Pierluigi Paganini March 26, 2018

The Internet Engineering Task Force (IETF) has finally announced the approval of TLS 1.3, the new version of the Transport Layer Security traffic encryption protocol.

It was a long journey, the IETF has been analyzing proposals for TLS 1.3 since April 2014, the final release is the result of the work on 28 drafts.

The TLS protocol was designed to allow client/server applications to communicate over the Internet in a secure way preventing message forgery, eavesdropping, and tampering.

TLS 1.2 and TLS 1.3 are quite different, the new version introduces many major features to improve performance and to make the protocol more resilient to certain attacks such as the ROBOT technique.

Below the description of one of the most important changes introduced with TLS 1.3:

• The list of supported symmetric algorithms has been pruned of all algorithms that are considered legacy. Those that remain all use Authenticated Encryption with Associated Data (AEAD) algorithms. The ciphersuite concept has been changed to separate the authentication and key exchange mechanisms from the record protection algorithm (including secret key length) and a hash to be used with the key derivation function and HMAC.

• A 0-RTT mode was added, saving a round-trip at connection setup for some application data, at the cost of certain security properties.

• Static RSA and Diffie-Hellman cipher suites have been removed; all public-key based key exchange mechanisms now provide forward secrecy.

• All handshake messages after the ServerHello are now encrypted. The newly introduced EncryptedExtension message allows various extensions previously sent in clear in the ServerHello to also enjoy confidentiality protection from active attackers.

• The key derivation functions have been re-designed. The new design allows easier analysis by cryptographers due to their improved key separation properties. The HMAC-based Extract-and-Expand Key Derivation Function (HKDF) is used as an underlying primitive.

• The handshake state machine has been significantly restructured to be more consistent and to remove superfluous messages such as ChangeCipherSpec (except when needed for middlebox compatibility).

• Elliptic curve algorithms are now in the base spec and new signature algorithms, such as ed25519 and ed448, are included. TLS 1.3 removed point format negotiation in favor of a single point format for each curve.

• Other cryptographic improvements including the removal of compression and custom DHE groups, changing the RSA padding to use RSASSA-PSS, and the removal of DSA.

• The TLS 1.2 version negotiation mechanism has been deprecated in favor of a version list in an extension. This increases compatibility with existing servers that incorrectly implemented version negotiation.

• Session resumption with and without server-side state as well as the PSK-based ciphersuites of earlier TLS versions have been replaced by a single new PSK exchange.

TLS 1.3 deprecates old cryptographic algorithms entirely, this is the best way to prevent the exploiting of vulnerabilities that affect the protocol and that can be mitigated only when users implement a correct configuration.

In the last few years, researchers discovered several critical issues in the protocol that have been exploited in attacks.

In February, the OpenSSL Project announced support for TLS 1.3 when it unveiled OpenSSL 1.1.1, which is currently in alpha.

One of the most debated problems when dealing with TLS is the role of so-called middleboxes, many companies need to inspect the traffic for security purposes and TLS 1.3 makes it very hard.

“The reductive answer to why TLS 1.3 hasn’t been deployed yet is middleboxes: network appliances designed to monitor and sometimes intercept HTTPS traffic inside corporate environments and mobile networks. Some of these middleboxes implemented TLS 1.2 incorrectly and now that’s blocking browsers from releasing TLS 1.3. However, simply blaming network appliance vendors would be disingenuous.” reads a blog post published by Cloudflare in December that explained the difficulties of mass deploying for the TLS 1.3.

According to the tests conducted by the IETF working group in December 2017, there was around a 3.25 percent failure rate of TLS 1.3 client connections.

Pierluigi Paganini

(Security Affairs – TLS 1.3, hacking)

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