CSRP implementation option

Author: feliwir

srp/src/server.rs

@@ -85,7 +85,8 @@ use subtle::ConstantTimeEq;
 
 use crate::types::{SrpAuthError, SrpGroup};
 use crate::utils::{
-    compute_hash, compute_k, compute_m1, compute_m1_rfc5054, compute_m2, compute_u,
+    compute_hash, compute_k, compute_k_nopad, compute_m1, compute_m1_csrp, compute_m1_rfc5054,
+    compute_m2, compute_u,
 };
 
 /// SRP server state
@@ -126,6 +127,13 @@ impl<'a, D: Digest> SrpServer<'a, D> {
         (inter + self.params.g.modpow(b, &self.params.n)) % &self.params.n
     }
 
+    //  k*v + g^b
+    #[must_use]
+    pub fn compute_b_pub_csrp(&self, b: &BigUint, k: &BigUint, v: &BigUint) -> BigUint {
+        let inter = k * v;
+        inter + self.params.g.modpow(b, &self.params.n)
+    }
+
     // <premaster secret> = (A * v^u) ^ b % N
     #[must_use]
     pub fn compute_premaster_secret(
@@ -151,6 +159,16 @@ impl<'a, D: Digest> SrpServer<'a, D> {
         .to_bytes_be()
     }
 
+    #[must_use]
+    pub fn compute_public_ephemeral_csrp(&self, b: &[u8], v: &[u8]) -> Vec<u8> {
+        self.compute_b_pub_csrp(
+            &BigUint::from_bytes_be(b),
+            &compute_k_nopad::<D>(self.params),
+            &BigUint::from_bytes_be(v),
+        )
+        .to_bytes_be()
+    }
+
     /// Process client reply to the handshake.
     /// b is a random value,
     /// v is the provided during initial user registration
@@ -240,6 +258,56 @@ impl<'a, D: Digest> SrpServer<'a, D> {
             session_key: session_key.to_vec(),
         })
     }
+
+    /// Process client reply to the handshake according to RFC 5054.
+    /// b is a random value,
+    /// v is the provided during initial user registration
+    pub fn process_reply_csrp(
+        &self,
+        username: &[u8],
+        salt: &[u8],
+        b: &[u8],
+        v: &[u8],
+        a_pub: &[u8],
+    ) -> Result<SrpServerVerifierRfc5054<D>, SrpAuthError> {
+        let b = BigUint::from_bytes_be(b);
+        let v = BigUint::from_bytes_be(v);
+        let a_pub = BigUint::from_bytes_be(a_pub);
+
+        let k = compute_k_nopad::<D>(self.params);
+        let b_pub = self.compute_b_pub_csrp(&b, &k, &v);
+
+        // Safeguard against malicious A
+        if &a_pub % &self.params.n == BigUint::default() {
+            return Err(SrpAuthError::IllegalParameter("a_pub".to_owned()));
+        }
+
+        let u = compute_u::<D>(&a_pub.to_bytes_be(), &b_pub.to_bytes_be());
+
+        let premaster_secret = self
+            .compute_premaster_secret(&a_pub, &v, &u, &b)
+            .to_bytes_be();
+
+        let session_key = compute_hash::<D>(&premaster_secret);
+
+        let m1 = compute_m1_csrp::<D>(
+            self.params,
+            username,
+            salt,
+            &a_pub.to_bytes_be(),
+            &b_pub.to_bytes_be(),
+            session_key.as_slice(),
+        );
+
+        let m2 = compute_m2::<D>(&a_pub.to_bytes_be(), &m1, session_key.as_slice());
+
+        Ok(SrpServerVerifierRfc5054 {
+            m1,
+            m2,
+            key: premaster_secret,
+            session_key: session_key.to_vec(),
+        })
+    }
 }
 
 impl<D: Digest> SrpServerVerifier<D> {

srp/src/utils.rs

@@ -27,6 +27,18 @@ pub fn compute_k<D: Digest>(params: &SrpGroup) -> BigUint {
     BigUint::from_bytes_be(d.finalize().as_slice())
 }
 
+// k = H(N | g)
+#[must_use]
+pub fn compute_k_nopad<D: Digest>(params: &SrpGroup) -> BigUint {
+    let n = params.n.to_bytes_be();
+    let g_bytes = params.g.to_bytes_be();
+
+    let mut d = D::new();
+    d.update(&n);
+    d.update(&g_bytes);
+    BigUint::from_bytes_be(d.finalize().as_slice())
+}
+
 // H(N) XOR H(PAD(g))
 #[must_use]
 pub fn compute_hash_n_xor_hash_g<D: Digest>(params: &SrpGroup) -> Vec<u8> {
@@ -45,6 +57,21 @@ pub fn compute_hash_n_xor_hash_g<D: Digest>(params: &SrpGroup) -> Vec<u8> {
         .collect()
 }
 
+// H(N) XOR H(g)
+#[must_use]
+pub fn compute_hash_n_xor_hash_g_nopad<D: Digest>(params: &SrpGroup) -> Vec<u8> {
+    let n = params.n.to_bytes_be();
+    let g_bytes = params.g.to_bytes_be();
+
+    let h_n = compute_hash::<D>(&n).to_vec();
+    let h_g = compute_hash::<D>(&g_bytes).to_vec();
+
+    h_n.iter()
+        .zip(h_g.iter())
+        .map(|(&x1, &x2)| x1 ^ x2)
+        .collect()
+}
+
 // M1 = H(A, B, K) this doesn't follow the spec but apparently no one does for M1
 #[must_use]
 pub fn compute_m1<D: Digest>(a_pub: &[u8], b_pub: &[u8], key: &[u8]) -> Output<D> {
@@ -82,6 +109,26 @@ pub fn compute_m1_rfc5054<D: Digest>(
     d.finalize()
 }
 
+// M1 = H(H(N) XOR H(g) | H(U) | s | A | B | K) following CSRP legacy mode
+#[must_use]
+pub fn compute_m1_csrp<D: Digest>(
+    params: &SrpGroup,
+    username: &[u8],
+    salt: &[u8],
+    a_pub: &[u8],
+    b_pub: &[u8],
+    key: &[u8],
+) -> Output<D> {
+    let mut d = D::new();
+    d.update(compute_hash_n_xor_hash_g_nopad::<D>(params));
+    d.update(compute_hash::<D>(username));
+    d.update(salt);
+    d.update(a_pub);
+    d.update(b_pub);
+    d.update(key);
+    d.finalize()
+}
+
 // M2 = H(A, M1, K)
 #[must_use]
 pub fn compute_m2<D: Digest>(a_pub: &[u8], m1: &Output<D>, key: &[u8]) -> Output<D> {