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4.2 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’.

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