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a007a7c6ba
The old flavored consensus URL format made it harder to decode URLs based on their prefixes, and didn't take into account our "only give it to me if it's signed by enough authorities" stuff.
189 lines
7.3 KiB
Plaintext
189 lines
7.3 KiB
Plaintext
Filename: 162-consensus-flavors.txt
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Title: Publish the consensus in multiple flavors
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Author: Nick Mathewson
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Created: 14-May-2009
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Target: 0.2.2
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Status: Open
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Overview:
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This proposal describes a way to publish each consensus in
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multiple simultaneous formats, or "flavors". This will reduce the
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amount of time needed to deploy new consensus-like documents, and
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reduce the size of consensus documents in the long term.
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Motivation:
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In the future, we will almost surely want different fields and
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data in the network-status document. Examples include:
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- Publishing hashes of microdescriptors instead of hashes of
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full descriptors (Proposal 158).
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- Including different digests of descriptors, instead of the
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perhaps-soon-to-be-totally-broken SHA1.
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Note that in both cases, from the client's point of view, this
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information _replaces_ older information. If we're using a
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SHA256 hash, we don't need to see the SHA1. If clients only want
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microdescriptors, they don't (necessarily) need to see hashes of
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other things.
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Our past approach to cases like this has been to shovel all of
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the data into the consensus document. But this is rather poor
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for bandwidth. Adding a single SHA256 hash to a consensus for
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each router increases the compressed consensus size by 47%. In
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comparison, replacing a single SHA1 hash with a SHA256 hash for
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each listed router increases the consensus size by only 18%.
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Design in brief:
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Let the voting process remain as it is, until a consensus is
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generated. With future versions of the voting algorithm, instead
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of just a single consensus being generated, multiple consensus
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"flavors" are produced.
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Consensuses (all of them) include a list of which flavors are
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being generated. Caches fetch and serve all flavors of consensus
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that are listed, regardless of whether they can parse or validate
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them, and serve them to clients. Thus, once this design is in
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place, we won't need to deploy more cache changes in order to get
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new flavors of consensus to be cached.
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Clients download only the consensus flavor they want.
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A note on hashes:
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Everything in this document is specified to use SHA256, and to be
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upgradeable to use better hashes in the future.
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Spec modifications:
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1. URLs and changes to the current consensus format.
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Every consensus flavor has a name consisting of a sequence of one
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or more alphanumeric characters and dashes. For compatibility
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current descriptor flavor is called "ns".
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The supported consensus flavors are defined as part of the
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authorities' consensus method.
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For each supported flavor, every authority calculates another
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consensus document of as-yet-unspecified format, and exchanges
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detached signatures for these documents as in the current consensus
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design.
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In addition to the consensus currently served at
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/tor/status-vote/(current|next)/consensus.z and
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/tor/status-vote/(current|next)/consensus/<FP1>+<FP2>+<FP3>+....z ,
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authorities serve another consensus of each flavor "F" from the
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locations /tor/status-vote/(current|next)/consensus-F.z. and
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/tor/status-vote/(current|next)/consensus-F/<FP1>+....z.
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When caches serve these documents, they do so from the same
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locations.
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2. Document format: generic consensus.
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The format of a flavored consensus is as-yet-unspecified, except
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that the first line is:
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"network-status-version" SP version SP flavor NL
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where version is 3 or higher, and the flavor is a string
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consisting of alphanumeric characters and dashes, matching the
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corresponding flavor listed in the unflavored consensus.
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3. Document format: detached signatures.
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We amend the detached signature format to include more than one
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consensus-digest line, and more than one set of signatures.
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After the consensus-digest line, we allow more lines of the form:
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"additional-digest" SP flavor SP algname SP digest NL
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Before the directory-signature lines, we allow more entries of the form:
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"additional-signature" SP flavor SP algname SP identity SP
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signing-key-digest NL signature.
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[We do not use "consensus-digest" or "directory-signature" for flavored
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consensuses, since this could confuse older Tors.]
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The consensus-signatures URL should contain the signatures
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for _all_ flavors of consensus.
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4. The consensus index:
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Authorities additionally generate and serve a consensus-index
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document. Its format is:
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Header ValidAfter ValidUntil Documents Signatures
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Header = "consensus-index" SP version NL
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ValidAfter = as in a consensus
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ValidUntil = as in a consensus
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Documents = Document*
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Document = "document" SP flavor SP SignedLength
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1*(SP AlgorithmName "=" Digest) NL
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Signatures = Signature*
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Signature = "directory-signature" SP algname SP identity
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SP signing-key-digest NL signature
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There must be one Document line for each generated consensus flavor.
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Each Document line describes the length of the signed portion of
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a consensus (the signatures themselves are not included), along
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with one or more digests of that signed portion. Digests are
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given in hex. The algorithm "sha256" MUST be included; others
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are allowed.
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The algname part of a signature describes what algorithm was
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used to hash the identity and signing keys, and to compute the
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signature. The algorithm "sha256" MUST be recognized;
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signatures with unrecognized algorithms MUST be ignored.
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(See below).
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The consensus index is made available at
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/tor/status-vote/(current|next)/consensus-index.z.
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Caches should fetch this document so they can check the
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correctness of the different consensus documents they fetch.
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They do not need to check anything about an unrecognized
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consensus document beyond its digest and length.
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4.1. The "sha256" signature format.
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The 'SHA256' signature format for directory objects is defined as
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the RSA signature of the OAEP+-padded SHA256 digest of the item to
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be signed. When checking signatures, the signature MUST be treated
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as valid if the signature material begins with SHA256(document);
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this allows us to add other data later.
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Considerations:
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- We should not create a new flavor of consensus when adding a
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field instead wouldn't be too onerous.
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- We should not proliferate flavors lightly: clients will be
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distinguishable based on which flavor they download.
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Migration:
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- Stage one: authorities begin generating and serving
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consensus-index files.
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- Stage two: Caches begin downloading consensus-index files,
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validating them, and using them to decide what flavors of
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consensus documents to cache. They download all listed
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documents, and compare them to the digests given in the
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consensus.
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- Stage three: Once we want to make a significant change to the
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consensus format, we deploy another flavor of consensus at the
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authorities. This will immediately start getting cached by the
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caches, and clients can start fetching the new flavor without
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waiting a version or two for enough caches to begin supporting
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it.
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Acknowledgements:
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Aspects of this design and its applications to hash migration were
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heavily influenced by IRC conversations with Marian.
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