tldr/pages/common/openssl.md

# openssl

> OpenSSL cryptographic toolkit.

- Generate a 2048bit RSA private key and save it to a file:

`openssl genrsa -out {{filename.key}} 2048`

- Generate a certificate signing request to be sent to a certificate authority:

`openssl req -new -sha256 -key {{filename.key}} -out {{filename.csr}}`

- Generate a self-signed certificate from a certificate signing request valid for some number of days:

`openssl x509 -req -days {{days}} -in {{filename.csr}} -signkey {{filename.key}} -out {{filename.crt}}`

- Display certificate information:

`openssl x509 -in {{filename.crt}} -noout -text`

- Display the start and expiry dates for a domain's certificate:

`openssl s_client -connect {{host}}:{{port}} 2>/dev/null | openssl x509 -noout -dates`

- Display the certificate presented by an SSL/TLS server:

`openssl s_client -connect {{host}}:{{port}} </dev/null`

- Display the complete certificate chain of an HTTPS server:

`openssl s_client -connect {{host}}:443 -showcerts </dev/null`