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Add cmov according to Milagro / hash_to_curve spec

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Mamy André-Ratsimbazafy 2020-02-16 22:18:22 +01:00
parent 6694023f16
commit 285b6aad1a
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1 changed files with 10 additions and 0 deletions
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constantine/math/bigints_raw.nim
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@ -217,6 +217,16 @@ func setZero*(a: BigIntViewMut) =
## It's bit size is unchanged
zeroMem(a[0].unsafeAddr, a.numLimbs() * sizeof(Word))
func cmov*(a: BigIntViewMut, b: BigIntViewAny, ctl: CTBool[Word]) =
## Constant-time conditional copy
## If ctl is true: b is copied into a
## if ctl is false: b is not copied and a is untouched
## Time and memory accesses are the same whether a copy occurs or not
checkMatchingBitlengths(a, b)
for i in 0 ..< a.numLimbs():
a[i] = ctl.mux(b[i], a[i])
# The arithmetic primitives all accept a control input that indicates
# if it is a placebo operation. It stills performs the
# same memory accesses to be side-channel attack resistant.