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#![no_std] #![forbid(unsafe_code)] use core::fmt; use curve25519_dalek::{ ristretto::{CompressedRistretto, RistrettoPoint}, scalar::Scalar, }; use getrandom::getrandom; use hmac_sha512::{Hash, BLOCKBYTES}; pub const SESSION_ID_BYTES: usize = 16; pub const STEP1_PACKET_BYTES: usize = 16 + 32; pub const STEP2_PACKET_BYTES: usize = 32; pub const SHARED_KEY_BYTES: usize = 32; const DSI1: &str = "CPaceRistretto255-1"; const DSI2: &str = "CPaceRistretto255-1"; #[derive(Debug)] pub enum Error { Overflow(&'static str), Random(getrandom::Error), InvalidPublicKey, } impl fmt::Display for Error { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "{:?}", &self) } } impl From<getrandom::Error> for Error { fn from(e: getrandom::Error) -> Self { Error::Random(e) } } #[derive(Debug, Copy, Clone)] pub struct SharedKeys { pub k1: [u8; SHARED_KEY_BYTES], pub k2: [u8; SHARED_KEY_BYTES], } #[derive(Debug, Clone)] pub struct CPace { session_id: [u8; SESSION_ID_BYTES], p: RistrettoPoint, r: Scalar, } pub struct Step1Out { ctx: CPace, step1_packet: [u8; STEP1_PACKET_BYTES], } impl Step1Out { pub fn packet(&self) -> [u8; STEP1_PACKET_BYTES] { self.step1_packet } pub fn step3(&self, step2_packet: &[u8; STEP2_PACKET_BYTES]) -> Result<SharedKeys, Error> { self.ctx.step3(step2_packet) } } pub struct Step2Out { shared_keys: SharedKeys, step2_packet: [u8; STEP2_PACKET_BYTES], } impl Step2Out { pub fn shared_keys(&self) -> SharedKeys { self.shared_keys } pub fn packet(&self) -> [u8; STEP2_PACKET_BYTES] { self.step2_packet } } impl CPace { fn new<T: AsRef<[u8]>>( session_id: [u8; SESSION_ID_BYTES], password: &str, id_a: &str, id_b: &str, ad: Option<T>, ) -> Result<Self, Error> { if id_a.len() > 0xff || id_b.len() > 0xff { return Err(Error::Overflow( "Identifiers must be at most 255 bytes long", )); } let zpad = [0u8; BLOCKBYTES]; let pad_len = zpad.len().wrapping_sub(DSI1.len() + password.len()) & (zpad.len() - 1); let mut st = Hash::new(); st.update(DSI1); st.update(password); st.update(&zpad[..pad_len]); st.update(session_id); st.update([id_a.len() as u8]); st.update(id_a); st.update([id_b.len() as u8]); st.update(id_b); st.update(ad.as_ref().map(|ad| ad.as_ref()).unwrap_or_default()); let h = st.finalize(); let mut p = RistrettoPoint::from_uniform_bytes(&h); let mut r = [0u8; 64]; getrandom(&mut r)?; let r = Scalar::from_bytes_mod_order_wide(&r); p *= r; Ok(CPace { session_id, p, r }) } fn finalize( &self, op: RistrettoPoint, ya: RistrettoPoint, yb: RistrettoPoint, ) -> Result<SharedKeys, Error> { let p = op * self.r; let mut st = Hash::new(); st.update(DSI2); st.update(p.compress().as_bytes()); st.update(ya.compress().as_bytes()); st.update(yb.compress().as_bytes()); let h = st.finalize(); let (mut k1, mut k2) = ([0u8; SHARED_KEY_BYTES], [0u8; SHARED_KEY_BYTES]); k1.copy_from_slice(&h[..SHARED_KEY_BYTES]); k2.copy_from_slice(&h[SHARED_KEY_BYTES..]); Ok(SharedKeys { k1, k2 }) } pub fn step1<T: AsRef<[u8]>>( password: &str, id_a: &str, id_b: &str, ad: Option<T>, ) -> Result<Step1Out, Error> { let mut session_id = [0u8; SESSION_ID_BYTES]; getrandom(&mut session_id)?; let ctx = CPace::new(session_id, password, id_a, id_b, ad)?; let mut step1_packet = [0u8; STEP1_PACKET_BYTES]; step1_packet[..SESSION_ID_BYTES].copy_from_slice(&ctx.session_id); step1_packet[SESSION_ID_BYTES..].copy_from_slice(ctx.p.compress().as_bytes()); Ok(Step1Out { ctx, step1_packet }) } pub fn step2<T: AsRef<[u8]>>( step1_packet: &[u8; STEP1_PACKET_BYTES], password: &str, id_a: &str, id_b: &str, ad: Option<T>, ) -> Result<Step2Out, Error> { let mut session_id = [0u8; SESSION_ID_BYTES]; session_id.copy_from_slice(&step1_packet[..SESSION_ID_BYTES]); let ya = &step1_packet[SESSION_ID_BYTES..]; let ctx = CPace::new(session_id, password, id_a, id_b, ad)?; let mut step2_packet = [0u8; STEP2_PACKET_BYTES]; step2_packet.copy_from_slice(ctx.p.compress().as_bytes()); let ya = CompressedRistretto::from_slice(ya) .decompress() .ok_or(Error::InvalidPublicKey)?; let shared_keys = ctx.finalize(ya, ya, ctx.p)?; Ok(Step2Out { shared_keys, step2_packet, }) } pub fn step3(&self, step2_packet: &[u8; STEP2_PACKET_BYTES]) -> Result<SharedKeys, Error> { let yb = CompressedRistretto::from_slice(step2_packet) .decompress() .ok_or(Error::InvalidPublicKey)?; self.finalize(yb, self.p, yb) } } #[test] fn test_cpace() { let client = CPace::step1("password", "client", "server", Some("ad")).unwrap(); let step2 = CPace::step2(&client.packet(), "password", "client", "server", Some("ad")).unwrap(); let shared_keys = client.step3(&step2.packet()).unwrap(); assert_eq!(shared_keys.k1, step2.shared_keys.k1); assert_eq!(shared_keys.k2, step2.shared_keys.k2); }