Key (cryptography)

When key information is referred to in cryptology in general, the parameterized a cryptographic algorithm. In the simplest case, in most symmetric methods of classical cryptography, such as the Vigenere cipher or Playfair method, it involves a password that is used to encrypt a plaintext and obtain a ciphertext. Conversely, this password will be required again as a key to get back to win from the ciphertext by decryption the plaintext. In modern, computer-based symmetric and asymmetric methods and the key is, however, a bit string.

Principle

In 1883, Auguste Kerckhoffs formulated his maxim, namely, that the security of a cryptographic method must not depend on the secrecy of the algorithm, but only on the secrecy of the key used ( Kerckhoffs ' principle). The key includes in a symmetric method ( others were not known at his time ) the information that must be kept secret, while the algorithm, so the encryption process itself, may be publicly known. In asymmetric encryption method, also referred to as "Public Key Cryptography ", takes on the role of the secret private key while the corresponding public key is widely known.

Key for symmetric methods

For symmetric process, ie for all classical methods of cryptography and also in modern algorithms such as the Data Encryption Standard (DES) or its successor, the Advanced Encryption Standard (AES), using both communication partners are the same ( secret) key both for encryption and also to decrypt. While classical methods in which the text must be encrypted and decrypted by hand, almost always use as key a password is the key in modern, ie computer-based, symmetric method usually consists of a sequence of bits.

The security of the process depends not only on the algorithm itself by the key length. When an attack is found against a method that is more efficient than the brute force method, trying all possible keys, the procedure is considered broken. The key length is at a safe procedure that is directly on the level of security.

Key in asymmetric methods

Asymmetric methods, such as the RSA cryptosystem, use key pairs consisting of a public key (english public key) and a private key ( private key german, German also " secret key " ) exist.

The public key is not secret, it should be known as many other users, for example by distributing over key server. With it public operations can be performed, ie encrypt messages and digital signatures are verified. It is important that a public key uniquely associated with a user. This is not the case, a message with another user's key is encrypted about who can read the message then, even though it was not intended for him. In order to name key easy to use a fingerprint, a short hash value that uniquely identifies a key.

To decrypt a ciphertext again or to sign a message, the private key is required. In contrast to symmetric methods in which multiple users share a secret key features in asymmetric method, only a user of the private ( secret) key. This circumstance allows only unique to a user with a signature. Therefore, it is essential that the private key can not be derived from the public.

Passwords

Especially advantageous when choosing a password such as " Maya the Bee " as the key is that it is easy to remember. This simple way of a key is often referred to as password (English: "password ") or in military parlance as a slogan ( from the Italian " la parola ", German: "the word" ) referred. For modern demands on safety, the choice of such a simple key is not sufficient because it is too short, so through trial and error ( brute force method ) can be found, and also consists of words and therefore relatively easy to guess can be respectively found by a dictionary attack. In contrast, to be resistant, a " strong" password should preferably be long and consist of an unusual string, for example, contain numerals and special characters, so that it can not be easily guessed. However, it can then usually also not easy to remember. When it is therefore listed or stored in any way, there is a danger that it can be spied possibly, bringing the supposedly secret communications would be compromised.

• Cryptology