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: (Similarly, is it accurate to say that "[a]pplications SHOULD provision separate PSKs [...], but the mechanism defined in this document provides a partial mitigation for use when that is not possible"? Also, we only use the word "interference" twice in this document (this is the first instance), so it might be worth a couple more words to clarify the nature of the potential interference. Section 3 non-imported keys for TLS versions prior to TLS 1.3. Non-imported and imported PSKs are distinct since their identities are different on the wire. See Section 6 for more details. (side note?) I think the precise story here is a little subtle -- for any given key, the imported and non-imported identities are distinct, but in principle one could construct a non-imported identity that overlaps with an imported identity for a different key (and we say as much at the end of Section 7). This is vanishingly rare to happen by chance, but makes the statement as written not quite categorically true. That said, the change to the binder key derivation (§ 4.2) seems to obviate any potential consequences to such a collision (though the connection attempt would presumably fail). Endpoints which import external keys MUST NOT use either the external keys or the derived keys for any other purpose. Moreover, each IIUC, this is "MUST NOT use the keys that are input to the import process for any purpose other than the importer, and MUST NOT use the derived keys for any purpose other than TLS PSKs". Is it worth spelling it out explicitly like that? external PSK MUST be associated with at most one hash function, as per the rules in Section 4.2.11 from [RFC8446]. See Section 7 for more discussion. Does this requirement apply to the inputs to the importer process, the outputs, or both? Section 3.1 Imported PSK (IPSK): A PSK derived from an EPSK, External Identity, optional context string, target protocol, and target KDF. There is an "External Identity" that is contained within the EPSK itself; is the "External Identity" listed here distinct from that?