math/big: Exp does not handle negative exponents correctly

[zzh1823]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

Go 1.10

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

linux_amd64

What did you do?

When using package math/big, specifically big.Int.Exp(), I found that it cannot handle negative exponents correctly but simply returns 1.

What did you expect to see?

Compared to ModPow() in Java, Exp(x, y, z) should return mod inverse of (x ^ Abs(y)) mod N when y is negative.

What did you see instead?

Exp(x, y, z) simply returns 1 when y is negative.

[gopherbot]

Change https://golang.org/cl/118562 mentions this issue: math/big: handle negative exponents in Exp

[zzh1823]

Thanks for the fix!
I checked the revision: Exp(x, y, m) = Exp(ModInv(x, m), y.abs, m). I wonder if ModInv(Exp(x, y.abs, m), m) is simpler.