wpa_tkip.c

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/*
 * Copyright (c) 2009 Joshua Oreman <oremanj@rwcr.net>.
 *
 * 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 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., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

FILE_LICENCE ( GPL2_OR_LATER );

#include <gpxe/net80211.h>
#include <gpxe/crypto.h>
#include <gpxe/hmac.h>
#include <gpxe/sha1.h>
#include <gpxe/md5.h>
#include <gpxe/crc32.h>
#include <gpxe/arc4.h>
#include <gpxe/wpa.h>
#include <byteswap.h>
#include <errno.h>

/** @file
 *
 * Backend for WPA using the TKIP encryption standard.
 */

/** Context for one direction of TKIP, either encryption or decryption */
struct tkip_dir_ctx
{
        /** High 32 bits of last sequence counter value used */
        u32 tsc_hi;

        /** Low 32 bits of last sequence counter value used */
        u16 tsc_lo;

        /** MAC address used to derive TTAK */
        u8 mac[ETH_ALEN];

        /** If TRUE, TTAK is valid */
        u16 ttak_ok;

        /** TKIP-mixed transmit address and key, depends on tsc_hi and MAC */
        u16 ttak[5];
};

/** Context for TKIP encryption and decryption */
struct tkip_ctx
{
        /** Temporal key to use */
        struct tkip_tk tk;

        /** State for encryption */
        struct tkip_dir_ctx enc;

        /** State for decryption */
        struct tkip_dir_ctx dec;
};

/** Header structure at the beginning of TKIP frame data */
struct tkip_head
{
        u8 tsc1;                /**< High byte of low 16 bits of TSC */
        u8 seed1;               /**< Second byte of WEP seed */
        u8 tsc0;                /**< Low byte of TSC */
        u8 kid;                 /**< Key ID and ExtIV byte */
        u32 tsc_hi;             /**< High 32 bits of TSC, as an ExtIV */
} __attribute__ (( packed ));


/** TKIP header overhead (IV + KID + ExtIV) */
#define TKIP_HEAD_LEN   8

/** TKIP trailer overhead (MIC + ICV) [assumes unfragmented] */
#define TKIP_FOOT_LEN   12

/** TKIP MIC length */
#define TKIP_MIC_LEN    8

/** TKIP ICV length */
#define TKIP_ICV_LEN    4


/** TKIP S-box */
static const u16 Sbox[256] = {
        0xC6A5, 0xF884, 0xEE99, 0xF68D, 0xFF0D, 0xD6BD, 0xDEB1, 0x9154,
        0x6050, 0x0203, 0xCEA9, 0x567D, 0xE719, 0xB562, 0x4DE6, 0xEC9A,
        0x8F45, 0x1F9D, 0x8940, 0xFA87, 0xEF15, 0xB2EB, 0x8EC9, 0xFB0B,
        0x41EC, 0xB367, 0x5FFD, 0x45EA, 0x23BF, 0x53F7, 0xE496, 0x9B5B,
        0x75C2, 0xE11C, 0x3DAE, 0x4C6A, 0x6C5A, 0x7E41, 0xF502, 0x834F,
        0x685C, 0x51F4, 0xD134, 0xF908, 0xE293, 0xAB73, 0x6253, 0x2A3F,
        0x080C, 0x9552, 0x4665, 0x9D5E, 0x3028, 0x37A1, 0x0A0F, 0x2FB5,
        0x0E09, 0x2436, 0x1B9B, 0xDF3D, 0xCD26, 0x4E69, 0x7FCD, 0xEA9F,
        0x121B, 0x1D9E, 0x5874, 0x342E, 0x362D, 0xDCB2, 0xB4EE, 0x5BFB,
        0xA4F6, 0x764D, 0xB761, 0x7DCE, 0x527B, 0xDD3E, 0x5E71, 0x1397,
        0xA6F5, 0xB968, 0x0000, 0xC12C, 0x4060, 0xE31F, 0x79C8, 0xB6ED,
        0xD4BE, 0x8D46, 0x67D9, 0x724B, 0x94DE, 0x98D4, 0xB0E8, 0x854A,
        0xBB6B, 0xC52A, 0x4FE5, 0xED16, 0x86C5, 0x9AD7, 0x6655, 0x1194,
        0x8ACF, 0xE910, 0x0406, 0xFE81, 0xA0F0, 0x7844, 0x25BA, 0x4BE3,
        0xA2F3, 0x5DFE, 0x80C0, 0x058A, 0x3FAD, 0x21BC, 0x7048, 0xF104,
        0x63DF, 0x77C1, 0xAF75, 0x4263, 0x2030, 0xE51A, 0xFD0E, 0xBF6D,
        0x814C, 0x1814, 0x2635, 0xC32F, 0xBEE1, 0x35A2, 0x88CC, 0x2E39,
        0x9357, 0x55F2, 0xFC82, 0x7A47, 0xC8AC, 0xBAE7, 0x322B, 0xE695,
        0xC0A0, 0x1998, 0x9ED1, 0xA37F, 0x4466, 0x547E, 0x3BAB, 0x0B83,
        0x8CCA, 0xC729, 0x6BD3, 0x283C, 0xA779, 0xBCE2, 0x161D, 0xAD76,
        0xDB3B, 0x6456, 0x744E, 0x141E, 0x92DB, 0x0C0A, 0x486C, 0xB8E4,
        0x9F5D, 0xBD6E, 0x43EF, 0xC4A6, 0x39A8, 0x31A4, 0xD337, 0xF28B,
        0xD532, 0x8B43, 0x6E59, 0xDAB7, 0x018C, 0xB164, 0x9CD2, 0x49E0,
        0xD8B4, 0xACFA, 0xF307, 0xCF25, 0xCAAF, 0xF48E, 0x47E9, 0x1018,
        0x6FD5, 0xF088, 0x4A6F, 0x5C72, 0x3824, 0x57F1, 0x73C7, 0x9751,
        0xCB23, 0xA17C, 0xE89C, 0x3E21, 0x96DD, 0x61DC, 0x0D86, 0x0F85,
        0xE090, 0x7C42, 0x71C4, 0xCCAA, 0x90D8, 0x0605, 0xF701, 0x1C12,
        0xC2A3, 0x6A5F, 0xAEF9, 0x69D0, 0x1791, 0x9958, 0x3A27, 0x27B9,
        0xD938, 0xEB13, 0x2BB3, 0x2233, 0xD2BB, 0xA970, 0x0789, 0x33A7,
        0x2DB6, 0x3C22, 0x1592, 0xC920, 0x8749, 0xAAFF, 0x5078, 0xA57A,
        0x038F, 0x59F8, 0x0980, 0x1A17, 0x65DA, 0xD731, 0x84C6, 0xD0B8,
        0x82C3, 0x29B0, 0x5A77, 0x1E11, 0x7BCB, 0xA8FC, 0x6DD6, 0x2C3A,
};

/**
 * Perform S-box mapping on a 16-bit value
 *
 * @v v         Value to perform S-box mapping on
 * @ret Sv      S-box mapped value
 */
static inline u16 S ( u16 v )
{
        return Sbox[v & 0xFF] ^ swap16 ( Sbox[v >> 8] );
}

/**
 * Rotate 16-bit value right
 *
 * @v v         Value to rotate
 * @v bits      Number of bits to rotate by
 * @ret rotv    Rotated value
 */
static inline u16 ror16 ( u16 v, int bits )
{
        return ( v >> bits ) | ( v << ( 16 - bits ) );
}

/**
 * Rotate 32-bit value right
 *
 * @v v         Value to rotate
 * @v bits      Number of bits to rotate by
 * @ret rotv    Rotated value
 */
static inline u32 ror32 ( u32 v, int bits )
{
        return ( v >> bits ) | ( v << ( 32 - bits ) );
}

/**
 * Rotate 32-bit value left
 *
 * @v v         Value to rotate
 * @v bits      Number of bits to rotate by
 * @ret rotv    Rotated value
 */
static inline u32 rol32 ( u32 v, int bits )
{
        return ( v << bits ) | ( v >> ( 32 - bits ) );
}


/**
 * Initialise TKIP state and install key
 *
 * @v crypto    TKIP cryptosystem structure
 * @v key       Pointer to tkip_tk to install
 * @v keylen    Length of key (32 bytes)
 * @v rsc       Initial receive sequence counter
 */
static int tkip_init ( struct net80211_crypto *crypto, const void *key,
                       int keylen, const void *rsc )
{
        struct tkip_ctx *ctx = crypto->priv;
        const u8 *rscb = rsc;

        if ( keylen != sizeof ( ctx->tk ) )
                return -EINVAL;

        if ( rscb ) {
                ctx->dec.tsc_lo =   ( rscb[1] <<  8 ) |   rscb[0];
                ctx->dec.tsc_hi = ( ( rscb[5] << 24 ) | ( rscb[4] << 16 ) |
                                    ( rscb[3] <<  8 ) |   rscb[2] );
        }

        memcpy ( &ctx->tk, key, sizeof ( ctx->tk ) );

        return 0;
}

/**
 * Perform TKIP key mixing, phase 1
 *
 * @v dctx      TKIP directional context
 * @v tk        TKIP temporal key
 * @v mac       MAC address of transmitter
 *
 * This recomputes the TTAK in @a dctx if necessary, and sets
 * @c dctx->ttak_ok.
 */
static void tkip_mix_1 ( struct tkip_dir_ctx *dctx, struct tkip_tk *tk, u8 *mac )
{
        int i, j;

        if ( dctx->ttak_ok && ! memcmp ( mac, dctx->mac, ETH_ALEN ) )
                return;

        memcpy ( dctx->mac, mac, ETH_ALEN );

        dctx->ttak[0] = dctx->tsc_hi & 0xFFFF;
        dctx->ttak[1] = dctx->tsc_hi >> 16;
        dctx->ttak[2] = ( mac[1] << 8 ) | mac[0];
        dctx->ttak[3] = ( mac[3] << 8 ) | mac[2];
        dctx->ttak[4] = ( mac[5] << 8 ) | mac[4];

        for ( i = 0; i < 8; i++ ) {
                j = 2 * ( i & 1 );

                dctx->ttak[0] += S ( dctx->ttak[4] ^ ( ( tk->key[1 + j] << 8 ) |
                                                         tk->key[0 + j] ) );
                dctx->ttak[1] += S ( dctx->ttak[0] ^ ( ( tk->key[5 + j] << 8 ) |
                                                         tk->key[4 + j] ) );
                dctx->ttak[2] += S ( dctx->ttak[1] ^ ( ( tk->key[9 + j] << 8 ) |
                                                         tk->key[8 + j] ) );
                dctx->ttak[3] += S ( dctx->ttak[2] ^ ( ( tk->key[13+ j] << 8 ) |
                                                         tk->key[12+ j] ) );
                dctx->ttak[4] += S ( dctx->ttak[3] ^ ( ( tk->key[1 + j] << 8 ) |
                                                         tk->key[0 + j] ) ) + i;
        }

        dctx->ttak_ok = 1;
}

/**
 * Perform TKIP key mixing, phase 2
 *
 * @v dctx      TKIP directional context
 * @v tk        TKIP temporal key
 * @ret key     ARC4 key, 16 bytes long
 */
static void tkip_mix_2 ( struct tkip_dir_ctx *dctx, struct tkip_tk *tk,
                         void *key )
{
        u8 *kb = key;
        u16 ppk[6];
        int i;

        memcpy ( ppk, dctx->ttak, sizeof ( dctx->ttak ) );
        ppk[5] = dctx->ttak[4] + dctx->tsc_lo;

        ppk[0] += S ( ppk[5] ^ ( ( tk->key[1] << 8 ) | tk->key[0] ) );
        ppk[1] += S ( ppk[0] ^ ( ( tk->key[3] << 8 ) | tk->key[2] ) );
        ppk[2] += S ( ppk[1] ^ ( ( tk->key[5] << 8 ) | tk->key[4] ) );
        ppk[3] += S ( ppk[2] ^ ( ( tk->key[7] << 8 ) | tk->key[6] ) );
        ppk[4] += S ( ppk[3] ^ ( ( tk->key[9] << 8 ) | tk->key[8] ) );
        ppk[5] += S ( ppk[4] ^ ( ( tk->key[11] << 8 ) | tk->key[10] ) );

        ppk[0] += ror16 ( ppk[5] ^ ( ( tk->key[13] << 8 ) | tk->key[12] ), 1 );
        ppk[1] += ror16 ( ppk[0] ^ ( ( tk->key[15] << 8 ) | tk->key[14] ), 1 );
        ppk[2] += ror16 ( ppk[1], 1 );
        ppk[3] += ror16 ( ppk[2], 1 );
        ppk[4] += ror16 ( ppk[3], 1 );
        ppk[5] += ror16 ( ppk[4], 1 );

        kb[0] = dctx->tsc_lo >> 8;
        kb[1] = ( ( dctx->tsc_lo >> 8 ) | 0x20 ) & 0x7F;
        kb[2] = dctx->tsc_lo & 0xFF;
        kb[3] = ( ( ppk[5] ^ ( ( tk->key[1] << 8 ) | tk->key[0] ) ) >> 1 )
                & 0xFF;

        for ( i = 0; i < 6; i++ ) {
                kb[4 + 2*i] = ppk[i] & 0xFF;
                kb[5 + 2*i] = ppk[i] >> 8;
        }
}

/**
 * Update Michael message integrity code based on next 32-bit word of data
 *
 * @v V         Michael code state (two 32-bit words)
 * @v word      Next 32-bit word of data
 */
static void tkip_feed_michael ( u32 *V, u32 word )
{
        V[0] ^= word;
        V[1] ^= rol32 ( V[0], 17 );
        V[0] += V[1];
        V[1] ^= ( ( V[0] & 0xFF00FF00 ) >> 8 ) | ( ( V[0] & 0x00FF00FF ) << 8 );
        V[0] += V[1];
        V[1] ^= rol32 ( V[0], 3 );
        V[0] += V[1];
        V[1] ^= ror32 ( V[0], 2 );
        V[0] += V[1];
}

/**
 * Calculate Michael message integrity code
 *
 * @v key       MIC key to use (8 bytes)
 * @v da        Destination link-layer address
 * @v sa        Source link-layer address
 * @v data      Start of data to calculate over
 * @v len       Length of header + data
 * @ret mic     Calculated Michael MIC (8 bytes)
 */
static void tkip_michael ( const void *key, const void *da, const void *sa,
                           const void *data, size_t len, void *mic )
{
        u32 V[2];               /* V[0] = "l", V[1] = "r" in 802.11 */
        union {
                u8 byte[12];
                u32 word[3];
        } cap;
        const u8 *ptr = data;
        const u8 *end = ptr + len;
        int i;

        memcpy ( V, key, sizeof ( V ) );
        V[0] = le32_to_cpu ( V[0] );
        V[1] = le32_to_cpu ( V[1] );

        /* Feed in header (we assume non-QoS, so Priority = 0) */
        memcpy ( &cap.byte[0], da, ETH_ALEN );
        memcpy ( &cap.byte[6], sa, ETH_ALEN );
        tkip_feed_michael ( V, le32_to_cpu ( cap.word[0] ) );
        tkip_feed_michael ( V, le32_to_cpu ( cap.word[1] ) );
        tkip_feed_michael ( V, le32_to_cpu ( cap.word[2] ) );
        tkip_feed_michael ( V, 0 );

        /* Feed in data */
        while ( ptr + 4 <= end ) {
                tkip_feed_michael ( V, le32_to_cpu ( *( u32 * ) ptr ) );
                ptr += 4;
        }

        /* Add unaligned part and padding */
        for ( i = 0; ptr < end; i++ )
                cap.byte[i] = *ptr++;
        cap.byte[i++] = 0x5a;
        for ( ; i < 8; i++ )
                cap.byte[i] = 0;

        /* Feed in padding */
        tkip_feed_michael ( V, le32_to_cpu ( cap.word[0] ) );
        tkip_feed_michael ( V, le32_to_cpu ( cap.word[1] ) );

        /* Output MIC */
        V[0] = cpu_to_le32 ( V[0] );
        V[1] = cpu_to_le32 ( V[1] );
        memcpy ( mic, V, sizeof ( V ) );
}

/**
 * Encrypt a packet using TKIP
 *
 * @v crypto    TKIP cryptosystem
 * @v iob       I/O buffer containing cleartext packet
 * @ret eiob    I/O buffer containing encrypted packet
 */
static struct io_buffer * tkip_encrypt ( struct net80211_crypto *crypto,
                                         struct io_buffer *iob )
{
        struct tkip_ctx *ctx = crypto->priv;
        struct ieee80211_frame *hdr = iob->data;
        struct io_buffer *eiob;
        struct arc4_ctx arc4;
        u8 key[16];
        struct tkip_head head;
        u8 mic[8];
        u32 icv;
        const int hdrlen = IEEE80211_TYP_FRAME_HEADER_LEN;
        int datalen = iob_len ( iob ) - hdrlen;

        ctx->enc.tsc_lo++;
        if ( ctx->enc.tsc_lo == 0 ) {
                ctx->enc.tsc_hi++;
                ctx->enc.ttak_ok = 0;
        }

        tkip_mix_1 ( &ctx->enc, &ctx->tk, hdr->addr2 );
        tkip_mix_2 ( &ctx->enc, &ctx->tk, key );

        eiob = alloc_iob ( iob_len ( iob ) + TKIP_HEAD_LEN + TKIP_FOOT_LEN );
        if ( ! eiob )
                return NULL;

        /* Copy frame header */
        memcpy ( iob_put ( eiob, hdrlen ), iob->data, hdrlen );
        hdr = eiob->data;
        hdr->fc |= IEEE80211_FC_PROTECTED;

        /* Fill in IV and key ID byte, and extended IV */
        memcpy ( &head, key, 3 );
        head.kid = 0x20;                /* have Extended IV, key ID 0 */
        head.tsc_hi = cpu_to_le32 ( ctx->enc.tsc_hi );
        memcpy ( iob_put ( eiob, sizeof ( head ) ), &head, sizeof ( head ) );

        /* Copy and encrypt the data */
        cipher_setkey ( &arc4_algorithm, &arc4, key, 16 );
        cipher_encrypt ( &arc4_algorithm, &arc4, iob->data + hdrlen,
                         iob_put ( eiob, datalen ), datalen );

        /* Add MIC */
        hdr = iob->data;
        tkip_michael ( &ctx->tk.mic.tx, hdr->addr3, hdr->addr2,
                       iob->data + hdrlen, datalen, mic );
        cipher_encrypt ( &arc4_algorithm, &arc4, mic,
                         iob_put ( eiob, sizeof ( mic ) ), sizeof ( mic ) );

        /* Add ICV */
        icv = crc32_le ( ~0, iob->data + hdrlen, datalen );
        icv = crc32_le ( icv, mic, sizeof ( mic ) );
        icv = cpu_to_le32 ( ~icv );
        cipher_encrypt ( &arc4_algorithm, &arc4, &icv,
                         iob_put ( eiob, TKIP_ICV_LEN ), TKIP_ICV_LEN );

        DBGC2 ( ctx, "WPA-TKIP %p: encrypted packet %p -> %p\n", ctx,
                iob, eiob );

        return eiob;
}

/**
 * Decrypt a packet using TKIP
 *
 * @v crypto    TKIP cryptosystem
 * @v eiob      I/O buffer containing encrypted packet
 * @ret iob     I/O buffer containing cleartext packet
 */
static struct io_buffer * tkip_decrypt ( struct net80211_crypto *crypto,
                                         struct io_buffer *eiob )
{
        struct tkip_ctx *ctx = crypto->priv;
        struct ieee80211_frame *hdr;
        struct io_buffer *iob;
        const int hdrlen = IEEE80211_TYP_FRAME_HEADER_LEN;
        int datalen = iob_len ( eiob ) - hdrlen - TKIP_HEAD_LEN - TKIP_FOOT_LEN;
        struct tkip_head *head;
        struct arc4_ctx arc4;
        u16 rx_tsc_lo;
        u8 key[16];
        u8 mic[8];
        u32 icv, crc;

        iob = alloc_iob ( hdrlen + datalen + TKIP_FOOT_LEN );
        if ( ! iob )
                return NULL;

        /* Copy frame header */
        memcpy ( iob_put ( iob, hdrlen ), eiob->data, hdrlen );
        hdr = iob->data;
        hdr->fc &= ~IEEE80211_FC_PROTECTED;

        /* Check and update TSC */
        head = eiob->data + hdrlen;
        rx_tsc_lo = ( head->tsc1 << 8 ) | head->tsc0;

        if ( head->tsc_hi < ctx->dec.tsc_hi ||
             ( head->tsc_hi == ctx->dec.tsc_hi &&
               rx_tsc_lo <= ctx->dec.tsc_lo ) ) {
                DBGC ( ctx, "WPA-TKIP %p: packet received out of order "
                       "(%08x:%04x <= %08x:%04x)\n", ctx, head->tsc_hi,
                       rx_tsc_lo, ctx->dec.tsc_hi, ctx->dec.tsc_lo );
                free_iob ( iob );
                return NULL;
        }
        ctx->dec.tsc_lo = rx_tsc_lo;
        if ( ctx->dec.tsc_hi != head->tsc_hi ) {
                ctx->dec.ttak_ok = 0;
                ctx->dec.tsc_hi = head->tsc_hi;
        }

        /* Calculate key */
        tkip_mix_1 ( &ctx->dec, &ctx->tk, hdr->addr2 );
        tkip_mix_2 ( &ctx->dec, &ctx->tk, key );

        /* Copy-decrypt data, MIC, ICV */
        cipher_setkey ( &arc4_algorithm, &arc4, key, 16 );
        cipher_decrypt ( &arc4_algorithm, &arc4,
                         eiob->data + hdrlen + TKIP_HEAD_LEN,
                         iob_put ( iob, datalen ), datalen + TKIP_FOOT_LEN );

        /* Check ICV */
        icv = le32_to_cpu ( *( u32 * ) ( iob->tail + TKIP_MIC_LEN ) );
        crc = ~crc32_le ( ~0, iob->data + hdrlen, datalen + TKIP_MIC_LEN );
        if ( crc != icv ) {
                DBGC ( ctx, "WPA-TKIP %p CRC mismatch: expect %08x, get %08x\n",
                       ctx, icv, crc );
                free_iob ( iob );
                return NULL;
        }

        /* Check MIC */
        tkip_michael ( &ctx->tk.mic.rx, hdr->addr1, hdr->addr3,
                       iob->data + hdrlen, datalen, mic );
        if ( memcmp ( mic, iob->tail, TKIP_MIC_LEN ) != 0 ) {
                DBGC ( ctx, "WPA-TKIP %p ALERT! MIC failure\n", ctx );
                /* XXX we should do the countermeasures here */
                free_iob ( iob );
                return NULL;
        }

        DBGC2 ( ctx, "WPA-TKIP %p: decrypted packet %p -> %p\n", ctx,
                eiob, iob );

        return iob;
}

/** TKIP cryptosystem */
struct net80211_crypto tkip_crypto __net80211_crypto = {
        .algorithm = NET80211_CRYPT_TKIP,
        .init = tkip_init,
        .encrypt = tkip_encrypt,
        .decrypt = tkip_decrypt,
        .priv_len = sizeof ( struct tkip_ctx ),
};




/**
 * Calculate HMAC-MD5 MIC for EAPOL-Key frame
 *
 * @v kck       Key Confirmation Key, 16 bytes
 * @v msg       Message to calculate MIC over
 * @v len       Number of bytes to calculate MIC over
 * @ret mic     Calculated MIC, 16 bytes long
 */
static void tkip_kie_mic ( const void *kck, const void *msg, size_t len,
                           void *mic )
{
        struct md5_ctx md5;
        u8 kckb[16];
        size_t kck_len = 16;

        memcpy ( kckb, kck, kck_len );

        hmac_init ( &md5_algorithm, &md5, kckb, &kck_len );
        hmac_update ( &md5_algorithm, &md5, msg, len );
        hmac_final ( &md5_algorithm, &md5, kckb, &kck_len, mic );
}

/**
 * Decrypt key data in EAPOL-Key frame
 *
 * @v kek       Key Encryption Key, 16 bytes
 * @v iv        Initialisation vector, 16 bytes
 * @v msg       Message to decrypt
 * @v len       Length of message
 * @ret msg     Decrypted message in place of original
 * @ret len     Unchanged
 * @ret rc      Always 0 for success
 */
static int tkip_kie_decrypt ( const void *kek, const void *iv,
                              void *msg, u16 *len )
{
        u8 key[32];
        memcpy ( key, iv, 16 );
        memcpy ( key + 16, kek, 16 );

        arc4_skip ( key, 32, 256, msg, msg, *len );

        return 0;
}


/** TKIP-style key integrity and encryption handler */
struct wpa_kie tkip_kie __wpa_kie = {
        .version = EAPOL_KEY_VERSION_WPA,
        .mic = tkip_kie_mic,
        .decrypt = tkip_kie_decrypt,
};