Sample Diffie-Hellman key exchange helper implementation in Bash (depends on dc for bigint and od for secret generation)

diffie.sh

#!/bin/bash

# RFC 3526 prime and base (id 14, 2048-bit)

PRIME='FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F4
4C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3DC2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F83655D23DCA3AD961C62F356208552BB9ED52907709
6966D670C354E4ABC9804F1746C08CA18217C32905E462E36CE3BE39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF6955817183995497CEA956AE515D2261898FA051015728E5A8AACAA68FFFF
FFFFFFFFFFFF'

BASE=2

# large hex number sanitizer

cleannum() {
    echo "$@"|tr -d ' \n\\'|tr a-f A-F
}

# actual modular exponentiation via dc
# modexp $base $exp $mod

modexp() {
    cleannum $(echo "16dio$(cleannum $1) $(cleannum $2) $(cleannum $3)|p"|dc)
}

#main action

case $1 in

secret)
    echo $(cleannum $(head -c 16 /dev/urandom|od -A n -v -t x1))
    ;;

public)
    echo $(modexp "$BASE" "$2" "$PRIME")
    ;;

session)
    echo $(modexp "$3" "$2" "$PRIME")
    ;;

*)
    echo -e "Usage: $0 (secret|public your_secret|session your_secret other_party_public)"
    ;;