[PPP]: add PPP MPPE encryption module

From: Matt Domsch <Matt_Domsch@dell.com>

The patch below implements the Microsoft Point-to-Point Encryption method
as a PPP compressor/decompressor.  This is necessary for Linux clients and
servers to interoperate with Microsoft Point-to-Point Tunneling Protocol
(PPTP) servers (either Microsoft PPTP servers or the poptop project) which
use MPPE to encrypt data when creating a VPN.

This patch differs from the kernel_ppp_mppe DKMS pacakge at
pptpclient.sourceforge.net by utilizing the kernel crypto routines rather
than providing its own SHA1 and arcfour implementations.

Minor changes to ppp_generic.c try to prevent a link from disabling
compression (in our case, the encryption) after it has started using
compression (encryption).

Feedback to <pptpclient-devel@lists.sourceforge.net> please.

Signed-off-by: Matt Domsch <Matt_Domsch@dell.com>
Cc: James Cameron <james.cameron@hp.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Brice Goglin <Brice.Goglin@ens-lyon.org>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 724 insertions, 0 deletions

diff --git a/drivers/net/ppp_mppe.c b/drivers/net/ppp_mppe.c
new file mode 100644
index 000000000000..1985d1b57c45
--- /dev/null
+++ b/drivers/net/ppp_mppe.c
@@ -0,0 +1,724 @@
+/*
+ * 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, write to the Free Software
+ *   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ *
+ * 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 <linux/config.h>
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/version.h>
+#include <linux/init.h>
+#include <linux/types.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <linux/crypto.h>
+#include <linux/mm.h>
+#include <linux/ppp_defs.h>
+#include <linux/ppp-comp.h>
+#include <asm/scatterlist.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 void
+setup_sg(struct scatterlist *sg, const void *address, unsigned int length)
+{
+        sg[0].page = virt_to_page(address);
+        sg[0].offset = offset_in_page(address);
+        sg[0].length = 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_tfm *arc4;
+        struct crypto_tfm *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, unsigned char *InterimKey)
+{
+        struct scatterlist sg[4];
+
+        setup_sg(&sg[0], state->master_key, state->keylen);
+        setup_sg(&sg[1], sha_pad->sha_pad1, sizeof(sha_pad->sha_pad1));
+        setup_sg(&sg[2], state->session_key, state->keylen);
+        setup_sg(&sg[3], sha_pad->sha_pad2, sizeof(sha_pad->sha_pad2));
+
+        crypto_digest_digest (state->sha1, sg, 4, state->sha1_digest);
+
+        memcpy(InterimKey, state->sha1_digest, state->keylen);
+}
+
+/*
+ * 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)
+{
+        unsigned char InterimKey[MPPE_MAX_KEY_LEN];
+        struct scatterlist sg_in[1], sg_out[1];
+
+        get_new_key_from_sha(state, InterimKey);
+        if (!initial_key) {
+                crypto_cipher_setkey(state->arc4, InterimKey, state->keylen);
+                setup_sg(sg_in, InterimKey, state->keylen);
+                setup_sg(sg_out, state->session_key, state->keylen);
+                if (crypto_cipher_encrypt(state->arc4, sg_out, sg_in,
+                                      state->keylen) != 0) {
+                    printk(KERN_WARNING "mppe_rekey: cipher_encrypt failed\n");
+                }
+        } else {
+                memcpy(state->session_key, InterimKey, 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_cipher_setkey(state->arc4, state->session_key, state->keylen);
+}
+
+/*
+ * 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 = (struct ppp_mppe_state *) kmalloc(sizeof(*state), GFP_KERNEL);
+        if (state == NULL)
+                goto out;
+
+        memset(state, 0, sizeof(*state));
+
+        state->arc4 = crypto_alloc_tfm("arc4", 0);
+        if (!state->arc4)
+                goto out_free;
+
+        state->sha1 = crypto_alloc_tfm("sha1", 0);
+        if (!state->sha1)
+                goto out_free;
+
+        digestsize = crypto_tfm_alg_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:
+            if (state->sha1_digest)
+                kfree(state->sha1_digest);
+            if (state->sha1)
+                crypto_free_tfm(state->sha1);
+            if (state->arc4)
+                crypto_free_tfm(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) {
+            if (state->sha1_digest)
+                kfree(state->sha1_digest);
+            if (state->sha1)
+                crypto_free_tfm(state->sha1);
+            if (state->arc4)
+                crypto_free_tfm(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) {
+                int i;
+                char mkey[sizeof(state->master_key) * 2 + 1];
+                char skey[sizeof(state->session_key) * 2 + 1];
+
+                printk(KERN_DEBUG "%s[%d]: initialized with %d-bit %s mode\n",
+                       debugstr, unit, (state->keylen == 16) ? 128 : 40,
+                       (state->stateful) ? "stateful" : "stateless");
+
+                for (i = 0; i < sizeof(state->master_key); i++)
+                        sprintf(mkey + i * 2, "%02x", state->master_key[i]);
+                for (i = 0; i < sizeof(state->session_key); i++)
+                        sprintf(skey + i * 2, "%02x", state->session_key[i]);
+                printk(KERN_DEBUG
+                       "%s[%d]: keys: master: %s initial session: %s\n",
+                       debugstr, unit, mkey, skey);
+        }
+
+        /*
+         * 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;
+        int proto;
+        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);
+        obuf[2] = PPP_COMP >> 8;        /* isize + MPPE_OVHD + 1 */
+        obuf[3] = PPP_COMP;     /* isize + MPPE_OVHD + 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);
+        obuf[0] = state->ccount >> 8;
+        obuf[1] = state->ccount & 0xff;
+
+        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 */
+        setup_sg(sg_in, ibuf, isize);
+        setup_sg(sg_out, obuf, osize);
+        if (crypto_cipher_encrypt(state->arc4, sg_out, sg_in, isize) != 0) {
+                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;
+        unsigned ccount;
+        int flushed = MPPE_BITS(ibuf) & MPPE_BIT_FLUSHED;
+        int sanity = 0;
+        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;
+                sanity = 1;
+        }
+        if (!state->stateful && !flushed) {
+                printk(KERN_DEBUG "mppe_decompress[%d]: FLUSHED bit not set in "
+                       "stateless mode!\n", state->unit);
+                state->sanity_errors += 100;
+                sanity = 1;
+        }
+        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;
+                sanity = 1;
+        }
+
+        if (sanity) {
+                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;
+        }
+
+        /*
+         * Check the coherency count.
+         */
+
+        if (!state->stateful) {
+                /* 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.
+         */
+        setup_sg(sg_in, ibuf, 1);
+        setup_sg(sg_out, obuf, 1);
+        if (crypto_cipher_decrypt(state->arc4, sg_out, sg_in, 1) != 0) {
+                printk(KERN_DEBUG "crypto_cypher_decrypt failed\n");
+                return DECOMP_ERROR;
+        }
+
+        /*
+         * 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);
+        if (crypto_cipher_decrypt(state->arc4, sg_out, sg_in, isize - 1) != 0) {
+                printk(KERN_DEBUG "crypto_cypher_decrypt failed\n");
+                return DECOMP_ERROR;
+        }
+
+        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;
+
+        return osize;
+}
+
+/*
+ * 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_alg_available("arc4", 0) &&
+              crypto_alg_available("sha1", 0)))
+                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);