Add initial warnings against OHTTP request reuse

[0xZaddyy]

Document OHTTP Request Non-Reusability

This PR addresses #819 by documenting that OHTTP-encapsulated requests must not be reused or retried, due to the risk of retransmitting ciphertext and violating OHTTP's privacy guarantees.

What's Included:

A top-level warning was added to the receive module docs (//!) to highlight the security implications of request reuse.

Closes #819

[coveralls]

Pull Request Test Coverage Report for Build 16027685870

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.896%

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Totals
Change from base Build 16014856746:	0.0%
Covered Lines:	7251
Relevant Lines:	8541

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💛 - Coveralls

[nothingmuch]

Is This the Right Direction?

Yes =)

I posted some comments elaborating on the rationale.

Something to keep in mind is not to over-state the benefit of avoiding reuse, for example in the event that both the sender and receiver use the same relay there would still be some concern about linking them together as participants in the same session and even inferring who is the receiver and who is the sender. If the encapsulated GET requests are reused that would make it much easier for a relay to infer that information, but ensuring each encapsulated request is fresh doesn't fully prevent that.

[nothingmuch]

Suggested change

	/// Retransmitting the same ciphertext breaks OHTTP security properties.
	/// Retransmitting the same ciphertext breaks OHTTP privacy properties.

The specific concern is that the relay can see that a request is being retried, which leaks that it's all the same polling request.

[nothingmuch]

Suggested change

	//! Reusing an encapsulated request may retransmit the same ciphertext, which breaks the
	//! Reusing an encapsulated request will retransmit the same ciphertext, which breaks the

the encapsulated request is a header followed by the ciphertext, so if it's POSTed twice to the relay, the relay can observe that it is the same as in the previous request

those two requests can aleady be linked by the relay due to metadata, e.g. ip address, but seeing repeats additionally reveals information about how the client is interacting with the directory

[nothingmuch]

git complains about trailing spaces, so best to filter these out for the whole file

Suggested change

	//! anonymity and unlinkability guarantees provided by the OHTTP protocol.
	//! anonymity and unlinkability guarantees provided by the OHTTP protocol.

[0xZaddyy]

Is This the Right Direction?

Yes =)

I posted some comments elaborating on the rationale.

Something to keep in mind is not to over-state the benefit of avoiding reuse, for example in the event that both the sender and receiver use the same relay there would still be some concern about linking them together as participants in the same session and even inferring who is the receiver and who is the sender. If the encapsulated GET requests are reused that would make it much easier for a relay to infer that information, but ensuring each encapsulated request is fresh doesn't fully prevent that.

Thanks! I’ve made a few corrections
Could you let me know if the locations I placed the warnings are appropriate?
Also, is the structure and phrasing of the warnings aligned with what you had in mind?

If there are other places where you'd prefer to see these comments added or removed I’d really appreciate the suggestions.

[nothingmuch]

Could you let me know if the locations I placed the warnings are appropriate?

Yeah, I think the module level stuff on v2 sender & receiver is the right place

Also, is the structure and phrasing of the warnings aligned with what you had in mind?

Yep! I've made some suggestions, and corrections re v1 stuff that doesn't use OHTTP

[0xZaddyy]

Alright boss, fixed.
Thanks ＾▽＾

[nothingmuch]

please verify that all methods with the warning are definitely related to v2, that the request is actually an OHTTP encapsulated one, and that "polling request" vs. just "request" is consistent

[0xZaddyy]

All corrections have been made, boss!