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116 lines
3.2 KiB
Plaintext
116 lines
3.2 KiB
Plaintext
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How to hand out bridges.
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Divide bridges into 'strategies' as they come in. Do this uniformly
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at random for now.
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For each strategy, we'll hand out bridges in a different way to
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clients. This document describes two strategies: email-based and
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IP-based.
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0. Notation:
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HMAC(k,v) : an HMAC of v using the key k.
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A|B: The string A concatenated with the string B.
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1. Email-based.
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Goal: bootstrap based on one or more popular email service's sybil
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prevention algorithms.
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Parameters:
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HMAC -- an HMAC function
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P -- a time period
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K -- the number of bridges to send in a period.
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Setup: Generate two nonces, N and M.
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As bridges arrive, put them into a ring according to HMAC(N,ID)
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where ID is the bridges's identity digest.
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Divide time into divisions of length P.
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When we get an email:
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If it's not from a supported email service, reject it.
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If we already sent a response to that email address (normalized)
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in this period, send _exactly_ the same response.
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If it is from a supported service, generate X = HMAC(M,PS|E) where E
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is the lowercased normalized email address for the user, and
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where PS is the start of the currrent period. Send
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the first K bridges in the ring after point X.
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To normalize an email address:
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Start with the RFC822 address. Consider only the mailbox {???}
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portion of the address (username@host). Put this into lowercase
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ascii.
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Questions:
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What to do with weird character encodings? Look up the RFC.
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Notes:
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Make sure that you can't force a single email address to appear
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in lots of different ways. IOW, if nickm@freehaven.net and
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NICKM@freehaven.net aren't treated the same, then I can get lots
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more bridges than I should.
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Make sure you can't construct a distinct address to match an
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existing one. IOW, if we treat nickm@X and nickm@Y as the same
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user, then anybody can register nickm@Z and use it to tell which
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bridges nickm@X got (or would get).
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Make sure that we actually check headers so we can't be trivially
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used to sapam people.
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2. IP-based.
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Goal: avoid handing out all the bridges to users in a similar IP
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space and time.
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Parameters:
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T_Flush -- how long it should take a user on a single network to
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see a whole cluster of bridges.
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N_C
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K -- the number of bridges we hand out in response to a single
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request.
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Setup: using an AS map or a geoip map or some other flawed input
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source, divide IP space into "areas" such that surveying a large
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collection of "areas" is hard. For v0, use /24 adress blocks.
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Group areas into N_C clusters.
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Generate nonces L, M, N.
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Set the period P such that P*(bridges-per-cluster/K) = T_flush.
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Don't set P to greater than a week, or less than three hours.
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When we get a bridge:
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Based on HMAC(L,ID), assign the bridge to a cluster. Within each
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cluster, keep the bridges in a ring based on HMAC(M,ID).
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When we get a connection:
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If it's http, redirect it to https.
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Let net be the incoming IP network. Let PS be the current
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period. Compute X = HMAC(N, PS|net). Return the next K bridges
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in the ring after X.
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3. Open issues
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Denial of service attacks
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A good view of network topology
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