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math/big: ModSqrt returns wrong value when aliasing parameters #22265

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FiloSottile opened this issue Oct 14, 2017 · 2 comments
Closed

math/big: ModSqrt returns wrong value when aliasing parameters #22265

FiloSottile opened this issue Oct 14, 2017 · 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
Go1.10

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

go 1.9

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

  • darwin_amd64
  • Playground

What did you do?

Aliased z and x in ModSqrt, as allowed by the package docs and not disallowed by the function docs.

y.ModSqrt(y, p.P)

https://play.golang.org/p/MsOief8jlg

What did you expect to see?

The same result as new(big.Int).ModSqrt(y, p.P).

What did you see instead?

An incorrect value is computed.
@ianlancetaylor ianlancetaylor added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Oct 14, 2017
@ianlancetaylor ianlancetaylor added this to the Go1.10 milestone Oct 14, 2017
@FiloSottile
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The optimized path modSqrt3Mod4Prime introduced by ea0491b starts with z.Set(p), which when aliasing will improperly set x = p, too.

@gopherbot
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Change https://golang.org/cl/70832 mentions this issue: math/big: fix ModSqrt optimized path for x = z

@golang golang locked and limited conversation to collaborators Oct 16, 2018
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@FiloSottile @ianlancetaylor @griesemer @gopherbot