-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 Hi Tor-Dev, I'm curious what the timing is of Tor's opening of preemptive circuits. Specifically, consider the following attack: 1. A new stream is assigned to a clean circuit. 2. Because of the above, that clean circuit is now a dirty circuit. 3. Because of the above, the number of clean circuits is now decreased by 1. 4. Because of the above, the number of clean circuits is now lower than the number that Tor wants to have open. 5. Because of the above, Tor opens a new preemptive circuit. 6. An attacker who can observe the circuit in (1) and the circuit in (5) can deduce by temporal proximity that those 2 circuits belong to the same client. This attack seemed obvious enough to me that I assumed that Tor must have some kind of countermeasure to it, e.g. random delays in opening preemptive circuits. However, the tor-path specification doesn't mention any such countermeasure, and based on a brief search through the Tor source code, all I can find is that Tor opens preemptive circuits using a function that always gets called once per second (with no mention of any delay beyond that one-second interval, random or otherwise). So, does Tor make any effort to mitigate the above attack? If so, what mitigations are present, and where would I find them (in both the spec and the source code)? If not, is there any documented reason (e.g. "the attack is too hard to pull off" or "we want to mitigate it but haven't gotten to it yet") for the lack of mitigation? Cheers, - -- - -Jeremy Rand Lead Application Engineer at Namecoin Mobile email: jeremyrandmobile@airmail.cc Mobile OpenPGP: 2158 0643 C13B B40F B0FD 5854 B007 A32D AB44 3D9C Send non-security-critical things to my Mobile with OpenPGP. Please don't send me unencrypted messages. My business email jeremy@veclabs.net is having technical issues at the moment. -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEVB3fdzAraEcoBtkSs/LRZXhtZXAFAl2CoycACgkQs/LRZXht ZXDyjhAAk3F6fJUvNl4FHws6IIBHWXby/zC7MLzr9zR3sIC3RY4f/I7XBDTKKfMg iOehJlBEksm5sRz6nlybUBivsJ0WRL8VCT3rEK3+LQM7JrYNvGajolEw2apBUeB3 +Vg+Quo5JzTZrjCtXljxdps/hhC1YwEyN6pxxn2137q4XAsRPQgSCcWpsBuDTffb LFRwyGrNxk6chbpQc0GIWQRoeElxFwaLbUrZdGtaaEjBXY2oWoan1TUB34MAVNzF 1o6DlaO2JQ7weHVHoXlTlXS7gt84/WyY0xaHUY9ONHwrhgDPGv3N5+Fa2Em+PvGx ik+4iSCzPf6aKY0iUv1AsWKiaq8rRsMToyNzNZ048uYHk2IK2Zrbqh4Nm44RTzKD 84tJU71calRdNhbwuvmgsYKh26Ad9ALf9BzDeiweA3c5ZP0JP0OlLcAN9c056WT3 FQbChA1edZkWkPvBjGtl4l7ROIx6wkNpWgaQsYUCMveszB2Rm38BQw5sYLh8i6ha AlT3UVs/GJscSSsfmHGrTisao444Etp4J/+SFe1aNJwfOogbEfgJ1jRf8V5dlYEb rcicgkwmnBf01vnVaUO9C6/lToUa9i7r3BD3s6OQW24qz2ixieS3K6NxxxqMrVQ1 vFArleJIYA/FsDM2NPbeaUQA9z5GJj5X3FcPUPLLF+y5G/0HE14= =aBRL -----END PGP SIGNATURE-----