irmd: Add strength-based crypto negotiation

Each side's configured cipher, KDF, and KEX algorithm now
represents a minimum security floor ("at least this strong").

Cipher and KDF use strongest-wins: the server compares ranks
and selects the stronger of client vs server config. The
negotiated values are sent in the response header. The client
verifies the server's response meets its own minimum, which
prevents downgrade attacks on the wire.

KEX uses a minimum-floor check: the server extracts the
client's algorithm from its public key and rejects if it
ranks below the server's configured algorithm. A server
configured with ML-KEM will reject all classical algorithms.

Special case: for client-encap KEM, the client has already
derived its key using its KDF, so the server must use the
same KDF and can only reject if it is too weak.

The supported_nids arrays are ordered weakest to strongest
and serve as the single source of truth for ranking.

Cipher ranking (weakest to strongest):
  aes-128-ctr, aes-192-ctr, aes-256-ctr,
  aes-128-gcm, aes-192-gcm, aes-256-gcm,
  chacha20-poly1305

KDF ranking (weakest to strongest):
  blake2s256, sha256, sha3-256, sha384,
  sha3-384, blake2b512, sha512, sha3-512

KEX ranking (weakest to strongest):
  ffdhe2048, prime256v1, X25519, ffdhe3072,
  secp384r1, ffdhe4096, X448, secp521r1,
  ML-KEM-512, ML-KEM-768, ML-KEM-1024,
  X25519MLKEM768, X448MLKEM1024

Negotiation outcomes:
  strong srv cipher + weak cli cipher  -> use strongest
  weak srv cipher + strong cli cipher  -> use strongest
  srv encryption + cli none            -> server rejects
  srv none + cli encryption            -> use client's
  strong srv KEX + weak cli KEX        -> server rejects
  weak srv KEX + strong cli KEX        -> succeeds
  wire tamper to weaker cipher         -> client rejects

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

4 files changed, 349 insertions, 68 deletions

diff --git a/src/irmd/oap/cli.c b/src/irmd/oap/cli.c
index ea2a25d1..9472e331 100644
--- a/src/irmd/oap/cli.c
+++ b/src/irmd/oap/cli.c
@@ -439,10 +439,16 @@ static int do_client_kex_complete(struct oap_cli_ctx *   s,
 {
         struct sec_config * kcfg = &s->kcfg;
         uint8_t *           id   = s->id.data;
+        int                 cipher_nid;
+        int                 kdf_nid;
 
         if (!IS_KEX_ALGO_SET(kcfg))
                 return 0;
 
+        /* Save client's configured minimums */
+        cipher_nid = kcfg->c.nid;
+        kdf_nid    = kcfg->k.nid;
+
         /* Accept server's cipher choice */
         if (peer_hdr->cipher_str == NULL) {
                 log_err_id(id, "Server did not provide cipher.");
@@ -456,7 +462,28 @@ static int do_client_kex_complete(struct oap_cli_ctx *   s,
                 return -ENOTSUP;
         }
 
-        log_dbg_id(id, "Accepted server cipher %s.", peer_hdr->cipher_str);
+        /* Verify server cipher >= client's minimum */
+        if (crypt_cipher_rank(kcfg->c.nid) < crypt_cipher_rank(cipher_nid)) {
+                log_err_id(id, "Server cipher %s too weak.",
+                           peer_hdr->cipher_str);
+                return -ECRYPT;
+        }
+
+        log_dbg_id(id, "Accepted server cipher %s.",
+                   peer_hdr->cipher_str);
+
+        /* Accept server's KDF for non-client-encap modes */
+        if (kcfg->x.mode != KEM_MODE_CLIENT_ENCAP
+            && peer_hdr->kdf_nid != NID_undef) {
+                if (crypt_kdf_rank(peer_hdr->kdf_nid)
+                    < crypt_kdf_rank(kdf_nid)) {
+                        log_err_id(id, "Server KDF too weak.");
+                        return -ECRYPT;
+                }
+                SET_KEX_KDF_NID(kcfg, peer_hdr->kdf_nid);
+                log_dbg_id(id, "Accepted server KDF %s.",
+                           md_nid_to_str(kcfg->k.nid));
+        }
 
         /* Derive shared secret */
         if (IS_KEM_ALGORITHM(kcfg->x.str))
diff --git a/src/irmd/oap/internal.h b/src/irmd/oap/internal.h
index 8363e3a2..f118cc33 100644
--- a/src/irmd/oap/internal.h
+++ b/src/irmd/oap/internal.h
@@ -34,24 +34,15 @@
 #include <stdbool.h>
 #include <stdint.h>
 
-/*
- * Authentication functions (auth.c)
- */
 int  oap_check_hdr(const struct oap_hdr * hdr);
 
 int  oap_auth_peer(char *                 name,
                    const struct oap_hdr * local_hdr,
                    const struct oap_hdr * peer_hdr);
 
-/*
- * Key exchange functions (kex.c)
- */
 int  oap_negotiate_cipher(const struct oap_hdr * peer_hdr,
                           struct sec_config *    kcfg);
 
-/*
- * Credential loading (oap.c) - shared between client and server
- */
 #ifndef OAP_TEST_MODE
 int  load_credentials(const char *                  name,
                       const struct name_sec_paths * paths,
@@ -63,9 +54,6 @@ int  load_kex_config(const char *        name,
                      struct sec_config * cfg);
 #endif
 
-/*
- * Server functions (srv.c)
- */
 #ifndef OAP_TEST_MODE
 int  load_srv_credentials(const struct name_info * info,
                           void **                  pkp,
@@ -94,9 +82,6 @@ int  do_server_kex(const struct name_info * info,
                    buffer_t *               kex,
                    struct crypt_sk *        sk);
 
-/*
- * Client functions (cli.c)
- */
 #ifndef OAP_TEST_MODE
 int  load_cli_credentials(const struct name_info * info,
                           void **                  pkp,
diff --git a/src/irmd/oap/srv.c b/src/irmd/oap/srv.c
index c5a4453f..93270c48 100644
--- a/src/irmd/oap/srv.c
+++ b/src/irmd/oap/srv.c
@@ -134,43 +134,70 @@ static int get_algo_from_peer_key(const struct oap_hdr * peer_hdr,
         return 0;
 }
 
-static int negotiate_kex(const struct oap_hdr * peer_hdr,
-                         struct sec_config *    kcfg)
+static int negotiate_cipher(const struct oap_hdr * peer_hdr,
+                            struct sec_config *    kcfg)
 {
         uint8_t * id = peer_hdr->id.data;
+        int       cli_nid;
+        int       cli_rank;
+        int       srv_rank;
+
+        /* Cipher: select the strongest of client and server */
+        cli_nid = peer_hdr->cipher_str != NULL
+                ? (int) crypt_str_to_nid(peer_hdr->cipher_str)
+                : NID_undef;
+
+        if (cli_nid != NID_undef
+            && crypt_cipher_rank(cli_nid) < 0) {
+                log_err_id(id, "Unsupported cipher '%s'.",
+                           peer_hdr->cipher_str);
+                return -ENOTSUP;
+        }
 
-        if (kcfg->c.nid == NID_undef) {
-                if (peer_hdr->cipher_str != NULL) {
-                        SET_KEX_CIPHER(kcfg, peer_hdr->cipher_str);
-                        if (kcfg->c.nid == NID_undef) {
-                                log_err_id(id, "Unsupported cipher '%s'.",
-                                           peer_hdr->cipher_str);
-                                return -ENOTSUP;
-                        }
-                        log_dbg_id(id, "Peer requested cipher %s.",
-                                   peer_hdr->cipher_str);
-                } else {
-                        log_err_id(id, "Encryption requested, no cipher.");
-                        return -ECRYPT;
-                }
+        cli_rank = crypt_cipher_rank(cli_nid);
+        srv_rank = crypt_cipher_rank(kcfg->c.nid);
+
+        if (cli_rank > srv_rank) {
+                SET_KEX_CIPHER_NID(kcfg, cli_nid);
+                log_dbg_id(id, "Selected client cipher %s.",
+                           kcfg->c.str);
+        } else if (srv_rank > 0) {
+                log_dbg_id(id, "Selected server cipher %s.",
+                           kcfg->c.str);
         } else {
-                log_dbg_id(id, "Using local cipher %s.", kcfg->c.str);
+                log_err_id(id, "Encryption requested, no cipher.");
+                return -ECRYPT;
         }
 
-        /* Negotiate KDF - server overrides client if configured */
-        if (kcfg->k.nid != NID_undef) {
-                log_dbg_id(id, "Using local KDF %s.",
-                           md_nid_to_str(kcfg->k.nid));
-        } else if (peer_hdr->kdf_nid != NID_undef) {
-                if (md_validate_nid(peer_hdr->kdf_nid) == 0) {
-                        kcfg->k.nid = peer_hdr->kdf_nid;
-                        log_dbg_id(id, "Using peer KDF %s.",
-                                   md_nid_to_str(peer_hdr->kdf_nid));
-                } else {
-                        log_err_id(id, "Unsupported KDF NID %d.",
-                                   peer_hdr->kdf_nid);
-                        return -ENOTSUP;
+        /* KDF: select the strongest of client and server */
+        if (peer_hdr->kdf_nid != NID_undef
+            && crypt_kdf_rank(peer_hdr->kdf_nid) < 0) {
+                log_err_id(id, "Unsupported KDF NID %d.",
+                           peer_hdr->kdf_nid);
+                return -ENOTSUP;
+        }
+
+        cli_rank = crypt_kdf_rank(peer_hdr->kdf_nid);
+        srv_rank = crypt_kdf_rank(kcfg->k.nid);
+
+        /*
+         * For client-encap KEM, the KDF is baked into
+         * the ciphertext. The server must use the client's
+         * KDF and can only verify the minimum.
+         */
+        if (OAP_KEX_ROLE(peer_hdr) == KEM_MODE_CLIENT_ENCAP) {
+                if (srv_rank > cli_rank) {
+                        log_err_id(id, "Client KDF too weak.");
+                        return -ECRYPT;
                 }
+                SET_KEX_KDF_NID(kcfg, peer_hdr->kdf_nid);
+        } else if (cli_rank > srv_rank) {
+                SET_KEX_KDF_NID(kcfg, peer_hdr->kdf_nid);
+                log_dbg_id(id, "Selected client KDF %s.",
+                           md_nid_to_str(kcfg->k.nid));
+        } else if (srv_rank > 0) {
+                log_dbg_id(id, "Selected server KDF %s.",
+                           md_nid_to_str(kcfg->k.nid));
         }
 
         if (IS_KEX_ALGO_SET(kcfg))
@@ -305,6 +332,7 @@ int do_server_kex(const struct name_info * info,
                   struct crypt_sk *        sk)
 {
         char      algo_buf[KEX_ALGO_BUFSZ];
+        int       srv_kex_nid;
         uint8_t * id;
 
         id = peer_hdr->id.data;
@@ -318,15 +346,26 @@ int do_server_kex(const struct name_info * info,
                 return 0;
         }
 
-        if (negotiate_kex(peer_hdr, kcfg) < 0)
+        if (negotiate_cipher(peer_hdr, kcfg) < 0)
                 return -ECRYPT;
 
+        /* Save server's configured KEX before overwriting */
+        srv_kex_nid = kcfg->x.nid;
+
         if (OAP_KEX_ROLE(peer_hdr) != KEM_MODE_CLIENT_ENCAP) {
                 /* Server encapsulation or DHE: extract algo from DER PK */
                 if (get_algo_from_peer_key(peer_hdr, algo_buf) < 0)
                         return -ECRYPT;
 
                 SET_KEX_ALGO(kcfg, algo_buf);
+
+                /* Reject if client KEX is weaker than server's */
+                if (crypt_kex_rank(kcfg->x.nid)
+                    < crypt_kex_rank(srv_kex_nid)) {
+                        log_err_id(id, "Client KEX %s too weak.",
+                                   kcfg->x.str);
+                        return -ECRYPT;
+                }
         }
 
         /* Dispatch based on algorithm type */
@@ -368,13 +407,11 @@ int oap_srv_process(const struct name_info * info,
 
         log_dbg("Processing OAP request for %s.", info->name);
 
-        /* Load server credentials */
         if (load_srv_credentials(info, &pkp, &crt) < 0) {
                 log_err("Failed to load security keys for %s.", info->name);
                 goto fail_cred;
         }
 
-        /* Load KEX config */
         if (load_srv_kex_config(info, &kcfg) < 0) {
                 log_err("Failed to load KEX config for %s.", info->name);
                 goto fail_kex;
@@ -392,13 +429,11 @@ int oap_srv_process(const struct name_info * info,
 
         id = peer_hdr.id.data; /* Logging */
 
-        /* Check for replay */
         if (oap_check_hdr(&peer_hdr) < 0) {
                 log_err_id(id, "OAP header failed replay check.");
                 goto fail_auth;
         }
 
-        /* Authenticate client before processing KEX data */
         oap_hdr_init(&local_hdr, peer_hdr.id, kex_buf, *data, NID_undef);
 
         if (oap_auth_peer(cli_name, &local_hdr, &peer_hdr) < 0) {
@@ -409,11 +444,15 @@ int oap_srv_process(const struct name_info * info,
         if (do_server_kex(info, &peer_hdr, &kcfg, &local_hdr.kex, sk) < 0)
                 goto fail_kex;
 
+        /* Update cipher NID after negotiation */
+        sk->nid = kcfg.c.nid;
+
         /* Build response header with hash of client request */
         local_hdr.nid = sk->nid;
 
         /* Use client's md_nid, defaulting to SHA-384 for PQC */
-        req_md_nid = peer_hdr.md_nid != NID_undef ? peer_hdr.md_nid : NID_sha384;
+        req_md_nid = peer_hdr.md_nid != NID_undef ?
+                     peer_hdr.md_nid : NID_sha384;
 
         /* Compute request hash using client's md_nid */
         hash_ret = md_digest(req_md_nid, req_buf, hash_buf);
diff --git a/src/irmd/oap/tests/oap_test.c b/src/irmd/oap/tests/oap_test.c
index 70f0a248..fc78ed9a 100644
--- a/src/irmd/oap/tests/oap_test.c
+++ b/src/irmd/oap/tests/oap_test.c
@@ -536,7 +536,7 @@ static int test_oap_roundtrip_all(void)
         return ret;
 }
 
-/* Cipher negotiation - client should accept server's chosen cipher */
+/* Cipher negotiation - strongest cipher and KDF are selected */
 static int test_oap_cipher_mismatch(void)
 {
         struct oap_test_ctx ctx;
@@ -545,17 +545,17 @@ static int test_oap_cipher_mismatch(void)
 
         memset(&test_cfg, 0, sizeof(test_cfg));
 
-        /* Server: ChaCha20-Poly1305, SHA3-256, SHA-384 */
+        /* Server: AES-128-GCM, SHA-256 */
         test_cfg.srv.kex    = NID_X25519;
-        test_cfg.srv.cipher = NID_chacha20_poly1305;
-        test_cfg.srv.kdf    = NID_sha3_256;
-        test_cfg.srv.md     = NID_sha384;
+        test_cfg.srv.cipher = NID_aes_128_gcm;
+        test_cfg.srv.kdf    = NID_sha256;
+        test_cfg.srv.md     = NID_sha256;
         test_cfg.srv.auth   = AUTH;
 
-        /* Client: AES-256-GCM, SHA-256, SHA-256 */
+        /* Client: AES-256-GCM, SHA-512 */
         test_cfg.cli.kex    = NID_X25519;
         test_cfg.cli.cipher = NID_aes_256_gcm;
-        test_cfg.cli.kdf    = NID_sha256;
+        test_cfg.cli.kdf    = NID_sha512;
         test_cfg.cli.md     = NID_sha256;
         test_cfg.cli.auth   = NO_AUTH;
 
@@ -577,17 +577,17 @@ static int test_oap_cipher_mismatch(void)
                 goto fail_cleanup;
         }
 
-        /* Verify: both should have the server's chosen cipher and KDF */
-        if (ctx.srv.nid != test_cfg.srv.cipher) {
+        /* Verify: both should have the strongest cipher */
+        if (ctx.srv.nid != NID_aes_256_gcm) {
                 printf("Server cipher mismatch: expected %s, got %s\n",
-                       crypt_nid_to_str(test_cfg.srv.cipher),
+                       crypt_nid_to_str(NID_aes_256_gcm),
                        crypt_nid_to_str(ctx.srv.nid));
                 goto fail_cleanup;
         }
 
-        if (ctx.cli.nid != test_cfg.srv.cipher) {
+        if (ctx.cli.nid != NID_aes_256_gcm) {
                 printf("Client cipher mismatch: expected %s, got %s\n",
-                       crypt_nid_to_str(test_cfg.srv.cipher),
+                       crypt_nid_to_str(NID_aes_256_gcm),
                        crypt_nid_to_str(ctx.cli.nid));
                 goto fail_cleanup;
         }
@@ -598,9 +598,9 @@ static int test_oap_cipher_mismatch(void)
                 /* KDF NID at offset 26: ID(16) + ts(8) + cipher(2) */
                 resp_kdf_nid = ntoh16(*(uint16_t *)(ctx.resp_hdr.data + 26));
 
-                if (resp_kdf_nid != test_cfg.srv.kdf) {
+                if (resp_kdf_nid != NID_sha512) {
                         printf("Response KDF mismatch: expected %s, got %s\n",
-                               md_nid_to_str(test_cfg.srv.kdf),
+                               md_nid_to_str(NID_sha512),
                                md_nid_to_str(resp_kdf_nid));
                         goto fail_cleanup;
                 }
@@ -618,6 +618,228 @@ static int test_oap_cipher_mismatch(void)
         return TEST_RC_FAIL;
 }
 
+/* Server encryption, client none: server rejects (no KEX data) */
+static int test_oap_srv_enc_cli_none(void)
+{
+        struct oap_test_ctx ctx;
+
+        TEST_START();
+
+        memset(&test_cfg, 0, sizeof(test_cfg));
+
+        /* Server: encryption configured */
+        test_cfg.srv.kex    = NID_X25519;
+        test_cfg.srv.cipher = NID_aes_256_gcm;
+        test_cfg.srv.kdf    = NID_sha256;
+        test_cfg.srv.md     = NID_sha256;
+        test_cfg.srv.auth   = AUTH;
+
+        /* Client: no encryption */
+        test_cfg.cli.md     = NID_sha256;
+        test_cfg.cli.auth   = NO_AUTH;
+
+        if (oap_test_setup(&ctx, root_ca_crt_ec, im_ca_crt_ec) < 0)
+                goto fail;
+
+        if (oap_cli_prepare_ctx(&ctx) < 0) {
+                printf("Client prepare failed.\n");
+                goto fail_cleanup;
+        }
+
+        /* Server should reject: KEX required but client sent none */
+        if (oap_srv_process_ctx(&ctx) == 0) {
+                printf("Server should have rejected.\n");
+                goto fail_cleanup;
+        }
+
+        oap_test_teardown(&ctx);
+
+        TEST_SUCCESS();
+        return TEST_RC_SUCCESS;
+
+ fail_cleanup:
+        oap_test_teardown(&ctx);
+ fail:
+        TEST_FAIL();
+        return TEST_RC_FAIL;
+}
+
+/* Client encryption, server none: use client settings */
+static int test_oap_cli_enc_srv_none(void)
+{
+        struct oap_test_ctx ctx;
+
+        TEST_START();
+
+        memset(&test_cfg, 0, sizeof(test_cfg));
+
+        /* Server: no encryption configured */
+        test_cfg.srv.md     = NID_sha256;
+        test_cfg.srv.auth   = AUTH;
+
+        /* Client: encryption configured */
+        test_cfg.cli.kex    = NID_X25519;
+        test_cfg.cli.cipher = NID_aes_256_gcm;
+        test_cfg.cli.kdf    = NID_sha256;
+        test_cfg.cli.md     = NID_sha256;
+        test_cfg.cli.auth   = NO_AUTH;
+
+        if (oap_test_setup(&ctx, root_ca_crt_ec, im_ca_crt_ec) < 0)
+                goto fail;
+
+        if (oap_cli_prepare_ctx(&ctx) < 0) {
+                printf("Client prepare failed.\n");
+                goto fail_cleanup;
+        }
+
+        if (oap_srv_process_ctx(&ctx) < 0) {
+                printf("Server process failed.\n");
+                goto fail_cleanup;
+        }
+
+        if (oap_cli_complete_ctx(&ctx) < 0) {
+                printf("Client complete failed.\n");
+                goto fail_cleanup;
+        }
+
+        if (memcmp(ctx.cli.key, ctx.srv.key, SYMMKEYSZ) != 0) {
+                printf("Key mismatch.\n");
+                goto fail_cleanup;
+        }
+
+        if (ctx.cli.nid != NID_aes_256_gcm) {
+                printf("Expected %s, got %s.\n",
+                       crypt_nid_to_str(NID_aes_256_gcm),
+                       crypt_nid_to_str(ctx.cli.nid));
+                goto fail_cleanup;
+        }
+
+        if (ctx.srv.nid != NID_aes_256_gcm) {
+                printf("Expected %s, got %s.\n",
+                       crypt_nid_to_str(NID_aes_256_gcm),
+                       crypt_nid_to_str(ctx.srv.nid));
+                goto fail_cleanup;
+        }
+
+        oap_test_teardown(&ctx);
+
+        TEST_SUCCESS();
+        return TEST_RC_SUCCESS;
+
+ fail_cleanup:
+        oap_test_teardown(&ctx);
+ fail:
+        TEST_FAIL();
+        return TEST_RC_FAIL;
+}
+
+/* Client rejects server response with downgraded cipher */
+static int test_oap_cli_rejects_downgrade(void)
+{
+        struct oap_test_ctx ctx;
+        uint16_t            weak;
+
+        TEST_START();
+
+        memset(&test_cfg, 0, sizeof(test_cfg));
+
+        test_cfg.srv.kex    = NID_X25519;
+        test_cfg.srv.cipher = NID_aes_256_gcm;
+        test_cfg.srv.kdf    = NID_sha256;
+        test_cfg.srv.md     = NID_sha256;
+        test_cfg.srv.auth   = AUTH;
+
+        test_cfg.cli.kex    = NID_X25519;
+        test_cfg.cli.cipher = NID_aes_256_gcm;
+        test_cfg.cli.kdf    = NID_sha256;
+        test_cfg.cli.md     = NID_sha256;
+        test_cfg.cli.auth   = NO_AUTH;
+
+        if (oap_test_setup(&ctx, root_ca_crt_ec, im_ca_crt_ec) < 0)
+                goto fail;
+
+        if (oap_cli_prepare_ctx(&ctx) < 0) {
+                printf("Client prepare failed.\n");
+                goto fail_cleanup;
+        }
+
+        if (oap_srv_process_ctx(&ctx) < 0) {
+                printf("Server process failed.\n");
+                goto fail_cleanup;
+        }
+
+        /* Tamper: replace cipher NID with weaker one */
+        weak = hton16(NID_aes_128_ctr);
+        memcpy(ctx.resp_hdr.data + OAP_CIPHER_NID_OFFSET,
+               &weak, sizeof(weak));
+
+        /* Client should reject the downgraded cipher */
+        if (oap_cli_complete_ctx(&ctx) == 0) {
+                printf("Client accepted downgrade.\n");
+                goto fail_cleanup;
+        }
+
+        oap_test_teardown(&ctx);
+
+        TEST_SUCCESS();
+        return TEST_RC_SUCCESS;
+
+ fail_cleanup:
+        oap_test_teardown(&ctx);
+ fail:
+        TEST_FAIL();
+        return TEST_RC_FAIL;
+}
+
+/* Server rejects client with weaker KEX algorithm */
+static int test_oap_srv_rejects_weak_kex(void)
+{
+        struct oap_test_ctx ctx;
+
+        TEST_START();
+
+        memset(&test_cfg, 0, sizeof(test_cfg));
+
+        /* Server: secp521r1 (strong) */
+        test_cfg.srv.kex    = NID_secp521r1;
+        test_cfg.srv.cipher = NID_aes_256_gcm;
+        test_cfg.srv.kdf    = NID_sha256;
+        test_cfg.srv.md     = NID_sha256;
+        test_cfg.srv.auth   = AUTH;
+
+        /* Client: ffdhe2048 (weakest) */
+        test_cfg.cli.kex    = NID_ffdhe2048;
+        test_cfg.cli.cipher = NID_aes_256_gcm;
+        test_cfg.cli.kdf    = NID_sha256;
+        test_cfg.cli.md     = NID_sha256;
+        test_cfg.cli.auth   = NO_AUTH;
+
+        if (oap_test_setup(&ctx, root_ca_crt_ec, im_ca_crt_ec) < 0)
+                goto fail;
+
+        if (oap_cli_prepare_ctx(&ctx) < 0) {
+                printf("Client prepare failed.\n");
+                goto fail_cleanup;
+        }
+
+        /* Server should reject: client KEX too weak */
+        if (oap_srv_process_ctx(&ctx) == 0) {
+                printf("Server should reject weak KEX.\n");
+                goto fail_cleanup;
+        }
+
+        oap_test_teardown(&ctx);
+
+        TEST_SUCCESS();
+        return TEST_RC_SUCCESS;
+
+ fail_cleanup:
+        oap_test_teardown(&ctx);
+ fail:
+        TEST_FAIL();
+        return TEST_RC_FAIL;
+}
+
 /* Test roundtrip with different signature digest algorithms */
 static int test_oap_roundtrip_md(int md)
 {
@@ -682,8 +904,8 @@ static int test_oap_roundtrip_md_all(void)
         int ret = 0;
         int i;
 
-        /* Test with default (0) */
-        ret |= test_oap_roundtrip_md(0);
+        /* Test with default */
+        ret |= test_oap_roundtrip_md(NID_undef);
 
         /* Test with all supported digest NIDs */
         for (i = 0; md_supported_nids[i] != NID_undef; i++)
@@ -919,6 +1141,10 @@ int oap_test(int    argc,
         ret |= test_oap_unsupported_nid();
 
         ret |= test_oap_cipher_mismatch();
+        ret |= test_oap_srv_enc_cli_none();
+        ret |= test_oap_cli_enc_srv_none();
+        ret |= test_oap_cli_rejects_downgrade();
+        ret |= test_oap_srv_rejects_weak_kex();
 
         ret |= test_oap_outdated_packet();
         ret |= test_oap_future_packet();
@@ -940,6 +1166,10 @@ int oap_test(int    argc,
         (void) test_oap_nid_without_kex;
         (void) test_oap_unsupported_nid;
         (void) test_oap_cipher_mismatch;
+        (void) test_oap_srv_enc_cli_none;
+        (void) test_oap_cli_enc_srv_none;
+        (void) test_oap_cli_rejects_downgrade;
+        (void) test_oap_srv_rejects_weak_kex;
         (void) test_oap_outdated_packet;
         (void) test_oap_future_packet;
         (void) test_oap_replay_packet;