Digital signature

2
What is
“Digital
Signature” ?
What is the
difference
between
Electronic and
Digital
Signature?
How does
it work?
How safe
is a
digital
signature
?
What is
Electronic
Signature
?
Is it hard to
use ?
Is a digital
signature
legally
enforceable
?
?

What is Digital Signature?What is Digital Signature?
 Digital Signature is a type of electronic signature that encrypts
documents with digital codes that are particularly difficult to
duplicate.
 A digital signature (standard electronic signature) takes the
concept of traditional paper-based signing and turns it into an
electronic “fingerprint.” This “fingerprint,” or coded message,
is unique to both the document and the signer and binds them
together.
 It is used to validate the authenticity and integrity of a
message, software or digital document. Digital signatures
cryptographically bind an electronic identity to an electronic
document and the signature cannot be copied to another
document.
3

What is Digital Signature?What is Digital Signature?
 Digital signature technique is based on public key
cryptography with a difference.
 In public key cryptography a pair of keys are used, one public
key and one private key. The public key is often used for
message encryption , and the private key is often used for
decrypting the message.
 However in case of digital signature message is encrypted with
the private key and decrypted with the public key.
 Only a specific person with the corresponding private key can
encrypt the message or in other words sign the message.
However any party who has the signatory’s public key can
encrypt the message, in other words can verify the message.

Confidentiality IssuesConfidentiality Issues
 It should be possible for the receiver of a message to ascertain
its origin. An intruder should not be able to masquerade as
someone else.
 It should be possible for the receiver of a message to verify
that it has not been modified in transit. An intruder should not
be able to substitute a false message for a legitimate one.
 A sender should not be able to falsely deny later that he sent a
message.

Attributes of Digital SignatureAttributes of Digital Signature
 Digital signature ensures the confidentiality via the following
three attributes:
1. Authentication
2. Integrity
3. Non-repudiation
8

Attributes of Digital SignatureAttributes of Digital Signature
 Authentication: Authentication means the act of proving who
you say you are. Authentication means that you know who
created and sent the message. Digital signature is used to
authenticate the source of messages. It ensures the user of the
sender.
 Integrity: Integrity ensures that when a message is sent over a
network, the data that arrives is the same as the data that was
originally sent. Integrity is the assurance that the information is
trustworthy and accurate. Digital signature ensures the integrity
of message.
 Non-repudiation: this is an important criteria of digital
signature. As digital signature ensures the authentication of the
message, so the sender can’t repudiate it later. At the same time
it also ensures the identity of the receiver, so the receiver can’t
repudiate it later. 9

What is Electronic Signature?What is Electronic Signature?
 An electronic signature is a typed name or a scanned image of a
handwritten signature.
 As a result, e-signatures are very problematic when it comes to
maintaining integrity and security, as nothing prevents one
individual from typing another individual’s name.
 Due to this reality, an electronic signature that does not
incorporate additional measures of security (the way digital
signatures do) is considered an insecure way of signing
documentation.
10

Difference Between Digital andDifference Between Digital and
Electronic SignatureElectronic Signature
 A digital signature, often referred to as advanced or standard
version of electronic signature, that provides the highest levels
of security and universal acceptance.
 Digital signatures are based on Public Key Infrastructure (PKI)
technology, and guarantee signer identity and intent, data
integrity, and the non-repudiation of signed documents. The
digital signature cannot be copied, tampered with or altered.
11

Difference Between Digital andDifference Between Digital and
Electronic SignatureElectronic Signature
 In addition, because digital signatures are based on standard
PKI technology, they can be validated by anyone without the
need for proprietary verification software.
 On the other hand, there is no standard format for electronic
signatures that may be a digitized image of a handwritten
signature, a symbol, voiceprint, etc., used to identify the
author(s) of an electronic message.
 An electronic signature is vulnerable to copying and
tampering, and invites forgery. In many cases, electronic
signatures are not legally binding and will require proprietary
software to validate the e-signature.
12

Is Digital Signature LegallyIs Digital Signature Legally
Enforceable?Enforceable?
“Yes”
In 2006, The parliament of Bangladesh enacted “ICT Act 2006”,
which made signed electronic contracts and documents as legally
binding as a paper-based contract.
Today, digital signature (standard electronic signature)
solutions carry recognized legal significance, enabling
organizations to comply with regulations worldwide.
13

What is Standard?What is Standard?
 There are three algorithms that are suitable for digital
signature generation under the DSS standard.
 They are the Digital Signature Algorithm (DSA, which I will talk
about more in depth later), the RSA algorithm, and the Elliptic
Curve Digital Signature Algorithm (ECDSA).
14

Digital Signature AlgorithmDigital Signature Algorithm
15

16
1. Sender takes the input
message.
2. Digest the input message
using message digest
algorithm
3. Apply DSA (using private key)
to encrypt the digested
message. Here the digital
signature is padded into the
digested message.
4. Create a data packet using the
original message plus the
encrypted message.
5. Send the packet to the
intended receiver.
Data Packet
+

17
1. Receiver receives the
packet.
2. Separate the original
message and encrypted
message.
3. Apply message digest
algorithm on the
original message.
4. Now Apply DSA (using
public key) on the
encrypted message to
decrypt it. At this stage
the digital signature is
removed from the
message and we get
digested message.
5. Then compare them
Data Packet
+ Separate two parts

DSA: ParametersDSA: Parameters
 Signature Computation: A DSA digital signature is computed
using a set of domain parameters:
1. A private key x,
2. A per-message or data secret number k,
3. Data to be signed,
4. And a hash function.
 Signature Verification: A digital signature is verified using the
same domain parameters:
1. A public key y that is mathematically associated with
the private key x used to generate the digital signature,
2. Data to be verified,
3. And the same hash function that was used during
signature generation. 18

 These parameters are defined as follows:
 The first three parameters, p, q, and g, are public and can be
common across a network of users. x is private and y is public key.
19

 Selection of Parameter Sizes and Hash Functions for DSA
20

DSA: How Signature isDSA: How Signature is
Generated by the Sender ?Generated by the Sender ?
 Let N be the bit length of q. And let min(N, outlen) denote the
minimum of the positive integers N and outlen. Outlen is the bit
length of the hash function output block.
 The signature of a message m consists of the pair of numbers r
and s that is computed according to the following equations:
r = (gk
mod p) mod q
z= H(m) [z = the leftmost min(N, outlen) bits of Hash(m)]
s = (k-1
(H(m) + xr) ) mod q.
 When computing s, the string z obtained from Hash(m) is
converted to an integer.
 (r , s) is considered to be the signature of the sender.
21

Generated by the Sender ?Generated by the Sender ?
 The values of r and s shall be checked to determine if r = 0 or
s = 0.
 If either r = 0 or s = 0, a new value of k shall be generated,
and the signature shall be recalculated.
 It is extremely rare that r = 0 or s = 0 if signatures are
generated properly.
 The signature (r, s) is transmitted along with the message to
the verifier.
22

Verified by the Receiver ?Verified by the Receiver ?
 Signature verification may be performed by any party the
signatory (sender), the intended receiver or any other party
using the signatory’s public key.
 A signatory may wish to verify that the computed signature is
correct or not, before sending the signed message to the
intended receiver.
 The intended receiver (or any other party) verifies the
signature to determine its authenticity upon on receiving the
message.
23

 Let m', r', and s' is the received versions of m, r, and s,
respectively. If every thing goes right then m‘=m, r‘=r, and s‘=s.
 Let y be the public key of the claimed signatory and let N be the
bit length of q.
 Also, let min(N, outlen) denote the minimum of the positive
integers N and outlen, where outlen is the bit length of the hash
function output block.
 The signature verification process is as follows:
– The verifier will check that 0 < r' < q and 0 < s' < q. If either
condition is violated, the signature shall be rejected as
invalid.
24

2. If the two conditions in step 1 are satisfied, the verifier
computes the following:
w = (s')–1
mod q.
z = H(m' )
u1 = (z * w) mod q = (H(m' ) * w) mod q
u2 = (r' * w) mod q.
v = [ ( (gu1
* yu2
) mod p) mod q ]
The string z obtained from Hash(m') is converted to an integer
2. If v = r', then the signature considered to be verified.
If m' = m, r' = r, and s'= s then v = r‘.
3. If v ≠ r', then the message or the signature may have been
modified. The signature is considered invalid. 25

ExampleExample
 Alice wants to send message m to Bob.
 Sender side:
1. Alice generates a random number, k, less than
2. Alice generates
r = (gk
mod p) mod q
s = (k-1
(H(m) + xr)) mod q
1. The parameters (r,s) are her signature; she sends these to
bob along with the signed message m'.
26

ExampleExample
 Receiver Side:
1. Let bob received the message and signatures as m', r', and s‘.
Then Bob verifies the signature by computing:
w = (s')–1
mod q.
z = H(m' )
u1 = (z * w) mod q = (H(m' ) * w) mod q
u2 = (r' * w) mod q.
v = [ ( (gu1
* yu2
) mod p) mod q ]
2. If v = r', then the signature and message considered to be
authenticated.
3. If v ≠ r', The signature and message is considered invalid.
27

ReferencesReferences
 http://en.wikipedia.org/wiki/
 Applied Cryptography by Bruce Schneier; 10th
Anniversary edition
 FIPS PUB 186-3; FEDERAL INFORMATION PROCESSING
STANDARDS PUBLICATION

29
Md. Shakhawat Hossain
Student of Department of Computer Science &
Engineering
University of Rajshahi
E-mail: mshimul86@gmail.com

Digital signature

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