In public key encryption, a key is a key pair consisting of a public key and a private key. Digital signature is encrypted with private key and decrypted with public key. Since the public key cannot be derived from the private key, the public key will not damage the security of the private key, and the public key can be spread publicly without confidentiality, while the private key must be kept confidential.
Therefore, when someone encrypts information with his own private key and can decrypt it correctly with TA's public key, it can be determined that the message has been signed, because it is impossible for others' public key to decrypt the encrypted information correctly, and it is impossible for others to have their own private key to generate encrypted information.
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 main way of digital signature is that the sender of a message generates a hash value (or message digest) from the message body, and the sender encrypts this hash value with his own private key to form the sender's digital signature, and then this digital signature will be sent to the receiver of the message as an attachment. The receiver of the message first calculates the hash value (or message digest) from the received original message, and then decrypts the digital signature attached to the message with the sender's public key. If two digital signatures are used,