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diff --git a/doc/design.md b/doc/design.md index 59cd7c8..9fcf4b6 100644 --- a/doc/design.md +++ b/doc/design.md @@ -2,9 +2,9 @@ We propose System Transparency logging. It is similar to Certificate Transparency, expect that cryptographically signed checksums are logged as opposed to X.509 certificates. Publicly logging signed checksums allow anyone -to discover which keys signed what. As such, malicious and unintended key-usage -can be _discovered_. We present our design and discuss how two possible -use-cases influenced it: binary transparency and reproducible builds. +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. @@ -12,20 +12,20 @@ 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. You may never have heard the term -_gossip-audit model_, or know how it is related to trust assumptions and -detectability properties. +Certificate Transparency solves and how. **Warning.** -This is a work-in-progress document that may be moved or modified. +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/). -System Transparency logging makes signed checksums transparent. The goal is to -_detect_ unwanted key-usage without making assumptions about the signed data. +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 @@ -33,7 +33,7 @@ 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 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 [\[SolarWinds\]](https://www.zdnet.com/article/third-malware-strain-discovered-in-solarwinds-supply-chain-attack/). +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 @@ -47,16 +47,172 @@ 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. Therefore, -we transparency log signed checksums. We assume that clients _fail closed_ if a -checksum does not appear in a public log. We also assume that the attacker -controls at most a threshold of independent parties to achieve our goal -("strength in numbers"). +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 signed 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 _courser-grained_ when compared to Certificate -Transparency. +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 _course-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 support 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. 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 +will be accepted. Once elapsed, the log can be shutdown. +- **checksum**: a cryptographic hash of some opaque data. The log never +sees the opaque data; just the hash. +- **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 public verification key that can be +used to verify the leaf's 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 a hashed release file or something else. + +The data publisher signs the selected shard hint and checksum using their 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 the good Samaritan cannot change it to log all leaves from an +earlier shard into a newer one. + +The hashed public verification key is also stored in the leaf. This makes it +easy to attribute the leaf to the signing entity. 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 force the +verifier to locate the key and trust it explicitly. Not disclosing the public +verification key in the leaf makes it more difficult to 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 +percent encoding. We decided to use percent encoding for requests and responses +because it is a simple format that is commonly used on the web. A more complex +parser like JSON is not needed if the exchanged data structures are basic +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 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 to be reliable. DNS also has built-in +caching that can be influenced by setting TTLs accordingly. + +The submitter's domain hint is not part of the leaf because key management is +more complex than that. The only service that the log provides is discovery of +signed checksums. Key transparency projects have their own merit. + +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 before concluding that the 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. It must be possible to +download the following collection (in)directly from the data publisher: +1. **Shard hint**: the data publisher's selected shard hint. +2. **Opaque data**: the data publisher's opaque data. +3. **Signature**: the data publisher's leaf signature. +5. **Cosigned tree head**: the log's tree head and a _list of signatures_ that +state it is consistent with prior history. +6. **Inclusion proof**: a proof of inclusion that is based on the leaf and tree +head in question. + +The 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 for a 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 shutdown 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 peak 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 +simpler and more secure. + +An immediate follow-up question is how that is supposed to work with existing +and future signature tooling. The key insight is that _additional tooling is +already required to verify Merkle tree proofs. That tooling should use SHA256. +That tooling should also verify all Ed25519 signatures that logs, witnesses, and +data publishers create_. + +For example, suppose that an ecosystem uses `gpg` which has its own incompatible +signature format and algorithms. The data publisher could _cross-sign_ using +Ed25519 as follows: +1. Sign the opaque data as you normally would with `gpg`. +2. Hash the opaque data and use that as the leaf's checksum. Sign the leaf +using Ed25519. + +First the end-user verifies that the `gpg` signature is valid. This is the +old verification process. Then the end-user uses the additional tooling to +verify proofs of logging, which involves SHA256 hashing and Ed25519 signatures. + +The downside is that the data publisher may need to manage an Ed25519 key _as +well_. TODO: motivate why that is a suboptimal but worth-while trade-off. + +#### What (de)serialization parsers are needed? +#### Why witness cosigning? +#### What policy should be used? +#### TODO +Add more key questions and answers. + +## Concluding remarks +Example of binary transparency and reproducible builds. |