General Information

Openssl is a tool to perform many certificate related tasks such as creating a CSR, verifying certs+keys, and converting formats.


  • Distro(s): Any

Certificate Encoding

  • Privacy Enhanced Mail (PEM) - One of the most common certificate encodings. ASCII format.
  • PKCS #7 B (P7B) - Represents a set of certificates. (IE a certificate chain)
  • PKCS #12/PFX/P12 - Lets you put a private key and certificate into a single file.
  • Distinguished Encoding Rules (DER) - Binary format most commonly used to represent certificates.

Common Extensions

  • .crt - Used for certificates, commonly on *nix systems.
  • .cer - Used for certificates, commonly on Windows.
  • .key - Public/private pkcs keys, encoded as binary DER or ASCII PEM.

Generate Certificate Signing Requests

Generating certificate signing requests to send to a certificate authority.

openssl req -new -newkey rsa:2048 -nodes -out MYSITE.csr -keyout MYSITE.key

openssl req -sha256 -new -key MYSITE.key -out MYSITE.csr

openssl x509 -x509toreq -in MYSITE.crt -signkey MYSITE.key -out MYSITE.csr

Self-Signed Certificates

Self-signed certificates are for development/home use. They encrypt traffic just fine, but end users will see a warning message since the cert is not signed by a valid certificate authority.

Generate a self-signed cert and private key from scratch

openssl req -newkey rsa:2048 -nodes -keyout MYSITE.key -x509 -days 365 -out MYSITE.crt

Generate a self-signed cert from an existing private key

openssl req -key MYSITE.key -new -x509 -days 365 -out MYSITE.crt

Generate a self-signed cert from an existing private key and existing CSR

openssl x509 -signkey MYSITE.key -in MYSITE.csr -req -days 365 -out MYSITE.crt

Certificate Conversions

Converting certificates from one type to another.

  • Extract Key
    openssl pkcs12 -in mycertpack.pfx -nocerts -nodes | sed -ne '/-BEGIN PRIVATE KEY-/,/-END PRIVATE KEY-/p' > mykey.key
  • Extract Certificate
    openssl pkcs12 -in mycertpack.pfx -clcerts -nokeys | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' > mycert.crt
  • Extract Certificate Authority
    openssl pkcs12 -in mycertpack.pfx -cacerts -nokeys -chain | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' > myCA.crt

openssl x509 -inform der -in MYSITE.cer -out MYSITE.pem

openssl x509 -outform der -in MYSITE.pem -out MYSITE.der

openssl pkcs12 -in MYSITE-KEYSTORE.pfx -out MYSITE.pem -nodes

openssl pkcs12 -in mysite.pfx -nocerts -out mysite.key.pem
openssl rsa -in mysite.key.pem -out mysite.key
openssl pkcs12 -in mysite.pfx -clcerts -nokeys -out mysite.crt

Convert p7b to PEM combined, then convert to bundle of certs

openssl pkcs7 -inform DER -outform PEM -in mysite.p7b -out mysite.p7b.pem
openssl pkcs7 -print_certs -in mysite.p7b.pem -out mysite.p7b.bundle

View the “mysite.p7b.bundle” file:

  • The top BEGIN/END block is the client cert
    • Copy that single BEGIN/END into a new file for the client cert
  • The rest is the certificate chain
    • Copy all the rest into a new file for the intermediate chain cert

Cert+Key Matching

Openssl can be used to very that a certificate and key match.

Compare to ensure they match

openssl x509 -noout -text -in mysite.crt
openssl rsa -noout -text -in mysite.key

Similar method, but running output through md5 hash for a shorter comparison

openssl x509 -noout -text -in mysite.crt | openssl md5
openssl rsa -noout -text -in mysite.key | openssl md5

Displaying Certificate Contents

Display Certificate Contents

openssl x509 -in mysite.crt -text

Display CSR Contents

openssl req -in mysite.csr -text


To verify that an intermediate cert and client certificate pass a chain of authority test:

openssl verify -CAfile mysites_intermediate.crt mysite.crt

Remotely check a site's certificate and fingerprint it

openssl s_client -connect <domain>:443 -showcerts | openssl x509 -text -fingerprint

  • linux_wiki/openssl.txt
  • Last modified: 2019/05/25 23:50
  • (external edit)