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 covers issues with current TLS (Transport Layer Security) related certificates generated and used by Tor. It also intends to cover some of the possible fingerprinting issues present in 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 that attempt to limit the ability of an attack who wishes to fingerprint and block Tor. 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. TLS certificate observatories should not be able to trivially detect Tor merely by receiving or observing the certificate used or advertised by a Tor relay. Additionally I propose that when these changes are implemented, a covert channel will be available to signal that a server 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. Beyond basic certificate examination or TLS setup issues, we attempt to make no guarantees about resisting other kinds of fingerprinting of Tor traffic. Implementation details Certificate Issues The CN or commonName ASN1 field Tor generates certificates with a commonName field that is predictable and the field is within a given value range that is specific to Tor. Additionally, the generated host names have other undesirable properties. The host names do not resolve as they are generally for host names that have not and will not ever exist in the DNS. They are also often host names that while RFC compliant, they have almost certainly 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 is 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 shall 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 is used. The commonName field for the issuer should be set to known issuer names, random words or left out entirely. Some host names may trigger certain censorship keyword filters and so it may be reasonable to provide an option to force certain values into 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. Certificate serial numbers Currently our certificate serial numbers is set to the of number of seconds since the epoch at creation time. I propose that we should ensure that our serial numbers are un-related to the epoch. I propose that we use a randomly generated number that is subsequently hashed with SHA-512 and 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 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. 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 or old and often expired. The current Tor certificate validity time is a very small time window starting at generation time and ending shortly thereafter as defined 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. The certificate values, such as expiration, should not be used for anything relating to security. 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 a member of the 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 blend 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. While amusing to have the power to make specific prime numbers into a new class of numbers (cf. imaginary, irrational, illegal) - our new friend prime p outlaw is not required. I propose that the function to initialize and generate DH parameters is split into two functions. First, init_dh_param() should be used only for OR to OR DH setup and communication. Secondly, it it 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 [0] sends (or “dh1024_apache_p”) and this change should be made immediately. This is a known good and safe prime number that is currently not known to be blocked. The second stage init_tls_dh_param() should randomly generate a new hard to outlaw or filter prime p (“evasive prime p”) on a regular basis; 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. Evasive prime p only needs to be generated by Tor relays as Tor clients will never send it. It 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 [1] 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 covert channel to signal support for the V3 handshake setup. 2048 is likely a more common certificate size 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. Appendix: Other issues What other obvious TLS certificate issues exist? What other static values are present in the Tor TLS setup? [0] httpd-2.2.17/modules/ss/ssl_engine_dh.c [1] http://tools.ietf.org/html/rfc4419