elliptic.c

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/*
 * Copyright (C) 2026 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 *
 * You can also choose to distribute this program under the terms of
 * the Unmodified Binary Distribution Licence (as given in the file
 * COPYING.UBDL), provided that you have satisfied its requirements.
 */
 
FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
FILE_SECBOOT ( PERMITTED );
 
/** @file
 *
 * Elliptic curves
 *
 */
 
#include <assert.h>
#include <errno.h>
#include <string.h>
#include <ipxe/crypto.h>
#include <ipxe/elliptic.h>
 
/**
 * Share public key
 *
 * @v exchange          Key exchange algorithm
 * @v private           Private key
 * @v public            Public key to fill in
 * @ret rc              Return status code
 */
int elliptic_share ( struct exchange_algorithm *exchange, const void *private,
                      void *public ) {
        struct elliptic_curve *curve = exchange->priv;
        size_t len = exchange->sharedsize;
        elliptic_uncompressed_t ( len ) *result = public;
        int rc;
 
        /* Sanity checks */
        assert ( curve->keysize == exchange->privsize );
        assert ( curve->pointsize == sizeof ( result->xy ) );
        assert ( sizeof ( *result ) == exchange->pubsize );
 
        /* Calculate public key */
        result->format = ELLIPTIC_FORMAT_UNCOMPRESSED;
        if ( ( rc = elliptic_multiply ( curve, curve->base, private,
                                        &result->xy ) ) != 0 ) {
                /* Can never fail when using the curve's own base point */
                assert ( 0 );
                return rc;
        }
 
        return rc;
}
 
/**
 * Agree shared secret
 *
 * @v exchange          Key exchange algorithm
 * @v private           Private key
 * @v partner           Partner public key
 * @v shared            Shared secret to fill in
 * @ret rc              Return status code
 */
int elliptic_agree ( struct exchange_algorithm *exchange, const void *private,
                     const void *partner, void *shared ) {
        struct elliptic_curve *curve = exchange->priv;
        size_t len = exchange->sharedsize;
        const elliptic_uncompressed_t ( len ) *base = partner;
        elliptic_uncompressed_t ( len ) result;
        int rc;
 
        /* Sanity checks */
        assert ( curve->keysize == exchange->privsize );
        assert ( curve->pointsize == sizeof ( base->xy ) );
        assert ( curve->pointsize == sizeof ( result.xy ) );
        assert ( sizeof ( *base ) == exchange->pubsize );
        assert ( sizeof ( result.x ) == exchange->sharedsize );
 
        /* Calculate shared secret */
        if ( ( rc = elliptic_multiply ( curve, &base->xy, private,
                                        result.xy ) ) != 0 )
                return rc;
        memcpy ( shared, &result.x, sizeof ( result.x ) );
 
        /* Check for point at infinity */
        if ( elliptic_is_infinity ( curve, &result.xy ) )
                return -EPERM;
 
        /* Check format byte */
        if ( base->format != ELLIPTIC_FORMAT_UNCOMPRESSED )
                return -EPERM;
 
        return 0;
}