solution.c

#include <stdio.h>
#include <memory.h>
#include <assert.h>

typedef unsigned char uint8;
typedef unsigned int uint32;

#ifdef __cplusplus
extern "C" {
#endif

typedef struct {
    uint8 se[256], sd[256];
    uint32 pose, posd;
    uint8 te, td;
} rc4_ctx;

void rc4_ks(rc4_ctx *ctx, const uint8 *key, uint32 key_len);
void rc4_encrypt(rc4_ctx *ctx, const uint8 *src, uint8 *dst, uint32 len);
void rc4_decrypt(rc4_ctx *ctx, const uint8 *src, uint8 *dst, uint32 len);

static const uint8 rc4_table[256] = {
    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b,
    0x0c, 0x0d, 0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
    0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, 0x20, 0x21, 0x22, 0x23,
    0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
    0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a, 0x3b,
    0x3c, 0x3d, 0x3e, 0x3f, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47,
    0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f, 0x50, 0x51, 0x52, 0x53,
    0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5a, 0x5b, 0x5c, 0x5d, 0x5e, 0x5f,
    0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6a, 0x6b,
    0x6c, 0x6d, 0x6e, 0x6f, 0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77,
    0x78, 0x79, 0x7a, 0x7b, 0x7c, 0x7d, 0x7e, 0x7f, 0x80, 0x81, 0x82, 0x83,
    0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
    0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9a, 0x9b,
    0x9c, 0x9d, 0x9e, 0x9f, 0xa0, 0xa1, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
    0xa8, 0xa9, 0xaa, 0xab, 0xac, 0xad, 0xae, 0xaf, 0xb0, 0xb1, 0xb2, 0xb3,
    0xb4, 0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba, 0xbb, 0xbc, 0xbd, 0xbe, 0xbf,
    0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xcb,
    0xcc, 0xcd, 0xce, 0xcf, 0xd0, 0xd1, 0xd2, 0xd3, 0xd4, 0xd5, 0xd6, 0xd7,
    0xd8, 0xd9, 0xda, 0xdb, 0xdc, 0xdd, 0xde, 0xdf, 0xe0, 0xe1, 0xe2, 0xe3,
    0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef,
    0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8, 0xf9, 0xfa, 0xfb,
    0xfc, 0xfd, 0xfe, 0xff};

#define SWAP(x, y) \
{                  \
    tmp = x;       \
    x = y;         \
    y = tmp;       \
}

#define RC4_CRYPT()                                             \
{                                                               \
    t += s[(uint8) i];                                          \
    SWAP(s[(uint8) i], s[t]);                                   \
    new_dst[i] = new_src[i] ^ s[(uint8) (s[(uint8) i] + s[t])]; \
}

void rc4_ks(rc4_ctx *ctx, const uint8 *key, uint32 key_len)
{
    uint32 i;
    uint8 *s;
    uint8 t, tmp;

    t = 0;
    s = ctx->se;

    assert(key_len > 0 && key_len <= 256);

    ctx->pose = 1;
    ctx->posd = 1;
    ctx->te = 0;
    ctx->td = 0;

    memcpy(s, rc4_table, 256);

    for (i = 0; i < 256; i++) {
        t += s[i] + key[i % key_len];
        SWAP(s[i], s[t]);
    }

    memcpy(ctx->sd, s, 256);
}

void rc4_encrypt(rc4_ctx *ctx, const uint8 *src, uint8 *dst, uint32 len)
{
    uint32 i;
    uint32 pos;
    const uint8 *new_src;
    uint8 *s, *new_dst;
    uint8 t, tmp;

    pos = ctx->pose;
    s = ctx->se;
    t = ctx->te;

    new_src = src - pos;
    new_dst = dst - pos;

    for (i = pos; i < len + pos; i++) {
        RC4_CRYPT();
    }

    ctx->pose = i;
    ctx->te = t;
}

void rc4_decrypt(rc4_ctx *ctx, const uint8 *src, uint8 *dst, uint32 len)
{
    uint32 i;
    uint32 pos;
    const uint8 *new_src;
    uint8 *s, *new_dst;
    uint8 t, tmp;

    pos = ctx->posd;
    s = ctx->sd;
    t = ctx->td;

    new_src = src - pos;
    new_dst = dst - pos;

    for (i = pos; i < len + pos; i++) {
        RC4_CRYPT();
    }

    ctx->posd = i;
    ctx->td = t;
}


int main() {
  rc4_ctx ctx;
  uint8 key1[8] = {0x20, 0x23, 0x20, 0x23, 0x20, 0x23, 0x20, 0x23};
  rc4_ks(&ctx, key1, sizeof(key1));
  char buf[81] = {0} ;
  const uint8 dst[80] = {0x62, 0x5c, 0x07, 0x52, 0x36, 0x9c, 0x2b, 0x7f, 0x2c, 0x05, 0xc3, 0x79, 0x8f, 0x4e, 0x15, 0xa7, 0xbe, 0x3e, 0x31, 0x8b, 0xe9, 0x02, 0xc9, 0xbd, 0x2a, 0xe2, 0xf0, 0x21, 0x33, 0x4f, 0x05, 0x3b, 0x7e, 0xee, 0xbd, 0xdb, 0x12, 0x99, 0xb6, 0x64, 0x95, 0x9e, 0xee, 0x53, 0x41, 0x25, 0xac, 0x87, 0x6a, 0x0c, 0x70, 0xba, 0xed, 0x06, 0x7c, 0x5b, 0xcc, 0x07, 0x68, 0x62, 0x15, 0xcb, 0x90, 0x90, 0x26, 0xae, 0xdf, 0xd2, 0x26, 0xa7, 0x25, 0x5a, 0xaf, 0x35, 0x6c, 0x3c, 0x6b, 0x62, 0xdc, 0x57};
  rc4_decrypt(&ctx, dst, buf, 80);
  
  printf("Keys: %s\n", buf);
  return 0;
}