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OpenSSL is a cryptography toolkit implementing the Secure Sockets Layer (SSL v2/v3) and Transport Layer Security (TLS v1) network protocols and related cryptography standards required by them.
The openssl program is a command line tool for using the various cryptography functions of OpenSSL’s crypto library from the shell. It can be used for:

• Creation and management of private keys, public keys and parameters
• Public key cryptographic operations
• Creation of X.509 certificates, CSRs and CRLs
• Calculation of Message Digests
• Encryption and Decryption with Ciphers
• SSL/TLS Client and Server Tests
• Handling of S/MIME signed or encrypted mail
• Time Stamp requests, generation and verification

OpenSSL Commands:

How to generate a new private key and Certificate Signing Request

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

How to generate a self-signed certificate

openssl req -x509 -sha256 -nodes -days 365 -newkey rsa:2048 -keyout privateKey.key -out certificate.crt

How to generate a certificate signing request (CSR) for an existing private key

openssl req -out CSR.csr -key privateKey.key -new

How to generate a certificate signing request based on an existing certificate

openssl x509 -x509toreq -in certificate.crt -out CSR.csr -signkey privateKey.key

How to remove a passphrase from a private key

openssl rsa -in privateKey.pem -out newPrivateKey.pem

How to a Certificate Signing Request (CSR)

openssl req -text -noout -verify -in CSR.csr

How to check a private key

openssl rsa -in privateKey.key -check

How to check a certificate

openssl x509 -in certificate.crt -text -noout

How to check CA certificate

openssl verify -CAfile <ca-bundle.crt> <certificate.crt>

Check a PKCS#12 file (.pfx or .p12)

openssl pkcs12 -info -in keyStore.p12

How to convert a DER file (.crt .cer .der) to PEM

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

How to convert a PEM file to DER

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

How to convert a PKCS#12 file (.pfx .p12) containing a private key and certificates to PEM

openssl pkcs12 -in keyStore.pfx -out keyStore.pem -nodes

You can add -nocerts to only output the private key or add -nokeys to only output the certificates.

How to convert a PEM certificate file and a private key to PKCS#12 (.pfx .p12)

openssl pkcs12 -export -out certificate.pfx -inkey privateKey.key -in certificate.crt -certfile CACert.crt

How to generate DH parameter

openssl dhparam -dsaparam -out dhparam.pem 4096