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C arm/ecc-224-modp.asm
ifelse(<
Copyright (C) 2013 Niels Möller
This file is part of GNU Nettle.
GNU Nettle is free software: you can redistribute it and/or
modify it under the terms of either:
* the GNU Lesser General Public License as published by the Free
Software Foundation; either version 3 of the License, or (at your
option) any later version.
or
* the GNU General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your
option) any later version.
or both in parallel, as here.
GNU Nettle is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received copies of the GNU General Public License and
the GNU Lesser General Public License along with this program. If
not, see http://www.gnu.org/licenses/.
>)
.file "ecc-224-modp.asm"
.arm
define(<RP>, <r1>)
define(<H>, <r0>) C Overlaps unused ecc argument
define(<T0>, <r2>)
define(<T1>, <r3>)
define(<T2>, <r4>)
define(<T3>, <r5>)
define(<T4>, <r6>)
define(<T5>, <r7>)
define(<T6>, <r8>)
define(<N3>, <r10>)
define(<L0>, <r11>)
define(<L1>, <r12>)
define(<L2>, <lr>)
C ecc_224_modp (const struct ecc_curve *ecc, mp_limb_t *rp)
.text
.align 2
PROLOGUE(nettle_ecc_224_modp)
push {r4,r5,r6,r7,r8,r10,r11,lr}
add L2, RP, #28
ldm L2, {T0,T1,T2,T3,T4,T5,T6}
mov H, #0
adds T0, T0, T4
adcs T1, T1, T5
adcs T2, T2, T6
adc H, H, #0
C This switch from adcs to sbcs takes carry into account with
C correct sign, but it always subtracts 1 too much. We arrange
C to also add B^7 + 1 below, so the effect is adding p. This
C addition of p also ensures that the result never is
C negative.
sbcs N3, T3, T0
sbcs T4, T4, T1
sbcs T5, T5, T2
sbcs T6, T6, H
mov H, #1 C This is the B^7
sbc H, #0
subs T6, T6, T3
sbc H, #0
C Now subtract from low half
ldm RP!, {L0,L1,L2}
C Clear carry, with the sbcs, this is the 1.
adds RP, #0
sbcs T0, L0, T0
sbcs T1, L1, T1
sbcs T2, L2, T2
ldm RP!, {T3,L0,L1,L2}
sbcs T3, T3, N3
sbcs T4, L0, T4
sbcs T5, L1, T5
sbcs T6, L2, T6
rsc H, H, #0
C Now -2 <= H <= 0 is the borrow, so subtract (B^3 - 1) |H|
C Use (B^3 - 1) H = <H, H, H> if -1 <=H <= 0, and
C (B^3 - 1) H = <1,B-1, B-1, B-2> if H = -2
subs T0, T0, H
asr L1, H, #1
sbcs T1, T1, L1
eor H, H, L1
sbcs T2, T2, L1
sbcs T3, T3, H
sbcs T4, T4, #0
sbcs T5, T5, #0
sbcs T6, T6, #0
sbcs H, H, H
C Final borrow, subtract (B^3 - 1) |H|
subs T0, T0, H
sbcs T1, T1, H
sbcs T2, T2, H
sbcs T3, T3, #0
sbcs T4, T4, #0
sbcs T5, T5, #0
sbcs T6, T6, #0
stmdb RP, {T0,T1,T2,T3,T4,T5,T6}
pop {r4,r5,r6,r7,r8,r10,r11,pc}
EPILOGUE(nettle_ecc_224_modp)
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