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RFC 9180: Hybrid Public Key Encryption (2022)

This document describes a scheme for hybrid public key encryption (HPKE). This scheme provides a variant of public key encryption of arbitrary-sized plaintexts for a recipient public key. It also includes three authenticated variants, including one that authenticates possession of a pre-shared key and two optional ones that authenticate possession of a key encapsulation mechanism (KEM) private key. HPKE works for any combination of an asymmetric KEM, key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. Some authenticated variants may not be supported by all KEMs. We provide instantiations of the scheme using widely used and efficient primitives, such as Elliptic Curve Diffie-Hellman (ECDH) key agreement, HMAC-based key derivation function (HKDF), and SHA2. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF.

All these existing schemes have problems, e.g., because they rely on outdated primitives, lack proofs of indistinguishable (adaptive) chosen-ciphertext attack (IND-CCA2) security, or fail to provide test vectors. A preliminary computational analysis of all HPKE modes has been done in [ HPKEAnalysis], indicating asymptotic security for the case where the KEM is DHKEM, the AEAD is any IND-CPA-secure and INT-CTXT-secure scheme, and the DH group and KDF satisfy the following conditions: ¶ [keyagreement] Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R. Davis, "Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography", NIST Special Publication 800-56A Revision 3, DOI 10.6028/nist.sp.800-56ar3, April 2018, < https://doi.org/10.6028/nist.sp.800-56ar3>.

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