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-rw-r--r--lib/rsa.go65
-rw-r--r--lib/rsa_test.go52
2 files changed, 117 insertions, 0 deletions
diff --git a/lib/rsa.go b/lib/rsa.go
new file mode 100644
index 0000000..86df861
--- /dev/null
+++ b/lib/rsa.go
@@ -0,0 +1,65 @@
+// Copyright © 2021 siddharth ravikumar <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package lib
+
+import (
+ "math/big"
+)
+
+type GCDResult struct {
+ Gcd *big.Int
+ X *big.Int // Bézout coefficient 'x'
+ Y *big.Int // Bézout coefficient 'y'
+}
+
+// Copy b to a.
+func biCopy(a, b *big.Int) *big.Int {
+ a.SetBytes(b.Bytes())
+ if b.Sign() == -1 {
+ a.Mul(a, big.NewInt(-1))
+ }
+ return a
+}
+
+// Extended Euclidian.
+func egcd(a, b *big.Int) GCDResult {
+ // Initialize.
+ s0 := big.NewInt(1)
+ s1 := big.NewInt(0)
+ r0 := biCopy(big.NewInt(0), a)
+ r1 := biCopy(big.NewInt(0), b)
+
+ for r1.Cmp(big.NewInt(0)) != 0 {
+ q := big.NewInt(0)
+ q.Div(r0, r1)
+
+ tr := big.NewInt(0)
+ tr = tr.Mul(q, r1)
+ tr = tr.Sub(r0, tr)
+
+ biCopy(r0, r1)
+ biCopy(r1, tr)
+
+ tr = big.NewInt(0)
+ tr = tr.Mul(q, s1)
+ tr = tr.Sub(s0, tr)
+
+ biCopy(s0, s1)
+ biCopy(s1, tr)
+ }
+
+ x := biCopy(big.NewInt(0), s0)
+ y := big.NewInt(0)
+ if b.Cmp(big.NewInt(0)) != 0 {
+ y = y.Mul(s0, a)
+ y = y.Sub(r0, y)
+ y = y.Div(y, b)
+ }
+
+ return GCDResult{
+ Gcd: biCopy(big.NewInt(0), r0),
+ X: x,
+ Y: y,
+ }
+}
diff --git a/lib/rsa_test.go b/lib/rsa_test.go
new file mode 100644
index 0000000..ee71755
--- /dev/null
+++ b/lib/rsa_test.go
@@ -0,0 +1,52 @@
+// Copyright © 2021 siddharth ravikumar <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package lib
+
+import (
+ "math/big"
+ "testing"
+)
+
+func TestEGCD(t *testing.T) {
+ a := big.NewInt(128)
+ b := big.NewInt(96)
+ r := egcd(a, b)
+ if r.Gcd.Cmp(big.NewInt(32)) != 0 {
+ t.Errorf("gcd(128, 96) != 32")
+ }
+ if r.X.Cmp(big.NewInt(1)) != 0 || r.Y.Cmp(big.NewInt(-1)) != 0 {
+ t.Errorf("bézout_coef(128, 96) != {1,-1}")
+ }
+
+ a = big.NewInt(360)
+ b = big.NewInt(210)
+ r = egcd(a, b)
+ if r.Gcd.Cmp(big.NewInt(30)) != 0 {
+ t.Errorf("gcd(360, 210) != 30")
+ }
+ if r.X.Cmp(big.NewInt(3)) != 0 || r.Y.Cmp(big.NewInt(-5)) != 0 {
+ t.Errorf("bézout_coef(360, 210) != {3,-5}")
+ }
+
+ a = big.NewInt(108)
+ b = big.NewInt(144)
+ r = egcd(a, b)
+ if r.Gcd.Cmp(big.NewInt(36)) != 0 {
+ t.Errorf("gcd(108, 144) != 36")
+ }
+ if r.X.Cmp(big.NewInt(-1)) != 0 || r.Y.Cmp(big.NewInt(1)) != 0 {
+ t.Errorf("bézout_coef(108, 144) != {-1,1}")
+ }
+
+ a = big.NewInt(240)
+ b = big.NewInt(46)
+ r = egcd(a, b)
+ if r.Gcd.Cmp(big.NewInt(2)) != 0 {
+ t.Errorf("gcd(240, 46) != 2")
+ }
+ if r.X.Cmp(big.NewInt(-9)) != 0 || r.Y.Cmp(big.NewInt(47)) != 0 {
+ t.Errorf("bézout_coef(240, 46) != {-9,47}")
+ }
+
+}