passphrase := "Hello world!".split("")
start := [0, 0, 0]
modulus := 10
base := byte("!")

i := 0
n := 0
while n < passphrase.length() {
    b := byte(passphrase.at(n))

    v = start.at(i) + b - base
    while v >= modulus {
        v = v - modulus
    }

    start.put(i, v)

    if i >= 3 {
        i = 0
    }
    n = n + 1
}