Digital Signature

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Digital Signature
By
•Neha Dabhi /
Parmar

Overview
 Difference between conventional signature and digital
signature
 Definition of digital signature
 Creation of digital signatures and schemes
 Services
 Attacks on digital signature
 Advantages and Disadvantages digital signatures

 COMPARISON
Let us begin by looking at the differences between
conventional signatures and digital signatures.

Topics discussed in this section:
1
2
3
4

Inclusion
Verification Method
Relationship
Duplicity

13.3

1 . Inclusion
A conventional signature is included in the document; it
is part of the document. But when we sign a document
digitally, we send the signature as a separate document.

2. Verification Method
For a conventional signature, when the recipient receives
a document, she compares the signature on the document
with the signature on file. For a digital signature, the
recipient receives the message and the signature. The
recipient needs to apply a verification technique to the
combination of the message and the signature to verify
13.4
the authenticity.

3. Relationship
For a conventional signature, there is normally a one-tomany relationship between a signature and documents.
For a digital signature, there is a one-to-one relationship
between a signature and a message.

13.5

4. Duplicity
In conventional signature, a copy of the signed document
can be distinguished from the original one on file. In
digital signature, there is no such distinction unless there
is a factor of time on the document.

13.6

Digital Signature

What is a digital signature?
A digital signature or digital signature scheme is a
mathematical scheme for demonstrating the
authenticity of a digital message or document.
A digital signature can be used with any kind of
message, whether it is encrypted or not, simply so that
the receiver can be sure of the sender's identity and
that the message arrived intact.

Creation Of Digital Signature

A digital signature scheme typically consists of three
algorithms:
• HASHING algorithm.
• Signature Generation Algorithm
• A signature verifying algorithm that, given a
message, public key and a signature, either accepts
or rejects the message's claim to authenticity.

1. Hashing
 Hashing is the transformation of a string
of characters into a usually shorter fixed-length value
or key that represents the original string.
 As a simple example of the using of hashing in
databases, a group of people could be arranged in a
database like this:
Abernathy Sara, Epperdingle Roscoe, Moore Wilfred,
Smith David (and many more sorted into alphabetical
order)
After Hashing, each of them will be replaced by a 4 digit
number (in this case)
7864=> Abernathy Sara 9802=> Epperdingle Roscoe
1990=> Moore Wilfred 8822=> Smith David (and
so forth).

1(Contd). Hashing Algorithm
The formula for hashing depends on two inputs:
 the sequence of characters representing the electronic data to
be signed
 a secret number referred to as a signature's private key
associated with the signing party and which only that party
has access to
Some simple Hash Functions
 The division-remainder method
 Folding
 Radix transformation
 Digit rearrangement

13.10

2. Prerequisite to create a digital signature
Public-private digital key pair
Certificate Authority.
The public key certificate creates proof of the identity of
the signer by using the services of a certificate
authority.
 A certificate authority uses a variety of processes to
associate the particular public key with an individual.
 The combination of public key and proof of identity
result in a public key certificate - also called a signer's
certificate.

3. Digital Signature Verification
It is the process of checking the digital signature
by the reference to the original message and a given
public key.
 Verifying also relies on a formula. Here, the
formula depends on three inputs:
 The sequence of characters representing
the supposedly originally signed electronic
data
 The public key of the signing party
 The value representing the supposedly
authentic digital signature.
The output of the formula is a simple answer: YES
or NO.


A cryptosystem uses the private and public keys of
the receiver: a digital signature uses
the private and public keys of the sender.

Figure : Digital Signature Process

Figure : Creation of the digital signature at sender and verifying at
receiver

Digital signature Schemes
The following are the widely used Schemes for
Digital Signatures
 RSA Scheme
 ElGamal Scheme
 Schnorr Scheme
 Digital Signature Standard (DSS)

 SERVICES
We discussed several security services in Chapter 1
including message confidentiality, message
authentication, message integrity, and nonrepudiation.
A digital signature can directly provide the last three;
for message confidentiality we still need
encryption/decryption.
Topics discussed in this section:
1
2
3
4

Message Authentication
Message Integrity
Nonrepudiation
Confidentiality

1. Message Authentication
A secure digital signature scheme, like a secure
conventional
signature
can
provide
message
authentication.
Note
A digital signature provides message authentication.

2. Message Integrity
The integrity of the message is preserved even if we sign
the whole message because we cannot get the same
signature if the message is changed.

Note
A digital signature provides message integrity.

3. Nonrepudiation
Figure : Using a trusted center for nonrepudiation

Note
Nonrepudiation can be provided using a trusted
party.

4. Confidentiality
Figure : Adding confidentiality to a digital signature scheme

Note
A digital signature does not provide privacy.
If there is a need for privacy, another layer of
encryption/decryption must be applied.

4.   ATTACKS
ON DIGITAL SIGNATURE
4.   
This section describes some attacks on digital
signatures and defines the types of forgery.

Topics discussed in this section:
1.
2.

Attack Types
Forgery Types
13.21

1. Attack Types
Key-Only Attack
Known-Message Attack
Chosen-Message Attack

2. Forgery Types
Existential Forgery

Selective Forgery

Advantages of digital signature

Authentication, identification of the person that signs.
Integrity of data, every change will be detected.
Non repudiation, because the author cannot be denied
of his work (he created and sent).
Imposter prevention
Elimination of possibility of committing fraud by an
imposter

Disadvantages of digital signature

The disadvantages of using digital signatures
involve the primary avenue for any business:
money.
Though the use of Digital Signatures is very powerful
way to secure and authenticate a message or document,
its advantages are hampered by lost or theft of keys and
the use of vulnerable storage facilities.
A number of Digital Signature standard exist which are
incompatible with each other and there is a strong need
of a standard through which these different methods
(keys) can interact.

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