autokey <- function(plain="", cipher="", keyletter="A")
{
# encode or decode an autokey cipher

if(missing(plain) && missing(cipher)) {
stop("Must include one of plaintext or ciphertext!")
}

if(!missing(plain) && !missing(cipher)) {
stop("Included both plaintext and ciphertext!")
}

if(missing(cipher)) {
# encode plaintext
plain <- toupper(plain)
keyletter <- toupper(keyletter)
textlen <- nchar(plain)

alphanum <- 1:26

cipher <- " "
for(i in 2:textlen) {
cipher <- paste(cipher, " ", sep="")
}
currkey <- keyletter

for(i in 1:textlen) {
currletter <- substring(plain, i, i)
if(currletter != " ") {
currkey <- alphanum[!is.na(match(LETTERS, currkey))] - 1
currletter <- alphanum[!is.na(match(LETTERS, currletter))]
currletter <- currletter + currkey
if(currletter > 26) {
currletter <- currletter - 26
}
currletter <- LETTERS[currletter]
substring(cipher, i, i) <- currletter
currkey <- substring(plain, i, i)
}
}
}

if(missing(plain)) {
# decode ciphertext
cipher <- toupper(cipher)
keyletter <- toupper(keyletter)
textlen <- nchar(cipher)

alphanum <- 1:26

plain <- " "
for(i in 2:textlen) {
plain <- paste(plain, " ", sep="")
}
currkey <- keyletter

for(i in 1:textlen) {
currletter <- substring(cipher, i, i)
if(currletter != " ") {
currkey <- alphanum[!is.na(match(LETTERS, currkey))] - 1
currletter <- alphanum[!is.na(match(LETTERS, currletter))]
currletter <- currletter - currkey
if(currletter < 1) {
currletter <- currletter + 26
}
currletter <- LETTERS[currletter]
substring(plain, i, i) <- currletter
currkey <- substring(plain, i, i)
}
}

}

list(plain=plain, cipher=cipher, keyletter=keyletter)
}