IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Communications Remarks
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S 5.64 Secure Shell
Initiation responsibility: Head of IT Section, IT Security Management
Implementation responsibility: Administrators, users
Without special extensions, the telnet and ftp protocols offer only rudimentary
mechanisms for authentication. As a rule, a simple request is issued for the
user ID and password, which then – in exactly the same way as the user data –
are sent as plain text. The confidentiality of the authentication data and user
data can therefore not be ensured. The related protocols rsh, rlogin and rcp,
which are often grouped under the term r-services, exhibit similar security
deficiencies.
Secure Shell (ssh) can be used as a substitute for the r-services. It makes use
of extensive functions designed to ensure secure authentication and to
maintain confidentiality and integrity. This is achieved with a hybrid
encryption technique, in other words a combination of asymmetric and
symmetric encryption. The Secure Shell resides on layer 7 (application layer)
of the ISO/OSI reference model; other protocols can also be transported via
ssh, however, such as the X11 protocol, which is used by the graphical user
interface X-Windows.
Currently Secure Shell is based on three protocols, one built upon the other.
An Internet draft has been drawn up for each one.
- The lowest protocol is the transport layer protocol. This protocol performs
the majority of the security functions of ssh, namely authentication at the
host level, encryption, and protection of data integrity. The cryptographic
algorithms are negotiable between the communication partners.
- The middle protocol is the user authentication protocol. This allows
authentication at the user level; in this case, too, the procedure can be
negotiated. If a method of simple transmission of user IDs and passwords
is used for the purpose of authentication, the confidentiality of the
information with respect to the communication route is ensured by the
underlying transport layer protocol. The recommended approach,
however, is to use a public key procedure for authentication.
- The connection protocol is built on the two preceding protocols and allows
several logical user information channels to be set up. The data on these
user information channels is transmitted via a common single Secure Shell
connection.
There are implementations of ssh clients and ssh servers for all commonly
used Unix operating systems. There are also ssh clients for 32-bit Windows,
OS/2 and Macintosh, among others, and as a Java applet.
Basically the use of Secure Shell is to be recommended if the functionalities
of the r-services are used via communication channels which are not
adequately protected against compromise and/or manipulation (for example
via the Internet). A few notes on the secure use of ssh are given below.
One risk that is particularly significant is that of attacks known as man-in-themiddle
attacks. These involve the attacker filtering all traffic between the
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IT-Baseline Protection Manual: Oktober 2000