Transposition Ciphers
Transposition Ciphers
Transposition Ciphers
Example:
Plain text: come home tomorrow
Key: ZEBRAS
plain text: welcome home
Order : 6 3 2 4 1 5
Diagonal plow
Row Transposition Ciphers
• This sort of thing would be trivial to cryptanalyze
• a more complex transposition is to write letters of message out in rows
over a specified number of columns
• then reorder the columns according to some key before reading off the
rows
• Attach postponed until two a.m
Key: 3 4 2 1 5 6 7
Plaintext: a t t a c k p
o s t p o n e
d u n t i l t
w o a m x y z
Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ
• A pure transposition cipher is easily recognized because it has the same
letter frequencies as the original plaintext.
• For the type of columnar transposition just shown, cryptanalysis is fairly
straightforward and involves laying out the ciphertext in a matrix and
playing around with column positions.
• Digram and trigram frequency tables can be useful.
• The transposition cipher can be made significantly more secure by
performing more than one stage of transposition. The result is a more
complex permutation that is not easily reconstructed. Thus, if the foregoing
message is re-encrypted using the same algorithm.
• Encrypted the above encrypted message again using the same algo.
• To visualize the result of this double transposition, designate the letters in
the original plaintext message by the numbers designating their position.
• This will be much difficult to crypt-analyze.
Scrambling with key-words
• Key words are easy to remember rather than the whole code.
Product Ciphers
• A pure transposition cipher is easily recognized because it has the
same letter frequencies as the original plaintext.
• ciphers using substitutions or transpositions are not secure because
of language characteristics
• hence consider using several ciphers in succession to make harder,
but:
• two substitutions make a more complex substitution
• two transpositions make more complex transposition
• but a substitution followed by a transposition makes a new much harder
cipher
• this is bridge from classical to modern ciphers
Summary
• have considered:
• classical cipher techniques and terminology
• monoalphabetic substitution ciphers
• cryptanalysis using letter frequencies
• Playfair cipher
• polyalphabetic ciphers
• transposition ciphers
• product ciphers