opensecura / 3p / lowrisc / opentitan / b2940d74880cf1c9026f76ec56186988bfd17e76 / . / sw / otbn / crypto / modexp.s

/* Copyright lowRISC Contributors. | |

* Copyright 2016 The Chromium OS Authors. All rights reserved. | |

* Use of this source code is governed by a BSD-style license that can be | |

* found in the LICENSE.dcrypto file. | |

* | |

* Derived from code in | |

* https://chromium.googlesource.com/chromiumos/platform/ec/+/refs/heads/cr50_stab/chip/g/dcrypto/dcrypto_bn.c | |

*/ | |

.text | |

.globl modexp_65537 | |

.globl modexp | |

.globl modload | |

/** | |

* Precomputation of a constant m0' for Montgomery modular arithmetic | |

* | |

* Word-wise Montgomery modular arithmetic requires a quantity m0' to be | |

* precomputed once per modulus M. m0' is the negative of the | |

* modular multiplicative inverse of the lowest limb m0 of the modulus M, in | |

* the field GF(2^w), where w is the number of bits per limb. w is set to 256 | |

* in this subroutine. | |

* | |

* Returns: m0' = -m0^(-1) mod 2^256 | |

* with m0 being the lowest limb of the modulus M | |

* | |

* This subroutine implements the Dusse-Kaliski method for computing the | |

* multiplicative modular inverse when the modulus is of the form 2^k. | |

* [Dus] DOI https://doi.org/10.1007/3-540-46877-3_21 section 3.2 | |

* (Algorithm "Modular Inverse" on p. 235) | |

* | |

* Flags: When leaving this subroutine, flags of FG0 depend on a | |

* the final subtraction and can be used if needed. | |

* FG0.M, FG0.L, FG0.Z depend directly on the value of the result m0'. | |

* FG0.C is always set. | |

* FG1 is not modified in this subroutine. | |

* | |

* @param[in] w28: m0, the lowest 256 bit limb of the modulus M | |

* @param[in] w31: all-zero. | |

* @param[out] w29: m0', negative of inverse of m0 in GF(2^256) | |

* | |

* clobbered registers: w0, w1, w29 | |

* clobbered flag groups: FG0 | |

*/ | |

m0inv: | |

/* w0 keeps track of loop iterations in one-hot encoding, i.e. | |

w0 = 2^i in the loop body below and initialized here with w0 = 1 | |

It is used for both the comparison in step 4 of [Dus] and the | |

addition in step 6 of [Dus] */ | |

bn.xor w0, w0, w0 | |

bn.addi w0, w0, 1 | |

/* according to [Dus] the result variable y is initialized with 1 */ | |

/* w29 = y_0 = 1 */ | |

bn.mov w29, w0 | |

/* iterate over all 256 bits of m0. | |

i refers to the loop cycle 0..255 in the loop body below. */ | |

loopi 256, 13 | |

/* y_i <= m*y_{i-1] */ | |

bn.mulqacc.z w28.0, w29.0, 0 | |

bn.mulqacc w28.1, w29.0, 64 | |

bn.mulqacc.so w1.L, w28.0, w29.1, 64 | |

bn.mulqacc w28.2, w29.0, 0 | |

bn.mulqacc w28.1, w29.1, 0 | |

bn.mulqacc w28.0, w29.2, 0 | |

bn.mulqacc w28.3, w29.0, 64 | |

bn.mulqacc w28.2, w29.1, 64 | |

bn.mulqacc w28.1, w29.2, 64 | |

bn.mulqacc.so w1.U, w28.0, w29.3, 64 | |

/* This checks if w1 = y_i = m0*y_(i-1) < 2^(i-1) mod 2^i | |

Due to the mathematical properties it can be shown that y_i at this point, | |

is either 1 or (10..0..01)_(i). Therefore, just probing the i_th bit is | |

the same as the full compare. */ | |

bn.and w1, w1, w0 | |

/* Compute | |

y_i=w29 <= w1=m0*y_(i-1) < 2^(i-1) mod 2^i y_i ? : y_{i-1}+2^i : y_{i-1} | |

there cannot be overlaps => or'ing is as good as adding */ | |

bn.or w29, w29, w1 | |

/* double w0 (w0 <= w0 << 1) i.e. w0=2^i */ | |

bn.add w0, w0, w0 | |

/* finally, compute m0' (negative of inverse) | |

w29 = m0' = -(m0^-1) mod 2^256 = -y_255 = 0 - y_255 = w31 - w29 */ | |

bn.sub w29, w31, w29 | |

ret | |

/** | |

* Constant time conditional subtraction of modulus from a bigint | |

* | |

* Returns C <= C-s*M | |

* with C being a bigint of length 256..4096 bit | |

* M being the modulus of length 256..4096 bit | |

* s being a boolean value [0,1] | |

* | |

* Conditionally subtracts the modulus located in dmem from the bigint | |

* located in a buffer in the wide regfile (starting at w5). The subtracted | |

* value is selected when FG1.C equals 1, otherwise the unmodified value is | |

* selected. | |

* | |

* Note that the interpretation of the subtrahend as a modulus is only | |

* contextual. In theory, it can be any bigint. However, the subtrahend is | |

* expected in dmem at a location that is reserved for the modulus according | |

* to the calling conventions within this library. | |

* | |

* Flags: When leaving this subroutine, flags of FG0 depend on a | |

* potentially discarded value and therefore are not usable after | |

* return. | |

* FG1 is not modified in this subroutine. | |

* | |

* @param[in] x16: dptr_m, pointer to 1st limb of modulus M | |

* @param[in] x30: N, number of 256 bit limbs in modulus and bigint | |

* @param[in] w31: all-zero | |

* @param[in] FG1.C: s, selection flag | |

* @param[out] [w[5+N-1]:w5]: new bigint value | |

* @param[in] FG0.C: needs to be set to 0 | |

* | |

* clobbered registers: x8, x10, x11, x16, w2, w3, w4, w5 to w[5+N-1] | |

* clobbered flag groups: FG0 | |

*/ | |

cond_sub_mod: | |

/* setup pointers */ | |

li x8, 5 | |

li x10, 3 | |

li x11, 2 | |

/* reset flags for FG0 */ | |

bn.add w31, w31, w31 | |

/* iterate over all limbs for limb-wise subtraction + conditional selection*/ | |

loop x30, 5 | |

/* load a limb of modulus from dmem to w3 */ | |

bn.lid x10, 0(x16++) | |

/* load the limb of bigint buffer to w2 */ | |

bn.movr x11, x8 | |

/* subtract the current limb of the modulus from current limb of bigint */ | |

bn.subb w4, w2, w3 | |

/* conditionally select subtraction result or unmodified limb */ | |

bn.sel w3, w4, w2, FG1.C | |

/* move back result from w3 to bigint buffer */ | |

bn.movr x8++, x10 | |

ret | |

/** | |

* Compute square of Montgomery modulus | |

* | |

* Returns RR = R^2 mod M | |

* with M being the Modulus of length 256..4096 bit | |

* R = 2^(256*N), N is the number of limbs per bigint | |

* | |

* The squared Montgomery modulus (RR) is needed to transform bigints to and | |

* from the Montgomery domain. | |

* | |

* RR is computed in this subroutine by iteratively doubling and reduction. | |

* | |

* Flags: The states of both FG0 and FG1 depend on intermediate values and are | |

* not usable after return. | |

* | |

* @param[in] x16: dptr_M, pointer to first limb of modulus in dmem | |

* @param[in] x18: dptr_RR: dmem pointer to first limb of output buffer for RR | |

* @param[in] x30: N, number of limbs | |

* @param[in] w31: all-zero | |

* @param[out] dmem[dptr_RR+N*32:dptr_RR]: computed RR | |

* | |

* clobbered registers: x3, x8, x10, x11, x22 | |

* w0, w2, w3, w4, w5 to w20 depending on N | |

* clobbered flag groups: FG0, FG1 | |

*/ | |

compute_rr: | |

/* save pointer to modulus */ | |

addi x22, x16, 0 | |

/* zeroize w3 */ | |

bn.xor w3, w3, w3 | |

/* compute full length of current bigint size in bits | |

N*w = x24 = N*256 = N*2^8 = x30 << 8 */ | |

slli x24, x30, 8 | |

/* reg pointers */ | |

li x8, 5 | |

li x10, 3 | |

/* zeroize w3 */ | |

bn.xor w3, w3, w3 | |

/* zeroize all limbs of bigint in regfile */ | |

loop x30, 1 | |

bn.movr x8++, x10 | |

/* compute R-M | |

since R = 2^(N*w), this can be computed as R-M = unsigned(0-M) */ | |

bn.addi w0, w31, 1 | |

bn.sub w3, w31, w0, FG1 | |

addi x16, x22, 0 | |

jal x1, cond_sub_mod | |

/* Compute R^2 mod M = R*2^(N*w) mod M. | |

=> R^2 mod M can be computed by performing N*w duplications of R. | |

We directly perform a modulo reduction in each step such that the | |

final result will already be reduced. */ | |

loop x24, 18 | |

/* reset pointer */ | |

li x8, 5 | |

/* zeroize w3 reset flags of FG1 */ | |

bn.sub w3, w3, w3, FG1 | |

/* Duplicate the intermediate bigint result. This can overflow such that | |

bit 2^(N*w) (represented by the carry bit after the final loop cycle) | |

is set. */ | |

loop x30, 3 | |

/* copy current limb of bigint to w2 */ | |

bn.movr x11, x8 | |

/* perform the doubling */ | |

bn.addc w2, w2, w2, FG1 | |

/* copy result back to bigint in regfile */ | |

bn.movr x8++, x11 | |

/* Conditionally subtract the modulus from the current bigint Y if there | |

was an overflow. Again, just considering the lowest N*w bits is | |

sufficient, since (in case of an overflow) we can write | |

2*Y as 2^(N*w) + X with M > X >= 0. | |

Then, 2*Y - M = 2^(N*w) + X - M = X + unsigned(0-M) */ | |

addi x16, x22, 0 | |

jal x1, cond_sub_mod | |

/* reset pointer to 1st limb of bigint in regfile */ | |

li x8, 5 | |

/* reset pointer to modulus in dmem */ | |

addi x16, x22, 0 | |

/* reset flags of FG1 */ | |

bn.sub w3, w3, w3, FG1 | |

/* compare intermediate bigint y with modulus | |

subtract modulus if Y > M */ | |

loop x30, 3 | |

bn.lid x10, 0(x16++) | |

bn.movr x11, x8++ | |

bn.cmpb w3, w2, FG1 | |

addi x16, x22, 0 | |

jal x1, cond_sub_mod | |

li x0, 0 | |

/* reset pointer to 1st limb of bigint in regfile */ | |

li x8, 5 | |

/* reset pointer to modulus */ | |

addi x16, x22, 0 | |

/* store computed RR in dmem */ | |

addi x3, x18, 0 | |

loop x30, 2 | |

bn.sid x8, 0(x3++) | |

addi x8, x8, 1 | |

ret | |

/** | |

* Unrolled 512=256x256 bit multiplication. | |

* | |

* Returns c = a x b. | |

* | |

* Flags: No flags are set in this subroutine | |

* | |

* @param[in] w30: a, first operand | |

* @param[in] w25: b, second operand | |

* @param[out] [w26, w27]: c, result | |

* | |

* clobbered registers: w26, w27 | |

* clobbered flag groups: none | |

*/ | |

mul256_w30xw25: | |

bn.mulqacc.z w30.0, w25.0, 0 | |

bn.mulqacc w30.1, w25.0, 64 | |

bn.mulqacc.so w27.L, w30.0, w25.1, 64 | |

bn.mulqacc w30.2, w25.0, 0 | |

bn.mulqacc w30.1, w25.1, 0 | |

bn.mulqacc w30.0, w25.2, 0 | |

bn.mulqacc w30.3, w25.0, 64 | |

bn.mulqacc w30.2, w25.1, 64 | |

bn.mulqacc w30.1, w25.2, 64 | |

bn.mulqacc.so w27.U, w30.0, w25.3, 64 | |

bn.mulqacc w30.3, w25.1, 0 | |

bn.mulqacc w30.2, w25.2, 0 | |

bn.mulqacc w30.1, w25.3, 0 | |

bn.mulqacc w30.3, w25.2, 64 | |

bn.mulqacc.so w26.L, w30.2, w25.3, 64 | |

bn.mulqacc.so w26.U, w30.3, w25.3, 0 | |

ret | |

/** | |

* Unrolled 512=256x256 bit multiplication. | |

* | |

* Returns c = a x b. | |

* | |

* Flags: No flags are set in this subroutine | |

* | |

* @param[in] w30: a, first operand | |

* @param[in] w2: b, second operand | |

* @param[out] [w26, w27]: c, result | |

* | |

* clobbered registers: w26, w27 | |

* clobbered flag groups: none | |

*/ | |

mul256_w30xw2: | |

bn.mulqacc.z w30.0, w2.0, 0 | |

bn.mulqacc w30.1, w2.0, 64 | |

bn.mulqacc.so w27.L, w30.0, w2.1, 64 | |

bn.mulqacc w30.2, w2.0, 0 | |

bn.mulqacc w30.1, w2.1, 0 | |

bn.mulqacc w30.0, w2.2, 0 | |

bn.mulqacc w30.3, w2.0, 64 | |

bn.mulqacc w30.2, w2.1, 64 | |

bn.mulqacc w30.1, w2.2, 64 | |

bn.mulqacc.so w27.U, w30.0, w2.3, 64 | |

bn.mulqacc w30.3, w2.1, 0 | |

bn.mulqacc w30.2, w2.2, 0 | |

bn.mulqacc w30.1, w2.3, 0 | |

bn.mulqacc w30.3, w2.2, 64 | |

bn.mulqacc.so w26.L, w30.2, w2.3, 64 | |

bn.mulqacc.so w26.U, w30.3, w2.3, 0 | |

ret | |

/** | |

* Constant time conditional bigint subtraction | |

* | |

* Returns C <= C-s*B | |

* with B being a bigint of length 256..4096 bit | |

* C being a bigint of length 256..4096 bit | |

* s being a boolean value [0,1] | |

* | |

* Depending on state of FG1.C subtracts a bigint B located in dmem from | |

* another bigint C, located in the wide reg file and stores result at same | |

* position in wide reg file. | |

* | |

* Flags: When leaving this subroutine, flags of FG0 depend on a | |

* potentially discarded value and therefore are not usable after | |

* return. FG1 is not modified in this subroutine. | |

* | |

* @param[in] x16: dmem pointer to first limb of subtrahend (B) | |

* @param[in] x8: regfile pointer to first limb of minuend and result (C) | |

* @param[in] FG.C: s, subtraction flag, subtract if 1 | |

* @param[in] x30: number of limbs | |

* @param[in] FG0.C: needs to be set to 0 | |

* | |

* clobbered registers: x8, x16, w24, w29, w30, w[x8] to w[x8+N-1] | |

* clobbered Flag Groups: FG0 | |

*/ | |

cond_sub_to_reg: | |

/* load pointers to temp regs */ | |

li x12, 30 | |

li x13, 24 | |

/* iterate over all limbs for conditional limb-wise subtraction */ | |

loop x30, 6 | |

/* load limb of subtrahend (input B) to w24 */ | |

bn.lid x13, 0(x16++) | |

/* load limb of minuend (input C) to w30 */ | |

bn.movr x12, x8 | |

/* perform subtraction for a limb */ | |

bn.subb w29, w30, w24 | |

bn.movr x8, x13 | |

/* conditionally select subtraction result or unmodified limb */ | |

bn.sel w24, w29, w30, FG1.C | |

/* store selection result in reg file */ | |

bn.movr x8++, x13 | |

ret | |

/** | |

* Main loop body for constant-time Montgomery Modular Multiplication | |

* | |

* Returns: C <= (C + A*b_i + M*m0'*(C[0] + A[0]*b_i))/(2^WLEN) mod R | |

* | |

* This implements the main loop body for the Montgomery Modular Multiplication | |

* as well as the conditional subtraction. See e.g. Handbook of Applied | |

* Cryptography (HAC) 14.36 (steps 2.1 and 2.2) and step 3. | |

* This subroutine has to be called for every iteration of the loop in step 2 | |

* of HAC 14.36, i.e. once per limb of operand B (x in HAC notation). The limb | |

* is supplied via w2. In the explanations below, the index i refers to the | |

* i_th call to this subroutine within one full Montgomery Multiplication run. | |

* Step 3 of HAC 14.36 is replaced by the approach to perform the conditional | |

* subtraction when the intermediate result is larger than R instead of m. See | |

* e.g. https://eprint.iacr.org/2017/1057 section 2.4.2 for a justification. | |

* This does not omit the conditional subtraction, but simplifies the | |

* comparison. The subtraction is carried out in constant time manner. | |

* Variable names of HAC are mapped as follows to the ones used in the | |

* this library: x=B, y=A, A=C, b=2^WLEN, m=M, R=R, m' = m0', n=N. | |

* | |

* Flags: The states of both FG0 and FG1 depend on intermediate values and are | |

* not usable after return. | |

* | |

* @param[in] x16: dmem pointer to first limb of modulus M | |

* @param[in] x19: dmem pointer to first limb operand A | |

* @param[in] x31: N-1, number of limbs minus one | |

* @param[in] w2: current limb of operand B, b_i | |

* @param[in] w3: Montgomery constant m0' | |

* @param[in] w31: all-zero | |

* @param[in] [w[4+N-1]:w4] intermediate result A | |

* @param[out] [w[4+N-1]:w4] intermediate result A | |

* | |

* clobbered registers: x8, x10, x12, x13, x16, x19 | |

* w24, w25, w26, w27, w28, w29, w30, w4 to w[4+N-1] | |

* clobbered Flag Groups: FG0, FG1 | |

*/ | |

mont_loop: | |

/* save pointer to modulus */ | |

addi x22, x16, 0 | |

/* pointers to temp. wregs */ | |

li x12, 30 | |

li x13, 24 | |

/* buffer read pointer */ | |

li x8, 4 | |

/* buffer write pointer */ | |

li x10, 4 | |

/* load 1st limb of input y (operand a): w30 = y[0] */ | |

bn.lid x12, 0(x19++) | |

/* This is x_i*y_0 in step 2.1 of HAC 14.36 */ | |

/* [w26, w27] = w30*w2 = y[0]*x_i */ | |

jal x1, mul256_w30xw2 | |

/* w24 = w4 = A[0] */ | |

bn.movr x13, x8++ | |

/* add A[0]: [w29, w30] = [w26, w27] + w24 = y[0]*x_i + A[0] */ | |

bn.add w30, w27, w24 | |

/* this serves as c_xy in the first cycle of the loop below */ | |

bn.addc w29, w26, w31 | |

/* w25 = w3 = m0' */ | |

bn.mov w25, w3 | |

/* multiply by m0', this concludes Step 2.1 of HAC 14.36 */ | |

/* [_, u_i] = [w26, w27] = w30*w25 = (y[0]*x_i + A[0])*m0'*/ | |

jal x1, mul256_w30xw25 | |

/* With the computation of u_i, the computations in a cycle 0 of the loop | |

below are already partly done. The following instructions (until the | |

start of the loop) implement the remainder, such that cycle 0 can be | |

omitted in the loop */ | |

/* [_, u_i] = [w28, w25] = [w26, w27] */ | |

bn.mov w25, w27 | |

bn.mov w28, w26 | |

/* w24 = w30 = y[0]*x_i + A[0] mod b */ | |

bn.mov w24, w30 | |

/* load first limb of modulus: w30 = m[0] */ | |

bn.lid x12, 0(x16++) | |

/* [w26, w27] = w30*w25 = m[0]*u_i*/ | |

jal x1, mul256_w30xw25 | |

/* [w28, w27] = [w26, w27] + w24 = m[0]*u_i + (y[0]*x_i + A[0] mod b) */ | |

bn.add w27, w27, w24 | |

/* this serves as c_m in the first cycle of the loop below */ | |

bn.addc w28, w26, w31 | |

/* This loop implements step 2.2 of HAC 14.36 with a word-by-word approach. | |

The loop body is subdivided into two steps. Each step performs one | |

multiplication and subsequently adds two WLEN sized words to the | |

2WLEN-sized result, such that there are no overflows at the end of each | |

step- | |

Two carry words are required between the cycles. Those are c_xy and c_m. | |

Assume that the variable j runs from 1 to N-1 in the explanations below. | |

A cycle 0 is omitted, since the results from the computations above are | |

re-used */ | |

loop x31, 14 | |

/* Step 1: First multiplication takes a limb of each of the operands and | |

computes the product. The carry word from the previous cycle c_xy and | |

the j_th limb of the buffer A, A[j] are added to the multiplication | |

result. | |

/* load limb of y (operand a) and mult. with x_i: [w26, w27] <= y[j]*x_i */ | |

bn.lid x12, 0(x19++) | |

jal x1, mul256_w30xw2 | |

/* add limb of buffer: [w26, w27] <= [w26,w27] + w24 = y[j]*x_i + A[j] */ | |

bn.movr x13, x8++ | |

bn.add w27, w27, w24 | |

bn.addc w26, w26, w31 | |

/* add carry word from previous cycle: | |

[c_xy, a_tmp] = [w29, w24] <= [w26,w27] + w29 = y[j]*x_i + A[j] + c_xy*/ | |

bn.add w24, w27, w29 | |

bn.addc w29, w26, w31 | |

/* Step 2: Second multiplication computes the product of a limb m[j] of | |

the modulus with u_i. The 2nd carry word from the previous loop cycle | |

c_m and the lower word a_tmp of the result of Step 1 are added. */ | |

/* load limb m[j] of modulus and multiply with u_i: | |

[w26, w27] = w30*w25 = m[j+1]*u_i */ | |

bn.lid x12, 0(x16++) | |

jal x1, mul256_w30xw25 | |

/* add result from first step | |

[w26, w27] <= [w26,w27] + w24 = m[j+1]*u_i + a_tmp */ | |

bn.add w27, w27, w24 | |

bn.addc w26, w26, w31 | |

/* [c_m, A[j]] = [w28, w24] = m[j+1]*u_i + a_tmp + c_m */ | |

bn.add w24, w27, w28, FG1 | |

/* store at w[4+j] = A[j-1] | |

This includes the reduction by 2^WLEN = 2^b in step 2.2 of HAC 14.36 */ | |

bn.movr x10++, x13 | |

bn.addc w28, w26, w31, FG1 | |

/* Most significant limb of A is sum of the carry words of last loop cycle | |

A[N-1] = w24 <= w29 + w28 = c_xy + c_m */ | |

bn.addc w24, w29, w28, FG1 | |

bn.movr x10++, x13 | |

/* restore pointers */ | |

addi x16, x22, 0 | |

li x8, 4 | |

li x10, 4 | |

/* This replaces Step 3 of HAC 14.36 and performs conditional constant-time | |

subtraction of the modulus from the output buffer. */ | |

jal x1, cond_sub_to_reg | |

/* restore pointer again */ | |

addi x16, x22, 0 | |

/* restore pointer */ | |

li x8, 4 | |

ret | |

/** | |

* Constant time conditional bigint subtraction | |

* | |

* Returns C = A-x*B | |

* with A being a bigint of length 256..4096 bit | |

* B being a bigint of length 256..4096 bit | |

* C being a bigint of length 256..4096 bit | |

* x being a boolean value [0,1] | |

* | |

* Depending on state of FG1.C subtracts a bigint B located in dmem from | |

* another bigint A, located in the wide reg file and stores result C in dmem. | |

* | |

* Flags: When leaving this subroutine, flags of FG0 depend on a | |

* potentially discarded value and therefore are not usable after | |

* return. FG1 is not modified in this subroutine. | |

* | |

* @param[in] x16: dmem pointer to first limb of subtrahend (B) | |

* @param[in] x8: regfile pointer to first limb of minuend (input A) | |

* @param[in] x21: dmem pointer to first limb of result (C) | |

* @param[in] x30: N, number of limbs | |

* @param[in] FG1.C: subtraction condition, subtract if 1 (x) | |

* @param[in] x9: pointer to temp reg, must be set to 3 | |

* @param[in] x11: pointer to temp reg, must be set to 2 | |

* @param[in] FG0.C: needs to be set to 0 | |

* | |

* clobbered registers: x8, x16, x21, w2, w3 | |

* clobbered Flag Groups: FG0 | |

*/ | |

cond_sub_to_dmem: | |

/* iterate over all limbs for conditional limb-wise subtraction */ | |

loop x30, 5 | |

/* load limb of subtrahend (input B): w3 = dmem[x16+i] */ | |

bn.lid x9, 0(x16++) | |

/* move limb from bignum bufer to w2 */ | |

bn.movr x11, x8++ | |

/* perform subtraction for a limb w3 = w2-1 */ | |

bn.subb w3, w2, w3 | |

/* conditionally select subtraction result or unmodified limb */ | |

bn.sel w2, w3, w2, FG1.C | |

/* store selection result in dmem */ | |

bn.sid x11, 0(x21++) | |

ret | |

/** | |

* Constant-time Montgomery modular multiply by one | |

* | |

* Returns: C = montmul(1,A) = A*R^(-1) mod M | |

* | |

* Routine for back-conversion from Montgomery domain. | |

* This implements the limb-by-limb interleaved Montgomery Modular | |

* Multiplication Algorithm, with one operand fixed to 1. This is only a | |

* wrapper around the main loop body. For algorithmic implementation details | |

* see the mont_loop subroutine. | |

* | |

* Flags: The states of both FG0 and FG1 depend on intermediate values and are | |

* not usable after return. | |

* | |

* @param[in] x16: dmem pointer to first limb of modulus M | |

* @param[in] x17: dptr_m0d: dmem pointer to Montgomery Constant m0' | |

* @param[in] x19: dmem pointer to first limb of operand A | |

* @param[in] x21: dmem pointer to first limb of result C | |

* @param[in] x30: N, number of limbs | |

* @param[in] x31: N-1, number of limbs minus one | |

* @param[in] x8: pointer to temp reg, must be set to 4 | |

* @param[in] x9: pointer to temp reg, must be set to 3 | |

* @param[in] x10: pointer to temp reg, must be set to 4 | |

* @param[in] x11: pointer to temp reg, must be set to 2 | |

* @param[in] w31: all-zero | |

* | |

* clobbered registers: x6, x7, x8, x9, x10, x12, x13, x16, x19, x21 | |

* w2. w3. w24, w25, w26, w27, w28, w29, w30 | |

* w4 to w[4+N-1] | |

* clobbered Flag Groups: FG0, FG1 | |

*/ | |

montmul_mul1: | |

/* load Montgomery constant: w3 = dmem[x17] = dmem[dptr_m0d] = m0' */ | |

bn.lid x9, 0(x17) | |

/* init regfile bigint buffer with zeros */ | |

bn.mov w2, w31 | |

loop x30, 1 | |

bn.movr x10++, x11 | |

/* w2=1 this is operand B */ | |

bn.xor w2, w2, w2 | |

bn.addi w2, w2, 1 | |

/* save dmem pointers for operand A and modulus */ | |

addi x6, x16, 0 | |

addi x7, x19, 0 | |

/* iterate over limbs of operand B */ | |

loop x30, 4 | |

/* restore dmem pointers for operand A and modulus */ | |

addi x16, x6, 0 | |

addi x19, x7, 0 | |

/* Main loop body of Montgomery Multiplication algorithm */ | |

/* 1[i]*A */ | |

jal x1, mont_loop | |

/* all subsequent limbs of operand B are zero since B=1 */ | |

bn.mov w2, w31 | |

/* restore dmem pointers for operand A and modulus */ | |

addi x16, x6, 0 | |

addi x19, x7, 0 | |

/* zeroize w2 and clear flags */ | |

bn.sub w2, w2, w2, FG1 | |

/* iterate over all limbs of bigint buffer for limbwise comparison of | |

buffer with the Modulus. After last loop cycle, FG1.C is set if bigint | |

in buffer is larger than Modulus */ | |

loop x30, 3 | |

/* load limb of limb of Modulus to w3 */ | |

bn.lid x9, 0(x16++) | |

/* load limb from bigint buffer to w2 */ | |

bn.movr x11, x8++ | |

/* compare limb of flag with limb of Modulus */ | |

bn.cmpb w3, w2, FG1 | |

/* restore pointers to bigint buffer in regfile */ | |

li x8, 4 | |

li x10, 4 | |

/* restore dmem pointers for operand A and modulus */ | |

addi x16, x6, 0 | |

addi x19, x7, 0 | |

/* conditionally subtract Modulus from buffer and store result in | |

dmem[x21] to dmem[x21+N] */ | |

jal x1, cond_sub_to_dmem | |

/* restore dmem pointers for operand A and modulus */ | |

addi x16, x6, 0 | |

addi x19, x7, 0 | |

ret | |

/** | |

* Constant-time Montgomery Modular Multiplication | |

* | |

* Returns: C = montmul(A,B) = A*B*R^(-1) mod M | |

* | |

* This implements the limb-by-limb interleaved Montgomery Modular | |

* Multiplication Algorithm. This is only a wrapper around the main loop body. | |

* For algorithmic implementation details see the mont_loop subroutine. | |

* | |

* Flags: The states of both FG0 and FG1 depend on intermediate values and are | |

* not usable after return. | |

* | |

* @param[in] x16: dptr_M, dmem pointer to first limb of modulus M | |

* @param[in] x17: dptr_m0d, dmem pointer to Montgomery Constant m0' | |

* @param[in] x19: dptr_a, dmem pointer to first limb of operand A | |

* @param[in] x20: dptr_b, dmem pointer to first limb of operand B | |

* @param[in] w31: all-zero | |

* @param[in] x30: N, number of limbs | |

* @param[in] x31: N-1, number of limbs minus one | |

* @param[in] x9: pointer to temp reg, must be set to 3 | |

* @param[in] x10: pointer to temp reg, must be set to 4 | |

* @param[in] x11: pointer to temp reg, must be set to 2 | |

* @param[out] [w[4+N-1]:w4]: result C | |

* | |

* clobbered registers: x5, x6, x7, x8, x10, x12, x13, x16, x17, x19, x20 | |

* w2, w3, w24 to w30, w4 to w[4+N-1] | |

* clobbered Flag Groups: FG0, FG1 | |

*/ | |

montmul: | |

/* load Montgomery constant: w3 = dmem[x17] = dmem[dptr_m0d] = m0' */ | |

bn.lid x9, 0(x17) | |

/* init regfile bigint buffer with zeros */ | |

bn.mov w2, w31 | |

loop x30, 1 | |

bn.movr x10++, x11 | |

/* iterate over limbs of operand B */ | |

loop x30, 8 | |

/* load limb of operand b */ | |

bn.lid x11, 0(x20++) | |

/* save some regs */ | |

addi x5, x20, 0 | |

addi x6, x16, 0 | |

addi x7, x19, 0 | |

/* Main loop body of Montgomery Multiplication algorithm */ | |

jal x1, mont_loop | |

/* restore regs */ | |

addi x20, x5, 0 | |

addi x16, x6, 0 | |

addi x19, x7, 0 | |

/* restore pointers */ | |

li x8, 4 | |

li x10, 4 | |

ret | |

/** | |

* Conditionally overwrite bigint in dmem | |

* | |

* Depending on state of FG0.C overwrites a bigint in dmem with one from | |

* a buffer in the wide reg file. | |

* | |

* Flags: Does not set any flags, does not use flags except FG0.C | |

* | |

* @param[in] x21: dptr, pointer to first limb of bigint in dmem | |

* @param[in] x8: rptr, pointer to first limb of bigint in regfile buffer | |

* @param[in] FG.C: selection condition, overwrite dmem when FG0.C==1 | |

* @param[in] x30: number of limbs | |

* @param[in] x9: pointer to temp reg, must be set to 3 | |

* @param[in] x11: pointer to temp reg, must be set to 2 | |

* | |

* clobbered registers: x8, x21, w0, w2 | |

* clobbered Flag Groups: none | |

*/ | |

sel_sqr_or_sqrmul: | |

/* iterate over all limbs */ | |

loop x30, 6 | |

/* load limb from dmem */ | |

bn.lid x9, 0(x21) | |

/* store limb to dmem */ | |

bn.sid x11, 0(x21) | |

/* load limb from regfile buffer */ | |

bn.movr x11, x8++ | |

bn.mov w0, w2 | |

/* conditional select: w2 = FG0.C?w[x8+i]:dmem[x21+i] */ | |

bn.sel w2, w0, w3, C | |

/* store selected limb to dmem */ | |

bn.sid x11, 0(x21++) | |

ret | |

/** | |

* Constant-time bigint modular exponentiation | |

* | |

* Returns: C = modexp(A,E) = A^E mod M | |

* | |

* This implements the square and multiply algorithm, i.e. for each bit of the | |

* exponent both the squared only and the squared with multiply results are | |

* computed but one result is discarded. | |

* Computation is carried out in the Montgomery domain, by using the montmul | |

* primitive. | |

* The squared Montgomery modulus RR and the Montgomery constant m0' have to | |

* be precomputed and provided at the appropriate locations in dmem. | |

* | |

* Flags: The states of both FG0 and FG1 depend on intermediate values and are | |

* not usable after return. | |

* | |

* The base bignum A is expected in the input buffer, the exponent E in the | |

* exp buffer, the result C is written to the output buffer. | |

* Note, that the content of both, the input buffer and the exp buffer is | |

* modified during execution. | |

* | |

* @param[in] x2: dptr_c, dmem pointer to buffer for output C | |

* @param[in] x14: dptr_a, dmem pointer to first limb of input A | |

* @param[in] x15: dptr_e, dmem pointer to first limb of exponent E | |

* @param[in] x16: dptr_M, dmem pointer to first limb of modulus M | |

* @param[in] x17: dptr_m0d, dmem pointer to first limb of m0' | |

* @param[in] x18: dptr_RR, dmem pointer to first limb of RR | |

* @param[in] x30: N, number of limbs per bignum | |

* @param[in] w31: all-zero | |

* @param[out] dmem[dptr_c:dptr_c+N*32] C, A^E mod M | |

* | |

* clobbered registers: x3 to x13, x16 to x31 | |

* w0 to w3, w24 to w30 | |

* w4 to w[4+N-1] | |

* clobbered Flag Groups: FG0, FG1 | |

*/ | |

modexp: | |

/* prepare pointers to temp regs */ | |

li x8, 4 | |

li x9, 3 | |

li x10, 4 | |

li x11, 2 | |

/* Compute (N-1). | |

x31 <= x30 - 1 = N - 1 */ | |

addi x31, x30, -1 | |

/* Convert input to montgomery domain. | |

dmem[dptr_a] <= montmul(A,RR) = A*R mod M */ | |

addi x19, x14, 0 | |

addi x20, x18, 0 | |

addi x21, x14, 0 | |

jal x1, montmul | |

loop x30, 2 | |

bn.sid x8, 0(x21++) | |

addi x8, x8, 1 | |

/* zeroize w2 and reset flags */ | |

bn.sub w2, w2, w2 | |

/* initialize the output buffer with -M */ | |

addi x3, x16, 0 | |

addi x21, x2, 0 | |

loop x30, 3 | |

/* load limb from modulus */ | |

bn.lid x11, 0(x3++) | |

/* subtract limb from 0 */ | |

bn.subb w2, w31, w2 | |

/* store limb in dmem */ | |

bn.sid x11, 0(x21++) | |

/* compute bit length of current bigint size */ | |

slli x24, x30, 8 | |

/* iterate over all bits of bigint */ | |

loop x24, 20 | |

/* square: out = montmul(out,out) */ | |

addi x19, x2, 0 | |

addi x20, x2, 0 | |

addi x21, x2, 0 | |

jal x1, montmul | |

/* Store result in dmem starting at dmem[dptr_c] */ | |

loop x30, 2 | |

bn.sid x8, 0(x21++) | |

addi x8, x8, 1 | |

/* multiply: out = montmul(in,out) */ | |

addi x19, x14, 0 | |

addi x20, x2, 0 | |

addi x21, x2, 0 | |

jal x1, montmul | |

/* w2 <= w2 << 1 */ | |

bn.add w2, w2, w2 | |

/* the loop performs a 1-bit left shift of the exponent. Last MSB moves | |

to FG0.C, such that it can be used for selection */ | |

addi x20, x15, 0 | |

loop x30, 3 | |

bn.lid x11, 0(x20) | |

/* w2 <= w2 << 1 */ | |

bn.addc w2, w2, w2 | |

bn.sid x11, 0(x20++) | |

/* select squared or squared+multiplied result */ | |

addi x21, x2, 0 | |

jal x1, sel_sqr_or_sqrmul | |

nop | |

/* convert back from montgomery domain */ | |

/* out = montmul(out,1) = out/R mod M */ | |

addi x19, x2, 0 | |

addi x21, x2, 0 | |

jal x1, montmul_mul1 | |

ret | |

/** | |

* Bigint modular exponentiation with fixed exponent of 65537 | |

* | |

* Returns: C = modexp(A,65537) = A^65537 mod M | |

* | |

* This implements the square and multiply algorithm for the fixed exponent | |

* of E=65537. Note that this implementation (in contrast to modexp) runs the | |

* multiplication step only for bits being actually set in the exponent. | |

* Since the exponent is fixed, this is inherently constant-time. | |

* | |

* The squared Montgomery modulus RR and the Montgomery constant m0' have to | |

* be precomputed and provided at the appropriate locations in dmem. | |

* | |

* Flags: The states of both FG0 and FG1 depend on intermediate values and are | |

* not usable after return. | |

* | |

* The base bignum A is expected in the input buffer, the result C is written | |

* to the output buffer. Note, that the content of the input buffer is | |

* modified during execution. | |

* | |

* @param[in] x2: dptr_c, dmem pointer to buffer for output C | |

* @param[in] x14: dptr_a, dmem pointer to first linb of input A | |

* @param[in] x16: dptr_M, dmem pointer to first limb of modulus M | |

* @param[in] x17: dptr_m0d, dmem pointer to Mongtgomery constant m0' | |

* @param[in] x18: dptr_RR, dmem pointer to Montgmery constant RR | |

* @param[in] x30: N, number of limbs per bignum | |

* @param[in] w31: all-zero | |

* @param[out] dmem[dptr_c:dptr_c+N*32] C, A^65537 mod M | |

* | |

* clobbered registers: x3 to x13, x16 to x31 | |

* w0 to w3, w24 to w30 | |

* w4 to w[4+N-1] | |

* clobbered Flag Groups: FG0, FG1 | |

*/ | |

modexp_65537: | |

/* prepare pointers to temp regs */ | |

li x8, 4 | |

li x9, 3 | |

li x10, 4 | |

li x11, 2 | |

/* Compute (N-1). | |

x31 <= x30 - 1 = N - 1 */ | |

addi x31, x30, -1 | |

/* convert to montgomery domain montmul(A,RR) | |

in = montmul(A,RR) montmul(A,RR) = C*R mod M */ | |

addi x19, x14, 0 | |

addi x20, x18, 0 | |

addi x21, x14, 0 | |

jal x1, montmul | |

/* Store result in dmem starting at dmem[dptr_a] */ | |

loop x30, 2 | |

bn.sid x8, 0(x21++) | |

addi x8, x8, 1 | |

/* pointer to out buffer */ | |

addi x21, x2, 0 | |

/* zeroize w2 and reset flags */ | |

bn.sub w2, w2, w2 | |

/* pointer to modulus */ | |

addi x3, x16, 0 | |

/* this loop initializes the output buffer with -M */ | |

loop x30, 3 | |

/* load limb from modulus */ | |

bn.lid x11, 0(x3++) | |

/* subtract limb from 0 */ | |

bn.subb w2, w31, w2 | |

/* store limb in dmem */ | |

bn.sid x11, 0(x21++) | |

/* TODO: Is this squaring necessary? */ | |

/* 65537 = 0b10000000000000001 | |

^ sqr + mult | |

out = montmul(out,out) */ | |

addi x19, x2, 0 | |

addi x20, x2, 0 | |

jal x1, montmul | |

/* Store result in dmem starting at dmem[dptr_c] */ | |

addi x21, x2, 0 | |

loop x30, 2 | |

bn.sid x8, 0(x21++) | |

addi x8, x8, 1 | |

/* out = montmul(in,out) */ | |

addi x19, x14, 0 | |

addi x20, x2, 0 | |

jal x1, montmul | |

/* store multiplication result in output buffer */ | |

addi x21, x2, 0 | |

li x8, 4 | |

loop x30, 2 | |

bn.sid x8, 0(x21++) | |

addi x8, x8, 1 | |

/* 65537 = 0b10000000000000001 | |

^<< 16 x sqr >>^ */ | |

loopi 16, 8 | |

/* square: out = montmul(out, out) */ | |

addi x19, x2, 0 | |

addi x20, x2, 0 | |

jal x1, montmul | |

/* Store result in dmem starting at dmem[dptr_c] */ | |

addi x21, x2, 0 | |

loop x30, 2 | |

bn.sid x8, 0(x21++) | |

addi x8, x8, 1 | |

nop | |

/* 65537 = 0b10000000000000001 | |

mult ^ | |

out = montmul(in,out) */ | |

addi x19, x14, 0 | |

addi x20, x2, 0 | |

jal x1, montmul | |

/* store multiplication result in output buffer */ | |

addi x21, x2, 0 | |

li x8, 4 | |

loop x30, 2 | |

bn.sid x8, 0(x21++) | |

addi x8, x8, 1 | |

/* convert back from montgomery domain */ | |

/* out = montmul(out,1) = out/R mod M */ | |

addi x19, x2, 0 | |

addi x21, x2, 0 | |

jal x1, montmul_mul1 | |

ret | |

/** | |

* Externally callable wrapper for computing Montgomery parameters | |

* | |

* Computes: | |

* - Montgomery Constant m0' | |

* - Squared Montgomery modulus RR mod N | |

* | |

* Needs to be executed once per constant Modulus. | |

* | |

* @param[in] x16: dptr_M, dmem pointer to first limb of modulus M | |

* @param[in] x17: dptr_m0d, dmem pointer to buffer for m0' | |

* @param[in] x18: dptr_RR, dmem pointer to buffer for RR | |

* @param[in] x30: N, number of limbs per bignum | |

* @param[in] w31: all-zero | |

* @param[out] [dmem[dptr_m0d+31]:dmem[dptr_m0d]] computed m0' | |

* @param[out] [dmem[dptr_RR+N*32-1]:dmem[dptr_RR]] computed RR | |

*/ | |

modload: | |

/* load lowest limb of modulus to w28 */ | |

li x8, 28 | |

bn.lid x8, 0(x16) | |

/* Compute Montgomery constant */ | |

jal x1, m0inv | |

/* Store Montgomery constant in dmem */ | |

li x9, 29 | |

bn.sid x9, 0(x17) | |

/* Compute square of Montgomery modulus */ | |

jal x1, compute_rr | |

ret |