Date: Fri, 13 Dec 2013 07:41:09 -0800
From: Ben Pfaff <blp@...stanford.edu>
To: full-disclosure@...ts.grok.org.uk
Subject: cryptographic flaws in IBM SPSS data file
	encryption

IBM SPSS 21 and later support a form of encrypted data files.  The
cryptographic scheme used, which is not publicly documented, has
recently come to my attention.  It is flawed enough that I feel I must
present it publicly.

The encrypted data file format is identical to the pre-existing
plaintext file format (which isn't important here), except that each
16-byte block is encrypted with AES-256 in ECB mode.  The AES-256 key
is derived from a password by a single AES-256 CMAC operation, as:

        CMAC-AES-256(password, constant)

where password is the literal password typed by the user (padded on
the right with zeros to fill out a 32-byte AES-256 key, since CMAC
needs a real cryptographic key not just any random string of bytes
like HMAC) and constant is a particular 73-byte constant.  This only
produces a 16-byte result.  AES-256 needs a 32-byte key, so the
16-byte result is repeated twice to expand it to 32 bytes.

(I think that the authors of the implementation must have thought they
were doing something smart, because the 73-byte constant is in the
right form for the NIST SP 800-108 key derivation function in counter
mode.  But that KDF is meant for deriving one cryptographic key from
another, not from a password.)

The problems I see:

    - ECB mode.

    - Cheap password derivation function (single round of CMAC)
      instead of an intentionally expensive function like PBKDF2 with
      thousands of iterations.

    - No salt, and the first 16 bytes of plaintext are essentially
      constant (as a magic number).  I believe that this means rainbow
      tables are possible.

    - Password is silently truncated after 10 bytes, limiting actual
      entropy in the key to 80 bits at the very most and probably more
      like 40 to 60 bits realistically.  (AES-256 is obviously
      overkill.)

Other issues:

    - Governments, universities, and companies use SPSS to analyze
      survey data that sometimes contain people's personal information
      that must not become public, so confidentiality is actually
      important here.  Mitigating that a little, this encrypted format
      is new in the last year or two, and a lot of organizations don't
      upgrade SPSS frequently because it is very expensive, so the
      encrypted format may not be widely used yet.

    - SPSS documentation talks about "encrypted passwords" that can be
      used in SPSS program syntax in place of plaintext passwords.
      However, calling these passwords "encrypted" is a misnomer,
      because the encoding algorithm is simple, fixed, and unkeyed.

I'm appending a program that decrypts such a data file, given the
plaintext or "encrypted" password.  Compile and link against libcrypto
(from OpenSSL).

--8<--------------------------cut here-------------------------->8--

#include <assert.h>
#include <errno.h>
#include <stdbool.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>

#include <openssl/aes.h>
#include <openssl/cmac.h>

/* Initializes AES from PASSWORD.  Returns true if CIPHERTEXT is the first
   ciphertext block in an encrypted .sav file for PASSWORD, false if PASSWORD
   is wrong. */
static bool
init(const char *password, const uint8_t ciphertext[16], AES_KEY *aes)
{
  /* NIST SP 800-108 fixed data. */
  static const uint8_t fixed[] = {
    /* i */
    0x00, 0x00, 0x00, 0x01,

    /* label */
    0x35, 0x27, 0x13, 0xcc, 0x53, 0xa7, 0x78, 0x89,
    0x87, 0x53, 0x22, 0x11, 0xd6, 0x5b, 0x31, 0x58,
    0xdc, 0xfe, 0x2e, 0x7e, 0x94, 0xda, 0x2f, 0x00,
    0xcc, 0x15, 0x71, 0x80, 0x0a, 0x6c, 0x63, 0x53,

    /* delimiter */
    0x00,

    /* context */
    0x38, 0xc3, 0x38, 0xac, 0x22, 0xf3, 0x63, 0x62,
    0x0e, 0xce, 0x85, 0x3f, 0xb8, 0x07, 0x4c, 0x4e,
    0x2b, 0x77, 0xc7, 0x21, 0xf5, 0x1a, 0x80, 0x1d,
    0x67, 0xfb, 0xe1, 0xe1, 0x83, 0x07, 0xd8, 0x0d,

    /* L */
    0x00, 0x00, 0x01, 0x00,
  };

  char padded_password[32];
  uint8_t plaintext[16];
  size_t password_len;
  uint8_t cmac[16];
  uint8_t key[32];
  size_t cmac_len;
  CMAC_CTX *ctx;
  int retval;

  /* Truncate password to at most 10 bytes. */
  password_len = strlen (password);
  if (password_len > 10)
    password_len = 10;

  /* padded_password = password padded with zeros to 32 bytes. */
  memset (padded_password, 0, sizeof padded_password);
  memcpy (padded_password, password, password_len);

  /* cmac = CMAC(padded_password, fixed). */
  ctx = CMAC_CTX_new ();
  assert (ctx != NULL);

  retval = CMAC_Init (ctx, padded_password, sizeof padded_password,
                      EVP_aes_256_cbc (), NULL);
  assert (retval == 1);

  retval = CMAC_Update (ctx, fixed, sizeof fixed);
  assert (retval == 1);

  cmac_len = sizeof cmac;
  retval = CMAC_Final (ctx, cmac, &cmac_len);
  assert (retval == 1);
  assert (cmac_len == 16);

  /* The key is the cmac repeated twice. */
  memcpy(key, cmac, 16);
  memcpy(key + 16, cmac, 16);

  /* Use key to initialize AES. */
  assert (sizeof key == 32);
  retval = AES_set_decrypt_key (key, sizeof key * 8, aes);
  assert (retval >= 0);

  /* Check for magic number "$FL" always present in SPSS .sav file. */
  AES_ecb_encrypt (ciphertext, plaintext, aes, AES_DECRYPT);
  return !memcmp (plaintext, "$FL", 3);
}
.
/* Password decoding. */

#define b(x) (1 << (x))

static const uint16_t m0[4][2] = {
  { b(2),                         b(2) | b(3) | b(6) | b(7) },
  { b(3),                         b(0) | b(1) | b(4) | b(5) },
  { b(4) | b(7),                  b(8) | b(9) | b(12) | b(14) },
  { b(5) | b(6),                  b(10) | b(11) | b(14) | b(15) },
};

static const uint16_t m1[4][2] = {
  { b(0) | b(3) | b(12) | b(15),  b(0) | b(1) | b(4) | b(5) },
  { b(1) | b(2) | b(13) | b(14),  b(2) | b(3) | b(6) | b(7) },
  { b(4) | b(7) | b(8) | b(11),   b(8) | b(9) | b(12) | b(13) },
  { b(5) | b(6) | b(9) | b(10),   b(10) | b(11) | b(14) | b(15) },
};

static const uint16_t m2[4][2] = {
  { b(2),                         b(1) | b(3) | b(9) | b(11) },
  { b(3),                         b(0) | b(2) | b(8) | b(10) },
  { b(4) | b(7),                  b(4) | b(6) | b(12) | b(14) },
  { b(5) | b(6),                  b(5) | b(7) | b(13) | b(15) },
};

static const uint16_t m3[4][2] = {
  { b(0) | b(3) | b(12) | b(15),  b(0) | b(2) | b(8) | b(10) },
  { b(1) | b(2) | b(13) | b(14),  b(1) | b(3) | b(9) | b(11) },
  { b(4) | b(7) | b(8) | b(11),   b(4) | b(6) | b(12) | b(14) },
  { b(5) | b(6) | b(9) | b(10),   b(5) | b(7) | b(13) | b(15) },
};

static int
decode_nibble (const uint16_t table[4][2], int nibble)
{
  int i;

  for (i = 0; i < 4; i++)
    if (table[i][0] & (1 << nibble))
      return table[i][1];

  return 0;
}

/* Returns true if X has exactly one 1-bit, false otherwise. */
static bool
is_pow2 (int x)
{
  return x && (x & (x - 1)) == 0;
}

/* If X has exactly one 1-bit, returns its index, where bit 0 is the LSB.
   Otherwise, returns 0. */
static int
find_1bit (uint16_t x)
{
  int i;

  if (!is_pow2 (x))
    return -1;

  for (i = 0; i < 16; i++)
    if (x & (1u << i))
      return i;

  abort ();
}

/* Attempts to decode a pair of encoded password characters A and B into a
   single byte of the plaintext password.  Returns 0 if A and B are not a valid
   encoded password pair, otherwise a byte of the plaintext password. */
static int
decode_password_2bytes (uint8_t a, uint8_t b)
{
  int x = find_1bit (decode_nibble (m0, a >> 4) & decode_nibble (m2, b >> 4));
  int y = find_1bit (decode_nibble (m1, a & 15) & decode_nibble (m3, b & 15));
  return x < 0 || y < 0 ? 0 : (x << 4) | y;
}

/* Decodes an SPSS so-called "encrypted" password INPUT into OUTPUT.

   An encoded password is always an even number of bytes long and no longer
   than 20 bytes.  A decoded password is never longer than 10 bytes plus a null
   terminator.

   Returns true if successful, otherwise false. */
static bool
decode_password (const char *input, char output[11])
{
  size_t len;

  len = strlen (input);
  if (len > 20 || len % 2)
    return false;

  for (; *input; input += 2)
    {
      int c = decode_password_2bytes (input[0], input[1]);
      if (!c)
        return false;
      *output++ = c;
    }
  *output = '\0';

  return true;
}
.
/* Main program. */

static void
xfread (void *data, size_t size, size_t count, FILE *stream)
{
  if (fread (data, size, count, stream) != count)
    {
      if (ferror (stream))
        fprintf (stderr, "read error (%s)", strerror (errno));
      else
        fprintf (stderr, "unexpected end of input\n");
      exit (1);
    }
}

int
main (int argc, char *argv[])
{
  uint8_t in[36];
  char pw[11];
  AES_KEY aes;

  if (argc != 2)
    {
      fprintf (stderr,
               "usage: %s PASSWORD < ENCRYPTED > DECRYPTED\n"
               "where ENCRYPTED is an encrypted system file,\n"
               "      DECRYPTED is the decrypted version,\n"
               "  and PASSWORD is the plaintext or encoded password.\n",
               argv[0]);
      exit (1);
    }
  if (isatty (1))
    {
      fprintf (stderr, "not writing output to a terminal, please "
               "redirect to a file\n");
      exit (1);
    }

  /* Read and discard 36-byte header, which only serves as a magic number. */
  xfread (in, 36, 1, stdin);

  /* Read first ciphertext block and use it to verify the password.  Try the
     password as plaintext first, then try decoding it. */
  xfread (in, 16, 1, stdin);
  if (!init (argv[1], in, &aes)
      && !(decode_password (argv[1], pw) && init (pw, in, &aes)))
    {
      fprintf (stderr, "wrong password, sorry\n");
      exit (1);
    }

  /* Decrypt entire input. */
  do
    {
      uint8_t out[16];

      AES_ecb_encrypt (in, out, &aes, AES_DECRYPT);
      fwrite (out, 16, 1, stdout);
    }
  while (fread (in, 16, 1, stdin));

  return 0;
}

/*
   Local variables:
   compile-command: "gcc -Wall -Wextra decrypt.c -o decrypt -lcrypto"
   End:
*/

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