CryptoSys PKI Pro Manual

Hybrid Public Key Encryption (HPKE)

[New in v22.0] This toolkit has all the basic cryptographic primitives required to carry out the hybrid public key encryption (HPKE) scheme described in [RFC9180]; namely Elliptic Curve Diffie-Hellman (ECDH) key agreement (see ECC_DHSharedSecret), HMAC-based key derivation function (HKDF) using SHA2 (see KDF_Bytes), and authenticated encryption with additional data (AEAD) (see CIPHER_EncryptAEAD).

Specific functions HPKE_LabeledExtract and HPKE_LabeledExpand are provided here to carry out LabeledExtract() and LabeledExpand(). The function HPKE_DerivePrivateKey is provided to derive an EC private key using the deterministic method described in HKPE. The corresponding public key can be derived using ECC_PublicKeyFromPrivate.

Note that these functions are intended to be used in an object-oriented language like C# or Python, not in raw ANSI C or VB6. See the code on our web site for an example.

In this implementation, the KDF algorithm is chosen automatically to match the KEM ECDH group curve as follows (from Table 2 in [RFC9180]).


Furthermore, the ciphersuite's KDF is assumed to be always equal to the DHKEM's associated KDF from the above table.

Because these are all standalone functions with no context, the ECDH curve group used in the scheme must be specified. This automatically defines the KDF and associated HMAC algorithm to be used as per the above table.

The LabeledExtract() and LabeledExpand() functions facilitate domain separation of KDF calls by incorporating a label and a suite_id which has a value derived from identifiers for the EC curve group, the KDF algorithm and, sometimes, the AEAD algorithm. Note that the suite_id value is different depending on where the KDF is used. In this implementation, specifying the curve name will automatically select the corresponding KDF algorithm, and the absence or presence of an option flag for an AEAD algorithm dictates whether the KDF is being used inside a KEM algorithm (zero flag) or in the remainder of HPKE (specific AEAD algorithm flag).

The string literal "HPKE-v1" is currently hardcoded into the LabeledExtract() and LabeledExpand() functions. Future implementations may offer an alternative if the specification is changed.

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