lib80211_crypt_wep.c

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/*
 * lib80211 crypt: host-based WEP encryption implementation for lib80211
 *
 * Copyright (c) 2002-2004, Jouni Malinen <[email protected]>
 * Copyright (c) 2008, John W. Linville <[email protected]>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation. See README and COPYING for
 * more details.
 */

#include <linux/err.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/random.h>
#include <linux/scatterlist.h>
#include <linux/skbuff.h>
#include <linux/mm.h>
#include <asm/string.h>

#include <net/lib80211.h>

#include <linux/crypto.h>
#include <linux/crc32.h>

MODULE_AUTHOR("Jouni Malinen");
MODULE_DESCRIPTION("lib80211 crypt: WEP");
MODULE_LICENSE("GPL");

struct lib80211_wep_data {
    u32 iv;
#define WEP_KEY_LEN 13
    u8 key[WEP_KEY_LEN + 1];
    u8 key_len;
    u8 key_idx;
    struct crypto_blkcipher *tx_tfm;
    struct crypto_blkcipher *rx_tfm;
};

static void *lib80211_wep_init(int keyidx)
{
    struct lib80211_wep_data *priv;

    priv = kzalloc(sizeof(*priv), GFP_ATOMIC);
    if (priv == NULL)
        goto fail;
    priv->key_idx = keyidx;

    priv->tx_tfm = crypto_alloc_blkcipher("ecb(arc4)", 0, CRYPTO_ALG_ASYNC);
    if (IS_ERR(priv->tx_tfm)) {
        priv->tx_tfm = NULL;
        goto fail;
    }

    priv->rx_tfm = crypto_alloc_blkcipher("ecb(arc4)", 0, CRYPTO_ALG_ASYNC);
    if (IS_ERR(priv->rx_tfm)) {
        priv->rx_tfm = NULL;
        goto fail;
    }
    /* start WEP IV from a random value */
    get_random_bytes(&priv->iv, 4);

    return priv;

      fail:
    if (priv) {
        if (priv->tx_tfm)
            crypto_free_blkcipher(priv->tx_tfm);
        if (priv->rx_tfm)
            crypto_free_blkcipher(priv->rx_tfm);
        kfree(priv);
    }
    return NULL;
}

static void lib80211_wep_deinit(void *priv)
{
    struct lib80211_wep_data *_priv = priv;
    if (_priv) {
        if (_priv->tx_tfm)
            crypto_free_blkcipher(_priv->tx_tfm);
        if (_priv->rx_tfm)
            crypto_free_blkcipher(_priv->rx_tfm);
    }
    kfree(priv);
}

/* Add WEP IV/key info to a frame that has at least 4 bytes of headroom */
static int lib80211_wep_build_iv(struct sk_buff *skb, int hdr_len,
                   u8 *key, int keylen, void *priv)
{
    struct lib80211_wep_data *wep = priv;
    u32 klen;
    u8 *pos;

    if (skb_headroom(skb) < 4 || skb->len < hdr_len)
        return -1;

    pos = skb_push(skb, 4);
    memmove(pos, pos + 4, hdr_len);
    pos += hdr_len;

    klen = 3 + wep->key_len;

    wep->iv++;

    /* Fluhrer, Mantin, and Shamir have reported weaknesses in the key
     * scheduling algorithm of RC4. At least IVs (KeyByte + 3, 0xff, N)
     * can be used to speedup attacks, so avoid using them. */
    if ((wep->iv & 0xff00) == 0xff00) {
        u8 B = (wep->iv >> 16) & 0xff;
        if (B >= 3 && B < klen)
            wep->iv += 0x0100;
    }

    /* Prepend 24-bit IV to RC4 key and TX frame */
    *pos++ = (wep->iv >> 16) & 0xff;
    *pos++ = (wep->iv >> 8) & 0xff;
    *pos++ = wep->iv & 0xff;
    *pos++ = wep->key_idx << 6;

    return 0;
}

/* Perform WEP encryption on given skb that has at least 4 bytes of headroom
 * for IV and 4 bytes of tailroom for ICV. Both IV and ICV will be transmitted,
 * so the payload length increases with 8 bytes.
 *
 * WEP frame payload: IV + TX key idx, RC4(data), ICV = RC4(CRC32(data))
 */
static int lib80211_wep_encrypt(struct sk_buff *skb, int hdr_len, void *priv)
{
    struct lib80211_wep_data *wep = priv;
    struct blkcipher_desc desc = { .tfm = wep->tx_tfm };
    u32 crc, klen, len;
    u8 *pos, *icv;
    struct scatterlist sg;
    u8 key[WEP_KEY_LEN + 3];

    /* other checks are in lib80211_wep_build_iv */
    if (skb_tailroom(skb) < 4)
        return -1;

    /* add the IV to the frame */
    if (lib80211_wep_build_iv(skb, hdr_len, NULL, 0, priv))
        return -1;

    /* Copy the IV into the first 3 bytes of the key */
    skb_copy_from_linear_data_offset(skb, hdr_len, key, 3);

    /* Copy rest of the WEP key (the secret part) */
    memcpy(key + 3, wep->key, wep->key_len);

    len = skb->len - hdr_len - 4;
    pos = skb->data + hdr_len + 4;
    klen = 3 + wep->key_len;

    /* Append little-endian CRC32 over only the data and encrypt it to produce ICV */
    crc = ~crc32_le(~0, pos, len);
    icv = skb_put(skb, 4);
    icv[0] = crc;
    icv[1] = crc >> 8;
    icv[2] = crc >> 16;
    icv[3] = crc >> 24;

    crypto_blkcipher_setkey(wep->tx_tfm, key, klen);
    sg_init_one(&sg, pos, len + 4);
    return crypto_blkcipher_encrypt(&desc, &sg, &sg, len + 4);
}

/* Perform WEP decryption on given buffer. Buffer includes whole WEP part of
 * the frame: IV (4 bytes), encrypted payload (including SNAP header),
 * ICV (4 bytes). len includes both IV and ICV.
 *
 * Returns 0 if frame was decrypted successfully and ICV was correct and -1 on
 * failure. If frame is OK, IV and ICV will be removed.
 */
static int lib80211_wep_decrypt(struct sk_buff *skb, int hdr_len, void *priv)
{
    struct lib80211_wep_data *wep = priv;
    struct blkcipher_desc desc = { .tfm = wep->rx_tfm };
    u32 crc, klen, plen;
    u8 key[WEP_KEY_LEN + 3];
    u8 keyidx, *pos, icv[4];
    struct scatterlist sg;

    if (skb->len < hdr_len + 8)
        return -1;

    pos = skb->data + hdr_len;
    key[0] = *pos++;
    key[1] = *pos++;
    key[2] = *pos++;
    keyidx = *pos++ >> 6;
    if (keyidx != wep->key_idx)
        return -1;

    klen = 3 + wep->key_len;

    /* Copy rest of the WEP key (the secret part) */
    memcpy(key + 3, wep->key, wep->key_len);

    /* Apply RC4 to data and compute CRC32 over decrypted data */
    plen = skb->len - hdr_len - 8;

    crypto_blkcipher_setkey(wep->rx_tfm, key, klen);
    sg_init_one(&sg, pos, plen + 4);
    if (crypto_blkcipher_decrypt(&desc, &sg, &sg, plen + 4))
        return -7;

    crc = ~crc32_le(~0, pos, plen);
    icv[0] = crc;
    icv[1] = crc >> 8;
    icv[2] = crc >> 16;
    icv[3] = crc >> 24;
    if (memcmp(icv, pos + plen, 4) != 0) {
        /* ICV mismatch - drop frame */
        return -2;
    }

    /* Remove IV and ICV */
    memmove(skb->data + 4, skb->data, hdr_len);
    skb_pull(skb, 4);
    skb_trim(skb, skb->len - 4);

    return 0;
}

static int lib80211_wep_set_key(void *key, int len, u8 * seq, void *priv)
{
    struct lib80211_wep_data *wep = priv;

    if (len < 0 || len > WEP_KEY_LEN)
        return -1;

    memcpy(wep->key, key, len);
    wep->key_len = len;

    return 0;
}

static int lib80211_wep_get_key(void *key, int len, u8 * seq, void *priv)
{
    struct lib80211_wep_data *wep = priv;

    if (len < wep->key_len)
        return -1;

    memcpy(key, wep->key, wep->key_len);

    return wep->key_len;
}

static char *lib80211_wep_print_stats(char *p, void *priv)
{
    struct lib80211_wep_data *wep = priv;
    p += sprintf(p, "key[%d] alg=WEP len=%d\n", wep->key_idx, wep->key_len);
    return p;
}

static struct lib80211_crypto_ops lib80211_crypt_wep = {
    .name = "WEP",
    .init = lib80211_wep_init,
    .deinit = lib80211_wep_deinit,
    .encrypt_mpdu = lib80211_wep_encrypt,
    .decrypt_mpdu = lib80211_wep_decrypt,
    .encrypt_msdu = NULL,
    .decrypt_msdu = NULL,
    .set_key = lib80211_wep_set_key,
    .get_key = lib80211_wep_get_key,
    .print_stats = lib80211_wep_print_stats,
    .extra_mpdu_prefix_len = 4, /* IV */
    .extra_mpdu_postfix_len = 4,    /* ICV */
    .owner = THIS_MODULE,
};

static int __init lib80211_crypto_wep_init(void)
{
    return lib80211_register_crypto_ops(&lib80211_crypt_wep);
}

static void __exit lib80211_crypto_wep_exit(void)
{
    lib80211_unregister_crypto_ops(&lib80211_crypt_wep);
}

module_init(lib80211_crypto_wep_init);
module_exit(lib80211_crypto_wep_exit);