Flexible and Scalable Public Key Security for SSH

A standard tool for secure remote access, the SSH protocol uses publickey cryptography to establish an encrypted and integrity-protected channel with a remote server. However, widely-deployed implementations of the protocol are vulnerable to man-in-the-middle attacks, where an adversary substitutes her public key for the server’s. This danger particularly threatens a traveling user Bob borrowing a client machine. Imposing a traditional X.509 PKI on all SSH servers and clients is neither flexible nor scalable nor (in the foreseeable future) practical. Requiring extensive work or an SSL server at Bob’s site is also not practical for many users. This paper presents our experiences designing and implementing an alternative scheme that solves the public-key security problem in SSH without requiring such an a priori universal trust structure or extensive sysadmin work—although it does require a modified SSH client. (The code is available for public download.)