path: root/contrib/restricted/aws/s2n/pq-crypto/bike_r3/decode.c

/* Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 * SPDX-License-Identifier: Apache-2.0"
 *
 * Written by Nir Drucker, Shay Gueron and Dusan Kostic,
 * AWS Cryptographic Algorithms Group.
 *
 * [1] The optimizations are based on the description developed in the paper:
 *     Drucker, Nir, and Shay Gueron. 2019. “A Toolbox for Software Optimization
 *     of QC-MDPC Code-Based Cryptosystems.” Journal of Cryptographic Engineering,
 *     January, 1–17. https://doi.org/10.1007/s13389-018-00200-4.
 *
 * [2] The decoder algorithm is the Black-Gray decoder in
 *     the early submission of CAKE (due to N. Sandrier and R Misoczki).
 *
 * [3] The analysis for the constant time implementation is given in
 *     Drucker, Nir, Shay Gueron, and Dusan Kostic. 2019.
 *     “On Constant-Time QC-MDPC Decoding with Negligible Failure Rate.”
 *     Cryptology EPrint Archive, 2019. https://eprint.iacr.org/2019/1289.
 *
 * [4] it was adapted to BGF in:
 *     Drucker, Nir, Shay Gueron, and Dusan Kostic. 2019.
 *     “QC-MDPC decoders with several shades of gray.”
 *     Cryptology EPrint Archive, 2019. To be published.
 *
 * [5] Chou, T.: QcBits: Constant-Time Small-Key Code-Based Cryptography.
 *     In: Gier-lichs, B., Poschmann, A.Y. (eds.) Cryptographic Hardware
 *     and Embedded Systems– CHES 2016. pp. 280–300. Springer Berlin Heidelberg,
 *     Berlin, Heidelberg (2016)
 *
 * [6] The rotate512_small funciton is a derivative of the code described in:
 *     Guimarães, Antonio, Diego F Aranha, and Edson Borin. 2019.
 *     “Optimized Implementation of QC-MDPC Code-Based Cryptography.”
 *     Concurrency and Computation: Practice and Experience 31 (18):
 *     e5089. https://doi.org/10.1002/cpe.5089.
 */

#include "decode.h"
#include "cleanup.h"
#include "decode_internal.h"
#include "gf2x.h"
#include "utilities.h"

// Decoding (bit-flipping) parameter
#if defined(BG_DECODER)
#  if(LEVEL == 1)
#    define MAX_IT 3
#  elif(LEVEL == 3)
#    define MAX_IT 4
#  else
#    error "Level can only be 1/3"
#  endif
#elif defined(BGF_DECODER)
#  if(LEVEL == 1)
#    define MAX_IT 5
#  elif(LEVEL == 3)
#    define MAX_IT 5
#  else
#    error "Level can only be 1/3"
#  endif
#endif

ret_t compute_syndrome(OUT syndrome_t *syndrome,
                       IN const pad_r_t *c0,
                       IN const pad_r_t *h0,
                       IN const decode_ctx *ctx)
{
  DEFER_CLEANUP(pad_r_t pad_s, pad_r_cleanup);

  gf2x_mod_mul(&pad_s, c0, h0);

  bike_memcpy((uint8_t *)syndrome->qw, pad_s.val.raw, R_BYTES);
  ctx->dup(syndrome);

  return SUCCESS;
}

_INLINE_ ret_t recompute_syndrome(OUT syndrome_t *syndrome,
                                  IN const pad_r_t *c0,
                                  IN const pad_r_t *h0,
                                  IN const pad_r_t *pk,
                                  IN const e_t *e,
                                  IN const decode_ctx *ctx)
{
  DEFER_CLEANUP(pad_r_t tmp_c0, pad_r_cleanup);
  DEFER_CLEANUP(pad_r_t e0 = {0}, pad_r_cleanup);
  DEFER_CLEANUP(pad_r_t e1 = {0}, pad_r_cleanup);

  e0.val = e->val[0];
  e1.val = e->val[1];

  // tmp_c0 = pk * e1 + c0 + e0
  gf2x_mod_mul(&tmp_c0, &e1, pk);
  gf2x_mod_add(&tmp_c0, &tmp_c0, c0);
  gf2x_mod_add(&tmp_c0, &tmp_c0, &e0);

  // Recompute the syndrome using the updated ciphertext
  POSIX_GUARD(compute_syndrome(syndrome, &tmp_c0, h0, ctx));

  return SUCCESS;
}

_INLINE_ uint8_t get_threshold(IN const syndrome_t *s)
{
  bike_static_assert(sizeof(*s) >= sizeof(r_t), syndrome_is_large_enough);

  const uint32_t syndrome_weight = r_bits_vector_weight((const r_t *)s->qw);

  // The equations below are defined in BIKE's specification p. 16, Section 5.2
  uint32_t       thr  = THRESHOLD_COEFF0 + (THRESHOLD_COEFF1 * syndrome_weight);
  const uint32_t mask = secure_l32_mask(thr, THRESHOLD_MIN);
  thr = (u32_barrier(mask) & thr) | (u32_barrier(~mask) & THRESHOLD_MIN);

  DMSG("    Threshold: %d\n", thr);
  return thr;
}

// Calculate the Unsatisfied Parity Checks (UPCs) and update the errors
// vector (e) accordingly. In addition, update the black and gray errors vector
// with the relevant values.
_INLINE_ void find_err1(OUT e_t *e,
                        OUT e_t *black_e,
                        OUT e_t *gray_e,
                        IN const syndrome_t *          syndrome,
                        IN const compressed_idx_d_ar_t wlist,
                        IN const uint8_t               threshold,
                        IN const decode_ctx *ctx)
{
  // This function uses the bit-slice-adder methodology of [5]:
  DEFER_CLEANUP(syndrome_t rotated_syndrome = {0}, syndrome_cleanup);
  DEFER_CLEANUP(upc_t upc, upc_cleanup);

  for(uint32_t i = 0; i < N0; i++) {
    // UPC must start from zero at every iteration
    bike_memset(&upc, 0, sizeof(upc));

    // 1) Right-rotate the syndrome for every secret key set bit index
    //    Then slice-add it to the UPC array.
    for(size_t j = 0; j < DV; j++) {
      ctx->rotate_right(&rotated_syndrome, syndrome, wlist[i].val[j]);
      ctx->bit_sliced_adder(&upc, &rotated_syndrome, LOG2_MSB(j + 1));
    }

    // 2) Subtract the threshold from the UPC counters
    ctx->bit_slice_full_subtract(&upc, threshold);

    // 3) Update the errors and the black errors vectors.
    //    The last slice of the UPC array holds the MSB of the accumulated values
    //    minus the threshold. Every zero bit indicates a potential error bit.
    //    The errors values are stored in the black array and xored with the
    //    errors Of the previous iteration.
    const r_t *last_slice = &(upc.slice[SLICES - 1].u.r.val);
    for(size_t j = 0; j < R_BYTES; j++) {
      const uint8_t sum_msb  = (~last_slice->raw[j]);
      black_e->val[i].raw[j] = sum_msb;
      e->val[i].raw[j] ^= sum_msb;
    }

    // Ensure that the padding bits (upper bits of the last byte) are zero so
    // they will not be included in the multiplication and in the hash function.
    e->val[i].raw[R_BYTES - 1] &= LAST_R_BYTE_MASK;

    // 4) Calculate the gray error array by adding "DELTA" to the UPC array.
    //    For that we reuse the rotated_syndrome variable setting it to all "1".
    for(size_t l = 0; l < DELTA; l++) {
      bike_memset((uint8_t *)rotated_syndrome.qw, 0xff, R_BYTES);
      ctx->bit_sliced_adder(&upc, &rotated_syndrome, SLICES);
    }

    // 5) Update the gray list with the relevant bits that are not
    //    set in the black list.
    for(size_t j = 0; j < R_BYTES; j++) {
      const uint8_t sum_msb = (~last_slice->raw[j]);
      gray_e->val[i].raw[j] = (~(black_e->val[i].raw[j])) & sum_msb;
    }
  }
}

// Recalculate the UPCs and update the errors vector (e) according to it
// and to the black/gray vectors.
_INLINE_ void find_err2(OUT e_t *e,
                        IN e_t * pos_e,
                        IN const syndrome_t *          syndrome,
                        IN const compressed_idx_d_ar_t wlist,
                        IN const uint8_t               threshold,
                        IN const decode_ctx *ctx)
{
  DEFER_CLEANUP(syndrome_t rotated_syndrome = {0}, syndrome_cleanup);
  DEFER_CLEANUP(upc_t upc, upc_cleanup);

  for(uint32_t i = 0; i < N0; i++) {
    // UPC must start from zero at every iteration
    bike_memset(&upc, 0, sizeof(upc));

    // 1) Right-rotate the syndrome, for every index of a set bit in the secret
    // key. Then slice-add it to the UPC array.
    for(size_t j = 0; j < DV; j++) {
      ctx->rotate_right(&rotated_syndrome, syndrome, wlist[i].val[j]);
      ctx->bit_sliced_adder(&upc, &rotated_syndrome, LOG2_MSB(j + 1));
    }

    // 2) Subtract the threshold from the UPC counters
    ctx->bit_slice_full_subtract(&upc, threshold);

    // 3) Update the errors vector.
    //    The last slice of the UPC array holds the MSB of the accumulated values
    //    minus the threshold. Every zero bit indicates a potential error bit.
    const r_t *last_slice = &(upc.slice[SLICES - 1].u.r.val);
    for(size_t j = 0; j < R_BYTES; j++) {
      const uint8_t sum_msb = (~last_slice->raw[j]);
      e->val[i].raw[j] ^= (pos_e->val[i].raw[j] & sum_msb);
    }

    // Ensure that the padding bits (upper bits of the last byte) are zero, so
    // they are not included in the multiplication, and in the hash function.
    e->val[i].raw[R_BYTES - 1] &= LAST_R_BYTE_MASK;
  }
}

ret_t decode(OUT e_t *e, IN const ct_t *ct, IN const sk_t *sk)
{
  // Initialize the decode methods struct
  decode_ctx ctx;
  decode_ctx_init(&ctx);

  DEFER_CLEANUP(e_t black_e = {0}, e_cleanup);
  DEFER_CLEANUP(e_t gray_e = {0}, e_cleanup);

  DEFER_CLEANUP(pad_r_t c0 = {0}, pad_r_cleanup);
  DEFER_CLEANUP(pad_r_t h0 = {0}, pad_r_cleanup);
  pad_r_t pk = {0};

  // Pad ciphertext (c0), secret key (h0), and public key (h)
  c0.val = ct->c0;
  h0.val = sk->bin[0];
  pk.val = sk->pk;

  DEFER_CLEANUP(syndrome_t s = {0}, syndrome_cleanup);
  DMSG("  Computing s.\n");
  POSIX_GUARD(compute_syndrome(&s, &c0, &h0, &ctx));
  ctx.dup(&s);

  // Reset (init) the error because it is xored in the find_err functions.
  bike_memset(e, 0, sizeof(*e));

  for(uint32_t iter = 0; iter < MAX_IT; iter++) {
    const uint8_t threshold = get_threshold(&s);

    DMSG("    Iteration: %d\n", iter);
    DMSG("    Weight of e: %lu\n",
         r_bits_vector_weight(&e->val[0]) + r_bits_vector_weight(&e->val[1]));
    DMSG("    Weight of syndrome: %lu\n", r_bits_vector_weight((r_t *)s.qw));

    find_err1(e, &black_e, &gray_e, &s, sk->wlist, threshold, &ctx);
    POSIX_GUARD(recompute_syndrome(&s, &c0, &h0, &pk, e, &ctx));
#if defined(BGF_DECODER)
    if(iter >= 1) {
      continue;
    }
#endif
    DMSG("    Weight of e: %lu\n",
         r_bits_vector_weight(&e->val[0]) + r_bits_vector_weight(&e->val[1]));
    DMSG("    Weight of syndrome: %lu\n", r_bits_vector_weight((r_t *)s.qw));

    find_err2(e, &black_e, &s, sk->wlist, ((DV + 1) / 2) + 1, &ctx);
    POSIX_GUARD(recompute_syndrome(&s, &c0, &h0, &pk, e, &ctx));

    DMSG("    Weight of e: %lu\n",
         r_bits_vector_weight(&e->val[0]) + r_bits_vector_weight(&e->val[1]));
    DMSG("    Weight of syndrome: %lu\n", r_bits_vector_weight((r_t *)s.qw));

    find_err2(e, &gray_e, &s, sk->wlist, ((DV + 1) / 2) + 1, &ctx);
    POSIX_GUARD(recompute_syndrome(&s, &c0, &h0, &pk, e, &ctx));
  }

  if(r_bits_vector_weight((r_t *)s.qw) > 0) {
    BIKE_ERROR(E_DECODING_FAILURE);
  }

  return SUCCESS;
}