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@@ -119,6 +119,39 @@ A key-hash is included in the leaf so that it can be attributed to the signing entity. A hash, rather than the full public verification key, is used to force the verifier to locate the appropriate key and make an explicit trust decision. +#### Shard hint +The log is only accepting new leaves during a predefined time interval. We +refer to this time interval as the log's _shard_. Sharding can simplify log +operations because it becomes explicit when the log can be shutdown. + +Unlike X.509 certificates that already have a validity range, a checksum does +not have any such information. Therefore, we require the submitter to sign a +_shard hint_. A shard hint is composed of a prefix and a tree leaf. + +``` +struct shard_hint { + u64 prefix; + struct tree_leaf leaf; +} +``` + +The log will check that the signed `shard_hint` can be verified using the +submitter's public verification key. The prefix could be anything and may +repeat. This API documentation assumes that the prefix is set to zero. + +As long as the `shard_hint` signature is not revealed, no one but the submitter +can submit a leaf that the log will accept. Therefore, the good Samaritan +cannot submit all leaves from an earlier shard into a newer one. The +`shard_hint` does not prevent the _legitimate submitter_ from reusing an earlier +submission in a future shard. + +Note the importance of letting the submitter decide if an entry is logged again +or not. If the log has a rate limiting function, replayed submissions could +deny service in a new shard. In practise we expect submitters to not log a +leaf again. Once an inclusion proof and a cosigned tree head is available, you +have all the necessary proofs. These proofs continue to be valid after the log +shuts down because the verification process is non-interactive. + ## Public endpoints Every log has a base URL that identifies it uniquely. The only constraint is that it must be a valid HTTP(S) URL that can have the `/st/v0/<endpoint>` suffix @@ -199,6 +232,7 @@ Input key-value pairs: - `leaf_checksum`: the checksum that the submitter wants to log in base64. - `signature_scheme`: the signature scheme that the submitter wants to use. - `tree_leaf_signature`: the submitter's `tree_leaf` signature in base64. +- `shard_hint_signature`: the submitter's `shard_hint` signature in base64. - `verification_key`: the submitter's public verification key. It is serialized as described in the corresponding RFC, then base64-encoded. - `domain_hint`: a domain name that indicates where the public verification-key |