In summary, there are four different ways to present certificates and their components:
NOTE: A general rule of thumb is:
Certificate Signing Request.
Some applications can generate these for submission to certificate-authorities.
The actual format is PKCS10 which is defined in RFC 2986.
It includes some/all of the key details of the requested certificate such as subject, organization, state, whatnot, as well as the public key of the certificate to get signed.
These get signed by the CA and a certificate is returned.
The returned certificate is the public certificate (which includes the public key but not the private key), which itself can be in a couple of formats.
The name is from Privacy Enhanced Mail (PEM), a failed method for secure email but the container format it used lives on, and is a base64 translation of the x509 ASN.1 keys.
Defined in RFCs 1421 through 1424.
This is a container format that may include just the public certificate (such as with Apache installs, and CA certificate files /etc/ssl/certs), or may include an entire certificate chain including public key, private key, and root certificates.
Confusingly, it may also encode a CSR (e.g. as used here) as the PKCS10 format can be translated into PEM.
NOTE: PEM files can actually include multiple certificates (up to or maybe more than 4 certificates).
This can actually be required for OpenSSL to verify the full chain of authority.
Example:
-----BEGIN CERTIFICATE----- (Your Primary SSL certificate: your_domain_name.crt) -----END CERTIFICATE----- -----BEGIN CERTIFICATE----- (Your Intermediate certificate: CertCA.crt) -----END CERTIFICATE----- -----BEGIN CERTIFICATE----- (Your Root certificate: TrustedRoot.crt) -----END CERTIFICATE-----
This is a PEM formatted file containing just the private-key of a specific certificate and is merely a conventional name and not a standardized one.
In Apache installs, this frequently resides in /etc/ssl/private.
The rights on these files are very important, and some programs will refuse to load these certificates if they are set wrong.
Originally defined by RSA in the Public-Key Cryptography Standards (abbreviated PKCS), the “12” variant was originally enhanced by Microsoft, and later submitted as RFC 7292.
This is a passworded container format that contains both public and private certificate pairs.
Unlike .pem files, this container is fully encrypted.
To convert a PKCS#12 file (.pfx .p12) containing a private key and public key (certificates) to PEM:
openssl pkcs12 -in keyStore.pfx -out keyStore.pem -nodes
or
openssl pkcs12 -in file-to-convert.p12 -out converted-file.pem -nodes
To convert a PEM certificate file and a private key to PKCS#12 (.pfx .p12):
openssl pkcs12 -export -out cert.pfx -inkey privateKey.key -in cert.crt -certfile CACert.crt
A way to encode ASN.1 syntax in binary, a .pem file is just a Base64 encoded .der file.
Windows sees these as Certificate files.
By default, Windows will export certificates as .DER formatted files with a different extension. Like…
To convert a DER file (.crt .cer .der) to PEM:
openssl x509 -inform der -in cert.cer -out cert.pem
To convert a PEM file to DER:
openssl x509 -outform der -in cert.pem -out cert.der
A .pem (or rarely .der) formatted file with a different extension, one that is recognized by Windows Explorer as a certificate, which .pem is not.
WARNING: Sometimes a .crt file is already a .pem.
Your keys may already be in PEM format, but just named with .crt or .key.
If the file's content begins with —–BEGIN and you can read it in a text editor:
Defined in RFC 2315 as PKCS number 7, this is a format used by Windows for certificate interchange.
Java understands these natively, and often uses .keystore as an extension instead.
Unlike .pem style certificates, this format has a defined way to include certification-path certificates.
A certificate revocation list.
Certificate Authorities produce these as a way to de-authorize certificates before expiration.
You can sometimes download them from CA websites.