Security level

In cryptography , The security level is an encryption primitive （ Such as a ciphertext or a hash function ） Measurement of the safety intensity that can be achieved , Its unit is usually bit. An encryption scheme is n-bit Safe , It means that an attacker wants to break the scheme , Must perform at least operations . for example AES-128（ The key length is 128bit） The security level of is 128-bit, It's about equal to 3072-bit Of RSA.

The target security level is , The security level that must be achieved in the design of this scheme . If the attacker can break this scheme at a lower cost , It means that the target security level is not reached .

Different encryption schemes are right “ Break through ” The definition of is not necessarily the same , It depends on the security requirements of the problem solved by the solution . For example, an encryption scheme , Such as RAS, According to some ciphertext pairs , find RSA The key of . A hash function , It may be to find an original image whose hash value is a fixed value , That is, the original image of hash function is stable , It is also possible to find that the hash values of two plaintext are the same , That is, the collision problem .

Generally speaking , For symmetric encryption scheme , The security level is the length of the key , such as AES-128 The security level of is 128bit. Of course, for hash functions , The original image problem is the length of the key , The collision problem is half the length of the key . however Phelix（ One basis MAC Stream cipher of XOR operation ） have 256bit The key of , But it only provides 128bit Security level of .SHA-3 Of SHAKE variant , about 256bit Output , The security level of the original image and collision is 128bit.

For asymmetric encryption , It's a little different . Usually come a Book , Asymmetric encryption is based on a difficult mathematical problem , The problem goes in one direction （ Encryption direction ） It's simple. , And its reverse direction （ Decryption direction ） It's difficult , Need some help information （ secret key ） Can be solved simply . Breaking this encryption scheme can often be prescribed to solve a difficult problem , such as RSA It can be reduced to the problem of large integer factorization . therefore , The security level of asymmetric encryption is defined according to the time required by the best algorithm to solve this difficult problem . and , These algorithms are often faster than brute force cracking .

about “ Break through ” An encryption scheme , It refers to finding an algorithm that can break the scheme , The operation required is lower than the security level he claims . however , In practice, it may not be possible .

Now, , Less than It can be solved . and To It may be solved in the future .

Reference resources ：

Wikipedia :security level