Code change:
6. <del> Recall that TLS 1.2 permits computing the TLS PRF with any hash algorithm and PSK.  Thus, an EPSK may be used with the same KDF (and underlying HMAC hash algorithm) as TLS 1.3 with importers.  However, critically, the derived PSK will not be the same since the importer differentiates the PSK via the identity and target KDF and protocol. Thus, PSKs imported for TLS 1.3 are distinct from those used in TLS 1.2, and thereby avoid cross-protocol collisions.  Note that this does not preclude endpoints from using non-imported PSKs for TLS 1.2. Indeed, this is necessary for incremental deployment.  Specifically, existing applications using TLS 1.2 with non-imported PSKs can safely enable TLS 1.3 with imported PSKs in clients and servers without interoperability risk. </del> <ins> The mechanism defined in this document requires that an EPSK is only ever used as an EPSK and not for any other purpose.  In particular, this requirement disallows direct use of the EPSK as a PSK in TLS 1.2.  The importer process produces distinct IPSKs derived from the target protocol and KDF, which in turn protects against cross- protocol collisions for protocol versions using this process by ensuring that each IPSK can only be used with one protocol and KDF. This is a distinct contrast to TLS 1.2, where a given PSK might be used with multiple KDFs in different handshakes, and importers are not available.  Furthermore, the KDF used in TLS 1.2 might be the same KDF used by the importer mechanism itself. In deployments that already have PSKs provisioned and in use with TLS 1.2, attempting to incrementally deploy the importer mechanism would then result in concurrent use of the already provisioned PSK both directly as a TLS 1.2 PSK and as an EPSK, which in turn could mean that the same KDF and key would be used in two different protocol contexts.  There are no known related outputs or security issues that would arise from this arrangement.  However, only limited analysis has been done, and as such is not a recommended configuration. However, the benefits of using TLS 1.3 and of using PSK importers may prove sufficiently compelling that existing deployments choose to enable this noncompliant configuration for a brief transition period while new software (using TLS 1.3 and importers) is deployed. Operators are advised to make any such transition period as short as possible. </ins> 7.

Related discussion:
text: Accordingly, a given EPSK might be both used directly as a TLS 1.2 PSK and used with TLS 1.3 via the importer mechanism (noting that this configuration is not recommended, per Section 1), using the same KDF algorithm. However, because the TLS 1.3 importer mechanism includes an additional key-derivation step, the actual PSK used in the TLS 1.3 key schedule will be distinct from the one used in the TLS 1.2 key schedule, so there are no cross-protocol collisions possible. Note that this does not preclude endpoints from using non-imported PSKs for TLS 1.2. For some reason this line implies to me that endpoints might use imported PSKs for TLS 1.2, but this is forbidden by Section 4.1 (as quoted above). Perhaps "does not preclude endpoints from continuing to use TLS 1.2 and the non-imported PSKs it requires"? Section 7 We should probably note that any information placed in ImportedIdentity.context will be visible on the wire in cleartext, and thus that confidential identifiers from other protocols should either not be used or should be protected (e.g., by hashing) prior to use. Externally provisioned PSKs imported into a TLS connection achieve compound authentication of the provisioning process and connection. Context-free PSKs only achieve authentication within the context of a single connection. Do we need to use pskdhke to get the strong compound authentication property, or is it attainable with pskke as well? Section 10.1 I'm not sure why [QUIC] and RFC 5246 are listed as normative references. Appendix B The Selfie attack [Selfie] relies on a misuse of the PSK interface. The PSK interface makes the implicit assumption that each PSK is known only to one client and one server. If multiple clients or multiple servers with distinct roles share a PSK, TLS only authenticates the entire group. A node successfully authenticates its peer as being in the group whether the peer is another node or itself.