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+/* seskey.c - make session keys etc.
+ * Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004,
+ * 2006, 2009, 2010 Free Software Foundation, Inc.
+ *
+ * This file is part of GnuPG.
+ *
+ * GnuPG 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 3 of the License, or
+ * (at your option) any later version.
+ *
+ * GnuPG 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 this program; if not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include <config.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "gpg.h"
+#include "../common/util.h"
+#include "options.h"
+#include "main.h"
+#include "../common/i18n.h"
+
+
+/* Generate a new session key in *DEK that is appropriate for the
+ * algorithm DEK->ALGO (i.e., ensure that the key is not weak).
+ *
+ * This function overwrites DEK->KEYLEN, DEK->KEY. The rest of the
+ * fields are left as is. */
+void
+make_session_key( DEK *dek )
+{
+ gcry_cipher_hd_t chd;
+ int i, rc;
+
+ dek->keylen = openpgp_cipher_get_algo_keylen (dek->algo);
+
+ if (openpgp_cipher_open (&chd, dek->algo, GCRY_CIPHER_MODE_CFB,
+ (GCRY_CIPHER_SECURE
+ | (dek->algo >= 100 ?
+ 0 : GCRY_CIPHER_ENABLE_SYNC))) )
+ BUG();
+ gcry_randomize (dek->key, dek->keylen, GCRY_STRONG_RANDOM );
+ for (i=0; i < 16; i++ )
+ {
+ rc = gcry_cipher_setkey (chd, dek->key, dek->keylen);
+ if (!rc)
+ {
+ gcry_cipher_close (chd);
+ return;
+ }
+ if (gpg_err_code (rc) != GPG_ERR_WEAK_KEY)
+ BUG();
+ log_info(_("weak key created - retrying\n") );
+ /* Renew the session key until we get a non-weak key. */
+ gcry_randomize (dek->key, dek->keylen, GCRY_STRONG_RANDOM);
+ }
+ log_fatal (_("cannot avoid weak key for symmetric cipher; "
+ "tried %d times!\n"), i);
+}
+
+
+/* Encode the session key stored in DEK as an MPI in preparation to
+ * encrypt it with the public key algorithm OPENPGP_PK_ALGO with a key
+ * whose length (the size of the public key) is NBITS.
+ *
+ * On success, returns an MPI, which the caller must free using
+ * gcry_mpi_release(). */
+gcry_mpi_t
+encode_session_key (int openpgp_pk_algo, DEK *dek, unsigned int nbits)
+{
+ size_t nframe = (nbits+7) / 8;
+ byte *p;
+ byte *frame;
+ int i,n;
+ u16 csum;
+ gcry_mpi_t a;
+
+ if (DBG_CRYPTO)
+ log_debug ("encode_session_key: encoding %d byte DEK", dek->keylen);
+
+ csum = 0;
+ for (p = dek->key, i=0; i < dek->keylen; i++)
+ csum += *p++;
+
+ /* Shortcut for ECDH. It's padding is minimal to simply make the
+ output be a multiple of 8 bytes. */
+ if (openpgp_pk_algo == PUBKEY_ALGO_ECDH)
+ {
+ /* Pad to 8 byte granulatiry; the padding byte is the number of
+ * padded bytes.
+ *
+ * A DEK(k bytes) CSUM(2 bytes) 0x 0x 0x 0x ... 0x
+ * +---- x times ---+
+ */
+ nframe = (( 1 + dek->keylen + 2 /* The value so far is always odd. */
+ + 7 ) & (~7));
+
+ /* alg+key+csum fit and the size is congruent to 8. */
+ log_assert (!(nframe%8) && nframe > 1 + dek->keylen + 2 );
+
+ frame = xmalloc_secure (nframe);
+ n = 0;
+ frame[n++] = dek->algo;
+ memcpy (frame+n, dek->key, dek->keylen);
+ n += dek->keylen;
+ frame[n++] = csum >> 8;
+ frame[n++] = csum;
+ i = nframe - n; /* Number of padded bytes. */
+ memset (frame+n, i, i); /* Use it as the value of each padded byte. */
+ log_assert (n+i == nframe);
+
+ if (DBG_CRYPTO)
+ log_debug ("encode_session_key: "
+ "[%d] %02x %02x %02x ... %02x %02x %02x\n",
+ (int) nframe, frame[0], frame[1], frame[2],
+ frame[nframe-3], frame[nframe-2], frame[nframe-1]);
+
+ if (gcry_mpi_scan (&a, GCRYMPI_FMT_USG, frame, nframe, &nframe))
+ BUG();
+ xfree(frame);
+ return a;
+ }
+
+ /* The current limitation is that we can only use a session key
+ * whose length is a multiple of BITS_PER_MPI_LIMB
+ * I think we can live with that.
+ */
+ if (dek->keylen + 7 > nframe || !nframe)
+ log_bug ("can't encode a %d bit key in a %d bits frame\n",
+ dek->keylen*8, nbits );
+
+ /* We encode the session key according to PKCS#1 v1.5 (see section
+ * 13.1.1 of RFC 4880):
+ *
+ * 0 2 RND(i bytes) 0 A DEK(k bytes) CSUM(2 bytes)
+ *
+ * (But how can we store the leading 0 - the external representaion
+ * of MPIs doesn't allow leading zeroes =:-)
+ *
+ * RND are (at least 1) non-zero random bytes.
+ * A is the cipher algorithm
+ * DEK is the encryption key (session key) length k depends on the
+ * cipher algorithm (20 is used with blowfish160).
+ * CSUM is the 16 bit checksum over the DEK
+ */
+
+ frame = xmalloc_secure( nframe );
+ n = 0;
+ frame[n++] = 0;
+ frame[n++] = 2;
+ /* The number of random bytes are the number of otherwise unused
+ bytes. See diagram above. */
+ i = nframe - 6 - dek->keylen;
+ log_assert( i > 0 );
+ p = gcry_random_bytes_secure (i, GCRY_STRONG_RANDOM);
+ /* Replace zero bytes by new values. */
+ for (;;)
+ {
+ int j, k;
+ byte *pp;
+
+ /* Count the zero bytes. */
+ for (j=k=0; j < i; j++ )
+ if (!p[j])
+ k++;
+ if (!k)
+ break; /* Okay: no zero bytes. */
+ k += k/128 + 3; /* Better get some more. */
+ pp = gcry_random_bytes_secure (k, GCRY_STRONG_RANDOM);
+ for (j=0; j < i && k ;)
+ {
+ if (!p[j])
+ p[j] = pp[--k];
+ if (p[j])
+ j++;
+ }
+ xfree (pp);
+ }
+ memcpy (frame+n, p, i);
+ xfree (p);
+ n += i;
+ frame[n++] = 0;
+ frame[n++] = dek->algo;
+ memcpy (frame+n, dek->key, dek->keylen );
+ n += dek->keylen;
+ frame[n++] = csum >>8;
+ frame[n++] = csum;
+ log_assert (n == nframe);
+ if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, n, &nframe))
+ BUG();
+ xfree (frame);
+ return a;
+}
+
+
+static gcry_mpi_t
+do_encode_md( gcry_md_hd_t md, int algo, size_t len, unsigned nbits,
+ const byte *asn, size_t asnlen )
+{
+ size_t nframe = (nbits+7) / 8;
+ byte *frame;
+ int i,n;
+ gcry_mpi_t a;
+
+ if (len + asnlen + 4 > nframe)
+ {
+ log_error ("can't encode a %d bit MD into a %d bits frame, algo=%d\n",
+ (int)(len*8), (int)nbits, algo);
+ return NULL;
+ }
+
+ /* We encode the MD in this way:
+ *
+ * 0 1 PAD(n bytes) 0 ASN(asnlen bytes) MD(len bytes)
+ *
+ * PAD consists of FF bytes.
+ */
+ frame = gcry_md_is_secure (md)? xmalloc_secure (nframe) : xmalloc (nframe);
+ n = 0;
+ frame[n++] = 0;
+ frame[n++] = 1; /* block type */
+ i = nframe - len - asnlen -3 ;
+ log_assert( i > 1 );
+ memset( frame+n, 0xff, i ); n += i;
+ frame[n++] = 0;
+ memcpy( frame+n, asn, asnlen ); n += asnlen;
+ memcpy( frame+n, gcry_md_read (md, algo), len ); n += len;
+ log_assert( n == nframe );
+
+ if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, n, &nframe ))
+ BUG();
+ xfree(frame);
+
+ /* Note that PGP before version 2.3 encoded the MD as:
+ *
+ * 0 1 MD(16 bytes) 0 PAD(n bytes) 1
+ *
+ * The MD is always 16 bytes here because it's always MD5. We do
+ * not support pre-v2.3 signatures, but I'm including this comment
+ * so the information is easily found in the future.
+ */
+
+ return a;
+}
+
+
+/****************
+ * Encode a message digest into an MPI.
+ * If it's for a DSA signature, make sure that the hash is large
+ * enough to fill up q. If the hash is too big, take the leftmost
+ * bits.
+ */
+gcry_mpi_t
+encode_md_value (PKT_public_key *pk, gcry_md_hd_t md, int hash_algo)
+{
+ gcry_mpi_t frame;
+ size_t mdlen;
+
+ log_assert (hash_algo);
+ log_assert (pk);
+
+ if (pk->pubkey_algo == PUBKEY_ALGO_EDDSA)
+ {
+ /* EdDSA signs data of arbitrary length. Thus no special
+ treatment is required. */
+ frame = gcry_mpi_set_opaque_copy (NULL, gcry_md_read (md, hash_algo),
+ 8*gcry_md_get_algo_dlen (hash_algo));
+ }
+ else if (pk->pubkey_algo == PUBKEY_ALGO_DSA
+ || pk->pubkey_algo == PUBKEY_ALGO_ECDSA)
+ {
+ /* It's a DSA signature, so find out the size of q. */
+
+ size_t qbits = gcry_mpi_get_nbits (pk->pkey[1]);
+
+ /* pkey[1] is Q for ECDSA, which is an uncompressed point,
+ i.e. 04 <x> <y> */
+ if (pk->pubkey_algo == PUBKEY_ALGO_ECDSA)
+ qbits = ecdsa_qbits_from_Q (qbits);
+
+ /* Make sure it is a multiple of 8 bits. */
+ if ((qbits%8))
+ {
+ log_error(_("DSA requires the hash length to be a"
+ " multiple of 8 bits\n"));
+ return NULL;
+ }
+
+ /* Don't allow any q smaller than 160 bits. This might need a
+ revisit as the DSA2 design firms up, but for now, we don't
+ want someone to issue signatures from a key with a 16-bit q
+ or something like that, which would look correct but allow
+ trivial forgeries. Yes, I know this rules out using MD5 with
+ DSA. ;) */
+ if (qbits < 160)
+ {
+ log_error (_("%s key %s uses an unsafe (%zu bit) hash\n"),
+ openpgp_pk_algo_name (pk->pubkey_algo),
+ keystr_from_pk (pk), qbits);
+ return NULL;
+ }
+
+
+ /* ECDSA 521 is special has it is larger than the largest hash
+ we have (SHA-512). Thus we change the size for further
+ processing to 512. */
+ if (pk->pubkey_algo == PUBKEY_ALGO_ECDSA && qbits > 512)
+ qbits = 512;
+
+ /* Check if we're too short. Too long is safe as we'll
+ automatically left-truncate. */
+ mdlen = gcry_md_get_algo_dlen (hash_algo);
+ if (mdlen < qbits/8)
+ {
+ log_error (_("%s key %s requires a %zu bit or larger hash "
+ "(hash is %s)\n"),
+ openpgp_pk_algo_name (pk->pubkey_algo),
+ keystr_from_pk (pk), qbits,
+ gcry_md_algo_name (hash_algo));
+ return NULL;
+ }
+
+ /* Note that we do the truncation by passing QBITS/8 as length to
+ mpi_scan. */
+ if (gcry_mpi_scan (&frame, GCRYMPI_FMT_USG,
+ gcry_md_read (md, hash_algo), qbits/8, NULL))
+ BUG();
+ }
+ else
+ {
+ gpg_error_t rc;
+ byte *asn;
+ size_t asnlen;
+
+ rc = gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, NULL, &asnlen);
+ if (rc)
+ log_fatal ("can't get OID of digest algorithm %d: %s\n",
+ hash_algo, gpg_strerror (rc));
+ asn = xtrymalloc (asnlen);
+ if (!asn)
+ return NULL;
+ if ( gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, asn, &asnlen) )
+ BUG();
+ frame = do_encode_md (md, hash_algo, gcry_md_get_algo_dlen (hash_algo),
+ gcry_mpi_get_nbits (pk->pkey[0]), asn, asnlen);
+ xfree (asn);
+ }
+
+ return frame;
+}