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Digital signatures are implemented through public-key encryption and are used to verify the origin and contents of a message.
One advantage of public-key encryption is that the recipient of successfully decrypted message knows that it was sent by the owner of the private key. This is known as authentication. However, encrypting messages with a private key is a relatively slow process particularly if the message is a long one. Instead a system of digital signatures is used.
A digital signature is prepared by first passing the message through a one-way cryptographic function to calculate the message digest. This digest is much smaller than the original message and can be quickly encrypted with the private key to produce a signature which is then added to the original message.
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