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Add TLS/cert normalization spec draft
This commit is contained in:
Jacob Appelbaum
Nick Mathewson
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Filename: xxx-draft-spec-for-TLS-normalization.txt
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Title: Draft spec for TLS certificate and handshake normalization
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Author: Jacob Appelbaum
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Created: 16-Feb-2011
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Status: Draft
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Draft spec for TLS certificate and handshake normalization
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Overview
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Scope
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This is a document that proposes improvements to problems with Tor's
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current TLS (Transport Layer Security) certificates and handshake that will
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reduce the distinguishability of Tor traffic from other encrypted traffic that
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uses TLS. It also addresses some of the possible fingerprinting attacks
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possible against the current Tor TLS protocol setup process.
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Motivation and history
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Censorship is an arms race and this is a step forward in the defense
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of Tor. This proposal outlines ideas to make it more difficult to
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fingerprint and block Tor traffic.
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Goals
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This proposal intends to normalize or remove easy-to-predict or static
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values in the Tor TLS certificates and with the Tor TLS setup process.
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These values can be used as criteria for the automated classification of
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encrypted traffic as Tor traffic. Network observers should not be able
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to trivially detect Tor merely by receiving or observing the certificate
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used or advertised by a Tor relay. I also propose the creation of
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a hard-to-detect covert channel through which a server can signal that it
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supports the third version ("V3") of the Tor handshake protocol.
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Non-Goals
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This document is not intended to solve all of the possible active or passive
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Tor fingerprinting problems. This document focuses on removing distinctive
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and predictable features of TLS protocol negotiation; we do not attempt to
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make guarantees about resisting other kinds of fingerprinting of Tor
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traffic, such as fingerprinting techniques related to timing or volume of
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transmitted data.
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Implementation details
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Certificate Issues
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The CN or commonName ASN1 field
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Tor generates certificates with a predictable commonName field; the
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field is within a given range of values that is specific to Tor.
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Additionally, the generated host names have other undesirable properties.
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The host names typically do not resolve in the DNS because the domain
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names referred to are generated at random. Although they are syntatically
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valid, they usually refer to domains that have never been registered by
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any domain name registrar.
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An example of the current commonName field: CN=www.s4ku5skci.net
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An example of OpenSSL’s asn1parse over a typical Tor certificate:
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0:d=0 hl=4 l= 438 cons: SEQUENCE
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4:d=1 hl=4 l= 287 cons: SEQUENCE
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8:d=2 hl=2 l= 3 cons: cont [ 0 ]
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10:d=3 hl=2 l= 1 prim: INTEGER :02
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13:d=2 hl=2 l= 4 prim: INTEGER :4D3C763A
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19:d=2 hl=2 l= 13 cons: SEQUENCE
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21:d=3 hl=2 l= 9 prim: OBJECT :sha1WithRSAEncryption
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32:d=3 hl=2 l= 0 prim: NULL
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34:d=2 hl=2 l= 35 cons: SEQUENCE
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36:d=3 hl=2 l= 33 cons: SET
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38:d=4 hl=2 l= 31 cons: SEQUENCE
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40:d=5 hl=2 l= 3 prim: OBJECT :commonName
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45:d=5 hl=2 l= 24 prim: PRINTABLESTRING :www.vsbsvwu5b4soh4wg.net
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71:d=2 hl=2 l= 30 cons: SEQUENCE
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73:d=3 hl=2 l= 13 prim: UTCTIME :110123184058Z
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88:d=3 hl=2 l= 13 prim: UTCTIME :110123204058Z
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103:d=2 hl=2 l= 28 cons: SEQUENCE
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105:d=3 hl=2 l= 26 cons: SET
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107:d=4 hl=2 l= 24 cons: SEQUENCE
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109:d=5 hl=2 l= 3 prim: OBJECT :commonName
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114:d=5 hl=2 l= 17 prim: PRINTABLESTRING :www.s4ku5skci.net
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133:d=2 hl=3 l= 159 cons: SEQUENCE
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136:d=3 hl=2 l= 13 cons: SEQUENCE
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138:d=4 hl=2 l= 9 prim: OBJECT :rsaEncryption
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149:d=4 hl=2 l= 0 prim: NULL
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151:d=3 hl=3 l= 141 prim: BIT STRING
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295:d=1 hl=2 l= 13 cons: SEQUENCE
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297:d=2 hl=2 l= 9 prim: OBJECT :sha1WithRSAEncryption
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308:d=2 hl=2 l= 0 prim: NULL
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310:d=1 hl=3 l= 129 prim: BIT STRING
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I propose that the commonName field be generated to match a specific property
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of the server in question. It is reasonable to set the commonName element to
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match either the hostname of the relay, the detected IP address of the relay,
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or for the relay operator to override certificate generation entirely by
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loading a custom certificate. For custom certificates, see the Custom
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Certificates section.
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I propose that the value for the commonName field be populated with the
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fully qualified host name as detected by reverse and forward resolution of the
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IP address of the relay. If the host name is in the DNS, this host name should
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be set as the common name. When forward and reverse DNS is not available, I
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propose that the IP address alone be used.
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The commonName field for the issuer should be set to known issuer names,
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random words or omitted entirely.
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Since some host names may themselves trigger censorship keyword filters,
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it may be reasonable to provide an option to override the defaults and
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force certain values in the commonName field.
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Considerations for commonName normalization
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Any host name supplied for the commonName field should resolve - even if it
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does not resolve to the IP address of the relay[0]. If the commonName field
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does include an IP address, it should be the current IP address of the relay as
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seen by other Internet hosts.
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Certificate serial numbers
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Currently our generated certificate serial number is set to the number of
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seconds since the epoch at the time of the certificate's creation. I propose
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that we should ensure that our serial numbers are unrelated to the epoch,
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since the generation methods are potentially recognizable as Tor-related.
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Instead, I propose that we use a randomly generated number that is
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subsequently hashed with SHA-512 and then truncated to a length chosen at
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random within a finite set of bounds. The length of the serial number should be
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chosen randomly at certificate generation time; it should be bound between the
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most commonly found bit lengths[1] in the wild. Random sixteen byte values
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appear to be the high bound for serial number as issued by Verisign and
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DigiCert. RapidSSL appears to be three bytes in length. Others common byte
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lengths appear to be between one and four bytes. I propose that we choose a
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byte length that is either 3, 4, or 16 bytes at certificate generation time.
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This randomly generated field may now serve as a covert channel that signals to
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the client that the OR will not support TLS renegotiation; this means that the
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client can expect to perform a V3 TLS handshake setup. Otherwise, if the serial
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number is a reasonable time since the epoch, we should assume the OR is
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using an earlier protocol version and hence that it expects renegotiation.
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As a security note, care must be taken to ensure that supporting this
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covert channel will not lead to an attacker having a method to downgrade client
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behavior.
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Certificate fingerprinting issues expressed as base64 encoding
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It appears that all deployed Tor certificates have the following strings in
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common:
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MIIB
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CCA
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gAwIBAgIETU
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ANBgkqhkiG9w0BAQUFADA
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YDVQQDEx
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3d3cu
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As expected these values correspond to specific ASN.1 OBJECT IDENTIFIER (OID)
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properties (sha1WithRSAEncryption, commonName, etc) of how we generate our
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certificates.
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As an illustrated example of the common bytes of all certificates used within
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the Tor network within a single one hour window, I have replaced the actual
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value with a wild card ('.') character here:
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-----BEGIN CERTIFICATE-----
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MIIB..CCA..gAwIBAgIETU....ANBgkqhkiG9w0BAQUFADA.M..w..YDVQQDEx.3
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d3cu............................................................
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................................................................
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................................................................
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................................................................
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................................................................
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................................................................
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................................................................
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................................................................
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........................... <--- Variable length and padding
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-----END CERTIFICATE-----
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This fine ascii art only illustrates the bytes that absolutely match in all
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cases. In many cases, it's likely that there is a high probability for a given
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byte to be only a small subset of choices.
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Using the above strings, the EFF's certificate observatory may trivially
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discover all known relays, known bridges and unknown bridges in a single SQL
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query. I propose that we ensure that we test our certificates to ensure that
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they do not have these kinds of statistical similarities without ensuring
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overlap with a very large cross section of the internet's certificates.
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Other certificate fields
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It may be advantageous to also generate values for the O, L, ST, C, and OU
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certificate fields. The C and ST fields may be populated from GeoIP information
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that is already available to Tor to reflect a plausible geographic location
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for the OR. The other fields should contain some semblance of a word or
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grouping of words. It has been suggested[2] that we should look to guides for
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certificate generation that use OpenSSL as a reasonable baseline for
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understanding these fields, as well as other certificate properties.
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Certificate dating and validity issues
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TLS certificates found in the wild are generally found to be long-lived;
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they are frequently old and often even expired. The current Tor certificate
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validity time is a very small time window starting at generation time and
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ending shortly thereafter, as defined in or.h by MAX_SSL_KEY_LIFETIME
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(2*60*60).
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I propose that the certificate validity time length is extended to a period of
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twelve Earth months, possibly with a small random skew to be determined by the
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implementer. Tor should randomly set the start date in the past or some
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currently unspecified window of time before the current date. This would
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more closely track the typical distribution of non-Tor TLS certificate
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expiration times.
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The certificate values, such as expiration, should not be used for anything
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relating to security; for example, if the OR presents an expired TLS
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certificate, this does not imply that the client should terminate the
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connection (as would be appropriate for an ordinary TLS implementation).
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Rather, I propose we use a TOFU style expiration policy - the certificate
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should never be trusted for more than a two hour window from first sighting.
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This policy should have two major impacts. The first is that an adversary will
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have to perform a differential analysis of all certificates for a given IP
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address rather than a single check. The second is that the server expiration
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time is enforced by the client and confirmed by keys rotating in the consensus.
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The expiration time should not be a fixed time that is simple to calculate by
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any Deep Packet Inspection device or it will become a new Tor TLS setup
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fingerprint.
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Custom Certificates
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It should be possible for a Tor relay operator to use a specifically supplied
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certificate and secret key. This will allow a relay or bridge operator to use a
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certificate signed by any member of any geographically relevant certificate
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authority racket; it will also allow for any other user-supplied certificate.
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This may be desirable in some kinds of filtered networks or when attempting to
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avoid attracting suspicion by blending in with the TLS web server certificate
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crowd.
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Problematic Diffie–Hellman parameters
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We currently send a static Diffie–Hellman parameter, prime p (or “prime p
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outlaw”) as specified in RFC2409 as part of the TLS Server Hello response.
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The use of this prime in TLS negotiations may, as a result, be filtered and
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effectively banned by certain networks. We do not have to use this particular
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prime in all cases.
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While amusing to have the power to make specific prime numbers into a new class
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of numbers (cf. imaginary, irrational, illegal [3]) - our new friend prime p
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outlaw is not required.
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The use of this prime in TLS negotiations may, as a result, be filtered and
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effectively banned by certain networks. We do not have to use this particular
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prime in all cases.
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I propose that the function to initialize and generate DH parameters be
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split into two functions.
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First, init_dh_param() should be used only for OR-to-OR DH setup and
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communication. Second, it is proposed that we create a new function
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init_tls_dh_param() that will have a two-stage development process.
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The first stage init_tls_dh_param() will use the same prime that
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Apache2.x [4] sends (or “dh1024_apache_p”), and this change should be
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made immediately. This is a known good and safe prime number (p-1 / 2
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is also prime) that is currently not known to be blocked.
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The second stage init_tls_dh_param() should randomly generate a new prime on a
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regular basis; this is designed to make the prime difficult to outlaw or
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filter. Call this a shape-shifting or "Rakshasa" prime. This should be added
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to the 0.2.3.x branch of Tor. This prime can be generated at setup or execution
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time and probably does not need to be stored on disk. Rakshasa primes only
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need to be generated by Tor relays as Tor clients will never send them. Such
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a prime should absolutely not be shared between different Tor relays nor
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should it ever be static after the 0.2.3.x release.
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As a security precaution, care must be taken to ensure that we do not generate
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weak primes or known filtered primes. Both weak and filtered primes will
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undermine the TLS connection security properties. OpenSSH solves this issue
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dynamically in RFC 4419 [5] and may provide a solution that works reasonably
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well for Tor. More research in this area including the applicability of
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Miller-Rabin or AKS primality tests[6] will need to be analyzed and probably
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added to Tor.
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Practical key size
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Currently we use a 1024 bit long RSA modulus. I propose that we increase the
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RSA key size to 2048 as an additional channel to signal support for the V3
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handshake setup. 2048 appears to be the most common key size[0] above 1024.
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Additionally, the increase in modulus size provides a reasonable security boost
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with regard to key security properties.
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The implementer should increase the 1024 bit RSA modulus to 2048 bits.
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Possible future filtering nightmares
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At some point it may cost effective or politically feasible for a network
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filter to simply block all signed or self-signed certificates without a known
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valid CA trust chain. This will break many applications on the internet and
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hopefully, our option for custom certificates will ensure that this step is
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simply avoided by the censors.
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The Rakshasa prime approach may cause censors to specifically allow only
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certain known and accepted DH parameters.
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Appendix: Other issues
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What other obvious TLS certificate issues exist? What other static values are
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present in the Tor TLS setup process?
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[0] http://archives.seul.org/or/dev/Jan-2011/msg00051.html
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[1] http://archives.seul.org/or/dev/Feb-2011/msg00016.html
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[2] http://archives.seul.org/or/dev/Feb-2011/msg00039.html
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[3] To be fair this is hardly a new class of numbers. History is rife with
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similar examples of inane authoritarian attempts at mathematical secrecy.
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Probably the most dramatic example is the story of the pupil Hipassus of
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Metapontum, pupil of the famous Pythagoras, who, legend goes, proved the
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fact that Root2 cannot be expressed as a fraction of whole numbers (now
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called an irrational number) and was assassinated for revealing this
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secret. Further reading on the subject may be found on the Wikipedia:
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http://en.wikipedia.org/wiki/Hippasus
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[4] httpd-2.2.17/modules/ss/ssl_engine_dh.c
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[5] http://tools.ietf.org/html/rfc4419
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[6] http://archives.seul.org/or/dev/Jan-2011/msg00037.html
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