Modified version of Debian's cryptsetup hook for GnuPG Smartcard unlock

decrypt_gnupg-sc.sh

#!/bin/sh

if [ -d "/cryptroot/gnupghome" ]; then
    export GNUPGHOME="/cryptroot/gnupghome"
fi

run_gpg() {
    /usr/bin/gpg --no-options --trust-model=always "$@"
}
decrypt_gpg () {
    if ! /lib/cryptsetup/askpass "Enter smartcard PIN: " | \
	    run_gpg --quiet --batch \
	    --pinentry-mode loopback --passphrase-fd 0 \
	    --no-tty --decrypt -- "$1"; then
        return 1
    fi
    return 0
}

if ! run_gpg --batch --quiet --no-tty --card-status >/dev/null; then
    echo "Please insert OpenPGP SmartCard..." >&2
fi

if [ ! -x /usr/bin/gpg ]; then
    echo "$0: /usr/bin/gpg is not available" >&2
    exit 1
fi

if [ -z "$1" ] || [ ! -f "$1" ]; then
    echo "$0: missing key as argument" >&2
    exit 1
fi

decrypt_gpg "$1"
exit $?

This version will work with a plymouth boot splash enabled. It can be used by following the setup process detailed here.

Also worth checking out this repo and also this pull request.