Create a Self-Signed Certificate Using OpenSSL

This article covers how to create a self-signed certificate using OpenSSL.

Self-signed certificates can be useful during software development, or for deployment of a web application to a low-risk internal network.
Self-signed certificates should not …


This content originally appeared on DEV Community and was authored by Thomas K

This article covers how to create a self-signed certificate using OpenSSL.

  • Self-signed certificates can be useful during software development, or for deployment of a web application to a low-risk internal network.
  • Self-signed certificates should not be used for publicly facing servers.

Create a Self-Signed Cert and Private Key

  • openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout private.key -out public.crt
    • req specifies that we want to use X.509 Certificate Signing Request (CSR) Management.
    • -x509 specifies that we want to output a self signed certificate instead of a certificate request.
    • -nodes tells OpenSSL not to encrypt the private key. This is important because the server will need to read it without user interaction.
    • -days 365 specifies the number of days to certify the certificate. In this example, we specify 365 days.
    • -newkey rsa:2048 creates a new certificate and new private key. In this example, we specify an RSA key of 2048 bits.
    • -keyout specifies the filename to write the private key to.
    • -out specifies the filename to write the certificate to.

View the Private Key

  • openssl rsa -noout -text -in private.key
    • The rsa command processes RSA keys.
    • -noout prevents output of the encoded version.
    • -text prints out the various public or private key components in plain text in addition to the encoded version.
    • -in specifies the input filename to read a key from.

View the Public Certificate

  • openssl x509 -noout -text -in public.crt
    • The x509 command is used to display certificate information.
    • -noout prevents output of the encoded version.
    • -text prints out the certificate in text form.
    • -in specifies the input filename to read a certificate from.

Check Compatibility

  • OpenSSL stores a modulus in both the private key and public certificate.

  • We can check compatibility of the pair by comparing the the MD5 checksums of each modulus.

    openssl x509 -noout -modulus -in public.crt | openssl md5 > public.md5
openssl rsa -noout -modulus -in private.key | openssl md5 > private.md5
diff public.md5 private.md5

  • If there are any differences in the md5 hashes, then the public cert and private key are not compatible.

Resources


This content originally appeared on DEV Community and was authored by Thomas K


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