ssharcf.c

/*
 * Arcfour (RC4) implementation for PuTTY.
 *
 * Coded from Schneier.
 */

#include <assert.h>
#include "ssh.h"

typedef struct {
  unsigned char i, j, s[256];
} ArcfourContext;

static void arcfour_block(void *handle, unsigned char *blk, int len)
{
  ArcfourContext *ctx = (ArcfourContext *)handle;
  unsigned k;
  unsigned char tmp, i, j, *s;

  s = ctx->s;
  i = ctx->i;
  j = ctx->j;
  for (k = 0; (int)k < len; k++) {
    i = (i + 1) & 0xff;
    j = (j + s[i]) & 0xff;
    tmp = s[i];
    s[i] = s[j];
    s[j] = tmp;
    blk[k] ^= s[(s[i] + s[j]) & 0xff];
  }
  ctx->i = i;
  ctx->j = j;
}

static void arcfour_setkey(ArcfourContext *ctx,
                           unsigned char const *key,
                           unsigned keybytes)
{
  unsigned char tmp, k[256], *s;
  unsigned i, j;

  s = ctx->s;
  assert(keybytes <= 256);
  ctx->i = ctx->j = 0;
  for (i = 0; i < 256; i++) {
    s[i] = i;
    k[i] = key[i % keybytes];
  }
  j = 0;
  for (i = 0; i < 256; i++) {
    j = (j + s[i] + k[i]) & 0xff;
    tmp = s[i];
    s[i] = s[j];
    s[j] = tmp;
  }
}

/* -- Interface with PuTTY -- */

/*
 * We don't implement Arcfour in SSH-1 because it's utterly insecure in
 * several ways.  See CERT Vulnerability Notes VU#25309, VU#665372,
 * and VU#565052.
 *
 * We don't implement the "arcfour" algorithm in SSH-2 because it doesn't
 * stir the cipher state before emitting keystream, and hence is likely
 * to leak data about the key.
 */

static void *arcfour_make_context(void)
{
  return snew(ArcfourContext);
}

static void arcfour_free_context(void *handle)
{
  sfree(handle);
}

static void arcfour_stir(ArcfourContext *ctx)
{
  unsigned char *junk = snewn(1536, unsigned char);
  memset(junk, 0, 1536);
  arcfour_block(ctx, junk, 1536);
  smemclr(junk, 1536);
  sfree(junk);
}

static void arcfour128_key(void *handle, unsigned char *key)
{
  ArcfourContext *ctx = (ArcfourContext *)handle;
  arcfour_setkey(ctx, key, 16);
  arcfour_stir(ctx);
}

static void arcfour256_key(void *handle, unsigned char *key)
{
  ArcfourContext *ctx = (ArcfourContext *)handle;
  arcfour_setkey(ctx, key, 32);
  arcfour_stir(ctx);
}

static void arcfour_iv(void *handle, unsigned char *key)
{
}

const struct ssh2_cipher ssh_arcfour128_ssh2 = {arcfour_make_context,
                                                arcfour_free_context,
                                                arcfour_iv,
                                                arcfour128_key,
                                                arcfour_block,
                                                arcfour_block,
                                                NULL,
                                                NULL,
                                                "arcfour128",
                                                1,
                                                128,
                                                16,
                                                0,
                                                "Arcfour-128",
                                                NULL};

const struct ssh2_cipher ssh_arcfour256_ssh2 = {arcfour_make_context,
                                                arcfour_free_context,
                                                arcfour_iv,
                                                arcfour256_key,
                                                arcfour_block,
                                                arcfour_block,
                                                NULL,
                                                NULL,
                                                "arcfour256",
                                                1,
                                                256,
                                                32,
                                                0,
                                                "Arcfour-256",
                                                NULL};

static const struct ssh2_cipher *const arcfour_list[] = {
    &ssh_arcfour256_ssh2,
    &ssh_arcfour128_ssh2,
};

const struct ssh2_ciphers ssh2_arcfour = {
    sizeof(arcfour_list) / sizeof(*arcfour_list), arcfour_list};