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-Filename: xxx-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