Hill cipher is an alternative cipher that uses the principles of basic matrix theory. It is now mostly used in classical cipher teaching and learning.
In the Hill password, each letter is treated as a 26-base number: A=0, B=1, C=2..., Z=25, and the original letter is converted into a number to form an N-dimensional vector, followed by an n Multiply the ×n key matrix, and then modulate the calculated result by 26 to obtain the encrypted ciphertext.
The key used for encryption (that is, the matrix) must be reversible, otherwise it will be impossible to decrypt it. Only the determinant of the matrix and 26 are mutually prime can be reversible.