math: Added parity check to ProbablyPrime

Fixes #9269

Change-Id: I25751632e95978537b656aedfa5c35ab2273089b
Reviewed-on: https://go-review.googlesource.com/1380
Reviewed-by: Robert Griesemer <gri@golang.org>

diff --git a/src/math/big/nat.go b/src/math/big/nat.go
index 16a87f5c537480efaeab5ecfe2a36ba26e9af2b4..6e65ea1cb44adb9d312740eb483dc1f4ff0ddb36 100644 (file)
--- a/src/math/big/nat.go
+++ b/src/math/big/nat.go
@@ -1404,6 +1404,10 @@ func (n nat) probablyPrime(reps int) bool {
                }
        }
 
+       if n[0]&1 == 0 {
+               return false // n is even
+       }
+
        const primesProduct32 = 0xC0CFD797         // Π {p ∈ primes, 2 < p <= 29}
        const primesProduct64 = 0xE221F97C30E94E1D // Π {p ∈ primes, 2 < p <= 53}