PKCS

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In cryptography, PKCS refers to a group of Public Key Cryptography Standards devised and published by RSA Security.

RSA Data Security Inc was assigned the licensing rights for the patent on the RSA asymmetric key algorithm and acquired the licensing rights to several other key patents as well (e.g., the Schnorr patent). As such, RSA Security, and its research division, RSA Labs, were interested in promoting and facilitating the use of public-key techniques. To that end, they developed the PKCS standards. They retained control over them, announcing that they would make changes/improvements as they deemed necessary, and so the PKCS standards were not, in a significant sense, actual industry standards, despite the name. Some, but not all, have in recent years begun to move into 'standards track' processes with one or more of the standards organizations (notably, the IETF PKIX working group).

PKCS Standards Summary
	Version	Name	Comments
PKCS #1	2.1	RSA Cryptography Standard	See RFC 3447. Defines the format of RSA encryption.
PKCS #2	-	Withdrawn	No longer active. Covered RSA encryption of message digests, but was merged into PKCS #1.
PKCS #3	1.4	Diffie-Hellman Key Agreement Standard	A cryptographic protocol that allows two parties that have no prior knowledge of each other to jointly establish a shared secret key over an insecure communications channel.
PKCS #4	-	Withdrawn	No longer active. Covered RSA key syntax, but was merged into PKCS #1.
PKCS #5	2.0	Password-based Encryption Standard	See RFC 2898 and PBKDF2.
PKCS #6	1.5	Extended-Certificate Syntax Standard	Defines extensions to the old v1 X.509 certificate specification. Obsoleted by v3 of the same.
PKCS #7	1.5	Cryptographic Message Syntax Standard	See RFC 2315. Used to sign and/or encrypt messages under a PKI. Used also for certificate dissemination (for instance as a response to a PKCS#10 message). Formed the basis for S/MIME, which is now based on RFC 3852, an updated Cryptographic Message Syntax Standard (CMS).
PKCS #8	1.2	Private-Key Information Syntax Standard
PKCS #9	2.0	Selected Attribute Types	Defines selected attribute types for use in PKCS #6 extended certificates, PKCS #7 digitally signed messages, PKCS #8 private-key information, and PKCS #10 certificate-signing requests.
PKCS #10	1.7	Certification Request Standard	See RFC 2986. Format of messages sent to a certification authority to request certification of a public key. See certificate signing request.
PKCS #11	2.20	Cryptographic Token Interface (Cryptoki)	An API defining a generic interface to cryptographic tokens (see also Hardware Security Module).
PKCS #12	1.0	Personal Information Exchange Syntax Standard	Defines a file format commonly used to store private keys with accompanying public key certificates, protected with a password-based symmetric key.
PKCS #13	–	Elliptic Curve Cryptography Standard	(Under development)
PKCS #14	–	Pseudo-random Number Generation	(Under development)
PKCS #15	1.1	Cryptographic Token Information Format Standard	Defines a standard allowing users of cryptographic tokens to identify themselves to applications, independent of the application's Cryptoki implementation (PKCS #11) or other API. RSA has relinquished IC-card-related parts of this standard to ISO/IEC 7816-15.[1]