esm - encrypted session manager

esm [-lrs] [-e command]

esm manages a simple encrypted session protocol. When first invoked from an interactive shell, it provides a transparent pseudo-terminal session on the local machine. When invoked from within an existing ESM session, however, the two ESM processes can automatically encrypt all traffic passed between them. Typically, the second session is executed on a remote networked machine reached by using the initial session to invoke, e.g., the telnet(1) program.

Ordinarily, ESM is run first on the local machine in "local" mode ("esm -l"). The resulting session is used to establish a connection (over a possibly insecure network) to a remote machine on which esm is run in "server" mode ("esm -s"). Encrypted sessions are always initiated by the remote esm process. Remote mode can be invoked in two ways. esm -s will attempt to initiate an encrypted session immediately after starting. esm -r will start the remote session in cleartext mode; a user escape sequence ("control-^" followed by "s") switches to the encrypted session mode. "Control-^ ?" provides a list of other options.

Encrypted sessions use two-key (112 bit) triple DES in 8-bit cipher feedback mode. A 1024-bit Diffie-Hellman key exchange protocol is used to establish the session key; the implementation of this protocol is based on the RSA Laboratories RSAREF 2.0 library. All encrypted traffic is encoded using a simple ASCII hexadecimal representation; this reduces encrypted terminal bandwidth by a factor of just over two compared with cleartext mode.

-l

Local mode. This establishes the initial session and provides a transparent pseudo-terminal interface until an encrypted session is initiated on the "master" (shell) side by a remote server. This is also the default mode if no command line flag is specified.

-r

Remote mode. Provides a transparent pseudo terminal session on the remote machine, with the ability to initiate an encrypted session with the "slave" (terminal) side of the connection.

-s

Remote mode. Immediately attempts to initiate an encrypted session with the "slave" (terminal) side of the connection. This mode is guaranteed to never operate in cleartext mode.

-e command

Executes command on the master side of the session. By default ESM executes the value of the SHELL environment or, if that is not set, /bin/sh.

ESM is not a replacement for a complete link, network or transport layer security protocol. In particular, nothing is authenticated and traffic is only protected once the session has started (which may occur after sensitive information like login passwords have already been sent in the clear).

Because nothing is authenticated, the protocol is vulnerable to several variations of the so-called "man-in-the-middle" active attack. ESM should be regarded as secure only against passive attacks.

ESM is slow. It takes a long time to generate the high-entropy random numbers required for the key agreement parameters. The ASCII encoding of the encrypted traffic noticeably reduces effective bandwidth and increases response time.

The session keys may not be random on all platforms. The random number generator relies on variations in clock rate and other hard to predict factors. Test these assumptions carefully before trusting ESM on a new machine or operating system.

Matt Blaze, mab@research.att.com (for ESM and CFS problems, use cfs@research.att.com).