The so-called digital signature can be simply understood as:

1)A signed the contract with digital signature and sent it to B;

2) Then, A's digital signature includes the original contract (in the form of hash value)+signature or seal appearance +A's private key.

At this moment,

1) The original contract is unique in content, signature or seal.

2)A's private key is only known to himself, and no one else can know it.

To sum up, the digital signature is unforgeable (the specific signature scenario is slightly different from the technical implementation for ease of understanding).