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Yorkville High School Computer Science

ASSIGNMENTS: Build a Computer - September 25, 2018 :: Network App - December 14, 2018

Computer Security :: Lessons :: Digital Signatures

Properties of Digital Signatures

A digital signature uses public-key cryptography as a way to verify a message. The diagram below shows a generic model of the process of making and using digital signatures.

A Generic Model of Digital Signatures

Message authentication can protect messages between two parties from a third party, but they do not protect those two parties against each other. Here are some forms of dispute that can arise:

  1. After receiving a message, the receiver could forge a different message and claim it is from the sender. The receiver would just have to create a message and append the sender's authentication code.
  2. The sender can deny sending the message since it is possible for the receiver to forge a message.

Because of these concerns a digital signature must have the following general properties:

The following attacks on digital signatures are listed in order of increasing severity, and will lead to more specific properties for a digital signature:

Success at breaking a signature scheme occurs when the attacker does any of the following:

Digital Signature Requirements

Based on the properties and attacks already discussed, the following requirements are necessary for a valid digital signature:

A direct digital signature is a digital signature scheme that involves only the communicating parties so it is assumed the destination knows the public key of the source. Confidentiality can be provided by encrypting the entire messages and signature with a shared secret key using symmetric encryption.

The Digital Signature Algorithm

The Digital Signature Algorithm

The Digital Signature Algorithm, or DSA, is based on the difficulty of computing discrete logarithms. The DSA was published in 1991 by the National Institute of Standards and Technology (NIST) and uses the Secure Hashing Algorithm (SHA). WHile it is a public-key technique, DSA cannot be used for encryption or key exchange. It is based on the ElGamal Digital Signature Scheme as well as the Schnorr Digital Signature Scheme.

RSA Algorithms

RSA Algorithms are widely used in the financial sector and are the most widely-used digital signature algorithm. The following video broadly explains how RSA works and the leadup to its development:

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