mppe.c

Go to the documentation of this file.

/*
 * mppe.c - interface MPPE to the PPP code.
 *
 * 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.
 *
 * 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 "lwip/opt.h"
#if PPP_SUPPORT && MPPE_SUPPORT  /* don't build if not configured for use in lwipopts.h */

#include <string.h>

#include "lwip/err.h"

#include "netif/ppp/ppp_impl.h"
#include "netif/ppp/ccp.h"
#include "netif/ppp/mppe.h"
#include "netif/ppp/pppdebug.h"

#if LWIP_INCLUDED_POLARSSL_SHA1
#include "netif/ppp/polarssl/sha1.h"
#else
#include "polarssl/sha1.h"
#endif

#if LWIP_INCLUDED_POLARSSL_ARC4
#include "netif/ppp/polarssl/arc4.h"
#else
#include "polarssl/arc4.h"
#endif

#define SHA1_SIGNATURE_SIZE 20

/* 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)[0] & 0xf0)
#define MPPE_CCOUNT(p) ((((p)[0] & 0x0f) << 8) + (p)[1])
#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 */

/*
 * 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(ppp_mppe_state * state, int initial_key)
{
    sha1_context sha1_ctx;
    u8_t sha1_digest[SHA1_SIGNATURE_SIZE];

    /*
     * Key Derivation, from RFC 3078, RFC 3079.
     * Equivalent to Get_Key() for MS-CHAP as described in RFC 3079.
     */
    sha1_starts(&sha1_ctx);
    sha1_update(&sha1_ctx, state->master_key, state->keylen);
    sha1_update(&sha1_ctx, mppe_sha1_pad1, SHA1_PAD_SIZE);
    sha1_update(&sha1_ctx, state->session_key, state->keylen);
    sha1_update(&sha1_ctx, mppe_sha1_pad2, SHA1_PAD_SIZE);
    sha1_finish(&sha1_ctx, sha1_digest);
    MEMCPY(state->session_key, sha1_digest, state->keylen);

    if (!initial_key) {
        arc4_setup(&state->arc4, sha1_digest, state->keylen);
        arc4_crypt(&state->arc4, state->session_key, state->keylen);
    }
    if (state->keylen == 8) {
        /* See RFC 3078 */
        state->session_key[0] = 0xd1;
        state->session_key[1] = 0x26;
        state->session_key[2] = 0x9e;
    }
    arc4_setup(&state->arc4, state->session_key, state->keylen);
}

/*
 * Set key, used by MSCHAP before mppe_init() is actually called by CCP so we
 * don't have to keep multiple copies of keys.
 */
void mppe_set_key(ppp_pcb *pcb, ppp_mppe_state *state, u8_t *key) {
    LWIP_UNUSED_ARG(pcb);
    MEMCPY(state->master_key, key, MPPE_MAX_KEY_LEN);
}

/*
 * Initialize (de)compressor state.
 */
void
mppe_init(ppp_pcb *pcb, ppp_mppe_state *state, u8_t options)
{
#if PPP_DEBUG
    const u8_t *debugstr = (const u8_t*)"mppe_comp_init";
    if (&pcb->mppe_decomp == state) {
        debugstr = (const u8_t*)"mppe_decomp_init";
    }
#endif /* PPP_DEBUG */

    /* Save keys. */
    MEMCPY(state->session_key, state->master_key, sizeof(state->master_key));

    if (options & MPPE_OPT_128)
        state->keylen = 16;
    else if (options & MPPE_OPT_40)
        state->keylen = 8;
    else {
        PPPDEBUG(LOG_DEBUG, ("%s[%d]: unknown key length\n", debugstr,
            pcb->netif->num));
        lcp_close(pcb, "MPPE required but peer negotiation failed");
        return;
    }
    if (options & MPPE_OPT_STATEFUL)
        state->stateful = 1;

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

#if PPP_DEBUG
    {
        int i;
        char mkey[sizeof(state->master_key) * 2 + 1];
        char skey[sizeof(state->session_key) * 2 + 1];

        PPPDEBUG(LOG_DEBUG, ("%s[%d]: initialized with %d-bit %s mode\n",
               debugstr, pcb->netif->num, (state->keylen == 16) ? 128 : 40,
               (state->stateful) ? "stateful" : "stateless"));

        for (i = 0; i < (int)sizeof(state->master_key); i++)
            sprintf(mkey + i * 2, "%02x", state->master_key[i]);
        for (i = 0; i < (int)sizeof(state->session_key); i++)
            sprintf(skey + i * 2, "%02x", state->session_key[i]);
        PPPDEBUG(LOG_DEBUG,
               ("%s[%d]: keys: master: %s initial session: %s\n",
               debugstr, pcb->netif->num, mkey, skey));
    }
#endif /* PPP_DEBUG */

    /*
     * 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;
}

/*
 * 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.)
 */
void mppe_comp_reset(ppp_pcb *pcb, ppp_mppe_state *state)
{
    LWIP_UNUSED_ARG(pcb);
    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.
 */
err_t
mppe_compress(ppp_pcb *pcb, ppp_mppe_state *state, struct pbuf **pb, u16_t protocol)
{
    struct pbuf *n, *np;
    u8_t *pl;
    err_t err;

    LWIP_UNUSED_ARG(pcb);

    /* TCP stack requires that we don't change the packet payload, therefore we copy
     * the whole packet before encryption.
     */
    np = pbuf_alloc(PBUF_RAW, MPPE_OVHD + sizeof(protocol) + (*pb)->tot_len, PBUF_POOL);
    if (!np) {
        return ERR_MEM;
    }

    /* Hide MPPE header + protocol */
    pbuf_header(np, -(s16_t)(MPPE_OVHD + sizeof(protocol)));

    if ((err = pbuf_copy(np, *pb)) != ERR_OK) {
        pbuf_free(np);
        return err;
    }

    /* Reveal MPPE header + protocol */
    pbuf_header(np, (s16_t)(MPPE_OVHD + sizeof(protocol)));

    *pb = np;
    pl = (u8_t*)np->payload;

    state->ccount = (state->ccount + 1) % MPPE_CCOUNT_SPACE;
    PPPDEBUG(LOG_DEBUG, ("mppe_compress[%d]: ccount %d\n", pcb->netif->num, state->ccount));
    /* FIXME: use PUT* macros */
    pl[0] = state->ccount>>8;
    pl[1] = state->ccount;

    if (!state->stateful || /* stateless mode     */
        ((state->ccount & 0xff) == 0xff) || /* "flag" packet      */
        (state->bits & MPPE_BIT_FLUSHED)) { /* CCP Reset-Request  */
        /* We must rekey */
        if (state->stateful) {
            PPPDEBUG(LOG_DEBUG, ("mppe_compress[%d]: rekeying\n", pcb->netif->num));
        }
        mppe_rekey(state, 0);
        state->bits |= MPPE_BIT_FLUSHED;
    }
    pl[0] |= state->bits;
    state->bits &= ~MPPE_BIT_FLUSHED;   /* reset for next xmit */
    pl += MPPE_OVHD;

    /* Add protocol */
    /* FIXME: add PFC support */
    pl[0] = protocol >> 8;
    pl[1] = protocol;

    /* Hide MPPE header */
    pbuf_header(np, -(s16_t)MPPE_OVHD);

    /* Encrypt packet */
    for (n = np; n != NULL; n = n->next) {
        arc4_crypt(&state->arc4, (u8_t*)n->payload, n->len);
        if (n->tot_len == n->len) {
            break;
        }
    }

    /* Reveal MPPE header */
    pbuf_header(np, (s16_t)MPPE_OVHD);

    return ERR_OK;
}

/*
 * We received a CCP Reset-Ack.  Just ignore it.
 */
void mppe_decomp_reset(ppp_pcb *pcb, ppp_mppe_state *state)
{
    LWIP_UNUSED_ARG(pcb);
    LWIP_UNUSED_ARG(state);
    return;
}

/*
 * Decompress (decrypt) an MPPE packet.
 */
err_t
mppe_decompress(ppp_pcb *pcb, ppp_mppe_state *state, struct pbuf **pb)
{
    struct pbuf *n0 = *pb, *n;
    u8_t *pl;
    u16_t ccount;
    u8_t flushed;

    /* MPPE Header */
    if (n0->len < MPPE_OVHD) {
        PPPDEBUG(LOG_DEBUG,
               ("mppe_decompress[%d]: short pkt (%d)\n",
               pcb->netif->num, n0->len));
        state->sanity_errors += 100;
        goto sanity_error;
    }

    pl = (u8_t*)n0->payload;
    flushed = MPPE_BITS(pl) & MPPE_BIT_FLUSHED;
    ccount = MPPE_CCOUNT(pl);
    PPPDEBUG(LOG_DEBUG, ("mppe_decompress[%d]: ccount %d\n",
           pcb->netif->num, ccount));

    /* sanity checks -- terminate with extreme prejudice */
    if (!(MPPE_BITS(pl) & MPPE_BIT_ENCRYPTED)) {
        PPPDEBUG(LOG_DEBUG,
               ("mppe_decompress[%d]: ENCRYPTED bit not set!\n",
               pcb->netif->num));
        state->sanity_errors += 100;
        goto sanity_error;
    }
    if (!state->stateful && !flushed) {
        PPPDEBUG(LOG_DEBUG, ("mppe_decompress[%d]: FLUSHED bit not set in "
               "stateless mode!\n", pcb->netif->num));
        state->sanity_errors += 100;
        goto sanity_error;
    }
    if (state->stateful && ((ccount & 0xff) == 0xff) && !flushed) {
        PPPDEBUG(LOG_DEBUG, ("mppe_decompress[%d]: FLUSHED bit not set on "
               "flag packet!\n", pcb->netif->num));
        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;
                ccp_resetrequest(pcb);
                return ERR_BUF;
            }
        } else {
            /* discard state */
            if (!flushed) {
                /* ccp.c will be silent (no additional CCP Reset-Requests). */
                return ERR_BUF;
            } 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);
    }

    /* Hide MPPE header */
    pbuf_header(n0, -(s16_t)(MPPE_OVHD));

    /* Decrypt the packet. */
    for (n = n0; n != NULL; n = n->next) {
        arc4_crypt(&state->arc4, (u8_t*)n->payload, n->len);
        if (n->tot_len == n->len) {
            break;
        }
    }

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

    return ERR_OK;

sanity_error:
    if (state->sanity_errors >= SANITY_MAX) {
        /*
         * 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.
         */
        lcp_close(pcb, "Too many MPPE errors");
    }
    return ERR_BUF;
}

#endif /* PPP_SUPPORT && MPPE_SUPPORT */