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1. Digital Signature
•Important application of public key
Cryptography
•Electronic form of Signature
•Serves the same purpose as of handwritten
signature but is much secure
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2. Why Digital Signature ?
– Authenticate identity of sender
– Ensure the original Content of Message
– Non-RepudiationVerifiability by
independent, public or third party
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3. Digital Signature

4. How digital Signature works?
User A
User B
Use A’s private key to sign the document
Transmit via the Internet
User B received
the document with
signature attachedVerify the signature
by A’s public key stored
at the directory

5. Digital Signature Generation and
Verification
Message Sender Message Receiver
Message Message
Hash function
Digest
Encryption
Signature
Hash function
Digest
Decryption
Expected Digest
Private
Key
Public
Key

6. 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.
– What if you receive a public key from someone you don’t
know?

7. CA model (Trust model)
Root Certificate
CA Certificate
Browser Cert.
CA Certificate
Server Cert.

8. Digital Certificates
• Digital Certificate is a data with digital
signature from one trusted Certification
Authority (CA).
• This data contains:
– Who owns this certificate
– Who signed this certificate
– The expired date
– User name & email address

9. Digital Certificate

10. Digital Signatures (1)
Required Conditions:
1.Receiver can verify claimed identity of sender.
2.Sender cannot later repudiate(deny) contents
of message.
3.Receiver cannot have concocted message
himself.

11. Digital Signatures (2)
• Asymmetric-key signatures
• Symmetry-key signatures
• Message digests
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