mirror of
https://git.tartarus.org/simon/putty.git
synced 2025-01-09 09:27:59 +00:00
1c039d0a7b
This code base has always been a bit confused about which spelling it likes to use to refer to that signature algorithm. The SSH protocol id is "ssh-dss". But everyone I know refers to it as the Digital Signature _Algorithm_, not the Digital Signature _Standard_. When I moved everything down into the crypto subdir, I took the opportunity to rename sshdss.c to dsa.c. Now I'm doing the rest of the job: all internal identifiers and code comments refer to DSA, and the spelling "dss" only survives in externally visible identifiers that have to remain constant. (Such identifiers include the SSH protocol id, and also the string id used to identify the key type in PuTTY's own host key cache. We can't change the latter without causing everyone a backwards-compatibility headache, and if we _did_ ever decide to do that, we'd surely want to do a much more thorough job of making the cache format more sensible!)
504 lines
14 KiB
C
504 lines
14 KiB
C
/*
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* Digital Signature Algorithm implementation for PuTTY.
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <assert.h>
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#include "ssh.h"
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#include "mpint.h"
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#include "misc.h"
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static void dsa_freekey(ssh_key *key); /* forward reference */
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static ssh_key *dsa_new_pub(const ssh_keyalg *self, ptrlen data)
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{
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BinarySource src[1];
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struct dsa_key *dsa;
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BinarySource_BARE_INIT_PL(src, data);
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if (!ptrlen_eq_string(get_string(src), "ssh-dss"))
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return NULL;
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dsa = snew(struct dsa_key);
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dsa->sshk.vt = &ssh_dsa;
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dsa->p = get_mp_ssh2(src);
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dsa->q = get_mp_ssh2(src);
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dsa->g = get_mp_ssh2(src);
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dsa->y = get_mp_ssh2(src);
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dsa->x = NULL;
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if (get_err(src) ||
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mp_eq_integer(dsa->p, 0) || mp_eq_integer(dsa->q, 0)) {
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/* Invalid key. */
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dsa_freekey(&dsa->sshk);
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return NULL;
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}
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return &dsa->sshk;
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}
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static void dsa_freekey(ssh_key *key)
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{
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struct dsa_key *dsa = container_of(key, struct dsa_key, sshk);
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if (dsa->p)
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mp_free(dsa->p);
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if (dsa->q)
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mp_free(dsa->q);
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if (dsa->g)
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mp_free(dsa->g);
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if (dsa->y)
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mp_free(dsa->y);
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if (dsa->x)
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mp_free(dsa->x);
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sfree(dsa);
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}
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static void append_hex_to_strbuf(strbuf *sb, mp_int *x)
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{
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if (sb->len > 0)
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put_byte(sb, ',');
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put_data(sb, "0x", 2);
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char *hex = mp_get_hex(x);
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size_t hexlen = strlen(hex);
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put_data(sb, hex, hexlen);
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smemclr(hex, hexlen);
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sfree(hex);
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}
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static char *dsa_cache_str(ssh_key *key)
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{
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struct dsa_key *dsa = container_of(key, struct dsa_key, sshk);
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strbuf *sb = strbuf_new();
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if (!dsa->p) {
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strbuf_free(sb);
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return NULL;
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}
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append_hex_to_strbuf(sb, dsa->p);
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append_hex_to_strbuf(sb, dsa->q);
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append_hex_to_strbuf(sb, dsa->g);
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append_hex_to_strbuf(sb, dsa->y);
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return strbuf_to_str(sb);
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}
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static key_components *dsa_components(ssh_key *key)
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{
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struct dsa_key *dsa = container_of(key, struct dsa_key, sshk);
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key_components *kc = key_components_new();
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key_components_add_text(kc, "key_type", "DSA");
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assert(dsa->p);
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key_components_add_mp(kc, "p", dsa->p);
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key_components_add_mp(kc, "q", dsa->q);
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key_components_add_mp(kc, "g", dsa->g);
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key_components_add_mp(kc, "public_y", dsa->y);
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if (dsa->x)
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key_components_add_mp(kc, "private_x", dsa->x);
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return kc;
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}
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static char *dsa_invalid(ssh_key *key, unsigned flags)
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{
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/* No validity criterion will stop us from using a DSA key at all */
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return NULL;
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}
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static bool dsa_verify(ssh_key *key, ptrlen sig, ptrlen data)
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{
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struct dsa_key *dsa = container_of(key, struct dsa_key, sshk);
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BinarySource src[1];
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unsigned char hash[20];
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bool toret;
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if (!dsa->p)
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return false;
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BinarySource_BARE_INIT_PL(src, sig);
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/*
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* Commercial SSH (2.0.13) and OpenSSH disagree over the format
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* of a DSA signature. OpenSSH is in line with RFC 4253:
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* it uses a string "ssh-dss", followed by a 40-byte string
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* containing two 160-bit integers end-to-end. Commercial SSH
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* can't be bothered with the header bit, and considers a DSA
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* signature blob to be _just_ the 40-byte string containing
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* the two 160-bit integers. We tell them apart by measuring
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* the length: length 40 means the commercial-SSH bug, anything
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* else is assumed to be RFC-compliant.
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*/
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if (sig.len != 40) { /* bug not present; read admin fields */
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ptrlen type = get_string(src);
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sig = get_string(src);
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if (get_err(src) || !ptrlen_eq_string(type, "ssh-dss") ||
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sig.len != 40)
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return false;
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}
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/* Now we're sitting on a 40-byte string for sure. */
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mp_int *r = mp_from_bytes_be(make_ptrlen(sig.ptr, 20));
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mp_int *s = mp_from_bytes_be(make_ptrlen((const char *)sig.ptr + 20, 20));
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if (!r || !s) {
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if (r)
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mp_free(r);
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if (s)
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mp_free(s);
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return false;
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}
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/* Basic sanity checks: 0 < r,s < q */
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unsigned invalid = 0;
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invalid |= mp_eq_integer(r, 0);
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invalid |= mp_eq_integer(s, 0);
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invalid |= mp_cmp_hs(r, dsa->q);
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invalid |= mp_cmp_hs(s, dsa->q);
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if (invalid) {
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mp_free(r);
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mp_free(s);
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return false;
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}
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/*
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* Step 1. w <- s^-1 mod q.
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*/
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mp_int *w = mp_invert(s, dsa->q);
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if (!w) {
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mp_free(r);
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mp_free(s);
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return false;
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}
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/*
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* Step 2. u1 <- SHA(message) * w mod q.
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*/
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hash_simple(&ssh_sha1, data, hash);
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mp_int *sha = mp_from_bytes_be(make_ptrlen(hash, 20));
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mp_int *u1 = mp_modmul(sha, w, dsa->q);
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/*
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* Step 3. u2 <- r * w mod q.
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*/
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mp_int *u2 = mp_modmul(r, w, dsa->q);
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/*
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* Step 4. v <- (g^u1 * y^u2 mod p) mod q.
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*/
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mp_int *gu1p = mp_modpow(dsa->g, u1, dsa->p);
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mp_int *yu2p = mp_modpow(dsa->y, u2, dsa->p);
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mp_int *gu1yu2p = mp_modmul(gu1p, yu2p, dsa->p);
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mp_int *v = mp_mod(gu1yu2p, dsa->q);
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/*
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* Step 5. v should now be equal to r.
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*/
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toret = mp_cmp_eq(v, r);
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mp_free(w);
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mp_free(sha);
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mp_free(u1);
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mp_free(u2);
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mp_free(gu1p);
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mp_free(yu2p);
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mp_free(gu1yu2p);
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mp_free(v);
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mp_free(r);
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mp_free(s);
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return toret;
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}
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static void dsa_public_blob(ssh_key *key, BinarySink *bs)
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{
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struct dsa_key *dsa = container_of(key, struct dsa_key, sshk);
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put_stringz(bs, "ssh-dss");
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put_mp_ssh2(bs, dsa->p);
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put_mp_ssh2(bs, dsa->q);
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put_mp_ssh2(bs, dsa->g);
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put_mp_ssh2(bs, dsa->y);
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}
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static void dsa_private_blob(ssh_key *key, BinarySink *bs)
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{
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struct dsa_key *dsa = container_of(key, struct dsa_key, sshk);
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put_mp_ssh2(bs, dsa->x);
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}
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static ssh_key *dsa_new_priv(const ssh_keyalg *self, ptrlen pub, ptrlen priv)
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{
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BinarySource src[1];
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ssh_key *sshk;
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struct dsa_key *dsa;
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ptrlen hash;
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unsigned char digest[20];
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mp_int *ytest;
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sshk = dsa_new_pub(self, pub);
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if (!sshk)
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return NULL;
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dsa = container_of(sshk, struct dsa_key, sshk);
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BinarySource_BARE_INIT_PL(src, priv);
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dsa->x = get_mp_ssh2(src);
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if (get_err(src)) {
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dsa_freekey(&dsa->sshk);
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return NULL;
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}
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/*
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* Check the obsolete hash in the old DSA key format.
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*/
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hash = get_string(src);
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if (hash.len == 20) {
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ssh_hash *h = ssh_hash_new(&ssh_sha1);
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put_mp_ssh2(h, dsa->p);
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put_mp_ssh2(h, dsa->q);
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put_mp_ssh2(h, dsa->g);
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ssh_hash_final(h, digest);
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if (!smemeq(hash.ptr, digest, 20)) {
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dsa_freekey(&dsa->sshk);
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return NULL;
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}
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}
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/*
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* Now ensure g^x mod p really is y.
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*/
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ytest = mp_modpow(dsa->g, dsa->x, dsa->p);
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if (!mp_cmp_eq(ytest, dsa->y)) {
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mp_free(ytest);
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dsa_freekey(&dsa->sshk);
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return NULL;
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}
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mp_free(ytest);
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return &dsa->sshk;
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}
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static ssh_key *dsa_new_priv_openssh(const ssh_keyalg *self,
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BinarySource *src)
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{
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struct dsa_key *dsa;
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dsa = snew(struct dsa_key);
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dsa->sshk.vt = &ssh_dsa;
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dsa->p = get_mp_ssh2(src);
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dsa->q = get_mp_ssh2(src);
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dsa->g = get_mp_ssh2(src);
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dsa->y = get_mp_ssh2(src);
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dsa->x = get_mp_ssh2(src);
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if (get_err(src) ||
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mp_eq_integer(dsa->q, 0) || mp_eq_integer(dsa->p, 0)) {
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/* Invalid key. */
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dsa_freekey(&dsa->sshk);
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return NULL;
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}
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return &dsa->sshk;
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}
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static void dsa_openssh_blob(ssh_key *key, BinarySink *bs)
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{
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struct dsa_key *dsa = container_of(key, struct dsa_key, sshk);
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put_mp_ssh2(bs, dsa->p);
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put_mp_ssh2(bs, dsa->q);
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put_mp_ssh2(bs, dsa->g);
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put_mp_ssh2(bs, dsa->y);
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put_mp_ssh2(bs, dsa->x);
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}
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static int dsa_pubkey_bits(const ssh_keyalg *self, ptrlen pub)
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{
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ssh_key *sshk;
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struct dsa_key *dsa;
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int ret;
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sshk = dsa_new_pub(self, pub);
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if (!sshk)
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return -1;
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dsa = container_of(sshk, struct dsa_key, sshk);
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ret = mp_get_nbits(dsa->p);
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dsa_freekey(&dsa->sshk);
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return ret;
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}
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mp_int *dsa_gen_k(const char *id_string, mp_int *modulus,
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mp_int *private_key,
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unsigned char *digest, int digest_len)
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{
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/*
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* The basic DSA signing algorithm is:
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*
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* - invent a random k between 1 and q-1 (exclusive).
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* - Compute r = (g^k mod p) mod q.
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* - Compute s = k^-1 * (hash + x*r) mod q.
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*
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* This has the dangerous properties that:
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*
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* - if an attacker in possession of the public key _and_ the
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* signature (for example, the host you just authenticated
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* to) can guess your k, he can reverse the computation of s
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* and work out x = r^-1 * (s*k - hash) mod q. That is, he
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* can deduce the private half of your key, and masquerade
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* as you for as long as the key is still valid.
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*
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* - since r is a function purely of k and the public key, if
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* the attacker only has a _range of possibilities_ for k
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* it's easy for him to work through them all and check each
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* one against r; he'll never be unsure of whether he's got
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* the right one.
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*
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* - if you ever sign two different hashes with the same k, it
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* will be immediately obvious because the two signatures
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* will have the same r, and moreover an attacker in
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* possession of both signatures (and the public key of
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* course) can compute k = (hash1-hash2) * (s1-s2)^-1 mod q,
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* and from there deduce x as before.
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*
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* - the Bleichenbacher attack on DSA makes use of methods of
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* generating k which are significantly non-uniformly
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* distributed; in particular, generating a 160-bit random
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* number and reducing it mod q is right out.
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*
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* For this reason we must be pretty careful about how we
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* generate our k. Since this code runs on Windows, with no
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* particularly good system entropy sources, we can't trust our
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* RNG itself to produce properly unpredictable data. Hence, we
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* use a totally different scheme instead.
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*
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* What we do is to take a SHA-512 (_big_) hash of the private
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* key x, and then feed this into another SHA-512 hash that
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* also includes the message hash being signed. That is:
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*
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* proto_k = SHA512 ( SHA512(x) || SHA160(message) )
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*
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* This number is 512 bits long, so reducing it mod q won't be
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* noticeably non-uniform. So
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*
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* k = proto_k mod q
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*
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* This has the interesting property that it's _deterministic_:
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* signing the same hash twice with the same key yields the
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* same signature.
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*
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* Despite this determinism, it's still not predictable to an
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* attacker, because in order to repeat the SHA-512
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* construction that created it, the attacker would have to
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* know the private key value x - and by assumption he doesn't,
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* because if he knew that he wouldn't be attacking k!
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*
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* (This trick doesn't, _per se_, protect against reuse of k.
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* Reuse of k is left to chance; all it does is prevent
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* _excessively high_ chances of reuse of k due to entropy
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* problems.)
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*
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* Thanks to Colin Plumb for the general idea of using x to
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* ensure k is hard to guess, and to the Cambridge University
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* Computer Security Group for helping to argue out all the
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* fine details.
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*/
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ssh_hash *h;
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unsigned char digest512[64];
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/*
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* Hash some identifying text plus x.
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*/
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h = ssh_hash_new(&ssh_sha512);
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put_asciz(h, id_string);
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put_mp_ssh2(h, private_key);
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ssh_hash_digest(h, digest512);
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/*
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* Now hash that digest plus the message hash.
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*/
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ssh_hash_reset(h);
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put_data(h, digest512, sizeof(digest512));
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put_data(h, digest, digest_len);
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ssh_hash_final(h, digest512);
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/*
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* Now convert the result into a bignum, and coerce it to the
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* range [2,q), which we do by reducing it mod q-2 and adding 2.
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*/
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mp_int *modminus2 = mp_copy(modulus);
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mp_sub_integer_into(modminus2, modminus2, 2);
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mp_int *proto_k = mp_from_bytes_be(make_ptrlen(digest512, 64));
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mp_int *k = mp_mod(proto_k, modminus2);
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mp_free(proto_k);
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mp_free(modminus2);
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mp_add_integer_into(k, k, 2);
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smemclr(digest512, sizeof(digest512));
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return k;
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}
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static void dsa_sign(ssh_key *key, ptrlen data, unsigned flags, BinarySink *bs)
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{
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struct dsa_key *dsa = container_of(key, struct dsa_key, sshk);
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unsigned char digest[20];
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int i;
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hash_simple(&ssh_sha1, data, digest);
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mp_int *k = dsa_gen_k("DSA deterministic k generator", dsa->q, dsa->x,
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digest, sizeof(digest));
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mp_int *kinv = mp_invert(k, dsa->q); /* k^-1 mod q */
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/*
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* Now we have k, so just go ahead and compute the signature.
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*/
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mp_int *gkp = mp_modpow(dsa->g, k, dsa->p); /* g^k mod p */
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mp_int *r = mp_mod(gkp, dsa->q); /* r = (g^k mod p) mod q */
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mp_free(gkp);
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mp_int *hash = mp_from_bytes_be(make_ptrlen(digest, 20));
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mp_int *xr = mp_mul(dsa->x, r);
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mp_int *hxr = mp_add(xr, hash); /* hash + x*r */
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mp_int *s = mp_modmul(kinv, hxr, dsa->q); /* s = k^-1 * (hash+x*r) mod q */
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mp_free(hxr);
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mp_free(xr);
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mp_free(kinv);
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mp_free(k);
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mp_free(hash);
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put_stringz(bs, "ssh-dss");
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put_uint32(bs, 40);
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for (i = 0; i < 20; i++)
|
|
put_byte(bs, mp_get_byte(r, 19 - i));
|
|
for (i = 0; i < 20; i++)
|
|
put_byte(bs, mp_get_byte(s, 19 - i));
|
|
mp_free(r);
|
|
mp_free(s);
|
|
}
|
|
|
|
const ssh_keyalg ssh_dsa = {
|
|
.new_pub = dsa_new_pub,
|
|
.new_priv = dsa_new_priv,
|
|
.new_priv_openssh = dsa_new_priv_openssh,
|
|
.freekey = dsa_freekey,
|
|
.invalid = dsa_invalid,
|
|
.sign = dsa_sign,
|
|
.verify = dsa_verify,
|
|
.public_blob = dsa_public_blob,
|
|
.private_blob = dsa_private_blob,
|
|
.openssh_blob = dsa_openssh_blob,
|
|
.cache_str = dsa_cache_str,
|
|
.components = dsa_components,
|
|
.pubkey_bits = dsa_pubkey_bits,
|
|
.ssh_id = "ssh-dss",
|
|
.cache_id = "dss",
|
|
};
|