PKCS, which stands for “Public-Key Cryptography Standards,” is a set of specifications and standards that define various aspects of public-key cryptography. PKCS was developed to promote interoperability between different systems and applications that use public-key cryptography for secure communication, digital signatures, and encryption. The standards are published by RSA Security, a well-known company in the field of cryptography.

The PKCS standards cover a range of cryptographic functions and protocols, each identified by a number. Here are some of the most notable PKCS standards:

  1. PKCS #1: Defines standards for RSA cryptography, including encryption, signature generation, and key exchange.
  2. PKCS #5: Specifies the syntax for password-based encryption schemes, such as password-based key derivation functions (PBKDF) used to derive encryption keys from passwords.
  3. PKCS #7: Defines the syntax for Cryptographic Message Syntax (CMS), which is used for digitally signing, encrypting, and authenticating messages. It’s also used in S/MIME (Secure/Multipurpose Internet Mail Extensions) for securing email communication.
  4. PKCS #8: Describes the syntax for private key information, allowing private keys to be stored in a portable and secure format.
  5. PKCS #10: Defines the syntax for Certificate Signing Requests (CSRs), which are used to request digital certificates from Certificate Authorities (CAs).
  6. PKCS #11: Also known as Cryptoki, this standard defines an API (Application Programming Interface) for accessing cryptographic tokens, such as hardware security modules (HSMs) and smart cards.
  7. PKCS #12: Specifies a standard for storing private keys, certificates, and other sensitive information in a portable format, often used for secure backup and migration.
  8. PKCS #15: Defines a standard for the structure of personal identity verification (PIV) cards, commonly used for secure access to physical and logical resources.
  9. PKCS #20: Specifies a syntax for cryptographic tokens that can be used for timestamping digital signatures.
  10. PKCS #21: Defines the syntax for attribute certificates, which provide additional information about a subject of a certificate.

These standards provide a common language for different applications and systems to communicate securely using public-key cryptography. PKCS standards are widely used in various security protocols, including HTTPS, S/MIME, and secure email, and are implemented in many cryptographic libraries and software solutions.