arc4.c

/*
 *  An implementation of the ARCFOUR algorithm
 *
 *  Copyright (C) 2006-2015, Kaspter Ju, All Rights Reserved
 *  SPDX-License-Identifier: Apache-2.0
 *
 *  Licensed under the Apache License, Version 2.0 (the "License"); you may
 *  not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 *
 */
/*
 *  The ARCFOUR algorithm was publicly disclosed on 94/09.
 *
 *  http://groups.google.com/group/sci.crypt/msg/10a300c9d21afca0
 */

#include "arc4.h"

#include <stdio.h>
#include <string.h>

/* Implementation that should never be optimized out by the compiler */
static void mbedtls_zeroize(void* v, size_t n)
{
    volatile unsigned char* p = (unsigned char*)v;
    while (n--)
        *p++ = 0;
}

void arc4_init(arc4_context* ctx)
{
    memset(ctx, 0, sizeof(arc4_context));
}

void arc4_free(arc4_context* ctx)
{
    if (ctx == NULL)
        return;

    mbedtls_zeroize(ctx, sizeof(arc4_context));
}

/*
 * ARC4 key schedule
 */
void arc4_setup(arc4_context* ctx, const unsigned char* key,
                unsigned int keylen)
{
    int            i, j, a;
    unsigned int   k;
    unsigned char* m;

    ctx->x = 0;
    ctx->y = 0;
    m      = ctx->m;

    for (i   = 0; i < 256; i++)
        m[i] = (unsigned char)i;

    j = k = 0;

    for (i = 0; i < 256; i++, k++) {
        if (k >= keylen)
            k = 0;

        a    = m[i];
        j    = (j + a + key[k]) & 0xFF;
        m[i] = m[j];
        m[j] = (unsigned char)a;
    }
}

/*
 * ARC4 cipher function
 */
int arc4_crypt(arc4_context* ctx, size_t length, const unsigned char* input,
               unsigned char* output)
{
    int            x, y, a, b;
    size_t         i;
    unsigned char* m;

    x = ctx->x;
    y = ctx->y;
    m = ctx->m;

    for (i = 0; i < length; i++) {
        x = (x + 1) & 0xFF;
        a = m[x];
        y = (y + a) & 0xFF;
        b = m[y];

        m[x] = (unsigned char)b;
        m[y] = (unsigned char)a;

        output[i] = (unsigned char)(input[i] ^ m[(unsigned char)(a + b)]);
    }

    ctx->x = x;
    ctx->y = y;

    return (0);
}

#if defined(ARC4_SELF_TEST)
/*
 * ARC4 tests vectors as posted by Eric Rescorla in sep. 1994:
 *
 * http://groups.google.com/group/comp.security.misc/msg/10a300c9d21afca0
 */
static const unsigned char arc4_test_key[3][8] = {
    { 0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF },
    { 0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF },
    { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 }
};

static const unsigned char arc4_test_pt[3][8] = {
    { 0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF },
    { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 },
    { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 }
};

static const unsigned char arc4_test_ct[3][8] = {
    { 0x75, 0xB7, 0x87, 0x80, 0x99, 0xE0, 0xC5, 0x96 },
    { 0x74, 0x94, 0xC2, 0xE7, 0x10, 0x4B, 0x08, 0x79 },
    { 0xDE, 0x18, 0x89, 0x41, 0xA3, 0x37, 0x5D, 0x3A }
};

/*
 * Checkup routine
 */
int arc4_self_test(int verbose)
{
    int           i, ret = 0;
    unsigned char ibuf[8];
    unsigned char obuf[8];
    arc4_context  ctx;

    arc4_init(&ctx);

    for (i = 0; i < 3; i++) {
        if (verbose != 0)
            printf("  ARC4 test #%d: ", i + 1);

        memcpy(ibuf, arc4_test_pt[i], 8);

        arc4_setup(&ctx, arc4_test_key[i], 8);
        arc4_crypt(&ctx, 8, ibuf, obuf);

        if (memcmp(obuf, arc4_test_ct[i], 8) != 0) {
            if (verbose != 0)
                printf("failed\n");

            ret = 1;
            goto exit;
        }

        if (verbose != 0)
            printf("passed\n");
    }

    if (verbose != 0)
        printf("\n");

exit:
    arc4_free(&ctx);

    return (ret);
}

#endif /* ARC4_SELF_TEST */