Digital Signature

INTRODUCTION

Why Signatures?Authenticates who created a documentAdds formality and finalityIn many cases, required by law or ruleDigital SignaturesNot simply a typed name or image of a

handwritten signatureBased on public-key encryptionAssociated with a digital document

Digital SignatureDigital signature can be used in all electronic

communicationsWeb, e-mail, e-commerce

It is an electronic stamp or seal that append to the document.

Ensure the document being unchanged during transmission.

Digital Signature

Digital Signature:- A special signature for signing electronic correspondence, produced by encrypting the message digest with the sender’s private key.

Message Digest:- A block of data or a sample of the message content that represents a private key.

Digital SignatureA digital signature’s main function is to verify that

a message of a document, in fact, comes from the claimed sender. This is called authentication.

When making a digital signature, cryptographic hash function are generally used to construct the message digest.

A hash function is a formula that converts a message of a given length into a string of digits (128 or more), called a message digest. Once the message digest is encrypted with the sender’s private key, it becomes a digital signature.

How digital Signature works?User A

User B

Use A’s private key to sign the document

Transmit via the Internet

User B receivedthe document withsignature attachedVerify the signature

by A’s public key storedat the directory

Digital Signature Generation and Verification

Message Sender Message Receiver

Message Message

Hash function

Digest

Encryption

Signature

Hash function

Digest

Decryption

Expected Digest

PrivateKey

PublicKey

8

Digital Signature

Reference

Key ManagementPrivate key are password-protected.

If someone want your private key:They need the file contains the keyThey need the passphrase for that key

If you have never written down your passphrase or told anyoneVery hard to crack

Digital CertificatesDigital Certificate is a data with digital

signature from one trusted Certification Authority (CA).

This data contains:Who owns this certificateWho signed this certificateThe expired dateUser name & email address

Certification Authority (CA)A trusted agent who certifies public keys

for general use (Corporation or Bank).User has to decide which CAs can be trusted.

The model for key certification based on friends and friends of friends is called “Web of Trust”.The public key is passing from friend to friend.Works well in small or high connected worlds.

Public Key Infrastructure (PKI)PKI is a system that uses public-key

encryption and digital certificates to achieve secure Internet services.

There are 4 major parts in PKI.Certification Authority (CA)A directory ServiceServices, Banks, Web serversBusiness Users

PKI StructureCertification Authority Directory services

UserServices,Banks,Webservers

Public/Private Keys

4 key servicesAuthentication – Digital Certificate

To identify a user who claim who he/she is, in order to access the resource.

Non-repudiation – Digital Signature To make the user becomes unable to deny that he/she has sent

the message, signed the document or participated in a transaction.

Confidentiality - Encryption To make the transaction secure, no one else is able to

read/retrieve the ongoing transaction unless the communicating parties.

Integrity - Encryption To ensure the information has not been tampered during

transmission.

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