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List:       tor-dev
Subject:    [tor-dev] Comparing handshake protocols for Tor: Ace vs ntor
From:       Esfandiar Mohammadi <post () esfandiar-mohammadi ! de>
Date:       2013-07-23 23:36:12
Message-ID: CA09E96A-0078-454B-9751-5ECA4358C709 () esfandiar-mohammadi ! de
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Hi,

in WPES 2012 [2], Aniket Kate and me introduced Ace, an alternative for Tor's current \
handshake protocol ntor (which was also briefly discussed on this list [4]). Back \
then, no implementation for the double scalar multiplication operation (a * b + c * \
d) on Curve25519 was available. But recently, Robert Ransom implemented in his \
celator library [1] a highly efficient double scalar multiplication which is suitable \
for our handshake protocol Ace.

In an internship with us, Shivanker Goel implemented and run comparative benchmarks \
of the Ace protocol, using Robert's celator library [1], and the current ntor \
implementation. Here are the benchmarks:

64-bit architecture:
Ace: 401,129 cycles,
ntor: 472,772 cycles,

32-bit architecture:
Ace: 2,797,303 cycles,
ntor: 5,179,069 cycles,

About the benchmarks:

* For cleaning the cache, the benchmarks runs the handshake 1000 times prior to \
starting the benchmarks. Then, we used a tsc_read() function to count the cycles, \
repeated the handshake 5000 times and computed the average of the cycles.

* In order not to be forced to import too many functions from the Tor library, we do \
not use the function dimap_search, which looks up the secret key for a given public \
key, in the benchmarks.

* We assume that ephemeral keys are precomputed, e.g., in idle cycles. In the \
benchmarks these ephemeral keys are received as an input.

* For the benchmarks hyper-threading and overclocking were disabled and gcc-4.8 with \
"-O2" optimizations was used.

The code is available online [2]. We also tried to formulate a tor-style proposal, \
which is also available online [3].

Any feedback, comments, or suggestions are welcome.

- Esfandiar

[1] http://www.infsec.cs.uni-saarland.de/~mohammadi/ace/celator.tar.gz
[2] http://www.infsec.cs.uni-saarland.de/~mohammadi/ace/ace-benchmarks.tar.gz
[3] http://www.infsec.cs.uni-saarland.de/~mohammadi/ace/ace-handshake.txt
[4] It was discussed in the thread "Another key exchange algorithm for extending \
circuits:	alternative to ntor?" from August 2012. \
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