OpenSSL - ArchWiki

# OpenSSL

Warning:

- A serious vulnerability was discovered affecting some of the OpenSSL 1.0.1 releases in April 2014. The vulnerability leaves private keys compromised and it is advisable to regenerate these and the corresponding certificates if an affected version of OpenSSL was deployed. Cf. http://heartbleed.com/ and FS#39775.
- Collaborated research into OpenSSL protocol usage, published in May 2015, showed further significant risks for SSL connections; named "Logjam" attack. See https://weakdh.org/ for results and https://weakdh.org/sysadmin.html for suggested server-side configuration changes.

OpenSSL is an open-source implementation of the SSL and TLS protocols, dual-licensed under the OpenSSL and the SSLeay 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.

## 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.

### 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 CA also distribute certificate revocation lists (CRL) which tell the end-user what certificates are no longer valid, and when the next CRL is due.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

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.

## Configuring

The OpenSSL configuration file, conventionally placed in `/etc/ssl/openssl.cnf`, may appear complicated at first. This isn't remedied by the fact that there's no include directive to split configuration into a modal setup. Nevertheless, this section covers the essential settings.

Remember that variables may be expanded in assignments, much like how shell scripts work. For a more thorough explanation of the configuration file format, see config(5). In some operating systems, this man page is named openssl-config or config(5ssl). 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`.

### Global variables

These settings are relevant in all sections. For that to happen, they can not be specified under a section header:

    DIR=		.			# Useful macro for populating real vars.
    RANDFILE=	${DIR}/private/.rnd	# Entropy source.
    default_md=	sha1			# Default message digest.

### ca section

These settings are used when signing CRLs, and signing and revoking certificates. Users that only want to generate requests can safely skip to the #req section.

    [ ca ]
    default_ca=	dft_ca	# Configuration files may have more than one CA
    			# section for different scenarios.

    [ dft_ca ]
    certificate=	${DIR}/cacert.pem	# The CA certificate.
    database=	${DIR}/index.txt	# Keeps tracks of valid/revoked certs.
    new_certs_dir=	${DIR}/newcerts		# Copies of signed certificates, for
    					# administrative purposes.
    private_key=	${DIR}/private/cakey.pem # The CA key.
    serial=		${DIR}/serial		# Should be populated with the next
    					# cert hex serial.

    # These govern the way certificates are displayed while confirming
    # the signing process.
    name_opt=	ca_default
    cert_opt=	ca_default

    default_days=	365	# How long to sign certificates for.
    default_crl_days=30	# The same, but for CRL.

    policy=		dft_policy	# The default policy should be lenient.
    x509_extensions=cert_v3		# For v3 certificates.

    [ dft_policy ]
    # A value of 'supplied' means the field must be present in the certificate,
    # whereas 'match' means the field must be populated with the same contents as
    # the CA certificate. 'optional' dictates that the field is entirely optional.

    C=	supplied	# Country
    ST=	supplied	# State or province
    L=	optional	# Locality
    O=	supplied	# Organization
    OU=	optional	# Organizational unit
    CN=	supplied	# Common name

    [ cert_v3 ]
    # With the exception of 'CA:FALSE', there are PKIX recommendations for end-user
    # certificates that should not be able to sign other certificates.
    # 'CA:FALSE' is explicitely set because some software will malfunction without.

    subjectKeyIdentifier=	hash
    basicConstraints=	CA:FALSE
    keyUsage=		nonRepudiation, digitalSignature, keyEncipherment

    nsCertType=		client, email
    nsComment=		"OpenSSL Generated Certificate"

    authorityKeyIdentifier=keyid:always,issuer:always

### req section

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's no chance that, between the possible combinations of certified end-users' CN and the CA certificate's, there's 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:

    # subject= /C=US/O=Equifax/OU=Equifax Secure Certificate Authority
    openssl x509 -subject -noout < /etc/ssl/certs/Equifax_Secure_CA.pem

Even though splitting the files isn't 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
    default_keyfile=private/cakey.pem

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

#### CA req settings

The settings should produce a standard CA capable of only singing other certificates:

    distinguished_name=ca_dn	# Distinguished name contents.
    x509_extensions=ca_v3		# For generating ca certificates.

    [ ca_dn ]
    # CN isn't needed for CA certificates
    C=	US
    ST=	New Jersey
    O=	localdomain

    [ ca_v3 ]
    subjectKeyIdentifier=	hash

    # PKIX says this should also contain the 'crucial' value, yet some programs
    # have trouble handling it.
    basicConstraints=	CA:TRUE

    keyUsage=	cRLSign, keyCertSign

    nsCertType=	sslCA
    nsComment=	"OpenSSL Generated CA Certificate"

    authorityKeyIdentifier=keyid:always,issuer:always

#### 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 ]
    C=	US
    ST=	New Jersey
    O=	localdomain
    CN=	localhost

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

### GOST engine support

First, be sure that libgost.so 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/libgost.so
    default_algorithms = ALL
    CRYPT_PARAMS = id-Gost28147-89-CryptoPro-A-ParamSet

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

## Generating keys

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

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

## Making requests

To obtain a certificate from a CA, whether it be a public one such as CAcert.org, or a locally managed solution, a request file must be delivered to the CA, and these are conventionally called `csr` files.

Make a new request and sign it with a previously generated key:

    openssl req -new -sha256 -key private/key.pem -out req.csr

## Signing certificates

Covers the process of local CA signing: directly self-signed certificates or through a local CA.

### Self-signed certificate

A significant amount of programs will not work with self-signed certificates, and maintaining more than one system with self-signed certificates is more trouble than investing the initial effort in setting up a certificate authority.

If a key was already generated as explained before, use this command to sign the new certificate with the aforementioned key:

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

### Certificate authority

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 #Configuring. Be sure to follow instructions relevant to CA administration; not request generation.

#### Makefile

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
    KEYMODE=	RSA

    CACERT=		cacert.pem
    CADAYS=		3650

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

    CADEPS=		${CNF} ${KEY} ${CACERT}

    all:	${CRL}

    ${CRL}:	${CADEPS}
    	${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

## See also

- Wikipedia page on OpenSSL, with background information.
- OpenSSL project page.
- FreeBSD Handbook
- Step-by-step guide to create a signed SSL certificate

Categories:

- Networking
- Encryption