math/big: support negative numbers in ModInverse
Fixes #16984
Change-Id: I3a330e82941a068ca6097985af4ab221275fd336
Reviewed-on: https://go-review.googlesource.com/29299
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
diff --git a/src/math/big/int.go b/src/math/big/int.go
index f2a75d1..6c08843 100644
--- a/src/math/big/int.go
+++ b/src/math/big/int.go
@@ -577,6 +577,11 @@
// ModInverse sets z to the multiplicative inverse of g in the ring ℤ/nℤ
// and returns z. If g and n are not relatively prime, the result is undefined.
func (z *Int) ModInverse(g, n *Int) *Int {
+ if g.neg {
+ // GCD expects parameters a and b to be > 0.
+ var g2 Int
+ g = g2.Mod(g, n)
+ }
var d Int
d.GCD(z, nil, g, n)
// x and y are such that g*x + n*y = d. Since g and n are
diff --git a/src/math/big/int_test.go b/src/math/big/int_test.go
index fcc2ebc..0cae4a1 100644
--- a/src/math/big/int_test.go
+++ b/src/math/big/int_test.go
@@ -1309,6 +1309,7 @@
}{
{"1234567", "458948883992"},
{"239487239847", "2410312426921032588552076022197566074856950548502459942654116941958108831682612228890093858261341614673227141477904012196503648957050582631942730706805009223062734745341073406696246014589361659774041027169249453200378729434170325843778659198143763193776859869524088940195577346119843545301547043747207749969763750084308926339295559968882457872412993810129130294592999947926365264059284647209730384947211681434464714438488520940127459844288859336526896320919633919"},
+ {"-10", "13"}, // issue #16984
}
func TestModInverse(t *testing.T) {
