Assemble ecdsa presignatures into a signature

assemble-presigs.hs

#!/usr/bin/env runhaskell
import Data.Foldable (foldl')
import Numeric (showHex)
import System.Environment (getArgs)

hex = flip showHex ""

m = 0xfffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141 :: Integer

sumMod m = foldl' (\a b -> (a + b) `mod` m) 0

main = do
    args <- getArgs
    let (r_hexs, s_hexs) = unzip . map (splitAt 64) $ args

    let r_hex = head r_hexs
    if not . all (== r_hex) $ r_hexs
    then error "Mismatching rs"
    else pure ()

    let ss = map (read . (<>) "0x") s_hexs :: [Integer]
    let s' = sumMod m ss
    let s = min s' (m - s')
    putStrLn $ r_hex <> hex s