Hash function h (m) also lists a list of aquary, it is the core of modern cryptography. The hash function has been used in computer science, and the hash function is a function that converts the variable input length string into a fixed length output value (called hash value). The one-way hash function is a hash function in one direction, which is easy to computers from the previewed value, but it is difficult to make its haveh value equal to a special value. A good hash function is also unable to conflict: it is difficult to generate two pre-mapped values, so that their hash value is the same.
The hash function is disclosed, which is not confidential, and the security of the one-way hash function is its unidirection, and its output does not depend on input. On average, a single bit of the premoded value will cause a change in half of the hash value. A hash value is known, to find a pre-mapped value, making it equal to the known hash value is not feasible, and can regard the one-way hashing function as a method constituting a fingerprint file. . If you verify that someone holds a specific file (you also hold the file), but you don't want him to pass the file to you, then you will notify him to pass the hash value of the file to you if he The transmitted hash value is correct, then he can affirm that he holds the document. The hash function can be used for digital signature, the integrity detection of messages, and authentication detection of message origin. Common hash algorithms include MD5, SHA, SNEFRU, and HVAL.
HASH is a message M acting on an arbitrary length, returns a fixed length has a hash value H: h = h (m). The length of H is m. The Hash function is mainly used in the process of packaging or digital signatures, it must have the following properties:
1. Given H, calculate M in calculation according to H (m) = H;
2. Given M, find another message M '. And satisfying H (m) = h (m ') is not active.
Any weaknesses in the above characteristics may destroy the security of various protocols that use the Hash function to package or sign, such as birthday attacks. The important part of the Hash function is to give M unique "fingerprint". If the user A is signed with a digital signature algorithm h (m), and B can generate another message M 'that satisfies H (m) = h (m'), then B can claim that the M is signed.
The Hash function requires the nature of the above nature:
3. Given M, it is easy to calculate H;
4. Anti-collision. That is, two messages M and M 'are found, so that H (m) = h (m') is not active.
64-bit Hash functions appear too small in front of birthday attack. Most haveh functions produce a 128-bit hash value. This forces an attacker attempting to make birthday attacks to have a hash of 264 random files to find two files having the same have the same value, so it is not sufficient to maintain the security of the hash function. NIST uses a 160-bit hash value in its secure hash standard (SHS). This makes birthday attacks more difficult to perform 280 random hash operations.
It is not difficult to analyze that the longer the hash value is, the better the security. Many actual one-way hash functions generate 128-bit hash values, such as the MD5 algorithm we will use, which makes any hackers who want to attack a one-time function, so we may wish to consider how to generate a long scattered Column value. The following is a method for generating a long column value:
(1) Use a one-way hash function to generate a hash value of a message.
(2) After the hash value is attached to the message.
(3) Generate a hash value of a series of values including hash values and messages.
(4) A larger hash value is generated in combining the hash value generated by the first step to generate a hash value generated by the third step.
(5) Repeat (1) to (3) steps. , Reading the full text, the technology area
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