September 20, 2013
by cologic
1. Summary
Several TLS attacks since 2011 impel a reassessment of the security of ADC’s usage of TLS to form ADCS. While the specific attacks tend not to be trivially replicated in a DC client as opposed to a web browser, remaining conservative with respect to security remains useful, the issues they exploit could cause problems regardless, and ADCS’s best response thus becomes to deprecate SSL 3.0 and TLS 1.0. Ideally, one should use TLS 1.2 with AES-GCM. Failing that, ensuring that TLS 1.1 runs and chooses AES-based ciphersuite works adequately.
2. HTTP-over-TLS Attacks
BEAST renders practical Rogaway’s 2002 attack on the security of CBC ciphersuites in SSL/TLS by using an SSL/TLS server’s CBC padding MAC acceptance/rejection as a timing oracle. Asking whether each possible byte in each position results in successful MAC, it decodes an entire message. One can avert BEAST either by avoiding CBC in lieu of RC4 or updating to TLS 1.1 or 1.2, which mitigate the timing oracle and generate new random IVs to undermine BEAST’s sequential attack.
CRIME and BREACH build on a 2002 compression and information leakage of plaintext-based attack. CRIME “requires on average 6 requests to decrypt 1 cookie byte” and, like BEAST, recognizes DEFLATE’s smaller output when it has found a pre-existing copy of the correct plaintext in its dictionary. Unlike BEAST, CRIME and BREACH depend not on TLS version or CBC versus RC4 ciphersuites but merely compression. Disabling HTTP and TLS compression therefore avoids CRIME and BREACH.
One backwards-compatible solution thus far involves avoiding compression due to CRIME/BREACH and avoiding BEAST with RC4-based TLS ciphersuites. However, a new attack against RC4 in TLS by AlFardan, Bernstein, et al exploits double-byte ciphertext biases to reconstruct messages using approximately 229 ciphertexts; as few as 225 achieve a 60+% recovery rate. RC4-based ciphersuites decreasingly inspire confidence as a backwards-compatible yet secure approach to TLS, enough that the IETF circulates an RFC draft prohibiting RC4 ciphersuites.
Thus far treating DC as sufficiently HTTP-like to borrow their threat model, options narrow to TLS 1.1 or TLS 1.2 with an AES-derived ciphersuite. One needs still beware: Lucky 13 weakens even TLS 1.1 and TLS 1.2 AES-CBC ciphers, leaving between it and the RC4 attack no unscathed TLS 1.1 configuration. Instead, AlFardan and Paterson recommend to “switch to using AEAD ciphersuites, such as AES-GCM” and/or “modify TLS’s CBC-mode decryption procedure so as to remove the timing side channel”. They observe that each major TLS library has addressed the latter point, so that AES-CBC might remain somewhat secure; certainly superior to RC4.
3. ADC-over-TLS-specific Concerns
ADCS clients’ and hubs’ vulnerability profiles and relevant threat models regarding each of BEAST, CRIME, BREACH, Lucky 13, and the RC4 break differ from that of a web browser using HTTP. BEAST and AlFardan, Bernstein, et al’s RC4 attack both point to adopting TLS 1.1, a ubiquitously supportable requirement worth satisfying regardless. OpenSSL, NSS, GnuTLS, PolarSSL, CyaSSL, MatrixSSL, BouncyCastle, and Oracle’s standard Java crypto library have all already “addressed” Lucky 13.
ADCS doesn’t use TLS compression, so that aspect of CRIME/BREACH does not apply. The ZLIB extension does operate analogously to HTTP compression. Indeed, the BREACH authors remark that:
there is nothing particularly special about HTTP and TLS in this side-channel. Any time an attacker has the ability to inject their own payload into plaintext that is compressed, the potential for a CRIME-like attack is there. There are many widely used protocols that use the composition of encryption with compression; it is likely that other instances of this vulnerability exist.
ADCS provides an attacker this capability via logging onto a hub and sending CTMs and B, D, and E-type messages. Weaponizing it, however, operates better when these injected payloads can discover cookie-like repeated secrets, which ADC lacks. GPA and PAS operate via a challenge-reponse system. CTM cookies find use at most once. Private IDs would presumably have left a client-hub connection’s compression dictionary by the time an attack might otherwise succeed and don’t appear in client-client connections. While a detailed analysis of the extent of practical feasibility remains wanting, I’m skeptical CRIME and BREACH much threaten ADCS.
4. Mitigation and Prevention in ADCS
Regardless, some of these attacks could be avoided entirely with specification updates incurring no ongoing cost and hindering implenetation on no common platforms. Three distinct categories emerge: BEAST and Lucky 13 attacks CBC in TLS; the RC4 break, well, attacks RC4; and CRIME and BREACH attack compression. Since one shouldn’t use RC4 regardless, that leaves AES-CBC attacks and compression attacks.
Disabling compression might incur substantial bandwidth cost for little thus-far demonstrated security benefit, so although ZLIB implementors should remain aware of CRIME and BREACH, continued usage seems unproblematic.
Separately, BEAST and Lucky 13 point to requiring TLS 1.1 and, following draft IETF recomendations for secure use of TLS and DTLS, preferring TLS 1.2 with the TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 or other AES-GCM ciphersuite if supported by both endpoints. cryptlib, CyaSSL, GnuTLS, MatrixSSL, NSS, OpenSSL, PolarSSL, SChannel, and JSSE support both TLS 1.1 and TLS 1.2 and all but Java’s supports AES-GCM.
Suggested responses:
- Consider how to communicate to ZLIB implementors the hazards and threat model, however minor, presented by CRIME and BREACH.
- Formally deprecate SSL 3.0 and TLS 1.0 in the ADCS extension specification.
- Discover which TLS versions and features clients (DC++ and variations, ncdc, Jucy, etc) and hubs (ADCH++, uHub, etc) support. If they use standard libraries, they probably all (except Jucy) already support TLS 1.2 with AES-GCM depending on how they configure their TLS libraries. Depending on results, one might already safely simply disable SSL 3.0 and TLS 1.0 in each such client and hub and prioritize TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 or a similar ciphersuite so that it finds use when mutually available. If this proves possible, the the ADCS extension specification should be updated to reflect this.
Direct Connect: Just These Guys, Ya Know? 
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