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dss.c
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dss.c
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/*
* Dropbear - a SSH2 server
*
* Copyright (c) 2002,2003 Matt Johnston
* All rights reserved.
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE. */
#include "includes.h"
#include "dbutil.h"
#include "bignum.h"
#include "dss.h"
#include "buffer.h"
#include "ssh.h"
#include "dbrandom.h"
/* Handle DSS (Digital Signature Standard), aka DSA (D.S. Algorithm),
* operations, such as key reading, signing, verification. Key generation
* is in gendss.c, since it isn't required in the server itself.
*
* See FIPS186 or the Handbook of Applied Cryptography for details of the
* algorithm */
#ifdef DROPBEAR_DSS
/* Load a dss key from a buffer, initialising the values.
* The key will have the same format as buf_put_dss_key.
* These should be freed with dss_key_free.
* Returns DROPBEAR_SUCCESS or DROPBEAR_FAILURE */
int buf_get_dss_pub_key(buffer* buf, dropbear_dss_key *key) {
TRACE(("enter buf_get_dss_pub_key"))
dropbear_assert(key != NULL);
m_mp_alloc_init_multi(&key->p, &key->q, &key->g, &key->y, NULL);
key->x = NULL;
buf_incrpos(buf, 4+SSH_SIGNKEY_DSS_LEN); /* int + "ssh-dss" */
if (buf_getmpint(buf, key->p) == DROPBEAR_FAILURE
|| buf_getmpint(buf, key->q) == DROPBEAR_FAILURE
|| buf_getmpint(buf, key->g) == DROPBEAR_FAILURE
|| buf_getmpint(buf, key->y) == DROPBEAR_FAILURE) {
TRACE(("leave buf_get_dss_pub_key: failed reading mpints"))
return DROPBEAR_FAILURE;
}
if (mp_count_bits(key->p) < MIN_DSS_KEYLEN) {
dropbear_log(LOG_WARNING, "DSS key too short");
TRACE(("leave buf_get_dss_pub_key: short key"))
return DROPBEAR_FAILURE;
}
TRACE(("leave buf_get_dss_pub_key: success"))
return DROPBEAR_SUCCESS;
}
/* Same as buf_get_dss_pub_key, but reads a private "x" key at the end.
* Loads a private dss key from a buffer
* Returns DROPBEAR_SUCCESS or DROPBEAR_FAILURE */
int buf_get_dss_priv_key(buffer* buf, dropbear_dss_key *key) {
int ret = DROPBEAR_FAILURE;
dropbear_assert(key != NULL);
ret = buf_get_dss_pub_key(buf, key);
if (ret == DROPBEAR_FAILURE) {
return DROPBEAR_FAILURE;
}
m_mp_alloc_init_multi(&key->x, NULL);
ret = buf_getmpint(buf, key->x);
if (ret == DROPBEAR_FAILURE) {
m_free(key->x);
}
return ret;
}
/* Clear and free the memory used by a public or private key */
void dss_key_free(dropbear_dss_key *key) {
TRACE2(("enter dsa_key_free"))
if (key == NULL) {
TRACE2(("enter dsa_key_free: key == NULL"))
return;
}
if (key->p) {
mp_clear(key->p);
m_free(key->p);
}
if (key->q) {
mp_clear(key->q);
m_free(key->q);
}
if (key->g) {
mp_clear(key->g);
m_free(key->g);
}
if (key->y) {
mp_clear(key->y);
m_free(key->y);
}
if (key->x) {
mp_clear(key->x);
m_free(key->x);
}
m_free(key);
TRACE2(("leave dsa_key_free"))
}
/* put the dss public key into the buffer in the required format:
*
* string "ssh-dss"
* mpint p
* mpint q
* mpint g
* mpint y
*/
void buf_put_dss_pub_key(buffer* buf, dropbear_dss_key *key) {
dropbear_assert(key != NULL);
buf_putstring(buf, SSH_SIGNKEY_DSS, SSH_SIGNKEY_DSS_LEN);
buf_putmpint(buf, key->p);
buf_putmpint(buf, key->q);
buf_putmpint(buf, key->g);
buf_putmpint(buf, key->y);
}
/* Same as buf_put_dss_pub_key, but with the private "x" key appended */
void buf_put_dss_priv_key(buffer* buf, dropbear_dss_key *key) {
dropbear_assert(key != NULL);
buf_put_dss_pub_key(buf, key);
buf_putmpint(buf, key->x);
}
#ifdef DROPBEAR_SIGNKEY_VERIFY
/* Verify a DSS signature (in buf) made on data by the key given.
* returns DROPBEAR_SUCCESS or DROPBEAR_FAILURE */
int buf_dss_verify(buffer* buf, dropbear_dss_key *key, buffer *data_buf) {
unsigned char msghash[SHA1_HASH_SIZE];
hash_state hs;
int ret = DROPBEAR_FAILURE;
DEF_MP_INT(val1);
DEF_MP_INT(val2);
DEF_MP_INT(val3);
DEF_MP_INT(val4);
char * string = NULL;
unsigned int stringlen;
TRACE(("enter buf_dss_verify"))
dropbear_assert(key != NULL);
m_mp_init_multi(&val1, &val2, &val3, &val4, NULL);
/* get blob, check length */
string = buf_getstring(buf, &stringlen);
if (stringlen != 2*SHA1_HASH_SIZE) {
goto out;
}
/* hash the data */
sha1_init(&hs);
sha1_process(&hs, data_buf->data, data_buf->len);
sha1_done(&hs, msghash);
/* create the signature - s' and r' are the received signatures in buf */
/* w = (s')-1 mod q */
/* let val1 = s' */
bytes_to_mp(&val1, (const unsigned char*) &string[SHA1_HASH_SIZE], SHA1_HASH_SIZE);
if (mp_cmp(&val1, key->q) != MP_LT) {
TRACE(("verify failed, s' >= q"))
goto out;
}
/* let val2 = w = (s')^-1 mod q*/
if (mp_invmod(&val1, key->q, &val2) != MP_OKAY) {
goto out;
}
/* u1 = ((SHA(M')w) mod q */
/* let val1 = SHA(M') = msghash */
bytes_to_mp(&val1, msghash, SHA1_HASH_SIZE);
/* let val3 = u1 = ((SHA(M')w) mod q */
if (mp_mulmod(&val1, &val2, key->q, &val3) != MP_OKAY) {
goto out;
}
/* u2 = ((r')w) mod q */
/* let val1 = r' */
bytes_to_mp(&val1, (const unsigned char*) &string[0], SHA1_HASH_SIZE);
if (mp_cmp(&val1, key->q) != MP_LT) {
TRACE(("verify failed, r' >= q"))
goto out;
}
/* let val4 = u2 = ((r')w) mod q */
if (mp_mulmod(&val1, &val2, key->q, &val4) != MP_OKAY) {
goto out;
}
/* v = (((g)^u1 (y)^u2) mod p) mod q */
/* val2 = g^u1 mod p */
if (mp_exptmod(key->g, &val3, key->p, &val2) != MP_OKAY) {
goto out;
}
/* val3 = y^u2 mod p */
if (mp_exptmod(key->y, &val4, key->p, &val3) != MP_OKAY) {
goto out;
}
/* val4 = ((g)^u1 (y)^u2) mod p */
if (mp_mulmod(&val2, &val3, key->p, &val4) != MP_OKAY) {
goto out;
}
/* val2 = v = (((g)^u1 (y)^u2) mod p) mod q */
if (mp_mod(&val4, key->q, &val2) != MP_OKAY) {
goto out;
}
/* check whether signatures verify */
if (mp_cmp(&val2, &val1) == MP_EQ) {
/* good sig */
ret = DROPBEAR_SUCCESS;
}
out:
mp_clear_multi(&val1, &val2, &val3, &val4, NULL);
m_free(string);
return ret;
}
#endif /* DROPBEAR_SIGNKEY_VERIFY */
/* Sign the data presented with key, writing the signature contents
* to the buffer */
void buf_put_dss_sign(buffer* buf, dropbear_dss_key *key, buffer *data_buf) {
unsigned char msghash[SHA1_HASH_SIZE];
unsigned int writelen;
unsigned int i;
DEF_MP_INT(dss_k);
DEF_MP_INT(dss_m);
DEF_MP_INT(dss_temp1);
DEF_MP_INT(dss_temp2);
DEF_MP_INT(dss_r);
DEF_MP_INT(dss_s);
hash_state hs;
TRACE(("enter buf_put_dss_sign"))
dropbear_assert(key != NULL);
/* hash the data */
sha1_init(&hs);
sha1_process(&hs, data_buf->data, data_buf->len);
sha1_done(&hs, msghash);
m_mp_init_multi(&dss_k, &dss_temp1, &dss_temp2, &dss_r, &dss_s,
&dss_m, NULL);
/* the random number generator's input has included the private key which
* avoids DSS's problem of private key exposure due to low entropy */
gen_random_mpint(key->q, &dss_k);
/* now generate the actual signature */
bytes_to_mp(&dss_m, msghash, SHA1_HASH_SIZE);
/* g^k mod p */
if (mp_exptmod(key->g, &dss_k, key->p, &dss_temp1) != MP_OKAY) {
dropbear_exit("DSS error");
}
/* r = (g^k mod p) mod q */
if (mp_mod(&dss_temp1, key->q, &dss_r) != MP_OKAY) {
dropbear_exit("DSS error");
}
/* x*r mod q */
if (mp_mulmod(&dss_r, key->x, key->q, &dss_temp1) != MP_OKAY) {
dropbear_exit("DSS error");
}
/* (SHA1(M) + xr) mod q) */
if (mp_addmod(&dss_m, &dss_temp1, key->q, &dss_temp2) != MP_OKAY) {
dropbear_exit("DSS error");
}
/* (k^-1) mod q */
if (mp_invmod(&dss_k, key->q, &dss_temp1) != MP_OKAY) {
dropbear_exit("DSS error");
}
/* s = (k^-1(SHA1(M) + xr)) mod q */
if (mp_mulmod(&dss_temp1, &dss_temp2, key->q, &dss_s) != MP_OKAY) {
dropbear_exit("DSS error");
}
buf_putstring(buf, SSH_SIGNKEY_DSS, SSH_SIGNKEY_DSS_LEN);
buf_putint(buf, 2*SHA1_HASH_SIZE);
writelen = mp_unsigned_bin_size(&dss_r);
dropbear_assert(writelen <= SHA1_HASH_SIZE);
/* need to pad to 160 bits with leading zeros */
for (i = 0; i < SHA1_HASH_SIZE - writelen; i++) {
buf_putbyte(buf, 0);
}
if (mp_to_unsigned_bin(&dss_r, buf_getwriteptr(buf, writelen))
!= MP_OKAY) {
dropbear_exit("DSS error");
}
mp_clear(&dss_r);
buf_incrwritepos(buf, writelen);
writelen = mp_unsigned_bin_size(&dss_s);
dropbear_assert(writelen <= SHA1_HASH_SIZE);
/* need to pad to 160 bits with leading zeros */
for (i = 0; i < SHA1_HASH_SIZE - writelen; i++) {
buf_putbyte(buf, 0);
}
if (mp_to_unsigned_bin(&dss_s, buf_getwriteptr(buf, writelen))
!= MP_OKAY) {
dropbear_exit("DSS error");
}
mp_clear(&dss_s);
buf_incrwritepos(buf, writelen);
mp_clear_multi(&dss_k, &dss_temp1, &dss_temp2, &dss_r, &dss_s,
&dss_m, NULL);
/* create the signature to return */
TRACE(("leave buf_put_dss_sign"))
}
#endif /* DROPBEAR_DSS */