Cryptography
Computer Science & Statistics at University of Rhode Island
Simple Substitution Cipher Description A monoalphabetic, or simple substitution, cipher is one
in which the ciphertext alphabet is a rearrangement of the plaintext
alphabet. Substitution ciphers, despite having 26! possible permutations,
are actually very insecure and are easily solved using letter
frequencies. As Thomas Barr explains, They are analogous to the cheap
locks that sometimes come with luggage: a deterrent to the casual intruder
but not to someone with even modest intentions of breaking in.
(Invitation to Cryptology, 84). Simple substitutions are a great way to
encipher messages that need to be visually enciphered but easily
deciphered. Example, Mixed Alphabet with Keywords The sender and receiver decides up on a keyword or
phrase. In
this case, lets use the phrase the cows go moo in the field. The
plaintext and ciphertext are laid out, with the key being made from the
distinct letters of the key phrase:
This gives the enciphered message LOOQLOTQWMUOAEFAEN
This gives the deciphered message meetmeatfiveoclock
Disadvantage The major problem with simple substitution ciphers is
that the frequencies of letters are not masked at all. If the enciphered
message LOOQLOTQWMUOAEFAEN was intercepted, the interceptor
could look at the frequencies of each letter and compare them to the
frequencies of English:
A quick deciphering test gives the answer
AEETAE?T???EON?ON? |