What is digital signature?

What is digital signature?

In its simplest form it is generating a one way hash of some information e.g. an email and encrypting this hash with a private key. This can be decrypted by anyone with the corresponding public key and the hash recalculated on the information. That the hash can be decrypted proves that the only the person with the corresponding private key signed the information. If the recalculated hash matches it proves the information has not been modified.
 
Therefore digitally signing something (e.g. a document, email) provides:

  • authentication (you know positively who the sender is),
  • non repudiation (the sender cannot say that they did not send the email), and
  • integrity (information has not been modified in transit by a third party).

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