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support for user-verified FIDO keys

FIDO2 supports a notion of "user verification" where the user is
required to demonstrate their identity to the token before particular
operations (e.g. signing). Typically this is done by authenticating
themselves using a PIN that has been set on the token.

This adds support for generating and using user verified keys where
the verification happens via PIN (other options might be added in the
future, but none are in common use now). Practically, this adds
another key generation option "verify-required" that yields a key that
requires a PIN before each authentication.

feedback markus@ and Pedro Martelletto; ok markus@

/*
 * Copyright (c) 2019 Google LLC
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#ifndef SSHSIG_H
#define SSHSIG_H

struct sshbuf;
struct sshkey;
struct sshsigopt;
struct sshkey_sig_details;

typedef int sshsig_signer(struct sshkey *, u_char **, size_t *,
    const u_char *, size_t, const char *, const char *, const char *,
    u_int, void *);

/* Buffer-oriented API */

/*
 * Creates a detached SSH signature for a given buffer.
 * Returns 0 on success or a negative SSH_ERR_* error code on failure.
 * out is populated with the detached signature, or NULL on failure.
 */
int sshsig_signb(struct sshkey *key, const char *hashalg,
    const char *sk_provider, const char *sk_pin, const struct sshbuf *message,
    const char *sig_namespace, struct sshbuf **out,
    sshsig_signer *signer, void *signer_ctx);

/*
 * Verifies that a detached signature is valid and optionally returns key
 * used to sign via argument.
 * Returns 0 on success or a negative SSH_ERR_* error code on failure.
 */
int sshsig_verifyb(struct sshbuf *signature,
    const struct sshbuf *message, const char *sig_namespace,
    struct sshkey **sign_keyp, struct sshkey_sig_details **sig_details);

/* File/FD-oriented API */

/*
 * Creates a detached SSH signature for a given file.
 * Returns 0 on success or a negative SSH_ERR_* error code on failure.
 * out is populated with the detached signature, or NULL on failure.
 */
int sshsig_sign_fd(struct sshkey *key, const char *hashalg,
    const char *sk_provider, const char *sk_pin,
    int fd, const char *sig_namespace,
    struct sshbuf **out, sshsig_signer *signer, void *signer_ctx);

/*
 * Verifies that a detached signature over a file is valid and optionally
 * returns key used to sign via argument.
 * Returns 0 on success or a negative SSH_ERR_* error code on failure.
 */
int sshsig_verify_fd(struct sshbuf *signature, int fd,
    const char *sig_namespace, struct sshkey **sign_keyp,
    struct sshkey_sig_details **sig_details);

/* Utility functions */

/*
 * Return a base64 encoded "ASCII armoured" version of a raw signature.
 */
int sshsig_armor(const struct sshbuf *blob, struct sshbuf **out);

/*
 * Decode a base64 encoded armoured signature to a raw signature.
 */
int sshsig_dearmor(struct sshbuf *sig, struct sshbuf **out);

/*
 * Checks whether a particular key/principal/namespace is permitted by
 * an allowed_keys file. Returns 0 on success.
 */
int sshsig_check_allowed_keys(const char *path, const struct sshkey *sign_key,
    const char *principal, const char *ns);

/* Parse zero or more allowed_keys signature options */
struct sshsigopt *sshsigopt_parse(const char *opts,
    const char *path, u_long linenum, const char **errstrp);

/* Free signature options */
void sshsigopt_free(struct sshsigopt *opts);

/* Get public key from signature */
int sshsig_get_pubkey(struct sshbuf *signature, struct sshkey **pubkey);

/* Find principal in allowed_keys file, given a sshkey. Returns
 * 0 on success.
 */
int sshsig_find_principals(const char *path, const struct sshkey *sign_key,
    char **principal);

#endif /* SSHSIG_H */