- djm@cvs.openbsd.org 2008/11/04 08:22:13

     [auth.h auth2.c monitor.c monitor.h monitor_wrap.c monitor_wrap.h]
     [readconf.c readconf.h servconf.c servconf.h ssh2.h ssh_config.5]
     [sshconnect2.c sshd_config.5 jpake.c jpake.h schnorr.c auth2-jpake.c]
     [Makefile.in]
     Add support for an experimental zero-knowledge password authentication
     method using the J-PAKE protocol described in F. Hao, P. Ryan,
     "Password Authenticated Key Exchange by Juggling", 16th Workshop on
     Security Protocols, Cambridge, April 2008.

     This method allows password-based authentication without exposing
     the password to the server. Instead, the client and server exchange
     cryptographic proofs to demonstrate of knowledge of the password while
     revealing nothing useful to an attacker or compromised endpoint.

     This is experimental, work-in-progress code and is presently
     compiled-time disabled (turn on -DJPAKE in Makefile.inc).

     "just commit it.  It isn't too intrusive." deraadt@

1 files changed, 604 insertions, 0 deletions

diff --git a/jpake.c b/jpake.c
new file mode 100644
index 000000000..565f2e255
--- /dev/null
+++ b/jpake.c
@@ -0,0 +1,604 @@
+/* $OpenBSD: jpake.c,v 1.1 2008/11/04 08:22:12 djm Exp $ */
+/*
+ * Copyright (c) 2008 Damien Miller.  All rights reserved.
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/*
+ * Shared components of zero-knowledge password auth using J-PAKE protocol
+ * as described in:
+ *
+ * F. Hao, P. Ryan, "Password Authenticated Key Exchange by Juggling",
+ * 16th Workshop on Security Protocols, Cambridge, April 2008
+ *
+ * http://grouper.ieee.org/groups/1363/Research/contributions/hao-ryan-2008.pdf
+ */
+
+#include "includes.h"
+
+#include <sys/types.h>
+
+#include <stdio.h>
+#include <string.h>
+#include <stdarg.h>
+
+#include <openssl/bn.h>
+#include <openssl/evp.h>
+
+#include "xmalloc.h"
+#include "ssh2.h"
+#include "key.h"
+#include "hostfile.h"
+#include "auth.h"
+#include "buffer.h"
+#include "packet.h"
+#include "dispatch.h"
+#include "log.h"
+
+#include "jpake.h"
+
+#ifdef JPAKE
+
+/* RFC3526 group 5, 1536 bits */
+#define JPAKE_GROUP_G "2"
+#define JPAKE_GROUP_P \
+        "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74" \
+        "020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F1437" \
+        "4FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED" \
+        "EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3DC2007CB8A163BF05" \
+        "98DA48361C55D39A69163FA8FD24CF5F83655D23DCA3AD961C62F356208552BB" \
+        "9ED529077096966D670C354E4ABC9804F1746C08CA237327FFFFFFFFFFFFFFFF"
+
+struct jpake_group *
+jpake_default_group(void)
+{
+        struct jpake_group *ret;
+
+        ret = xmalloc(sizeof(*ret));
+        ret->p = ret->q = ret->g = NULL;
+        if (BN_hex2bn(&ret->p, JPAKE_GROUP_P) == 0 ||
+            BN_hex2bn(&ret->g, JPAKE_GROUP_G) == 0)
+                fatal("%s: BN_hex2bn", __func__);
+        /* Subgroup order is p/2 (p is a safe prime) */
+        if ((ret->q = BN_new()) == NULL)
+                fatal("%s: BN_new", __func__);
+        if (BN_rshift1(ret->q, ret->p) != 1)
+                fatal("%s: BN_rshift1", __func__);
+
+        return ret;
+}
+
+/*
+ * Generate uniformly distributed random number in range (1, high).
+ * Return number on success, NULL on failure.
+ */
+BIGNUM *
+bn_rand_range_gt_one(const BIGNUM *high)
+{
+        BIGNUM *r, *tmp;
+        int success = -1;
+
+        if ((tmp = BN_new()) == NULL) {
+                error("%s: BN_new", __func__);
+                return NULL;
+        }
+        if ((r = BN_new()) == NULL) {
+                error("%s: BN_new failed", __func__);
+                goto out;
+        }
+        if (BN_set_word(tmp, 2) != 1) {
+                error("%s: BN_set_word(tmp, 2)", __func__);
+                goto out;
+        }
+        if (BN_sub(tmp, high, tmp) == -1) {
+                error("%s: BN_sub failed (tmp = high - 2)", __func__);
+                goto out;
+        }
+        if (BN_rand_range(r, tmp) == -1) {
+                error("%s: BN_rand_range failed", __func__);
+                goto out;
+        }
+        if (BN_set_word(tmp, 2) != 1) {
+                error("%s: BN_set_word(tmp, 2)", __func__);
+                goto out;
+        }
+        if (BN_add(r, r, tmp) == -1) {
+                error("%s: BN_add failed (r = r + 2)", __func__);
+                goto out;
+        }
+        success = 0;
+ out:
+        BN_clear_free(tmp);
+        if (success == 0)
+                return r;
+        BN_clear_free(r);
+        return NULL;
+}
+
+/*
+ * Hash contents of buffer 'b' with hash 'md'. Returns 0 on success,
+ * with digest via 'digestp' (caller to free) and length via 'lenp'.
+ * Returns -1 on failure.
+ */
+int
+hash_buffer(const u_char *buf, u_int len, const EVP_MD *md,
+    u_char **digestp, u_int *lenp)
+{
+        u_char digest[EVP_MAX_MD_SIZE];
+        u_int digest_len;
+        EVP_MD_CTX evp_md_ctx;
+        int success = -1;
+
+        EVP_MD_CTX_init(&evp_md_ctx);
+
+        if (EVP_DigestInit_ex(&evp_md_ctx, md, NULL) != 1) {
+                error("%s: EVP_DigestInit_ex", __func__);
+                goto out;
+        }
+        if (EVP_DigestUpdate(&evp_md_ctx, buf, len) != 1) {
+                error("%s: EVP_DigestUpdate", __func__);
+                goto out;
+        }
+        if (EVP_DigestFinal_ex(&evp_md_ctx, digest, &digest_len) != 1) {
+                error("%s: EVP_DigestFinal_ex", __func__);
+                goto out;
+        }
+        *digestp = xmalloc(digest_len);
+        *lenp = digest_len;
+        memcpy(*digestp, digest, *lenp);
+        success = 0;
+ out:
+        EVP_MD_CTX_cleanup(&evp_md_ctx);
+        bzero(digest, sizeof(digest));
+        digest_len = 0;
+        return success;
+}
+
+/* print formatted string followed by bignum */
+void
+jpake_debug3_bn(const BIGNUM *n, const char *fmt, ...)
+{
+        char *out, *h;
+        va_list args;
+
+        out = NULL;
+        va_start(args, fmt);
+        vasprintf(&out, fmt, args);
+        va_end(args);
+        if (out == NULL)
+                fatal("%s: vasprintf failed", __func__);
+
+        if (n == NULL)
+                debug3("%s(null)", out);
+        else {
+                h = BN_bn2hex(n);
+                debug3("%s0x%s", out, h);
+                free(h);
+        }
+        free(out);
+}
+
+/* print formatted string followed by buffer contents in hex */
+void
+jpake_debug3_buf(const u_char *buf, u_int len, const char *fmt, ...)
+{
+        char *out, h[65];
+        u_int i, j;
+        va_list args;
+
+        out = NULL;
+        va_start(args, fmt);
+        vasprintf(&out, fmt, args);
+        va_end(args);
+        if (out == NULL)
+                fatal("%s: vasprintf failed", __func__);
+
+        debug3("%s length %u%s", out, len, buf == NULL ? " (null)" : "");
+        free(out);
+        if (buf == NULL)
+                return;
+
+        *h = '\0';
+        for (i = j = 0; i < len; i++) {
+                snprintf(h + j, sizeof(h) - j, "%02x", buf[i]);
+                j += 2;
+                if (j >= sizeof(h) - 1 || i == len - 1) {
+                        debug3("    %s", h);
+                        *h = '\0';
+                        j = 0;
+                }
+        }
+}
+
+struct jpake_ctx *
+jpake_new(void)
+{
+        struct jpake_ctx *ret;
+
+        ret = xcalloc(1, sizeof(*ret));
+
+        ret->grp = jpake_default_group();
+
+        ret->s = ret->k = NULL;
+        ret->x1 = ret->x2 = ret->x3 = ret->x4 = NULL;
+        ret->g_x1 = ret->g_x2 = ret->g_x3 = ret->g_x4 = NULL;
+        ret->a = ret->b = NULL;
+
+        ret->client_id = ret->server_id = NULL;
+        ret->h_k_cid_sessid = ret->h_k_sid_sessid = NULL;
+
+        debug3("%s: alloc %p", __func__, ret);
+
+        return ret;
+}
+
+
+void
+jpake_free(struct jpake_ctx *pctx)
+{
+        debug3("%s: free %p", __func__, pctx);
+
+#define JPAKE_BN_CLEAR_FREE(v)                  \
+        do {                                    \
+                if ((v) != NULL) {              \
+                        BN_clear_free(v);       \
+                        (v) = NULL;             \
+                }                               \
+        } while (0)
+#define JPAKE_BUF_CLEAR_FREE(v, l)              \
+        do {                                    \
+                if ((v) != NULL) {              \
+                        bzero((v), (l));        \
+                        xfree(v);               \
+                        (v) = NULL;             \
+                        (l) = 0;                \
+                }                               \
+        } while (0)
+
+        JPAKE_BN_CLEAR_FREE(pctx->s);
+        JPAKE_BN_CLEAR_FREE(pctx->k);
+        JPAKE_BN_CLEAR_FREE(pctx->x1);
+        JPAKE_BN_CLEAR_FREE(pctx->x2);
+        JPAKE_BN_CLEAR_FREE(pctx->x3);
+        JPAKE_BN_CLEAR_FREE(pctx->x4);
+        JPAKE_BN_CLEAR_FREE(pctx->g_x1);
+        JPAKE_BN_CLEAR_FREE(pctx->g_x2);
+        JPAKE_BN_CLEAR_FREE(pctx->g_x3);
+        JPAKE_BN_CLEAR_FREE(pctx->g_x4);
+        JPAKE_BN_CLEAR_FREE(pctx->a);
+        JPAKE_BN_CLEAR_FREE(pctx->b);
+
+        JPAKE_BUF_CLEAR_FREE(pctx->client_id, pctx->client_id_len);
+        JPAKE_BUF_CLEAR_FREE(pctx->server_id, pctx->server_id_len);
+        JPAKE_BUF_CLEAR_FREE(pctx->h_k_cid_sessid, pctx->h_k_cid_sessid_len);
+        JPAKE_BUF_CLEAR_FREE(pctx->h_k_sid_sessid, pctx->h_k_sid_sessid_len);
+
+#undef JPAKE_BN_CLEAR_FREE
+#undef JPAKE_BUF_CLEAR_FREE
+
+        bzero(pctx, sizeof(pctx));
+        xfree(pctx);
+}
+
+/* dump entire jpake_ctx. NB. includes private values! */
+void
+jpake_dump(struct jpake_ctx *pctx, const char *fmt, ...)
+{
+        char *out;
+        va_list args;
+
+        out = NULL;
+        va_start(args, fmt);
+        vasprintf(&out, fmt, args);
+        va_end(args);
+        if (out == NULL)
+                fatal("%s: vasprintf failed", __func__);
+
+        debug3("%s: %s (ctx at %p)", __func__, out, pctx);
+        if (pctx == NULL) {
+                free(out);
+                return;
+        }
+
+#define JPAKE_DUMP_BN(a)        do { \
+                if ((a) != NULL) \
+                        JPAKE_DEBUG_BN(((a), "%s = ", #a)); \
+        } while (0)
+#define JPAKE_DUMP_BUF(a, b)    do { \
+                if ((a) != NULL) \
+                        JPAKE_DEBUG_BUF((a, b, "%s", #a)); \
+        } while (0)
+
+        JPAKE_DUMP_BN(pctx->s);
+        JPAKE_DUMP_BN(pctx->k);
+        JPAKE_DUMP_BN(pctx->x1);
+        JPAKE_DUMP_BN(pctx->x2);
+        JPAKE_DUMP_BN(pctx->x3);
+        JPAKE_DUMP_BN(pctx->x4);
+        JPAKE_DUMP_BN(pctx->g_x1);
+        JPAKE_DUMP_BN(pctx->g_x2);
+        JPAKE_DUMP_BN(pctx->g_x3);
+        JPAKE_DUMP_BN(pctx->g_x4);
+        JPAKE_DUMP_BN(pctx->a);
+        JPAKE_DUMP_BN(pctx->b);
+
+        JPAKE_DUMP_BUF(pctx->client_id, pctx->client_id_len);
+        JPAKE_DUMP_BUF(pctx->server_id, pctx->server_id_len);
+        JPAKE_DUMP_BUF(pctx->h_k_cid_sessid, pctx->h_k_cid_sessid_len);
+        JPAKE_DUMP_BUF(pctx->h_k_sid_sessid, pctx->h_k_sid_sessid_len);
+
+        debug3("%s: %s done", __func__, out);
+        free(out);
+}
+
+/* Shared parts of step 1 exchange calculation */
+void
+jpake_step1(struct jpake_group *grp,
+    u_char **id, u_int *id_len,
+    BIGNUM **priv1, BIGNUM **priv2, BIGNUM **g_priv1, BIGNUM **g_priv2,
+    u_char **priv1_proof, u_int *priv1_proof_len,
+    u_char **priv2_proof, u_int *priv2_proof_len)
+{
+        BN_CTX *bn_ctx;
+
+        if ((bn_ctx = BN_CTX_new()) == NULL)
+                fatal("%s: BN_CTX_new", __func__);
+
+        /* Random nonce to prevent replay */
+        *id = xmalloc(KZP_ID_LEN);
+        *id_len = KZP_ID_LEN;
+        arc4random_buf(*id, *id_len);
+
+        /*
+         * x1/x3 is a random element of Zq
+         * x2/x4 is a random element of Z*q
+         * We also exclude [1] from x1/x3 candidates and [0, 1] from
+         * x2/x4 candiates to avoid possible degeneracy (i.e. g^0, g^1).
+         */
+        if ((*priv1 = bn_rand_range_gt_one(grp->q)) == NULL ||
+            (*priv2 = bn_rand_range_gt_one(grp->q)) == NULL)
+                fatal("%s: bn_rand_range_gt_one", __func__);
+
+        /*
+         * client: g_x1 = g^x1 mod p / server: g_x3 = g^x3 mod p
+         * client: g_x2 = g^x2 mod p / server: g_x4 = g^x4 mod p
+         */
+        if ((*g_priv1 = BN_new()) == NULL ||
+            (*g_priv2 = BN_new()) == NULL)
+                fatal("%s: BN_new", __func__);
+        if (BN_mod_exp(*g_priv1, grp->g, *priv1, grp->p, bn_ctx) == -1)
+                fatal("%s: BN_mod_exp", __func__);
+        if (BN_mod_exp(*g_priv2, grp->g, *priv2, grp->p, bn_ctx) == -1)
+                fatal("%s: BN_mod_exp", __func__);
+
+        /* Generate proofs for holding x1/x3 and x2/x4 */
+        if (schnorr_sign(grp->p, grp->q, grp->g,
+            *priv1, *g_priv1, *id, *id_len,
+            priv1_proof, priv1_proof_len) != 0)
+                fatal("%s: schnorr_sign", __func__);
+        if (schnorr_sign(grp->p, grp->q, grp->g,
+            *priv2, *g_priv2, *id, *id_len,
+            priv2_proof, priv2_proof_len) != 0)
+                fatal("%s: schnorr_sign", __func__);
+
+        BN_CTX_free(bn_ctx);
+}
+
+/* Shared parts of step 2 exchange calculation */
+void
+jpake_step2(struct jpake_group *grp, BIGNUM *s,
+    BIGNUM *mypub1, BIGNUM *theirpub1, BIGNUM *theirpub2, BIGNUM *mypriv2,
+    const u_char *theirid, u_int theirid_len,
+    const u_char *myid, u_int myid_len,
+    const u_char *theirpub1_proof, u_int theirpub1_proof_len,
+    const u_char *theirpub2_proof, u_int theirpub2_proof_len,
+    BIGNUM **newpub,
+    u_char **newpub_exponent_proof, u_int *newpub_exponent_proof_len)
+{
+        BN_CTX *bn_ctx;
+        BIGNUM *tmp, *exponent;
+
+        /* Validate peer's step 1 values */
+        if (BN_cmp(theirpub1, BN_value_one()) <= 0)
+                fatal("%s: theirpub1 <= 1", __func__);
+        if (BN_cmp(theirpub2, BN_value_one()) <= 0)
+                fatal("%s: theirpub2 <= 1", __func__);
+
+        if (schnorr_verify(grp->p, grp->q, grp->g, theirpub1,
+            theirid, theirid_len, theirpub1_proof, theirpub1_proof_len) != 1)
+                fatal("%s: schnorr_verify theirpub1 failed", __func__);
+        if (schnorr_verify(grp->p, grp->q, grp->g, theirpub2,
+            theirid, theirid_len, theirpub2_proof, theirpub2_proof_len) != 1)
+                fatal("%s: schnorr_verify theirpub2 failed", __func__);
+
+        if ((bn_ctx = BN_CTX_new()) == NULL)
+                fatal("%s: BN_CTX_new", __func__);
+
+        if ((*newpub = BN_new()) == NULL ||
+            (tmp = BN_new()) == NULL ||
+            (exponent = BN_new()) == NULL)
+                fatal("%s: BN_new", __func__);
+
+        /*
+         * client: exponent = x2 * s mod p
+         * server: exponent = x4 * s mod p
+         */
+        if (BN_mod_mul(exponent, mypriv2, s, grp->q, bn_ctx) != 1)
+                fatal("%s: BN_mod_mul (exponent = mypriv2 * s mod p)",
+                    __func__);
+
+        /*
+         * client: tmp = g^(x1 + x3 + x4) mod p
+         * server: tmp = g^(x1 + x2 + x3) mod p
+         */
+        if (BN_mod_mul(tmp, mypub1, theirpub1, grp->p, bn_ctx) != 1)
+                fatal("%s: BN_mod_mul (tmp = mypub1 * theirpub1 mod p)",
+                    __func__);
+        if (BN_mod_mul(tmp, tmp, theirpub2, grp->p, bn_ctx) != 1)
+                fatal("%s: BN_mod_mul (tmp = tmp * theirpub2 mod p)", __func__);
+
+        /*
+         * client: a = tmp^exponent = g^((x1+x3+x4) * x2 * s) mod p
+         * server: b = tmp^exponent = g^((x1+x2+x3) * x4 * s) mod p
+         */
+        if (BN_mod_exp(*newpub, tmp, exponent, grp->p, bn_ctx) != 1)
+                fatal("%s: BN_mod_mul (newpub = tmp^exponent mod p)", __func__);
+
+        JPAKE_DEBUG_BN((tmp, "%s: tmp = ", __func__));
+        JPAKE_DEBUG_BN((exponent, "%s: exponent = ", __func__));
+
+        /* Note the generator here is 'tmp', not g */
+        if (schnorr_sign(grp->p, grp->q, tmp, exponent, *newpub,
+            myid, myid_len,
+            newpub_exponent_proof, newpub_exponent_proof_len) != 0)
+                fatal("%s: schnorr_sign newpub", __func__);
+
+        BN_clear_free(tmp); /* XXX stash for later use? */
+        BN_clear_free(exponent); /* XXX stash for later use? (yes, in conf) */
+
+        BN_CTX_free(bn_ctx);
+}
+
+/* Confirmation hash calculation */
+void
+jpake_confirm_hash(const BIGNUM *k,
+    const u_char *endpoint_id, u_int endpoint_id_len,
+    const u_char *sess_id, u_int sess_id_len,
+    u_char **confirm_hash, u_int *confirm_hash_len)
+{
+        Buffer b;
+
+        /*
+         * Calculate confirmation proof:
+         *     client: H(k || client_id || session_id)
+         *     server: H(k || server_id || session_id)
+         */
+        buffer_init(&b);
+        buffer_put_bignum2(&b, k);
+        buffer_put_string(&b, endpoint_id, endpoint_id_len);
+        buffer_put_string(&b, sess_id, sess_id_len);
+        if (hash_buffer(buffer_ptr(&b), buffer_len(&b), EVP_sha256(),
+            confirm_hash, confirm_hash_len) != 0)
+                fatal("%s: hash_buffer", __func__);
+        buffer_free(&b);
+}
+
+/* Shared parts of key derivation and confirmation calculation */
+void
+jpake_key_confirm(struct jpake_group *grp, BIGNUM *s, BIGNUM *step2_val,
+    BIGNUM *mypriv2, BIGNUM *mypub1, BIGNUM *mypub2,
+    BIGNUM *theirpub1, BIGNUM *theirpub2,
+    const u_char *my_id, u_int my_id_len,
+    const u_char *their_id, u_int their_id_len,
+    const u_char *sess_id, u_int sess_id_len,
+    const u_char *theirpriv2_s_proof, u_int theirpriv2_s_proof_len,
+    BIGNUM **k,
+    u_char **confirm_hash, u_int *confirm_hash_len)
+{
+        BN_CTX *bn_ctx;
+        BIGNUM *tmp;
+
+        if ((bn_ctx = BN_CTX_new()) == NULL)
+                fatal("%s: BN_CTX_new", __func__);
+        if ((tmp = BN_new()) == NULL ||
+            (*k = BN_new()) == NULL)
+                fatal("%s: BN_new", __func__);
+
+        /* Validate step 2 values */
+        if (BN_cmp(step2_val, BN_value_one()) <= 0)
+                fatal("%s: step2_val <= 1", __func__);
+
+        /*
+         * theirpriv2_s_proof is calculated with a different generator:
+         * tmp = g^(mypriv1+mypriv2+theirpub1) = g^mypub1*g^mypub2*g^theirpub1
+         * Calculate it here so we can check the signature.
+         */
+        if (BN_mod_mul(tmp, mypub1, mypub2, grp->p, bn_ctx) != 1)
+                fatal("%s: BN_mod_mul (tmp = mypub1 * mypub2 mod p)", __func__);
+        if (BN_mod_mul(tmp, tmp, theirpub1, grp->p, bn_ctx) != 1)
+                fatal("%s: BN_mod_mul (tmp = tmp * theirpub1 mod p)", __func__);
+
+        JPAKE_DEBUG_BN((tmp, "%s: tmp = ", __func__));
+
+        if (schnorr_verify(grp->p, grp->q, tmp, step2_val, 
+            their_id, their_id_len,
+            theirpriv2_s_proof, theirpriv2_s_proof_len) != 1)
+                fatal("%s: schnorr_verify theirpriv2_s_proof failed", __func__);
+
+        /*
+         * Derive shared key:
+         *     client: k = (b / g^(x2*x4*s))^x2 = g^((x1+x3)*x2*x4*s)
+         *     server: k = (a / g^(x2*x4*s))^x4 = g^((x1+x3)*x2*x4*s)
+         *
+         * Computed as:
+         *     client: k = (g_x4^(q - (x2 * s)) * b)^x2 mod p
+         *     server: k = (g_x2^(q - (x4 * s)) * b)^x4 mod p
+         */
+        if (BN_mul(tmp, mypriv2, s, bn_ctx) != 1)
+                fatal("%s: BN_mul (tmp = mypriv2 * s)", __func__);
+        if (BN_mod_sub(tmp, grp->q, tmp, grp->q, bn_ctx) != 1)
+                fatal("%s: BN_mod_sub (tmp = q - tmp mod q)", __func__);
+        if (BN_mod_exp(tmp, theirpub2, tmp, grp->p, bn_ctx) != 1)
+                fatal("%s: BN_mod_exp (tmp = theirpub2^tmp) mod p", __func__);
+        if (BN_mod_mul(tmp, tmp, step2_val, grp->p, bn_ctx) != 1)
+                fatal("%s: BN_mod_mul (tmp = tmp * step2_val) mod p", __func__);
+        if (BN_mod_exp(*k, tmp, mypriv2, grp->p, bn_ctx) != 1)
+                fatal("%s: BN_mod_exp (k = tmp^mypriv2) mod p", __func__);
+        
+        BN_CTX_free(bn_ctx);
+        BN_clear_free(tmp);
+
+        jpake_confirm_hash(*k, my_id, my_id_len, sess_id, sess_id_len,
+            confirm_hash, confirm_hash_len);
+}
+
+/*
+ * Calculate and check confirmation hash from peer. Returns 1 on success
+ * 0 on failure/mismatch.
+ */
+int
+jpake_check_confirm(const BIGNUM *k,
+    const u_char *peer_id, u_int peer_id_len,
+    const u_char *sess_id, u_int sess_id_len,
+    const u_char *peer_confirm_hash, u_int peer_confirm_hash_len)
+{
+        u_char *expected_confirm_hash;
+        u_int expected_confirm_hash_len;
+        int success = 0;
+
+        /* Calculate and verify expected confirmation hash */
+        jpake_confirm_hash(k, peer_id, peer_id_len, sess_id, sess_id_len,
+            &expected_confirm_hash, &expected_confirm_hash_len);
+
+        JPAKE_DEBUG_BUF((expected_confirm_hash, expected_confirm_hash_len,
+            "%s: expected confirm hash", __func__));
+        JPAKE_DEBUG_BUF((peer_confirm_hash, peer_confirm_hash_len,
+            "%s: received confirm hash", __func__));
+
+        if (peer_confirm_hash_len != expected_confirm_hash_len)
+                error("%s: confirmation length mismatch (my %u them %u)",
+                    __func__, expected_confirm_hash_len, peer_confirm_hash_len);
+        else if (memcmp(peer_confirm_hash, expected_confirm_hash,
+            expected_confirm_hash_len) == 0)
+                success = 1;
+        bzero(expected_confirm_hash, expected_confirm_hash_len);
+        xfree(expected_confirm_hash);
+        debug3("%s: success = %d", __func__, success);
+        return success;
+}
+
+/* XXX main() function with tests */
+
+#endif /* JPAKE */
+