How to generate a self-signed certificate

This tutorial outlines how to generate your own keys and certificates for your system.

Mbed TLS includes the core and applications for generating keys and certificates without relying on other libraries and applications, giving you a command-line alternative to OpenSSL for generating their keys and (self-signed) certificates.

This article assumes you have compiled and installed the Mbed TLS library on your system.

Generating a RSA key file

The first step for generating a self-signed certificate is to generate a private/public key pair for the certificate.

For generating key files, Mbed TLS includes the gen_key application in programs/pkey.

This key generation application accepts the following arguments:

 usage: gen_key param=<>...

 acceptable parameters:
    type=rsa              default: rsa
    rsa_keysize=%d        default: 4096
    filename=%s           default: keyfile.key
    format=pem|der        default: pem

The following command generates a 4096 bit RSA key file, as explained here:

programs/pkey/gen_key type=rsa rsa_keysize=4096 filename=our_key.key

Generating a self-signed certificate

For generating and writing certificate files, Mbed TLS includes the cert_write application (located in programs/x509).

Before generating the self-signed certificate, determine the different values in it.

In the case of a self-signed certificate, indicate it with selfsign=1.

Key to use in the certificate

First and foremost a certificate binds a public-private key pair to an identity. To indicate which key to use to sign the certificate we use the issuer_key argument, like so issuer_key=our_key.key.

Because this is a self-signed certificate the subject_key is automatically identical.

Subject name / Issuer name

Each certificate is identified with a subject name (the identity that is being signed) and an issuer name (the party doing the signing). For self-signed certificates, only the issuer name is used.

In case we want to name the issuer myserver from the organization myorganization and the country NL, we should use issuer_name=CN=myserver,O=myorganization,C=NL on the command-line.

Note: If you want to use a space in one of the names you have to either nullify it (issuer_name=CN=my\ server) or contain the entire argument in quotes ("issuer_name=CN=my server").

Note: Commas inside names need to be nullified with a backslash as well. You need to protect the backslash from your shell, for example, issuer_name=CN=my\\\,server or issuer_name='CN=my\,server'.

The supported items in a subject_name are:

C = Country
CN = Common Name
L = Locality
O = Organization
OU = Organizational Unit
R = e-mail address
ST = State
serialNumber
postalAddress
postalCode

Validity

Each certificate contains within it the validity period for that certificate indicated by the not_before and not_after values.

The applications accepts values of the format YYYYMMDDHHMMSS, so to indicate that a certificate is not valid before 2013, you can use not_before=20130101000000 and not valid after 2015, you can use not_after=20151231235959.

Is it a CA certificate?

If you want this self-signed certificate to be a CA certificate capable of signing other certificates, enable it with is_ca=1.

You then need to decide if this CA certificate can only sign regular certificates (max_pathlen=0) or if it is at the top of other intermediate CA certificates.

Command to generate a self-signed certificate

Example:

The full command for generating a self-signed CA certificate that cannot sign other CA certificates for our_key.key with the name CN=myserver,O=myorganization,C=NL and valid only in the years 2013, 2014 and 2015 would be:

programs/x509/cert_write selfsign=1 issuer_key=our_key.key                    \
                         issuer_name=CN=myserver,O=myorganization,C=NL        \
                         not_before=20130101000000 not_after=20151231235959   \
                         is_ca=1 max_pathlen=0 output_file=my_crt.crt

The file my_crt.crt now contains your PEM-encoded certificate.

Key usage and NS cert type extension

You can set the key usage extension and the NS Cert type extension in the certificate on the command-line as well with:

    key_usage=%s        default: (empty)
                        Comma-separated-list of values:
                          digital_signature
                          non_repudiation
                          key_encipherment
                          data_encipherment
                          key_agreement
                          key_certificate_sign
                          crl_sign
    ns_cert_type=%s     default: (empty)
                        Comma-separated-list of values:
                          ssl_client
                          ssl_server
                          email
                          object_signing
                          ssl_ca
                          email_ca
                          object_signing_ca