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Cleanup in Montgomery Mul, Square, Pow

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Mamy André-Ratsimbazafy 2020-03-22 13:24:37 +01:00
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commit 8b7374f405
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4 changed files with 61 additions and 45 deletions
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1
benchmarks/bench_fp.nim
View File

@ -9,6 +9,7 @@
import
# Internals
../constantine/config/curves,
../constantine/arithmetic,
# Helpers
../helpers/static_for,
./bench_fields_template,

12
constantine/arithmetic/bigints.nim
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@ -315,7 +315,7 @@ func montySquare*(r: var BigInt, a, M: BigInt, negInvModWord: static BaseType, c
func montyPow*[mBits, eBits: static int](
a: var BigInt[mBits], exponent: BigInt[eBits],
M, one: BigInt[mBits], negInvModWord: static BaseType, windowSize: static int,
canUseNoCarryMontyMul: static bool
canUseNoCarryMontyMul, canUseNoCarryMontySquare: static bool
) =
## Compute a <- a^exponent (mod M)
## ``a`` in the Montgomery domain
@ -334,12 +334,12 @@ func montyPow*[mBits, eBits: static int](
const scratchLen = if windowSize == 1: 2
else: (1 shl windowSize) + 1
var scratchSpace {.noInit.}: array[scratchLen, Limbs[mBits.wordsRequired]]
montyPow(a.limbs, expBE, M.limbs, one.limbs, negInvModWord, scratchSpace, canUseNoCarryMontyMul)
montyPow(a.limbs, expBE, M.limbs, one.limbs, negInvModWord, scratchSpace, canUseNoCarryMontyMul, canUseNoCarryMontySquare)
func montyPowUnsafeExponent*[mBits, eBits: static int](
a: var BigInt[mBits], exponent: BigInt[eBits],
M, one: BigInt[mBits], negInvModWord: static BaseType, windowSize: static int,
canUseNoCarryMontyMul: static bool
canUseNoCarryMontyMul, canUseNoCarryMontySquare: static bool
) =
## Compute a <- a^exponent (mod M)
## ``a`` in the Montgomery domain
@ -362,12 +362,12 @@ func montyPowUnsafeExponent*[mBits, eBits: static int](
const scratchLen = if windowSize == 1: 2
else: (1 shl windowSize) + 1
var scratchSpace {.noInit.}: array[scratchLen, Limbs[mBits.wordsRequired]]
montyPowUnsafeExponent(a.limbs, expBE, M.limbs, one.limbs, negInvModWord, scratchSpace, canUseNoCarryMontyMul)
montyPowUnsafeExponent(a.limbs, expBE, M.limbs, one.limbs, negInvModWord, scratchSpace, canUseNoCarryMontyMul, canUseNoCarryMontySquare)
func montyPowUnsafeExponent*[mBits: static int](
a: var BigInt[mBits], exponent: openarray[byte],
M, one: BigInt[mBits], negInvModWord: static BaseType, windowSize: static int,
canUseNoCarryMontyMul: static bool
canUseNoCarryMontyMul, canUseNoCarryMontySquare: static bool
) =
## Compute a <- a^exponent (mod M)
## ``a`` in the Montgomery domain
@ -386,7 +386,7 @@ func montyPowUnsafeExponent*[mBits: static int](
const scratchLen = if windowSize == 1: 2
else: (1 shl windowSize) + 1
var scratchSpace {.noInit.}: array[scratchLen, Limbs[mBits.wordsRequired]]
montyPowUnsafeExponent(a.limbs, exponent, M.limbs, one.limbs, negInvModWord, scratchSpace, canUseNoCarryMontyMul)
montyPowUnsafeExponent(a.limbs, exponent, M.limbs, one.limbs, negInvModWord, scratchSpace, canUseNoCarryMontyMul, canUseNoCarryMontySquare)
{.pop.} # inline
{.pop.} # raises no exceptions

25
constantine/arithmetic/finite_fields.nim
View File

@ -170,7 +170,8 @@ func pow*(a: var Fp, exponent: BigInt) =
exponent,
Fp.C.Mod, Fp.C.getMontyOne(),
Fp.C.getNegInvModWord(), windowSize,
Fp.C.canUseNoCarryMontyMul()
Fp.C.canUseNoCarryMontyMul(),
Fp.C.canUseNoCarryMontySquare()
)
func powUnsafeExponent*(a: var Fp, exponent: BigInt) =
@ -189,9 +190,29 @@ func powUnsafeExponent*(a: var Fp, exponent: BigInt) =
exponent,
Fp.C.Mod, Fp.C.getMontyOne(),
Fp.C.getNegInvModWord(), windowSize,
Fp.C.canUseNoCarryMontyMul()
Fp.C.canUseNoCarryMontyMul(),
Fp.C.canUseNoCarryMontySquare()
)
func powUnsafeExponent*(a: var Fp, exponent: openarray[byte]) =
## Exponentiation modulo p
## ``a``: a field element to be exponentiated
## ``exponent``: a big integer
##
## Warning ⚠️ :
## This is an optimization for public exponent
## Otherwise bits of the exponent can be retrieved with:
## - memory access analysis
## - power analysis
## - timing analysis
const windowSize = 5 # TODO: find best window size for each curves
a.mres.montyPowUnsafeExponent(
exponent,
Fp.C.Mod, Fp.C.getMontyOne(),
Fp.C.getNegInvModWord(), windowSize,
Fp.C.canUseNoCarryMontyMul(),
Fp.C.canUseNoCarryMontySquare()
)
# ############################################################
#

68
constantine/arithmetic/limbs_montgomery.nim
View File

@ -85,9 +85,6 @@ macro staticFor(idx: untyped{nkIdent}, start, stopEx: static int, body: untyped)
# Implementation
# ------------------------------------------------------------
# Note: the low-level implementations should not use static parameter
# the code generated is already big enough for curve with different
# limb sizes, we want to use the same codepath when limbs lenght are compatible.
func montyMul_CIOS_nocarry(r: var Limbs, a, b, M: Limbs, m0ninv: BaseType) =
## Montgomery Multiplication using Coarse Grained Operand Scanning (CIOS)
@ -266,7 +263,7 @@ func montySquare_CIOS(r: var Limbs, a, M: Limbs, m0ninv: BaseType) =
func montyMul*(
r: var Limbs, a, b, M: Limbs,
m0ninv: static BaseType, canUseNoCarryMontyMul: static bool) {.inline.} =
m0ninv: static BaseType, canUseNoCarryMontyMul: static bool) =
## Compute r <- a*b (mod M) in the Montgomery domain
## `m0ninv` = -1/M (mod Word). Our words are 2^32 or 2^64
##
@ -278,7 +275,7 @@ func montyMul*(
## Assuming 64-bit words, the magic constant should be:
##
## - µ ≡ -1/M[0] (mod 2^64) for a general multiplication
## This can be precomputed with `negInvModWord`
## This can be precomputed with `m0ninv`
## - 1 for conversion from Montgomery to canonical representation
## The library implements a faster `redc` primitive for that use-case
## - R^2 (mod M) for conversion from canonical to Montgomery representation
@ -286,27 +283,22 @@ func montyMul*(
# i.e. c'R <- a'R b'R * R^-1 (mod M) in the natural domain
# as in the Montgomery domain all numbers are scaled by R
# Nim doesn't like static Word, so we pass static BaseType up to here
# Then we passe them as Word again for the final processing.
# Many curve moduli are "Montgomery-friendly" which means that m0inv is 1
# Many curve moduli are "Montgomery-friendly" which means that m0ninv is 1
# This saves N basic type multiplication and potentially many register mov
# as well as unless using "mulx" instruction, x86 "mul" requires very specific registers.
# Compilers should be able to constant-propagate, but this prevents reusing code
# for example between secp256k1 (friendly) and BN254 (non-friendly).
# Here, as "montyMul" is inlined at the call site, the compiler shouldn't constant fold, saving size.
# Inlining the implementation instead (and no-inline this "montyMul" proc) would allow constant propagation
# of Montgomery-friendly m0ninv if the compiler deems it interesting,
# or we use `when m0ninv == 1` and enforce the inlining.
#
# The implementation is visible from here, the compiler can make decision whether to:
# - specialize/duplicate code for m0ninv == 1 (especially if only 1 curve is needed)
# - keep it generic and optimize code size
when canUseNoCarryMontyMul:
montyMul_CIOS_nocarry(r, a, b, M, m0ninv)
else:
montyMul_CIOS(r, a, b, M, m0ninv)
func montySquare*(r: var Limbs, a, M: Limbs,
m0ninv: static BaseType, canUseNoCarryMontySquare: static bool) {.inline.} =
m0ninv: static BaseType, canUseNoCarryMontySquare: static bool) =
## Compute r <- a^2 (mod M) in the Montgomery domain
## `negInvModWord` = -1/M (mod Word). Our words are 2^31 or 2^63
## `m0ninv` = -1/M (mod Word). Our words are 2^31 or 2^63
when canUseNoCarryMontySquare:
montySquare_CIOS_nocarry(r, a, M, m0ninv)
@ -314,7 +306,7 @@ func montySquare*(r: var Limbs, a, M: Limbs,
montySquare_CIOS(r, a, M, m0ninv)
func redc*(r: var Limbs, a, one, M: Limbs,
m0ninv: static BaseType, canUseNoCarryMontyMul: static bool) {.inline.} =
m0ninv: static BaseType, canUseNoCarryMontyMul: static bool) =
## Transform a bigint ``a`` from it's Montgomery N-residue representation (mod N)
## to the regular natural representation (mod N)
##
@ -325,7 +317,7 @@ func redc*(r: var Limbs, a, one, M: Limbs,
##
## This is called a Montgomery Reduction
## The Montgomery Magic Constant is µ = -1/N mod M
## is used internally and can be precomputed with negInvModWord(Curve)
## is used internally and can be precomputed with m0ninv(Curve)
# References:
# - https://eprint.iacr.org/2017/1057.pdf (Montgomery)
# page: Radix-r interleaved multiplication algorithm
@ -336,7 +328,7 @@ func redc*(r: var Limbs, a, one, M: Limbs,
montyMul(r, a, one, M, m0ninv, canUseNoCarryMontyMul)
func montyResidue*(r: var Limbs, a, M, r2modM: Limbs,
m0ninv: static BaseType, canUseNoCarryMontyMul: static bool) {.inline.} =
m0ninv: static BaseType, canUseNoCarryMontyMul: static bool) =
## Transform a bigint ``a`` from it's natural representation (mod N)
## to a the Montgomery n-residue representation
##
@ -424,12 +416,12 @@ func montyPowSquarings(
a: var Limbs,
exponent: openarray[byte],
M: Limbs,
negInvModWord: static BaseType,
m0ninv: static BaseType,
tmp: var Limbs,
window: uint,
acc, acc_len: var uint,
e: var int,
canUseNoCarryMontyMul: static bool
canUseNoCarryMontySquare: static bool
): tuple[k, bits: uint] {.inline.}=
## Squaring step of exponentiation by squaring
## Get the next k bits in range [1, window)
@ -455,7 +447,7 @@ func montyPowSquarings(
# We have k bits and can do k squaring
for i in 0 ..< k:
tmp.montySquare(a, M, negInvModWord, canUseNoCarryMontyMul)
tmp.montySquare(a, M, m0ninv, canUseNoCarryMontySquare)
a = tmp
return (k, bits)
@ -464,9 +456,10 @@ func montyPow*(
a: var Limbs,
exponent: openarray[byte],
M, one: Limbs,
negInvModWord: static BaseType,
m0ninv: static BaseType,
scratchspace: var openarray[Limbs],
canUseNoCarryMontyMul: static bool
canUseNoCarryMontyMul: static bool,
canUseNoCarryMontySquare: static bool
) =
## Modular exponentiation r = a^exponent mod M
## in the Montgomery domain
@ -479,7 +472,7 @@ func montyPow*(
## Use ``exportRawUint`` for conversion
## - ``M`` is the modulus
## - ``one`` is 1 (mod M) in montgomery representation
## - ``negInvModWord`` is the montgomery magic constant "-1/M[0] mod 2^WordBitSize"
## - ``m0ninv`` is the montgomery magic constant "-1/M[0] mod 2^WordBitSize"
## - ``scratchspace`` with k the window bitsize of size up to 5
## This is a buffer that can hold between 2^k + 1 big-ints
## A window of of 1-bit (no window optimization) requires only 2 big-ints
@ -494,7 +487,7 @@ func montyPow*(
## A window of size 5 requires (2^5 + 1)*(381 + 7)/8 = 33 * 48 bytes = 1584 bytes
## of scratchspace (on the stack).
let window = montyPowPrologue(a, M, one, negInvModWord, scratchspace, canUseNoCarryMontyMul)
let window = montyPowPrologue(a, M, one, m0ninv, scratchspace, canUseNoCarryMontyMul)
# We process bits with from most to least significant.
# At each loop iteration with have acc_len bits in acc.
@ -506,10 +499,10 @@ func montyPow*(
e = 0
while acc_len > 0 or e < exponent.len:
let (k, bits) = montyPowSquarings(
a, exponent, M, negInvModWord,
a, exponent, M, m0ninv,
scratchspace[0], window,
acc, acc_len, e,
canUseNoCarryMontyMul
canUseNoCarryMontySquare
)
# Window lookup: we set scratchspace[1] to the lookup value.
@ -524,16 +517,17 @@ func montyPow*(
# Multiply with the looked-up value
# we keep the product only if the exponent bits are not all zero
scratchspace[0].montyMul(a, scratchspace[1], M, negInvModWord, canUseNoCarryMontyMul)
scratchspace[0].montyMul(a, scratchspace[1], M, m0ninv, canUseNoCarryMontyMul)
a.ccopy(scratchspace[0], Word(bits).isNonZero())
func montyPowUnsafeExponent*(
a: var Limbs,
exponent: openarray[byte],
M, one: Limbs,
negInvModWord: static BaseType,
m0ninv: static BaseType,
scratchspace: var openarray[Limbs],
canUseNoCarryMontyMul: static bool
canUseNoCarryMontyMul: static bool,
canUseNoCarryMontySquare: static bool
) =
## Modular exponentiation r = a^exponent mod M
## in the Montgomery domain
@ -547,26 +541,26 @@ func montyPowUnsafeExponent*(
# TODO: scratchspace[1] is unused when window > 1
let window = montyPowPrologue(a, M, one, negInvModWord, scratchspace, canUseNoCarryMontyMul)
let window = montyPowPrologue(a, M, one, m0ninv, scratchspace, canUseNoCarryMontyMul)
var
acc, acc_len: uint
e = 0
while acc_len > 0 or e < exponent.len:
let (k, bits) = montyPowSquarings(
a, exponent, M, negInvModWord,
a, exponent, M, m0ninv,
scratchspace[0], window,
acc, acc_len, e,
canUseNoCarryMontyMul
canUseNoCarryMontySquare
)
## Warning ⚠️: Exposes the exponent bits
if bits != 0:
if window > 1:
scratchspace[0].montyMul(a, scratchspace[1+bits], M, negInvModWord, canUseNoCarryMontyMul)
scratchspace[0].montyMul(a, scratchspace[1+bits], M, m0ninv, canUseNoCarryMontyMul)
else:
# scratchspace[1] holds the original `a`
scratchspace[0].montyMul(a, scratchspace[1], M, negInvModWord, canUseNoCarryMontyMul)
scratchspace[0].montyMul(a, scratchspace[1], M, m0ninv, canUseNoCarryMontyMul)
a = scratchspace[0]
{.pop.} # raises no exceptions