Filename: xxx-draft-spec-for-TLS-normalization.txt Title: Draft spec for TLS certificate and handshake normalization Author: Jacob Appelbaum Created: 24-Jan-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. 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 of 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 un-related 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. The serial number should be similar in bit width to commonly found certificate serial numbers in the wild. 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. 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. 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). 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 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 [0]) - 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 [1] 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 [2] and may provide a solution that works reasonably well for Tor. More research in this area including Miller-Rabin primality tests will need to be analyzed and probably added to Tor. Practical key size Currently we use 1024-bit RSA keys. I propose that we increase the RSA key size to 1280 or to 2048 as an additional channel to signal support for the V3 handshake setup. 2048 is likely a more common key size in certificates today and also provides a reasonable security boost with regard to key security properties. The implementer should choose a key size that is common and meaningfully above 1024 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 unsigned 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] 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 [1] httpd-2.2.17/modules/ss/ssl_engine_dh.c [2] http://tools.ietf.org/html/rfc4419