git.ozlabs.org Git - ccan/blob - ccan/crypto/shachain/shachain.c

   1 /* MIT (BSD) license - see LICENSE file for details */
   2 #include <ccan/crypto/shachain/shachain.h>
   3 #include <ccan/ilog/ilog.h>
   4 #include <limits.h>
   5 #include <string.h>
   6 #include <assert.h>
   7
   8 static void change_bit(unsigned char *arr, size_t index)
   9 {
  10         arr[index / CHAR_BIT] ^= (1 << (index % CHAR_BIT));
  11 }
  12
  13 /* We can only ever *unset* bits, so to must only have bits in from. */
  14 static bool can_derive(shachain_index_t from, shachain_index_t to)
  15 {
  16         return (~from & to) == 0;
  17 }
  18
  19 static void derive(shachain_index_t from, shachain_index_t to,
  20                    const struct sha256 *from_hash,
  21                    struct sha256 *hash)
  22 {
  23         shachain_index_t branches;
  24         int i;
  25
  26         assert(can_derive(from, to));
  27
  28         /* We start with the first hash. */
  29         *hash = *from_hash;
  30
  31         /* This represents the bits set in from, and not to. */
  32         branches = from ^ to;
  33         for (i = ilog64(branches) - 1; i >= 0; i--) {
  34                 if (((branches >> i) & 1)) {
  35                         change_bit(hash->u.u8, i);
  36                         sha256(hash, hash, sizeof(*hash));
  37                 }
  38         }
  39 }
  40
  41 void shachain_from_seed(const struct sha256 *seed, shachain_index_t index,
  42                         struct sha256 *hash)
  43 {
  44         derive((shachain_index_t)-1ULL, index, seed, hash);
  45 }
  46
  47 void shachain_init(struct shachain *chain)
  48 {
  49         chain->num_valid = 0;
  50         chain->max_index = 0;
  51 }
  52
  53 bool shachain_add_hash(struct shachain *chain,
  54                        shachain_index_t index, const struct sha256 *hash)
  55 {
  56         int i;
  57
  58         /* You have to insert them in order! */
  59         assert(index == chain->max_index + 1 ||
  60                (index == 0 && chain->num_valid == 0));
  61
  62         for (i = 0; i < chain->num_valid; i++) {
  63                 /* If we could derive this value, we don't need it,
  64                  * not any others (since they're in order). */
  65                 if (can_derive(index, chain->known[i].index)) {
  66                         struct sha256 expect;
  67
  68                         /* Make sure the others derive as expected! */
  69                         derive(index, chain->known[i].index, hash, &expect);
  70                         if (memcmp(&expect, &chain->known[i].hash,
  71                                    sizeof(expect)) != 0)
  72                                 return false;
  73                         break;
  74                 }
  75         }
  76
  77         /* This can happen if you skip indices! */
  78         assert(i < sizeof(chain->known) / sizeof(chain->known[0]));
  79         chain->known[i].index = index;
  80         chain->known[i].hash = *hash;
  81         chain->num_valid = i+1;
  82         chain->max_index = index;
  83         return true;
  84 }
  85
  86 bool shachain_get_hash(const struct shachain *chain,
  87                        shachain_index_t index, struct sha256 *hash)
  88 {
  89         int i;
  90
  91         for (i = 0; i < chain->num_valid; i++) {
  92                 /* If we can get from key to index only by resetting bits,
  93                  * we can derive from it => index has no bits key doesn't. */
  94                 if (!can_derive(chain->known[i].index, index))
  95                         continue;
  96
  97                 derive(chain->known[i].index, index, &chain->known[i].hash,
  98                        hash);
  99                 return true;
 100         }
 101         return false;
 102 }