IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Communications Remarks
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Only version 3 or higher of SSL should be used, because the additional server
authentication these versions provide means that man-in-the-middle attacks
are no longer possible, as was the case with SSLv2.
Key length
Various cryptographic algorithms with various key lengths can be used with
SSL, for example RC2 or RC4 with a 40-bit or 128-bit key length, DES with a
56-bit key length, triple DES with a 112-bit key length, or IDEA with a 128bit
key length, and MD5 or SHA-1 for example as hash functions (see also S
3.23 Introduction to basic cryptographic terms in this connection). The client
and server must agree on the procedures to be used in the session at the time
when the connection is set up.
In browsers from US vendors the integrated encryption procedures have only
extremely short key lengths (40-bit), on account of the US export restrictions.
These do not stand up to brute-force attacks for long, i.e. attacks involving
simply trying out all possible keys. If the protection requirements for the
transmitted data are low, this short key length may be adequate, and it at least
protects against opportunist offenders. Otherwise, to overcome this
shortcoming use should be made of add-on products from local vendors,
which also allow the use of longer keys even within standard browsers. Public
domain software such as SSLeay or OpenSSL can also be used for this
purpose.
Certificates
One difficult problem with data communications across open networks is how
to check the identity of the communication partners, because it cannot be
relied upon that specified names will be correct. With SSL, the identity of the
communication partners is checked by means of certificates. Certificates
contain their public keys together with verification from another entity of the
correct assignment of the public key to its "owner", in this case therefore a
server or client. The value of a certificate is therefore dependent not least on
the trustworthiness of this verification entity (also known as a trust centre or
certification body). The genuineness of the certificate can, in turn, be checked
using the public key of the verification entity.
A distinction is drawn between three different types of certificates with SSL:
- User certificates, which are required for client authentication
- Certificates from certification bodies, although some certification bodies
have several certificates, depending on their underlying security policy
- Certificates from software producers or from operators of Web pages
All browsers already contain SSL certificates from certain certification bodies
when they are installed. These certification bodies have very different security
guidelines and conditions under which they grant certificates. Initially,
therefore, all certificates should be deactivated, and only reactivated when you
are convinced that their security policy satisfies your own security needs.
When a new certificate is adopted, care should be taken that it is not activated
until after its fingerprint has been checked. The fingerprint is a hexadecimal
number that is transmitted together with the certificate. It should also be
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IT-Baseline Protection Manual: Oktober 2000