feat(lib): use new challenge creation flow (#749)
* feat(decaymap): add Delete method Signed-off-by: Xe Iaso <me@xeiaso.net> * chore(lib/challenge): refactor Validate to take ValidateInput Signed-off-by: Xe Iaso <me@xeiaso.net> * feat(lib): implement store interface Signed-off-by: Xe Iaso <me@xeiaso.net> * feat(lib/store): all metapackage to import all store implementations Signed-off-by: Xe Iaso <me@xeiaso.net> * chore(policy): import all store backends Signed-off-by: Xe Iaso <me@xeiaso.net> * feat(lib): use new challenge creation flow Previously Anubis constructed challenge strings from request metadata. This was a good idea in spirit, but has turned out to be a very bad idea in practice. This new flow reuses the Store facility to dynamically create challenge values with completely random data. This is a fairly big rewrite of how Anubis processes challenges. Right now it defaults to using the in-memory storage backend, but on-disk (boltdb) and valkey-based adaptors will come soon. Signed-off-by: Xe Iaso <me@xeiaso.net> * chore(decaymap): fix documentation typo Signed-off-by: Xe Iaso <me@xeiaso.net> * chore(lib): fix SA4004 Signed-off-by: Xe Iaso <me@xeiaso.net> * test(lib/store): make generic storage interface test adaptor Signed-off-by: Xe Iaso <me@xeiaso.net> * chore: spelling Signed-off-by: Xe Iaso <me@xeiaso.net> * fix(decaymap): invert locking process for Delete Signed-off-by: Xe Iaso <me@xeiaso.net> * feat(lib/store): add bbolt store implementation Signed-off-by: Xe Iaso <me@xeiaso.net> * chore: spelling Signed-off-by: Xe Iaso <me@xeiaso.net> * chore: go mod tidy Signed-off-by: Xe Iaso <me@xeiaso.net> * chore(devcontainer): adapt to docker compose, add valkey service Signed-off-by: Xe Iaso <me@xeiaso.net> * fix(lib): make challenges live for 30 minutes by default Signed-off-by: Xe Iaso <me@xeiaso.net> * feat(lib/store): implement valkey backend Signed-off-by: Xe Iaso <me@xeiaso.net> * test(lib/store/valkey): disable tests if not using docker Signed-off-by: Xe Iaso <me@xeiaso.net> * test(lib/policy/config): ensure valkey stores can be loaded Signed-off-by: Xe Iaso <me@xeiaso.net> * Update metadata check-spelling run (pull_request) for Xe/store-interface Signed-off-by: check-spelling-bot <check-spelling-bot@users.noreply.github.com> on-behalf-of: @check-spelling <check-spelling-bot@check-spelling.dev> * chore(devcontainer): remove port forwards because vs code handles that for you Signed-off-by: Xe Iaso <me@xeiaso.net> * docs(default-config): add a nudge to the storage backends section of the docs Signed-off-by: Xe Iaso <me@xeiaso.net> * chore(docs): listen on 0.0.0.0 for dev container support Signed-off-by: Xe Iaso <me@xeiaso.net> * docs(policy): document storage backends Signed-off-by: Xe Iaso <me@xeiaso.net> * docs: update CHANGELOG and internal links Signed-off-by: Xe Iaso <me@xeiaso.net> * docs(admin/policies): don't start a sentence with as Signed-off-by: Xe Iaso <me@xeiaso.net> * chore: fixes found in review Signed-off-by: Xe Iaso <me@xeiaso.net> --------- Signed-off-by: Xe Iaso <me@xeiaso.net> Signed-off-by: check-spelling-bot <check-spelling-bot@users.noreply.github.com>
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title: Why does Anubis use Proof-of-Work?
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---
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Anubis uses a [proof of work](https://en.wikipedia.org/wiki/Proof_of_work) in order to validate that clients are genuine. The reason Anubis does this was inspired by [Hashcash](https://en.wikipedia.org/wiki/Hashcash), a suggestion from the early 2000's about extending the email protocol to avoid spam. The idea is that genuine people sending emails will have to do a small math problem that is expensive to compute, but easy to verify such as hashing a string with a given number of leading zeroes. This will have basically no impact on individuals sending a few emails a week, but the company churning out industrial quantities of advertising will be required to do prohibitively expensive computation. This is also how Bitcoin's consensus algorithm works.
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Anubis uses [proof of work](https://en.wikipedia.org/wiki/Proof_of_work) in order to validate that clients are genuine. The reason Anubis does this was inspired by [Hashcash](https://en.wikipedia.org/wiki/Hashcash), a suggestion from the early 2000's about extending the email protocol to avoid spam. The idea is that genuine people sending emails will have to do a small math problem that is expensive to compute, but easy to verify such as hashing a string with a given number of leading zeroes. This will have basically no impact on individuals sending a few emails a week, but the company churning out industrial quantities of advertising will be required to do prohibitively expensive computation. This is also how Bitcoin's consensus algorithm works.
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## How Anubis' proof of work scheme works
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@ -21,16 +21,3 @@ const hash = await sha256(`${challenge}${nonce}`);
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In order to pass a challenge, the `hash` has to have the right number of leading zeros (the "difficulty"). When a client requests to pass the challenge, they include the nonce they used. The server then only has to do one sha256 operation: the one that confirms that the challenge (generated from request metadata) and the nonce (provided by the client) match the difficulty number of leading zeroes.
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Ultimately, this is a hack whose real purpose is to give a "good enough" placeholder solution so that more time can be spent on fingerprinting and identifying headless browsers (EG via how they do font rendering) so that the challenge proof of work page doesn't need to be presented to known legitimate users.
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## Challenge format
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Anubis generates challenges based on browser metadata, including but not limited to the following:
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- The contents of your [`Accept-Language` header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Accept-Language)
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- The IP address of your client
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- Your browser's [`User-Agent` string](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/User-Agent)
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- The date of the current week, rooted on Sundays
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- Anubis' ed25519 public signing key for [JSON web tokens](https://jwt.io/) (JWTs)
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- The challenge difficulty
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This is intended to be a random value that is difficult for attackers to forge and guess, but also deterministic enough that it will naturally reset itself.
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