Easy for the signer to sign a message There is no point in having a digital signature scheme that involves the signer needing to use slow and complex operations to compute a digital signature. Easy for anyone to verify a message Similarly we would like the verification of a digital signature to be as efficient as possible. Hard for anyone to forge a digital signature It should be practically impossible for anyone who is not the legitimate signer to compute a digital signature on a message that appears to be valid. By “appears to be valid” we mean that anyone who attempts to verify the digital signature is led to believe that they have just successfully verified a valid digital signature on a message.

1. E- Business
&
need of Digital Signature

2. Electronic payments: the players
Consumer Merchant Financial institutions
Physical tokens
representing value

3. Digital cash
01011010110101011101011010101101
01101010110101101010110101011010
10110111101011111011010000000110
101010110101
Since digital cash is represented by data, it is
easily replicated. How do we prevent:
•Counterfeiting?
•Multiple spending?

4. 4
Electronic signatures
The European Community Directive on electronic signatures
refers to the concept of an electronic signature as:
data in electronic form attached to, or logically connected with,
other electronic data and which serves as a method of
authentication
What different things can you think of that might
satisfy this rather vague notion of an electronic
signature?

5. 5
Advanced electronic signatures
The European Community Directive on electronic signatures
also refers to the concept of an advanced electronic
signature as:
an electronic signature that is:
1. uniquely linked to the signatory
2. capable of identifying the signatory
3. created using means under the sole control of the signatory
4. linked to data to which it relates in such a way that subsequent
changes in the data is detectable

6. 6
Security requirements
• Data origin authentication of the signer
– A digital signature validates the message in the sense that
assurance is provided about the integrity of the message and
of the identity of the entity that signed the message.
• Non-repudiation
– A digital signature can be stored by anyone who receives
the signed message as evidence that the message was
sent and of who sent it. This evidence could later be
presented to a third party who could use the evidence to
resolve any dispute that relates to the contents and/or origin
of the message.
We will define a digital signature on a message to be some data that
provides:

7. 7
Symmetric-key
ciphers:
Block ciphers
Stream ciphers
Public-key
ciphers
Cryptographic goals
Confidentiality Data integrity Authentication Non-repudiation
Message authentication
Entity authenticationArbitrary length
hash functions
Message
Authentication
codes (MACs)
Digital signatures
Authentication
primitives
Digital signatures
MACs
Digital
signatures

8. 8
Input to a digital signature
• The message
– Since a digital signature needs to offer data origin authentication
(and non-repudiation) it is clear that the digital signature itself must
be a piece of data that depends on the message, and cannot be a
completely separate identifier.
– It may be sent as a separate piece of data to the message, but its
computation must involve the message.
• A secret parameter known only by the signer
– Since a digital signature needs to offer non-repudiation, its
calculation must involve a secret parameter that is known only by
the signer.
– The only possible exception to this rule is if the other entity is totally
trusted by all parties involved in the signing and verifying of digital
signatures.

9. 9
Properties of a digital
signature
• Easy for the signer to sign a message
– There is no point in having a digital signature scheme that involves
the signer needing to use slow and complex operations to compute
a digital signature.
• Easy for anyone to verify a message
– Similarly we would like the verification of a digital signature to be as
efficient as possible.
• Hard for anyone to forge a digital signature
– It should be practically impossible for anyone who is not the
legitimate signer to compute a digital signature on a message that
appears to be valid. By “appears to be valid” we mean that anyone
who attempts to verify the digital signature is led to believe that they
have just successfully verified a valid digital signature on a
message.

10. •is a type of asymmetric cryptography used to simulate the security
properties of a signature in digital, rather than written, form. Digital signature
schemes normally give two algorithms, one for signing which involves the user's
secret or private key, and one for verifying signatures which involves the user's
public key. The output of the signature process is called the "digital signature.“
•is an electronic signature that can be used to authenticate the identity of
the sender of a message or the signer of a document, and possibly to ensure that
the original content of the message or document that has been sent is
unchanged. Digital signatures are easily transportable, cannot be imitated by
someone else, and can be automatically time-stamped. The ability to ensure that
the original signed message arrived means that the sender cannot easily
repudiate it later.
What is a digital
signature?

11. • The use of digital signatures usually involves two
processes, one performed by the signer and the other
by the receiver of the digital signature:
• Digital signature creation uses a hash result derived from and
unique to both the signed message and a given private key. For the hash
result to be secure, there must be only a negligible possibility that the same
digital signature could be created by the combination of any other message
or private key.
• Digital signature verification is the process of checking the digital
signature by reference to the original message and a given public key, thereby
determining whether the digital signature was created for that same message using
the private key that corresponds to the referenced public key.
How it works

12. Paper Signatures V/s Digital Signatures
Parameter Paper Electronic
Authenticity May be forged Can not be copied
Integrity Signature
independent of the
document
Signature depends
on the contents of
the document
Non-
repudiation
a. Handwriting
expert needed
b. Error prone
a. Any computer
user
b. Error free
V/s

13. Electronic RecordElectronic Record
1. Very easy to make copies
2. Very fast distribution
3. Easy archiving and retrieval
4. Copies are as good as original
5. Easily modifiable
6. Environmental Friendly
Because of 4 & 5 together, these lack authenticity

14. Digital signatures employ a type of Asymmetric Cryptography. The
Scheme typically consists of three Algorithms
 A key generation algorithm that selects a private key
uniformly at random from a set of possible private
keys. The algorithm outputs the private key and a
corresponding public key.
 A signing algorithm that, given a message and a
private key, produces a signature.
 A signature verifying algorithm that, given a message,
public key and a signature, either accepts or rejects
the message's claim to authenticity
Hash value of a message when encrypted with the private key of a person is his
digital signature on that e-Document

15. Digital SignaturesDigital Signatures
Each individual generates his own key pair
[Public key known to everyone
&
Private key only to the owner]
Private Key – Used for making Digital Signature
Public Key – Used to verify the Digital Signature

16. Smart Card
iKey
Hardware Tokens

17. Example
• Assume you were going to send the draft of a contract to your
lawyer in another town. You want to give your lawyer the assurance
that it was unchanged from what you sent and that it is really from
you.
1. You copy-and-paste the contract (it's a short one!) into an e-mail
note.
2. Using special software, you obtain a message hash (mathematical
summary) of the contract.
3. You then use a private key that you have previously obtained from a
public-private key authority to encrypt the hash.
4. The encrypted hash becomes your digital signature of the message.
(Note that it will be different each time you send a message.)
• At the other end, your lawyer receives the message.
1. To make sure it's intact and from you, your lawyer makes a hash of
the received message.
2. Your lawyer then uses your public key to decrypt the message hash
or summary.
3. If the hashes match, the received message is valid.

18. 18
Hash functions
1. Condenses arbitrary long inputs into a
fixed length output
– You stuff as much data as you want into the function, and it
churns out an output (or hash) that is always the same
fixed length.
– In general this hash is much smaller than the data that was
put into the function.
– Because the hash is a smaller thing that represents a
larger thing, it sometimes referred to as a digest, and the
hash function as a message digest function.
A hash function is a mathematical function that generally has the
following three properties:

19. 19
Hash functions
2. Is one-way
– The hash function should be easy to compute, but given the
hash of some data it should be very hard to recover the
original data from the hash.
3. It is hard to find two inputs with the same
output
– It should be hard to find two different inputs (of any length) that
when fed into the hash function result in the same hash
(collision free).
– Note that it is impossible for a hash function not to have
collisions. If arbitrarily large inputs are all being reduced to a
fixed length hash then there will be lots of collisions. (For
example - it is impossible to give each of 60 million people a
different 4 digit PIN.) The point is that these collisions should
be hard to find.

21. Message Hash
Function
Message
Digest
Signature
Function
Digital
Signature
Message
Signature Private Key

22. Message Hash Function Message
Digest
Digest
Signature
Signature
Function
Message
Digest
If the message
digest are identical,
the signature is valid.
If they are different,
the signature is not
valid.
Signer’s Public Key

24. Digital Signatures
• Digital Signatures are numbers
• Same Length – 40 digits
• They are document content dependent
I am George kutty
efcc61c1c03db8d8ea8569545c073c814a0ed755
My place of birth is at Thrissur.
fe1188eecd44ee23e13c4b6655edc8cd5cdb6f25
I am 27 years old.
0e6d7d56c4520756f59235b6ae981cdb5f9820a0
I am an Engineer.
ea0ae29b3b2c20fc018aaca45c3746a057b893e7
I am a Engineer.
01f1d8abd9c2e6130870842055d97d315dff1ea3
• These are digital signatures of same person on different documents

25. • Application Request
• Issuance of Digital Signatures
• Accessing Website with Membership
ID for Enrollment of Request On line
• Driver Downloading
• Member Login
How To Get & Use Digital
Signature

26. Benefits of digital signatures
These are common reasons for applying a digital signature to communications:
• Authentication
Although messages may often include information about the entity sending
a message, that information may not be accurate. Digital signatures can be
used to authenticate the source of messages. When ownership of a digital
signature secret key is bound to a specific user, a valid signature shows that
the message was sent by that user. The importance of high confidence in
sender authenticity is especially obvious in a financial context. For example,
suppose a bank's branch office sends instructions to the central office
requesting a change in the balance of an account. If the central office is not
convinced that such a message is truly sent from an authorized source,
acting on such a request could be a grave mistake.
• Integrity
In many scenarios, the sender and receiver of a message may have a need
for confidence that the message has not been altered during transmission.
Although encryption hides the contents of a message, it may be possible to
change an encrypted message without understanding it. (Some encryption
algorithms, known as nonmalleable ones, prevent this, but others do not.)
However, if a message is digitally signed, any change in the message will
invalidate the signature. Furthermore, there is no efficient way to modify a
message and its signature to produce a new message with a valid
signature, because this is still considered to be computationally infeasible by
most cryptographic hash functions.

27. Drawbacks of digital signatures
Despite their usefulness, digital signatures do not alone solve all
the problems we might wish them to.
Non-repudiation
In a cryptographic context, the word repudiation refers to the act
of disclaiming responsibility for a message. A message's
recipient may insist the sender attach a signature in order to
make later repudiation more difficult, since the recipient can
show the signed message to a third party (eg, a court) to
reinforce a claim as to its signatories and integrity. However,
loss of control over a user's private key will mean that all digital
signatures using that key, and so ostensibly 'from' that user, are
suspect. Nonetheless, a user cannot repudiate a signed
message without repudiating their signature key.

28. Prevention From Misuse

29. Prevention From Misuse
 Don’t Hand over you DSC Media
USB/Card to any one
 Don’t tell your PIN to anyone
 Document Digitally Signed carries
same legal status as manually
signing as per the IT Act

30. To prevent this misuse…
• Assign each user fixed keys
• Usually be done by third party: VeriSign…
• Moreover, common good practice: sign
first, encrypt later.

31. Questions?
1. In the digital signature who use the private key and
who use the public key?
Private key: sender
Public key: receiver
2. What are the benefits of digital signatures?
Authentication and Integrity