fakekey

#!/usr/bin/env bash

# Requires:
# * der2ascii/ascii2der tools by David Benjamin:
#   https://github.com/google/der-ascii
# * OpenSSL

set -eu -o pipefail

type der2ascii &>/dev/null || echo "Need der2ascii/ascii2der tools from https://github.com/google/der-ascii"

if [[ ${#} -ne 1 ]] && [[ ${#} -ne 2 ]]; then
    echo "Usage:"
    echo "    fakekey [certificate] [output]"
    echo ""
    echo "If [output] isn't given we'll write to fake.key."
    exit 1
fi

TMP="$(mktemp -d)"

[[ "${#}" -eq 2 ]] && out="${2}" || out="fake.key"

openssl x509 -noout -pubkey -in "${1}" | openssl pkey -outform der -pubin > "${TMP}/cert.der"

openssl genrsa 2048 | openssl pkey -outform der -out "${TMP}/newkey.der"

der2ascii -i "${TMP}/cert.der" -o "${TMP}/cert.ascii"
der2ascii -i "${TMP}/newkey.der" -o "${TMP}/newkey.ascii"

head -n 2 "${TMP}/newkey.ascii" > "${TMP}/fake.ascii"
grep INTEGER "${TMP}/cert.ascii" >> "${TMP}/fake.ascii"
tail -n +5 "${TMP}/newkey.ascii" >> "${TMP}/fake.ascii"

ascii2der -i "${TMP}/fake.ascii" -o "${TMP}/fake.der"

openssl pkey -inform der -in "${TMP}/fake.der" -out "${out}"

echo "checking ${out} (errors are to be expected!)"
openssl rsa -in "${out}" -check -noout

rm -rf "${TMP}"