Hash is an important hash function in cryptography. It maps an input of arbitrary length (also called a message) to an output of fixed length (also called a hash value or digest), usually represented by a hexadecimal string. The output value of the hash function is a fixed-length binary sequence, usually 128 bits or 256 bits, which uniquely represents the input value.

The hash function is a one-way function, that is, the input value cannot be deduced from the hash value. The design of hash function requires that a unique hash value can be generated for any input value. At the same time, a different hash value should be generated for any slight change in the input value.

Application of hash

Hash function is widely used in cryptography, including digital signature, message authentication code, password storage, random number generation and so on. Their characteristic is that they need to encrypt, verify and compare data, and hash function is an important tool to realize these operations.

Digital signature is a technology used to verify data integrity and identity authentication. Digital signature is realized by using hash function to generate message digest and encrypting the digest with private key. Message authentication code is a technology used to verify the integrity and authenticity of messages. Message authentication code is also realized by using hash function to generate message digest and encrypting the digest with key.

Password storage is a technology to store user passwords in a database. In order to protect the user's password from being leaked, the password is usually encrypted by hash function and then stored in the database. When the user logs in, the system will hash the password entered by the user, and then compare it with the hash value in the database to verify the identity of the user.

Operating steps of hash

The operation steps of hash function usually include the following steps:

1. Initialize: Initialize the internal state of the hash function.

2. Input the message: input the message into the hash function in blocks.

3. Compression: each message block is compressed to update the internal state of the hash function.

4. Output hash value: when all message blocks are input, output the hash value of hash function.

Security of hash

The security of hash function refers to the ability of hash function to resist attackers. An attacker may try to crack the hash function through a collision attack (that is, two different input values with the same hash value are found) or a pre-image attack (that is, an input value matching a given hash value is found).

In order to ensure the security of hash function, the design of hash function needs to meet the following conditions:

1. Anti-collision: For any two different input values, their hash values should be different.

2. One-way: the input value cannot be derived from the hash value.

3. Randomness: For any slight change in the input value, different hash values should be generated.

4. Anti-image attack: The attacker cannot find the input value matching the given hash value.