The AES (Advanced Encryption Standard) specifies a FIPS-approved cryptographic algorithm that can be used to protect electronic data. The AES algorithm is a symmetric block cipher that can encrypt and decrypt information. The NIST published a formal request for this DES successor; in October 2000 Rijndael was chosen. Requirements included, among others, that it should be a symmetric block cipher with a block length of 128 and key lengths of 128, 192 and 256 bit. While approx. 15 candidates were presented in the beginning, in the last round 5 algorithms were still in the game. The finalists were MARS, RC6, Serpent, Twofish and Rijndael.


An algorithm is a procedure (a finite set of well-defined instructions) for accomplishing some task which, given an initial state, will terminate in a defined end-state. Its purpose is to hide the meaning of a message rather than its existence.

Alice and Bob

Probably the two most popular cryptography users. While end points are often marked as A and B in technical descriptions, it has become common in cryptography to use these names. Further frequently encountered persons are: Eve ("E", eavesdropper), the listener; and Mallory ("M", malicious) or Oscar ("O", opponent) the active attackers. Charly ("C") is used often as "enemy" synonym of a third party listening to a secret dialogue, a reminiscence to US soldiers fighting in Vietnam, in time with the mathematical proof of asymmetric cryptography at the Universities of Stanford and Harvard.


The ANSSI (Agence nationale de la sécurité des systèmes d'information, eng. National Agency for the Security of Information Technology) was founded in 2001. ANSSI could be compared to » BSI, promotes IT security in different areas and provides pertinent support. ANSSI issued product certifications are comparably recognized at BSI.


An application programming interface (API) is the interface that a computer system, library or application provides in order to allow requests for services to be made of it by other computer programs, and/or to allow data to be exchanged between them.


Abbreviation for Abstract Syntax Notation One. ASN.1 is a widely used standard for the decryption of abstract objects. In encoding (rules describing how such objects are to be produced as a string) it is distinguished between Basic Encoding Rules (BER) and Distinguished Encoding Rules (DER).

Asymmetric Cipher

Encryption algorithms employing two different keys (in contrast to symmetric cipher): One publicly known key - the public key - for data encryption and one key only known to the message receiver - the private key - for decryption.

Asymmetric Encryption

See Asymmetric Cipher.


By authentication an entity, e.g. a user, proves his identity. Authentication types include: authentication by knowledge (password), possession (cryptographic token), or biometric characteristics (fingerprint, etc.). The most elegant method is based on the use of so called digital signatures.


Authorization (in computer systems) is granting access to a network resource. It is the step that follows after successful authentication.

Avalanche Effect

The avalanche effect describes the fact that in case of a good cipher, changes in plaintext have a preferably quick effect on the cipher text (within the enciphering function). This is a strong diffusion of a block cipher and a counter measure against differential cryptanalysis. The strict avalanche criterion means, that if one input bit is changed, exactly half of the output bits are changed.


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