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sig-check.c
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/* sig-check.c - Check a signature
* Copyright (C) 1998 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 2 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, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include "util.h"
#include "packet.h"
#include "memory.h"
#include "mpi.h"
#include "keydb.h"
#include "cipher.h"
#include "main.h"
#include "status.h"
static int do_check( PKT_public_cert *pkc, PKT_signature *sig,
MD_HANDLE digest );
/****************
* Check the signature which is contained in SIG.
* The md5handle should be currently open, so that this function
* is able to append some data, before finalizing the digest.
*/
int
signature_check( PKT_signature *sig, MD_HANDLE digest )
{
PKT_public_cert *pkc = m_alloc_clear( sizeof *pkc );
int rc=0;
#ifndef HAVE_RSA_CIPHER
if( is_RSA(sig->pubkey_algo) )
write_status(STATUS_RSA_OR_IDEA);
#endif
if( get_pubkey( pkc, sig->keyid ) )
rc = G10ERR_NO_PUBKEY;
else
rc = do_check( pkc, sig, digest );
free_public_cert( pkc );
return rc;
}
static int
do_check( PKT_public_cert *pkc, PKT_signature *sig, MD_HANDLE digest )
{
MPI result = NULL;
int rc=0;
if( pkc->version == 4 && pkc->pubkey_algo == PUBKEY_ALGO_ELGAMAL_E ) {
log_info("this is a PGP generated "
"ElGamal key which is NOT secure for signatures!\n");
return G10ERR_PUBKEY_ALGO;
}
if( pkc->timestamp > sig->timestamp )
return G10ERR_TIME_CONFLICT; /* pubkey newer that signature */
if( is_ELGAMAL(pkc->pubkey_algo) ) {
if( (rc=check_digest_algo(sig->digest_algo)) )
goto leave;
/* make sure the digest algo is enabled (in case of a detached
* signature */
md_enable( digest, sig->digest_algo );
/* complete the digest */
md_putc( digest, sig->sig_class );
{ u32 a = sig->timestamp;
md_putc( digest, (a >> 24) & 0xff );
md_putc( digest, (a >> 16) & 0xff );
md_putc( digest, (a >> 8) & 0xff );
md_putc( digest, a & 0xff );
}
md_final( digest );
result = encode_md_value( digest, sig->digest_algo,
mpi_get_nbits(pkc->d.elg.p));
if( DBG_CIPHER )
log_mpidump("calc sig frame (elg): ", result);
if( !elg_verify( sig->d.elg.a, sig->d.elg.b, result, &pkc->d.elg ) )
rc = G10ERR_BAD_SIGN;
}
else if( pkc->pubkey_algo == PUBKEY_ALGO_DSA ) {
if( (rc=check_digest_algo(sig->digest_algo)) )
goto leave;
/* make sure the digest algo is enabled (in case of a detached
* signature */
md_enable( digest, sig->digest_algo );
/* complete the digest */
if( sig->version >= 4 )
md_putc( digest, sig->version );
md_putc( digest, sig->sig_class );
if( sig->version < 4 ) {
u32 a = sig->timestamp;
md_putc( digest, (a >> 24) & 0xff );
md_putc( digest, (a >> 16) & 0xff );
md_putc( digest, (a >> 8) & 0xff );
md_putc( digest, a & 0xff );
}
else {
byte buf[6];
size_t n;
md_putc( digest, sig->pubkey_algo );
md_putc( digest, sig->digest_algo );
if( sig->hashed_data ) {
n = (sig->hashed_data[0] << 8) | sig->hashed_data[1];
md_write( digest, sig->hashed_data, n+2 );
n += 6;
}
else
n = 6;
/* add some magic */
buf[0] = sig->version;
buf[1] = 0xff;
buf[2] = n >> 24;
buf[3] = n >> 16;
buf[4] = n >> 8;
buf[5] = n;
md_write( digest, buf, 6 );
}
md_final( digest );
result = mpi_alloc( (md_digest_length(sig->digest_algo)
+BYTES_PER_MPI_LIMB-1) / BYTES_PER_MPI_LIMB );
mpi_set_buffer( result, md_read(digest, sig->digest_algo),
md_digest_length(sig->digest_algo), 0 );
if( DBG_CIPHER )
log_mpidump("calc sig frame: ", result);
if( !dsa_verify( sig->d.dsa.r, sig->d.dsa.s, result, &pkc->d.dsa ) )
rc = G10ERR_BAD_SIGN;
}
#ifdef HAVE_RSA_CIPHER
else if( pkc->pubkey_algo == PUBKEY_ALGO_RSA
|| pkc->pubkey_algo == PUBKEY_ALGO_RSA_S ) {
int i, j, c, old_enc;
byte *dp;
const byte *asn;
size_t mdlen, asnlen;
result = mpi_alloc(40);
rsa_public( result, sig->d.rsa.rsa_integer, &pkc->d.rsa );
old_enc = 0;
for(i=j=0; (c=mpi_getbyte(result, i)) != -1; i++ ) {
if( !j ) {
if( !i && c != 1 )
break;
else if( i && c == 0xff )
; /* skip the padding */
else if( i && !c )
j++;
else
break;
}
else if( ++j == 18 && c != 1 )
break;
else if( j == 19 && c == 0 ) {
old_enc++;
break;
}
}
if( old_enc ) {
log_error("old encoding scheme is not supported\n");
rc = G10ERR_GENERAL;
goto leave;
}
if( (rc=check_digest_algo(sig->digest_algo)) )
goto leave; /* unsupported algo */
md_enable( digest, sig->digest_algo );
asn = md_asn_oid( sig->digest_algo, &asnlen, &mdlen );
for(i=mdlen,j=asnlen-1; (c=mpi_getbyte(result, i)) != -1 && j >= 0;
i++, j-- )
if( asn[j] != c )
break;
if( j != -1 || mpi_getbyte(result, i) ) { /* ASN is wrong */
rc = G10ERR_BAD_PUBKEY;
goto leave;
}
for(i++; (c=mpi_getbyte(result, i)) != -1; i++ )
if( c != 0xff )
break;
i++;
if( c != sig->digest_algo || mpi_getbyte(result, i) ) {
/* Padding or leading bytes in signature is wrong */
rc = G10ERR_BAD_PUBKEY;
goto leave;
}
if( mpi_getbyte(result, mdlen-1) != sig->digest_start[0]
|| mpi_getbyte(result, mdlen-2) != sig->digest_start[1] ) {
/* Wrong key used to check the signature */
rc = G10ERR_BAD_PUBKEY;
goto leave;
}
/* complete the digest */
md_putc( digest, sig->sig_class );
{ u32 a = sig->timestamp;
md_putc( digest, (a >> 24) & 0xff );
md_putc( digest, (a >> 16) & 0xff );
md_putc( digest, (a >> 8) & 0xff );
md_putc( digest, a & 0xff );
}
md_final( digest );
dp = md_read( digest, sig->digest_algo );
for(i=mdlen-1; i >= 0; i--, dp++ ) {
if( mpi_getbyte( result, i ) != *dp ) {
rc = G10ERR_BAD_SIGN;
goto leave;
}
}
}
#endif/*HAVE_RSA_CIPHER*/
else {
/*log_debug("signature_check: unsupported pubkey algo %d\n",
pkc->pubkey_algo );*/
rc = G10ERR_PUBKEY_ALGO;
goto leave;
}
leave:
mpi_free( result );
return rc;
}
static void
hash_uid_node( KBNODE unode, MD_HANDLE md, PKT_signature *sig )
{
PKT_user_id *uid = unode->pkt->pkt.user_id;
assert( unode->pkt->pkttype == PKT_USER_ID );
if( sig->version >=4 ) {
byte buf[5];
buf[0] = 0xb4; /* indicates a userid packet */
buf[1] = uid->len >> 24; /* always use 4 length bytes */
buf[2] = uid->len >> 16;
buf[3] = uid->len >> 8;
buf[4] = uid->len;
md_write( md, buf, 5 );
}
md_write( md, uid->name, uid->len );
}
/****************
* check the signature pointed to by NODE. This is a key signature.
* If the function detects a self-signature, it uses the PKC from
* NODE and does not read any public key.
*/
int
check_key_signature( KBNODE root, KBNODE node, int *is_selfsig )
{
MD_HANDLE md;
PKT_public_cert *pkc;
PKT_signature *sig;
int algo;
int rc;
if( is_selfsig )
*is_selfsig = 0;
assert( node->pkt->pkttype == PKT_SIGNATURE );
assert( root->pkt->pkttype == PKT_PUBLIC_CERT );
pkc = root->pkt->pkt.public_cert;
sig = node->pkt->pkt.signature;
algo = sig->digest_algo;
if( (rc=check_digest_algo(algo)) )
return rc;
if( sig->sig_class == 0x20 ) {
md = md_open( algo, 0 );
hash_public_cert( md, pkc );
rc = do_check( pkc, sig, md );
md_close(md);
}
else if( sig->sig_class == 0x18 ) {
KBNODE snode = find_prev_kbnode( root, node, PKT_PUBKEY_SUBCERT );
if( snode ) {
if( is_selfsig ) {
u32 keyid[2];
keyid_from_pkc( pkc, keyid );
if( keyid[0] == sig->keyid[0] && keyid[1] == sig->keyid[1] )
*is_selfsig = 1;
}
md = md_open( algo, 0 );
hash_public_cert( md, pkc );
hash_public_cert( md, snode->pkt->pkt.public_cert );
rc = do_check( pkc, sig, md );
md_close(md);
}
else {
log_error("no subkey for key signature packet\n");
rc = G10ERR_SIG_CLASS;
}
}
else if( sig->sig_class >= 0x14 && sig->sig_class <= 0x17 ) {
/* a gnupg extension: calculate the signature over all
* preceding userids */
KBNODE unode = find_prev_kbnode( root, node, PKT_USER_ID );
u32 keyid[2];
int any = 0;
keyid_from_pkc( pkc, keyid );
md = md_open( algo, 0 );
hash_public_cert( md, pkc );
for( unode=root->next; unode && unode != node; unode = unode->next ) {
if( unode->pkt->pkttype == PKT_USER_ID ) {
hash_uid_node( unode, md, sig );
any++;
}
}
if( any ) {
if( keyid[0] == sig->keyid[0] && keyid[1] == sig->keyid[1] ) {
if( is_selfsig )
*is_selfsig = 1;
rc = do_check( pkc, sig, md );
}
else
rc = signature_check( sig, md );
}
else {
log_error("no user id for key signature packet\n");
rc = G10ERR_SIG_CLASS;
}
md_close(md);
}
else {
KBNODE unode = find_prev_kbnode( root, node, PKT_USER_ID );
if( unode ) {
u32 keyid[2];
keyid_from_pkc( pkc, keyid );
md = md_open( algo, 0 );
/*md_start_debug(md, "check");*/
hash_public_cert( md, pkc );
hash_uid_node( unode, md, sig );
if( keyid[0] == sig->keyid[0] && keyid[1] == sig->keyid[1] ) {
if( is_selfsig )
*is_selfsig = 1;
rc = do_check( pkc, sig, md );
}
else
rc = signature_check( sig, md );
md_close(md);
}
else {
log_error("no user id for key signature packet\n");
rc = G10ERR_SIG_CLASS;
}
}
return rc;
}

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