#!/bin/dash

die() { echo "$0: Error: $*" >&2; exit 1; }

DEFAULT_PEM_FILE=/etc/ssh/ssh_host_rsa_key.pub
if [ "$1" = -h ]
then
    cat >&2 <<EOF
Usage: $0 [pem-file]

    Default pem-file is $DEFAULT_PEM_FILE
EOF
    exit
fi

b16_to_b32()
{
    echo -n "$1" | basez -x -d | basez -j -l | tr -d =
}

pem_to_host()
{
    local INPUT="$1"
    DNS_FMT=$(ssh-keygen -r . -f "$INPUT")
    HEX_FMT=$(echo -n "$DNS_FMT" | sed -ne 's/^. IN SSHFP [0-9]* 2 //p')
    B32_FMT=$(b16_to_b32 "$HEX_FMT")

    read keytype keydata < "${INPUT}" || die "reading from INPUT=$INPUT"
    case "$keytype" in
        ssh-rsa|ssh-dss|ecdsa-sha2-nistp256|ssh-ed25519)
            domain=$keytype.cryptonomic.net ;;
        *)
            die "Unsupported key type: $keytype" ;;
    esac
#   echo $HEX_FMT.$domain
    echo $B32_FMT.$domain
}

INPUT=${1:-$DEFAULT_PEM_FILE}
[ -f "$INPUT" ] || die "not a file: $INPUT"

pem_to_host "$INPUT"