Note
TLS is the successor to SSL but may still be incorrectly referred to as SSL.
Before you can do anything with TLS certificates, you need to choose a Certificate Authority (CA) to use. For public facing websites with HTTPS, you need to use a Root Certificate Authority which will not be covered by this document. For internal services using self-signed certificates, a self-signed Certificate Authority is sufficient because you control every end.
The following command is required:
openssl
This can be installed on Debian based distributions (e.g., Ubuntu) with apt-get:
sudo apt-get install openssl
Every certificate should have a subject which describes the owner and use of the certificate:
/C=<country>/ST=<state>/L=<locality>/O=<organization>/OU=<organization-unit>/CN=<common-name>/emailAddress=<email>
Where:
- <country> is the country (e.g., "US").
- <state> is the state or province (e.g., "Michigan").
- <locality> is the locality or city (e.g., "Grand Rapids").
- <organization> is the name of the organization or individual (e.g., "Acme Inc.").
- <organization-unit> is the organization unit (e.g., "IT").
- <common-name> is the common name for the certificate (e.g., "Internal Server Certificate").
- <email> optionally is a contact email address (e.g., "[email protected]").
Example:
/C=US/ST=Michigan/L=Grand Rapids/O=Acme Inc./OU=IT/CN=Internal Server Certificate/[email protected]
See Certificate subject X.509 for more information.
A self-signed root certificate can be created with:
openssl req -new -newkey rsa:4096 -nodes -x509 -days <days> -keyout <key> -out <cert> -subj <subject>
Where:
- <days> is the number of days the root certificate is valid. It is impossible to create a certificate which is valid indefinitely, but you can any duration.
- <key> is the file to store the private key of the certificate (e.g.,
ca_key.pem). - <cert> is the file to store the public certificate (e.g.,
ca_cert.pem). - <subject> is as described in the Subjects section above.
Example:
openssl req -new -newkey rsa:4096 -nodes -x509 -days 365 -keyout ca_key.pem -out ca_cert.pem -subj '/C=US/ST=Michigan/L=Grand Rapids/O=Acme Inc./OU=IT/CN=Internal Root Certificate/[email protected]'
This creates a root private key ca_key.pem and public certificate
ca_cert.pem.
Now we can create certificates which are signed by the CA certificate. Regular certificates are created in 2 steps.
- A private key and Certificate Signing Request (CSR) for that key must be created.
- The CSR must be signed by the CA to generate the actual signed certificate for the private key.
First, create a private key:
openssl genpkey -algorithm rsa -pkeyopt rsa_keygen_bits:2048 -out <key>
Then, create the CSR for the private key:
openssl req -new -key <key> -out <csr> -subj <subject>
Or, create a private key and CSR in one step:
openssl req -new -newkey rsa:2048 -nodes -keyout <key> -out <csr> -subj <subject>
Where:
- <key> is the file to store the private key of the future certificate (e.g., "server_key.pem").
- <csr> is the file to store the CSR which needs to be signed (e.g., "server_csr.pem")
- <subject> is as described in the Subjects section above.
Example:
openssl req -new -newkey rsa:2048 -nodes -keyout server_key.pem -out server_csr.pem -subj '/C=US/ST=Michigan/L=Grand Rapids/O=Acme Inc./OU=IT/CN=Internal Server Certificate/[email protected]'
This creates a new private key server_key.pem and a CSR server_csr.pem
which still needs to be signed.
Now, sign the CSR with the CA:
openssl x509 -req -days <days> -extfile /etc/ssl/openssl.cnf -extensions usr_cert -CAcreateserial -CA <ca-cert> -CAkey <ca-key> -in <csr> -out <cert>
Where:
- <days> is the number of days the signed certificate should be valid for (e.g., 365).
- <ca-cert> is the CA (or signing) public certificate (e.g.,
ca_cert.pem). - <ca-key> is the CA (or signing) private key (e.g.,
ca_key.pem). - <csr> is the CSR being signed (e.g.,
server_csr.pem). - <cert> is the new certificate generated from the CSR which was signed by
the CA (e.g.,
server_cert.pem).
Note
This will create a serial file with the same base name as
<ca-cert> but with a ".srl" extension (e.g., ca_cert.srl). The
serial file is created the first time a certificate is signed by the CA,
and is used to give a unique serial number to every certificate signed by
the CA. KEEP THIS FILE WHERE YOU SIGN CERTIFICATES WITH THE CA.
Example:
openssl x509 -req -days 365 -extfile /etc/ssl/openssl.cnf -extensions usr_cert -CAcreateserial -CA ca_cert.pem -CAkey ca_key.pem -in server_csr.pem -out server_cert.pem
This creates the new public certificate server_cert.pem from the CSR
server_csr.pem which was signed by the CA using ca_key.pem,
ca_cert.pem, and ca_cert.srl.
Certificates can be created and signed using a chain of CAs. Any signed
certificate can act as a CA. A certificate client_cert.pem could be
created by creating a new private key client_key.pem and CSR
client_csr.pem, and by signing the CSR with the server certificate acting
as the CA using server_key.pem, server_cert.pem, and
server_cert.srl (automatically generated).
Example:
openssl req -new -nodes -keyout client_key.pem -out client_csr.pem -subj '/C=US/ST=Michigan/L=Grand Rapids/O=Acme Inc./OU=IT/CN=Internal Client Certificate/[email protected]' openssl x509 -req -days 365 -extfile /etc/ssl/openssl.cnf -extensions usr_cert -CAcreateserial -CA server_cert.pem -CAkey server_key.pem -in client_csr.pem -out client_cert.pem
A certificate can be verified, but the whole certificate chain (excluding the one being verified) must be concatenated into a single file:
openssl verify -CAfile <ca-chain> <cert>
Where:
- <ca-chain> is a file containing each public certificate in the chain which signed the certificate being verified.
- <cert> is the file containing the certificate to verify.
Example:
cat ca_cert.pem server_cert.pem >> ca_chain.pem openssl verify -CAfile ca_chain.pem client_cert.pem
.. TODO:: How to verify a public and private key pair.