The security services provided by digital signature system for communication parties are: preventing the signature from being tampered with and preventing the signature file from being tampered with. Details are as follows:

First of all, digital signature is not a digital image signed in writing, but an electronic signature of electronic documents through cryptographic technology. In fact, people can deny that they signed the document, and the accuracy of handwriting identification is not 100%, but it is difficult to deny a digital signature. Because the generation of digital signature needs to use the private key and the corresponding public key to verify the signature, some existing schemes, such as digital certificate, bind the identity of the entity (legal subject) with the pair of private key and public key, which makes it difficult for the subject to deny the digital signature.

Secondly, digital signature is essentially a security measure. The receiver can prove the authenticity of the received message and the sender to a third party, and its use can ensure that the sender cannot deny and forge information.

The following are the main ways of digital signature: the sender of a message generates a hash value (or message digest) from the message text. The sender encrypts this hash value with his own private key to form the sender's digital signature. Then, the digital signature will be sent to the recipient of the email as an attachment. The receiver of the message first calculates the hash value (or message digest) from the received original message, and then decrypts and verifies the digital signature attached to the message with the sender's public key.

If the two hash values (also called hash values) are the same, the receiver can confirm that the digital signature belongs to the sender. The length of hash value is fixed and the operation is irreversible. The hash values of different plaintext are different, but the hash values of the same plaintext are the same and unique. Any change in the original text will change its hash value. Through this principle, we can identify whether the file has been tampered with.

In fact, the tampered digital signature data message is easy to find, and even the document can be identified in appearance without authentication, unless the defendant can submit a digital signature data message with different contents and no tampering is found.