IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Communications Remarks
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Key length
Various cryptographic algorithms with different 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, as well as hash functions such as MD5 or SHA-1 (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 established.
Some browsers from US vendors the integrated encryption procedures have
only extremely short key lengths (40-bit), on account of the US export
restrictions. These cannot withstand a brute-force attack for long, i.e. an attack
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, browser versions
which offer encryption procedures based on at least an 80-bit key length
should be used. International versions of the commonly used browsers which
support 128-bit key lengths are now available.
Alternatively, add-on products developed by German companies which
likewise permit the use of longer keys inside standard browsers should be
used. Public domain software such as SSLeay or OpenSSL can also be used
for this purpose.
Certificates
One difficult problem with data communication across open networks is how
to check the identity of communication partners, because one cannot rely on
the stated name actually being correct. With SSL, the identity of
communication partners is checked by means of certificates. Certificates
contain their public keys together with confirmation provided from another
authority 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.
Three different types of certificates may be distinguished 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,
and
- Certificates from software producers or from operators of Web pages
All browsers come supplied with 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.
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IT-Baseline Protection Manual: Oktober 2000
Minimum key length 80
bits