[or-cvs] [tor/master] Add TLS/cert normalization spec draft
nickm at torproject.org
nickm at torproject.org
Fri Feb 18 21:04:28 UTC 2011
commit 400a5a7ddd3adf8ef798302e60db8a0a0ff94b9f
Author: Jacob Appelbaum <jacob at appelbaum.net>
Date: Fri Feb 18 16:04:23 2011 -0500
Add TLS/cert normalization spec draft
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+Filename: xxx-draft-spec-for-TLS-normalization.txt
+Title: Draft spec for TLS certificate and handshake normalization
+Author: Jacob Appelbaum
+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 the commonName field be generated to match a specific property
+of the server in question. It is reasonable to set the commonName element to
+match either the hostname of the relay, the detected IP address of the relay,
+or for the relay operator to override certificate generation entirely by
+loading a custom certificate. For custom certificates, see the Custom
+Certificates section.
+
+I propose that the value for the commonName field be populated with the
+fully qualified host name as detected by reverse and forward resolution of the
+IP address of the relay. If the host name is in the DNS, this host name should
+be set as the common name. When forward and reverse DNS is not available, I
+propose that the IP address alone be used.
+
+The commonName field for the issuer should be set to known issuer names,
+random words or omitted entirely.
+
+Since some host names may themselves trigger censorship keyword filters,
+it may be reasonable to provide an option to override the defaults and
+force certain values in the commonName field.
+
+Considerations for commonName normalization
+
+Any host name supplied for the commonName field should resolve - even if it
+does not resolve to the IP address of the relay[0]. If the commonName field
+does include an IP address, it should be the current IP address of the relay as
+seen by other Internet hosts.
+
+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 truncated to a length chosen at
+random within a finite set of bounds. The length of the serial number should be
+chosen randomly at certificate generation time; it should be bound between the
+most commonly found bit lengths[1] in the wild. 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. I propose that we choose a
+byte length that is either 3, 4, or 16 bytes at certificate generation time.
+
+This randomly generated 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.
+
+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.
+
+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.
+
+Other certificate fields
+
+It may be advantageous to also generate values for the O, L, ST, C, and OU
+certificate fields. The C and ST fields may be populated from GeoIP information
+that is already available to Tor to reflect a plausible geographic location
+for the OR. The other fields should contain some semblance of a word or
+grouping of words. It has been suggested[2] that we should look to guides for
+certificate generation that use OpenSSL as a reasonable baseline for
+understanding these fields, as well as other certificate properties.
+
+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.
+
+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
+outlawâ€) 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_pâ€), 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
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