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Barrett-based quotient computation for compression functions #83

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Barrett-based quotient computation for compression functions #83

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61 changes: 34 additions & 27 deletions ref/poly.c
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Original file line number Diff line number Diff line change
Expand Up @@ -20,22 +20,23 @@ void poly_compress(uint8_t r[KYBER_POLYCOMPRESSEDBYTES], const poly *a)
{
unsigned int i,j;
int16_t u;
uint32_t d0;
uint8_t t[8];

#if (KYBER_POLYCOMPRESSEDBYTES == 128)

for(i=0;i<KYBER_N/8;i++) {
for(j=0;j<8;j++) {
// map to positive standard representatives
u = a->coeffs[8*i+j];
u += (u >> 15) & KYBER_Q;
/* t[j] = ((((uint16_t)u << 4) + KYBER_Q/2)/KYBER_Q) & 15; */
d0 = u << 4;
d0 += 1665;
d0 *= 80635;
d0 >>= 28;
t[j] = d0 & 0xf;

// 16-bit precision suffices for round(2^4 x / q)
// inputs are in [-q/2, ..., q/2]
// 315 = round(16 * 2^16 / q)
t[j] = (int16_t)(((int32_t)u * 315 + (1 << 15)) >> 16) & 0xf;

// this is equivalent to first mapping to positive
// standard representatives followed by
// t[j] = ((((uint16_t)u << 4) + KYBER_Q/2)/KYBER_Q) & 0xf;

}

r[0] = t[0] | (t[1] << 4);
Expand All @@ -47,15 +48,17 @@ void poly_compress(uint8_t r[KYBER_POLYCOMPRESSEDBYTES], const poly *a)
#elif (KYBER_POLYCOMPRESSEDBYTES == 160)
for(i=0;i<KYBER_N/8;i++) {
for(j=0;j<8;j++) {
// map to positive standard representatives
u = a->coeffs[8*i+j];
u += (u >> 15) & KYBER_Q;
/* t[j] = ((((uint32_t)u << 5) + KYBER_Q/2)/KYBER_Q) & 31; */
d0 = u << 5;
d0 += 1664;
d0 *= 40318;
d0 >>= 27;
t[j] = d0 & 0x1f;

// 15-bit precision suffices for round(2^5 x / q)
// inputs are in [-q/2, ..., q/2]
// 315 = round(32 * 2^15 / q)
t[j] = (int16_t)(((int32_t)u * 315 + (1 << 14)) >> 15) & 0x1f;

// this is equivalent to first mapping to positive
// standard representatives followed by
// t[j] = ((((uint32_t)u << 5) + KYBER_Q/2)/KYBER_Q) & 0x1f;

}

r[0] = (t[0] >> 0) | (t[1] << 5);
Expand Down Expand Up @@ -192,20 +195,24 @@ void poly_frommsg(poly *r, const uint8_t msg[KYBER_INDCPA_MSGBYTES])
void poly_tomsg(uint8_t msg[KYBER_INDCPA_MSGBYTES], const poly *a)
{
unsigned int i,j;
uint32_t t;
int16_t u;

for(i=0;i<KYBER_N/8;i++) {
msg[i] = 0;
for(j=0;j<8;j++) {
t = a->coeffs[8*i+j];
// t += ((int16_t)t >> 15) & KYBER_Q;
// t = (((t << 1) + KYBER_Q/2)/KYBER_Q) & 1;
t <<= 1;
t += 1665;
t *= 80635;
t >>= 28;
t &= 1;
msg[i] |= t << j;
u = a->coeffs[8*i+j];

// 19-bit precision suffices for round(2 x / q)
// inputs are in [-q/2, ..., q/2]
// 315 = round(2 * 2^19 / q)
u = (int16_t)(((int32_t)u * 315 + (1 << 18)) >> 19) & 1;

// this is equivalent to first mapping to positive
// standard representatives followed by
// u = ((((uint16_t)u << 1) + KYBER_Q/2)/KYBER_Q) & 1;

msg[i] |= u << j;

}
}
}
Expand Down
42 changes: 23 additions & 19 deletions ref/polyvec.c
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Original file line number Diff line number Diff line change
Expand Up @@ -15,22 +15,24 @@
void polyvec_compress(uint8_t r[KYBER_POLYVECCOMPRESSEDBYTES], const polyvec *a)
{
unsigned int i,j,k;
uint64_t d0;
int16_t u;

#if (KYBER_POLYVECCOMPRESSEDBYTES == (KYBER_K * 352))
uint16_t t[8];
for(i=0;i<KYBER_K;i++) {
for(j=0;j<KYBER_N/8;j++) {
for(k=0;k<8;k++) {
t[k] = a->vec[i].coeffs[8*j+k];
t[k] += ((int16_t)t[k] >> 15) & KYBER_Q;
/* t[k] = ((((uint32_t)t[k] << 11) + KYBER_Q/2)/KYBER_Q) & 0x7ff; */
d0 = t[k];
d0 <<= 11;
d0 += 1664;
d0 *= 645084;
d0 >>= 31;
t[k] = d0 & 0x7ff;
u = a->vec[i].coeffs[8*j+k];

// 21-bit suffices for round(2048 x / q)
// inputs are in [-q/2, ..., q/2]
// 1290167 = round(2048 * 2^21 / q)
t[k] = ((int16_t)(((int32_t)u * 1290167 + (1 << 20)) >> 21)) & 0x7ff;

// this is equivalent to first mapping to positive
// standard representatives followed by
// t[k] = ((((uint32_t)u << 11) + KYBER_Q/2)/KYBER_Q) & 0x7ff;

}

r[ 0] = (t[0] >> 0);
Expand All @@ -52,15 +54,17 @@ void polyvec_compress(uint8_t r[KYBER_POLYVECCOMPRESSEDBYTES], const polyvec *a)
for(i=0;i<KYBER_K;i++) {
for(j=0;j<KYBER_N/4;j++) {
for(k=0;k<4;k++) {
t[k] = a->vec[i].coeffs[4*j+k];
t[k] += ((int16_t)t[k] >> 15) & KYBER_Q;
/* t[k] = ((((uint32_t)t[k] << 10) + KYBER_Q/2)/ KYBER_Q) & 0x3ff; */
d0 = t[k];
d0 <<= 10;
d0 += 1665;
d0 *= 1290167;
d0 >>= 32;
t[k] = d0 & 0x3ff;
u = a->vec[i].coeffs[4*j+k];

// 22-bit suffices for round(1024 x / q)
// inputs are in [-q/2, ..., q/2]
// 1290167 = round(1024 * 2^22 / q)
t[k] = ((int16_t)(((int32_t)u * 1290167 + (1 << 21)) >> 22)) & 0x3ff;

// this is equivalent to first mapping to positive
// standard representatives followed by
// t[k] = ((((uint32_t)u << 10) + KYBER_Q/2)/ KYBER_Q) & 0x3ff;

}

r[0] = (t[0] >> 0);
Expand Down