Tabula recta

Due to the variable shifting, natural letter frequencies are hidden.

The cipher is vulnerable to attack because it lacks a key, thus violating Kerckhoffs's principle of cryptology.

[1] In 1553, an important extension to Trithemius's method was developed by Giovan Battista Bellaso, now called the Vigenère cipher.

[3] Bellaso added a key, which is used to dictate the switching of cipher alphabets with each letter.

This method was misattributed to Blaise de Vigenère, who published a similar autokey cipher in 1586.

Within the body of the tabula recta, each alphabet is shifted one letter to the left from the one above it.

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.

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 ).