Hardening DC++ Cryptography: TLS, HTTPS, and KEYP

BEAST, CRIME, BREACH, and Lucky 13 together left DC++ with no secure TLS support. Since then, the triple handshake attack, Heartbleed, POODLE for both SSL 3 and TLS, FREAK, and Logjam have multiplied hazards.

Fortunately, in the intervening year and a half, in response:

  • poy introduces direct, encrypted private messages in DC++ 0.830.
  • DC++ 0.840 sees substantial, wide-ranging improvements in KEYP and HTTPS support from Crise, anticipating Google sunsetting SHA1 by several months and detecting man-in-the-middle attempts across both KEYP and HTTPS.
  • OpenSSL 1.0.1g, included in DC++ 0.842, fixes Heartbleed.
  • DC++ 0.850 avoids CRIME and BREACH by disabling TLS compression; avoids RC4 vulnerabilities by removing support for RC4; prevents BEAST by supporting TLS 1.1 and 1.2; mitigates Lucky 13 through preferring AES-GCM ciphersuites; removes support for increasingly factorable 512-bit and 1024-bit DH and RSA ephemeral TLS keys; and with all but one ciphersuite, AES128-SHA, deprecated and included for DC++ pre-0.850 compatibility, uses either DHE or ECDHE ciphersuites to provide perfect forward secrecy, mitigating any future Heartbleed-like vulnerabilities.
  • DC++ 0.851 uses a new OpenSSL 1.0.2 API to constrain allowed elliptic curves to those for which OpenSSL provides constant-time assembly code to avoid timing side-channel attacks.

These KEYP, TLS, and HTTPS improvements have not only fixed known weaknesses, but prevent DC++ 0.850 and 0.851 from ever having been vulnerable to either FREAK or Logjam. As with perfect forward secrecy, these changes increase DC++’s ongoing security against yet-unknown cryptographic developments.

The upcoming version switches URLs in documentation, in menu items, and of the GeoIP downloads from HTTP to HTTPS. While these changes do not and cannot prevent attacks perfectly, it should now provide users with improved and still-improving cryptographic security for the benefit of all DC++ users.

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