In modern cryptography more than twenty different encryption algorithms are known. If they are properly implemented and used with a key long enough, they can be quite strong. The most commonly used algorithms are hash functions (GOST R 34.11-94, SHA-1, SHA-2, MD4, MD5, MD6), asymmetric encryption system (RSA, Digital Signature ElGamal), symmetrical system (GOST 28147-89 , DES, AES, Twofish, Blowfish, Camellia, RC4, IDEA). In many countries, some of these encryption systems were approved as national standards.
Symmetric encryption
Symmetric cryptographic schemes use the same key sequence, to encrypt the plaintext, and to subsequently decrypt the message. Such algorithms have a long history of development, and differ from other ways of encryption by a relative simplicity of use and high speed. They can be divided into two broad categories - Stream and Block ciphers. When Stream encryption is used the conversion method depends not only on the key, but also on the location of the encrypted character in the text, and, therefore, on the entire text. In Block algorithms text is broken into fragments, the ciphertext of one such fragment or block does not depend on the rest of the text.
Stream algorithms can be successfully attacked using the methods of linear and differential analysis and linear algebra, the structure itself of the stream cipher key is also more vulnerable. But in the same time, these algorithms are much faster than those of block and regardless of the amount of the incoming information they can provide a real-time encryption, excluding the error propagation effect.While cryptanalysing the symmetric schemes different variants of attacks can be used : with known or chosen plaintext, a chosen ciphertext, with a chosen or a related-key, differential cryptanalysis, attacks using only ciphertext - in case of a block algorithm; correlated, statistical and analytical attacks, "time-memory" compromise, attacks with allegedly known range and number of shifts of the register – while cracking stream algorithms. In both cases brute force attacks can also be used.
While cryptanalysing the symmetric schemes different variants of attacks can be used : with known or chosen plaintext, a chosen ciphertext, with a chosen or a related-key, differential cryptanalysis, attacks using only ciphertext - in case of a block algorithm; correlated, statistical and analytical attacks, "time-memory" compromise, attacks with allegedly known range and number of shifts of the register – while cracking stream algorithms. In both cases brute force attacks can also be used.
Asymmetric encryption
Asymmetric encryption uses two keys. Recipient's public key can freely circulate on insecure channels; its transmission does not require any precautions. This key encrypts all messages, and to convert the ciphertext to the original a different private key is used, which is kept secret by the recipient. Both of cryptographic keys - public and private - are formed on the basis of a single master key. As a result, both of them are dependent on this source key, but none of them can be calculated on the basis of another. To maintain the secrecy after generating the public and private keys the master key is deleted.
The Cryptographic strength of this method is based on two mathematical problems - factoring large numbers and computing discrete logarithms in finite fields. In asymmetric encryption, are used so-called unilateral (non-inverting) functions with a secret value; the value of the function is relatively easy to obtain, while the reverse mathematical manipulation without knowing the secret, is extremely difficult.
When using an encryption with a public-key the users have to choose between security and performance. The longer the crypto key, the longer it will take to brute-force attacks to succeed, and in this case, the practical encryption will be extremely slow. For this reason, this algorithm is used primarily used to control digital signatures, which are encrypted with a small block of data.
The most common types of attacks when working with asymmetric encryptions are the interceptions of the communications during the transmission and their substitution, attempts to calculate the private key with known open key, attacks on probable messages.