mirror of
https://git.tartarus.org/simon/putty.git
synced 2025-07-01 11:32:48 -05:00
Add support for DSA authentication in SSH2, following clever ideas
on how to get round the problem of generating a good k. [originally from svn r1284]
This commit is contained in:
Simon Tatham
317
sshdss.c
317
sshdss.c
@ -3,6 +3,7 @@
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#include <assert.h>
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#include "ssh.h"
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#include "misc.h"
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#define GET_32BIT(cp) \
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(((unsigned long)(unsigned char)(cp)[0] << 24) | \
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@ -16,11 +17,35 @@
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(cp)[2] = (unsigned char)((value) >> 8); \
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(cp)[3] = (unsigned char)(value); }
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#if 0
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#define DEBUG_DSS
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#else
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#define diagbn(x,y)
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#endif
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static void sha_mpint(SHA_State * s, Bignum b)
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{
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unsigned char *p;
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unsigned char lenbuf[4];
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int len;
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len = (bignum_bitcount(b) + 8) / 8;
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PUT_32BIT(lenbuf, len);
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SHA_Bytes(s, lenbuf, 4);
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while (len-- > 0) {
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lenbuf[0] = bignum_byte(b, len);
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SHA_Bytes(s, lenbuf, 1);
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}
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memset(lenbuf, 0, sizeof(lenbuf));
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}
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static void sha512_mpint(SHA512_State * s, Bignum b)
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{
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unsigned char *p;
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unsigned char lenbuf[4];
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int len;
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len = (bignum_bitcount(b) + 8) / 8;
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PUT_32BIT(lenbuf, len);
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SHA512_Bytes(s, lenbuf, 4);
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while (len-- > 0) {
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lenbuf[0] = bignum_byte(b, len);
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SHA512_Bytes(s, lenbuf, 1);
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}
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memset(lenbuf, 0, sizeof(lenbuf));
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}
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static void getstring(char **data, int *datalen, char **p, int *length)
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{
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@ -62,10 +87,6 @@ static Bignum get160(char **data, int *datalen)
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return b;
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}
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struct dss_key {
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Bignum p, q, g, y;
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};
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static void *dss_newkey(char *data, int len)
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{
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char *p;
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@ -236,14 +257,8 @@ static int dss_verifysig(void *key, char *sig, int siglen,
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}
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sig += 4, siglen -= 4; /* skip yet another length field */
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}
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diagbn("p=", dss->p);
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diagbn("q=", dss->q);
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diagbn("g=", dss->g);
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diagbn("y=", dss->y);
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r = get160(&sig, &siglen);
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diagbn("r=", r);
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s = get160(&sig, &siglen);
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diagbn("s=", s);
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if (!r || !s)
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return 0;
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@ -251,7 +266,6 @@ static int dss_verifysig(void *key, char *sig, int siglen,
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* Step 1. w <- s^-1 mod q.
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*/
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w = modinv(s, dss->q);
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diagbn("w=", w);
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/*
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* Step 2. u1 <- SHA(message) * w mod q.
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@ -260,28 +274,20 @@ static int dss_verifysig(void *key, char *sig, int siglen,
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p = hash;
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slen = 20;
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sha = get160(&p, &slen);
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diagbn("sha=", sha);
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u1 = modmul(sha, w, dss->q);
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diagbn("u1=", u1);
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/*
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* Step 3. u2 <- r * w mod q.
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*/
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u2 = modmul(r, w, dss->q);
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diagbn("u2=", u2);
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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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gu1p = modpow(dss->g, u1, dss->p);
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diagbn("gu1p=", gu1p);
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yu2p = modpow(dss->y, u2, dss->p);
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diagbn("yu2p=", yu2p);
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gu1yu2p = modmul(gu1p, yu2p, dss->p);
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diagbn("gu1yu2p=", gu1yu2p);
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v = modmul(gu1yu2p, One, dss->q);
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diagbn("gu1yu2q=v=", v);
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diagbn("r=", r);
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/*
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* Step 5. v should now be equal to r.
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@ -347,28 +353,281 @@ static unsigned char *dss_public_blob(void *key, int *len)
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static unsigned char *dss_private_blob(void *key, int *len)
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{
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return NULL; /* can't handle DSS private keys */
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struct dss_key *dss = (struct dss_key *) key;
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int xlen, bloblen;
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int i;
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unsigned char *blob, *p;
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SHA_State s;
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unsigned char digest[20];
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xlen = (bignum_bitcount(dss->x) + 8) / 8;
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/*
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* mpint x, string[20] the SHA of p||q||g. Total 28 + xlen.
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* (two length fields and twenty bytes, 20+8=28).
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*/
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bloblen = 28 + xlen;
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blob = smalloc(bloblen);
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p = blob;
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PUT_32BIT(p, xlen);
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p += 4;
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for (i = xlen; i--;)
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*p++ = bignum_byte(dss->x, i);
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PUT_32BIT(p, 20);
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SHA_Init(&s);
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sha_mpint(&s, dss->p);
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sha_mpint(&s, dss->q);
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sha_mpint(&s, dss->g);
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SHA_Final(&s, digest);
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p += 4;
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for (i = 0; i < 20; i++)
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*p++ = digest[i];
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assert(p == blob + bloblen);
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*len = bloblen;
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return blob;
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}
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static void *dss_createkey(unsigned char *pub_blob, int pub_len,
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unsigned char *priv_blob, int priv_len)
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{
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return NULL; /* can't handle DSS private keys */
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struct dss_key *dss;
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char *pb = (char *) priv_blob;
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char *hash;
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int hashlen;
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SHA_State s;
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unsigned char digest[20];
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Bignum ytest;
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dss = dss_newkey((char *) pub_blob, pub_len);
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dss->x = getmp(&pb, &priv_len);
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getstring(&pb, &priv_len, &hash, &hashlen);
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/*
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* Verify details of the key. First check that the hash is
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* indeed a hash of p||q||g.
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*/
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if (hashlen != 20) {
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dss_freekey(dss);
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return NULL;
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}
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SHA_Init(&s);
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sha_mpint(&s, dss->p);
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sha_mpint(&s, dss->q);
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sha_mpint(&s, dss->g);
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SHA_Final(&s, digest);
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if (0 != memcmp(hash, digest, 20)) {
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dss_freekey(dss);
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return NULL;
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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 = modpow(dss->g, dss->x, dss->p);
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if (0 != bignum_cmp(ytest, dss->y)) {
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dss_freekey(dss);
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return NULL;
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}
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freebn(ytest);
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return dss;
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}
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static void *dss_openssh_createkey(unsigned char **blob, int *len)
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{
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return NULL; /* can't handle DSS private keys */
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char **b = (char **) blob;
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struct dss_key *dss;
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dss = smalloc(sizeof(struct dss_key));
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if (!dss)
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return NULL;
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dss->p = getmp(b, len);
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dss->q = getmp(b, len);
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dss->g = getmp(b, len);
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dss->y = getmp(b, len);
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dss->x = getmp(b, len);
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if (!dss->p || !dss->q || !dss->g || !dss->y || !dss->x) {
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sfree(dss->p);
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sfree(dss->q);
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sfree(dss->g);
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sfree(dss->y);
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sfree(dss->x);
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sfree(dss);
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return NULL;
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}
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return dss;
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}
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static int dss_openssh_fmtkey(void *key, unsigned char *blob, int len)
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{
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return -1; /* can't handle DSS private keys */
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struct dss_key *dss = (struct dss_key *) key;
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int bloblen, i;
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bloblen =
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ssh2_bignum_length(dss->p) +
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ssh2_bignum_length(dss->q) +
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ssh2_bignum_length(dss->g) +
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ssh2_bignum_length(dss->y) +
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ssh2_bignum_length(dss->x);
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if (bloblen > len)
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return bloblen;
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bloblen = 0;
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#define ENC(x) \
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PUT_32BIT(blob+bloblen, ssh2_bignum_length((x))-4); bloblen += 4; \
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for (i = ssh2_bignum_length((x))-4; i-- ;) blob[bloblen++]=bignum_byte((x),i);
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ENC(dss->p);
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ENC(dss->q);
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ENC(dss->g);
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ENC(dss->y);
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ENC(dss->x);
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return bloblen;
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}
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unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
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{
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return NULL; /* can't handle DSS private keys */
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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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* (It doesn't, _per se_, protect against reuse of k. Reuse of
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* k is left to chance; all it does is prevent _excessively
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* high_ chances of reuse of k due to entropy 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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struct dss_key *dss = (struct dss_key *) key;
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SHA512_State ss;
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unsigned char digest[20], digest512[64];
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Bignum proto_k, k, gkp, hash, kinv, hxr, r, s;
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unsigned char *bytes;
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int nbytes, i;
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SHA_Simple(data, datalen, digest);
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/*
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* Hash some identifying text plus x.
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*/
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SHA512_Init(&ss);
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SHA512_Bytes(&ss, "DSA deterministic k generator", 30);
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sha512_mpint(&ss, dss->x);
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SHA512_Final(&ss, 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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SHA512_Init(&ss);
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SHA512_Bytes(&ss, digest512, sizeof(digest512));
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SHA512_Bytes(&ss, digest, sizeof(digest));
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SHA512_Final(&ss, digest512);
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memset(&ss, 0, sizeof(ss));
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/*
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* Now convert the result into a bignum, and reduce it mod q.
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*/
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proto_k = bignum_from_bytes(digest512, 64);
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k = bigmod(proto_k, dss->q);
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freebn(proto_k);
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memset(digest512, 0, sizeof(digest512));
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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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gkp = modpow(dss->g, k, dss->p); /* g^k mod p */
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r = bigmod(gkp, dss->q); /* r = (g^k mod p) mod q */
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freebn(gkp);
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hash = bignum_from_bytes(digest, 20);
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kinv = modinv(k, dss->q); /* k^-1 mod q */
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hxr = bigmuladd(dss->x, r, hash); /* hash + x*r */
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s = modmul(kinv, hxr, dss->q); /* s = k^-1 * (hash + x*r) mod q */
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freebn(hxr);
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freebn(kinv);
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freebn(hash);
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/*
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* Signature blob is
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*
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* string "ssh-dss"
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* string two 20-byte numbers r and s, end to end
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*
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* i.e. 4+7 + 4+40 bytes.
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*/
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nbytes = 4 + 7 + 4 + 40;
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bytes = smalloc(nbytes);
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PUT_32BIT(bytes, 7);
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memcpy(bytes + 4, "ssh-dss", 7);
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PUT_32BIT(bytes + 4 + 7, 40);
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for (i = 0; i < 20; i++) {
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bytes[4 + 7 + 4 + i] = bignum_byte(r, 19 - i);
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bytes[4 + 7 + 4 + 20 + i] = bignum_byte(s, 19 - i);
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}
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freebn(r);
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freebn(s);
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*siglen = nbytes;
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return bytes;
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}
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const struct ssh_signkey ssh_dss = {
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