTIAL_ID_LEN];
+    OsRng.fill_bytes(&mut credential_id);
+    let key = SigningKey::random(&mut OsRng);
+    from_signing_key(&key, credential_id, rp_id, user_handle, user_name, now)
+}
+
+/// Builds a passkey around an existing key (used by [`generate`] and known-answer tests).
+pub fn from_signing_key(
+    key: &SigningKey,
+    credential_id: Vec<u8>,
+    rp_id: &str,
+    user_handle: &[u8],
+    user_name: &str,
+    now: i64,
+) -> Result<Passkey, PasskeyError> {
+    let pkcs8 = key.to_pkcs8_der().map_err(|_| PasskeyError::BadKey)?;
+    Ok(Passkey {
+        credential_id,
+        rp_id: rp_id.to_owned(),
+        user_handle: user_handle.to_vec(),
+        user_name: user_name.to_owned(),
+        private_key: pkcs8.as_bytes().to_vec(),
+        sign_count: 0,
+        created_at: now,
+    })
+}
+
+fn signing_key(pk: &Passkey) -> Result<SigningKey, PasskeyError> {
+    // `SigningKey` zeroizes its scalar on drop.
+    SigningKey::from_pkcs8_der(&pk.private_key).map_err(|_| PasskeyError::BadKey)
+}
+
+/// The registration response for a freshly generated passkey.
+pub fn registration(pk: &Passkey) -> Result<Registration, PasskeyError> {
+    validate_rp_id(&pk.rp_id)?;
+    let vk = *signing_key(pk)?.verifying_key();
+    let cose_public_key = cose_key(&vk);
+    let mut authenticator_data = auth_data_header(&pk.rp_id, REGISTRATION_FLAGS, pk.sign_count);
+    authenticator_data.extend_from_slice(KESTROVE_AAGUID.as_bytes());
+    let id_len = u16::try_from(pk.credential_id.len()).map_err(|_| PasskeyError::BadKey)?;
+    authenticator_data.extend_from_slice(&id_len.to_be_bytes());
+    authenticator_data.extend_from_slice(&pk.credential_id);
+    authenticator_data.extend_from_slice(&cose_public_key);
+    let public_key_spki = vk.to_public_key_der().map_err(|_| PasskeyError::BadKey)?.into_vec();
+    Ok(Registration {
+        credential_id: pk.credential_id.clone(),
+        attestation_object: attestation_object_none(&authenticator_data),
+        cose_public_key,
+        public_key_spki,
+        authenticator_data,
+    })
+}
+
+/// Signs an assertion for `rp_id`. Fails with [`PasskeyError::RpMismatch`] unless `rp_id` is
+/// exactly the passkey's RP ID.
+pub fn assert(pk: &Passkey, rp_id: &str, client_data_hash: &[u8]) -> Result<Assertion, PasskeyError> {
+    validate_rp_id(rp_id)?;
+    check_client_data_hash(client_data_hash)?;
+    if pk.rp_id != rp_id {
+        return Err(PasskeyError::RpMismatch);
+    }
+    let authenticator_data = auth_data_header(rp_id, ASSERTION_FLAGS, pk.sign_count);
+    let mut message = authenticator_data.clone();
+    message.extend_from_slice(client_data_hash);
+    let signature = sign_der(&signing_key(pk)?, &message);
+    Ok(Assertion { authenticator_data, signature })
+}
+
+/// ECDSA P-256 / SHA-256 (RFC 6979 deterministic nonce), DER-encoded.
+pub fn sign_der(key: &SigningKey, message: &[u8]) -> Vec<u8> {
+    let sig: Signature = key.sign(message);
+    sig.to_der().as_bytes().to_vec()
+}
+
+/// `rpIdHash || flags || signCount` (37 bytes).
+fn auth_data_header(rp_id: &str, flags: u8, sign_count: u32) -> Vec<u8> {
+    let mut out = Vec::with_capacity(37 + 16 + 2 + CREDENTIAL_ID_LEN + 77);
+    out.extend_from_slice(&Sha256::digest(rp_id.as_bytes()));
+    out.push(flags);
+    out.extend_from_slice(&sign_count.to_be_bytes());
+    out
+}
+
+// ---- CTAP2 canonical CBOR (only the few shapes needed here) ----
+
+fn cbor_head(out: &mut Vec<u8>, major: u8, len: u64) {
+    let m = major << 5;
+    match len {
+        0..=23 => out.push(m | len as u8),
+        24..=0xff => out.extend_from_slice(&[m | 24, len as u8]),
+        0x100..=0xffff => {
+            out.push(m | 25);
+            out.extend_from_slice(&(len as u16).to_be_bytes());
+        }
+        _ => {
+            out.push(m | 26);
+            out.extend_from_slice(&(len as u32).to_be_bytes());
+        }
+    }
+}
+
+fn cbor_int(out: &mut Vec<u8>, v: i64) {
+    if v >= 0 {
+        cbor_head(out, 0, v as u64);
+    } else {
+        cbor_head(out, 1, (-1 - v) as u64);
+    }
+}
+
+fn cbor_bytes(out: &mut Vec<u8>, b: &[u8]) {
+    cbor_head(out, 2, b.len() as u64);
+    out.extend_from_slice(b);
+}
+
+fn cbor_text(out: &mut Vec<u8>, s: &str) {
+    cbor_head(out, 3, s.len() as u64);
+    out.extend_from_slice(s.as_bytes());
+}
+
+/// COSE_Key `{1: 2 (EC2), 3: -7 (ES256), -1: 1 (P-256), -2: x, -3: y}` (RFC 9053), keys in
+/// canonical order.
+pub fn cose_key(vk: &VerifyingKey) -> Vec<u8> {
+    let point = vk.to_encoded_point(false);
+    let (Some(x), Some(y)) = (point.x(), point.y()) else { unreachable!("uncompressed point has coordinates") };
+    let mut out = Vec::with_capacity(77);
+    cbor_head(&mut out, 5, 5);
+    cbor_int(&mut out, 1);
+    cbor_int(&mut out, 2);
+    cbor_int(&mut out, 3);
+    cbor_int(&mut out, COSE_ALG_ES256);
+    cbor_int(&mut out, -1);
+    cbor_int(&mut out, 1);
+    cbor_int(&mut out, -2);
+    cbor_bytes(&mut out, x);
+    cbor_int(&mut out, -3);
+    cbor_bytes(&mut out, y);
+    out
+}
+
+fn attestation_object_none(authenticator_data: &[u8]) -> Vec<u8> {
+    let mut out = Vec::with_capacity(authenticator_data.len() + 32);
+    cbor_head(&mut out, 5, 3);
+    cbor_text(&mut out, "fmt");
+    cbor_text(&mut out, "none");
+    cbor_text(&mut out, "attStmt");
+    cbor_head(&mut out, 5, 0);
+    cbor_text(&mut out, "authData");
+    cbor_bytes(&mut out, authenticator_data);
+    out
+}
+
+#[cfg(test)]
+mod tests;
diff --git a/crates/kv-model/src/passkey/tests.rs b/crates/kv-model/src/passkey/tests.rs
new file mode 100644
index 0000000..ff604e8
--- /dev/null
+++ b/crates/kv-model/src/passkey/tests.rs
@@ -0,0 +1,146 @@
+//! Checks responses with an independent verifier: authenticatorData parsed by hand, CBOR decoded with
+//! `ciborium`, signatures verified with `p256` against the COSE key.
+
+use ciborium::Value;
+use p256::{
+    ecdsa::{signature::Verifier, Signature, SigningKey, VerifyingKey},
+    EncodedPoint,
+};
+use sha2::{Digest, Sha256};
+
+use super::*;
+
+const CDH: [u8; 32] = [7; 32];
+
+struct AuthData {
+    rp_id_hash: [u8; 32],
+    flags: u8,
+    sign_count: u32,
+    attested: Option<(Vec<u8>, Vec<u8>, Value)>,
+}
+
+fn parse_auth_data(d: &[u8]) -> AuthData {
+    assert!(d.len() >= 37);
+    let flags = d[32];
+    let attested = (flags & flags::AT != 0).then(|| {
+        let aaguid = d[37..53].to_vec();
+        let n = u16::from_be_bytes([d[53], d[54]]) as usize;
+        let cred_id = d[55..55 + n].to_vec();
+        let mut rest = &d[55 + n..];
+        let key: Value = ciborium::from_reader(&mut rest).unwrap();
+        assert!(rest.is_empty(), "no trailing data (ED is not set)");
+        (aaguid, cred_id, key)
+    });
+    if flags & flags::AT == 0 {
+        assert_eq!(d.len(), 37);
+    }
+    AuthData {
+        rp_id_hash: d[..32].try_into().unwrap(),
+        flags,
+        sign_count: u32::from_be_bytes(d[33..37].try_into().unwrap()),
+        attested,
+    }
+}
+
+fn map_get<'a>(m: &'a Value, k: &Value) -> &'a Value {
+    m.as_map().unwrap().iter().find(|(kk, _)| kk == k).map(|(_, v)| v).unwrap()
+}
+
+fn verifying_key_from_cose(key: &Value) -> VerifyingKey {
+    let i = |n: i64| Value::Integer(n.into());
+    assert_eq!(map_get(key, &i(1)), &i(2));
+    assert_eq!(map_get(key, &i(3)), &i(-7));
+    assert_eq!(map_get(key, &i(-1)), &i(1));
+    let x = map_get(key, &i(-2)).as_bytes().unwrap();
+    let y = map_get(key, &i(-3)).as_bytes().unwrap();
+    let point = EncodedPoint::from_affine_coordinates(x.as_slice().into(), y.as_slice().into(), false);
+    VerifyingKey::from_encoded_point(&point).unwrap()
+}
+
+#[test]
+fn registration_and_assertion_verify_independently() {
+    let pk = generate("example.com", b"user-1", "alice", 1_700_000_000).unwrap();
+    assert_eq!(pk.credential_id.len(), CREDENTIAL_ID_LEN);
+    let reg = registration(&pk).unwrap();
+
+    let att: Value = ciborium::from_reader(reg.attestation_object.as_slice()).unwrap();
+    assert_eq!(map_get(&att, &Value::Text("fmt".into())), &Value::Text("none".into()));
+    assert_eq!(map_get(&att, &Value::Text("attStmt".into())), &Value::Map(vec![]));
+    let auth_data = map_get(&att, &Value::Text("authData".into())).as_bytes().unwrap();
+    assert_eq!(auth_data, &reg.authenticator_data);
+
+    let ad = parse_auth_data(auth_data);
+    assert_eq!(ad.rp_id_hash.as_slice(), Sha256::digest(b"example.com").as_slice());
+    assert_eq!(ad.flags, 0x5d);
+    assert_eq!(ad.sign_count, 0);
+    let (aaguid, cred_id, key) = ad.attested.unwrap();
+    assert_eq!(aaguid, KESTROVE_AAGUID.as_bytes());
+    assert_eq!(cred_id, pk.credential_id);
+    let vk = verifying_key_from_cose(&key);
+    let cose: Value = ciborium::from_reader(reg.cose_public_key.as_slice()).unwrap();
+    assert_eq!(cose, key);
+    assert_eq!(vk, VerifyingKey::from_public_key_der_for_test(&reg.public_key_spki));
+
+    let a = super::assert(&pk, "example.com", &CDH).unwrap();
+    let ad = parse_auth_data(&a.authenticator_data);
+    assert_eq!(ad.rp_id_hash.as_slice(), Sha256::digest(b"example.com").as_slice());
+    assert_eq!((ad.flags, ad.sign_count), (0x1d, 0));
+    assert!(ad.attested.is_none());
+    let mut msg = a.authenticator_data.clone();
+    msg.extend_from_slice(&CDH);
+    vk.verify(&msg, &Signature::from_der(&a.signature).unwrap()).unwrap();
+    msg[0] ^= 1;
+    assert!(vk.verify(&msg, &Signature::from_der(&a.signature).unwrap()).is_err());
+}
+
+trait SpkiForTest {
+    fn from_public_key_der_for_test(der: &[u8]) -> Self;
+}
+impl SpkiForTest for VerifyingKey {
+    fn from_public_key_der_for_test(der: &[u8]) -> Self {
+        use p256::pkcs8::DecodePublicKey;
+        VerifyingKey::from_public_key_der(der).unwrap()
+    }
+}
+
+#[test]
+fn rp_id_is_matched_exactly() {
+    let pk = generate("example.com", b"u", "a", 0).unwrap();
+    for other in ["login.example.com", "example.org", "xample.com"] {
+        assert_eq!(super::assert(&pk, other, &CDH), Err(PasskeyError::RpMismatch));
+    }
+    for bad in ["", "Example.com", "https://example.com", "example.com:443", "example.com.", "a..b"] {
+        assert_eq!(validate_rp_id(bad), Err(PasskeyError::InvalidRpId), "{bad}");
+    }
+    assert_eq!(super::assert(&pk, "example.com", &[0; 31]), Err(PasskeyError::InvalidClientDataHash));
+    assert_eq!(generate("example.com", b"", "a", 0).unwrap_err(), PasskeyError::InvalidUserHandle);
+    assert_eq!(generate("example.com", &[0; 65], "a", 0).unwrap_err(), PasskeyError::InvalidUserHandle);
+}
+
+fn hex(s: &str) -> Vec<u8> {
+    (0..s.len()).step_by(2).map(|i| u8::from_str_radix(&s[i..i + 2], 16).unwrap()).collect()
+}
+
+/// RFC 6979 A.2.5 (P-256, SHA-256, message "sample") and the well-known rpIdHash of example.com.
+#[test]
+fn known_answer_vectors() {
+    let key = SigningKey::from_slice(&hex("c9afa9d845ba75166b5c215767b1d6934e50c3db36e89b127b8a622b120f6721")).unwrap();
+    let cose = cose_key(key.verifying_key());
+    let mut expected = hex("a5010203262001215820");
+    expected.extend(hex("60fed4ba255a9d31c961eb74c6356d68c049b8923b61fa6ce669622e60f29fb6"));
+    expected.extend(hex("225820"));
+    expected.extend(hex("7903fe1008b8bc99a41ae9e95628bc64f2f1b20c2d7e9f5177a3c294d4462299"));
+    assert_eq!(cose, expected);
+
+    let sig = Signature::from_der(&sign_der(&key, b"sample")).unwrap();
+    let (r, s) = (sig.r().to_bytes().to_vec(), sig.s().to_bytes().to_vec());
+    assert_eq!(r, hex("efd48b2aacb6a8fd1140dd9cd45e81d69d2c877b56aaf991c34d0ea84eaf3716"));
+    assert_eq!(s, hex("f7cb1c942d657c41d436c7a1b6e29f65f3e900dbb9aff4064dc4ab2f843acda8"));
+
+    let pk = from_signing_key(&key, vec![0xab; 16], "example.com", b"u", "a", 0).unwrap();
+    let a = super::assert(&pk, "example.com", &CDH).unwrap();
+    let mut ad = hex("a379a6f6eeafb9a55e378c118034e2751e682fab9f2d30ab13d2125586ce1947");
+    ad.extend([0x1d, 0, 0, 0, 0]);
+    assert_eq!(a.authenticator_data, ad);
+    assert_eq!(KESTROVE_AAGUID.to_string(), "e63b1a47-0244-453d-aef9-981639609295");
+}