First iteration of vanguards lite proposal.

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+```
+Filename: 332-vanguards-lite.md
+Title: Vanguards lite
+Author: George Kadianakis, Mike Perry
+Created: 2021-05-20
+Status: Draft
+```
+
+# 0. Introduction & Motivation
+
+  This proposal specifies a simplified version of Proposal 292 "Mesh-based
+  vanguards" for the purposes of implementing it directly into the C Tor
+  codebase.
+
+  For more details on guard discovery attacks and how vanguards defend against
+  it, we refer to Proposal 292 [PROP292_REF].
+
+# 1. Overview
+
+  We propose an identical system to the Mesh-based Vanguards from proposal 292,
+  but with the following differences:
+
+  - No third layer of guards is used.
+  - The Layer2 lifetime uses the max(x,x) distribution with a minimum of one
+    day and maximum of 12 days. This makes the average lifetime approximately a
+    week. We let NUM_LAYER2_GUARDS=4.
+  - We don't write guards on disk. This means that the guard topology resets
+    when tor restarts.
+
+  By avoiding a third-layer of guards we reduce the linkability issues
+  of Proposal 292, which means that we don't have to add an extra hop on top of
+  our paths. This simplifies engineering.
+
+# 2. Rotation Period Analysis
+
+  From the table in Section 3.1 of Proposal 292, with NUM_LAYER2_GUARDS=4 it
+  can be seen that this means that the Sybil attack on Layer2 will complete
+  with 50% chance in 18*7 days (126 days) for the 1% adversary, 4*7 days (one
+  month) for the 5% adversary, and 2*7 days (two weeks) for the 10% adversary.
+
+# 3. Tradeoffs from Proposal 292
+
+  This proposal has several advantages over Proposal 292:
+
+  By avoiding a third-layer of guards we reduce the linkability issues of
+  Proposal 292, which means that we don't have to add an extra hop on top of
+  our paths. This simplifies engineering and makes paths shorter by default:
+  this means less latency and quicker page load times.
+
+  This proposal also comes with disadvantages:
+
+  The lack of third-layer guards makes it easier to launch guard discovery
+  attacks against clients and onion services. Long-lived services are not well
+  protected, and this proposal might provide those services with a false sense
+  of security. Such services should still use the vanguards addon [VANGUARDS_REF].
+
+# 4. References
+
+  [PROP292_REF]: https://gitlab.torproject.org/tpo/core/torspec/-/blob/main/proposals/292-mesh-vanguards.txt
+  [VANGUARDS_REF]: https://github.com/mikeperry-tor/vanguards