Find Key Length Algorithm

A program to match sections of the cipher text in an attempt to find the length of the key used to produce it.

Main

Global Constant:

1. loMatchLim: The minimum number of characters that must match before we are interested.

Local variables:

1. mesgLen: Number of characters in cipher text

2. beginMatch: Loop index which goes through cipher text one position at a time

3. firstMatch: The position in the cipher text where the first matching character to the character at beginMatch is found.

4. matchLen: The number of characters that match

Basic Algorithm:

1. Call readCipher returning the number of characters to mesgLen.

2. For Loop: beginMatch takes on [1 ® loMatchLim]

   a. Assign to firstMatch the value returned from calling findFirstMatch with 3

parameters:

                        i. The character at position beginMatch

                        ii. The first position at which to look for a match: beginMatch+loMatchLim

                        iii. The length of the message

   b. If a matching string has been found (i.e., firstMatch is greater than 0):

                        i. Assign to matchLen the value returned from calling findMatchLen with 2

   parameters: beginMatch and firstMatch

                        ii. If matchLen is greater than or equal to loMatchLim then:

                                    - Print out information about the matching strings found

                                    - Increment beginMatch by the appropriate amount

findFirstMatch

Parameters:

1. The character for which match is sought

2. The position at which to begin looking

3. The position at which to stop looking

Local variables:

1. lookIndex: Index to iterate through positions of cipher text seeking at match

2. matchPosn: The position at which a match was found (if there was a match)

Basic Algorithm:

1. Set matchPosn to -1 (in case no match is found)

2. For Loop: lookIndex takes on [beginning position ® ending position]

   a. If the character at lookIndex matches the character sought then:

                        i. Set matchPosn to lookIndex

                        ii. Exit the For Loop

   b. Return the value of matchPosn

          [Note: If no match was found, then this is still -1; otherwise it is the position at

which the first match was found].

findMatchLen

Parameters:

1. The position at which the first character is found

2. The position at which its matching character is found

Local variables:

1. matchLen: The number of characters that match at both positions

Basic Algorithm:

1. Set matchLen to 1

2. While Loop

with condition: the character at the first position + matchLen matches the

character at the second position + matchLen

   a. Increment matchLen by 1

3. Return matchLen