From f613d9d03211efed74dcc9e5674f7eb8d9a94325 Mon Sep 17 00:00:00 2001
From: siddharth <s@ricketyspace.net>
Date: Sun, 10 Apr 2022 12:30:20 -0400
Subject: lib: add srp scrambling parameter functions

---
 lib/srp.go      | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++
 lib/srp_test.go | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 124 insertions(+)

diff --git a/lib/srp.go b/lib/srp.go
index 05b61ab..2303793 100644
--- a/lib/srp.go
+++ b/lib/srp.go
@@ -170,6 +170,35 @@ func (u *SRPUser) EphemeralKeyPub() (*big.Int, error) {
 	return pub, nil
 }
 
+func (u *SRPUser) SetScramblingParam(a *big.Int) error {
+	b, err := u.EphemeralKeyPub()
+	if err != nil {
+		return err
+	}
+	bb := b.Bytes()
+	ab := a.Bytes()
+
+	// Make M=A+B
+	m := make([]byte, 0)
+	m = append(m, ab...)
+	m = append(m, bb...)
+	if len(m) != (len(ab) + len(bb)) {
+		return CPError{"length of m is incorrect"}
+	}
+
+	// Hash M
+	u.h.Message(m)
+	h := u.h.Hash()
+
+	// Set scrambling paramter u
+	u.u = new(big.Int)
+	u.u.SetBytes(h)
+	if u.u.Cmp(big.NewInt(0)) != 1 {
+		return CPError{"u is invalid"}
+	}
+	return nil
+}
+
 func NewSRPClientSession(n, g, k, ident string) (*SRPClientSession, error) {
 	var ok bool
 
@@ -212,3 +241,32 @@ func (s *SRPClientSession) EphemeralKeyPub() (*big.Int, error) {
 
 	return pub, nil
 }
+
+func (s *SRPClientSession) SetScramblingParam(b *big.Int) error {
+	a, err := s.EphemeralKeyPub()
+	if err != nil {
+		return err
+	}
+	ab := a.Bytes()
+	bb := b.Bytes()
+
+	// Make M=A+B
+	m := make([]byte, 0)
+	m = append(m, ab...)
+	m = append(m, bb...)
+	if len(m) != (len(ab) + len(bb)) {
+		return CPError{"length of m is incorrect"}
+	}
+
+	// Hash M
+	s.h.Message(m)
+	h := s.h.Hash()
+
+	// Set scrambling paramter u
+	s.u = new(big.Int)
+	s.u.SetBytes(h)
+	if s.u.Cmp(big.NewInt(0)) != 1 {
+		return CPError{"u is invalid"}
+	}
+	return nil
+}
diff --git a/lib/srp_test.go b/lib/srp_test.go
index 3e3c8d6..a819b15 100644
--- a/lib/srp_test.go
+++ b/lib/srp_test.go
@@ -182,3 +182,69 @@ func TestSRPClientSessionEphemeralKeyPub(t *testing.T) {
 		return
 	}
 }
+
+func TestSRPScramblingParamter(t *testing.T) {
+	n := StripSpaceChars(
+		`ffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024
+                 e088a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd
+                 3a431b302b0a6df25f14374fe1356d6d51c245e485b576625e7ec
+                 6f44c42e9a637ed6b0bff5cb6f406b7edee386bfb5a899fa5ae9f
+                 24117c4b1fe649286651ece45b3dc2007cb8a163bf0598da48361
+                 c55d39a69163fa8fd24cf5f83655d23dca3ad961c62f356208552
+                 bb9ed529077096966d670c354e4abc9804f1746c08ca237327fff
+                 fffffffffffff`)
+	g := "2"
+	k := "3"
+	ident := "s@ricketyspace.net"
+	pass := "d59d6c93af0f37f272d924979"
+
+	// Init srp server user.
+	user, err := NewSRPUser(n, g, k, ident, pass)
+	if err != nil {
+		t.Errorf("Error: %v\n", err)
+		return
+	}
+
+	// Get server's pub for user.
+	user.EphemeralKeyGen()
+	pubB, err := user.EphemeralKeyPub()
+	if err != nil {
+		t.Errorf("Error: %v\n", err)
+		return
+	}
+
+	// Init srp client session.
+	session, err := NewSRPClientSession(n, g, k, ident)
+	if err != nil {
+		t.Errorf("Error: %v\n", err)
+		return
+	}
+
+	// Get client's pub for user.
+	pubA, err := session.EphemeralKeyPub()
+	if err != nil {
+		t.Errorf("Error: %v\n", err)
+		return
+	}
+
+	// Compute server's scrambling parameter for user.
+	err = user.SetScramblingParam(pubA)
+	if err != nil {
+		t.Errorf("Error: %v\n", err)
+		return
+	}
+
+	// Compute client's scrambling paramter for user.
+	err = session.SetScramblingParam(pubB)
+	if err != nil {
+		t.Errorf("Error: %v\n", err)
+		return
+	}
+
+	// The server's and client's scrambling parameter must be the
+	// same.
+	if user.u.Cmp(session.u) != 0 {
+		t.Error("Error: scrambling parameter of server != client\n")
+		return
+	}
+}
-- 
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