NCC Group's Cryptography Services Complete an Audit of Cloudflare's TLS1.3

NCC Group's Cryptography Services practice recently completed a two-week audit of Cloudflare's TLS 1.3 implementation. The audit took place between November 11, 2016 and December 9, 2016.

The TLS standard was last updated almost ten years ago and this version brings new features and a simplified handshake to the protocol. Many old cryptographic algorithms have been replaced with more modern ones, key exchanges have forward secrecy by default, the handshake phase will be faster, certificates will be able to enjoy security-proven signature schemes, MAC-then-Encrypt constructions are out—the weakest features of older TLS versions have been updated or removed.

Cryptography Services analyzed Cloudflare's TLS 1.3 implementation for protocol-level flaws and for deviations from the draft specification. The team found a small number of issues during the review—all of which were promptly fixed—and was pleased with the quality of the code.

Cloudflare built their implementation of TLS 1.3 on the Go programming language's standard TLS library, making use of the existing base to correctly and safely parse TLS packets. While building on top of older versions can be challenging, Cloudflare has added TLS 1.3 code in a safe and segregated way, with new defenses against downgrade attacks being added in the final implementation of the specification. This permits support for older versions of TLS while being free from unexpected conflicts or downgrades.

Using Go and its standard libraries enables Cloudflare to avoid common implementation issues stemming from vulnerable strcpy and memcpy operations, pointer arithmetic and manual memory management while providing a best-in-class crypto API.

Cloudflare implemented a conservative subset of the TLS 1.3 specification. State-of-the-art algorithms, such as Curve25519, are given priority over legacy algorithms. Session resumption is limited to the forward secure option. Cloudflare's implementation also considers efficiency, using AES-GCM if it detects accelerated hardware support and the faster-in-software Chacha20-Poly1305 in its absence.

There is still work to be done before TLS 1.3 enjoys large scale adoption. Cloudflare is paving the way with its reliable server implementation of TLS 1.3, and Firefox and Chrome's client implementations make end-to-end testing of the draft specification possible. NCC Group applauds the work of the IETF and these early implementers.


Published date:  14 February 2017

Written by:  Scott Stender

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