More Monoalphabetic Ciphers

The shift cipher is one of the simplest of the 'monoalphabetic' ciphers. Remember, this class of ciphers substitutes one encoded letter for each letter from the unencoded message. There is also a 1-to-1 correspondence between the alphabets used to transform a message into an encrypted message.

A variant of the shift cipher is the keyword-shift cipher. Recall that, for a shift cipher, we simply 'rotate' the alphabet several characters in a particular direction. The 'key' to the cipher is the number of positions you shift the alphabet. In a keyword-shift cipher, the 'key' is the chosen 'keyword'. You pick a word without any duplicate letters (remember that we want our alphabets to map each character to a unique character in the ciphertext alphabet. You start the new alphabet with the keyword, and then, list the remaining characters from a normal alphabet. Let's look at this with the keyword SIMPLE.

A - S
B - I
C - M
D - P
E - L
F - E
G - A
H - B
I - C
J - D
K - F
L - G
M - H
N - J
O - K
P - N
Q - O
R - Q
S - R
T - T
U - U
V - V
W - W
X - X
Y - Y
Z - Z

Given the above 'mapping', if you encode the message : HAPPY DAYS ARE HERE AGAIN. results in : BSNNY PSYRS QLBLQ LSASC J

Remember, we want to obscure word boundaries so we just regroup the message into 5 character groups.

You may notice that the last several characters translate to the same character. This is because the keyword uses letters up to the letter 'S' but no further. It is best if you can use a keyword that assures that no letter translates to itself. Thus, they keyword we picked was acceptable but could be improved by using letters later in the alphabet (ideally including 'Z').

In cryptography, a 'key' that does a poor job of scrambling data is called a weak key. Any key that does a good job of scrambling data is a strong key. So SIMPLE is not a weak key but is also not the best. BEAD would be a very weak key whereas ZEBRA would be a strong key (the Z forces the entire alphabet to shift by at least 1 character.)

Another monoalphabetic cipher is the random substitution cipher. Instead of shifting or using a short keyword, we scramble all 26 letters of the alphabet into a different ordering. Whereas a short keyword (which is easy to remember) can allow two people to create the correct mapping to encrypt and decrypt, this cipher requires 26 characters (the reordered alphabet) to be shared between people using it to communicate. That makes it a little less practical to use but it is harder to accidently use a weak key as in the type of cipher above.

Here is an example:

ZAXQSCWDVEFBRGNTHMYJUKILOP

I only showed the alphabet we are encoding 'to' here. To translate a message with this alphabet, all 'A's become Z, all 'B's become A, all 'C's become X, and so forth up to all 'Z's become P.

Try using this to encode a short message. Then work it backwards (Z->A, A->B, etc.) to see if you get back your original message again.

The actual ordering above isn't quite 'random'. There is a pattern to it. See if you can figure out what that is. I'll mention it a few entries down the road as it will nicely lead into other types of ciphers I want to talk about.