9.6. Domain Separation

9.6. Domain Separation

HPKE allows combining a DHKEM variant DHKEM(Group, KDF') and a KDF such that both KDFs are instantiated by the same KDF. By design, the calls to Extract() and Expand() inside DHKEM and the remainder of HPKE use separate input domains. This justifies modeling them as independent functions even if instantiated by the same KDF. This domain separation between DHKEM and the remainder of HPKE is achieved by using prefix-free sets of suite_id values in LabeledExtract() and LabeledExpand() (KEM... in DHKEM and HPKE... in the remainder of HPKE). Recall that a set is prefix-free if no element is a prefix of another within the set.

Future KEM instantiations MUST ensure, should Extract() and Expand() be used internally, that they can be modeled as functions independent from the invocations of Extract() and Expand() in the remainder of HPKE. One way to ensure this is by using LabeledExtract() and LabeledExpand() with a suite_id as defined in Section 4, which will ensure input domain separation, as outlined above. Particular attention needs to be paid if the KEM directly invokes functions that are used internally in HPKE's Extract() or Expand(), such as Hash() and HMAC() in the case of HKDF. It MUST be ensured that inputs to these invocations cannot collide with inputs to the internal invocations of these functions inside Extract() or Expand(). In HPKE's KeySchedule() this is avoided by using Extract() instead of Hash() on the arbitrary-length inputs info and psk_id.

The string literal "HPKE-v1" used in LabeledExtract() and LabeledExpand() ensures that any secrets derived in HPKE are bound to the scheme's name and version, even when possibly derived from the same Diffie-Hellman or KEM shared secret as in another scheme or version.