IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Communications Remarks
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Key recovery
If the keys used for encryption are lost, this generally also means that the data
protected by the keys is also lost. In the commercial versions, 5.0 or higher,
PGP includes functions for retrieving data in such instances. These functions
are also referred to as key recovery functions. The functionality they offer can
prevent the loss of data by recovering stored, encrypted data in the event of a
key or access password being lost.
If it is intended to use the recovery function, either one or two additional keys
(ADKs, additional decryption keys) have to be generated. During key
generation, these additional keys are attached to the newly created keys, and
all data that is encrypted with the new keys additionally incorporates
encryption of the session key with the ADKs. In this way it is possible in an
emergency to decrypt the data using the ADKs, without using the original key.
PGP is therefore able to offer a message recovery function without the need
for central storage of retrieval information.
Use of key recovery can be enforced by making presettings to that effect on
the clients, ensuring that this functionality cannot be circumvented by
individual users. In that case, however, the security of the whole encryption
system is dependent on the confidentiality of the ADKs. If the ADKs are
revealed, they can be used to decrypt all of the data.
In order to prevent misuse of this highly sensitive function it is essential that
the ADKs should be protected by a particularly carefully chosen, safely kept
password. In addition, as of PGP Version 6.0, keys can also be divided into
several parts, which means that several people have to take action jointly in
order to use them. This form of the two-person control rule should always be
used when ADKs are used. To give further protection, provision can be made
for users to be warned every time that they encrypt data with a key to which
ADKs are attached.
Before PGP is used with key recovery, the advantages and disadvantages
should be weighed up against each other. On the one hand it protects against
the loss of data as a result of losing a key, but on the other it creates a central
weak point in the encryption system. This function should therefore only be
used if PGP is used for encrypting stored data. If it is used solely for securing
communications, in the event of the loss of a key is it also possible simply to
request that the e-mail be sent again. It should also be examined whether as an
alternative it would not be preferable to keep the password in a safe place in a
sealed envelope and to create backup copies of the private key files.
Additional controls:
- Are users trained in the use of PGP?
- Are the data and keys stored separately?
- Are backup copies made of the private keys? Are these kept at a secure
location?
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IT-Baseline Protection Manual: Oktober 2000