In cryptography, a substitution cipher is a method of encryption by which units of plaintext are replaced with ciphertext according to a regular system; the “units” may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing an inverse substitution.
Substitution ciphers can be compared with transposition ciphers. In a transposition cipher, the units of the plaintext are rearranged in a different and usually quite complex order, but the units themselves are left unchanged. By contrast, in a substitution cipher, the units of the plaintext are retained in the same sequence in the ciphertext, but the units themselves are altered.
There are a number of different types of substitution cipher. If the cipher operates on single letters, it is termed a simple substitution cipher; a cipher that operates on larger groups of letters is termed polygraphic. A monoalphabetic cipher uses fixed substitution over the entire message, whereas a polyalphabetic cipher uses a number of substitutions at different times in the message, where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and vice-versa.
Here are some attempts, classic methods are indicated with the appropriate name.
I attempted a forced Foursquare solution.
I attempted a Bifid cipher solution.
I attempted a Digraphic Substitution Cipher solution
I even tried to see if K4 was a simple Caesar shift cipher.
I toyed with the idea, with some success, that there are segments within K4 that are enciphered with different substitution ciphers creating a layering of different ciphers.
I tried converting the K4 97 letters into numbers to look for patterns.
I tried a Caesar shift cipher.
I tried a keyed-Caesar shift cipher.
I attempted a Gronsfield cipher.
I tried a Playfair Cipher.
I contemplated a Straddling Checkerboard Cipher.
I debated if K4 was another Vigenere Cipher.
I checked an Affine Cipher.
I looked at an Atbash Cipher.
I even tried Roman numerals.
I checked to see if K4 used Pig Latin.
I invented the Castor cipher to try on K4.
I tried someone’s suggestion to approach it as an SMS cipher.
I pondered a Runic cipher.