wordkey <- function(plain="", cipher="", keyword)
{
# 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(keyword)) keyword <- paste(LETTERS, sep="", collapse="")
keyword <- toupper(keyword)

if(missing(cipher)) {
# encode plaintext

plain <- toupper(plain)
textlen <- nchar(plain)

alphanum <- 1:26

## set up keyword
longkey <- paste(rep(0, textlen), sep="", collapse="")
# remove spaces
strindex <- 1
for(i in 1:textlen) {
	while(is.na(any(match(LETTERS, substring(keyword, strindex, strindex))))) {
		strindex <- strindex + 1
		if(strindex > nchar(keyword)) strindex <- 1
	}
	substring(longkey, i, i) <- substring(keyword, strindex, strindex)
	strindex <- strindex + 1
	if(strindex > nchar(keyword)) strindex <- 1
}


cipher <- paste(rep(0, textlen), sep="", collapse="")
strindex <- 1

for(i in 1:textlen) {
currletter <- substring(plain, i, i)
if(currletter != " ") {
currkey <- substring(longkey, strindex, strindex)
strindex <- strindex + 1
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)
}
else {
	substring(cipher, i, i) <- " "
}
}
}

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

textlen <- nchar(cipher)

alphanum <- 1:26

## set up keyword
longkey <- paste(rep(0, textlen), sep="", collapse="")
# remove spaces
strindex <- 1
for(i in 1:textlen) {
	while(is.na(any(match(LETTERS, substring(keyword, strindex, strindex))))) {
		strindex <- strindex + 1
		if(strindex > nchar(keyword)) strindex <- 1
	}
	substring(longkey, i, i) <- substring(keyword, strindex, strindex)
	strindex <- strindex + 1
	if(strindex > nchar(keyword)) strindex <- 1
}


plain <- paste(rep(0, textlen), sep="", collapse="")
strindex <- 1

for(i in 1:textlen) {
currletter <- substring(cipher, i, i)
if(currletter != " ") {
currkey <- substring(longkey, strindex, strindex)
strindex <- strindex + 1
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)
}
else {
	substring(plain, i, i) <- " "
}
}


}

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