Digital signature is to encrypt data with the holder's private key, because the private key only belongs to the holder and cannot be forged by others, so digital signature can ensure the integrity, authenticity and non-repudiation of data.

Symmetric keys are easy to be stolen or cracked, so the integrity, authenticity and non-repudiation of data cannot be guaranteed.

The biggest advantage of public-private key system is to solve the problem of the biggest symmetric key-key distribution.

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