Public Key Cryptography Standards
PKCS is a family of numbered standards, originally published by RSA Laboratories, that define how public-key cryptography is used in practice. Well-known members include PKCS #1 for RSA encryption and signatures, PKCS #7 for signed and encrypted messages, PKCS #11 for talking to hardware tokens, and PKCS #12 for bundling keys and certificates into a single file. Many of these have since been republished as IETF RFCs and remain foundational to certificates, smart cards, and TLS deployments.