From 01189f114bafa2a6ad68dacc2b7418bb303bdd35 Mon Sep 17 00:00:00 2001 From: Rasmus Dahlberg Date: Tue, 22 Jun 2021 23:25:09 +0200 Subject: imported logging docs without any changes --- doc/design.md | 251 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 251 insertions(+) create mode 100644 doc/design.md (limited to 'doc/design.md') diff --git a/doc/design.md b/doc/design.md new file mode 100644 index 0000000..2e01a34 --- /dev/null +++ b/doc/design.md @@ -0,0 +1,251 @@ +# System Transparency Logging: Design v0 +We propose System Transparency logging. It is similar to Certificate +Transparency, except that cryptographically signed checksums are logged as +opposed to X.509 certificates. Publicly logging signed checksums allow anyone +to discover which keys produced what signatures. As such, malicious and +unintended key-usage can be _detected_. We present our design and conclude by +providing two use-cases: binary transparency and reproducible builds. + +**Target audience.** +You are most likely interested in transparency logs or supply-chain security. + +**Preliminaries.** +You have basic understanding of cryptographic primitives like digital +signatures, hash functions, and Merkle trees. You roughly know what problem +Certificate Transparency solves and how. + +**Warning.** +This is a work-in-progress document that may be moved or modified. A future +revision of this document will bump the version number to v1. Please let us +know if you have any feedback. + +## Introduction +Transparency 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/). +A System Transparency log makes signed checksums transparent. The overall goal +is to facilitate detection of unwanted key-usage. + +## Threat model and (non-)goals +We consider a powerful attacker that gained control of a target's signing and +release infrastructure. This covers a weaker form of attacker that is able to +sign data and distribute it to a subset of isolated users. For example, this is +essentially what the FBI requested from Apple in the San Bernardino case [\[FBI-Apple\]](https://www.eff.org/cases/apple-challenges-fbi-all-writs-act-order). +The fact that signing keys and related infrastructure components get +compromised should not be controversial these days [\[SolarWinds\]](https://www.zdnet.com/article/third-malware-strain-discovered-in-solarwinds-supply-chain-attack/). + +The attacker can also gain control of the transparency log's signing key and +infrastructure. This covers a weaker form of attacker that is able to sign log +data and distribute it to a subset of isolated users. For example, this could +have been the case when a remote code execution was found for a Certificate +Transparency Log [\[DigiCert\]](https://groups.google.com/a/chromium.org/g/ct-policy/c/aKNbZuJzwfM). + +Any attacker that is able to position itself to control these components will +likely be _risk-averse_. This is at minimum due to two factors. First, +detection would result in a significant loss of capability that is by no means +trivial to come by. Second, detection means that some part of the attacker's +malicious behavior will be disclosed publicly. + +Our goal is to facilitate _detection_ of compromised signing keys. We consider +a signing key compromised if an end-user accepts an unwanted signature as valid. +The solution that we propose is that signed checksums are transparency logged. +For security we need a collision resistant hash function and an unforgeable +signature scheme. We also assume that at most a threshold of seemingly +independent parties are adversarial. + +It is a non-goal to disclose the data that a checksum represents. For example, +the log cannot distinguish between a checksum that represents a tax declaration, +an ISO image, or a Debian package. This means that the type of detection we +support is more _coarse-grained_ when compared to Certificate Transparency. + +## Design +We consider a data publisher that wants to digitally sign their data. The data +is of opaque type. We assume that end-users have a mechanism to locate the +relevant public verification keys. Data and signatures can also be retrieved +(in)directly from the data publisher. We make little assumptions about the +signature tooling. The ecosystem at large can continue to use `gpg`, `openssl`, +`ssh-keygen -Y`, `signify`, or something else. + +We _have to assume_ that additional tooling can be installed by end-users that +wish to enforce transparency logging. For example, none of the existing +signature tooling supports verification of Merkle tree proofs. A side-effect of +our design is that this additional tooling makes no outbound connections. The +above data flows are thus preserved. + +### A bird's view +A central part of any transparency log is the data stored by the log. The data is stored by the +leaves of an append-only Merkle tree. Our leaf structure contains four fields: +- **shard_hint**: a number that binds the leaf to a particular _shard interval_. +Sharding means that the log has a predefined time during which logging requests +are accepted. Once elapsed, the log can be shut down. +- **checksum**: a cryptographic hash of some opaque data. The log never +sees the opaque data; just the hash made by the data publisher. +- **signature**: a digital signature that is computed by the data publisher over +the leaf's shard hint and checksum. +- **key_hash**: a cryptographic hash of the data publisher's public verification key that can be +used to verify the signature. + +#### Step 1 - preparing a logging request +The data publisher selects a shard hint and a checksum that should be logged. +For example, the shard hint could be "logs that are active during 2021". The +checksum might be the hash of a release file. + +The data publisher signs the selected shard hint and checksum using a secret +signing key. Both the signed message and the signature is stored +in the leaf for anyone to verify. Including a shard hint in the signed message +ensures that a good Samaritan cannot change it to log all leaves from an +earlier shard into a newer one. + +A hash of the public verification key is also stored in the leaf. This makes it +possible to attribute the leaf to the data publisher. For example, a data publisher +that monitors the log can look for leaves that match their own key hash(es). + +A hash, rather than the full public verification key, is used to motivate the +verifier to locate the key and make an explicit trust decision. Not disclosing the public +verification key in the leaf makes it more unlikely that someone would use an untrusted key _by +mistake_. + +#### Step 2 - submitting a logging request +The log implements an HTTP(S) API. Input and output is human-readable and uses +a simple key-value format. A more complex parser like JSON is not needed +because the exchanged data structures are primitive enough. + +The data publisher submits their shard hint, checksum, signature, and public +verification key as key-value pairs. The log will use the public verification +key to check that the signature is valid, then hash it to construct the `key_hash` part of the leaf. + +The data publisher also submits a _domain hint_. The log will download a DNS +TXT resource record based on the provided domain name. The downloaded result +must match the public verification key hash. By verifying that the submitter +controls a domain that is aware of the public verification key, rate limits can +be applied per second-level domain. As a result, you would need a large number +of domain names to spam the log in any significant way. + +Using DNS to combat spam is convenient because many data publishers already have +a domain name. A single domain name is also relatively cheap. Another +benefit is that the same anti-spam mechanism can be used across several +independent logs without coordination. This is important because a healthy log +ecosystem needs more than one log in order to be reliable. DNS also has built-in +caching which data publishers can influence by setting TTLs accordingly. + +The submitter's domain hint is not part of the leaf because key management is +more complex than that. A separate project should focus on transparent key +management. The scope of our work is transparent _key-usage_. + +The log will _try_ to incorporate a leaf into the Merkle tree if a logging +request is accepted. There are no _promises of public logging_ as in +Certificate Transparency. Therefore, the submitter needs to wait for an +inclusion proof to appear before concluding that the logging request succeeded. Not having +inclusion promises makes the log less complex. + +#### Step 3 - distributing proofs of public logging +The data publisher is responsible for collecting all cryptographic proofs that +their end-users will need to enforce public logging. The collection below +should be downloadable from the same place that published data is normally hosted. +1. **Opaque data**: the data publisher's opaque data. +2. **Shard hint**: the data publisher's selected shard hint. +3. **Signature**: the data publisher's leaf signature. +4. **Cosigned tree head**: the log's tree head and a _list of signatures_ that +state it is consistent with prior history. +5. **Inclusion proof**: a proof of inclusion based on the logged leaf and tree +head in question. + +The data publisher's public verification key is known. Therefore, the first three fields are +sufficient to reconstruct the logged leaf. The leaf's signature can be +verified. The final two fields then prove that the leaf is in the log. If the +leaf is included in the log, any monitor can detect that there is a new +signature made by a given data publisher, 's public verification key. + +The catch is that the proof of logging is only as convincing as the tree head +that the inclusion proof leads up to. To bypass public logging, the attacker +needs to control a threshold of independent _witnesses_ that cosign the log. A +benign witness will only sign the log's tree head if it is consistent with prior +history. + +#### Summary +The log is sharded and will shut down at a predefined time. The log can shut +down _safely_ because end-user verification is not interactive. The difficulty +of bypassing public logging is based on the difficulty of controlling a +threshold of independent witnesses. Witnesses cosign tree heads to make them +trustworthy. + +Submitters, monitors, and witnesses interact with the log using an HTTP(S) API. +Submitters must prove that they own a domain name as an anti-spam mechanism. +End-users interact with the log _indirectly_ via a data publisher. It is the +data publisher's job to log signed checksums, distribute necessary proofs of +logging, and monitor the log. + +### A peek into the details +Our bird's view introduction skipped many details that matter in practise. Some +of these details are presented here using a question-answer format. A +question-answer format is helpful because it is easily modified and extended. + +#### What cryptographic primitives are supported? +The only supported hash algorithm is SHA256. The only supported signature +scheme is Ed25519. Not having any cryptographic agility makes the protocol less +complex and more secure. + +We can be cryptographically opinionated because of a key insight. Existing +signature tools like `gpg`, `ssh-keygen -Y`, and `signify` cannot verify proofs +of public logging. Therefore, _additional tooling must already be installed by +end-users_. That tooling should verify hashes using the log's hash function. +That tooling should also verify signatures using the log's signature scheme. +Both tree heads and tree leaves are being signed. + +#### Why not let the data publisher pick their own signature scheme and format? +Agility introduces complexity and difficult policy questions. For example, +which algorithms and formats should (not) be supported and why? Picking Ed25519 +is a current best practise that should be encouraged if possible. + +There is not much we can do if a data publisher _refuses_ to rely on the log's +hash function or signature scheme. + +#### What if the data publisher must use a specific signature scheme or format? +They may _cross-sign_ the data as follows. +1. Sign the data as they're used to. +2. Hash the data and use the result as the leaf's checksum to be logged. +3. Sign the leaf using the log's signature scheme. + +For verification, the end-user first verifies that the usual signature from step 1 is valid. Then the +end-user uses the additional tooling (which is already required) to verify the rest. +Cross-signing should be a relatively comfortable upgrade path that is backwards +compatible. The downside is that the data publisher may need to manage an +additional key-pair. + +#### What (de)serialization parsers are needed? +#### What policy should be used? +#### Why witness cosigning? +#### Why sharding? +Unlike X.509 certificates which already have validity ranges, a +checksum does not carry any such information. Therefore, we require +that the submitter selects a _shard hint_. The selected shard hint +must be in the log's _shard interval_. A shard interval is defined by +a start time and an end time. Both ends of the shard interval are +inclusive and expressed as the number of seconds since the UNIX epoch +(January 1, 1970 00:00 UTC). + +Sharding simplifies log operations because it becomes explicit when a +log can be shutdown. A log must only accept logging requests that +have valid shard hints. A log should only accept logging requests +during the predefined shard interval. Note that _the submitter's +shard hint is not a verified timestamp_. The submitter should set the +shard hint as large as possible. If a roughly verified timestamp is +needed, a cosigned tree head can be used. + +Without a shard hint, the good Samaritan could log all leaves from an +earlier shard into a newer one. Not only would that defeat the +purpose of sharding, but it would also become a potential +denial-of-service vector. + +#### TODO +Add more key questions and answers. +- Log spamming +- Log poisoning +- Why we removed identifier field from the leaf +- Explain `latest`, `stable` and `cosigned` tree head. +- Privacy aspects +- How does this whole thing work with more than one log? + +## Concluding remarks +Example of binary transparency and reproducible builds. -- cgit v1.2.3