Encrypted File Wrappers
***********************

SPSS 21 and later can package multiple kinds of files inside an encrypted wrapper.  The wrapper has a common format, regardless of the kind of the file that it contains.

     The SPSS encryption wrapper is poorly designed.  When the password is unknown, it is much cheaper and faster to decrypt a file encrypted this way than if a well designed alternative were used.  If you must use this format, use a 10-byte randomly generated password.

Common Wrapper Format
=====================

An encrypted file wrapper begins with the following 36-byte header, where xxx identifies the type of file encapsulated: SAV for a system file, SPS for a syntax file, SPV for a viewer file.  PSPP code for identifying these files just checks for the ENCRYPTED keyword at offset 8, but the other bytes are also fixed in practice:

     0000  1c 00 00 00 00 00 00 00  45 4e 43 52 59 50 54 45  |........ENCRYPTE|
     0010  44 xx xx xx 15 00 00 00  00 00 00 00 00 00 00 00  |Dxxx............|
     0020  00 00 00 00                                       |....|

Following the fixed header is essentially the regular contents of the encapsulated file in its usual format, with each 16-byte block encrypted with AES-256 in ECB mode.

To make the plaintext an even multiple of 16 bytes in length, the encryption process appends PKCS #7 padding, as specified in RFC 5652 section 6.3.  Padding appends 1 to 16 bytes to the plaintext, in which each byte of padding is the number of padding bytes added.  If the plaintext is, for example, 2 bytes short of a multiple of 16, the padding is 2 bytes with value 02; if the plaintext is a multiple of 16 bytes in length, the padding is 16 bytes with value 0x10.

The AES-256 key is derived from a password in the following way:

 1. Start from the literal password typed by the user.  Truncate it to at most 10 bytes, then append as many null bytes as necessary until there are exactly 32 bytes.  Call this PASSWORD.

 2. Let CONSTANT be the following 73-byte constant:

          0000  00 00 00 01 35 27 13 cc  53 a7 78 89 87 53 22 11
          0010  d6 5b 31 58 dc fe 2e 7e  94 da 2f 00 cc 15 71 80
          0020  0a 6c 63 53 00 38 c3 38  ac 22 f3 63 62 0e ce 85
          0030  3f b8 07 4c 4e 2b 77 c7  21 f5 1a 80 1d 67 fb e1
          0040  e1 83 07 d8 0d 00 00 01  00

 3. Compute CMAC-AES-256(PASSWORD, CONSTANT).  Call the 16-byte result CMAC.

 4. The 32-byte AES-256 key is CMAC || CMAC, that is, CMAC repeated twice.

Example
-------

Consider the password pspp.  PASSWORD is:

     0000  70 73 70 70 00 00 00 00  00 00 00 00 00 00 00 00  |pspp............|
     0010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

CMAC is:

     0000  3e da 09 8e 66 04 d4 fd  f9 63 0c 2c a8 6f b0 45

The AES-256 key is:

     0000  3e da 09 8e 66 04 d4 fd  f9 63 0c 2c a8 6f b0 45
     0010  3e da 09 8e 66 04 d4 fd  f9 63 0c 2c a8 6f b0 45

Checking Passwords
------------------

A program reading an encrypted file may wish to verify that the password it was given is the correct one.  One way is to verify that the PKCS #7 padding at the end of the file is well formed.  However, any plaintext that ends in byte 01 is well formed PKCS #7, meaning that about 1 in 256 keys will falsely pass this test.  This might be acceptable for interactive use, but the false positive rate is too high for a brute-force search of the password space.

A better test requires some knowledge of the file format being wrapped, to obtain a ``magic number'' for the beginning of the file.

  * The plaintext of system files begins with $FL2@(#) or $FL3@(#).

  * Before encryption, a syntax file is prefixed with a line at the beginning of the form * Encoding: ENCODING., where ENCODING is the encoding used for the rest of the file, e.g. windows-1252.  Thus, * Encoding may be used as a magic number for system files.

  * The plaintext of viewer files begins with 50 4b 03 04 14 00 08 (50 4b is PK).

Password Encoding
=================

SPSS also supports what it calls ``encrypted passwords.''  These are not encrypted.  They are encoded with a simple, fixed scheme.  An encoded password is always a multiple of 2 characters long, and never longer than 20 characters.  The characters in an encoded password are always in the graphic ASCII range 33 through 126.  Each successive pair of characters in the password encodes a single byte in the plaintext password.

Use the following algorithm to decode a pair of characters:

 1. Let A be the ASCII code of the first character, and B be the ASCII code of the second character.

 2. Let AH be the most significant 4 bits of A.  Find the line in the table below that has AH on the left side.  The right side of the line is a set of possible values for the most significant 4 bits of the decoded byte.

          2  => 2367
          3  => 0145
          47 => 89cd
          56 => abef

 3. Let BH be the most significant 4 bits of B.  Find the line in the second table below that has BH on the left side.  The right side of the line is a set of possible values for the most significant 4 bits of the decoded byte.  Together with the results of the previous step, only a single possibility is left.

          2  => 139b
          3  => 028a
          47 => 46ce
          56 => 57df

 4. Let AL be the least significant 4 bits of A.  Find the line in the table below that has AL on the left side.  The right side of the line is a set of possible values for the least significant 4 bits of the decoded byte.

          03cf => 0145
          12de => 2367
          478b => 89cd
          569a => abef

 5. Let BL be the least significant 4 bits of B.  Find the line in the table below that has BL on the left side.  The right side of the line is a set of possible values for the least significant 4 bits of the decoded byte.  Together with the results of the previous step, only a single possibility is left.

          03cf => 028a
          12de => 139b
          478b => 46ce
          569a => 57df

Example
-------

Consider the encoded character pair -|.  A is 0x2d and B is 0x7c, so AH is 2, BH is 7, AL is 0xd, and BL is 0xc.  AH means that the most significant four bits of the decoded character is 2, 3, 6, or 7, and BH means that they are 4, 6, 0xc, or 0xe.  The single possibility in common is 6, so the most significant four bits are 6.  Similarly, AL means that the least significant four bits are 2, 3, 6, or 7, and BL means they are 0, 2, 8, or 0xa, so the least significant four bits are 2.  The decoded character is therefore 0x62, the letter b.