MD5, the message digest algorithm MD5, is a widely used hash function in the field of computer security, which is used to protect the integrity of messages.

MD5 algorithm has the following characteristics:

1. Compressibility: For data with any length, the length of the calculated MD5 value is fixed.

2. Easy calculation: It is easy to calculate the MD5 value from the original data.

3. Anti-modification: any modification to the original data, even if only 1 byte is modified, the MD5 values obtained are quite different.

4. Strong collision resistance: knowing the original data and its MD5 value, it is difficult to find a data with the same MD5 value (that is, forged data).

The function of MD5 is to "compress" a large amount of information into a secure format before signing the private key with digital signature software (that is, to convert a byte string with any length into a hexadecimal digit string with a certain length). Besides MD5, sha- 1, RIPEMD and Harvard are also famous.