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+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
+
+
+        Draft spec for TLS certificate and handshake normalization
+
+
+                                    Overview
+
+Scope
+
+This is a document that proposes improvements to problems with Tor's
+current TLS (Transport Layer Security) certificates and handshake that will
+reduce the distinguishability of Tor traffic from other encrypted traffic that
+uses TLS.  It also addresses some of the possible fingerprinting attacks
+possible against the current Tor TLS protocol setup process.
+
+Motivation and history
+
+Censorship is an arms race and this is a step forward in the defense
+of Tor.  This proposal outlines ideas to make it more difficult to
+fingerprint and block Tor traffic.
+
+Goals
+
+This proposal intends to normalize or remove easy-to-predict or static
+values in the Tor TLS certificates and with the Tor TLS setup process.
+These values can be used as criteria for the automated classification of
+encrypted traffic as Tor traffic. Network observers should not be able
+to trivially detect Tor merely by receiving or observing the certificate
+used or advertised by a Tor relay. I also propose the creation of
+a hard-to-detect covert channel through which a server can signal that it
+supports the third version ("V3") of the Tor handshake protocol.
+
+Non-Goals
+
+This document is not intended to solve all of the possible active or passive
+Tor fingerprinting problems. This document focuses on removing distinctive
+and predictable features of TLS protocol negotiation; we do not attempt to
+make guarantees about resisting other kinds of fingerprinting of Tor
+traffic, such as fingerprinting techniques related to timing or volume of
+transmitted data.
+
+                                Implementation details
+
+
+Certificate Issues
+
+The CN or commonName ASN1 field
+
+Tor generates certificates with a predictable commonName field; the
+field is within a given range of values that is specific to Tor.
+Additionally, the generated host names have other undesirable properties.
+The host names typically do not resolve in the DNS because the domain
+names referred to are generated at random. Although they are syntatically
+valid, they usually refer to domains that have never been registered by
+any domain name registrar.
+
+An example of the current commonName field: CN=www.s4ku5skci.net
+
+An example of OpenSSL’s asn1parse over a typical Tor certificate:
+
+   0:d=0  hl=4 l= 438 cons: SEQUENCE
+    4:d=1  hl=4 l= 287 cons: SEQUENCE
+    8:d=2  hl=2 l=   3 cons: cont [ 0 ]
+   10:d=3  hl=2 l=   1 prim: INTEGER           :02
+   13:d=2  hl=2 l=   4 prim: INTEGER           :4D3C763A
+   19:d=2  hl=2 l=  13 cons: SEQUENCE
+   21:d=3  hl=2 l=   9 prim: OBJECT            :sha1WithRSAEncryption
+   32:d=3  hl=2 l=   0 prim: NULL
+   34:d=2  hl=2 l=  35 cons: SEQUENCE
+   36:d=3  hl=2 l=  33 cons: SET
+   38:d=4  hl=2 l=  31 cons: SEQUENCE
+   40:d=5  hl=2 l=   3 prim: OBJECT            :commonName
+   45:d=5  hl=2 l=  24 prim: PRINTABLESTRING   :www.vsbsvwu5b4soh4wg.net
+   71:d=2  hl=2 l=  30 cons: SEQUENCE
+   73:d=3  hl=2 l=  13 prim: UTCTIME           :110123184058Z
+   88:d=3  hl=2 l=  13 prim: UTCTIME           :110123204058Z
+  103:d=2  hl=2 l=  28 cons: SEQUENCE
+  105:d=3  hl=2 l=  26 cons: SET
+  107:d=4  hl=2 l=  24 cons: SEQUENCE
+  109:d=5  hl=2 l=   3 prim: OBJECT            :commonName
+  114:d=5  hl=2 l=  17 prim: PRINTABLESTRING   :www.s4ku5skci.net
+  133:d=2  hl=3 l= 159 cons: SEQUENCE
+  136:d=3  hl=2 l=  13 cons: SEQUENCE
+  138:d=4  hl=2 l=   9 prim: OBJECT            :rsaEncryption
+  149:d=4  hl=2 l=   0 prim: NULL
+  151:d=3  hl=3 l= 141 prim: BIT STRING
+  295:d=1  hl=2 l=  13 cons: SEQUENCE
+  297:d=2  hl=2 l=   9 prim: OBJECT            :sha1WithRSAEncryption
+  308:d=2  hl=2 l=   0 prim: NULL
+  310:d=1  hl=3 l= 129 prim: BIT STRING
+
+I propose that we match OpenSSL's default self-signed certificates. I hypothesise
+that they are the most common self-signed certificates. If this turns out not
+to be the case, then we should use whatever the most common turns out to be.
+
+Certificate serial numbers
+
+Currently our generated certificate serial number is set to the number of
+seconds since the epoch at the time of the certificate's creation. I propose
+that we should ensure that our serial numbers are unrelated to the epoch,
+since the generation methods are potentially recognizable as Tor-related.
+
+Instead, I propose that we use a randomly generated number that is
+subsequently hashed with SHA-512 and then truncate the data to eight bytes[1].
+
+Random sixteen byte values appear to be the high bound for serial number as
+issued by Verisign and DigiCert.  RapidSSL appears to be three bytes in length.
+Others common byte lengths appear to be between one and four bytes. The default
+OpenSSL certificates are eight bytes and we should use this length with our
+self-signed certificates.
+
+This randomly generated serial number field may now serve as a covert channel
+that signals to the client that the OR will not support TLS renegotiation; this
+means that the client can expect to perform a V3 TLS handshake setup.
+Otherwise, if the serial number is a reasonable time since the epoch, we should
+assume the OR is using an earlier protocol version and hence that it expects
+renegotiation.
+
+We also have a need to signal properties with our certificates for a possible
+v3 handshake in the future. Therefore I propose that we match OpenSSL default
+self-signed certificates (a 64-bit random number), but reserve the two least-
+significant bits for signaling. For the moment, these two bits will be zero.
+
+This means that an attacker may be able to identify Tor certificates from default
+OpenSSL certificates with a 75% probability.
+
+As a security note, care must be taken to ensure that supporting this
+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.
+
+Certificate fingerprinting issues expressed as base64 encoding
+
+It appears that all deployed Tor certificates have the following strings in
+common:
+
+MIIB
+CCA
+gAwIBAgIETU
+ANBgkqhkiG9w0BAQUFADA
+YDVQQDEx
+3d3cu
+
+As expected these values correspond to specific ASN.1 OBJECT IDENTIFIER (OID)
+properties (sha1WithRSAEncryption, commonName, etc) of how we generate our
+certificates.
+
+As an illustrated example of the common bytes of all certificates used within
+the Tor network within a single one hour window, I have replaced the actual
+value with a wild card ('.') character here:
+
+-----BEGIN CERTIFICATE-----
+MIIB..CCA..gAwIBAgIETU....ANBgkqhkiG9w0BAQUFADA.M..w..YDVQQDEx.3
+d3cu............................................................
+................................................................
+................................................................
+................................................................
+................................................................
+................................................................
+................................................................
+................................................................
+........................... <--- Variable length and padding
+-----END CERTIFICATE-----
+
+This fine ascii art only illustrates the bytes that absolutely match in all
+cases.  In many cases, it's likely that there is a high probability for a given
+byte to be only a small subset of choices.
+
+Using the above strings, the EFF's certificate observatory may trivially
+discover all known relays, known bridges and unknown bridges in a single SQL
+query.  I propose that we ensure that we test our certificates to ensure that
+they do not have these kinds of statistical similarities without ensuring
+overlap with a very large cross section of the internet's certificates.
+
+Certificate dating and validity issues
+
+TLS certificates found in the wild are generally found to be long-lived;
+they are frequently old and often even expired. The current Tor certificate
+validity time is a very small time window starting at generation time and
+ending shortly thereafter, as defined in or.h by MAX_SSL_KEY_LIFETIME
+(2*60*60).
+
+I propose that the certificate validity time length is extended to a period of
+twelve Earth months, possibly with a small random skew to be determined by the
+implementer. Tor should randomly set the start date in the past or some
+currently unspecified window of time before the current date. This would
+more closely track the typical distribution of non-Tor TLS certificate
+expiration times.
+
+The certificate values, such as expiration, should not be used for anything
+relating to security; for example, if the OR presents an expired TLS
+certificate, this does not imply that the client should terminate the
+connection (as would be appropriate for an ordinary TLS implementation).
+Rather, I propose we use a TOFU style expiration policy - the certificate
+should never be trusted for more than a two hour window from first sighting.
+
+This policy should have two major impacts. The first is that an adversary will
+have to perform a differential analysis of all certificates for a given IP
+address rather than a single check. The second is that the server expiration
+time is enforced by the client and confirmed by keys rotating in the consensus.
+
+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.
+
+Proposed certificate form
+
+The following output from openssl asn1parse results from the proposed
+certificate generation algorithm. It matches the results of generating a
+default self-signed certificate:
+
+    0:d=0  hl=4 l= 513 cons: SEQUENCE          
+    4:d=1  hl=4 l= 362 cons: SEQUENCE          
+    8:d=2  hl=2 l=   9 prim: INTEGER           :DBF6B3B864FF7478
+   19:d=2  hl=2 l=  13 cons: SEQUENCE          
+   21:d=3  hl=2 l=   9 prim: OBJECT            :sha1WithRSAEncryption
+   32:d=3  hl=2 l=   0 prim: NULL              
+   34:d=2  hl=2 l=  69 cons: SEQUENCE          
+   36:d=3  hl=2 l=  11 cons: SET               
+   38:d=4  hl=2 l=   9 cons: SEQUENCE          
+   40:d=5  hl=2 l=   3 prim: OBJECT            :countryName
+   45:d=5  hl=2 l=   2 prim: PRINTABLESTRING   :AU
+   49:d=3  hl=2 l=  19 cons: SET               
+   51:d=4  hl=2 l=  17 cons: SEQUENCE          
+   53:d=5  hl=2 l=   3 prim: OBJECT            :stateOrProvinceName
+   58:d=5  hl=2 l=  10 prim: PRINTABLESTRING   :Some-State
+   70:d=3  hl=2 l=  33 cons: SET               
+   72:d=4  hl=2 l=  31 cons: SEQUENCE          
+   74:d=5  hl=2 l=   3 prim: OBJECT            :organizationName
+   79:d=5  hl=2 l=  24 prim: PRINTABLESTRING   :Internet Widgits Pty Ltd
+  105:d=2  hl=2 l=  30 cons: SEQUENCE          
+  107:d=3  hl=2 l=  13 prim: UTCTIME           :110217011237Z
+  122:d=3  hl=2 l=  13 prim: UTCTIME           :120217011237Z
+  137:d=2  hl=2 l=  69 cons: SEQUENCE          
+  139:d=3  hl=2 l=  11 cons: SET               
+  141:d=4  hl=2 l=   9 cons: SEQUENCE          
+  143:d=5  hl=2 l=   3 prim: OBJECT            :countryName
+  148:d=5  hl=2 l=   2 prim: PRINTABLESTRING   :AU
+  152:d=3  hl=2 l=  19 cons: SET               
+  154:d=4  hl=2 l=  17 cons: SEQUENCE          
+  156:d=5  hl=2 l=   3 prim: OBJECT            :stateOrProvinceName
+  161:d=5  hl=2 l=  10 prim: PRINTABLESTRING   :Some-State
+  173:d=3  hl=2 l=  33 cons: SET               
+  175:d=4  hl=2 l=  31 cons: SEQUENCE          
+  177:d=5  hl=2 l=   3 prim: OBJECT            :organizationName
+  182:d=5  hl=2 l=  24 prim: PRINTABLESTRING   :Internet Widgits Pty Ltd
+  208:d=2  hl=3 l= 159 cons: SEQUENCE          
+  211:d=3  hl=2 l=  13 cons: SEQUENCE          
+  213:d=4  hl=2 l=   9 prim: OBJECT            :rsaEncryption
+  224:d=4  hl=2 l=   0 prim: NULL              
+  226:d=3  hl=3 l= 141 prim: BIT STRING        
+  370:d=1  hl=2 l=  13 cons: SEQUENCE          
+  372:d=2  hl=2 l=   9 prim: OBJECT            :sha1WithRSAEncryption
+  383:d=2  hl=2 l=   0 prim: NULL              
+  385:d=1  hl=3 l= 129 prim: BIT STRING        
+
+
+Custom Certificates
+
+It should be possible for a Tor relay operator to use a specifically supplied
+certificate and secret key. This will allow a relay or bridge operator to use a
+certificate signed by any member of any geographically relevant certificate
+authority racket; it will also allow for any other user-supplied certificate.
+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.
+
+Problematic Diffie–Hellman parameters
+
+We currently send a static Diffie–Hellman parameter, prime p (or “prime p
+outlaw”) as specified in RFC2409 as part of the TLS Server Hello response.
+
+The use of this prime in TLS negotiations may, as a result, be filtered and
+effectively banned by certain networks. We do not have to use this particular
+prime in all cases.
+
+While amusing to have the power to make specific prime numbers into a new class
+of numbers (cf. imaginary, irrational, illegal [3]) - our new friend prime p
+outlaw is not required.
+
+The use of this prime in TLS negotiations may, as a result, be filtered and
+effectively banned by certain networks. We do not have to use this particular
+prime in all cases.
+
+I propose that the function to initialize and generate DH parameters be
+split into two functions.
+
+First, init_dh_param() should be used only for OR-to-OR DH setup and
+communication. Second, it is proposed that we create a new function
+init_tls_dh_param() that will have a two-stage development process.
+
+The first stage init_tls_dh_param() will use the same prime that
+Apache2.x [4] sends (or “dh1024_apache_p”), and this change should be
+made immediately. This is a known good and safe prime number (p-1 / 2
+is also prime) that is currently not known to be blocked.
+
+The second stage init_tls_dh_param() should randomly generate a new prime on a
+regular basis; this is designed to make the prime difficult to outlaw or
+filter.  Call this a shape-shifting or "Rakshasa" prime.  This should be added
+to the 0.2.3.x branch of Tor. This prime can be generated at setup or execution
+time and probably does not need to be stored on disk. Rakshasa primes only
+need to be generated by Tor relays as Tor clients will never send them. Such
+a prime should absolutely not be shared between different Tor relays nor
+should it ever be static after the 0.2.3.x release.
+
+As a security precaution, care must be taken to ensure that we do not generate
+weak primes or known filtered primes. Both weak and filtered primes will
+undermine the TLS connection security properties. OpenSSH solves this issue
+dynamically in RFC 4419 [5] and may provide a solution that works reasonably
+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.
+
+Practical key size
+
+Currently we use a 1024 bit long RSA modulus. I propose that we increase the
+RSA key size to 2048 as an additional channel to signal support for the V3
+handshake setup.  2048 appears to be the most common key size[0] above 1024.
+Additionally, the increase in modulus size provides a reasonable security boost
+with regard to key security properties.
+
+The implementer should increase the 1024 bit RSA modulus to 2048 bits.
+
+Possible future filtering nightmares
+
+At some point it may cost effective or politically feasible for a network
+filter to simply block all signed or self-signed certificates without a known
+valid CA trust chain. This will break many applications on the internet and
+hopefully, our option for custom certificates will ensure that this step is
+simply avoided by the censors.
+
+The Rakshasa prime approach may cause censors to specifically allow only
+certain known and accepted DH parameters.
+
+
+Appendix: Other issues
+
+What other obvious TLS certificate issues exist? What other static values are
+present in the Tor TLS setup process?
+
+[0] http://archives.seul.org/or/dev/Jan-2011/msg00051.html
+[1] http://archives.seul.org/or/dev/Feb-2011/msg00016.html
+[2] http://archives.seul.org/or/dev/Feb-2011/msg00039.html
+[3] To be fair this is hardly a new class of numbers. History is rife with
+    similar examples of inane authoritarian attempts at mathematical secrecy.
+    Probably the most dramatic example is the story of the pupil Hipassus of
+    Metapontum, pupil of the famous Pythagoras, who, legend goes, proved the
+    fact that Root2 cannot be expressed as a fraction of whole numbers (now
+    called an irrational number) and was assassinated for revealing this
+    secret.  Further reading on the subject may be found on the Wikipedia:
+    http://en.wikipedia.org/wiki/Hippasus
+
+[4] httpd-2.2.17/modules/ss/ssl_engine_dh.c
+[5] http://tools.ietf.org/html/rfc4419
+[6] http://archives.seul.org/or/dev/Jan-2011/msg00037.html