picture of tbs certificates
picture of tbs certificates
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Certificates extension

Here are the most used file formats to store X509 certificates, cryptographic keys or cryptographic operations results:


Used to encode X509 certificates with ASN.1 notation
Usual extensions: .der, .cer, .crt, .cert


Can contain private keys, public keys and X509 certificates. PEM format is base64 encoded DER to which are added ASCII headers.
Usual extensions: .pem, .cer, .crt, .cert


A Microsoft specification, ancestor of PKCS#12. It is not used anymore excepted in old products (Netscape 4.03 and former ones).
Note though that Microsoft still uses the .pfx extension to designate files in PKCS#12 format. It can be confusing.


This specification is part of RSA's Public-Key Cryptography Standards.
PKCS#7 describes the syntax of data that has been subject to cryptographic operations such as electronic signature or encryption. It is recursivity-compatible: encryption of signed data for example. The version 1.5 is specified in RFC 2315. The Cryptographic Message Syntax (CMS, RFC 2630) specification, used by S/MIME 3, comes from PKCS#7.
PKCS#7 files usual extensions: .p7b, .p7s (signed data), .p7m (enciphered data)


This specification is part of RSA's Public-Key Cryptography Standards. It is used to store private keys, public keys and certificate safely in terms of confidentiality and integrity (either with password - in most cases - or via asymmetric cryptography - more scarce and not used by general public products). Data is stored in binary format.

It is the commonly used format to store a certificate and its private key in a file protected with a password (confidentiality and integrity). This format is used by Mozilla and Internet Explorer/Outlook to import and export a certificate and its private key.
PKCS#12 files usual extensions: .p12, .pfx (Microsoft uses this extension to designate PKCS#12 files; see PFX section)


It is the proprietary format used by Microsoft to store signature private keys in several of its products. The associated public keys are stored in .spc files.

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