Get the first PIV public key in SSH format in the same way that PuTTY-CAC does on Windows

get-piv-ssh-pubkey

#!/bin/bash
set -eu

# This script prints the first PIV certificate found from a smart card in SSH
# format in the same way that putty-cac does on windows. (yeah.)

# LICENSE
#
# As a work of the United States Government, this project is in the public
# domain within the United States.
#
# Additionally, we waive copyright and related rights in the work worldwide
# through the CC0 1.0 Universal public domain dedication.

run() {
    echo >&2 "+ $*"
    "$@"
}

get_piv_pubkey_mac() {
    lastfirst_str="$(id -F)"
    lastfirst_arr=( ${lastfirst_str//,/} )

    fullname="${lastfirst_arr[*]:1} ${lastfirst_arr[0]}"
    fullname_upper="$(echo "$fullname" | tr '[:lower:]' '[:upper:]')"

    # Contractor format
    fullname_upper_affiliate="$fullname_upper (affiliate)"

    cert="$(run security find-certificate -p "PIV-$fullname_upper")" || true
    if [ -z "$cert" ]; then
        cert="$(run security find-certificate -p "PIV-$fullname_upper_affiliate")"
    fi

    print_pubkey_from_cert "$cert"
}

print_pubkey_from_cert() {
    if [ $# -lt 1 ]; then
        echo >&2 "usage: print_pubkey_from_cert X509_CERT_CONTENT"
        return 1
    fi
    cert="$1"

    pubkey="$(ssh-keygen -i -m pkcs8 -f <(
        openssl x509 -noout -pubkey <<< "$cert"))"

    sha1="$(openssl x509 -noout -fingerprint <<< "$cert" \
        | cut -d'=' -f2 | tr -d ':')"

    echo "$pubkey CAPI:User\\MY\\$sha1"
}


get_piv_pubkey_opensc() {
    # we could use `--read-ssh-key 1` instead, but putty uses the SHA1
    # fingerprint of the full X.509 certificate, not just the RSA public key

    # use the first certificate found
    cert="$(run pkcs15-tool --read-certificate 1)"

    print_pubkey_from_cert "$cert"
}

trap 'echo ERROR' EXIT

# use opensc if it is installed
if which pkcs15-tool >/dev/null 2>&1; then
    get_piv_pubkey_opensc
    trap - EXIT
    exit
fi

case "$OSTYPE" in
    darwin*)
        get_piv_pubkey_mac
        ;;
    linux*)
        get_piv_pubkey_opensc
        ;;
    *)
        trap - EXIT
        echo >&2 "Not yet implemented for $OSTYPE. Sorry!"
        exit 1
        ;;
esac

trap - EXIT