OpenSSL is an open-source implementation of the SSL and TLS protocols, designed to be as flexible as possible. It is supported on a variety of platforms, including BSD, Linux, OpenVMS, Solaris and Windows.
openssl is installed by default on Arch Linux (as a dependency of coreutils).
There are various OpenSSL library bindings available for developers:
- lua-sec, lua52-sec, lua51-sec
On Arch Linux the
The OpenSSL configuration file, conventionally placed in
/etc/ssl/openssl.cnf, may appear complicated at first. Remember that variables may be expanded in assignments, much like how shell scripts work. For a thorough explanation of the configuration file format, see config(5ssl).
Settings related to generating keys, requests and self-signed certificates.
The req section is responsible for the DN prompts. A general misconception is the Common Name (CN) prompt, which suggests that it should have the user's proper name as a value. End-user certificates need to have the machine hostname as CN, whereas CA should not have a valid TLD, so that there is no chance that, between the possible combinations of certified end-users' CN and the CA certificate's, there is a match that could be misinterpreted by some software as meaning that the end-user certificate is self-signed. Some CA certificates do not even have a CN, such as Equifax:
$ openssl x509 -subject -noout < /etc/ssl/certs/Equifax_Secure_CA.pem
subject= /C=US/O=Equifax/OU=Equifax Secure Certificate Authority
This sections assumes you have read Transport Layer Security#Obtaining a certificate.
Generate a Curve25519 private key
$ openssl genpkey -algorithm x25519 -out file
Generate an ECDSA private key
$ openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 -out file
Generate an RSA private key
With genpkey(1ssl), which supersedes genrsa according to openssl(1ssl):
$ openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:keysize -out file
If an encrypted key is desired, use the
Generate a certificate signing request
$ openssl req -new -sha256 -key private_key -out filename
Show a certificate signing request
Certificate signing requests are stored in an encoded format. To view the request in human readable format:
$ openssl req -noout -text -in filename
Generate a self-signed certificate
$ openssl req -key private_key -x509 -new -days days -out filename
Generate a self-signed certificate with private key in a single command
You can combine the above command in OpenSSL into a single command which might be convenient in some cases:
$ openssl req -x509 -newkey rsa:4096 -days days -keyout key_filename -out cert_filename
Generate Diffie–Hellman parameters
See Diffie–Hellman key exchange for more information.
Current best practice is to use one of the standard DH groups from RFC:7919, eg. ffdhe2048.
Alternatively you can generate a random group of your own:
$ openssl dhparam -out filename 2048
Show certificate information
$ openssl x509 -text -in cert_filename
Show certificate fingerprint
$ openssl x509 -noout -in cert_filename -fingerprint -digest
-digest is optional and one of
-sha512. See "-digest" in x509(1ssl) § Input, Output, and General Purpose Options for when the digest is unspecified.
Convert certificate format
openssl x509 to convert certificates from binary (DER) format to PEM format (the text format with
BEGIN CERTIFICATE headers)：
$ openssl x509 -inform DER < myCA.crt > myCA_pem.crt
"bad decrypt" while decrypting
OpenSSL 1.1.0 changed the default digest algorithm for the dgst and enc commands from MD5 to SHA256. 
Therefore if a file has been encrypted using OpenSSL 1.0.2 or older, trying to decrypt it with an up to date version may result in an error like:
error:06065064:digital envelope routines:EVP_DecryptFinal_ex:bad decrypt:crypto/evp/evp_enc.c:540
-md md5 option should solve the issue:
$ openssl enc -d -md md5 -in encrypted -out decrypted
Python 3.10 and "ca md too weak" errors
In Python 3.10 by default there is a hardcoded list of allowed OpenSSL ciphers. Some of the less secure, like MD5, have been disabled at the
ssl module level, ignoring the system-wide configuration of OpenSSL. It results sometimes in strange errors on older certificates, sometimes even when establishing
https connections, like:
requests.exceptions.SSLError: HTTPSConnectionPool(host='a.kind.of.example.com', port=443): Max retries exceeded with url: / (Caused by SSLError(SSLError(398, '[SSL: CA_MD_TOO_WEAK] ca md too weak (_ssl.c:3862)')))
To make Python follow the system configuration, you may have to rebuild it, adding
--with-ssl-default-suites=openssl parameter to
./configure. The issue has been also reported as FS#73549.
- Wikipedia page on OpenSSL, with background information.
- OpenSSL project page.
- FreeBSD Handbook
- Step-by-step guide to create a signed SSL certificate
- OpenSSL Certificate Authority: A guide demonstrating how to act as your own certificate authority.
- Bulletproof SSL and TLS by Ivan Ristić, a more formal introduction to SSL/TLS