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Table Of Contents  CertiGuide to Security+
 9  Chapter 4:  Basics of Cryptography (Domain 4.0; 15%)
      9  4.2  Concepts in Using Cryptography
           9  4.2.2  Integrity

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 4.2.3  Authentication
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4.2.2.1  Digital Signatures
(Page 2 of 3)

Using Digital Signatures



To create a digital signature, you would:

  • Compute the hash value of the message, generally using a standardized algorithm like MD5 or SHA-1.

  • Encrypt the hash value of the message, using the sender’s private key, with an algorithm like RSA.

  • Associate or attach the encrypted hash value to the message in some way.

To verify that a message has not been tampered with and was indeed sent by the specified sender, the receiver would:

  • Decrypt the encrypted hash value, using the sender’s public key and the same asymmetric algorithm used by the sender.

  • Compute the hash value of the message using the same algorithm used by its sender.

  • Compare the hash value of the received message, with the hash value associated with the message by the sender.

A Digital Signature Is Created By...

Typically, a digital signature is created by computing a hash value (using an algorithm like MD5) for a document. Then, the hash value is encrypted using sender’s private key, using an asymmetric algorithm like RSA. This signature value is sent along with the message, to the intended recipients.

To verify that the message has not been tampered with, and was sent by the specified sender, the receiver would decrypt the signature value using the sender’s public key, compute the hash value for the received message (using the same algorithm the sender used), and verify that both the received hash value and the computed hash value are identical.


Figure 42: A digitally signed message receives a Message Digest to prove it has not been changed. It may be encrypted as in this example.

 

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