1 //! SRP server implementation
4 //! First receive user's username and public value `a_pub`, retrieve from a
5 //! database `UserRecord` for a given username, generate `b` (e.g. 512 bits
6 //! long) and initialize SRP server instance:
9 //! use srp::groups::G_2048;
11 //! let (username, a_pub) = conn.receive_handshake();
12 //! let user = db.retrieve_user_record(username);
13 //! let b = [0u8; 64];
14 //! rng.fill_bytes(&mut b);
15 //! let server = SrpServer::<Sha256>::new(&user, &a_pub, &b, &G_2048)?;
18 //! Next send to user `b_pub` and `salt` from user record:
21 //! let b_pub = server.get_b_pub();
22 //! conn.reply_to_handshake(&user.salt, b_pub);
25 //! And finally recieve user proof, verify it and send server proof in the
29 //! let user_proof = conn.receive_proof();
30 //! let server_proof = server.verify(user_proof)?;
31 //! conn.send_proof(server_proof);
34 //! To get the shared secret use `get_key` method. As alternative to using
35 //! `verify` method it's also possible to use this key for authentificated
37 use std::marker::PhantomData;
39 use digest::{Digest, Output};
40 use num_bigint::BigUint;
42 use crate::tools::powm;
43 use crate::types::{SrpAuthError, SrpGroup};
45 /// Data provided by users upon registration, usually stored in the database.
46 pub struct UserRecord<'a> {
47 pub username: &'a [u8],
50 pub verifier: &'a [u8],
54 pub struct SrpServer<D: Digest> {
64 impl<D: Digest> SrpServer<D> {
65 /// Create new server state.
67 user: &UserRecord<'_>,
71 ) -> Result<Self, SrpAuthError> {
72 let a_pub = BigUint::from_bytes_be(a_pub);
73 // Safeguard against malicious A
74 if &a_pub % ¶ms.n == BigUint::default() {
75 return Err(SrpAuthError {
76 description: "Malicious a_pub value",
79 let v = BigUint::from_bytes_be(user.verifier);
80 let b = BigUint::from_bytes_be(b) % ¶ms.n;
81 let k = params.compute_k::<D>();
83 let interm = (k * &v) % ¶ms.n;
84 let b_pub = (interm + ¶ms.powm(&b)) % ¶ms.n;
88 d.update(&a_pub.to_bytes_be());
89 d.update(&b_pub.to_bytes_be());
92 let d = Default::default();
95 let u = BigUint::from_bytes_be(u.as_slice());
96 let t = (&a_pub * powm(&v, &u, ¶ms.n)) % ¶ms.n;
97 let s = powm(&t, &b, ¶ms.n);
98 D::digest(&s.to_bytes_be())
109 /// Get private `b` value. (see `new_with_b` documentation)
110 pub fn get_b(&self) -> Vec<u8> {
114 /// Get public `b_pub` value for sending to the user.
115 pub fn get_b_pub(&self) -> Vec<u8> {
116 self.b_pub.to_bytes_be()
119 /// Get shared secret between user and the server. (do not forget to verify
120 /// that keys are the same!)
121 pub fn get_key(&self) -> Output<D> {
125 /// Process user proof of having the same shared secret and compute
126 /// server proof for sending to the user.
127 pub fn verify(&self, user_proof: &[u8]) -> Result<Output<D>, SrpAuthError> {
129 let mut d = D::new();
130 d.update(&self.a_pub.to_bytes_be());
131 d.update(&self.b_pub.to_bytes_be());
134 if user_proof == d.finalize().as_slice() {
136 let mut d = D::new();
137 d.update(&self.a_pub.to_bytes_be());
138 d.update(user_proof);
143 description: "Incorrect user proof",