Create a OpenVPN Certificate Authority

Part of configuring OpenVPN involves the creation of a certificate authority (CA), also known as a public key infrastructure (PKI) (the public refers to public-key cryptography). You can not use an existing public key infrastructure; you would allow anyone with a certificate of that PKI to connect to your server (the tls-remote and tls-verify options can limits the allowed clients). You either need to to set up your own certificate infrastructure, or limit OpenVPN to use password-based authentication (see the options auth-user-pass-verify, client-cert-not-required and username-as-common-name).

The easiest method it to use easy-rsa, as described in the OpenVPN How-to.

This article describes the steps taken by easy-rsa in more detail, but basically gives the same result. This article is partly based on the excellent tutorial by Phil Dibowitz on creating an CA.

Certificates, when deployed correctly, are much more secure than passwords, since the secret (key) does not need to be exchanged or shared between the different hosts. However, doing so requires a security hygiene which may not be required for a small-scale deployment.

Files

The following files are used by OpenSSL:

Filename	Needed by	Purpose	Secret
ca.cnf	CA	CA configuration of its common name (CN)	No
ca-sign.cnf	CA	CA configuration for Signing	No
ca.key	CA	Root CA key	Yes
ca.crt	CA + server + all clients	Root CA certificate	No
index.txt	CA	List of all signed certificates	No
serial	CA	First free serial number of certificates	No
crl.pem	CA + server (+ all clients)	Certificate revokation list	No
crlnumber	CA	First free serial number of CRLs	No
.rand	CA	Root CA certificate	Yes?
dh2048.pem	server only	Diffie Hellman parameters	No
server.cnf	server only during certificate generation	Server configuration	No
client1.req	CA only during certificate generation	Client1 certificate signing request	No
server.key	server only	Server key	Yes
server.crt	server only	Server certificate	No
client1.cnf	client 1 only during certificate generation	Client 1 configuration	No
client1.req	CA only during certificate generation	Client1 certificate signing request	No
client1.key	client 1 only	Client1 key	Yes
client1.crt	client 1 only	Client1 certificate	No
client2.cnf	client 2 only during certificate generation	Client 2 configuration	No
client2.req	CA only during certificate generation	Client2 certificate signing request	No
client2.key	client 2 only	Client2 key	Yes
client2.crt	client 2 only	Client2 certificate	No

I'll use these directories to denote the files that are required on different host:

Directory	Needed by
ca	CA
server	server
client1	client 1
client2	client 2

This is a compromise between the completely flat file structure of easy-rsa, and the recommendations set forth by most CA tutorials, which creates directories for certificate requests (.req), signed certificates (.crt), certificate revokations (crl), and private keys (ca.key).

If you are serious about security, you should not run these commands on the same machine, but run commands in the server, client1, and client2 directory of respectively the server and client machines, and run commands in the CA directory on a machine that is not connected to the Internet.

Initialize

(roughly equivalent to easy-rsa/clean-all)

mkdir ca server client1 client2
cd ca
touch index.txt

CA Configuration Files

Create configuration files. In our setup, ca-sign.cnf contains the configuration for signing certificates. We only use it in conjunction with the openssl ca command. It described the folder structure within the ca directory, the location of support files for the CA, as well as properties of the signed certificates (duration, restricted usage) as well as the policy for the name ("distinguished name") of signed certificates. Finally, it lists the policy for certification revocation lists.

# ca-sign.cnf
# This configuration file is used by the 'ca' command, to create signed certificates.
[ ca ]
default_ca              = CA_default                    # The default ca section

[ CA_default ]
dir                     = /path/to/ca                   # Where everything is kept
certs                   = $dir/                         # Where the issued certs are kept
crl_dir                 = $dir/                         # Where the issued crl are kept
new_certs_dir           = $dir/                         # default place for new certs

private_key             = $dir/ca.key                   # The private key
certificate             = $dir/ca.crt                   # The CA root certificate
database                = $dir/index.txt                # List of signed certificates
serial                  = $dir/serial                   # The current serial number
crlnumber               = $dir/crlnumber                # the current crl number
crl                     = $dir/crl.pem                  # The current CRL
RANDFILE                = $dir/.rand                    # private random number file

unique_subject          = no                            # allow multiple certificates with same subject.
default_md              = default                       # Use hash algorithm specified in the request
default_days            = 3650                          # client certificates last 10 years
#default_crl_days        = 30                            # How often clients should download the CRL

#x509_extensions         = X509_ca                       # The x509 extensions for the root certificate
#x509_extensions         = X509_server                   # The x509 extensions for a server certificate
x509_extensions         = X509_client                   # The x509 extensions for a client certificate

# These options control what fields from the distinguished name to show before signing.
# They are required to make sure all fields are shown.
name_opt                = ca_default                    # Subject Name options
cert_opt                = ca_default                    # Certificate field options

copy_extensions         = copy                          # Copy extensions, such as subjectAltName, from the request

policy                  = policy_dn

[ X509_ca ]
# X509v3 extensions for the root certificate
basicConstraints        = CA:TRUE
subjectKeyIdentifier    = hash
authorityKeyIdentifier  = keyid:always,issuer:always
#subjectAltName         = email:move                    # Move the email address from the DN in the request to here
#crlDistributionPoints   = URI:http://www.example.com/example_ca.crl

[ X509_server ]
# X509v3 extensions for server certificates
basicConstraints        = CA:FALSE
nsCertType              = server
subjectKeyIdentifier    = hash
authorityKeyIdentifier  = keyid,issuer
#subjectAltName         = email:move                    # Move the email address from the DN in the request to here
#crlDistributionPoints   = URI:http://www.example.com/example_ca.crl

[ X509_client ]
# X509v3 extensions for client certificates
basicConstraints        = CA:FALSE
nsCertType              = client
subjectKeyIdentifier    = hash
authorityKeyIdentifier  = keyid,issuer
#subjectAltName         = email:move                    # Move the email address from the DN in the request to here
#crlDistributionPoints   = URI:http://www.example.com/example_ca.crl

[ policy_dn ]
countryName             = supplied                      # required parameter, any value allowed
stateOrProvinceName     = optional
localityName            = optional
organizationName        = match                         # required, and must match root certificate
organizationalUnitName  = optional
commonName              = supplied                      # required parameter, any value allowed
emailAddress            = optional                      # email address in DN is deprecated, use subjectAltName

ca.cnf defines the distinguished name for the certificate authority. It also contains the key length (2048 is recommended nowadays, over the default of 1024), and if the key should be encrypted.

# ca.cnf
# This configuration file is used by the 'req' command when the root certificates is created.
[ req ]
default_bits            = 2048                          # default strength of client certificates
default_md              = sha1
encrypt_key             = yes                           # "no" is equivalent to -nodes
prompt                  = no
string_mask             = utf8only
distinguished_name      = ca_distinguished_name         # root certificate name
req_extensions          = req_cert_extensions
# attributes              = req_attributes

[ ca_distinguished_name ]
# root certificate name
countryName             = NL
#stateOrProvinceName    = Utrecht
localityName            = Hometown
organizationName        = My Organisation
#organizationalUnitName  = My Department Name
commonName              = OpenVPN-CA
#emailAddress            = hostmaster@example.org       # email in DN is deprecated, use subjectAltName

[ req_cert_extensions ]
# nsCertType              = server
subjectAltName          = email:hostmaster@example.org

Traditionally, the email address was part of the distinguished name, like so:

Subject: C=NL, O=MyOrganisation, CN="www.example.org", emailAddress=hostmaster@macfreek.nl

However, RFC 3850 (section 3) specifies that nowadays:

The email address SHOULD be in the subjectAltName extension, and SHOULD NOT be in the subject distinguished name.

Like so:

Subject: C=NL, O=MyOrganisation, CN="www.example.org"
X509v3 extensions:
    X509v3 Subject Alternative Name: 
        email:hostmaster@macfreek.nl

In the above request, the email is already specified in the request. The SubjectAltName (and all other extensions) are copied by this setting:

copy_extensions = copy

An alternative is to specify the email address in the distinguished name (DN) in the request, but copy or move it to the subjectAltName by one of these two settings:

subjectAltName=email:copy
subjectAltName=email:move

Build CA certificate

(roughly equivalent to easy-rsa/build-ca or pkitool --interact --initca)

If your CA should be valid after the year 2038, be sure to use openssl 0.9.9 or higher.

First create a request with the correct name, and then self-sign a certificate and create a serial number file.

cd ca
openssl req -new -config ca.cnf -keyout ca.key -out ca.req
openssl ca -config ca-sign.cnf -extensions X509_ca -days 21700 -create_serial -selfsign \
    -keyfile ca.key -in ca.req -out ca.crt
chmod 400 ca.key
chmod 444 ca.crt

These actions create ca.key and ca.crt. ca.key should be kept secret. ca.crt should be distributed to all servers and clients.

cp ca/ca.crt server/
cp ca/ca.crt client1/
cp ca/ca.crt client2/

Generate Prime Numbers

(roughly equivalent to build-dh)

cd server
openssl dhparam -out dh2048.pem 2048

Build server certificate

(roughly equivalent to build-key-server myserver or pkitool --interact --server myserver)

First create a configuration for the server:

# server.cnf
# This configuration file is used by the 'req' command when the server certificate is created.
[ req ]
default_bits            = 2048
default_keyfile         = server.key
default_md              = sha1
encrypt_key             = no
prompt                  = no
distinguished_name      = server_distinguished_name
req_extensions          = req_cert_extensions
# attributes              = req_attributes
x509_extensions         = usr_cert

[ server_distinguished_name ]
countryName             = NL
#stateOrProvinceName    = Utrecht
localityName            = Breukelen
organizationName        = MacFreek
#organizationalUnitName  = My Department Name
commonName              = gateway.macfreek.nl
emailAddress            = hostmaster@macfreek.nl

[ req_cert_extensions ]
nsCertType              = server

[ v3_ca ]
subjectAltName          = email:hostmaster@macfreek.nl

Create the server request and private key:

cd server
openssl req -new -config server.cnf -keyout server.key -out server.req
chmod 400 server.key

Copy server.req to the CA machine, and run on the CA machine:

cp server/server.req ca/
cd ca
openssl ca -config -extensions X509_server ca.cnf -in server.req -out server.crt

Copy server.crt to the client machine.

cp ca/server.crt server/

You can delete server.req. Only server.crt and server.key are required on the server.

If you have multiple servers, repeat this step for all servers.

Build client certificate

(roughly equivalent to pkitool --interact --csr myclient)

On the client machine

openssl req -new -config client1.cnf -keyout client1.key -out client1.req
chmod go-rw client1.key

Copy client1.req to the CA machine, and run on the CA machine:

openssl ca -config ca.cnf -out client1.crt -in client1.req

Copy client1.crt to the client machine.

You can delete client1.req. Only client1.crt and client1.key are required on the client.

Repeat this step for all client hosts.

Revoke a client certificate

if the file crlnumber does not exist:

echo '0000000000000001' > crlnumber

openssl ca -config ca.cnf -crl_reason superseded -revoke 5FE840894254A22.crt
openssl ca -config ca.cnf -gencrl -out crl.pem

where the reason is one of the following

• unspecified
• keyCompromise
• CACompromise
• affiliationChanged
• superseded
• cessationOfOperation
• certificateHold

Test if you receive an error (code 23):

cat ca.crt crl.pem > revoke-test.pem
openssl verify -CAfile revoke-test.pem -crl_check 5FE840894254A22.crt