S 5.110 Protection of e-mail with SPHINX (S/MIME)

Initiation responsibility: Administrator, IT Security Officer

Implementation responsibility: User, Administrator

The increasing importance of e-mail requires the use of safeguards ensuring confidentiality and non-repudiation. This can be achieved through the wide use of products for encryption and digital signature of e-mails. The electronic signature ensures that the e-mail originates from the sender specified and is unchanged. The purpose of encrypting the information is to ensure that only the authorised recipient can read the e-mail.

To this end the BSI initiated the SPHINX project in whose context cryptographic products based on international standards were developed further. The interoperability, i.e. the exact interchangeability of cryptographically treated messages, of products by different manufacturers and for different platforms, is examined by a test laboratory on a quarterly basis and the result is published.

Cryptographic methods

In order to achieve manufacturer-independent interoperability, SPHINX only uses products based on the S/MIME and "MailTrusT" industrial standards. These standards use a combination of different cryptographic methods for generation of secure e-mails. The symmetric method used for encrypting the data is the triple-DES algorithm with a key length of 112 bits. The public key procedure used for the electronic signature and for encryption is the RSA algorithm with a key length of at least 1024 bits. SHA-1 is the recommended hash algorithm used for unique mapping of the message to a value with a defined length.

The allocation of cryptographic keys to persons is controlled by means of digital certificates. A certificate is an electronic document essentially containing the public key and the name of the key owner. By means of its electronic signature the certification authority (trust centre) certifies the allocation between key and person. SPHINX uses standardised certificates according to ITU recommendation X.509 version 3.

The trust between the communication partners essentially consists of the reliance on the digital certificates and the credibility of all information contained therein. A number of trust centres have already issued certificates for public administration. These trust centres are audited by the superior root certificate authority of the BSI and combined in the PKI (Public Key Infrastructure) of the public administration. All certificates issued are thus subject to the IT-Grundschutz standard in all questions relating to information security. For the contact with citizens and companies the administration PKI was integrated in the European Bridge-CA which connects independent PKI in a trustworthy manner.

Another requirement for development of trust is the protection of a user's secret key. For this purpose, the secret (or personal) key can be stored either in a special file or a chip card. In general, this file or chip card is referred to as Personal Security Environment (PSE). PSEs are cryptographically protected means and can only be activated for use by means of a password. The secure handling of the password and the file or chip card is the owner's responsibility.

Secure installation and use

In general, SPHINX products are so-called plug-in products. They complement the existing e-mail product by cryptographic methods recognised as secure.

Incorrect configuration or improper use, however, can result in a lower security level.

As for most more complex cryptographic products, the configuration of SPHINX products is not self-explanatory. In order to prevent administration errors from arising, familiarisation with the SPHINX product used is required. In companies and public agencies, an employee of the IT administration should be familiarised with the use of the SPHINX product and be available as a technical contact.

In order to ensure that the users understand the new functionality, it is necessary to teach them some basic cryptographic terms. Training should be provided in the procedures for applying for a certificate and the use of the SPHINX product. In companies and public agencies, selected users should be familiarised with the use of the SPHINX product and instruct the other users in the use of the product as multipliers. Training provided by the manufacturer or distributor should be preferred. In particular, the generation of signed and encrypted e-mails or the receipt of such e-mails should be practised before the user uses the program.

It is recommendable to use a uniform SPHINX product within an organisational unit, or even better, a uniform program version. This helps to keep the effort for administration, training, support and software maintenance to a minimum.

Every SPHINX product comes with a comprehensive documentation which should be read before use. Before distribution, it should be adapted to the particularities of the organisation. This helps to achieve a higher level of acceptance during product introduction.

Storage of keys

The private keys are stored in the Personal Security Environment (PSE). A deciding factor for the secure use is that the content of the PSE remains confidential and protected against manipulation. The password used must be created in accordance with the password rules described in S 2.11 Provisions governing the use of passwords and stored securely. Unintentional or deliberate disclosure of the password enables other persons to electronically sign on behalf of the owner.

If the PSE is a file it is referred to as a soft PSE. This file is cryptographically protected. It is recommended not to store it on network drives as otherwise further security safeguards must be taken. The use of chip cards for storing the key should be preferred. Nevertheless, the password used must also be stored securely when chip cards are used. It is not possible to create a copy of the chip cards used in SPHINX.

In case of the soft PSE, a backup copy should be created and the password should be noted down. The backup copy and the password should be stored securely, preferably in separate locations. This ensures that the PSE is not lost in case of a hard drive crash or improper use. If the PSE is lost, it will no longer be possible to decrypt encrypted messages.

Noting down and depositing the password in a protected location should be considered a critical process which exclusively serves for contingency planning. The locked drawer of a desk or similar "secure" locations can in no event be recommended as a storage location for the PSE or the password.

Key distribution

For a recipient to be able to verify the electronic signature of the sender of a file or the sender of the message to be able to encrypt a message for a certain recipient they require the digital certificate of their communication partner. This certificate can be obtained in different ways, for example, as an attachment to an e-mail or from a special internet server (directory), sometimes also from a WWW server.

SPHINX products assist the user in verifying the digital certificates. With most products, the user is required to manually allocate the certificate of their communication partner to an e-mail address when it is received for the first time. In addition to the certificate of the communication partner, the certificate of the issuing trust centre is required for automatic verification. In most cases, the required certificates are included in the signed e-mail as an attachment. The certificate of the root certificate authority should be available or pre-installed by the IT service.

In order for a user to obtain their own certificate they are required to submit a certificate application and to provide identification. Both will be dealt with in collaboration with the registration authority. In public agencies, companies, and organisations these can usually be found with the Internal Services or factory safety personnel. Trust centres usually have registration authorities in their branches. The registration authority checks the certificate applications for correctness and identifies the user based on his/her service card or identity card. If chip cards are issued these will normally also be available there. Soft PSE are sent electronically, usually by e-mail.

Review questions:

© Federal Office for Information Security. All rights reserved.