Code change:
TLS_AES_128_GCM_SHA256 and TLS_AES_256_GCM_SHA384 would yield two PSKs, one for HKDF-SHA256 and another for HKDF-SHA384.  In contrast, if TLS_AES_128_GCM_SHA256 and TLS_CHACHA20_POLY1305_SHA256 are <del> supported, only one derived key is necessary. </del> <ins> supported, only one derived key is necessary.  Each ciphersuite uniquely identifies the target KDF.  Future specifications that change the way the KDF is negotiated will need to update this specification to make clear how target KDFs are determined for the import process. </ins> EPSKs MAY be imported before the start of a connection if the target KDFs, protocols, and context string(s) are known a priori.  EPSKs MAY <del> also be imported for early data use if they are bound to protocol settings and configurations that would otherwise be required for early data with normal (ticket-based PSK) resumption.  Minimally, that means Application-Layer Protocol Negotiation RFC7301, QUIC transport parameters (if used for QUIC), and any other relevant parameters that are negotiated for early data MUST be provisioned alongside these EPSKs. </del> <ins> also be imported for early data use if they are bound to the protocol settings and configuration that are required for sending early data. Minimally, that means Application-Layer Protocol Negotiation value RFC7301, QUIC transport parameters (if used for QUIC), and any other relevant parameters that are negotiated for early data MUST be provisioned alongside these EPSKs. </ins> 4.2.

Related discussion:
text: Section 4.1 The PSK Importer interface takes as input an EPSK with External Identity "externalidentity" and base key "epsk", as defined in Section 3.1, along with an optional context, and transforms it into a set of PSKs and imported identities for use in a connection based on target protocols and KDFs. In particular, for each supported target protocol "targetprotocol" and KDF "targetkdf", the importer constructs an ImportedIdentity structure as follows: If I understand correctly the "targetkdf" is supposed to be the KDF associated with the cipher suite(s) the derived PSK will be usable for. This is something of a divergence from RFC 8446, where we assume that the KDF will be HKDF and associate only a hash function with the cipher suite. While discussing a more generic KDF concept seems reasonable (and is directly in line with the cryptographic principles that motivate this work), I'd recommend adding a few more words somewhere in this section to clarify the relationship between the targetkdf and the cipher suite(s), probably in the paragraph that starts "[e]ndpoints SHOULD generate a compatible 'ipskx' for each target ciphersuite they offer". struct { opaque externalidentity; opaque context; uint16 targetprotocol; uint16 targetkdf; } ImportedIdentity; Should we say that this is using the TLS presentation language? ImportedIdentity.context MUST include the context used to derive the EPSK, if any exists. For example, ImportedIdentity.context may (nit?) "derive" suggests a cryptographic key derivation process, but the rest of the prose suggests that this is more a process of "identification" or "determination". Is there a more appropriate word to use? Given an ImportedIdentity and corresponding EPSK with base key "epsk", an Imported PSK IPSK with base key "ipskx" is computed as follows: epskx = HKDF-Extract(0, epsk) ipskx = HKDF-Expand-Label(epskx, "derived psk", Hash(ImportedIdentity), L) I think we need to say that the HKDF-Expand-Label used (i.e., the URL prefix) is the one corresponding to ImportedIdentity.targetprotocol.