Don't make scary messages just because one key didn't work on a message:

[tripe] / keyset.c

/* -*-c-*-
 *
 * $Id: keyset.c,v 1.7 2003/05/17 11:00:47 mdw Exp $
 *
 * Handling of symmetric keysets
 *
 * (c) 2001 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------* 
 *
 * This file is part of Trivial IP Encryption (TrIPE).
 *
 * TrIPE is free software; you can redistribute it and/or modify
 * it under the terms of 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.
 * 
 * TrIPE 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 a copy of the GNU General Public License
 * along with TrIPE; if not, write to the Free Software Foundation,
 * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */

/*----- Revision history --------------------------------------------------* 
 *
 * $Log: keyset.c,v $
 * Revision 1.7  2003/05/17 11:00:47  mdw
 * Don't make scary messages just because one key didn't work on a message:
 * only be frightened if they all fail.  Set initial keyset refcount
 * correctly.
 *
 * Revision 1.6  2003/04/06 10:26:35  mdw
 * Report peer name on decrypt errors.
 *
 * Revision 1.5  2001/06/19 22:07:43  mdw
 * Change the encrypted packet format to be non-malleable.
 *
 * Revision 1.4  2001/06/16 14:06:40  mdw
 * Quantify collision probabilities for the stated data volume bounds.
 *
 * Revision 1.3  2001/02/16 21:39:55  mdw
 * Major overhaul.  Separate functions for manipulating keysets from
 * functions for manipulating keyset lists.  Introduce a concept of
 * listening-only keys.
 *
 * Revision 1.2  2001/02/05 19:53:23  mdw
 * Add sequence number protection.
 *
 * Revision 1.1  2001/02/03 20:26:37  mdw
 * Initial checkin.
 *
 */

/*----- Header files ------------------------------------------------------*/

#include "tripe.h"

/*----- Tunable parameters ------------------------------------------------*/

/* --- Note on size limits --- *
 *
 * For a 64-bit block cipher (e.g., Blowfish), the probability of a collision
 * occurring after 32 MB is less than %$2^{-21}$%, and the probability of a
 * collision occurring after 64 MB is less than %$2^{-19}$%.
 */

#define T_EXP MIN(60)                   /* Expiry time for a key */
#define T_REGEN MIN(45)                 /* Regeneration time for a key */
#define SZ_EXP MEG(64)                  /* Expiry data size for a key */
#define SZ_REGEN MEG(32)                /* Data size threshold for regen */

/*----- Handy macros ------------------------------------------------------*/

#define KEYOK(ks, now) ((ks)->sz_exp > 0 && (ks)->t_exp > now)

/*----- Low-level packet encryption and decryption ------------------------*/

/* --- Encrypted data format --- *
 *
 * Let %$p_i$% be the %$i$%-th plaintext message.  We first compute
 *
 *   %$c_i = \mathcal{E}\textrm{-CBC}_{K_{\text{E}}}(p_i)$%
 *
 * as the CBC-ciphertext of %$p_i$%, and then
 *
 *   %$\sigma_i = \mathcal{T}_{K_{\text{M}}}(i, c_i)$%
 *
 * as a MAC on the %%\emph{ciphertext}%%.  The message sent is then the pair
 * %$(\sigma_i, c_i)$%.  This construction is provably secure in the NM-CCA
 * sense (assuming that the cipher is IND-CPA, and the MAC is SUF-CMA)
 * [Bellare and Namprempre].
 *
 * This also ensures that, assuming the key is good, we have a secure channel
 * [Krawczyk].  Actually, [Krawczyk] shows that, if the cipher is either a
 * simple stream cipher or a block cipher in CBC mode, we can use the MAC-
 * then-encrypt scheme and still have a secure channel.  However, I like the
 * NM-CCA guarantee from [Bellare and Namprempre].  I'm less worried about
 * the Horton Principle [Wagner and Schneier].
 */

/* --- @doencrypt@ --- *
 *
 * Arguments:   @keyset *ks@ = pointer to keyset to use
 *              @buf *b@ = pointer to an input buffer
 *              @buf *bb@ = pointer to an output buffer
 *
 * Returns:     Zero if OK, nonzero if a new key is required.
 *
 * Use:         Encrypts a message with the given key.  We assume that the
 *              keyset is OK to use.
 */

static int doencrypt(keyset *ks, buf *b, buf *bb)
{
  ghash *h;
  gcipher *c;
  const octet *p = BCUR(b);
  size_t sz = BLEFT(b);
  octet *qmac, *qseq, *qiv, *qpk;
  uint32 oseq;
  size_t osz, nsz;
  int rc = 0;

  /* --- Allocate the required buffer space --- */

  c = ks->cout;
  if (buf_ensure(bb, MACSZ + SEQSZ + IVSZ + sz))
    return (0); /* Caution! */
  qmac = BCUR(bb); qseq = qmac + MACSZ; qiv = qseq + SEQSZ; qpk = qiv + IVSZ;
  BSTEP(bb, MACSZ + SEQSZ + IVSZ + sz);

  /* --- Encrypt the packet --- */

  oseq = ks->oseq++; STORE32(qseq, oseq);
  rand_get(RAND_GLOBAL, qiv, IVSZ);
  c->ops->setiv(c, qiv);
  c->ops->encrypt(c, p, qpk, sz);
  IF_TRACING(T_KEYSET, {
    trace(T_KEYSET, "keyset: encrypting packet %lu using keyset %u",
          (unsigned long)oseq, ks->seq);
    trace_block(T_CRYPTO, "crypto: encrypted packet", qpk, sz);
  })

  /* --- Now compute the MAC --- */

  h = ks->mout->ops->init(ks->mout);
  h->ops->hash(h, qseq, SEQSZ + IVSZ + sz);
  memcpy(qmac, h->ops->done(h, 0), MACSZ);
  h->ops->destroy(h);
  IF_TRACING(T_KEYSET, {
    trace_block(T_CRYPTO, "crypto: computed MAC", qmac, MACSZ);
  })

  /* --- Deduct the packet size from the key's data life --- */

  osz = ks->sz_exp;
  if (osz > sz)
    nsz = osz - sz;
  else
    nsz = 0;
  if (osz >= SZ_REGEN && nsz < SZ_REGEN) {
    T( trace(T_KEYSET, "keyset: keyset %u data regen limit exceeded -- "
             "forcing exchange", ks->seq); )
    rc = -1;
  }
  ks->sz_exp = nsz;
  return (rc);  
}

/* --- @dodecrypt@ --- *
 *
 * Arguments:   @keyset *ks@ = pointer to keyset to use
 *              @buf *b@ = pointer to an input buffer
 *              @buf *bb@ = pointer to an output buffer
 *              @uint32 *seq@ = where to store the sequence number
 *
 * Returns:     Zero if OK, nonzero if it failed.
 *
 * Use:         Attempts to decrypt a message with the given key.  No other
 *              checking (e.g., sequence number checks) is performed.  We
 *              assume that the keyset is OK to use, and that there is
 *              sufficient output buffer space reserved.  If the decryption
 *              is successful, the buffer pointer is moved past the decrypted
 *              packet, and the packet's sequence number is stored in @*seq@.
 */

static int dodecrypt(keyset *ks, buf *b, buf *bb, uint32 *seq)
{
  const octet *pmac, *piv, *pseq, *ppk;
  size_t psz = BLEFT(b);
  size_t sz;
  octet *q = BCUR(bb);
  ghash *h;
  gcipher *c = ks->cin;
  size_t ivsz = c->ops->c->blksz;
  octet *mac;
  int eq;

  /* --- Break up the packet into its components --- */

  if (psz < ivsz + 4) {
    T( trace(T_KEYSET, "keyset: block too small for keyset %u", ks->seq); )
    return (-1);
  }
  sz = psz - IVSZ - SEQSZ - MACSZ;
  pmac = BCUR(b); pseq = pmac + MACSZ; piv = pseq + SEQSZ; ppk = piv + IVSZ;

  /* --- Verify the MAC on the packet --- */

  h = ks->min->ops->init(ks->min);
  h->ops->hash(h, pseq, SEQSZ + IVSZ + sz);
  mac = h->ops->done(h, 0);
  eq = !memcmp(mac, pmac, MACSZ);
  IF_TRACING(T_KEYSET, {
    trace(T_KEYSET, "keyset: decrypting using keyset %u", ks->seq);
    trace_block(T_CRYPTO, "crypto: computed MAC", mac, MACSZ);
  })
  h->ops->destroy(h);
  if (!eq) {
    IF_TRACING(T_KEYSET, {
      trace(T_KEYSET, "keyset: incorrect MAC: decryption failed");
      trace_block(T_CRYPTO, "crypto: expected MAC", pmac, MACSZ);
    })
    return (-1);
  }

  /* --- Decrypt the packet --- */

  c->ops->setiv(c, piv);
  c->ops->decrypt(c, ppk, q, sz);
  if (seq)
    *seq = LOAD32(pseq);
  IF_TRACING(T_KEYSET, {
    trace(T_KEYSET, "keyset: decrypted OK (sequence = %lu)",
          (unsigned long)LOAD32(pseq));
    trace_block(T_CRYPTO, "crypto: decrypted packet", q, sz);
  })
  BSTEP(bb, sz);
  return (0);
}

/* --- @dosequence@ --- *
 *
 * Arguments:   @keyset *ks@ = pointer to a keyset
 *              @uint32 seq@ = a sequence number from a packet
 *
 * Returns:     Zero if the sequence number is OK, nonzero if it's not.
 *
 * Use:         Checks a sequence number.  The data in the keyset which keeps
 *              track of valid sequence numbers is updated if the sequence
 *              number given is good.  It's assumed that the sequence number
 *              has already been checked for authenticity.
 */

static int dosequence(keyset *ks, uint32 seq)
{
  uint32 seqbit;
  uint32 n;

  if (seq < ks->iseq) {
    a_warn("received packet has old sequence number (possible replay)");
    return (-1);
  }
  if (seq >= ks->iseq + KS_SEQWINSZ) {
    n = seq - (ks->iseq + KS_SEQWINSZ - 1);
    if (n < KS_SEQWINSZ)
      ks->iwin >>= n;
    else
      ks->iwin = 0;
    ks->iseq += n;
  }
  seqbit = 1 << (seq - ks->iseq);
  if (ks->iwin & seqbit) {
    a_warn("received packet repeats old sequence number");
    return (-1);
  }
  ks->iwin |= seqbit;
  return (0);
}

/*----- Operations on a single keyset -------------------------------------*/

/* --- @ks_drop@ --- *
 *
 * Arguments:   @keyset *ks@ = pointer to a keyset
 *
 * Returns:     ---
 *
 * Use:         Decrements a keyset's reference counter.  If the counter hits
 *              zero, the keyset is freed.
 */

void ks_drop(keyset *ks)
{
  if (--ks->ref)
    return;
  ks->cin->ops->destroy(ks->cin);
  ks->cout->ops->destroy(ks->cout);
  ks->min->ops->destroy(ks->min);
  ks->mout->ops->destroy(ks->mout);
  DESTROY(ks);
}

/* --- @ks_gen@ --- *
 *
 * Arguments:   @const void *k@ = pointer to key material
 *              @size_t x, y, z@ = offsets into key material (see below)
 *              @peer *p@ = pointer to peer information 
 *
 * Returns:     A pointer to the new keyset.
 *
 * Use:         Derives a new keyset from the given key material.  The
 *              offsets @x@, @y@ and @z@ separate the key material into three
 *              parts.  Between the @k@ and @k + x@ is `my' contribution to
 *              the key material; between @k + x@ and @k + y@ is `your'
 *              contribution; and between @k + y@ and @k + z@ is a shared
 *              value we made together.  These are used to construct two
 *              pairs of symmetric keys.  Each pair consists of an encryption
 *              key and a message authentication key.  One pair is used for
 *              outgoing messages, the other for incoming messages.
 *
 *              The new key is marked so that it won't be selected for output
 *              by @ksl_encrypt@.  You can still encrypt data with it by
 *              calling @ks_encrypt@ directly.
 */

keyset *ks_gen(const void *k, size_t x, size_t y, size_t z, peer *p)
{
  HASH_CTX h;
  octet buf[HASHSZ];
  keyset *ks = CREATE(keyset);
  time_t now = time(0);
  const octet *pp = k;
  T( static unsigned seq = 0; )

  T( trace(T_KEYSET, "keyset: adding new keyset %u", seq); )

  /* --- Construct the various keys --- *
   *
   * This is done with macros, because it's quite tedious.
   */

#define MINE HASH(&h, pp, x)
#define YOURS HASH(&h, pp + x, y - x)
#define OURS HASH(&h, pp + y, z - y)

#define IN MINE; YOURS; OURS
#define OUT YOURS; MINE; OURS
#define STR_IN "incoming"
#define STR_OUT "outgoing"

#define GETHASH(str, dir) do {                                          \
  HASH_INIT(&h);                                                        \
  HASH_STRING(&h, "tripe-" str);                                        \
  dir;                                                                  \
  HASH_DONE(&h, buf);                                                   \
  IF_TRACING(T_KEYSET, {                                                \
    trace_block(T_CRYPTO, "crypto: " STR_##dir " key " str,             \
                buf, sizeof(buf));                                      \
  })                                                                    \
} while (0)

  GETHASH("encryption", IN); ks->cin = CIPHER->init(buf, sizeof(buf));
  GETHASH("integrity", IN); ks->min = MAC->key(buf, sizeof(buf));
  GETHASH("encryption", OUT); ks->cout = CIPHER->init(buf, sizeof(buf));
  GETHASH("integrity", OUT); ks->mout = MAC->key(buf, sizeof(buf));

#undef MINE
#undef YOURS
#undef OURS
#undef IN
#undef OUT
#undef STR_IN
#undef STR_OUT
#undef GETHASH

  T( ks->seq = seq++; )
  ks->ref = 1;
  ks->t_exp = now + T_EXP;
  ks->sz_exp = SZ_EXP;
  ks->oseq = ks->iseq = 0;
  ks->iwin = 0;
  ks->next = 0;
  ks->p = p;
  ks->f = KSF_LISTEN;
  BURN(buf);
  return (ks);
}

/* --- @ks_tregen@ --- *
 *
 * Arguments:   @keyset *ks@ = pointer to a keyset
 *
 * Returns:     The time at which moves ought to be made to replace this key.
 */

time_t ks_tregen(keyset *ks) { return (ks->t_exp - T_EXP + T_REGEN); }

/* --- @ks_activate@ --- *
 *
 * Arguments:   @keyset *ks@ = pointer to a keyset
 *
 * Returns:     ---
 *
 * Use:         Activates a keyset, so that it can be used for encrypting
 *              outgoing messages.
 */

void ks_activate(keyset *ks)
{
  if (ks->f & KSF_LISTEN) {
    T( trace(T_KEYSET, "keyset: activating keyset %u", ks->seq); )
    ks->f &= ~KSF_LISTEN;
  }
}

/* --- @ks_encrypt@ --- *
 *
 * Arguments:   @keyset *ks@ = pointer to a keyset
 *              @buf *b@ = pointer to input buffer
 *              @buf *bb@ = pointer to output buffer
 *
 * Returns:     Zero if OK, nonzero if the key needs replacing.  If the
 *              encryption failed, the output buffer is broken and zero is
 *              returned.
 *
 * Use:         Encrypts a block of data using the key.  Note that the `key
 *              ought to be replaced' notification is only ever given once
 *              for each key.  Also note that this call forces a keyset to be
 *              used even if it's marked as not for data output.
 */

int ks_encrypt(keyset *ks, buf *b, buf *bb)
{
  time_t now = time(0);

  if (!KEYOK(ks, now)) {
    buf_break(bb);
    return (0);
  }
  return (doencrypt(ks, b, bb));
}

/* --- @ks_decrypt@ --- *
 *
 * Arguments:   @keyset *ks@ = pointer to a keyset
 *              @buf *b@ = pointer to an input buffer
 *              @buf *bb@ = pointer to an output buffer
 *
 * Returns:     Zero on success, or nonzero if there was some problem.
 *
 * Use:         Attempts to decrypt a message using a given key.  Note that
 *              requesting decryption with a key directly won't clear a
 *              marking that it's not for encryption.
 */

int ks_decrypt(keyset *ks, buf *b, buf *bb)
{
  time_t now = time(0);
  uint32 seq;

  if (!KEYOK(ks, now) ||
      buf_ensure(bb, BLEN(b)) ||
      dodecrypt(ks, b, bb, &seq) ||
      dosequence(ks, seq))
    return (-1);
  return (0);
}

/*----- Keyset list handling ----------------------------------------------*/

/* --- @ksl_free@ --- *
 *
 * Arguments:   @keyset **ksroot@ = pointer to keyset list head
 *
 * Returns:     ---
 *
 * Use:         Frees (releases references to) all of the keys in a keyset.
 */

void ksl_free(keyset **ksroot)
{
  keyset *ks, *ksn;
  for (ks = *ksroot; ks; ks = ksn) {
    ksn = ks->next;
    ks->f &= ~KSF_LINK;
    ks_drop(ks);
  }
}

/* --- @ksl_link@ --- *
 *
 * Arguments:   @keyset **ksroot@ = pointer to keyset list head
 *              @keyset *ks@ = pointer to a keyset
 *
 * Returns:     ---
 *
 * Use:         Links a keyset into a list.  A keyset can only be on one list
 *              at a time.  Bad things happen otherwise.
 */

void ksl_link(keyset **ksroot, keyset *ks)
{
  assert(!(ks->f & KSF_LINK));
  ks->next = *ksroot;
  *ksroot = ks;
  ks->f |= KSF_LINK;
  ks->ref++;
}

/* --- @ksl_prune@ --- *
 *
 * Arguments:   @keyset **ksroot@ = pointer to keyset list head
 *
 * Returns:     ---
 *
 * Use:         Prunes the keyset list by removing keys which mustn't be used
 *              any more.
 */

void ksl_prune(keyset **ksroot)
{
  time_t now = time(0);

  while (*ksroot) {
    keyset *ks = *ksroot;

    if (ks->t_exp <= now) {
      T( trace(T_KEYSET, "keyset: expiring keyset %u (time limit reached)",
               ks->seq); )
      goto kill;
    } else if (ks->sz_exp == 0) {
      T( trace(T_KEYSET, "keyset: expiring keyset %u (data limit reached)",
               ks->seq); )
      goto kill;
    } else {
      ksroot = &ks->next;
      continue;
    }

  kill:
    *ksroot = ks->next;
    ks->f &= ~KSF_LINK;
    ks_drop(ks);
  }
}

/* --- @ksl_encrypt@ --- *
 *
 * Arguments:   @keyset **ksroot@ = pointer to keyset list head
 *              @buf *b@ = pointer to input buffer
 *              @buf *bb@ = pointer to output buffer
 *
 * Returns:     Nonzero if a new key is needed.
 *
 * Use:         Encrypts a packet.
 */

int ksl_encrypt(keyset **ksroot, buf *b, buf *bb)
{
  time_t now = time(0);
  keyset *ks = *ksroot;

  for (;;) {
    if (!ks) {
      T( trace(T_KEYSET, "keyset: no suitable keysets found"); )
      buf_break(bb);
      return (-1);
    }
    if (KEYOK(ks, now) && !(ks->f & KSF_LISTEN))
      break;
    ks = ks->next;
  }

  return (doencrypt(ks, b, bb));
}

/* --- @ksl_decrypt@ --- *
 *
 * Arguments:   @keyset **ksroot@ = pointer to keyset list head
 *              @buf *b@ = pointer to input buffer
 *              @buf *bb@ = pointer to output buffer
 *
 * Returns:     Nonzero if the packet couldn't be decrypted.
 *
 * Use:         Decrypts a packet.
 */

int ksl_decrypt(keyset **ksroot, buf *b, buf *bb)
{
  time_t now = time(0);
  keyset *ks;
  uint32 seq;

  if (buf_ensure(bb, BLEN(b)))
    return (-1);

  for (ks = *ksroot; ks; ks = ks->next) {
    if (!KEYOK(ks, now))
      continue;
    if (!dodecrypt(ks, b, bb, &seq)) {
      if (ks->f & KSF_LISTEN) {
        T( trace(T_KEYSET, "keyset: implicitly activating keyset %u",
                 ks->seq); )
        ks->f &= ~KSF_LISTEN;
      }
      return (dosequence(ks, seq));
    }
  }
  T( trace(T_KEYSET, "keyset: no matching keys, or incorrect MAC"); )
  return (-1);
}

/*----- That's all, folks -------------------------------------------------*/