The Hill cipher (Lester S. Hill, 1929) treats blocks of letters as vectors and multiplies them by an invertible key matrix modulo 26. This tool supports 2×2 and 3×3 keys, automatically computes the modular inverse for decryption, and pads input as needed.
Each letter A–Z is mapped to 0–25. Plaintext is split into vectors of length n (the block size) and multiplied by an n×n key matrix mod 26. To decrypt, multiply by the matrix inverse mod 26 — which exists only when gcd(det(K), 26) = 1, i.e. the determinant is coprime to 26 (so it must be odd and not a multiple of 13). The Hill cipher is one of the earliest practical polygraphic ciphers and was the first to use linear algebra; however, it falls quickly to a known-plaintext attack since enough pairs of plain/cipher blocks let an attacker solve for the key.