Unit 3_Digital Signature Model Details.pdf

Digital Signatures in contexts of Asymmetric key cryptography, Digital Signature, Digital Signature Model

1. Cryptography and Cyber Security
[IT311]
Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon-423603
(An Autonomous Institute Affiliated to Savitribai Phule Pune University, Pune)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 Certified
Department of Information Technology
(NBA Accredited)
Mrs. Kanchan D. Patil
Assistant Professor

2. Unit 3: Message Digest & Key Management

•Hash Algorithms: SHA-1, MD5, Key Management: Introduction, Key
Management: Generations, Distribution, Updation, Digital Certificate,
Digital Signature, Kerberos 5.0.
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

3. Digital Signatures in contexts of Asymmetric
key cryptography

•If A is the sender of a message and B is the receiver.
•A encrypts the message with B's public key and sends the encrypted
message to B.
•Assume that the whole message is encrypted with the receiver's pubic key
•A's public key would be well, public, i.e. accessible to anybody.
•This means that anybody who is interested in knowing the contents of the
message sent by A to B can simply use A's public key to decrypt the
message, thus causing the failure of this encryption scheme
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

4. Digital Signatures in contexts of Asymmetric
key cryptography
•If A is the sender of a message and B is the receiver.
•A encrypts the message with A's private key and sends the encrypted
message to B.

Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

5. Digital Signatures in contexts of Asymmetric
key cryptography

•when A encrypts the message with her private key, her intention not to
hide the contents of the message (ie. not to achieve confidentiality), but it
is something else
•If the receiver (B) receives such a message encrypted with A's private key B
can use A's public key to decrypt it and therefore, access the plain text.
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

6. Digital Signatures in contexts of Asymmetric
key cryptography
•If the decryption is successful, it assures B that this message was indeed
sent by A.
•This is because
•if B can decrypt a message with A's public key, it means that the
message must have been initially encrypted with A's private key
•only A knows her private key Therefore, someone posing as A (say C)
could not have sent a message encrypted with A's private key to B.
•A must have sent it.
•Therefore, although this scheme does not achieve confidentiality, it
achieve authentication (identifying and proving A as the sender).
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

7. Digital Signatures in contexts of Asymmetric
key cryptography

•Even if someone (say C) manages to intercept and access the encrypted
message while it is in transit, then uses A's public key to decrypt the
message, changes the message, that would not achieve any purpose.
because C does not have A's private key.
•C cannot encrypt the changed message with A's private key again.
•Therefore, even if C now forwards this changed message to B, B will not be
fooled into believing that it came from A, as it was not encrypted with A's
private key.
•This forms the basis of Digital signature
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

8. Digital Signatures
•Digital Signature is a value calculated from the data along with a secret key that only
the signer is aware of.
•The receiver needs to be assured that the message belongs to the sender. This is crucial
in businesses as the chances of disputes over data exchange are high.
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

9. Digital Signatures
•Digital signatures are the public-key primitives of message authentication.
•In the physical world, it is common to use handwritten signatures on
handwritten or typed messages. They are used to bind signatory to the
message.

•Digital signature is a technique that binds a person/entity to the digital
data.
•This binding can be independently verified by receiver as well as any third
party.

Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

10. Digital Signature Model in detail
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

11. Digital Signature Model in detail
•Each person adopting this scheme has a public-private key pair.
•Generally, the key pairs used for encryption/decryption and
signing/verifying are different.
•The private key used for signing is referred to as the signature key and the
public key as the verification key.
•Signer feeds data to the hash function and generates hash of data.
•Hash value and signature key are then fed to the signature algorithm which
produces the digital signature on given hash.
•Signature is appended to the data and then both are sent to the verifier.
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

12. Digital Signature Model in detail
•Verifier feeds the digital signature and the verification key into the
verification algorithm.
•The verification algorithm gives some value as output.
•Verifier also runs same hash function on received data to generate hash
value.
•For verification, this hash value and output of verification algorithm are
compared.
•Based on the comparison result, verifier decides whether the digital
signature is valid.
•Since digital signature is created by ‘private’ key of signer and no one else
can have this key. the signer cannot repudiate signing the data in future.
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

13. Importance of Digital Signatures
•Message Authentication
•When the verifier validates the digital signature using public key of a
sender, he is assured that signature has been created only by sender
who possess the corresponding secret private key and no one else.

•Data Integrity
•In case an attacker has access to the data and modifies it, the digital
signature verification at receiver end fails.
•The hash of modified data and the output provided by the verification
algorithm will not match.
•Hence, receiver can safely deny the message assuming that data
integrity has been breached.
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

14. Importance of Digital Signatures
•Non-repudiation
•Since it is assumed that only the signer has the knowledge of the
signature key, he can only create unique signature on a given data.
•Thus the receiver can present data and the digital signature to a third
party as evidence if any dispute arises in the future.
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

15. References:
Cryptography & Cyber Security Mrs. Kanchan Patil Department of Information Technology

•Atul Kahate,”Cryptography and Network Security”, second edition, Tata
McGraw Hill
•William Stallings, “Cryptography and Network Security-Principles and
practice”
•https://www.tutorialspoint.com/cryptography/cryptography_digital_sig
natures.htm