commit 28a78381e4cee7d9fe20415d5c2d1391510a87f8 Author: Nick Mathewson nickm@torproject.org Date: Tue Mar 6 18:04:04 2012 -0500

Draft-status proposal 195: normalize TLS in 0.2.4 --- proposals/000-index.txt | 6 +- .../179-TLS-cert-and-parameter-normalization.txt | 27 +++- proposals/195-TLS-normalization-for-024.txt | 170 ++++++++++++++++++++ 3 files changed, 200 insertions(+), 3 deletions(-)

diff --git a/proposals/000-index.txt b/proposals/000-index.txt index 95dd4d4..fc5c508 100644 --- a/proposals/000-index.txt +++ b/proposals/000-index.txt @@ -99,7 +99,7 @@ Proposals by number: 176 Proposed version-3 link handshake for Tor [CLOSED] 177 Abstaining from votes on individual flags [OPEN] 178 Require majority of authorities to vote for consensus parameters [CLOSED] -179 TLS certificate and parameter normalization [DRAFT] +179 TLS certificate and parameter normalization [CLOSED] 180 Pluggable transports for circumvention [OPEN] 181 Optimistic Data for Tor: Client Side [CLOSED] 182 Credit Bucket [DRAFT] @@ -115,6 +115,7 @@ Proposals by number: 192 Automatically retrieve and store information about bridges [OPEN] 193 Safe cookie authentication for Tor controllers [OPEN] 194 Mnemonic .onion URLs [OPEN] +195 TLS certificate normalization for Tor 0.2.4.x [DRAFT]

Proposals by status: @@ -126,9 +127,9 @@ Proposals by status: 141 Download server descriptors on demand 144 Increase the diversity of circuits by detecting nodes belonging the same provider 175 Automatically promoting Tor clients to nodes - 179 TLS certificate and parameter normalization [for 0.2.3.x] 182 Credit Bucket 186 Multiple addresses for one OR or bridge + 195 TLS certificate normalization for Tor 0.2.4.x [for 0.2.4.x] NEEDS-REVISION: 131 Help users to verify they are using Tor 190 Bridge Client Authorization Based on a Shared Secret @@ -205,6 +206,7 @@ Proposals by status: 174 Optimistic Data for Tor: Server Side [in 0.2.3.1-alpha] 176 Proposed version-3 link handshake for Tor [for 0.2.3] 178 Require majority of authorities to vote for consensus parameters [in 0.2.3.9-alpha] + 179 TLS certificate and parameter normalization [for 0.2.3.x] 181 Optimistic Data for Tor: Client Side [in 0.2.3.3-alpha] 183 Refill Intervals [in 0.2.3.5-alpha] 184 Miscellaneous changes for a v3 Tor link protocol [for 0.2.3.x] diff --git a/proposals/179-TLS-cert-and-parameter-normalization.txt b/proposals/179-TLS-cert-and-parameter-normalization.txt index 7ad3ed8..4272d4f 100644 --- a/proposals/179-TLS-cert-and-parameter-normalization.txt +++ b/proposals/179-TLS-cert-and-parameter-normalization.txt @@ -2,7 +2,7 @@ Filename: 179-TLS-cert-and-parameter-normalization.txt Title: TLS certificate and parameter normalization Author: Jacob Appelbaum, Gladys Shufflebottom Created: 16-Feb-2011 -Status: Draft +Status: Closed Target: 0.2.3.x

@@ -11,6 +11,12 @@ Target: 0.2.3.x

Overview

+ STATUS NOTE: + + This document is implemented in part in 0.2.3.x, deferred in part, and + rejected in part. See indented bracketed comments in individual + sections below for more information. -NM + Scope

This is a document that proposes improvements to problems with Tor's @@ -135,6 +141,11 @@ covert channel will not lead to an attacker having a method to downgrade client behavior. This shouldn't be a risk because the TLS Finished message hashes over all the bytes of the handshake, including the certificates.

+ [Randomized serial numbers are implemented in 0.2.3.9-alpha. We probably + shouldn't do certificate tagging by a covert channel in serial numbers, + since doing so would mean we could never have an externally signed + cert. -NM] + Certificate fingerprinting issues expressed as base64 encoding

It appears that all deployed Tor certificates have the following strings in @@ -209,6 +220,8 @@ The expiration time should not be a fixed time that is simple to calculate by any Deep Packet Inspection device or it will become a new Tor TLS setup fingerprint.

+ [Deferred and needs revision; see proposal XXX. -NM] + Proposed certificate form

The following output from openssl asn1parse results from the proposed @@ -260,6 +273,10 @@ default self-signed certificate: 383:d=2 hl=2 l= 0 prim: NULL 385:d=1 hl=3 l= 129 prim: BIT STRING

+ [Rejected pending more evidence; this pattern is trivially detectable, + and there is just not enough reason at the moment to think that this + particular certificate pattern is common enough for sites that matter + that the censors wouldn't be willing to block it. -NM]

Custom Certificates

@@ -271,6 +288,8 @@ This may be desirable in some kinds of filtered networks or when attempting to avoid attracting suspicion by blending in with the TLS web server certificate crowd.

+ [Deferred; see proposal XXX] + Problematic Diffie–Hellman parameters

We currently send a static Diffie–Hellman parameter, prime p (or “prime p @@ -317,6 +336,8 @@ well for Tor. More research in this area including the applicability of Miller-Rabin or AKS primality tests[6] will need to be analyzed and probably added to Tor.

+ [Randomized DH groups are implemented in 0.2.3.9-alpha. -NM] + Practical key size

Currently we use a 1024 bit long RSA modulus. I propose that we increase the @@ -327,6 +348,10 @@ with regard to key security properties.

The implementer should increase the 1024 bit RSA modulus to 2048 bits.

+ [Deferred and needs performance analysis. See proposal + XXX. Additionally, DH group strength seems far more crucial. Still, this + is out-of-scope for a "normalization" question. -NM] + Possible future filtering nightmares

At some point it may cost effective or politically feasible for a network diff --git a/proposals/195-TLS-normalization-for-024.txt b/proposals/195-TLS-normalization-for-024.txt new file mode 100644 index 0000000..37ee546 --- /dev/null +++ b/proposals/195-TLS-normalization-for-024.txt @@ -0,0 +1,170 @@ +Filename: 195-TLS-normalization-for-024.txt +Title: TLS certificate normalization for Tor 0.2.4.x +Author: Jacob Appelbaum, Gladys Shufflebottom, Nick Mathewson, Tim Wilde +Created: 6-Mar-2012 +Status: Draft +Target: 0.2.4.x + + +0. Introduction + + The TLS (Transport Layer Security) protocol was designed for security + and extensibility, not for uniformity. Because of this, it's not + hard for an attacker to tell one application's use of TLS from + another's. + + We proposes improvements to Tor's current TLS certificates to + reduce the distinguishability of Tor traffic. + +0.1. History + + This draft is based on parts of Proposal 179, by Jacob Appelbaum + and Gladys Shufflebottom, but removes some already implemented parts + and replaces others. + +0.2. Non-Goals + + We do not address making TLS harder to distinguish after the + handshake is done. We also do not discuss TLS improvements not + related to distinguishability (such as increased key size, algorithm + choice, and so on). + +1. Certificate Issues + + Currently, Tor generates certificates according to a fixed pattern, + where lifetime is fairly small, the certificate Subject DN is a + single randomly generated CN, and the certificate Issuer DN is a + different single randomly generated CN. + + We propose several ways to improve this below. + +1.1. Separate initial certificate from link certificate + + When Tor is using the v2 or v3 link handshake (see tor-spec.txt), it + currently presents an initial handshake authenticating the link key + with the identity key. + + We propose instead that Tor should be able to present an arbitrary + initial certificate (so long as its key matches the link key used in + the actual TLS handshake), and then present the real certificate + authenticating the link key during the Tor handshake. (That is, + during the v2 handshake's renegotiation step, or in the v3 + handshake's CERTS cell.) + + The TLS protocol and the Tor handshake protocol both allow this, and + doing so will give us more freedom for the alternative certificate + presentation ideas below. + +1.2. Allow externally generated certificates + + It should be possible for a Tor relay operator to generate and + provide their own certificate and secret key. This will allow a relay or + bridge operator to use a certificate signed by any member of the "SSL + mafia,"[*] to generate their own self-signed certificate, and so on. + + For compatibility, we need to require that the key be an RSA secret + key, of at least 1024 bits, generated with e=65537. + + As a proposed interface, let's require that the certificate be stored + in ${DataDir}/tls_cert/tls_certificate.crt , that the secret key be + stored in ${DataDir}/tls_cert/private_tls_key.key , and that they be + used instead of generating our own certificate whenever the new + boolean option "ProvidedTLSCert" is set to true. + + (Alternative interface: Allow the cert and key cert to be stored + wherever, and have the user provide their respective locations with + TLSCertificateFile and TLSCertificateKeyFile options.) + +1.3. Longer certificate lifetimes + + Tor's current certificates aren't long-lived, which makes them + different from most other certificates in the wild. + + Typically, certificates are valid for a year, so let's use that as + our default lifetime. [TODO: investigate whether "a year" for most + CAs and self-signed certs have their validity dates running for a + calendar year ending at the second of issue, one calendar year + ending at midnight, or 86400*(365.5 +/- .5) seconds, or what.] + + There are two ways to approach this. We could continue our current + certificate management approach where we frequently generate new + certificates (albeit with longer lifetimes), or we could make a cert, + store it to disk, and use it for all or most of its declared + lifetime. + + If we continue to use fairly short lifetimes for the _true_ link + certificates (the ones presented during the Tor handshake), then + presenting long-lived certificates doesn't hurt us much: in the event + of a link-key-only compromise, the adversary still couldn't actually + impersonate a server for long.[**] + + Using shorter-lived certificates with long nominal lifetimes doesn't + seem to buy us much. It would let us rotate link keys more + frequently, but we're already getting forward secrecy from our use of + diffie-hellman key agreement. Further, it would make our behavior + look less like regular TLS behavior, where certificates are typically + used for most of their nominal lifetime. Therefore, let's store and + use certs and link keys for the full year. + +1.4. Self-signed certificates with better DNs + + When we generate our own certificates, we currently set no DN fields + other than the commonName. This behavior isn't terribly common: + users of self-signed certs usually/often set other fields too. + [TODO: find out frequency.] + + Unfortunately, it appears that no particular other set of fields or + way of filling them out _is_ universal for self-signed certificates, + or even particularly common. The most common schema seem to be for + things most censors wouldn't mind blocking, like embedded devices. + Even the default openssl schema, though common, doesn't appear to + represent a terribly large fraction of self-signed websites. [TODO: + get numbers here.] + + So the best we can do here is probably to reproduce the process that + results in self-signed certificates originally: let the bridge and relay + operators to pick the DN fields themselves. This is an annoying + interface issue, and wants a better solution. + +1.5. Better commonName values + + Our current certificates set the commonName to a randomly generated + field like www.rmf4h4h.net. This is also a weird behavior: nearly + all TLS certs used for web purposes will have a hostname that + resolves to their IP. + + The simplest way to get a plausible commonName here would be to do a + reverse lookup on our IP and try to find a good hostname. It's not + clear whether this would actually work out in practice, or whether + we'd just get dynamic-IP-pool hostnames everywhere blocked when they + appear in certificates. + + Alternatively, if we are told a hostname in our Torrc (possibly in + the Address field), we could try to use that. + +2. TLS handshake issues + +2.1. Session ID. + + Currently we do not send an SSL session ID, as we do not support session + resumption. However, Apache (and likely other major SSL servers) do have + this support, and do send a 32 byte SSLv3/TLSv1 session ID in their Server + Hello cleartext. We should do the same to avoid an easy fingerprinting + opportunity. It may be necessary to lie to OpenSSL to claim that we are + tracking session IDs to cause it to generate them for us. + + (We should not actually support session resumption.) + + + + +[*] "Hey buddy, it's a nice website you've got there. Sure would be a + shame if somebody started poppin' up warnings on all your user's + browsers, tellin' everbody that you're _insecure_..." + +[**] Furthermore, a link-key-only compromise isn't very realistic atm; + nearly any attack that would let an adversary learn a link key would + probably let the adversary learn the identity key too. The most + plausible way would probably be an implementation bug in OpenSSL or + something. +