First commit

13 files changed, 826 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..6aa1064
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+/target/
+**/*.rs.bk
+Cargo.lock
diff --git a/.travis.yml b/.travis.yml
new file mode 100644
index 0000000..7f3fa40
--- /dev/null
+++ b/.travis.yml
@@ -0,0 +1,9 @@
+language: rust
+rust:
+  - stable
+  - beta
+  - nightly
+matrix:
+  allow_failures:
+    - rust: nightly
+script: cargo test --release --verbose
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..a176edd
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,13 @@
+[package]
+name = "srp"
+version = "0.1.0"
+authors = ["RustCrypto Developers"]
+
+[dependencies]
+num = "*"
+generic-array = "*"
+rand = "0.3"
+digest = "*"
+
+[dev-dependencies]
+sha2 = "*"
diff --git a/LICENSE-APACHE b/LICENSE-APACHE
new file mode 100644
index 0000000..78173fa
--- /dev/null
+++ b/LICENSE-APACHE
@@ -0,0 +1,201 @@
+                              Apache License
+                        Version 2.0, January 2004
+                     http://www.apache.org/licenses/
+
+TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+1. Definitions.
+
+   "License" shall mean the terms and conditions for use, reproduction,
+   and distribution as defined by Sections 1 through 9 of this document.
+
+   "Licensor" shall mean the copyright owner or entity authorized by
+   the copyright owner that is granting the License.
+
+   "Legal Entity" shall mean the union of the acting entity and all
+   other entities that control, are controlled by, or are under common
+   control with that entity. For the purposes of this definition,
+   "control" means (i) the power, direct or indirect, to cause the
+   direction or management of such entity, whether by contract or
+   otherwise, or (ii) ownership of fifty percent (50%) or more of the
+   outstanding shares, or (iii) beneficial ownership of such entity.
+
+   "You" (or "Your") shall mean an individual or Legal Entity
+   exercising permissions granted by this License.
+
+   "Source" form shall mean the preferred form for making modifications,
+   including but not limited to software source code, documentation
+   source, and configuration files.
+
+   "Object" form shall mean any form resulting from mechanical
+   transformation or translation of a Source form, including but
+   not limited to compiled object code, generated documentation,
+   and conversions to other media types.
+
+   "Work" shall mean the work of authorship, whether in Source or
+   Object form, made available under the License, as indicated by a
+   copyright notice that is included in or attached to the work
+   (an example is provided in the Appendix below).
+
+   "Derivative Works" shall mean any work, whether in Source or Object
+   form, that is based on (or derived from) the Work and for which the
+   editorial revisions, annotations, elaborations, or other modifications
+   represent, as a whole, an original work of authorship. For the purposes
+   of this License, Derivative Works shall not include works that remain
+   separable from, or merely link (or bind by name) to the interfaces of,
+   the Work and Derivative Works thereof.
+
+   "Contribution" shall mean any work of authorship, including
+   the original version of the Work and any modifications or additions
+   to that Work or Derivative Works thereof, that is intentionally
+   submitted to Licensor for inclusion in the Work by the copyright owner
+   or by an individual or Legal Entity authorized to submit on behalf of
+   the copyright owner. For the purposes of this definition, "submitted"
+   means any form of electronic, verbal, or written communication sent
+   to the Licensor or its representatives, including but not limited to
+   communication on electronic mailing lists, source code control systems,
+   and issue tracking systems that are managed by, or on behalf of, the
+   Licensor for the purpose of discussing and improving the Work, but
+   excluding communication that is conspicuously marked or otherwise
+   designated in writing by the copyright owner as "Not a Contribution."
+
+   "Contributor" shall mean Licensor and any individual or Legal Entity
+   on behalf of whom a Contribution has been received by Licensor and
+   subsequently incorporated within the Work.
+
+2. Grant of Copyright License. Subject to the terms and conditions of
+   this License, each Contributor hereby grants to You a perpetual,
+   worldwide, non-exclusive, no-charge, royalty-free, irrevocable
+   copyright license to reproduce, prepare Derivative Works of,
+   publicly display, publicly perform, sublicense, and distribute the
+   Work and such Derivative Works in Source or Object form.
+
+3. Grant of Patent License. Subject to the terms and conditions of
+   this License, each Contributor hereby grants to You a perpetual,
+   worldwide, non-exclusive, no-charge, royalty-free, irrevocable
+   (except as stated in this section) patent license to make, have made,
+   use, offer to sell, sell, import, and otherwise transfer the Work,
+   where such license applies only to those patent claims licensable
+   by such Contributor that are necessarily infringed by their
+   Contribution(s) alone or by combination of their Contribution(s)
+   with the Work to which such Contribution(s) was submitted. If You
+   institute patent litigation against any entity (including a
+   cross-claim or counterclaim in a lawsuit) alleging that the Work
+   or a Contribution incorporated within the Work constitutes direct
+   or contributory patent infringement, then any patent licenses
+   granted to You under this License for that Work shall terminate
+   as of the date such litigation is filed.
+
+4. Redistribution. You may reproduce and distribute copies of the
+   Work or Derivative Works thereof in any medium, with or without
+   modifications, and in Source or Object form, provided that You
+   meet the following conditions:
+
+   (a) You must give any other recipients of the Work or
+       Derivative Works a copy of this License; and
+
+   (b) You must cause any modified files to carry prominent notices
+       stating that You changed the files; and
+
+   (c) You must retain, in the Source form of any Derivative Works
+       that You distribute, all copyright, patent, trademark, and
+       attribution notices from the Source form of the Work,
+       excluding those notices that do not pertain to any part of
+       the Derivative Works; and
+
+   (d) If the Work includes a "NOTICE" text file as part of its
+       distribution, then any Derivative Works that You distribute must
+       include a readable copy of the attribution notices contained
+       within such NOTICE file, excluding those notices that do not
+       pertain to any part of the Derivative Works, in at least one
+       of the following places: within a NOTICE text file distributed
+       as part of the Derivative Works; within the Source form or
+       documentation, if provided along with the Derivative Works; or,
+       within a display generated by the Derivative Works, if and
+       wherever such third-party notices normally appear. The contents
+       of the NOTICE file are for informational purposes only and
+       do not modify the License. You may add Your own attribution
+       notices within Derivative Works that You distribute, alongside
+       or as an addendum to the NOTICE text from the Work, provided
+       that such additional attribution notices cannot be construed
+       as modifying the License.
+
+   You may add Your own copyright statement to Your modifications and
+   may provide additional or different license terms and conditions
+   for use, reproduction, or distribution of Your modifications, or
+   for any such Derivative Works as a whole, provided Your use,
+   reproduction, and distribution of the Work otherwise complies with
+   the conditions stated in this License.
+
+5. Submission of Contributions. Unless You explicitly state otherwise,
+   any Contribution intentionally submitted for inclusion in the Work
+   by You to the Licensor shall be under the terms and conditions of
+   this License, without any additional terms or conditions.
+   Notwithstanding the above, nothing herein shall supersede or modify
+   the terms of any separate license agreement you may have executed
+   with Licensor regarding such Contributions.
+
+6. Trademarks. This License does not grant permission to use the trade
+   names, trademarks, service marks, or product names of the Licensor,
+   except as required for reasonable and customary use in describing the
+   origin of the Work and reproducing the content of the NOTICE file.
+
+7. Disclaimer of Warranty. Unless required by applicable law or
+   agreed to in writing, Licensor provides the Work (and each
+   Contributor provides its Contributions) on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+   implied, including, without limitation, any warranties or conditions
+   of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A
+   PARTICULAR PURPOSE. You are solely responsible for determining the
+   appropriateness of using or redistributing the Work and assume any
+   risks associated with Your exercise of permissions under this License.
+
+8. Limitation of Liability. In no event and under no legal theory,
+   whether in tort (including negligence), contract, or otherwise,
+   unless required by applicable law (such as deliberate and grossly
+   negligent acts) or agreed to in writing, shall any Contributor be
+   liable to You for damages, including any direct, indirect, special,
+   incidental, or consequential damages of any character arising as a
+   result of this License or out of the use or inability to use the
+   Work (including but not limited to damages for loss of goodwill,
+   work stoppage, computer failure or malfunction, or any and all
+   other commercial damages or losses), even if such Contributor
+   has been advised of the possibility of such damages.
+
+9. Accepting Warranty or Additional Liability. While redistributing
+   the Work or Derivative Works thereof, You may choose to offer,
+   and charge a fee for, acceptance of support, warranty, indemnity,
+   or other liability obligations and/or rights consistent with this
+   License. However, in accepting such obligations, You may act only
+   on Your own behalf and on Your sole responsibility, not on behalf
+   of any other Contributor, and only if You agree to indemnify,
+   defend, and hold each Contributor harmless for any liability
+   incurred by, or claims asserted against, such Contributor by reason
+   of your accepting any such warranty or additional liability.
+
+END OF TERMS AND CONDITIONS
+
+APPENDIX: How to apply the Apache License to your work.
+
+   To apply the Apache License to your work, attach the following
+   boilerplate notice, with the fields enclosed by brackets "[]"
+   replaced with your own identifying information. (Don't include
+   the brackets!)  The text should be enclosed in the appropriate
+   comment syntax for the file format. We also recommend that a
+   file or class name and description of purpose be included on the
+   same "printed page" as the copyright notice for easier
+   identification within third-party archives.
+
+Copyright [yyyy] [name of copyright owner]
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
diff --git a/LICENSE-MIT b/LICENSE-MIT
new file mode 100644
index 0000000..8dcb85b
--- /dev/null
+++ b/LICENSE-MIT
@@ -0,0 +1,25 @@
+Copyright (c) 2017 Artyom Pavlov
+
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..d0225ae
--- /dev/null
+++ b/README.md
@@ -0,0 +1,34 @@
+# SRP [![Build Status](https://travis-ci.org/RustCrypto/SRP.svg?branch=master)](https://travis-ci.org/RustCrypto/SRP) [![crates.io](https://img.shields.io/crates/v/srp.svg)](https://crates.io/crates/srp) [![Documentation](https://docs.rs/srp/badge.svg)](https://docs.rs/srp)
+[Secure Remote Password][1] (SRP) protocol implementation.
+
+This implementation uses little-endian representation of big integers and is
+generic over hash functions using [`Digest`][2] trait, so you will need to
+choose a hash function, e.g. `Sha256` from [`sha2`][3] crate. Additionally this
+crate allows to use a specialized password hashing algorithms for private key
+computation instead of method described in the SRP literature.
+
+Currently compatability with over implementations was not tested.
+
+## Warnings
+
+This crate have not yet received any formal cryptographic and security reviews.
+
+No efforts were yet taken in regards of [blinding][4] or erasing secrets from
+the memory.
+
+**USE AT YOUR OWN RISK.**
+
+## License
+
+This crate licensed under either of
+
+ * [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0)
+ * [MIT license](http://opensource.org/licenses/MIT)
+
+at your option.
+
+
+[1]: https://en.wikipedia.org/wiki/Secure_Remote_Password_protocol
+[2]: https://crates.io/crates/digest
+[3]: https://crates.io/crates/sha2
+[4]: https://en.wikipedia.org/wiki/Blinding_(cryptography)
\ No newline at end of file
diff --git a/src/client.rs b/src/client.rs
new file mode 100644
index 0000000..ee19e88
--- /dev/null
+++ b/src/client.rs
@@ -0,0 +1,211 @@
+//! SRP client implementation.
+//! 
+//! # Usage
+//! First create SRP client struct by passing to it SRP parameters (shared
+//! between client and server) and RNG instance (OS RNG is recommended):
+//! 
+//! ```ignore
+//! let srp_params = SrpParams{n, g, k};
+//! let mut rng = rand::os::OsRng::new().unwrap();
+//! let client = SrpClient::<Sha256>::new(&srp_params, &mut rng);
+//! ```
+//! 
+//! Next send handshake data (username and `a_pub`) to the server and receive
+//! `salt` and `b_pub`:
+//! 
+//! ```ignore
+//! let a_pub = client.get_a_pub();
+//! let (salt, b_pub) = conn.send_handshake(username, a_pub);
+//! ```
+//! 
+//! Compute private key using `salt` with any password hashing function.
+//! You can use method from SRP-6a, but it's recommended to use specialized
+//! password hashing algorithm instead (e.g. PBKDF2, argon2 or scrypt).
+//! Next create verifier instance, note that `get_verifier` consumes client and
+//! can return error in case of malicious `b_pub`.
+//! 
+//! ```ignore
+//! let private_key = srp6a_private_key::<Sha256>(username, password, salt);
+//! let verifier = client.get_verifier(&private_key, &b_pub)?;
+//! ```
+//!
+//! Finally verify the server: first generate user proof,
+//! send it to the server and verify server proof in the reply. Note that
+//! `verify_server` method will return error in case of incorrect server reply.
+//! 
+//! ```ignore
+//! let user_proof = verifier.get_proof();
+//! let server_proof = conn.send_proof(user_proof);
+//! let key = verifier.verify_server(server_proof)?;
+//! ```
+//! 
+//! `key` contains shared secret key between user and the server. Alternatively
+//! you can directly extract shared secret key using `get_key()` method and
+//! handle authentification through different (secure!) means (e.g. by using
+//! authentificated cipher mode).
+//! 
+//! For user registration on the server first generate salt (e.g. 32 bytes long)
+//! and get password verifier which depends on private key. Send useranme, salt
+//! and password verifier over protected channel to protect against MitM for
+//! registration.
+//! 
+//! ```ignore
+//! let pwd_verifier = SrpClient::<Sha256>::register(&private_key, &srp_params);
+//! conn.send_registration_data(username, salt, pwd_verifier);
+//! ```
+use std::marker::PhantomData;
+
+use rand::Rng;
+use num::{BigUint, Zero};
+use digest::Digest;
+use generic_array::GenericArray;
+
+use tools::powm;
+use types::{SrpAuthError, SrpParams};
+
+/// SRP client state before handshake with the server.
+pub struct SrpClient<'a, D: Digest> {
+    params: &'a SrpParams,
+
+    a: BigUint,
+    a_pub: BigUint,
+
+    d: PhantomData<D>
+}
+
+/// SRP client state after handshake with the server.
+pub struct SrpClientVerifier<D: Digest> {
+    proof: GenericArray<u8, D::OutputSize>,
+    server_proof: GenericArray<u8, D::OutputSize>,
+    key: GenericArray<u8, D::OutputSize>,
+}
+
+/// Compute user private key as described in the SRP6a. Consider using proper
+/// password hashing algorithm instead.
+pub fn srp6a_private_key<D: Digest>(username: &[u8], password: &[u8],
+        salt: &[u8]
+    ) -> GenericArray<u8, D::OutputSize>
+{
+    let p = {
+        let mut d = D::new();
+        d.input(username);
+        d.input(b":");
+        d.input(password);
+        d.result()
+    };
+    let mut d = D::new();
+    d.input(salt);
+    d.input(&p);
+    d.result()
+}
+
+impl<'a, D: Digest> SrpClient<'a, D> {
+    /// Create new SRP client instance.
+    pub fn new<R: Rng>(params: &'a SrpParams, rng: &mut R) -> Self {
+        let l = params.n.bits()/8; 
+        let buf = rng.gen_iter::<u8>().take(l).collect::<Vec<u8>>();
+        let a = BigUint::from_bytes_le(&buf);
+        let a_pub = params.powm(&a);
+
+        Self { params, a, a_pub, d: Default::default() }
+    }
+
+    /// Get password verfier for user registration on the server
+    pub fn get_password_verifier(&self, private_key: &[u8]) -> Vec<u8> {
+        let x = BigUint::from_bytes_le(&private_key);
+        let v = self.params.powm(&x);
+        v.to_bytes_le()
+    }
+
+    fn calc_key(&self, b_pub: &BigUint, x: &BigUint, u: &BigUint)
+        -> GenericArray<u8, D::OutputSize>
+    {
+        let n = &self.params.n;
+        let interm = (&self.params.k * self.params.powm(x)) % n;
+        // Because we do operation in modulo N we can get: (kv + g^b) < kv
+        let v = if b_pub > &interm {
+            (b_pub - &interm) % n
+        } else {
+            (n + b_pub - &interm) % n
+        };
+        // S = |B - kg^x| ^ (a + ux)
+        let s = powm(&v, &(&self.a + (u*x) % n ), n);
+        D::digest(&s.to_bytes_le())
+    }
+
+    /// Process server reply to the handshake.
+    pub fn process_reply(self, private_key: &[u8], b_pub: &[u8])
+        -> Result<SrpClientVerifier<D>, SrpAuthError>
+    {
+        let u = {
+            let mut d = D::new();
+            d.input(&self.a_pub.to_bytes_le());
+            d.input(b_pub);
+            BigUint::from_bytes_le(&d.result())
+        };
+
+        let b_pub = BigUint::from_bytes_le(b_pub);
+
+        // Safeguard against malicious B
+        if &b_pub % &self.params.n == BigUint::zero() {
+            return Err(SrpAuthError{ description: "Malicious b_pub value" })
+        }
+
+        let x = BigUint::from_bytes_le(&private_key);
+        let key = self.calc_key(&b_pub, &x, &u);
+        // M1 = H(A, B, K)
+        let proof = {
+            let mut d = D::new();
+            d.input(&self.a_pub.to_bytes_le());
+            d.input(&b_pub.to_bytes_le());
+            d.input(&key);
+            d.result()
+        };
+
+        // M2 = H(A, M1, K)
+        let server_proof = {
+            let mut d = D::new();
+            d.input(&self.a_pub.to_bytes_le());
+            d.input(&proof);
+            d.input(&key);
+            d.result()
+        };
+
+        Ok(SrpClientVerifier {
+            proof: proof,
+            server_proof: server_proof,
+            key: key,
+        })
+    }
+
+    /// Get public ephemeral value for handshaking with the server.
+    pub fn get_a_pub(&self) -> Vec<u8> {
+        self.a_pub.to_bytes_le()
+    }
+}
+
+impl<D: Digest> SrpClientVerifier<D> {
+    /// Get shared secret key without authenticating server, e.g. for using with
+    /// authenticated encryption modes. DO NOT USE this method without
+    /// some kind of secure authentification.
+    pub fn get_key(self) -> GenericArray<u8, D::OutputSize> {
+        self.key
+    }
+
+    /// Verification data for sending to the server.
+    pub fn get_proof(&self) -> GenericArray<u8, D::OutputSize> {
+        self.proof.clone()
+    }
+
+    /// Verify server reply to verification data. It will return shared secret
+    /// key in case of success.
+    pub fn verify_server(self, reply: &[u8])
+        -> Result<GenericArray<u8, D::OutputSize>, SrpAuthError>
+    {
+        if self.server_proof.as_slice() != reply {
+            Err(SrpAuthError{ description: "Incorrect server proof" })
+        } else {
+            Ok(self.key)
+        }
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..9401514
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,59 @@
+//! [Secure Remote Password][1] (SRP) protocol implementation.
+//! 
+//! This implementation uses little-endian serialization of big integers and is
+//! generic over hash functions using `Digest` trait, so you will need to choose
+//! a hash function, e.g. `Sha256` from `sha2` crate. Additionally this crate
+//! allows to use a specialized password hashing algorithms for private key
+//! computation instead of method described in the SRP literature.
+//! 
+//! Currently compatability with over implementations was not tested.
+//! 
+//! # Algorithm description
+//! Here we briefly describe implemented algroithm. For additionall information
+//! refer to SRP literature. All arithmetic is done modulo `N`, where `N` is a 
+//! large safe prime (`N = 2q+1`, where `q` is prime).
+//! 
+//! Client |       | Server
+//! -------|-------|--------
+//!                         |    — `I` —>   | (lookup `s`, `v`)
+//! `x = PH(P, s)`          | <— `s`, `v` — | 
+//! `a_pub = g^a`           |  — `a_pub` —> | `b_pub = k*v + g^b`
+//! `u = H(a_pub || b_pub)` |  <— `b_pub` — | `u = H(a_pub || b_pub)`
+//! `s = (b_pub - k*g^x)^(a+u*x)` |         | `S = (b_pub - k*g^x)^(a+u*x)`
+//! `K = H(s)`              |               | `K = H(s)`
+//! `M1 = H(A || B || K)`   |   — `M1` —>   | (verify `M1`)
+//! (verify `M2`)           |   <— `M2` —   | `M2 = H(A || M1 || K)`
+//! 
+//! `||` denotes concatenation, variables and notations have the following
+//! meaning:
+//! 
+//! - `I` — user identity (username)
+//! - `P` — user password
+//! - `H` — one-way hash function
+//! - `HP` — password hashing algroithm, in the SRP described as
+//! `H(s || I || P)` or `H(s || P)`
+//! - `^` — (modular) exponentiation
+//! - `x` — user private key
+//! - `s` — salt generated by user and stored on the server
+//! - `v` — password verifier equal to `g^x` and stored on the server
+//! - `a`, `b` — secret ephemeral values
+//! - `A`, `B` — Public ephemeral values
+//! - `u` — scrambling parameter
+//! - `g` — a generator modulo `N`
+//! - `k` — multiplier parameter (`k = H(N || g)` in SRP-6a)
+//!
+//! [1]: https://en.wikipedia.org/wiki/Secure_Remote_Password_protocol
+extern crate num;
+extern crate digest;
+extern crate generic_array;
+extern crate rand;
+
+mod tools;
+pub mod client;
+pub mod server;
+pub mod types;
+
+/// 1024 bit prime number which can be used as `n` in the `SrpParams`.
+///
+/// For conversion to `BigUint` use `BigUint::from_bytes_le(SRP_PRIME)`.
+pub const PRIME: &'static [u8] = include_bytes!("prime.bin");
diff --git a/src/prime.bin b/src/prime.bin
new file mode 100644
index 0000000..d2109b7
--- /dev/null
+++ b/src/prime.bin
@@ -0,0 +1 @@
+�_�P<���=�èyCp^	>H�h�w
��B��b{+2,Z�u��HEX����r�Ӌt���o �'��Tw����;��䪗�D��l'$����2�/A�-�t�c��K2�-懘2��u�7�
\ No newline at end of file
diff --git a/src/server.rs b/src/server.rs
new file mode 100644
index 0000000..e82ebf6
--- /dev/null
+++ b/src/server.rs
@@ -0,0 +1,147 @@
+//! SRP server implementation
+//!
+//! # Usage
+//! First receive user's username and public value `a_pub`, retrieve from a
+//! database `UserRecord` for a given username and initialize SRP server state:
+//! 
+//! ```ignore
+//! let mut rng = rand::os::OsRng::new().unwrap();
+//! let (username, a_pub) = conn.receive_handshake();
+//! let user = db.retrieve_user_record(username);
+//! let server = SrpServer::<Sha256>::new(&user, &a_pub, &srp_params, &mut rng)?;
+//! ```
+//! 
+//! Next send to user `b_pub` and `salt` from user record:
+//! 
+//! ```ignore
+//! let b_pub = server.get_b_pub();
+//! conn.reply_to_handshake(&user.salt, b_pub);
+//! ```
+//! 
+//! And finally recieve user proof, verify it and send server proof as reply:
+//! 
+//! ```ignore
+//! let user_proof = conn.receive_proof();
+//! let server_proof = server.verify(user_proof)?;
+//! conn.send_proof(server_proof);
+//! ```
+//! 
+//! To get the shared secret use `get_key` method. As alternative to using
+//! `verify` method it's also possible to use this key for authentificated
+//! encryption.
+use std::marker::PhantomData;
+
+use rand::Rng;
+use num::{BigUint, Zero};
+use digest::Digest;
+use generic_array::GenericArray;
+
+use tools::powm;
+use types::{SrpAuthError, SrpParams};
+
+/// Data provided by users upon registration, usually stored in the database.
+pub struct UserRecord<'a> {
+    pub username: &'a [u8],
+    pub salt: &'a [u8],
+    /// Password verifier
+    pub verifier: &'a [u8],
+}
+
+/// SRP server state
+pub struct SrpServer<D: Digest> {
+    b: BigUint,
+    a_pub: BigUint,
+    b_pub: BigUint,
+
+    key: GenericArray<u8, D::OutputSize>,
+
+    d: PhantomData<D>
+}
+
+impl< D: Digest> SrpServer< D> {
+    /// Create new server state with randomly generated `b`.
+    pub fn new<R: Rng>(user: &UserRecord, a_pub: &[u8], params: &SrpParams,
+            rng: &mut R)
+        -> Result<Self, SrpAuthError>
+    {
+        let l = params.n.bits()/8; 
+        let b = rng.gen_iter::<u8>().take(l).collect::<Vec<u8>>();
+        Self::new_with_b(user, a_pub, &b, params)
+    }
+
+    /// Create new server state with given `b`.
+    ///
+    /// Usefull if it's not convenient to keep `SrpServer` state between
+    /// handshake and verification steps. (e.g. when working over HTTP and
+    /// storing `b` in a database)
+    pub fn new_with_b(user: &UserRecord, a_pub: &[u8], b: &[u8],
+                params: &SrpParams)
+        -> Result<Self, SrpAuthError>
+    {
+        let a_pub = BigUint::from_bytes_le(a_pub);
+        // Safeguard against malicious A
+        if &a_pub % &params.n == BigUint::zero() {
+            return Err(SrpAuthError { description: "Malicious a_pub value" })
+        }
+        let v = BigUint::from_bytes_le(user.verifier);
+        let b = BigUint::from_bytes_le(b)  % &params.n;
+        // kv + g^b
+        let interm = (&params.k * &v) % &params.n;
+        let b_pub = (interm + &params.powm(&b)) % &params.n;
+        // H(A || B)
+        let u = {
+            let mut d = D::new();
+            d.input(&a_pub.to_bytes_le());
+            d.input(&b_pub.to_bytes_le());
+            d.result()
+        };
+        let d = Default::default();
+        //(Av^u) ^ b
+        let key = {
+            let u =  BigUint::from_bytes_le(&u);
+            let t = (&a_pub * powm(&v, &u, &params.n)) % &params.n;
+            let s = powm(&t, &b, &params.n);
+            D::digest(&s.to_bytes_le())
+        };
+        Ok(Self { b, a_pub, b_pub, key, d})
+    }
+
+    /// Get private `b` value. (see `new_with_b` documentation)
+    pub fn get_b(&self) -> Vec<u8> {
+        self.b.to_bytes_le()
+    }
+
+    /// Get public `b_pub` value for sending to the user.
+    pub fn get_b_pub(&self) -> Vec<u8> {
+        self.b_pub.to_bytes_le()
+    }
+
+    /// Get shared secret between user and the server. (do not forget to verify
+    /// that keys are the same!)
+    pub fn get_key(&self) -> GenericArray<u8, D::OutputSize> {
+        self.key.clone()
+    }
+
+    /// Process user proof of having the same shared secret and compute
+    /// server proof for sending to the user.
+    pub fn verify(&self, user_proof: &[u8])
+        -> Result<GenericArray<u8, D::OutputSize>, SrpAuthError>
+    {
+        // M = H(A, B, K)
+        let mut d = D::new();
+        d.input(&self.a_pub.to_bytes_le());
+        d.input(&self.b_pub.to_bytes_le());
+        d.input(&self.key);
+
+        if user_proof == d.result().as_slice() {
+            // H(A, M, K)
+            let mut d = D::new();
+            d.input(&self.a_pub.to_bytes_le());
+            d.input(user_proof);
+            d.input(&self.key);
+            Ok(d.result())
+        } else {
+            Err(SrpAuthError { description: "Incorrect user proof" })
+        }
+    }
+}
diff --git a/src/tools.rs b/src/tools.rs
new file mode 100644
index 0000000..8cb6910
--- /dev/null
+++ b/src/tools.rs
@@ -0,0 +1,19 @@
+use num::BigUint;
+
+pub fn powm(base: &BigUint, exp: &BigUint, modulus: &BigUint) -> BigUint {
+    let zero = BigUint::new(vec![0]);
+    let one = BigUint::new(vec![1]);
+    let two = BigUint::new(vec![2]);
+    let mut exp = exp.clone();
+    let mut result = one.clone();
+    let mut base = base % modulus;
+
+    while exp > zero {
+        if &exp % &two == one {
+            result = (result * &base) % modulus;
+        }
+        exp = exp >> 1;
+        base = (&base * &base) % modulus;
+    }
+    result
+}
diff --git a/src/types.rs b/src/types.rs
new file mode 100644
index 0000000..aa8a0dc
--- /dev/null
+++ b/src/types.rs
@@ -0,0 +1,39 @@
+//! Additional SRP types.
+use std::{fmt, error};
+use num::BigUint;
+use tools::powm;
+
+/// SRP authentification error.
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub struct SrpAuthError {
+    pub(crate) description: &'static str
+}
+
+impl fmt::Display for SrpAuthError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "SRP authentification error")
+    }
+}
+
+impl error::Error for SrpAuthError {
+    fn description(&self) -> &str {
+        self.description
+    }
+}
+
+/// Parameters of SRP shared between client and server.
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct SrpParams {
+    /// A large safe prime (N = 2q+1, where q is prime)
+    pub n: BigUint,
+    /// A generator modulo N (e.g. 2)
+    pub g: BigUint,
+    /// Multiplier parameter (k = H(N, g) in SRP-6a, k = 3 for legacy SRP-6)
+    pub k: BigUint,
+}
+
+impl SrpParams {
+    pub(crate) fn powm(&self, v: &BigUint) -> BigUint {
+        powm(&self.g, v, &self.n)
+    }
+}
diff --git a/tests/mod.rs b/tests/mod.rs
new file mode 100644
index 0000000..f7810e8
--- /dev/null
+++ b/tests/mod.rs
@@ -0,0 +1,65 @@
+extern crate num;
+extern crate sha2;
+extern crate rand;
+extern crate srp;
+
+use num::BigUint;
+use sha2::Sha256;
+use rand::Rng;
+
+use srp::types::SrpParams;
+use srp::client::{ SrpClient, srp6a_private_key };
+use srp::server::{SrpServer, UserRecord};
+
+fn auth_test(reg_pwd: &[u8], auth_pwd: &[u8]) {
+    let mut rng = rand::os::OsRng::new().unwrap();
+    let username = "john".as_bytes();
+    let srp_params = SrpParams{
+        n: BigUint::from_bytes_le(srp::PRIME),
+        k: BigUint::from_bytes_be(&[1, 2, 3]),
+        g: BigUint::new(vec![2]),
+    };
+
+    // Client instance creation
+    let client = SrpClient::<Sha256>::new(&srp_params, &mut rng);
+
+    // Registration
+    let salt: [u8; 16] = rng.gen();
+    let reg_priv_key = srp6a_private_key::<Sha256>(username, reg_pwd, &salt);
+    let verif = client.get_password_verifier(&reg_priv_key);
+
+    // User sends handshake
+    let a_pub = client.get_a_pub();
+
+    // Server retrieve user record from db and processes handshake
+    let user = UserRecord { username, salt: &salt, verifier: &verif };
+    let server = SrpServer::<Sha256>::new(&user, &a_pub, &srp_params, &mut rng)
+        .unwrap();
+    let (salt, b_pub) = (&user.salt, server.get_b_pub());
+
+    // Client processes handshake reply
+    let auth_priv_key = srp6a_private_key::<Sha256>(username, auth_pwd, &salt);
+    let client2 = client.process_reply(&auth_priv_key, &b_pub).unwrap();
+    let proof = client2.get_proof();
+
+    // Server processes verification data
+    println!("Client verification");
+    let proof2 = server.verify(&proof).unwrap();
+    let server_key = server.get_key();
+
+    // Client verifies server
+    println!("Server verification");
+    let user_key = client2.verify_server(&proof2).unwrap();
+    assert_eq!(server_key, user_key, "server and client keys are not equal");
+}
+
+#[test]
+fn good_password() {
+    auth_test("password".as_bytes(), "password".as_bytes());
+}
+
+#[test]
+#[should_panic]
+fn bad_password() {
+    auth_test("password".as_bytes(), "paSsword".as_bytes());
+}