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>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/ouroboros/crypt.h b/include/ouroboros/crypt.h
index 5b39fb5f..1f2640e8 100644
--- a/include/ouroboros/crypt.h
+++ b/include/ouroboros/crypt.h
@@ -276,6 +276,12 @@ const char *       crypt_nid_to_str(uint16_t nid);
 
 uint16_t           crypt_str_to_nid(const char * cipher);
 
+int                crypt_cipher_rank(int nid);
+
+int                crypt_kdf_rank(int nid);
+
+int                crypt_kex_rank(int nid);
+
 int                md_validate_nid(int nid);
 
 const char *       md_nid_to_str(uint16_t nid);