dcrypt_kem/ecdh/p521/
mod.rs

1// File: crates/kem/src/ecdh/p521/mod.rs
2//! ECDH-KEM with NIST P-521
3//!
4//! This module provides a Key Encapsulation Mechanism (KEM) based on the
5//! Elliptic Curve Diffie-Hellman (ECDH) protocol using the NIST P-521 curve.
6//! This is a dcrypt-specific ECDH-plus-HKDF construction, not RFC 9180 HPKE.
7//! Invalid inputs return errors. No blanket constant-time or IND-CCA claim is
8//! made; arithmetic behavior depends on the backend, compiler, and target.
9//!
10//! This implementation uses compressed point format for optimal bandwidth efficiency.
11//!
12//! Public points are validated and the shared point is processed with
13//! HKDF-SHA512. Protocols needing HPKE or implicit rejection must use a vetted
14//! implementation of that construction instead.
15
16use super::concat_kdf_ikm;
17use crate::error::Error as KemError;
18use alloc::vec::Vec;
19use dcrypt_algorithms::ec::p521 as ec_p521;
20use dcrypt_api::{
21    error::Error as ApiError,
22    traits::serialize::{Serialize, SerializeSecret},
23    Kem, Key as ApiKey, Result as ApiResult, ZeroizingBytes,
24};
25use dcrypt_common::security::SecretBuffer;
26use dcrypt_internal::random::{CryptoRng, RngCore};
27use dcrypt_internal::zeroing::Zeroizing;
28
29const KDF_INFO: &[u8] = b"dcrypt-v3/ECDH-P521-KEM/shared-secret";
30
31/// ECDH KEM with P-521 curve
32pub struct EcdhP521;
33
34/// Public key for ECDH-P521 KEM (compressed EC point)
35#[derive(Clone)]
36pub struct EcdhP521PublicKey([u8; ec_p521::P521_POINT_COMPRESSED_SIZE]);
37
38impl_zeroize_tuple!(EcdhP521PublicKey);
39
40impl AsRef<[u8]> for EcdhP521PublicKey {
41    fn as_ref(&self) -> &[u8] {
42        &self.0
43    }
44}
45
46impl AsMut<[u8]> for EcdhP521PublicKey {
47    fn as_mut(&mut self) -> &mut [u8] {
48        &mut self.0
49    }
50}
51
52/// Secret key for ECDH-P521 KEM (scalar value)
53#[derive(Clone)]
54pub struct EcdhP521SecretKey(SecretBuffer<{ ec_p521::P521_SCALAR_SIZE }>);
55
56impl_zeroize_on_drop_tuple!(EcdhP521SecretKey);
57
58impl AsRef<[u8]> for EcdhP521SecretKey {
59    fn as_ref(&self) -> &[u8] {
60        self.0.as_ref()
61    }
62}
63
64/// Shared secret from ECDH-P521 KEM
65#[derive(Clone)]
66pub struct EcdhP521SharedSecret(ApiKey);
67
68impl_zeroize_on_drop_tuple!(EcdhP521SharedSecret);
69
70impl AsRef<[u8]> for EcdhP521SharedSecret {
71    fn as_ref(&self) -> &[u8] {
72        self.0.as_ref()
73    }
74}
75
76/// Ciphertext for ECDH-P521 KEM (compressed ephemeral public key)
77#[derive(Clone)]
78pub struct EcdhP521Ciphertext([u8; ec_p521::P521_POINT_COMPRESSED_SIZE]);
79
80impl AsRef<[u8]> for EcdhP521Ciphertext {
81    fn as_ref(&self) -> &[u8] {
82        &self.0
83    }
84}
85
86impl AsMut<[u8]> for EcdhP521Ciphertext {
87    fn as_mut(&mut self) -> &mut [u8] {
88        &mut self.0
89    }
90}
91
92// --- Public key methods ---
93impl EcdhP521PublicKey {
94    pub fn from_bytes(bytes: &[u8]) -> ApiResult<Self> {
95        if bytes.len() != ec_p521::P521_POINT_COMPRESSED_SIZE {
96            return Err(ApiError::InvalidLength {
97                context: "EcdhP521PublicKey::from_bytes",
98                expected: ec_p521::P521_POINT_COMPRESSED_SIZE,
99                actual: bytes.len(),
100            });
101        }
102        let point = ec_p521::Point::deserialize_compressed(bytes)
103            .map_err(|e| ApiError::from(KemError::from(e)))?;
104        if point.is_identity() {
105            return Err(ApiError::InvalidKey {
106                context: "EcdhP521PublicKey::from_bytes",
107                #[cfg(feature = "std")]
108                message: "Public key cannot be the identity point".to_string(),
109            });
110        }
111        let mut key_bytes = [0u8; ec_p521::P521_POINT_COMPRESSED_SIZE];
112        key_bytes.copy_from_slice(bytes);
113        Ok(Self(key_bytes))
114    }
115    pub fn to_bytes(&self) -> Vec<u8> {
116        self.0.to_vec()
117    }
118}
119
120impl Serialize for EcdhP521PublicKey {
121    fn from_bytes(bytes: &[u8]) -> ApiResult<Self> {
122        Self::from_bytes(bytes)
123    }
124    fn to_bytes(&self) -> Vec<u8> {
125        self.to_bytes()
126    }
127}
128
129// --- Secret key methods ---
130impl EcdhP521SecretKey {
131    pub fn from_bytes(bytes: &[u8]) -> ApiResult<Self> {
132        if bytes.len() != ec_p521::P521_SCALAR_SIZE {
133            return Err(ApiError::InvalidLength {
134                context: "EcdhP521SecretKey::from_bytes",
135                expected: ec_p521::P521_SCALAR_SIZE,
136                actual: bytes.len(),
137            });
138        }
139        let mut buffer = SecretBuffer::zeroed();
140        buffer.as_mut().copy_from_slice(bytes);
141        let scalar = ec_p521::Scalar::from_secret_buffer(buffer.clone())
142            .map_err(|e| ApiError::from(KemError::from(e)))?;
143        drop(scalar);
144        Ok(Self(buffer))
145    }
146    pub fn to_bytes(&self) -> ZeroizingBytes {
147        self.0.to_bytes_zeroizing_boxed()
148    }
149}
150
151impl SerializeSecret for EcdhP521SecretKey {
152    fn from_bytes(bytes: &[u8]) -> ApiResult<Self> {
153        Self::from_bytes(bytes)
154    }
155    fn to_bytes_zeroizing(&self) -> ZeroizingBytes {
156        self.to_bytes()
157    }
158}
159
160// --- Shared secret methods ---
161impl EcdhP521SharedSecret {
162    pub fn to_bytes(&self) -> ZeroizingBytes {
163        self.0.to_bytes_zeroizing_boxed()
164    }
165}
166
167impl SerializeSecret for EcdhP521SharedSecret {
168    fn from_bytes(bytes: &[u8]) -> ApiResult<Self> {
169        if bytes.len() != ec_p521::P521_KEM_SHARED_SECRET_KDF_OUTPUT_SIZE {
170            return Err(ApiError::InvalidLength {
171                context: "EcdhP521SharedSecret::from_bytes",
172                expected: ec_p521::P521_KEM_SHARED_SECRET_KDF_OUTPUT_SIZE,
173                actual: bytes.len(),
174            });
175        }
176        Ok(Self(ApiKey::new(bytes)))
177    }
178    fn to_bytes_zeroizing(&self) -> ZeroizingBytes {
179        self.to_bytes()
180    }
181}
182
183// --- Ciphertext methods ---
184impl EcdhP521Ciphertext {
185    pub fn from_bytes(bytes: &[u8]) -> ApiResult<Self> {
186        if bytes.len() != ec_p521::P521_POINT_COMPRESSED_SIZE {
187            return Err(ApiError::InvalidLength {
188                context: "EcdhP521Ciphertext::from_bytes",
189                expected: ec_p521::P521_POINT_COMPRESSED_SIZE,
190                actual: bytes.len(),
191            });
192        }
193        let point = ec_p521::Point::deserialize_compressed(bytes)
194            .map_err(|e| ApiError::from(KemError::from(e)))?;
195        if point.is_identity() {
196            return Err(ApiError::InvalidCiphertext {
197                context: "EcdhP521Ciphertext::from_bytes",
198                #[cfg(feature = "std")]
199                message: "Ephemeral public key cannot be the identity point".to_string(),
200            });
201        }
202        let mut ct_bytes = [0u8; ec_p521::P521_POINT_COMPRESSED_SIZE];
203        ct_bytes.copy_from_slice(bytes);
204        Ok(Self(ct_bytes))
205    }
206    pub fn to_bytes(&self) -> Vec<u8> {
207        self.0.to_vec()
208    }
209}
210
211impl Serialize for EcdhP521Ciphertext {
212    fn from_bytes(bytes: &[u8]) -> ApiResult<Self> {
213        Self::from_bytes(bytes)
214    }
215    fn to_bytes(&self) -> Vec<u8> {
216        self.to_bytes()
217    }
218}
219
220impl Kem for EcdhP521 {
221    type PublicKey = EcdhP521PublicKey;
222    type SecretKey = EcdhP521SecretKey;
223    type SharedSecret = EcdhP521SharedSecret;
224    type Ciphertext = EcdhP521Ciphertext;
225    type KeyPair = (Self::PublicKey, Self::SecretKey);
226
227    fn name() -> &'static str {
228        "ECDH-P521"
229    }
230
231    fn keypair<R: CryptoRng + RngCore>(rng: &mut R) -> ApiResult<Self::KeyPair> {
232        let (sk_scalar, pk_point) =
233            ec_p521::generate_keypair(rng).map_err(|e| ApiError::from(KemError::from(e)))?;
234        let public_key = EcdhP521PublicKey(pk_point.serialize_compressed());
235        let secret_key = EcdhP521SecretKey(sk_scalar.as_secret_buffer().clone());
236        Ok((public_key, secret_key))
237    }
238
239    fn public_key(keypair: &Self::KeyPair) -> Self::PublicKey {
240        keypair.0.clone()
241    }
242
243    fn secret_key(keypair: &Self::KeyPair) -> Self::SecretKey {
244        keypair.1.clone()
245    }
246
247    fn encapsulate<R: CryptoRng + RngCore>(
248        rng: &mut R,
249        public_key_recipient: &Self::PublicKey,
250    ) -> ApiResult<(Self::Ciphertext, Self::SharedSecret)> {
251        let pk_r_point = ec_p521::Point::deserialize_compressed(&public_key_recipient.0)
252            .map_err(|e| ApiError::from(KemError::from(e)))?;
253        if pk_r_point.is_identity() {
254            return Err(ApiError::InvalidKey {
255                context: "ECDH-P521 encapsulate",
256                #[cfg(feature = "std")]
257                message: "Recipient public key cannot be the identity point".to_string(),
258            });
259        }
260        let (ephemeral_scalar, ephemeral_point) =
261            ec_p521::generate_keypair(rng).map_err(|e| ApiError::from(KemError::from(e)))?;
262        let ciphertext = EcdhP521Ciphertext(ephemeral_point.serialize_compressed());
263        let shared_point = ec_p521::scalar_mult(&ephemeral_scalar, &pk_r_point)
264            .map_err(|e| ApiError::from(KemError::from(e)))?;
265        if shared_point.is_identity() {
266            return Err(ApiError::DecryptionFailed {
267                context: "ECDH-P521 encapsulate",
268                #[cfg(feature = "std")]
269                message: "Shared point is the identity".to_string(),
270            });
271        }
272        let x_coord_bytes = Zeroizing::new(shared_point.x_coordinate_bytes());
273        let kdf_ikm = concat_kdf_ikm(
274            x_coord_bytes.as_ref(),
275            &ephemeral_point.serialize_compressed(),
276            &public_key_recipient.0,
277        );
278        let ss_bytes = ec_p521::kdf_hkdf_sha512_for_ecdh_kem(&kdf_ikm, Some(KDF_INFO))
279            .map_err(|e| ApiError::from(KemError::from(e)))?;
280        let shared_secret = EcdhP521SharedSecret(ApiKey::new(&ss_bytes[..]));
281        drop(ephemeral_scalar);
282        Ok((ciphertext, shared_secret))
283    }
284
285    fn decapsulate(
286        secret_key_recipient: &Self::SecretKey,
287        ciphertext_ephemeral_pk: &Self::Ciphertext,
288    ) -> ApiResult<Self::SharedSecret> {
289        let scalar_result = ec_p521::Scalar::from_secret_buffer(secret_key_recipient.0.clone());
290        let sk_r_scalar = match scalar_result {
291            Ok(scalar) => scalar,
292            Err(e) => return Err(ApiError::from(KemError::from(e))),
293        };
294        let q_e_point = ec_p521::Point::deserialize_compressed(&ciphertext_ephemeral_pk.0)
295            .map_err(|e| ApiError::from(KemError::from(e)))?;
296        if q_e_point.is_identity() {
297            return Err(ApiError::InvalidCiphertext {
298                context: "ECDH-P521 decapsulate",
299                #[cfg(feature = "std")]
300                message: "Ephemeral public key cannot be the identity point".to_string(),
301            });
302        }
303        let shared_point = ec_p521::scalar_mult(&sk_r_scalar, &q_e_point)
304            .map_err(|e| ApiError::from(KemError::from(e)))?;
305        if shared_point.is_identity() {
306            return Err(ApiError::DecryptionFailed {
307                context: "ECDH-P521 decapsulate",
308                #[cfg(feature = "std")]
309                message: "Shared point is the identity".to_string(),
310            });
311        }
312        let x_coord_bytes = Zeroizing::new(shared_point.x_coordinate_bytes());
313        let q_r_point = ec_p521::scalar_mult_base_g(&sk_r_scalar)
314            .map_err(|e| ApiError::from(KemError::from(e)))?;
315        let kdf_ikm = concat_kdf_ikm(
316            x_coord_bytes.as_ref(),
317            &ciphertext_ephemeral_pk.0,
318            &q_r_point.serialize_compressed(),
319        );
320        let ss_bytes = ec_p521::kdf_hkdf_sha512_for_ecdh_kem(&kdf_ikm, Some(KDF_INFO))
321            .map_err(|e| ApiError::from(KemError::from(e)))?;
322        let shared_secret = EcdhP521SharedSecret(ApiKey::new(&ss_bytes[..]));
323        Ok(shared_secret)
324    }
325}
326
327#[cfg(test)]
328mod tests;
dcrypt_kem/ecdh/p521/mod.rs

dcrypt_kem/ecdh/p521/
mod.rs
