[tor-bugs] #13538 [Tor]: Stop signed left shift overflows in curve25519-donna (non-64-bit)

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#13538: Stop signed left shift overflows in curve25519-donna (non-64-bit)
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     Reporter:  teor    |      Owner:  teor
         Type:  defect  |     Status:  assigned
     Priority:  normal  |  Milestone:  Tor: 0.2.6.x-final
    Component:  Tor     |    Version:  Tor: unspecified
   Resolution:          |   Keywords:  tor-router integer-safety
Actual Points:          |  Parent ID:
       Points:          |
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Comment (by teor):

 I agree it looks ugly, but I can't imagine a less messy patch.

 We could duplicate all the signed variables (needed for multiplication and
 carry) as unsigned variables (needed for shifts), and somehow keep then in
 sync. But that could be just as ugly.

 Or we could access the array as unsigned, but that is just as undefined,
 isn't it?

 What C really needs is an unsigned-left-shift-cast-back-to-signed
 operator... or something!

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/13538#comment:10>
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