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Warning: Collaborated research into OpenSSL protocol usage, published in May 2015, showed further significant risks for SSL connections; named "Logjam" attack. See for results and for suggested server-side configuration changes.

OpenSSL is an open-source implementation of the SSL and TLS protocols, dual-licensed under the OpenSSL (Apache License 1.0) and the SSLeay (4-clause BSD) licenses. It is supported on a variety of platforms, including BSD, Linux, OpenVMS, Solaris and Windows. It is designed to be as flexible as possible, and is free to use for both personal and commercial uses. It is based on the earlier SSLeay library. Version 1.0.0 of OpenSSL was released on March 29, 2010.

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

SSL introduction

In order to focus on setting up a SSL/TLS solution, rather than explaining the bare basics regarding the subject, the approach used throughout the article to explain SSL concepts is by and large file-oriented.

Consult both Wikipedia:Certificate authority and Wikipedia:Public key infrastructure for more information.

Tango-edit-cut.pngThis section is being considered for removal.Tango-edit-cut.png

Reason: Wikipedia does a better job at explaining. These definitions are at least partially wrong. (Discuss in Talk:OpenSSL#Plan)
Certificate authority (CA)
Certificate authorities return certificates from end-user requests. In order to do this, the returned end-user certificate is signed with the CA private key and CA certificate, which in turn contains the CA public key. CA also distribute certificate revocation lists (CRL) which tell the end-user what certificates are no longer valid, and when the next CRL is due.
CA private key
The CA private key is the crucial part of the trifecta. Exposing it would defeat the purpose of designating a central authority that validates and revokes permissions, and at the same time, it is the signed counter part to the CA public key used to certify against the CA certificate. An exposed CA private key could allow an attacker to replicate the CA certificate since the CA private key signature is embedded in the CA certificate itself.
CA certificate and public key
These are distributed in a single file to all end-users. They are used to certify other end-user certificates that claimed to be signed by the matching CA, such as mail servers or websites.
End-users submit certificate requests to the CA which contain a distinguished name (DN). Normally, CA do not allow more than one valid certificate with the same DN without revoking the previous one. End-user certificates may be revoked if they are not renewed when due, among other reasons.
End-user generated key
End-users generate keys in order to sign certificate requests that are submitted to the CA. As with the CA private key, an exposed user-key could facilitate impersonating the user to the point where an attacker could submit a request under the user's name, resulting in the CA revoking the former, legitimate, user certificate.
Certificate requests
These contain the user's DN and public key. As their name implies, they fully represent the initial part of the process of acquiring certification from a CA.
End-user certificate
The main distinction between an end-user certificate and CA certificate is that end-user ones cannot sign certificates themselves; they merely provide means of identification in exchanges of information.
Certificate revocation list (CRL)
CRLs are also signed with the CA key, but they only dictate information regarding end-user certificates. Usually, a 30 day span is given between new CRL submissions.


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). In some operating systems, this man page is named config(5) or openssl-config(5). Sometimes, it may not even be available through the man hierarchy at all, for example, it may be placed in the following location /usr/share/openssl.

req section

Merge-arrows-2.pngThis article or section is a candidate for merging with #Creating certificate signing requests.Merge-arrows-2.png

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

Even though splitting the files is not strictly necessary to normal functioning, it is very confusing to handle request generation and CA administration from the same configuration file, so it is advised to follow the convention of clearly separating the settings into two cnf files and into two containing directories.

Here are the settings that are common to both tasks:

[ req ]
# Default bit encryption and out file for generated keys.
default_bits=	2048

string_mask=	utf8only	# Only allow utf8 strings in request/ca fields.
prompt=		no		# Do not prompt for field value confirmation.

End-user req settings

Makes a v3 request suitable for most circumstances:

distinguished_name=ca_dn	# Distinguished name contents.
req_extensions=req_v3		# For generating ca certificates.

[ ca_dn ]
ST=	New Jersey
O=	localdomain
CN=	localhost

[ req_v3 ]
basicConstraints=	CA:FALSE
keyUsage=		nonRepudiation, digitalSignature, keyEncipherment

GOST engine support

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Reason: What is GOST, why would you want to use it? (Discuss in Talk:OpenSSL#)

First, be sure that exist on your system

$ pacman -Ql openssl | grep libgost

In case everything is fine, add the following lines to the config:

openssl_conf = openssl_def # this must be a top-level declaration

Put the following lines in the end of the document:

[ openssl_def ]
engines = engine_section

[ engine_section ]
gost = gost_section

[ gost_section ]
engine_id = gost
soft_load = 1
dynamic_path = /usr/lib/engines/
default_algorithms = ALL
CRYPT_PARAMS = id-Gost28147-89-CryptoPro-A-ParamSet

The official README.gost should contain more examples on this.

Generating private keys

Warning: The openssl package doesn't properly safeguard the /etc/ssl/private/ directory like most other distributions do, see FS#43059.

Before generating the key, make a secure directory to host it:

$ mkdir -m0700 private

Followed by preemptively assigning secure permissions for the key itself:

$ touch private/key.pem
$ chmod 0600 private/key.pem

Alternatively set umask to restrict permissions of newly created files and directories:

$ umask 077

An example genpkey key generation:

$ openssl genpkey -algorithm RSA -out private/key.pem -pkeyopt rsa_keygen_bits:4096

If an encrypted key is desired, use the following command. Password will be prompted for:

$ openssl genpkey -aes-256-cbc -algorithm RSA -out private/key.pem -pkeyopt rsa_keygen_bits:4096


If you want to communicate securely with a server for the first time, you need to trust an unknown public key. TLS solves this using the Public Key Infrastructrue. Basically clients trust a set of certificate authorities (CAs) (on Arch Linux the ca-certificates packages). When a certificate is received from a server, your client (mostly gnutls) verifies that it is signed by a certificate authority you trust.

Creating certificate signing requests

To obtain a certificate from a certificate authority, you need to create a Certificate Signing Request (CSR) and sign it with a previously generated private key:

$ openssl req -new -sha256 -key private/key.pem -out req.csr
Tip: You can get free certificates from the Let's Encrypt certificate authority using an ACME client.

Self-signed certificate

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Reason: It might be more educational to split up the command into openssl req and openssl x509. The former could then just be referenced as #Creating certificate signing requests. (Discuss in Talk:OpenSSL#)

Clients reject self-signed certificates by default, requiring you to manually configure every client to trust your self-signed certificate. Maintaining more than one self-signed certificate is more trouble than investing the initial effort in setting up a certificate authority.

To create a self-signed certificate with a previously generated private key:

$ openssl req -key private/key.pem -x509 -new -days 3650 -out selfcert.pem

Certificate authority

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Reason: It's not the style of the ArchWiki to tell the reader to copy and paste 43 lines of a Makefile. (Discuss in Talk:OpenSSL#Plan)

OpenSSL Certificate Authority is a detailed guide on using OpenSSL to act as a CA.

The method shown in this section is mostly meant to show how signing works; it is not suited for large deployments that need to automate signing a large number of certificates. Consider installing an SSL server for that purpose.

Before using the Makefile, make a configuration file according to #Configuration. Be sure to follow instructions relevant to CA administration; not request generation.


Saving the file as Makefile and issuing make in the containing directory will generate the initial CRL along with its prerequisites:

OPENSSL=	openssl
CNF=		openssl.cnf
CA=		${OPENSSL} ca -config ${CNF}
REQ=		${OPENSSL} req -config ${CNF}

KEY=		private/cakey.pem

CACERT=		cacert.pem
CADAYS=		3650

CRL=		crl.pem
INDEX=		index.txt
SERIAL=		serial


all:	${CRL}

	${CA} -gencrl -out ${CRL}

${CACERT}: ${CNF} ${KEY}
	${REQ} -key ${KEY} -x509 -new -days ${CADAYS} -out ${CACERT}
	rm -f ${INDEX}
	touch ${INDEX}
	echo 100001 > ${SERIAL}

${KEY}: ${CNF}
	mkdir -m0700 -p $(dir ${KEY})
	touch ${KEY}
	chmod 0600 ${KEY}
	${OPENSSL} genpkey -algorithm ${KEYMODE} -out ${KEY}

revoke:	${CADEPS} ${item}
	@test -n $${item:?'usage: ${MAKE} revoke item=cert.pem'}
	${CA} -revoke ${item}
	${MAKE} ${CRL}

sign:	${CADEPS} ${item}
	@test -n $${item:?'usage: ${MAKE} sign item=request.csr'}
	mkdir -p newcerts
	${CA} -in ${item} -out ${item:.csr=.crt}

To sign certificates:

$ make sign item=req.csr

To revoke certificates:

$ make revoke item=cert.pem


"bad decrypt" while decrypting

OpenSSL 1.1.0 changed the default digest algorithm for the dgst and enc commands from MD5 to SHA256. [1]

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

See also