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from: https://www.sslshopper.com/article-most-common-openssl-commands.html

>> Check a certificate

# openssl x509 -in certificate.crt -text -noout

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

# openssl pkcs12 -info -in keyStore.p12

>> Check an MD5 hash of the public key to ensure that it matches with what is in a CSR or private key

# openssl x509 -noout -modulus -in certificate.crt | openssl md5

# openssl rsa -noout -modulus -in privateKey.key | openssl md5

# openssl req -noout -modulus -in CSR.csr | openssl md5

>> Check an SSL connection. All the certificates (including Intermediates) should be displayed

# openssl s_client -connect www.paypal.com:443

>> Convert a DER file (.crt .cer .der) to PEM

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

>> Convert a PEM file to DER

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

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

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

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