lib: implement diffie-hellman

2 files changed, 77 insertions, 0 deletions

diff --git a/lib/dh.go b/lib/dh.go
new file mode 100644
index 0000000..0782033
--- /dev/null
+++ b/lib/dh.go
@@ -0,0 +1,41 @@
+// Copyright © 2021 siddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package lib
+
+import "math/big"
+
+type DH struct {
+	p  *big.Int
+	g  *big.Int
+	pk *big.Int // Private key
+}
+
+func NewDH(ps, gs string) (*DH, bool) {
+	p, ok := new(big.Int).SetString(StripSpaceChars(ps), 16)
+	if !ok {
+		return nil, false
+	}
+	g, ok := new(big.Int).SetString(StripSpaceChars(gs), 16)
+	if !ok {
+		return nil, false
+	}
+
+	// Init DH.
+	dh := new(DH)
+	dh.p = p
+	dh.g = g
+	dh.pk = big.NewInt(RandomInt(1, 10000000))
+	return dh, true
+}
+
+// Return our public key.
+func (dh *DH) Pub() *big.Int {
+	return new(big.Int).Exp(dh.g, dh.pk, dh.p)
+}
+
+// Return shared secret between us and the other party.
+// `pub` is the other party's public key.
+func (dh *DH) SharedSecret(pub *big.Int) *big.Int {
+	return new(big.Int).Exp(pub, dh.pk, dh.p)
+}
diff --git a/lib/dh_test.go b/lib/dh_test.go
new file mode 100644
index 0000000..f313d82
--- /dev/null
+++ b/lib/dh_test.go
@@ -0,0 +1,36 @@
+// Copyright © 2021 siddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package lib
+
+import "testing"
+
+func TestDH(t *testing.T) {
+	p := `ffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024
+e088a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd
+3a431b302b0a6df25f14374fe1356d6d51c245e485b576625e7ec
+6f44c42e9a637ed6b0bff5cb6f406b7edee386bfb5a899fa5ae9f
+24117c4b1fe649286651ece45b3dc2007cb8a163bf0598da48361
+c55d39a69163fa8fd24cf5f83655d23dca3ad961c62f356208552
+bb9ed529077096966d670c354e4abc9804f1746c08ca237327fff
+fffffffffffff`
+	g := "2"
+
+	bobDH, ok := NewDH(p, g)
+	if !ok {
+		t.Errorf("Error: Unable to initialize Bob's DH\n")
+		return
+	}
+
+	aliceDH, ok := NewDH(p, g)
+	if !ok {
+		t.Errorf("Error: Unable to initialize Alice's DH\n")
+		return
+	}
+
+	bobSK := bobDH.SharedSecret(aliceDH.Pub())
+	aliceSK := aliceDH.SharedSecret(bobDH.Pub())
+	if bobSK.Cmp(aliceSK) != 0 {
+		t.Errorf("Error: Shared Secret for Bob and Alice is not the same\n")
+	}
+}