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- Transport Layer Security

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.

## Installation

openssl is installed by default on Arch Linux (as a dependency of coreutils).

There are various OpenSSL library bindings available for developers:

- python-pyopenssl
- perl-net-ssleay
- lua-sec, lua52-sec, lua51-sec
- haskell-hsopenssl
- haskell-openssl-streams

## Configuration

On Arch Linux the `OPENSSLDIR` is `/etc/ssl`.

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).

### req section

This article or section is a candidate for merging with #Generate a certificate signing request.

Notes: Same topic. (Discuss in Talk:OpenSSL#Plan)

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

## Usage

This sections assumes you have read Transport Layer Security#Obtaining a certificate.

### Generate a Curve25519 private key

    $ openssl genpkey -algorithm x25519 -out filename

### Generate an ECDSA private key

    $ openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 -out filename

### 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 filename

If an encrypted key is desired, use the `-aes-256-cbc` option.

### Generate a certificate signing request

Use req(1ssl):

    $ 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

This article or section needs expansion.

Reason: This produces a certificate for the (root) Certificate Authority, which you are acting as. Most web browsers do not seem to accept CA certificates, deeming it necessary to request another certificate and sign it with the CA cert and CA key. The "Generate a certificate issued by own CA" procedure in this forum post is what seems to satisfy browsers. (Discuss in Talk:OpenSSL)

    $ 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

### Sign a certificate signing request with a CA certificate

    $ openssl x509 -req -in cert_req_filename -days days -CA CA_cert -CAkey CA_cert_private_key -CAserial CA_cert_serial_file -out cert_out

### 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

Tip: To speed up generating, especially when not on high-end hardware, add the `-dsaparam` option [1].

### 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 `-md5`, `-sha1`, `-sha256`, or `-sha512`. See "-digest" in x509(1ssl) § Input, Output, and General Purpose Options for when the digest is unspecified.

### Convert certificate format

Use `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

### Use third-party providers

OpenSSL 3 introduced providers as a new concept for OpenSSL plugability. It is possible to use algorithms not included in OpenSSL without having to recompile it. For example, to test the NIST Post-Quantum Cryptography algorithms, you can install the Open Quantum Safe provider oqsproviderAUR. As an example, you can generate a quantum-safe self-signed certificate with private key using one of the variants of the Dilithium signature algorithm:

    $ openssl req -provider oqsprovider -x509 -newkey dilithium3 -days days -keyout key -out cert

## Troubleshooting

### "bad decrypt" while decrypting

OpenSSL 1.1.0 changed the default digest algorithm for the dgst and enc commands from MD5 to SHA256. [2]

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

Supplying the `-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.

### Error setting cipher XXX

If you try to use a "retired" cipher, you'll get an error of this type:

    $ openssl bf -d -in cipher_file -K passphrase
    Error setting cipher BF-CBC
    4087A97A8A7F0000:error:0308010C:digital envelope routines:inner_evp_generic_fetch:unsupported:crypto/evp/evp_fetch.c:341:Global default library context, Algorithm (BF-CBC : 12)

Since OpenSSL 3.0, crypto algorithms are supplied through "providers". Oldest or least used algorithms belong to the legacy provider. [3]

If you need to use retired algorithms like DES, RC4, Blowfish, etc., you must add the option `-provider legacy` in your command line.

Here is a complete example for decoding a Blowfish cipher.

    $ openssl bf -d -in cipher_file -provider legacy -provider default -K passphrase

## See also

- 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

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