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OpenSSL Overview

Symmetric Cryptography


openssl enc -aes-256-cbc -md md5 -in $1 -out $1.txt


openssl enc -d -aes-256-cbc -md md5 -in $1.txt -out $1


  • OpenSSL 1.1.0 implicitly uses -md md5
  • OpenSSL 1.1.1 implicitly uses -md sha256

SSH Cryptography

Sometimes you want to asymmetrically protect an encryption without having to create a new key-pair. With a little finesse you can use an existing SSH key-pair to accomplish this.

I've found that one use case for this is for storage of a .env file that you want to be able to use to start a service. We want to keep the file stored securely but in a manner where you will not be able to decrypt without knowing a password (i.e. the SSH private key passphrase). Additionally, the actual SSH private key could be thought of as the thing you have. Bam!, now you have 2 factor authentication to unlock the .env.

Encrypt data.txt to data.txt.enc with aes256 using an SSH public key:

openssl rand 32 | \
tee >(openssl rsautl -encrypt -oaep -pubin -inkey <(ssh-keygen -e -f ~/.ssh/ -m PKCS8) -out secret.key | \
openssl enc -aes-256-cbc -base64 -in data.txt -out data.txt.enc -pass stdin

Decrypt data.txt.enc to data.txt.dec with aes256 using SSH private key:

openssl rsautl -decrypt -oaep -inkey ~/.ssh/id_rsa -in secret.key | \
openssl enc -d -aes-256-cbc -base64 -in data.txt.enc -out data.txt.dec -pass stdin

Given an encrypted file (as encrypted above) that can be sourced into your shell environment, you can grab the file from a remote source, decrypt, and load the .env into your environment with the following one-liner:

(ssh -T -q [email protected] cat /path/to/key) | \
openssl rsautl -decrypt -oaep -inkey ~/.ssh/id_rsa | \
openssl enc -d -aes-256-cbc -base64 -in data.txt.enc -pass stdin | \
(eval `cat`;exec ./printsecret ANOTHER)

Note: Replace exec ./printsecret ANOTHER with the service command you want to run with the new environment.

Certificate Utilities

Create a CA certificate chain for client certificate verification:

cat root.cert.pem intermediate.cert.pem > cachain.cert.pem

Note: Include all applicable intermediate certificates in the concatenation.

Verify the client certificate matches the CA certificate chain:

openssl verify -verbose -CAfile cachain.cert.pem client.cert.pem

PKCS12 / .p12 / .pfx

Create a PKCS12 file:

openssl pkcs12 -export -out client_certs.p12 -inkey client.key.pem \
-in client.cert.pem -certfile cachain.cert.pem -name "Friendly Name"

Note: Set Friendly Name to easily locate the loaded certificate in GUI listings.

Setup A Certificate Authority

OpenSSL is a toolbox that has what you need to setup your own CA. The issue is that it has a number of configuration file and database file setups to accomplish correctly. This are non-portable configurations that are fragile and not intended for production. It is recommended to use another more complete CA system that comes with a well defined flow of operations (in contrast to a lose set of tools). Something that also includes ACME support is a major benefit. If you insist on using OpenSSL for CA setup, simply refer to OpenSSL Certificate Authority.

Adhoc Certificate Authority

Assuming you wanted to create a root certificate for some servers and one of the servers was test.home.domain. Using OpenSSL you could do the following.

  1. Create Root CA certificate and key PEM files:
openssl req -x509 -days 18250  -nodes \
-newkey rsa:4096 -keyout ca.key.pem \
-out ca.cert.pem -extensions v3_ca
  1. Create certificate signing request PEM:
openssl req -nodes \
-new -newkey rsa:2048 -keyout test.home.domain.key.pem \
-out test.home.domain.csr.pem -subj "/CN=test.home.domain"
  1. Sign certificate with Root CA key:
openssl x509 -req \
-in test.home.domain.csr.pem \
-days 18250 \
-CA ca.cert.pem -CAkey ca.key.pem -CAcreateserial \
-out test.home.domain.cert-only.pem \
-extfile <(printf "subjectAltName=DNS:test.home.domain")
  1. Generate full certificate chain:
cat test.home.domain.cert-only.pem ca.cert.pem > test.home.domain.cert.pem
  1. Rinse and repeat from step 2 for all servers.

  2. Use the test.home.domain.cert.pem and test.home.domain.key.pem in your HTTPS services (e.g. web servers).

  3. Use the ca.cert.pem in your intended clients (e.g. browsers).

This process has the advantage of not requiring a complex or hard to managed CA service for some quick modern browser compliant web service security behaviors.

Note: This process uses -nodes. Remove this argument to protect signing keys with a password (recommended for long lived services.)

Utility Script

For those that find openssl's numerous arguments unappealing, I've written an opinionated utility to simplify the process.

Equivalent command for all of the above would look like: ./ test.home.domain.

  • It automatically generates CA certificate and key if not provided or not already created.
  • It automatically generates the server certificate and key into a similar folder.
  • Provide --dns (i.e. FQDN) and/or --ip arguments to specify additional domain names for a single certificate/key pair.
  • You can use --dns home.domain --dns '*.home.domain' to generate a wild card certificate for entire domain.
  • Developed and tested with OpenSSL 3.0.2 15 Mar 2022 (Library: OpenSSL 3.0.2 15 Mar 2022).
#!/usr/bin/env python3

usage: [-h] [--prefix PREFIX] [--cert-only CERT_ONLY] [--key KEY]
[--csr CSR] [--cert CERT] [--cacert CACERT] [--cakey CAKEY]
[--caname CANAME] [--caprefix CAPREFIX] [--dns DNS] [--ip IP]

Create adhoc (signed) x509 certificates.

positional arguments:
name Server/Client (fqdn) value for implicit behavior. ([a-z][A-Z] String).

-h, --help show this help message and exit
--prefix PREFIX Server/Client Prefix Output Path
--cert-only CERT_ONLY
Certificate Output Path (PEM)
--key KEY Private Key Output Path (PEM)
--csr CSR Certificate Signing Request Output Path (PEM)
--cert CERT Full Certificate Chain Output Path (PEM)
--cacert CACERT CA Certificate Output/Input Path (PEM)
--cakey CAKEY CA Signing Key Output/Input Path (PEM)
--caname CANAME CA Name for implicit path generation ([a-z][A-Z] String)
--caprefix CAPREFIX CA Prefix Output Path
--dns DNS DNS Name Entry for Certificate
--ip IP IP Entry for Certificate

from argparse import ArgumentParser
from subprocess import run, PIPE, DEVNULL
from tempfile import NamedTemporaryFile, mkstemp
import os
import shutil

from pprint import pprint
from pdb import set_trace


class ArgumentProcessor(object):

def __init__(s):
s.do_create_ca_pair = False
s._args = s.parser.parse_args()

def config_parser(s):
s.parser = ArgumentParser(
description='Create adhoc (signed) x509 certificates.'

# Positional Argument
help='Server/Client (fqdn) value for implicit behavior. ([a-z][A-Z] String).'
# TODO: Include password in env, cli, readline?
default=os.environ.get('ADHOC_PREFIX', None),
help='Server/Client Prefix Output Path'
default=os.environ.get('ADHOC_CERTONLY', None),
help='Certificate Output Path (PEM)'
# TODO: Include password in env, cli, readline?
default=os.environ.get('ADHOC_KEY', None),
help='Private Key Output Path (PEM)'
default=os.environ.get('ADHOC_CSR', None),
help='Certificate Signing Request Output Path (PEM)'
default=os.environ.get('ADHOC_CERT', None),
help='Full Certificate Chain Output Path (PEM)'

default=os.environ.get('ADHOC_CACERT', None),
help='CA Certificate Output/Input Path (PEM)'
# TODO: Include password in env, cli, readline?
default=os.environ.get('ADHOC_CAKEY', None),
help='CA Signing Key Output/Input Path (PEM)'
default=os.environ.get('ADHOC_CANAME', DEFAULT_CANAME),
help='CA Name for implicit path generation ([a-z][A-Z] String)'
default=os.environ.get('ADHOC_CAPREFIX', DEFAULT_CANAME),
help='CA Prefix Output Path'

# s.parser.add_argument(
# '--dn',
# required=True,
# default=None,
# help='Distinguished Name For Certificate'
# )
help='DNS Name Entry for Certificate'
help='IP Entry for Certificate'

def process_ca_args(s):
s.cacert = s._args.cacert
s.cakey = s._args.cakey
s.caname = s._args.caname
s.caprefix = s._args.caprefix

# 1. If set, check for existing s.cacert and s.cakey
if s.cakey and s.cacert:
if not os.path.exists(s.cacert) or not os.path.exists(s.cakey):
print("Failed to find given cacert and cakey, recreating.")
s.do_create_ca_pair = True
print("Using user provided cacert and cakey")
# todo: check access (no access throws error)

# When either cakey or cacert not set, clobber both with defaults
s.cacert = os.path.join(s.caprefix, f'{DEFAULT_CANAME}.cert.pem')
s.cakey = os.path.join(s.caprefix, f'{DEFAULT_CANAME}.key.pem')
# 2. If default exists, use default.
if os.path.exists(s.cacert) and os.path.exists(s.cakey):
# todo: check access (no access throws error)
print("Using default cacert or cakey.")
s.do_create_ca_pair = False
# 3. If no existing defaults, recreate
print("No given cacert or cakey, recreating.")
s.do_create_ca_pair = True

def process_cert_args(s): =[0]
s.prefix = s._args.prefix
s.cert = s._args.cert
s.key = s._args.key
s.csr = s._args.csr
s.cert_only = s._args.cert_only
s.dns = s._args.dns
s.ip = s._args.ip
s.subject_alt_names = []

if not s.prefix:
s.prefix =
if not s.cert:
s.cert = os.path.join(s.prefix, f'{}.cert.pem')
if not s.key:
s.key = os.path.join(s.prefix, f'{}.key.pem')
if not s.csr:
s.csr = os.path.join(s.prefix, f'{}.csr.pem')
if not s.cert_only:
s.cert_only = os.path.join(s.prefix, f'{}.cert-only.pem')

if len(s.dns) == 0:
for entry in s.dns:
for entry in s.ip:

class CertManager(object):
def __init__(s):
s.args = ArgumentProcessor()

if s.args.do_create_ca_pair:


def concat_files(s, fpaths, dst):
with open(dst, "w") as dstobj:
for fpath in fpaths:
with open(fpath, "r") as fobj:

def create_ca_pair(s):
# todo: os.environ.get('CAPASS', None)

# Create the subdirs for the ca prefixes
os.makedirs(os.path.dirname(s.args.cakey), exist_ok=True)
os.makedirs(os.path.dirname(s.args.cacert), exist_ok=True)

cmd = [
'openssl', 'req', '-x509',
'-days', '18250',
'-newkey', 'rsa:4096',
'-keyout', s.args.cakey,
'-out', s.args.cacert,
'-extensions', 'v3_ca',
'-subj', f'/CN={s.args.caname}',

ca_run = run(cmd, stderr=DEVNULL)
if ca_run.returncode != 0:
print(f"cmd({ca_run.returncode}):{' '.join(cmd)}\n{ca_run.stdout}")

def create_server_cert(s):
# todo: os.environ.get('PASS', None)

os.makedirs(os.path.dirname(s.args.key), exist_ok=True)
os.makedirs(os.path.dirname(s.args.cert), exist_ok=True)

cmd = [
'openssl', 'req', '-nodes', '-new',
'-newkey', 'rsa:2048',
'-keyout', s.args.key,
'-out', s.args.csr,
'-subj', f'/CN={}',

csr_run = run(cmd, stderr=DEVNULL)
if csr_run.returncode != 0:
print(f"cmd({csr_run.returncode}):{' '.join(cmd)}\n{csr_run.stdout}")

extfile_out = f'subjectAltName={",".join(s.args.subject_alt_names)}\n'
(fd, fpath) = mkstemp()
with os.fdopen(fd, 'w') as fobj:

cmd = [
'openssl', 'x509', '-req',
'-in', s.args.csr,
'-CA', s.args.cacert,
'-CAkey', s.args.cakey, '-CAcreateserial',
'-out', s.args.cert_only,
'-extfile', fpath,

crt_run = run(cmd, stderr=DEVNULL)
if crt_run.returncode != 0:
print(f"cmd({crt_run.returncode}):{' '.join(cmd)}\n{crt_run.stdout}")

# Merge cert and cacert for "full chain" cert
s.concat_files([s.args.cert_only, s.args.cacert], s.args.cert)