1. Public Integrity Auditing -
Digital Signatures
by G Jaya Rao
Roll No:716178,Research Scholar ,NITW&IDRBT,Hyd
2. Public Integrity Auditing -Digital
Signatures
Digital Signatures
• MAC
Security services provided by a digital signature
Digital signature scheme-RSA
Attacks on digital signatures
Some applications of digital signatures
References
3. Message Integrity-Message
Authentication Code (MAC)
MAC algorithm is a symmetric key cryptographic
technique to provide message authentication.
For establishing MAC process, the sender and
receiver share a symmetric key K.
Essentially, a MAC is an encrypted checksum
generated on the underlying message that is sent
along with a message to ensure message
authentication.
7. Limitations of MAC
Establishment of Shared Secret. This requires
establishment of shared secret among pre-decided
legitimate users prior to use of MAC.
Inability to Provide Non-Repudiation: Non-repudiation
is the assurance that a message originator cannot
deny any previously sent messages and
commitments or actions. MAC technique does not
provide a non-repudiation service.
If the sender and receiver get involved in a dispute
over message origination, MACs cannot provide a
proof that a message was indeed sent by the sender.
Both these limitations can be overcome by using the
public key based digital signatures discussed in
following section.
8. DIGITAL SIGNATURES
A person signs a document to show that it
originated from him or was approved by him.
The signature is proof to the recipient that the
document comes from the correct entity and
nobody else.
A sign of authentication: A verified signature on a
document.
A message can be signed electronically.
The electronic signature can prove the authenticity
of the sender of the message --->digital signature.
11. Security Services
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.
12. 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.
13. 13.13
Key Generation
Key generation in the RSA digital signature scheme is
exactly the same as key generation in the RSA
RSA digital signature scheme
In the RSA digital signature scheme, d is private;
e and n are public.
Note
14. Key-Generation for RSA
1. Generate two large random distinct primes
p and q, each roughly the same size
2. Compute n = pq and
3. Select random integer e:
4. Compute unique integer d:
5. Public key is (n, e); Private key is d
( ) ( 1)( 1)
n p q
1 , such that gcd( , ) 1
e e
1 , such that 1mod
d ed
15. 13.15
RSA Signature on the Message Digest
Continued
The RSA signature on the message digest
22. Limitations
The private key must be kept in a secured
manner.
The process of generation and verification of
digital signature requires considerable amount of
time.
For using the digital signature the user has to
obtain private and public key, the receiver has to
obtain the digital signature certificate also.
