Code change:
To generate the report, the client begins by sharding its measurement into a sequence of input shares as specified by the VDAF in use.  To <del> encrypt an input share, the client first generates an HPKE RFC9180 context for the aggregator by running </del> <ins> encrypt an input share, the client generates an HPKE RFC9180 ciphertext and encapsulated key for the aggregator by running </ins> <del> where "pk" is the aggregator's public key and "server_role" is the Role of the intended recipient ("0x02" for the leader and "0x03" for the helper).  In general, the info string for computing the HPKE context is suffixed by two bytes, the first of which identifies the role of the sender and the second of which identifies the role of the intended recipient. "enc" is the HPKE encapsulated key and "context" is the HPKE context used by the client for encryption.  The payload is encrypted as where "input_share" is the aggregator's input share and "nonce" and "extensions" are the corresponding fields of "Report".  Clients MUST NOT use the same "enc" for multiple reports. </del> <ins> where "pk" is the aggregator's public key; "server_role" is the Role of the intended recipient ("0x02" for the leader and "0x03" for the helper); "task_id", "nonce", and "extensions" are the corresponding fields of "Report"; and "input_share" is the aggregator's input share. </ins> The leader responds to well-formed requests to "[leader]/upload" with HTTP status code 200 OK and an empty body.  Malformed requests are

Related discussion:
text: Specifically: Specify use of the "one-shot" HPKE APIs defined in RFC9180, replacing equivalent use of the "multi-shot" APIs. (This change is intended to be a clarification, not a functional change.) Move the task_id parameter from the "info" parameter to the "aad" parameter. This is intended to protect against key-commitment-related attacks[1]. (This is a functional change.) Clarify that the URL field is an encapsulated key, rather than an encryption context. (This is a clarification, not a functional change.) [1] See discussion in URL
Per URL section 8.1: PPM effectively uses the single-shot APIs; we should consider refactoring PPM's usage of HPKE to use only one of applicationinfo & AAD. Following the advice of the HPKE RFC, we would want to use applicationinfo. This would permit slightly simpler implementations. I am not sure how much it would simplify the HPKE computations.
Should we then formally use the single shot HPKE APIs from ?
That would be my recommendation (though, somewhat confusingly given the advice quoted above, those APIs still allow specifying both application info & AAD). Implementations can & do already use the single-shot APIs, so I strongly suspect specifying the single-shot APIs will be feasible. & .]
+1 to using single-shot APIs, though I'm not sure I agree that using either info or add (but not both) is simpler. What's the reasoning?
The reasoning is: a) it follows the advice of the HPKE spec (quoted above) b) AFAICT, in a single-shot setting, application info & AAD serve exactly the same purpose, so using both is arbitrary/confusing/a potential source of mistakes. (in a multi-shot setting, application info is bound to all encryptions/decryptions done by the same context; AAD is bound to a single encryption/decryption)
TBH I'm not sure I agree with the quoted guidance here. There is a concept in protocol design known as "domain separation", the goal of which is to provide some kind of binding of long-term secret key operations to the context in which they were used. Imagine, for example, that someone used an HPKE secret key for PPM and some other protocol. Suppose further that both are using the empty string for .