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catlfish design (in Emacs -*- org -*- mode)
This document describes the design of catlfish, an implementation of a
Certificate Transparency (RFC6962) log server.
We have
- a primary database storing x509 certificate chains [replicating r/o
copies to a number of frontend nodes?]
- a hash tree kept in RAM
- one secondary database per frontend node, storing the most recently
submitted data
- a cluster of backend nodes with an elected leader which periodically
updates the primary db with data from the secondary db's
- a number of frontend nodes accepting http requests, updating
secondary db's and reading from local r/o copy of the primary db
- a private key used for signing SCT's and STH's, kept (in HSM:s) on
backend nodes
Backend nodes
- are either asleep, functioning as storage only
or
- store submitted cert chains in persistent media
- have write access to the primary database holding cert chains
- periodically append new cert chains to the hash tree and sign the
tree head
Frontend nodes
- reply to the http requests specified in RFC 6962
- write submitted cert chains to their own, single master, secondary
database
- have read access to (a local copy of) the primary database
- defer signing of SCT's (and STH's) to backend nodes
The primary database
- stores cert chains and their corresponding SCT's
- is indexed on a running integer (primary) and a hash of the cert
chain (secondary)
- runs on backend nodes
- is persistently stored on disk on several other backend nodes in
separate data centers
- grows with 5 GB per year, based on 5,000 3 kB submissions per day
- max size is 300 GB, based on 100e6 certificates
The secondary databases
- store cert chains, unordered, between hash tree generation
- run on frontend nodes
- are persistently stored on disk on several other frontend nodes
- are typically kept in RAM too
- max size is around 128 MB, based on 10 submissions (รก 3 kB) per
second for an hour
Scaling, performance, estimates
- submissions: less than 0.1 qps, based on 5,000 submissions per day
- monitors: 6 qps, based on 100 monitors
- auditors: 8,000 qps, based on 2.5e9 browsers visiting 100 sites
(with a 1y certificate) per month (assuming a single combined
request for doing get-sth + get-sth-consistency + get-proof-by-hash)
Open questions
- What's a good MMD? Google seems to sign a new tree after 60-90
minutes (early 2014). They don't promise an MMD but aim to sign at
least once a day.
A picture
+-----------------------------------------------+
| front end nodes |
+-----------------------------------------------+
^ ^ ^ ^
| | | |
| v | |
| short term long term |
| cert db cert db copy |
| ^ |
| | v
+-----------------------------------------------+
| tree makers | mergers | signers |
+-----------------------------------------------+
^ ^
\ |
\ v
------------- long term
cert db
[TODO: Update terms in text or picture so they match:
secondary database == short term cert db
primary database == long term cert db
backend nodes == box with tree makers, mergers and signers]
[TODO: Move the picture to the top of the document.]
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