Question

3. When signing and verifying a plaintext document electronically using digital signatures, describe the function of the message digest or hash function in this process. Describe also how the X.509 Digital Certificate and the chain of trust concept could be related to digital signatures.

Answer 1

Digital signatures are used for message authentication in digital world like handwritten sign in physical world.

A) Message digest function

Hash function is the key element in cryptographic procedures. It take some input and produce a small number shorter than the input. A message digest is also a hsh function. If we are giving same input, it always produce the same output. The key features of message digest is

1) message digest algorithm cannot be reversd

2) small change in input result variation of the output

How to use message digest

* publish your document with message digest

* if we want to check the integrity of our document, download it from Internet and recalculate the message digest.

*compare it with original message digest

*if both are. Matching you got the same document as original

B) x509 certificate

It is a digital certificate accepted by international X509 public key infrastructure. It possess information about the identity which a certificate is issues it. It include the following fields

Version

Serial number

Algorithm information

Issuer name

Validity period

Name of the identity the certificate is issued to

Public key

Extension

C) Chain of trust concept

The chain of trust concept is used to verify the receivers that sender and all intermediate are genuine. It prove that a particular certificate originate from a trusted source. It consist of

1)root certificate

2)intermediate certificate

3)server certificate