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diff --git a/doc/design.md b/doc/design.md index 7ef8a4e..da1bcf2 100644 --- a/doc/design.md +++ b/doc/design.md @@ -18,38 +18,51 @@ Please let us know if you have any feedback. ## 1 - Introduction Transparent logs make it possible to detect unwanted events. For example, - are there any (mis-)issued TLS certificates [\[CT\]](https://tools.ietf.org/html/rfc6962), - did you get a different Go module than everyone else [\[ChecksumDB\]](https://go.googlesource.com/proposal/+/master/design/25530-sumdb.md), - or is someone running unexpected commands on your server [\[AuditLog\]](https://transparency.dev/application/reliably-log-all-actions-performed-on-your-servers/). +are there any (mis-)issued TLS certificates + [\[CT\]](https://tools.ietf.org/html/rfc6962), +did you get a different Go module than everyone else + [\[ChecksumDB\]](https://go.googlesource.com/proposal/+/master/design/25530-sumdb.md), +or is someone running unexpected commands on your server + [\[AuditLog\]](https://transparency.dev/application/reliably-log-all-actions-performed-on-your-servers/). + A sigsum log brings transparency to **sig**ned check**sum**s. ### 1.1 - Problem description Suppose you are an entity that distributes some opaque data. For example, the opaque data might be a provenance file, - an executable binary, - an automatic software update, or - a TPM quote. -You claim to distribute the right opaque data to everyone. -However, past incidents taught us that word is cheap and sometimes things go -wrong. Trusted parties get compromised and lie about it [\[DigiNotar\]](), or -they might not even realize it until later on because the break-in was stealthy -[\[SolarWinds\]](https://www.zdnet.com/article/third-malware-strain-discovered-in-solarwinds-supply-chain-attack/). - -The goal of sigsum logging is to make your claims verifiable by you and -others. To keep the design simple and general, we want to achieve this goal -with few assumptions about the opaque data or the involved claims. You can -think of this as some sort of bottom-line for what it takes to apply a + an executable binary, or + a javascript. +You claim to distribute the right opaque data to everyone. However, past +incidents taught us that word is cheap and sometimes things go wrong. +Trusted parties get compromised and lie about it + [\[DigiNotar\]](https://roselabs.nl/files/audit_reports/Fox-IT_-_DigiNotar.pdf), +or they might not even realize it until later on because the break-in was +stealthy + [\[SolarWinds\]](https://www.zdnet.com/article/third-malware-strain-discovered-in-solarwinds-supply-chain-attack/). + +The goal of sigsum logging is to facilitate verification of your claims. To +keep the design simple and general, we want to achieve this goal with few +assumptions about the opaque data or the involved claims. + +You can think of this as some sort of bottom-line for what it takes to apply a transparent logging pattern. Past use-cases that wanted to piggy-back on an -existing reliable log ecosystem fit well into our scope [\[BinTrans\]](https://wiki.mozilla.org/Security/Binary_Transparency). +existing reliable log ecosystem fit well into our scope + [\[BinTrans\]](https://wiki.mozilla.org/Security/Binary_Transparency). + +We also want our design to be easy from many different perspectives, for example +log operations and verification in constrained environments. This includes +considerations such as simple parsing, protection against log spam and +poisoning, and a well-defined gossip protocol without complex auditing logic. -We also want the design to be easy from many different perspectives, for example log operations and -verification in constrained environments. This includes considerations such as -simple parsing, protection against log spam and poisoning, and a -well-defined gossip protocol without any complex auditing logic. +This is in contrast to Certificate Transparency, which requires ASN.1 +parsing, storage of arbitrary certificate fields, reactive auditing of +complicated log promises, and deployment of a gossip protocol that suits the web + [\[G1,](https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7346853&casa_token=5bqmfaLp0Y8AAAAA:pzOJyu_BgWFxwEcM0r_ikDLhofU61PM8PWQnjcalxA3oXo7BxgKcOn0aIKorP02imZOG5i5Ew5sF&tag=1) + [G2\]](https://datatracker.ietf.org/doc/html/draft-ietf-trans-gossip-05). ### 1.2 - Abstract setting -You would like users of the opaque data to _believe_ your claims. Therefore, +You would like users of some opaque data to _believe_ your claims. Therefore, we refer to you as a _claimant_ and your users as _believers_. Belief is going to be reasonable because each claim is expressed as a _signed statement_ that is transparency logged. The opaque data and relevant proofs of public logging are @@ -61,8 +74,8 @@ a _verifier_ can discover any statement that you as a claimant produced. If it turns out that a statement contains a false claim, an _arbiter_ is notified that can act on it. An overview of these _roles_ and how they interact are shown in Figure 1. A party may play multiple roles. A role may also be fulfilled by -multiple parties. Refer to the [claimant model](https://github.com/google/trillian/blob/master/docs/claimantmodel/CoreModel.md) -for additional detail. +multiple parties. This is heavily inspired by the + [claimant model](https://github.com/google/trillian/blob/master/docs/claimantmodel/CoreModel.md). ``` statement +----------+ @@ -90,16 +103,15 @@ a certain cryptographic hash_. It is stored in a sigsum log for discoverability claimant may add additional claims that are _implicit_ for each statement. An implicit claim is not stored by the log and therefore communicated through policy. Examples of implicit claims: -- The opaque data can be located in repository Y using X as an identifier. +- The opaque data can be located in repository X using Y as an identifier. - The opaque data is a `.buildinfo` file that facilitates a reproducible build [\[R-B\]](https://wiki.debian.org/ReproducibleBuilds/BuildinfoFiles). Detailed examples of use-case specific claimant models are defined in a separate -document [\[CM-Examples\]](https://git.sigsum.org/sigsum/blob/main/doc/claimant.md). +document [\[CM-Examples\]](https://git.sigsum.org/sigsum/tree/doc/claimant.md). ### 1.3 - Design considerations -Our main contribution is in the details that surround the log role in practise. -Below is a brief summary. +Below is a summary of design considerations that were considered. - **Preserved data flows:** a believer can enforce sigsum logging without making additional outbound network connections. Proofs of public logging are provided using the same distribution mechanism as is used for distributing the opaque data. @@ -113,13 +125,14 @@ little metadata as possible to combat log poisoning. We piggyback on DNS to combat log spam. - **Built-in mechanisms that ensure a globally consistent log:** transparent logs rely on gossip protocols to detect forks. We built a proactive gossip -protocol directly into the log. It is a variant of [witness cosigning](). +protocol directly into the log. It is a variant of + [witness cosigning](https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7546521&casa_token=W2hlJFylY3sAAAAA:lzSJGF_OlAjHGQf3dCefW2RbYad18U__hGo48BQvOdHuS4zWM2jW7j9ksgXVWlPjxh0nAT28GD4B). - **No cryptographic agility**: the only supported signature scheme is Ed25519. The only supported hash function is SHA256. Not having any cryptographic agility makes protocols and data formats simpler and more secure. - **Simple (de)serialization parsers:** complex (de)serialization parsers increase attack surfaces and make the system more difficult to use in -constrained environments. A sigsum statement can be (de)serialized using +constrained environments. Signed and logged data can be (de)serialized using [Trunnel](https://gitlab.torproject.org/tpo/core/trunnel/-/blob/main/doc/trunnel.md), or "by hand" in many modern programming languages. This is the only parsing that a believer is required to support. Claimants and verifiers additionally |