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PEM
PEM ("Privacy Enhanced Mail") is the common format for X.509 certificates, CSRs ("Certificate Signing Request"), and cryptographic keys. A PEM file is a text file containing one or more items in Base64 ASCII encoding, each with plain-text headers and footers (e.g. -----BEGIN CERTIFICATE----- and -----END CERTIFICATE-----). A single PEM file could contain an end-entity certificate, a private key, or multiple certificates forming a complete chain of trust.
PEM Filename Extensions
PEM files are usually seen with the extensions .crt, .pem, .cer, and .key (for private keys)
# You can read the contents of a PEM certificate (<file>.cer) using the
# 'openssl' command on Linux or Windows as follows:
openssl x509 -in <file>.cer -text
DER
DER ("Distinguished Encoding Rules") is a binary encoding for X.509 certificates and private keys. Unlike PEM, DER-encoded files do not contain plain text statements such as -----BEGIN CERTIFICATE-----. DER files are most commonly seen in Java contexts.
DER Filename Extensions
DER-encoded files are usually found with the extensions .der and .cer.
# You can't read the contents of a DER certificate (<file>.der) in the
# way as descriped for the PEM certificat. You will get an error if you do so:
openssl x509 -in <file>.cer -text
# You have to use the following 'openssl' command:
openssl x509 -inform der -in <file>.der -text -noout
P7B
PKCS#7 (also known as P7B) is a container format for digital certificates that is most found in Windows and Java server contexts, and usually has the extension .p7b. PKCS#7 files are not used to store private keys. In the example below, you can see that the PB7- file contains 3 certificates (in this case a complete chain - the server - , intermediate - and root - certificate).
# You can read the contents of a PB7 File (<file>.pb7) using the
# 'openssl' command on Linux or Windows as follows:
openssl pkcs7 -print_certs -in <file>.p7b
PFX
PKCS#12 (also known as PKCS12 or PFX) is a common binary format for storing a certificate chain and private key in a single, encryptable file, and usually have the filename extensions .p12 or .pfx.
# You can read the contents of a PFX File (<file>.pfx) using the
# 'openssl' command on Linux or Windows as follows:
openssl pkcs12 -info -in <file>.pfx
Certificate conversion
# Conversion of PKCS#12 ( .pfx .p12, typically used on Microsoft Windows)
# files with private key and certificate to PEM (typically used on Linux):
openssl pkcs12 -nodes -in <file>.pfx -out <file>.crt
# PKCS#12 Key Extraction
openssl pkcs12 -in <file>.pfx -out <file>.key -nodes -nocerts
# Conversion of PEM to PKCS#12:
openssl pkcs12 -export -in <file>.crt -inkey <file>.key -out <file>.pfx
# Conversion of PKCS#7 format ( .p7b .p7c ) to PEM:
openssl pkcs7 -print_certs -in <file>.p7b -out <file>.cer
# Conversion of PEM format to PKCS#7:
openssl crl2pkcs7 -nocrl -certfile <file>.crt -out <file>.p7b
# Conversion of DER (.crt .cer or .der) to PEM:
openssl x509 -inform der -in <file>.der -out <file>.pem
# Conversion from PEM to DER format:
openssl x509 -outform der -in certificate.pem -out certificate.cer
References:
MSXFAQ
PEM, DER, CRT, and CER: X.509 Encodings and Conversions
What Is an X.509 Certificate?
useful commands