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diff --git a/doc/design.md b/doc/design.md index 401ba2e..2723ae8 100644 --- a/doc/design.md +++ b/doc/design.md @@ -334,13 +334,7 @@ The signing format for logs, witnesses, and signers is based on a subset of what is supported by OpenSSH. Ed25519 and SHA256 must be used as primitives. ### 4 - Frequently Asked Questions -#### 4.1 - What parts of the design are up for debate? -A brief summary appeared in our archive on - [2021-10-05](https://git.sigsum.org/sigsum/tree/archive/2021-10-05-open-design-thoughts?id=5c02770b5bd7d43b9327623d3de9adeda2468e84). -It may be incomplete, but covers some details that are worth thinking more -about. We are still open to remove, add, or change things. - -# 4.2 - Why use the OpenSSH signing format? +#### 4.1 - Why use the OpenSSH signing format? Our main criteria for a signing format is that it can express signing contexts without any complex parsers. A magic preamble is good for overall hygiene as well. We sketched on such a format using Trunnel. We realized that by @@ -348,7 +342,7 @@ tweaking a few constants it would be compatible with SSH's signing format. If it is possible to share format with an existing reliable and widely deployed ecosystem, great! -#### 4.3 - What is the point of submitting a checksum's preimage? +#### 4.2 - What is the point of submitting a checksum's preimage? Logging arbitrary bytes can poison a log with inappropriate content. While a leaf is already light-weight in Sigsum, a stream of leaves could be made to carry more meaning. Disallowing checksums to contain arbitrary bytes, by having logs compute @@ -360,7 +354,7 @@ is `H(D)`. The resulting checksum would be `H(H(D))`. The log will not be in a position to observe the data `D`, thereby removing power in the form of trivial data mining while at the same time making the overall protocol less heavy. -#### 4.4 - What is the point of having a domain hint? +#### 4.3 - What is the point of having a domain hint? Domain hints help log operators combat spam. By verifying that every signer controls a domain name that is aware of their public key, rate limits can be applied per second-level domain. You would need a large number of domain names @@ -387,7 +381,7 @@ that added this criteria. We are considering if additional anti-spam mechanisms should be supported in v1. -#### 4.5 - What is the point of having a shard hint? +#### 4.4 - What is the point of having a shard hint? Unlike TLS certificates which already have validity ranges, a checksum does not carry any such information. Therefore, we require that the signer selects a shard hint. The selected shard hint must be within a log's shard interval. @@ -414,7 +408,7 @@ A log operator that shuts down a completed shard will not affect end-users. In other words, a signer can continue to distribute proofs that were once collected. This is important because a checksum does not necessarily expire. -#### 4.6 - What parts of witness cosigning are not done? +#### 4.5 - What parts of witness cosigning are not done? There are interesting policy aspects that relate to witness cosigning. For example, what witnessing policy should an end-user use and how are trustworthy witnesses discovered. This is somewhat analogous to a related policy question @@ -435,6 +429,42 @@ the original proposal by [Syta et al.](https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7546521), which puts an authority right in the middle of a slowly evolving witnessing policy. +#### 4.6 - What parts of the design are up for debate? +Several parts are under consideration for the v1 design. Any feedback on what +we have now or might (not) have in the future would be appreciated. We are not +in any particular rush to bump the version and want to experiment more with v0. + +A list of some debated topics: + + - Support any additional cryptographic algorithms? For example, is it a + worth-while complexity trade-off to add a NIST curve? Note that this question + is about cryptographic primitives; not additional signature formats like PGP. + - Accept or reject the [checkpoint format][] that the TrustFabric team uses + for (co)signed tree heads? It is also a relatively simple ASCII format, and + we support it as an experimental endpoint (`<log URL>/checkpoint`). The main + differences from our current cosigned tree head format are: + - The signed blob is not based on SSH + - Base64 instead of hex + - Extensibility in an "other data" section + - No timestamp verification without defining an extension + - A signature-line format that is less minimal + - Keep or remove tree head timestamps? A monitor can convince itself of + freshness by submitting a leaf request, observing that it gets merged by a + cosigned tree head. There are pros and cons, see [timestamp reflections][]. + - Is it a worthwhile complexity trade-off to have a "quick" cosigned tree head + endpoint that provides cosignatures as fast as possible? Right now there is a + "slow" cosigned tree head endpoint that makes all cosignatures available at + once. The motivation for this is that a ["quick" endpoint adds complexity][] + while only giving _lower latency_; not _low latency_. The "slow" cosigned + tree head endpoint could also be implemented to just rotate a bit faster. + - Should there be support for any alternative rate-limiting mechanism? A + domain hint could be viewed as a subset of several possible _ownership hints_. + - Should there be any IANA registration for the future `_sigsum_v1` DNS label? + +[checkpoint format]: https://github.com/google/trillian-examples/blob/master/formats/log/README.md +["quick" endpoint adds complexity]: https://git.sigsum.org/sigsum/tree/doc/proposals/2022-01-no-quick-tree-head-endpoint +[timestamp reflections]: https://git.sigsum.org/sigsum/tree/archive/2022-02-08-timestamp-reflections.md + #### 4.7 - More questions - What are the privacy concerns? - Add more questions here! |