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Symmetric key algorithm and asymmetric key algorithm

Two common cryptographic algorithms in cryptography are symmetric cryptographic algorithm (single key cryptographic algorithm) and asymmetric cryptographic algorithm (public key cryptographic algorithm).

The so-called symmetric key algorithm means that if the encryption key and decryption key of an encryption algorithm are the same, or although different, the other can be easily deduced from either of them, that is, the key is shared by both parties.

Asymmetric key algorithm means that the encryption key and decryption key of an encryption algorithm are different, or the other key cannot be deduced from one key. One of the two keys, called the public key, is used for encryption, is public, and the other, called the private key, is used for decryption and is confidential. It is not feasible to calculate the private key from the public key.

The main differences between the two cryptographic algorithms are as follows:

1. The difference of keys used in encryption and decryption: from the above description of symmetric key algorithm and asymmetric key algorithm, we can see that the same key is used in symmetric key encryption and decryption, or it is easy to derive the decryption key from the encryption key; while the asymmetric key algorithm uses different keys, one of which is difficult to derive the other key.

2. The difference in algorithm: ① symmetric key algorithm uses packet encryption technology, that is, the plaintext to be processed is grouped according to a fixed length, and the grouping is iteratively encoded several times by using the key, and finally the ciphertext is obtained. Similarly, under the control of a fixed-length key, the plaintext is obtained by iterative decoding several times in the unit of a packet. The idea of asymmetric key algorithm is that it is easy to evaluate from one direction, but its reverse calculation is very difficult, or it is not feasible. When encrypting, the plaintext is encrypted and transformed by the public key to get the ciphertext. When decrypting the ciphertext, the private key is used to decrypt the ciphertext to get the plaintext. ② symmetric key algorithm has the characteristics of simple encryption, fast encryption and decryption speed, short key, long development history and so on. Asymmetric key algorithm has the characteristics of slow encryption and decryption speed, large key size and short development history.

3. The difference of key management security: because the algorithm of symmetric key is public, its confidentiality depends on the secrecy of the key. Because the keys used by both encryption and decryption are the same, it is difficult to distribute and replace the keys. Because the key has been distributed in advance, the asymmetric key algorithm does not need to transmit the key in the process of communication, so the security is greatly improved, and the problem of key management is solved.

4. Security: because its algorithm is public, the security of the symmetric key algorithm depends on the length of the packet and the length of the key. common methods include: exhaustive key search, dictionary *, look-up table *, differential cryptanalysis, linear cryptanalysis, of which the most effective is differential cryptanalysis, which recovers some key bits by analyzing the influence of plaintext on the difference of ciphertext pairs. The security of asymmetric key algorithm is based on the difficulty of one-way function, such as elliptic curve cryptography algorithm, which is considered by many cryptographic experts to be exponential. From the known algorithms, the security of 160bit elliptic curve cryptography algorithm is equivalent to that of 1024bit RSA algorithm.

Symmetric algorithms are: DES IEDS

2009-03-24

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