/*
 * ppp_mppe.c - interface MPPE to the PPP code.
 * This version is for use with Linux kernel 2.6.14+
 *
 * By Frank Cusack <fcusack@fcusack.com>.
 * Copyright (c) 2002,2003,2004 Google, Inc.
 * All rights reserved.
 *
 * License:
 * Permission to use, copy, modify, and distribute this software and its
 * documentation is hereby granted, provided that the above copyright
 * notice appears in all copies.  This software is provided without any
 * warranty, express or implied.
 *
 * ALTERNATIVELY, provided that this notice is retained in full, this product
 * may be distributed under the terms of the GNU General Public License (GPL),
 * in which case the provisions of the GPL apply INSTEAD OF those given above.
 *
 *   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
 *   (at your option) 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, see <http://www.gnu.org/licenses/>.
 *
 *
 * Changelog:
 *      08/12/05 - Matt Domsch <Matt_Domsch@dell.com>
 *                 Only need extra skb padding on transmit, not receive.
 *      06/18/04 - Matt Domsch <Matt_Domsch@dell.com>, Oleg Makarenko <mole@quadra.ru>
 *                 Use Linux kernel 2.6 arc4 and sha1 routines rather than
 *                 providing our own.
 *      2/15/04 - TS: added #include <version.h> and testing for Kernel
 *                    version before using
 *                    MOD_DEC_USAGE_COUNT/MOD_INC_USAGE_COUNT which are
 *                    deprecated in 2.6
 */

#include <crypto/hash.h>
#include <crypto/skcipher.h>
#include <linux/err.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/types.h>
#include <linux/slab.h>
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/ppp_defs.h>
#include <linux/ppp-comp.h>
#include <linux/scatterlist.h>
#include <asm/unaligned.h>

#include "ppp_mppe.h"

MODULE_AUTHOR("Frank Cusack <fcusack@fcusack.com>");
MODULE_DESCRIPTION("Point-to-Point Protocol Microsoft Point-to-Point Encryption support");
MODULE_LICENSE("Dual BSD/GPL");
MODULE_ALIAS("ppp-compress-" __stringify(CI_MPPE));
MODULE_VERSION("1.0.2");

static unsigned int
setup_sg(struct scatterlist *sg, const void *address, unsigned int length)
{
        sg_set_buf(sg, address, length);
        return length;
}

#define SHA1_PAD_SIZE 40

/*
 * kernel crypto API needs its arguments to be in kmalloc'd memory, not in the module
 * static data area.  That means sha_pad needs to be kmalloc'd.
 */

struct sha_pad {
        unsigned char sha_pad1[SHA1_PAD_SIZE];
        unsigned char sha_pad2[SHA1_PAD_SIZE];
};
static struct sha_pad *sha_pad;

static inline void sha_pad_init(struct sha_pad *shapad)
{
        memset(shapad->sha_pad1, 0x00, sizeof(shapad->sha_pad1));
        memset(shapad->sha_pad2, 0xF2, sizeof(shapad->sha_pad2));
}

/*
 * State for an MPPE (de)compressor.
 */
struct ppp_mppe_state {
        struct crypto_skcipher *arc4;
        struct crypto_ahash *sha1;
        unsigned char *sha1_digest;
        unsigned char master_key[MPPE_MAX_KEY_LEN];
        unsigned char session_key[MPPE_MAX_KEY_LEN];
        unsigned keylen;        /* key length in bytes             */
        /* NB: 128-bit == 16, 40-bit == 8! */
        /* If we want to support 56-bit,   */
        /* the unit has to change to bits  */
        unsigned char bits;     /* MPPE control bits */
        unsigned ccount;        /* 12-bit coherency count (seqno)  */
        unsigned stateful;      /* stateful mode flag */
        int discard;            /* stateful mode packet loss flag */
        int sanity_errors;      /* take down LCP if too many */
        int unit;
        int debug;
        struct compstat stats;
};

/* struct ppp_mppe_state.bits definitions */
#define MPPE_BIT_A      0x80    /* Encryption table were (re)inititalized */
#define MPPE_BIT_B      0x40    /* MPPC only (not implemented) */
#define MPPE_BIT_C      0x20    /* MPPC only (not implemented) */
#define MPPE_BIT_D      0x10    /* This is an encrypted frame */

#define MPPE_BIT_FLUSHED        MPPE_BIT_A
#define MPPE_BIT_ENCRYPTED      MPPE_BIT_D

#define MPPE_BITS(p) ((p)[4] & 0xf0)
#define MPPE_CCOUNT(p) ((((p)[4] & 0x0f) << 8) + (p)[5])
#define MPPE_CCOUNT_SPACE 0x1000        /* The size of the ccount space */

#define MPPE_OVHD       2       /* MPPE overhead/packet */
#define SANITY_MAX      1600    /* Max bogon factor we will tolerate */

/*
 * Key Derivation, from RFC 3078, RFC 3079.
 * Equivalent to Get_Key() for MS-CHAP as described in RFC 3079.
 */
static void get_new_key_from_sha(struct ppp_mppe_state * state)
{
        AHASH_REQUEST_ON_STACK(req, state->sha1);
        struct scatterlist sg[4];
        unsigned int nbytes;

        sg_init_table(sg, 4);

        nbytes = setup_sg(&sg[0], state->master_key, state->keylen);
        nbytes += setup_sg(&sg[1], sha_pad->sha_pad1,
                           sizeof(sha_pad->sha_pad1));
        nbytes += setup_sg(&sg[2], state->session_key, state->keylen);
        nbytes += setup_sg(&sg[3], sha_pad->sha_pad2,
                           sizeof(sha_pad->sha_pad2));

        ahash_request_set_tfm(req, state->sha1);
        ahash_request_set_callback(req, 0, NULL, NULL);
        ahash_request_set_crypt(req, sg, state->sha1_digest, nbytes);

        crypto_ahash_digest(req);
        ahash_request_zero(req);
}

/*
 * Perform the MPPE rekey algorithm, from RFC 3078, sec. 7.3.
 * Well, not what's written there, but rather what they meant.
 */
static void mppe_rekey(struct ppp_mppe_state * state, int initial_key)
{
        struct scatterlist sg_in[1], sg_out[1];
        SKCIPHER_REQUEST_ON_STACK(req, state->arc4);

        skcipher_request_set_tfm(req, state->arc4);
        skcipher_request_set_callback(req, 0, NULL, NULL);

        get_new_key_from_sha(state);
        if (!initial_key) {
                crypto_skcipher_setkey(state->arc4, state->sha1_digest,
                                       state->keylen);
                sg_init_table(sg_in, 1);
                sg_init_table(sg_out, 1);
                setup_sg(sg_in, state->sha1_digest, state->keylen);
                setup_sg(sg_out, state->session_key, state->keylen);
                skcipher_request_set_crypt(req, sg_in, sg_out, state->keylen,
                                           NULL);
                if (crypto_skcipher_encrypt(req))
                    printk(KERN_WARNING "mppe_rekey: cipher_encrypt failed\n");
        } else {
                memcpy(state->session_key, state->sha1_digest, state->keylen);
        }
        if (state->keylen == 8) {
                /* See RFC 3078 */
                state->session_key[0] = 0xd1;
                state->session_key[1] = 0x26;
                state->session_key[2] = 0x9e;
        }
        crypto_skcipher_setkey(state->arc4, state->session_key, state->keylen);
        skcipher_request_zero(req);
}

/*
 * Allocate space for a (de)compressor.
 */
static void *mppe_alloc(unsigned char *options, int optlen)
{
        struct ppp_mppe_state *state;
        unsigned int digestsize;

        if (optlen != CILEN_MPPE + sizeof(state->master_key) ||
            options[0] != CI_MPPE || options[1] != CILEN_MPPE)
                goto out;

        state = kzalloc(sizeof(*state), GFP_KERNEL);
        if (state == NULL)
                goto out;


        state->arc4 = crypto_alloc_skcipher("ecb(arc4)", 0, CRYPTO_ALG_ASYNC);
        if (IS_ERR(state->arc4)) {
                state->arc4 = NULL;
                goto out_free;
        }

        state->sha1 = crypto_alloc_ahash("sha1", 0, CRYPTO_ALG_ASYNC);
        if (IS_ERR(state->sha1)) {
                state->sha1 = NULL;
                goto out_free;
        }

        digestsize = crypto_ahash_digestsize(state->sha1);
        if (digestsize < MPPE_MAX_KEY_LEN)
                goto out_free;

        state->sha1_digest = kmalloc(digestsize, GFP_KERNEL);
        if (!state->sha1_digest)
                goto out_free;

        /* Save keys. */
        memcpy(state->master_key, &options[CILEN_MPPE],
               sizeof(state->master_key));
        memcpy(state->session_key, state->master_key,
               sizeof(state->master_key));

        /*
         * We defer initial key generation until mppe_init(), as mppe_alloc()
         * is called frequently during negotiation.
         */

        return (void *)state;

out_free:
        kfree(state->sha1_digest);
        crypto_free_ahash(state->sha1);
        crypto_free_skcipher(state->arc4);
        kfree(state);
out:
        return NULL;
}

/*
 * Deallocate space for a (de)compressor.
 */
static void mppe_free(void *arg)
{
        struct ppp_mppe_state *state = (struct ppp_mppe_state *) arg;
        if (state) {
                kfree(state->sha1_digest);
                crypto_free_ahash(state->sha1);
                crypto_free_skcipher(state->arc4);
                kfree(state);
        }
}

/*
 * Initialize (de)compressor state.
 */
static int
mppe_init(void *arg, unsigned char *options, int optlen, int unit, int debug,
          const char *debugstr)
{
        struct ppp_mppe_state *state = (struct ppp_mppe_state *) arg;
        unsigned char mppe_opts;

        if (optlen != CILEN_MPPE ||
            options[0] != CI_MPPE || options[1] != CILEN_MPPE)
                return 0;

        MPPE_CI_TO_OPTS(&options[2], mppe_opts);
        if (mppe_opts & MPPE_OPT_128)
                state->keylen = 16;
        else if (mppe_opts & MPPE_OPT_40)
                state->keylen = 8;
        else {
                printk(KERN_WARNING "%s[%d]: unknown key length\n", debugstr,
                       unit);
                return 0;
        }
        if (mppe_opts & MPPE_OPT_STATEFUL)
                state->stateful = 1;

        /* Generate the initial session key. */
        mppe_rekey(state, 1);

        if (debug) {
                printk(KERN_DEBUG "%s[%d]: initialized with %d-bit %s mode\n",
                       debugstr, unit, (state->keylen == 16) ? 128 : 40,
                       (state->stateful) ? "stateful" : "stateless");
                printk(KERN_DEBUG
                       "%s[%d]: keys: master: %*phN initial session: %*phN\n",
                       debugstr, unit,
                       (int)sizeof(state->master_key), state->master_key,
                       (int)sizeof(state->session_key), state->session_key);
        }

        /*
         * Initialize the coherency count.  The initial value is not specified
         * in RFC 3078, but we can make a reasonable assumption that it will
         * start at 0.  Setting it to the max here makes the comp/decomp code
         * do the right thing (determined through experiment).
         */
        state->ccount = MPPE_CCOUNT_SPACE - 1;

        /*
         * Note that even though we have initialized the key table, we don't
         * set the FLUSHED bit.  This is contrary to RFC 3078, sec. 3.1.
         */
        state->bits = MPPE_BIT_ENCRYPTED;

        state->unit = unit;
        state->debug = debug;

        return 1;
}

static int
mppe_comp_init(void *arg, unsigned char *options, int optlen, int unit,
               int hdrlen, int debug)
{
        /* ARGSUSED */
        return mppe_init(arg, options, optlen, unit, debug, "mppe_comp_init");
}

/*
 * We received a CCP Reset-Request (actually, we are sending a Reset-Ack),
 * tell the compressor to rekey.  Note that we MUST NOT rekey for
 * every CCP Reset-Request; we only rekey on the next xmit packet.
 * We might get multiple CCP Reset-Requests if our CCP Reset-Ack is lost.
 * So, rekeying for every CCP Reset-Request is broken as the peer will not
 * know how many times we've rekeyed.  (If we rekey and THEN get another
 * CCP Reset-Request, we must rekey again.)
 */
static void mppe_comp_reset(void *arg)
{
        struct ppp_mppe_state *state = (struct ppp_mppe_state *) arg;

        state->bits |= MPPE_BIT_FLUSHED;
}

/*
 * Compress (encrypt) a packet.
 * It's strange to call this a compressor, since the output is always
 * MPPE_OVHD + 2 bytes larger than the input.
 */
static int
mppe_compress(void *arg, unsigned char *ibuf, unsigned char *obuf,
              int isize, int osize)
{
        struct ppp_mppe_state *state = (struct ppp_mppe_state *) arg;
        SKCIPHER_REQUEST_ON_STACK(req, state->arc4);
        int proto;
        int err;
        struct scatterlist sg_in[1], sg_out[1];

        /*
         * Check that the protocol is in the range we handle.
         */
        proto = PPP_PROTOCOL(ibuf);
        if (proto < 0x0021 || proto > 0x00fa)
                return 0;

        /* Make sure we have enough room to generate an encrypted packet. */
        if (osize < isize + MPPE_OVHD + 2) {
                /* Drop the packet if we should encrypt it, but can't. */
                printk(KERN_DEBUG "mppe_compress[%d]: osize too small! "
                       "(have: %d need: %d)\n", state->unit,
                       osize, osize + MPPE_OVHD + 2);
                return -1;
        }

        osize = isize + MPPE_OVHD + 2;

        /*
         * Copy over the PPP header and set control bits.
         */
        obuf[0] = PPP_ADDRESS(ibuf);
        obuf[1] = PPP_CONTROL(ibuf);
        put_unaligned_be16(PPP_COMP, obuf + 2);
        obuf += PPP_HDRLEN;

        state->ccount = (state->ccount + 1) % MPPE_CCOUNT_SPACE;
        if (state->debug >= 7)
                printk(KERN_DEBUG "mppe_compress[%d]: ccount %d\n", state->unit,
                       state->ccount);
        put_unaligned_be16(state->ccount, obuf);

        if (!state->stateful || /* stateless mode     */
            ((state->ccount & 0xff) == 0xff) || /* "flag" packet      */
            (state->bits & MPPE_BIT_FLUSHED)) { /* CCP Reset-Request  */
                /* We must rekey */
                if (state->debug && state->stateful)
                        printk(KERN_DEBUG "mppe_compress[%d]: rekeying\n",
                               state->unit);
                mppe_rekey(state, 0);
                state->bits |= MPPE_BIT_FLUSHED;
        }
        obuf[0] |= state->bits;
        state->bits &= ~MPPE_BIT_FLUSHED;       /* reset for next xmit */

        obuf += MPPE_OVHD;
        ibuf += 2;              /* skip to proto field */
        isize -= 2;

        /* Encrypt packet */
        sg_init_table(sg_in, 1);
        sg_init_table(sg_out, 1);
        setup_sg(sg_in, ibuf, isize);
        setup_sg(sg_out, obuf, osize);

        skcipher_request_set_tfm(req, state->arc4);
        skcipher_request_set_callback(req, 0, NULL, NULL);
        skcipher_request_set_crypt(req, sg_in, sg_out, isize, NULL);
        err = crypto_skcipher_encrypt(req);
        skcipher_request_zero(req);
        if (err) {
                printk(KERN_DEBUG "crypto_cypher_encrypt failed\n");
                return -1;
        }

        state->stats.unc_bytes += isize;
        state->stats.unc_packets++;
        state->stats.comp_bytes += osize;
        state->stats.comp_packets++;

        return osize;
}

/*
 * Since every frame grows by MPPE_OVHD + 2 bytes, this is always going
 * to look bad ... and the longer the link is up the worse it will get.
 */
static void mppe_comp_stats(void *arg, struct compstat *stats)
{
        struct ppp_mppe_state *state = (struct ppp_mppe_state *) arg;

        *stats = state->stats;
}

static int
mppe_decomp_init(void *arg, unsigned char *options, int optlen, int unit,
                 int hdrlen, int mru, int debug)
{
        /* ARGSUSED */
        return mppe_init(arg, options, optlen, unit, debug, "mppe_decomp_init");
}

/*
 * We received a CCP Reset-Ack.  Just ignore it.
 */
static void mppe_decomp_reset(void *arg)
{
        /* ARGSUSED */
        return;
}

/*
 * Decompress (decrypt) an MPPE packet.
 */
static int
mppe_decompress(void *arg, unsigned char *ibuf, int isize, unsigned char *obuf,
                int osize)
{
        struct ppp_mppe_state *state = (struct ppp_mppe_state *) arg;
        SKCIPHER_REQUEST_ON_STACK(req, state->arc4);
        unsigned ccount;
        int flushed = MPPE_BITS(ibuf) & MPPE_BIT_FLUSHED;
        struct scatterlist sg_in[1], sg_out[1];

        if (isize <= PPP_HDRLEN + MPPE_OVHD) {
                if (state->debug)
                        printk(KERN_DEBUG
                               "mppe_decompress[%d]: short pkt (%d)\n",
                               state->unit, isize);
                return DECOMP_ERROR;
        }

        /*
         * Make sure we have enough room to decrypt the packet.
         * Note that for our test we only subtract 1 byte whereas in
         * mppe_compress() we added 2 bytes (+MPPE_OVHD);
         * this is to account for possible PFC.
         */
        if (osize < isize - MPPE_OVHD - 1) {
                printk(KERN_DEBUG "mppe_decompress[%d]: osize too small! "
                       "(have: %d need: %d)\n", state->unit,
                       osize, isize - MPPE_OVHD - 1);
                return DECOMP_ERROR;
        }
        osize = isize - MPPE_OVHD - 2;  /* assume no PFC */

        ccount = MPPE_CCOUNT(ibuf);
        if (state->debug >= 7)
                printk(KERN_DEBUG "mppe_decompress[%d]: ccount %d\n",
                       state->unit, ccount);

        /* sanity checks -- terminate with extreme prejudice */
        if (!(MPPE_BITS(ibuf) & MPPE_BIT_ENCRYPTED)) {
                printk(KERN_DEBUG
                       "mppe_decompress[%d]: ENCRYPTED bit not set!\n",
                       state->unit);
                state->sanity_errors += 100;
                goto sanity_error;
        }
        if (!state->stateful && !flushed) {
                printk(KERN_DEBUG "mppe_decompress[%d]: FLUSHED bit not set in "
                       "stateless mode!\n", state->unit);
                state->sanity_errors += 100;
                goto sanity_error;
        }
        if (state->stateful && ((ccount & 0xff) == 0xff) && !flushed) {
                printk(KERN_DEBUG "mppe_decompress[%d]: FLUSHED bit not set on "
                       "flag packet!\n", state->unit);
                state->sanity_errors += 100;
                goto sanity_error;
        }

        /*
         * Check the coherency count.
         */

        if (!state->stateful) {
                /* Discard late packet */
                if ((ccount - state->ccount) % MPPE_CCOUNT_SPACE
                                                > MPPE_CCOUNT_SPACE / 2) {
                        state->sanity_errors++;
                        goto sanity_error;
                }

                /* RFC 3078, sec 8.1.  Rekey for every packet. */
                while (state->ccount != ccount) {
                        mppe_rekey(state, 0);
                        state->ccount = (state->ccount + 1) % MPPE_CCOUNT_SPACE;
                }
        } else {
                /* RFC 3078, sec 8.2. */
                if (!state->discard) {
                        /* normal state */
                        state->ccount = (state->ccount + 1) % MPPE_CCOUNT_SPACE;
                        if (ccount != state->ccount) {
                                /*
                                 * (ccount > state->ccount)
                                 * Packet loss detected, enter the discard state.
                                 * Signal the peer to rekey (by sending a CCP Reset-Request).
                                 */
                                state->discard = 1;
                                return DECOMP_ERROR;
                        }
                } else {
                        /* discard state */
                        if (!flushed) {
                                /* ccp.c will be silent (no additional CCP Reset-Requests). */
                                return DECOMP_ERROR;
                        } else {
                                /* Rekey for every missed "flag" packet. */
                                while ((ccount & ~0xff) !=
                                       (state->ccount & ~0xff)) {
                                        mppe_rekey(state, 0);
                                        state->ccount =
                                            (state->ccount +
                                             256) % MPPE_CCOUNT_SPACE;
                                }

                                /* reset */
                                state->discard = 0;
                                state->ccount = ccount;
                                /*
                                 * Another problem with RFC 3078 here.  It implies that the
                                 * peer need not send a Reset-Ack packet.  But RFC 1962
                                 * requires it.  Hopefully, M$ does send a Reset-Ack; even
                                 * though it isn't required for MPPE synchronization, it is
                                 * required to reset CCP state.
                                 */
                        }
                }
                if (flushed)
                        mppe_rekey(state, 0);
        }

        /*
         * Fill in the first part of the PPP header.  The protocol field
         * comes from the decrypted data.
         */
        obuf[0] = PPP_ADDRESS(ibuf);    /* +1 */
        obuf[1] = PPP_CONTROL(ibuf);    /* +1 */
        obuf += 2;
        ibuf += PPP_HDRLEN + MPPE_OVHD;
        isize -= PPP_HDRLEN + MPPE_OVHD;        /* -6 */
        /* net osize: isize-4 */

        /*
         * Decrypt the first byte in order to check if it is
         * a compressed or uncompressed protocol field.
         */
        sg_init_table(sg_in, 1);
        sg_init_table(sg_out, 1);
        setup_sg(sg_in, ibuf, 1);
        setup_sg(sg_out, obuf, 1);

        skcipher_request_set_tfm(req, state->arc4);
        skcipher_request_set_callback(req, 0, NULL, NULL);
        skcipher_request_set_crypt(req, sg_in, sg_out, 1, NULL);
        if (crypto_skcipher_decrypt(req)) {
                printk(KERN_DEBUG "crypto_cypher_decrypt failed\n");
                osize = DECOMP_ERROR;
                goto out_zap_req;
        }

        /*
         * Do PFC decompression.
         * This would be nicer if we were given the actual sk_buff
         * instead of a char *.
         */
        if ((obuf[0] & 0x01) != 0) {
                obuf[1] = obuf[0];
                obuf[0] = 0;
                obuf++;
                osize++;
        }

        /* And finally, decrypt the rest of the packet. */
        setup_sg(sg_in, ibuf + 1, isize - 1);
        setup_sg(sg_out, obuf + 1, osize - 1);
        skcipher_request_set_crypt(req, sg_in, sg_out, isize - 1, NULL);
        if (crypto_skcipher_decrypt(req)) {
                printk(KERN_DEBUG "crypto_cypher_decrypt failed\n");
                osize = DECOMP_ERROR;
                goto out_zap_req;
        }

        state->stats.unc_bytes += osize;
        state->stats.unc_packets++;
        state->stats.comp_bytes += isize;
        state->stats.comp_packets++;

        /* good packet credit */
        state->sanity_errors >>= 1;

out_zap_req:
        skcipher_request_zero(req);
        return osize;

sanity_error:
        if (state->sanity_errors < SANITY_MAX)
                return DECOMP_ERROR;
        else
                /* Take LCP down if the peer is sending too many bogons.
                 * We don't want to do this for a single or just a few
                 * instances since it could just be due to packet corruption.
                 */
                return DECOMP_FATALERROR;
}

/*
 * Incompressible data has arrived (this should never happen!).
 * We should probably drop the link if the protocol is in the range
 * of what should be encrypted.  At the least, we should drop this
 * packet.  (How to do this?)
 */
static void mppe_incomp(void *arg, unsigned char *ibuf, int icnt)
{
        struct ppp_mppe_state *state = (struct ppp_mppe_state *) arg;

        if (state->debug &&
            (PPP_PROTOCOL(ibuf) >= 0x0021 && PPP_PROTOCOL(ibuf) <= 0x00fa))
                printk(KERN_DEBUG
                       "mppe_incomp[%d]: incompressible (unencrypted) data! "
                       "(proto %04x)\n", state->unit, PPP_PROTOCOL(ibuf));

        state->stats.inc_bytes += icnt;
        state->stats.inc_packets++;
        state->stats.unc_bytes += icnt;
        state->stats.unc_packets++;
}

/*************************************************************
 * Module interface table
 *************************************************************/

/*
 * Procedures exported to if_ppp.c.
 */
static struct compressor ppp_mppe = {
        .compress_proto = CI_MPPE,
        .comp_alloc     = mppe_alloc,
        .comp_free      = mppe_free,
        .comp_init      = mppe_comp_init,
        .comp_reset     = mppe_comp_reset,
        .compress       = mppe_compress,
        .comp_stat      = mppe_comp_stats,
        .decomp_alloc   = mppe_alloc,
        .decomp_free    = mppe_free,
        .decomp_init    = mppe_decomp_init,
        .decomp_reset   = mppe_decomp_reset,
        .decompress     = mppe_decompress,
        .incomp         = mppe_incomp,
        .decomp_stat    = mppe_comp_stats,
        .owner          = THIS_MODULE,
        .comp_extra     = MPPE_PAD,
};

/*
 * ppp_mppe_init()
 *
 * Prior to allowing load, try to load the arc4 and sha1 crypto
 * libraries.  The actual use will be allocated later, but
 * this way the module will fail to insmod if they aren't available.
 */

static int __init ppp_mppe_init(void)
{
        int answer;
        if (!(crypto_has_skcipher("ecb(arc4)", 0, CRYPTO_ALG_ASYNC) &&
              crypto_has_ahash("sha1", 0, CRYPTO_ALG_ASYNC)))
                return -ENODEV;

        sha_pad = kmalloc(sizeof(struct sha_pad), GFP_KERNEL);
        if (!sha_pad)
                return -ENOMEM;
        sha_pad_init(sha_pad);

        answer = ppp_register_compressor(&ppp_mppe);

        if (answer == 0)
                printk(KERN_INFO "PPP MPPE Compression module registered\n");
        else
                kfree(sha_pad);

        return answer;
}

static void __exit ppp_mppe_cleanup(void)
{
        ppp_unregister_compressor(&ppp_mppe);
        kfree(sha_pad);
}

module_init(ppp_mppe_init);
module_exit(ppp_mppe_cleanup);