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math/big: Exp does not handle negative exponents correctly #25865

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zzh1823 opened this issue Jun 13, 2018 · 2 comments
Closed

math/big: Exp does not handle negative exponents correctly #25865

zzh1823 opened this issue Jun 13, 2018 · 2 comments
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FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
Milestone
Go1.12

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@zzh1823
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zzh1823 commented Jun 13, 2018 •
edited

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.
@ianlancetaylor ianlancetaylor changed the title math/big Exp() does not handle negative exponents correctly math/big: Exp does not handle negative exponents correctly Jun 13, 2018
@ianlancetaylor ianlancetaylor added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jun 13, 2018
@ianlancetaylor ianlancetaylor added this to the Go1.12 milestone Jun 13, 2018
@gopherbot
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Change https://golang.org/cl/118562 mentions this issue: math/big: handle negative exponents in Exp

@zzh1823
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zzh1823 commented Jun 14, 2018

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.

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