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43 lines
1.1 KiB
Go
43 lines
1.1 KiB
Go
// reduce.go - Montgomery, Barret, and Full reduction.
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//
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// To the extent possible under law, Yawning Angel has waived all copyright
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// and related or neighboring rights to the software, using the Creative
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// Commons "CC0" public domain dedication. See LICENSE or
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// <http://creativecommons.org/publicdomain/zero/1.0/> for full details.
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package kyber
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const (
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qinv = 7679 // -inverse_mod(q,2^18)
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rlog = 18
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)
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// Montgomery reduction; given a 32-bit integer a, computes 16-bit integer
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// congruent to a * R^-1 mod q, where R=2^18 (see value of rlog).
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func montgomeryReduce(a uint32) uint16 {
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u := a * qinv
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u &= (1 << rlog) - 1
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u *= kyberQ
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a += u
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return uint16(a >> rlog)
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}
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// Barrett reduction; given a 16-bit integer a, computes 16-bit integer
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// congruent to a mod q in {0,...,11768}.
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func barrettReduce(a uint16) uint16 {
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u := uint32(a >> 13) // ((uint32_t) a * sinv) >> 16
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u *= kyberQ
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a -= uint16(u)
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return a
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}
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// Full reduction; given a 16-bit integer a, computes unsigned integer a mod q.
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func freeze(x uint16) uint16 {
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r := barrettReduce(x)
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m := r - kyberQ
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c := int16(m)
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c >>= 15
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r = m ^ ((r ^ m) & uint16(c))
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return r
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}
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