Dealing with boilerplate in Haskell

This is kind of a soft question, but in the following code, there's a lot of duplication in the section marked "caesar ciphers." What's the "Haskell" way to deal with this? Should I make a higher order function? I thought about that, but I don't know what makes sense. Is there a "cipher" type that I could define for making ciphers?

Also, I know it may appear a little bit overengineered, in the sense that I'm doing the same error checking in two places, but I think it makes sense from the perspective of what each of the functions "means." Suggestions?

import Data.Char
import Control.Applicative
import Control.Monad
import Math.NumberTheory.Powers

--Helpers

extendedGcd::Integer->Integer->(Integer, Integer)
extendedGcd a b | r == 0 = (0, 1)
                | otherwise = (y, x - (y * d))
                where
                    (d, r) = a `divMod` b
                    (x, y) = extendedGcd b r

modularInverse::Integer->Integer->Maybe Integer
modularInverse n b | relativelyPrime n b = Just . fst $ extGcd n b
                   | otherwise = Nothing
                   where
                        extGcd = extendedGcd

relativelyPrime::Integer->Integer->Bool
relativelyPrime m n = gcd m n == 1 

textToDigits::String->[Integer]
textToDigits = map (\x->toInteger (ord x - 97)) 

digitsToText::[Integer]->String
digitsToText = map (\x->chr (fromIntegral x + 97)) 

--Caesar Ciphers

caesarEncipher::Integer->Integer->Integer->Maybe Integer
caesarEncipher r s p | relativelyPrime r 26 = Just $ mod (r * p + s) 26
                     | otherwise = Nothing

caesarDecipher::Integer->Integer->Integer->Maybe Integer
caesarDecipher r s c | relativelyPrime r 26 = mod <$> ((*) <$> q <*> pure (c - s)) <*> pure 26
                     | otherwise = Nothing
    where
        q = modularInverse r 26

caesarEncipherString::Integer->Integer->String->Maybe String
caesarEncipherString r s