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diff --git a/link/seal.h b/link/seal.h
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+#ifndef PURE_SEAL_H
+#define PURE_SEAL_H
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+/* Sends sealed end to end (docs/research/086-sealed-sends.md, 088): a
+ * receiving node's key opens what a sender sealed to its public key, and
+ * nothing between them (a relay, a proxy, a mailbox) can read or change it.
+ * HPKE (RFC 9180): X25519, HKDF-SHA256, and AES-128-GCM where the CPU has AES
+ * instructions, else ChaCha20-Poly1305; auth mode when the sender has a key
+ * of its own, so the receiver knows who sealed. A sender keeps one session
+ * (one encapsulation) and numbers its seals in it; a receiver opens a
+ * session once and then each seal by its number. A private key lives in
+ * OpenSSL's secure heap: locked in memory, out of core dumps, cleared when
+ * freed. OpenSSL comes with pure-crypto.so (link/crypto.c), mapped the
+ * first time a key is read or a session made. */
+enum { PURE_SEAL_KEY = 32, PURE_SEAL_OVERHEAD = 16 }; /* a public key; a seal's tag */
+
+typedef struct PureSealKey PureSealKey;
+typedef struct PureSealer PureSealer; /* a sender's session */
+typedef struct PureOpener PureOpener; /* a receiver's session */
+
+/* Whether this machine can seal (pure-crypto.so maps), with why not in error. */
+bool pure_seal_ready(char *error, size_t size);
+/* A node's key: read from path, or made and written there (0600) when there
+ * is none; its public half in hex. NULL with why in error. */
+PureSealKey *pure_seal_key(const char *path, char public_hex[2 * PURE_SEAL_KEY + 1], char *error, size_t size);
+void pure_seal_key_free(PureSealKey *);
+/* A public key from its hex. */
+bool pure_seal_peer(const char *hex, unsigned char out[PURE_SEAL_KEY]);
+/* The suite this machine seals with, as the X-Pure-Seal header names it. */
+const char *pure_seal_suite(void);
+
+/* A session sealing to peer, authenticated by own when given: its
+ * encapsulated key into enc (what a receiver opens the session with). */
+PureSealer *pure_sealer(const unsigned char peer[PURE_SEAL_KEY], const PureSealKey *own,
+                        unsigned char enc[PURE_SEAL_KEY]);
+/* plain sealed as the session's next seal, bound to aad: size +
+ * PURE_SEAL_OVERHEAD bytes into out, and its number into n. */
+bool pure_seal(PureSealer *, const char *aad, const unsigned char *plain, size_t size, unsigned char *out,
+               uint64_t *n);
+void pure_sealer_free(PureSealer *);
+
+/* A session a sender made to key's public half, in suite (this machine's or
+ * the other known one), from its enc; from is the sender's public key when
+ * it sealed with its own (NULL otherwise). NULL when it does not open. */
+PureOpener *pure_opener(const PureSealKey *key, const char *suite, const unsigned char enc[PURE_SEAL_KEY],
+                        const unsigned char *from);
+/* The session's seal number n: size - PURE_SEAL_OVERHEAD bytes into plain;
+ * false when it does not open (another session, another aad, or changed on
+ * the way). */
+bool pure_unseal(PureOpener *, uint64_t n, const char *aad, const unsigned char *sealed, size_t size,
+                 unsigned char *plain);
+void pure_opener_free(PureOpener *);
+
+/* Links (link/link.h): the secret two nodes share (X25519 of this node's
+ * private key and the other's public key: the same on both sides, and
+ * never all zeros), keys from it (HKDF-SHA256 with the info given), and
+ * AES-128-GCM under such a key with the nonce the caller gives (sealing may
+ * be in place: out may be plain). A context is used by one thread at a time;
+ * sealing or opening with a key given uses that key from then on, so one
+ * context serves many links (a key costs about as much as a small packet),
+ * and NULL keeps the key it has. */
+typedef struct PureAead PureAead;
+enum { PURE_AEAD_KEY = 16, PURE_AEAD_NONCE = 12, PURE_AEAD_TAG = 16 };
+bool pure_link_secret(const PureSealKey *own, const unsigned char peer[PURE_SEAL_KEY], unsigned char out[32]);
+bool pure_link_derive(const unsigned char secret[32], const unsigned char *info, size_t info_size, unsigned char *out,
+                      size_t out_size);
+PureAead *pure_aead(const unsigned char key[PURE_AEAD_KEY]);
+/* size + PURE_AEAD_TAG bytes into out. */
+bool pure_aead_seal(PureAead *, const unsigned char *key, const unsigned char nonce[PURE_AEAD_NONCE],
+                    const unsigned char *aad, size_t aad_size, const unsigned char *plain, size_t size,
+                    unsigned char *out);
+/* size - PURE_AEAD_TAG bytes into plain; false when it does not open. */
+bool pure_aead_open(PureAead *, const unsigned char *key, const unsigned char nonce[PURE_AEAD_NONCE],
+                    const unsigned char *aad, size_t aad_size, const unsigned char *sealed, size_t size,
+                    unsigned char *plain);
+void pure_aead_free(PureAead *);
+#endif