Its kinda like subtitution cipher, where the plain text are converted to its ascii/utf counterpart (or simply 1 to 26), then apply a mathematical formula to each numbers, and convert it back to letters.
ex.
plaintext: HALLO
formula: 3x+1
to number: 8 1 12 12 15
apply formula: 25 4 37 37 45
mod 26: 25 4 11 11 19
ciphertext: Y D K K S
