IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Communications Remarks
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- Pass phrase
A pass phrase (also referred to as a "mantra") must be entered when
generating a key; this protects the file with the private keys from
unauthorised access. As with every password, this should also not be easy
to guess.
There are Trojan horses in circulation, for example, which selectively
target the file with the private keys (SECRING.PGP) and send it to an
external address by e-mail. If the chosen pass phrase is too simple, it will
not offer sufficient resistance to brute force attacks (automated password
guessing). The pass phrase should therefore consist of at least ten
characters and include special characters.
- User ID
The public PGP keys are associated with a user ID, which if possible
should be unique and also contain the e-mail address, e.g. userA@bsi.de.
- To generate keys, PGP requires starting values that are as random as
possible. The user is therefore asked to type in an arbitrary text. It is better
in this case to enter "real" text; for example, it would be possible to type in
this paragraph. Simply "typing away" on the keyboard usually produces
worse results.
Safekeeping of keys
The private keys are stored in the file named SECRING.PGP. Although access
to this file is protected by the pass phrase, it should not be stored on local
networks, not even on insufficiently secure standalone systems. Key rings
(collections of keys) should be stored on floppy disk, which the user must
keep in a safe place. Preference is to be given to using chip cards for storing
keys.
A backup copy of the SECRING.PGP file should also be created, and a note
made of the pass phrase. The backup copy and the pass phrase should be
stored securely, and best of all separately, to ensure that the private key will
not be lost as a result of a hard disk crash or because of an operator error.
Messages which have been encrypted with the public key can no longer be
decrypted if this happens.
Writing down the pass phrase and placing it in safekeeping in a secure
location should be seen as a critical process serving solely the purpose of
contingency planning. A locked drawer in a desk or a similar "secure" location
can under no circumstances be recommended as a storage location for the
secret key or for the pass phrase.
Distribution of keys
For a recipient to be able to check the digital signature of the sender of a file
or for the sender of a message to be able encrypt a message for a certain
recipient, it is necessary to have the public key of the communication partner
in each case. This can be obtained in various ways: for example as an
attachment to an e-mail or from a WWW server. However, the user must
satisfy himself that the key really belongs to the specified person. Certificates
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IT-Baseline Protection Manual: Oktober 2000