srp: rebuild library (#79)

Complete rewrite of the SRP library.

Includes many improvements over the old library:

- Improved file and code organization
- Access to individual SRP computations 
- Consistent sever and client API
- Simpler API
- Improved documentation with tests in documentation
- New tests for compatibility with the RFC
- Bumps dependencies
- Timing safe verification comparisons
- Modernized error handling 

1 files changed, 45 insertions, 0 deletions

diff --git a/srp/src/utils.rs b/srp/src/utils.rs
new file mode 100644
index 0000000..a9372bd
--- /dev/null
+++ b/srp/src/utils.rs
@@ -0,0 +1,45 @@
+use digest::{Digest, Output};
+use num_bigint::BigUint;
+
+use crate::types::SrpGroup;
+
+// u = H(PAD(A) | PAD(B))
+pub fn compute_u<D: Digest>(a_pub: &[u8], b_pub: &[u8]) -> BigUint {
+    let mut u = D::new();
+    u.update(a_pub);
+    u.update(b_pub);
+    BigUint::from_bytes_be(&u.finalize())
+}
+
+// k = H(N | PAD(g))
+pub fn compute_k<D: Digest>(params: &SrpGroup) -> BigUint {
+    let n = params.n.to_bytes_be();
+    let g_bytes = params.g.to_bytes_be();
+    let mut buf = vec![0u8; n.len()];
+    let l = n.len() - g_bytes.len();
+    buf[l..].copy_from_slice(&g_bytes);
+
+    let mut d = D::new();
+    d.update(&n);
+    d.update(&buf);
+    BigUint::from_bytes_be(d.finalize().as_slice())
+}
+
+// M1 = H(A, B, K) this doesn't follow the spec but apparently no one does for M1
+// M1 should equal =  H(H(N) XOR H(g) | H(U) | s | A | B | K) according to the spec
+pub fn compute_m1<D: Digest>(a_pub: &[u8], b_pub: &[u8], key: &[u8]) -> Output<D> {
+    let mut d = D::new();
+    d.update(a_pub);
+    d.update(b_pub);
+    d.update(key);
+    d.finalize()
+}
+
+// M2 = H(A, M1, K)
+pub fn compute_m2<D: Digest>(a_pub: &[u8], m1: &Output<D>, key: &[u8]) -> Output<D> {
+    let mut d = D::new();
+    d.update(&a_pub);
+    d.update(&m1);
+    d.update(&key);
+    d.finalize()
+}