The release of OpenSSH 9.5 has been published, providing an open implementation of the client and server for working with SSH 2.0 and SFTP protocols.
Key Changes:
- In ssh-keygen, key generation based on the Ed25519 digital signature, developed by Daniel Bernstein and standardized in RFC 8709, is enabled by default. Ed25519 keys have been supported since the release of OpenSSH 6.5 (2014) and are more convenient due to their small size. Moreover, Ed25519 digital signatures offer a higher level of security than ECDSA and DSA, demonstrating very high signature verification and creation speeds. The resistance to brute force attacks for Ed25519 is approximately 2^128 (on average, an attack on Ed25519 would require 2^140 bit operations), comparable to the strength of algorithms like NIST P-256 and RSA with a key size of 375 bytes or the 128-bit block cipher. Additionally, Ed25519 is immune to collision problems in hashes, is not sensitive to cache-timing attacks, and side-channel attacks.
- The ssh utility has added protection against side-channel attacks that analyze delays between keystrokes to reconstruct input. Such attacks are based on the premise that the delays between keystrokes depend on the keyboard layout (for example, the reaction time when typing the letter 'F' is faster than when typing 'Q' or 'X' because less finger movement is required). SSH was vulnerable to these attacks as it sent information about the entered character in a separate packet immediately after each key press, and thus the delays between packet transmissions correlated with the delays between keystrokes.
To obscure the characteristics of interactive input in the traffic, ssh implements the transmission of data not as it is typed, but only at fixed intervals (by default, 20 ms). Additionally, to confuse attackers, fake keystrokes are sent at random moments after transmitting real data. To configure this protection, the parameter 'ObscureKeystrokeTiming' has been added to ssh_config.
- In ssh and sshd, support has been implemented for the SSH protocol extension 'ping@openssh.com', which adds a new type of message SSH2_MSG_PING and SSH2_MSG_PONG for periodically sending packets at regular intervals. This extension is necessary for the aforementioned protection against side-channel attacks.
- In sshd, it is allowed to override the Sybsystem directives through Match blocks.
- In sshd, the handling of quotes in the Subsystem directive has been changed, which are now preserved for commands and arguments, potentially leading to compatibility issues with very rare configurations.
Source: opennet.ru
