upstream: Add support for a PQC KEX/KEM:

sntrup4591761x25519-sha512@tinyssh.org using the Streamlined NTRU Prime
4591^761 implementation from SUPERCOP coupled with X25519 as a stop-loss. Not
enabled by default.

introduce KEM API; a simplified framework for DH-ish KEX methods.

from markus@ feedback & ok djm@

OpenBSD-Commit-ID: d687f76cffd3561dd73eb302d17a1c3bf321d1a7

1 files changed, 213 insertions, 0 deletions

diff --git a/kexsntrup4591761x25519.c b/kexsntrup4591761x25519.c
new file mode 100644
index 000000000..ffe05f420
--- /dev/null
+++ b/kexsntrup4591761x25519.c
@@ -0,0 +1,213 @@
+/* $OpenBSD: kexsntrup4591761x25519.c,v 1.1 2019/01/21 10:20:12 djm Exp $ */
+/*
+ * Copyright (c) 2019 Markus Friedl.  All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <sys/types.h>
+
+#include <stdio.h>
+#include <string.h>
+#include <signal.h>
+
+#include "sshkey.h"
+#include "kex.h"
+#include "sshbuf.h"
+#include "digest.h"
+#include "ssherr.h"
+
+int
+kex_kem_sntrup4591761x25519_keypair(struct kex *kex)
+{
+        struct sshbuf *buf = NULL;
+        u_char *cp = NULL;
+        size_t need;
+        int r;
+
+        if ((buf = sshbuf_new()) == NULL)
+                return SSH_ERR_ALLOC_FAIL;
+        need = crypto_kem_sntrup4591761_PUBLICKEYBYTES + CURVE25519_SIZE;
+        if ((r = sshbuf_reserve(buf, need, &cp)) != 0)
+                goto out;
+        crypto_kem_sntrup4591761_keypair(cp, kex->sntrup4591761_client_key);
+#ifdef DEBUG_KEXECDH
+        dump_digest("client public key sntrup4591761:", cp,
+            crypto_kem_sntrup4591761_PUBLICKEYBYTES);
+#endif
+        cp += crypto_kem_sntrup4591761_PUBLICKEYBYTES;
+        kexc25519_keygen(kex->c25519_client_key, cp);
+#ifdef DEBUG_KEXECDH
+        dump_digest("client public key c25519:", cp, CURVE25519_SIZE);
+#endif
+        kex->kem_client_pub = buf;
+        buf = NULL;
+ out:
+        sshbuf_free(buf);
+        return r;
+}
+
+int
+kex_kem_sntrup4591761x25519_enc(struct kex *kex, const u_char *pkblob,
+   size_t pklen, struct sshbuf **server_blobp, struct sshbuf **shared_secretp)
+{
+        struct sshbuf *server_blob = NULL;
+        struct sshbuf *buf = NULL;
+        u_char *kem_key, *ciphertext, *server_pub;
+        u_char server_key[CURVE25519_SIZE];
+        u_char hash[SSH_DIGEST_MAX_LENGTH];
+        size_t need;
+        int r;
+
+        *server_blobp = NULL;
+        *shared_secretp = NULL;
+
+        /* pkblob contains both KEM and ECDH client pubkeys */
+        need = crypto_kem_sntrup4591761_PUBLICKEYBYTES + CURVE25519_SIZE;
+        if (pklen != need) {
+                r = SSH_ERR_SIGNATURE_INVALID;
+                goto out;
+        }
+#ifdef DEBUG_KEXECDH
+        dump_digest("client public key sntrup4591761:", pkblob,
+            crypto_kem_sntrup4591761_PUBLICKEYBYTES);
+        dump_digest("client public key 25519:",
+            pkblob + crypto_kem_sntrup4591761_PUBLICKEYBYTES, CURVE25519_SIZE);
+#endif
+        /* allocate buffer for concatenation of KEM key and ECDH shared key */
+        /* the buffer will be hashed and the result is the shared secret */
+        if ((buf = sshbuf_new()) == NULL) {
+                r = SSH_ERR_ALLOC_FAIL;
+                goto out;
+        }
+        if ((r = sshbuf_reserve(buf, crypto_kem_sntrup4591761_BYTES,
+            &kem_key)) != 0)
+                goto out;
+        /* allocate space for encrypted KEM key and ECDH pub key */
+        if ((server_blob = sshbuf_new()) == NULL) {
+                r = SSH_ERR_ALLOC_FAIL;
+                goto out;
+        }
+        need = crypto_kem_sntrup4591761_CIPHERTEXTBYTES + CURVE25519_SIZE;
+        if ((r = sshbuf_reserve(server_blob, need, &ciphertext)) != 0)
+                goto out;
+        /* generate and encrypt KEM key with client key */
+        crypto_kem_sntrup4591761_enc(ciphertext, kem_key, pkblob);
+        /* generate ECDH key pair, store server pubkey after ciphertext */
+        server_pub = ciphertext + crypto_kem_sntrup4591761_CIPHERTEXTBYTES;
+        kexc25519_keygen(server_key, server_pub);
+        /* append ECDH shared key */
+        if ((r = kexc25519_shared_key_ext(server_key,
+            pkblob + crypto_kem_sntrup4591761_PUBLICKEYBYTES, buf, 1)) < 0)
+                goto out;
+        if ((r = ssh_digest_buffer(kex->hash_alg, buf, hash, sizeof(hash))) != 0)
+                goto out;
+#ifdef DEBUG_KEXECDH
+        dump_digest("server public key 25519:", server_pub, CURVE25519_SIZE);
+        dump_digest("server cipher text:", ciphertext,
+            crypto_kem_sntrup4591761_CIPHERTEXTBYTES);
+        dump_digest("server kem key:", kem_key, sizeof(kem_key));
+        dump_digest("concatenation of KEM key and ECDH shared key:",
+            sshbuf_ptr(buf), sshbuf_len(buf));
+#endif
+        /* string-encoded hash is resulting shared secret */
+        sshbuf_reset(buf);
+        if ((r = sshbuf_put_string(buf, hash,
+            ssh_digest_bytes(kex->hash_alg))) != 0)
+                goto out;
+#ifdef DEBUG_KEXECDH
+        dump_digest("encoded shared secret:", sshbuf_ptr(buf), sshbuf_len(buf));
+#endif
+        *server_blobp = server_blob;
+        *shared_secretp = buf;
+        server_blob = NULL;
+        buf = NULL;
+ out:
+        explicit_bzero(hash, sizeof(hash));
+        explicit_bzero(server_key, sizeof(server_key));
+        sshbuf_free(server_blob);
+        sshbuf_free(buf);
+        return r;
+}
+
+int
+kex_kem_sntrup4591761x25519_dec(struct kex *kex, const u_char *pkblob,
+    size_t pklen, struct sshbuf **shared_secretp)
+{
+        struct sshbuf *buf = NULL;
+        u_char *kem_key = NULL;
+        const u_char *ciphertext, *server_pub;
+        u_char hash[SSH_DIGEST_MAX_LENGTH];
+        size_t need;
+        int r, decoded;
+
+        *shared_secretp = NULL;
+
+        need = crypto_kem_sntrup4591761_CIPHERTEXTBYTES + CURVE25519_SIZE;
+        if (pklen != need) {
+                r = SSH_ERR_SIGNATURE_INVALID;
+                goto out;
+        }
+        ciphertext = pkblob;
+        server_pub = pkblob + crypto_kem_sntrup4591761_CIPHERTEXTBYTES;
+#ifdef DEBUG_KEXECDH
+        dump_digest("server cipher text:", ciphertext,
+            crypto_kem_sntrup4591761_CIPHERTEXTBYTES);
+        dump_digest("server public key c25519:", server_pub, CURVE25519_SIZE);
+#endif
+        /* hash concatenation of KEM key and ECDH shared key */
+        if ((buf = sshbuf_new()) == NULL) {
+                r = SSH_ERR_ALLOC_FAIL;
+                goto out;
+        }
+        if ((r = sshbuf_reserve(buf, crypto_kem_sntrup4591761_BYTES,
+            &kem_key)) != 0)
+                goto out;
+        decoded = crypto_kem_sntrup4591761_dec(kem_key, ciphertext,
+            kex->sntrup4591761_client_key);
+        if ((r = kexc25519_shared_key_ext(kex->c25519_client_key, server_pub,
+            buf, 1)) < 0)
+                goto out;
+        if ((r = ssh_digest_buffer(kex->hash_alg, buf, hash, sizeof(hash))) != 0)
+                goto out;
+#ifdef DEBUG_KEXECDH
+        dump_digest("client kem key:", kem_key, sizeof(kem_key));
+        dump_digest("concatenation of KEM key and ECDH shared key:",
+            sshbuf_ptr(buf), sshbuf_len(buf));
+#endif
+        sshbuf_reset(buf);
+        if ((r = sshbuf_put_string(buf, hash,
+            ssh_digest_bytes(kex->hash_alg))) != 0)
+                goto out;
+#ifdef DEBUG_KEXECDH
+        dump_digest("encoded shared secret:", sshbuf_ptr(buf), sshbuf_len(buf));
+#endif
+        if (decoded != 0) {
+                r = SSH_ERR_SIGNATURE_INVALID;
+                goto out;
+        }
+        *shared_secretp = buf;
+        buf = NULL;
+ out:
+        explicit_bzero(hash, sizeof(hash));
+        sshbuf_free(buf);
+        return r;
+}