Posts Tagged ‘cipher text

Direct Cryptanalytic Attacks

During a cryptanalytic attack the adversary observes the output and tries to gain any information about the inner state or future output of the generator. Many RNGs (Random Number Generators) use cryptographic primitives like hash functions (e.g. SHA-1 or MD5) or block ciphers (DES,Triple-DES, AES) to prevent this kind of attacks. The underlying assumption is that the cryptographic security of the primitives transfers to the generators which employ them.Generally, the con dence into the security of this primitives is based only partially on mathematical analysis but mainly on empirical results and statistical tests. Since most of the applications that apply cryptographic RNGs rely on those primitives, we may have con dence in their security as well.

Nevertheless, it is not advisable to blindly trust generators that are built on cryptographic primitives as we will see by the example of the Kerberos 4 session key generator. The speci c method of employing the primitives has a main impact on the security of the generator as well. The Kerberos 4 generator produces a 56-bit key fora DES block cipher by two successive calls of the UNIX random function which uses onlya 32 bit key. The random function is seeded every time a key is requested. Consequently,the strength of the encryption and, thus, the resistance against cryptanalytic attacks is reduced from 56 to 32 bits. It still takes about 6 hours on a DEC Alpha to gain the proper key of a plain text-cipher text pair by brute force, but we see that the 56 bit strength of the encryption is only an illusion. It is the weakest link in the chain that counts.

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XML Encryption and Access Control

Subtree encryption (element wise)

The two published proposals by [Imamura] and [Simon, LaMacchia] have in common that they take a complete sub tree (descendant-or-self(), maybe with of without attributes of self()), serialize this subtree into a text representation, encrypt it using some encryption mechanism like a symmetric cipher and replace the unencrypted part of the document with the resulting cipher text.

The subtree encryption is an end-to-end-security approach, in which the document includes all sensitive information in encrypted (secured) form. It allows to include multiple encrypted subtrees, and depending on the choosen model and granularity, it is possible to select even single attributes for encryption. In the following illustration, the “Public Nodes” do not need to be confidential (encrypted),but the one at the bottom is encrypted in the subtree.

To encrypt a subtree, the nodes that should be secured are selected:

 

Server-side Access Control

The server-side access control scenarios with flexible in their content model:

Server-side AC can completely restructure and rebuild the tree, based on the access control lists. It is not forced to make a complete subtree opaque, but it can let some elements childs visible (unencrypted) to the client without enforcing the root of the subtree (self()) being visible.

 

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