Let links make an X25519 pair for once, and agree with it

An epoch's keys (docs/design/011-pure-transport.md): a pair made for the
next epoch, held raw (a link may hold one for long and a node many, so
not in OpenSSL's 16 KiB secure heap), and the secret its private half
shares with the other side's public half, never all zeros. The crypto
table's version is 5.

1 files changed, 28 insertions, 1 deletions

diff --git a/link/crypto-openssl.c b/link/crypto-openssl.c
index 6bdf6fb..96daa82 100644
--- a/link/crypto-openssl.c
+++ b/link/crypto-openssl.c
@@ -207,6 +207,32 @@ static bool link_secret(const PureSealKey *own, const unsigned char peer[PURE_SE
     return ok && any;
 }
 
+/* A pair made for once (X25519): an epoch's half, raw. */
+static bool link_ephemeral(unsigned char private_key[32], unsigned char public_key[PURE_SEAL_KEY]) {
+    EVP_PKEY *key = EVP_PKEY_Q_keygen(NULL, NULL, "X25519");
+    size_t a = 32, b = PURE_SEAL_KEY;
+    bool ok = key && EVP_PKEY_get_raw_private_key(key, private_key, &a) == 1 &&
+              EVP_PKEY_get_raw_public_key(key, public_key, &b) == 1 && a == 32 && b == PURE_SEAL_KEY;
+    EVP_PKEY_free(key);
+    return ok;
+}
+
+/* The secret a raw private half shares with a peer's public key. */
+static bool link_agree(const unsigned char private_key[32], const unsigned char peer[PURE_SEAL_KEY], unsigned char out[32]) {
+    EVP_PKEY *mine = EVP_PKEY_new_raw_private_key(EVP_PKEY_X25519, NULL, private_key, 32);
+    EVP_PKEY *theirs = EVP_PKEY_new_raw_public_key(EVP_PKEY_X25519, NULL, peer, PURE_SEAL_KEY);
+    EVP_PKEY_CTX *context = mine && theirs ? EVP_PKEY_CTX_new(mine, NULL) : NULL;
+    size_t length = 32;
+    bool ok = context && EVP_PKEY_derive_init(context) == 1 && EVP_PKEY_derive_set_peer(context, theirs) == 1 &&
+              EVP_PKEY_derive(context, out, &length) == 1 && length == 32;
+    EVP_PKEY_CTX_free(context);
+    EVP_PKEY_free(mine);
+    EVP_PKEY_free(theirs);
+    unsigned char any = 0; /* a small-order key makes zeros: no secret */
+    for (size_t i = 0; ok && i < 32; ++i) any |= out[i];
+    return ok && any;
+}
+
 /* HKDF, fetched once. */
 static EVP_KDF *hkdf(void) {
     static EVP_KDF *kept;
@@ -281,4 +307,5 @@ static bool aead_open(PureAead *a, const unsigned char *key, const unsigned char
 
 const PureCrypto pure_crypto = {PURE_CRYPTO_VERSION, tls_start, tls_open, tls_read, tls_write, tls_close,
                                 seal_key, seal_key_free, sealer, seal, sealer_free, opener, unseal, opener_free,
-                                link_secret, link_derive, aead, aead_seal, aead_open, aead_free};
+                                link_secret, link_ephemeral, link_agree, link_derive, aead, aead_seal, aead_open,
+                                aead_free};