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;
40 use generic_array::GenericArray;
41 use num::{BigUint, Zero};
43 use crate::tools::powm;
44 use crate::types::{SrpAuthError, SrpGroup};
46 /// Data provided by users upon registration, usually stored in the database.
47 pub struct UserRecord<'a> {
48 pub username: &'a [u8],
51 pub verifier: &'a [u8],
55 pub struct SrpServer<D: Digest> {
60 key: GenericArray<u8, D::OutputSize>,
65 impl<D: Digest> SrpServer<D> {
66 /// Create new server state.
68 user: &UserRecord<'_>,
72 ) -> Result<Self, SrpAuthError> {
73 let a_pub = BigUint::from_bytes_be(a_pub);
74 // Safeguard against malicious A
75 if &a_pub % ¶ms.n == BigUint::zero() {
76 return Err(SrpAuthError {
77 description: "Malicious a_pub value",
80 let v = BigUint::from_bytes_be(user.verifier);
81 let b = BigUint::from_bytes_be(b) % ¶ms.n;
82 let k = params.compute_k::<D>();
84 let interm = (k * &v) % ¶ms.n;
85 let b_pub = (interm + ¶ms.powm(&b)) % ¶ms.n;
89 d.input(&a_pub.to_bytes_be());
90 d.input(&b_pub.to_bytes_be());
93 let d = Default::default();
96 let u = BigUint::from_bytes_be(&u);
97 let t = (&a_pub * powm(&v, &u, ¶ms.n)) % ¶ms.n;
98 let s = powm(&t, &b, ¶ms.n);
99 D::digest(&s.to_bytes_be())
110 /// Get private `b` value. (see `new_with_b` documentation)
111 pub fn get_b(&self) -> Vec<u8> {
115 /// Get public `b_pub` value for sending to the user.
116 pub fn get_b_pub(&self) -> Vec<u8> {
117 self.b_pub.to_bytes_be()
120 /// Get shared secret between user and the server. (do not forget to verify
121 /// that keys are the same!)
122 pub fn get_key(&self) -> GenericArray<u8, D::OutputSize> {
126 /// Process user proof of having the same shared secret and compute
127 /// server proof for sending to the user.
131 ) -> Result<GenericArray<u8, D::OutputSize>, SrpAuthError> {
133 let mut d = D::new();
134 d.input(&self.a_pub.to_bytes_be());
135 d.input(&self.b_pub.to_bytes_be());
138 if user_proof == d.result().as_slice() {
140 let mut d = D::new();
141 d.input(&self.a_pub.to_bytes_be());
147 description: "Incorrect user proof",