Change:
supports, as well as providing a public key that others can use for key agreement.  The client's identity key is intended to be stable throughout the lifetime of the group; there is no mechanism to change <del> it.  Init keys are intended to be used a very limited number of times, ideally only once.  (See init-key-reuse).  Clients MAY generate and publish multiple ClientInitKey objects to support multiple ciphersuites, or to reduce the likelihood of init key reuse. ClientInitKeys contain an identifier chosen by the client, which the client MUST ensure uniquely identifies a given ClientInitKey object among the set of ClientInitKeys created by this client. </del> <ins> it.  Init keys are intended to be used only once and SHOULD NOT be reused except in case of last resort.  (See init-key-reuse).  Clients MAY generate and publish multiple ClientInitKey objects to support multiple ciphersuites.  ClientInitKeys contain an identifier chosen by the client, which the client MUST ensure uniquely identifies a given ClientInitKey object among the set of ClientInitKeys created by this client. </ins> The value for init_key MUST be a public key for the asymmetric encryption scheme defined by cipher_suite.  The whole structure is

Discussion:
This has been reported unclear multiple times. I strongly prefer forbidding reuse altogether. A validation mechanism is needed for the formal security proof, it can be implemented according to an expiration mechanism (see Architecture - 4.2. Delivery Service Compromise)
We need to allow Last Resort keys. Not sure what you mean by "replay attacks"
I rewrote it in favor of using "SHOULD NOT reuse", which is true, except in case of last resort to avoid DOS, which we are willing to allow. Normal meaning... If I remember correctly, there is a cross channel forwarding attack (replay) possible on the first message in Signal when reusing prekeys (the last resort key) because it lacks some channel binding but I would have to check. I am working to make sure we don't suffer the same defect but I am documenting it in the mean time.
I can live with this for now; we can refine later.
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