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Implement the diffie-hellman-group-exchange-sha256 key exchange method
using the SHA256 code in libc (and wrapper to make it into an OpenSSL
EVP), interop tested against CVS PuTTY

/*
 * Copyright (c) 2005 Damien Miller <djm@openbsd.org>
 *
 * 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.
 */

/* EVP wrapper for SHA256 */

#include "includes.h"
#include <openssl/evp.h>
#include <sha2.h>

RCSID("$OpenBSD: md-sha256.c,v 1.1 2006/03/07 09:07:40 djm Exp $");

const EVP_MD *evp_ssh_sha256(void);

static int
ssh_sha256_init(EVP_MD_CTX *ctxt)
{
	SHA256_Init(ctxt->md_data);
	return (1);
}

static int
ssh_sha256_update(EVP_MD_CTX *ctxt, const void *data, unsigned long len)
{
	SHA256_Update(ctxt->md_data, data, len);
	return (1);
}

static int
ssh_sha256_final(EVP_MD_CTX *ctxt, unsigned char *digest)
{
	SHA256_Final(digest, ctxt->md_data);
	return (1);
}

static int
ssh_sha256_cleanup(EVP_MD_CTX *ctxt)
{
	memset(ctxt->md_data, 0, sizeof(SHA256_CTX));
	return (1);
}

const EVP_MD *
evp_ssh_sha256(void)
{
	static EVP_MD ssh_sha256;

	memset(&ssh_sha256, 0, sizeof(ssh_sha256));
	ssh_sha256.type = NID_undef;
	ssh_sha256.md_size = SHA256_DIGEST_LENGTH;
	ssh_sha256.init = ssh_sha256_init;
	ssh_sha256.update = ssh_sha256_update;
	ssh_sha256.final = ssh_sha256_final;
	ssh_sha256.cleanup = ssh_sha256_cleanup;
	ssh_sha256.block_size = SHA256_BLOCK_LENGTH;
	ssh_sha256.ctx_size = sizeof(SHA256_CTX);

	return (&ssh_sha256);
}