Editing Tabula recta (section)

== Usage ==
Within the body of the tabula recta, each alphabet is shifted one letter to the left from the one above it. This forms 26 rows of shifted alphabets, ending with an alphabet starting with Z (as shown in image). Separate from these 26 alphabets are a header row at the top and a header column on the left, each containing the letters of the alphabet in A-Z order.

The tabula recta can be used in several equivalent ways to encrypt and decrypt text. Most commonly, the left-side header column is used for the plaintext letters, both with encryption and decryption. That usage will be described herein. In order to decrypt a Trithemius cipher, one first locates in the tabula recta the letters to decrypt: first letter in the first interior column, second letter in the second column, etc.; the letter directly to the far left, in the header column, is the corresponding decrypted plaintext letter. Assuming a standard shift of 1 with no key used, the encrypted text HFNOS would be decrypted to HELLO (H->H, F->E, N->L, O->L, S->O ). So, for example, to decrypt the second letter of this text, first find the F within the second interior column, then move directly to the left, all the way to the leftmost header column, to find the corresponding plaintext letter: E.

[[Plaintext|Data]] is encrypted in the opposite fashion, by first locating each plaintext letter of the message in the leftmost header column of the tabula recta, and mapping it to the appropriate corresponding letter in the interior columns. For example, the first letter of the message is found within the left header column, and then mapped to the letter directly across in the column headed by "A". The next letter is then mapped to the corresponding letter in the column headed by "B", and this continues until the entire message is encrypted.<ref>{{Citation |last= Rodriguez-Clark |first= Dan |title= Polyalphabetic Substitution Ciphers |publisher= Crypto Corner |url= https://crypto.interactive-maths.com/polyalphabetic-substitution-ciphers.html}}</ref> If the Trithemius cipher is thought of as having the key ABCDEFGHIJKLMNOPQRSTUVWXYZ, the encryption process can also be conceptualized as finding, for each letter, the intersection of the row containing the letter to be encrypted with the column corresponding to the current letter of the key. The letter where this row and column cross is the ciphertext letter.

Programmatically, the cipher is computable, assigning <math>A = 0, B = 1 ...</math>, then the encryption process is <math>ciphertext = (plaintext + key)\!\!\!\!\pmod {26}</math>. Decryption follows the same process, exchanging ciphertext and plaintext. {{mvar|key}} may be defined as the value of a letter from a companion ciphertext in a [[running key cipher]], a constant for a [[Caesar cipher]], or a zero-based counter with some period in Trithemius's usage.<ref name="Kahn, page 136">Kahn, page 136</ref>