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f133abe521
The ssh_signkey vtable has grown a new method ssh_key_invalid(), which checks whether the key is going to be usable for constructing a signature at all. Currently the only way this can fail is if it's an RSA key so short that there isn't room to put all the PKCS#1 formatting in the signature preimage integer, but the return value is an arbitrary error message just in case more reasons are needed later. This is tested separately rather than at key-creation time because of the signature flags system: an RSA key of intermediate length could be valid for SHA-1 signing but not for SHA-512. So really this method should be called at the point where you've decided what sig flags you want to use, and you're checking if _those flags_ are OK. On the verification side, there's no need for a separate check. If someone presents us with an RSA key so short that it's impossible to encode a valid signature using it, then we simply regard all signatures as invalid.
483 lines
13 KiB
C
483 lines
13 KiB
C
/*
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* Digital Signature Standard 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 dss_freekey(ssh_key *key); /* forward reference */
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static ssh_key *dss_new_pub(const ssh_keyalg *self, ptrlen data)
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{
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BinarySource src[1];
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struct dss_key *dss;
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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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dss = snew(struct dss_key);
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dss->sshk.vt = &ssh_dss;
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dss->p = get_mp_ssh2(src);
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dss->q = get_mp_ssh2(src);
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dss->g = get_mp_ssh2(src);
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dss->y = get_mp_ssh2(src);
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dss->x = NULL;
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if (get_err(src) ||
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mp_eq_integer(dss->p, 0) || mp_eq_integer(dss->q, 0)) {
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/* Invalid key. */
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dss_freekey(&dss->sshk);
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return NULL;
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}
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return &dss->sshk;
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}
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static void dss_freekey(ssh_key *key)
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{
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struct dss_key *dss = container_of(key, struct dss_key, sshk);
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if (dss->p)
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mp_free(dss->p);
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if (dss->q)
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mp_free(dss->q);
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if (dss->g)
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mp_free(dss->g);
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if (dss->y)
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mp_free(dss->y);
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if (dss->x)
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mp_free(dss->x);
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sfree(dss);
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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 *dss_cache_str(ssh_key *key)
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{
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struct dss_key *dss = container_of(key, struct dss_key, sshk);
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strbuf *sb = strbuf_new();
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if (!dss->p)
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return NULL;
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append_hex_to_strbuf(sb, dss->p);
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append_hex_to_strbuf(sb, dss->q);
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append_hex_to_strbuf(sb, dss->g);
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append_hex_to_strbuf(sb, dss->y);
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return strbuf_to_str(sb);
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}
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static char *dss_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 dss_verify(ssh_key *key, ptrlen sig, ptrlen data)
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{
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struct dss_key *dss = container_of(key, struct dss_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 (!dss->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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if (mp_eq_integer(s, 0)) {
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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, dss->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, dss->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, dss->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(dss->g, u1, dss->p);
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mp_int *yu2p = mp_modpow(dss->y, u2, dss->p);
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mp_int *gu1yu2p = mp_modmul(gu1p, yu2p, dss->p);
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mp_int *v = mp_mod(gu1yu2p, dss->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 dss_public_blob(ssh_key *key, BinarySink *bs)
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{
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struct dss_key *dss = container_of(key, struct dss_key, sshk);
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put_stringz(bs, "ssh-dss");
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put_mp_ssh2(bs, dss->p);
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put_mp_ssh2(bs, dss->q);
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put_mp_ssh2(bs, dss->g);
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put_mp_ssh2(bs, dss->y);
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}
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static void dss_private_blob(ssh_key *key, BinarySink *bs)
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{
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struct dss_key *dss = container_of(key, struct dss_key, sshk);
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put_mp_ssh2(bs, dss->x);
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}
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static ssh_key *dss_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 dss_key *dss;
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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 = dss_new_pub(self, pub);
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if (!sshk)
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return NULL;
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dss = container_of(sshk, struct dss_key, sshk);
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BinarySource_BARE_INIT_PL(src, priv);
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dss->x = get_mp_ssh2(src);
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if (get_err(src)) {
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dss_freekey(&dss->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 DSS 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, dss->p);
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put_mp_ssh2(h, dss->q);
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put_mp_ssh2(h, dss->g);
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ssh_hash_final(h, digest);
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if (!smemeq(hash.ptr, digest, 20)) {
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dss_freekey(&dss->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(dss->g, dss->x, dss->p);
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if (!mp_cmp_eq(ytest, dss->y)) {
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mp_free(ytest);
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dss_freekey(&dss->sshk);
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return NULL;
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}
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mp_free(ytest);
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return &dss->sshk;
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}
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static ssh_key *dss_new_priv_openssh(const ssh_keyalg *self,
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BinarySource *src)
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{
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struct dss_key *dss;
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dss = snew(struct dss_key);
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dss->sshk.vt = &ssh_dss;
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dss->p = get_mp_ssh2(src);
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dss->q = get_mp_ssh2(src);
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dss->g = get_mp_ssh2(src);
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dss->y = get_mp_ssh2(src);
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dss->x = get_mp_ssh2(src);
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if (get_err(src) ||
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mp_eq_integer(dss->q, 0) || mp_eq_integer(dss->p, 0)) {
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/* Invalid key. */
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dss_freekey(&dss->sshk);
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return NULL;
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}
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return &dss->sshk;
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}
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static void dss_openssh_blob(ssh_key *key, BinarySink *bs)
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{
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struct dss_key *dss = container_of(key, struct dss_key, sshk);
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put_mp_ssh2(bs, dss->p);
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put_mp_ssh2(bs, dss->q);
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put_mp_ssh2(bs, dss->g);
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put_mp_ssh2(bs, dss->y);
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put_mp_ssh2(bs, dss->x);
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}
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static int dss_pubkey_bits(const ssh_keyalg *self, ptrlen pub)
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{
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ssh_key *sshk;
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struct dss_key *dss;
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int ret;
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sshk = dss_new_pub(self, pub);
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if (!sshk)
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return -1;
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dss = container_of(sshk, struct dss_key, sshk);
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ret = mp_get_nbits(dss->p);
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dss_freekey(&dss->sshk);
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return ret;
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}
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mp_int *dss_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 DSS 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_final(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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h = ssh_hash_new(&ssh_sha512);
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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 dss_sign(ssh_key *key, ptrlen data, unsigned flags, BinarySink *bs)
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{
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struct dss_key *dss = container_of(key, struct dss_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 = dss_gen_k("DSA deterministic k generator", dss->q, dss->x,
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digest, sizeof(digest));
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mp_int *kinv = mp_invert(k, dss->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(dss->g, k, dss->p); /* g^k mod p */
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mp_int *r = mp_mod(gkp, dss->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(dss->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, dss->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++)
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put_byte(bs, mp_get_byte(r, 19 - i));
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for (i = 0; i < 20; i++)
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put_byte(bs, mp_get_byte(s, 19 - i));
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mp_free(r);
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mp_free(s);
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}
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const ssh_keyalg ssh_dss = {
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dss_new_pub,
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dss_new_priv,
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dss_new_priv_openssh,
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dss_freekey,
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dss_invalid,
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dss_sign,
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dss_verify,
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dss_public_blob,
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dss_private_blob,
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dss_openssh_blob,
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dss_cache_str,
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dss_pubkey_bits,
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"ssh-dss",
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"dss",
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NULL,
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0, /* no supported flags */
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};
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