Ubuntu - Certificates - Certificate Types

Common filename extensions for X.509 certificates are:

Filename Extension	Description
.pem	Privacy-enhanced Electronic Mail. Base64 encoded DER certificate, enclosed between “—–BEGIN CERTIFICATE—–” and “—–END CERTIFICATE—–“
.cer, .crt, .der	Usually in binary DER form, but Base64-encoded certificates are common too (see .pem above)
.p7b, .p7c	PKCS#7 SignedData structure without data, just certificate(s) or CRL(s)
.p12	PKCS#12, may contain certificate(s) (public) and private keys (password protected)
.pfx	PFX, predecessor of PKCS#12 (usually contains data in PKCS#12 format, e.g., with PFX files generated in IIS)

PKCS#7 is a standard for signing or encrypting (officially called “enveloping”) data. Since the certificate is needed to verify signed data, it is possible to include them in the SignedData structure. A .P7C file is a degenerated SignedData structure, without any data to sign.[citation needed]
PKCS#12 evolved from the personal information exchange (PFX) standard and is used to exchange public and private objects in a single file.