Original text:
For each stored security context, the first time after boot the server receives an OSCORE request, the server uses the Repeat option I-D.amsuess-core-repeat-request-tag to get a request with <del> verifiable freshness and uses that to synchronize the replay window.  If the server can verify the fresh request, the Partial IV in the fresh request is set as the lower limit of the replay </del>  window. 6.5.3.

Reviewer comments:
This prescribes explicitly one of the possible ways that the response is sent, and states the (hopefully) obvious about accepting new requests. The alternative to "MUST NOT be encrypted" would be "MUST be encrypted and MUST carry an explicit IV"; even both would work, but clients need to know what they need to be prepared for. I'd be fine with either of the two (or three) versions of the request encryption, but not stating anything might result in incompatible implementations where one only works this and one only works that way. Being explicit about (to us) obvious things like not responding with an implied IV or not accepting Repeat as Class U (or in an unencrypted request that might then clear the way for any replays) might not be necessary from a specification point of view, but could avoid implementation errors I see very vividly before my inner eye.
Come to think of it (from the considerations in URL), we have to go the other way and say that the response "MUST be encrypted, and MUST use an explicit partial IV". As we want to use this for HTTP too, this is by far the easier option; otherwise, we'd have to additionally specify how the option is transported in HTTP, so a HC proxy can convert it, and then the HC proxy would need to know whether it's transporting OSCORE (so it'd forward the Repeat option for the HTTP client to include it in a new encrypted request) or whether it's working for an OSCORE-unaware client that would not be equipped to answer the Repeat challenge (so it'd repeat the known fresh (because HTTP) request with a Repeat option by itself without bothering the client).
Did I understand right, that you take back the proposal in the PR? Partial IV generated by the server is allowed in any response. With the resolution of issue : "if Partial IV present in the message -> use that Partial IV" Hence the replay draft can specify that replay syncing MUST use the server generated PIV, in which case it will be encrypted. I probably missed the issue?
Yes; I'll still need to update the PR to say the desired. We don't need to change anything about mechanism, but we should be explicit that the server MUST reply with an encrypted Repeat and a generated partial IV.
Sorry, still not clear: OSCORE now allows the use of server-generated Partial IV. The processing for Repeat is defined in the RRT-draft. Why does anything else need to change in the OSCORE draft?
Wrote the promised changes as it's much easier to talk with concrete text; the PR is now updated as 81d47927. This doesn't change any mechanism (at least not in implementations that are based on the current text and written in full awareness of all the relevant considerations). It is just a text change that prescribes what will be obvious to some implementors but not to others. (Especially since it's not so obvious as that I would have gotten it right in the first attempt.)
(This issue takes some context from issue ). The current draft states that replay protection violation triggers a 5.03 with Max-Age 0 (Section 6.4). While this is a practical solution to issue (which is not mitigated by encrypting the code because FETCH is still safe), it could still be caught by a proxy that'd try to be smart about it (and would, AFAICT, be within its rights). I suggest that the response code be 4.01 Unauthorized with Max-Age=0. The semantics of Unauthorized are "SHOULD NOT repeat the request without first improving its authentication status to the server", and while it's a bit of a stretch to say that increasing the sequence number is sufficient to improve the authentication status, the Max-Age gives a good hint ("Come on, try again right away, don't look for other keys"), and clients can use the presence of the Max-Age option as a discriminator to tell "Sorry, no such context" apart from "Retry with a new seqno".
We also just identified this response code as not appropriate. The proposal is good, unless it results in overloading the semantics of Max-Age=0. There is already at least one requirement when Max-Age has to be zero: RFC7252, Section 5.6.1: "If an origin server wishes to prevent caching, it MUST explicitly include a Max-Age Option with a value of zero seconds." Is there a setting where the the server would not want the response 4.01 to be cached even though it is not a replay? (Obviously we have to differentiate between Inner and Outer 4.01, but is that enough?)
I don't know. I briefly thought about using an Echo here, but an HTTP proxy would react to that as a CoAP proxy would to 5.03,Max-Age=0. (See comments 2ff). I did give that some thought, and considering the Echo option thought about whether the server should just reply with an encrypted ,Max-Age=0 (or Echo=short). I decided against suggesting that because responding to a possibly fraudulent message with an owned partial IV means that any adversary that has one valid-but-used message can thus drive any server into partial-IV exhaustion.
By the way, just FYI, that Max-Age=0 with 5.03 means "service unavailable, retry in 0 seconds", which is wrong (and not what we want). (see: URL)
The current text where there MAY be a Max-Age of 0 both on "4.01, Security context not found" and on "4.01, Security context not found" means that a client can not know any more which is the case (and it should not look into the message). I see the reason for the MAY set Max-Age for most error cases in , but a client needs to keep those apart (otherwise it'd need to so something like try 3 times with a new PIV, and if the server still responds the same, assume that it doesn't know the KID). I suggest that for those two responses, the Max-Age values are enforced as "MUST NOT set a Max-Age option" and "MUST set the Max-Age option to 0", or find any other discriminator, but an implementation should be able to tell them apart.
Granted, with the current setup of POST and FETCH there are much fewer cases where replay protection errors can happen (they'd only legally happen under particular package loss situations when starting an observation via a proxy that does not support observation but does do optimizations for safe requests), but I still think that there'd be more clarity if there were a discriminant between the two error cases.
The document, as of now, specifies that Partial IV SHOULD NOT be present in responses without observe. (This refer to: https://core-URL ) The processing for protecting and verifying responses, as of now, specifies that: if Observe -> use the Partial IV in the response if not Observe -> use Partial IV of the request Should this part be changed to: if Partial IV present in the message -> use that Partial IV if not present -> use Partial IV of the request
Need to read text, but this reflects what I wanted to see.
Sorry NAME when you say "this" do you mean That's not currently included (so no text about it yet). The text right now is:
I was talking about the should change to
Section 5 - You have defined a 'sid' and said that it could be placed in the unprotected field, however you have also said that the unprotected field must be empty. These statements are contradictory
As per Christian Amsüss' message from 2017-01-05 there still seems to be confusing text in that section.
Answering both to NAME and NAME (URL) The point here is: when defining the 'sid' as a new COSE Header parameter, there is no reason related to security that would make this a "MUST be placed in the protected bucket", so instead it is a MAY. But when used in OSCOAP, the 'sid' is used in a way that it MUST be in the protected part, so the unprotected SHALL be empty. The 'sid' is analogous to 'kid' from this point of view, it is defined as a COSE parameter that MAY be unprotected, but is used protected.
Multicast removed in branch v-02 Hello OSCOAP authors, some more questions came up: 5.2 gives the context for the Encstructure as "Encrypted", while cose-msg-24 would set a "Encrypt0". Why does that differ here? states that the '"unprotected" field [...] SHALL be empty', but above states that sid 'can be placed in the unprotected headers bucket'. How do those statements go together? How are sequence numbers (which are sorted integers) mapped to partial IVs? Which endianness is used? (The padding direction follows from cose-msg-24 3.1). I'd assume (and implement) that big-endian is used because it aligns naturally with COSE's left-padding, but as I see it, that does not strictly follow from the draft. I concluded that the tag is always the last bytes of the ciphertext from example A.4, but did not find text in the spec to support that; what made me especially unsure about this is that the COSEEncrypt0 definition of cose-msg-24 5.2 only mentions the ciphertext there, which may be nil. Best regards Christian To use raw power is to make yourself infinitely vulnerable to greater powers. -- Bene Gesserit axiom

Revised text:
For each stored security context, the first time after boot the server receives an OSCORE request, the server uses the Repeat option I-D.amsuess-core-repeat-request-tag to get a request with  <ins> verifiable freshness. The Request option in the server's response MUST be encrypted, and the server MUST NOT use the same nonce as the request in its reply. If the server can verify a second request as fresh, the Partial IV of the second request is set as the lower limit of the replay </ins> window. 6.5.3.