Digital Signatures 2

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Cryptography and Network SecurityChapter 13,
Fifth Editionby William Stallings
Digital Signature

• BSIT-7th
• University of Okara
• Instructor Assistant Prof. Inam ul Haq

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Contents

• Digital Signatures
• 3-Properties
• Attacks and Forgeries
• Digital Signature Model
• Digital Signature Requirements
• Digital Signature Standard
• Elgamal Schnorr Schemes
• DSS vs RSA
• Digital Signature Algorithm

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Digital Signatures

• A digital signature is an authentication
  mechanism that enables the creator of a message
  to attach a code that acts as a signature
• Hash (message)
• Cryptographic Hash Function is a
  mathematical algorithm that maps data of
  arbitrary size to a data of fixed size
• Encrypting message with creator private key,
  always unique
• Question In situations where there is not
  complete trust between sender and receiver,
  something more than authentication is needed. The
  most attractive solution to this problem is?

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3 Properties of Digital Signature

• It must verify the author, date and time of
  the signature. It must authenticate the
  contents at the time of the signature. It must
  be verifiable by third parties, to resolve
  disputes
• What is solution if signature is lost or
  stolen?
• Solution digital certificate (because it
  contains copy of public key of sender)

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Attacks and Forgeries

• It lists following types of attacks where A
  denotes victim while C denotes attacker.
• Key-only attack C only knows As public key
• Total break C determines As private key
• Existential forgery C forges a signature for at
  least one message. C has no control over the
  message.

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Digital Signature Model
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Digital Signature Model
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Digital Signature Requirements

• must depend on the message signed
• must use information unique to sender
• to prevent both forgery and denial
• must be relatively easy to produce
• must be relatively easy to recognize verify
• be computationally infeasible to forge
• with new message for existing digital signature
• with fraudulent digital signature for given
  message
• be practical save digital signature in storage

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Direct Digital Signatures

• Involve only sender receiver (digital door
  closer)
• Assumed receiver has senders public-key
• Digital signature made by sender signing entire
  message or hash with private-key
• Can encrypt using receivers public-key
• Important that sign first then encrypt message
  signature
• Security depends on senders private-key

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Digital Signature Standard (DSS)

• US Govt approved signature scheme in 1991,
  designed by NIST
• Uses the SHA hash algorithm
• DSS is the standard, DSA is the algorithm
• DSA is digital signature only, while RSA is a
  public key encryption method widely used in data
  transmission
• A public-key technique
• E.g. of DSA RSA-based signature scheme, Elgamal,
  Schnorr and Robin Signatures

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ElGamal Digital Signatures

• The ElGamal signature scheme involves the use of
  the private key for encryption and the public key
  for decryption
• The ElGamal signature scheme is a digital
  signature scheme which is based on the difficulty
  of computing discrete algorithms. It was
  described by Taher ElGamal in 1984
• The ElGamal signature algorithm is rarely used in
  practice. A variant developed at NSA and known as
  the Digital Signature Algorithm is much more
  widely used. There are several other
  variants. The ElGamal signature scheme must not
  be confused with ElGamal encryption which was
  also invented by Taher ElGamal
• The ElGamal signature scheme allows a third-party
  to confirm the authenticity of a message sent
  over an insecure channel. (explore wikipedia)

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Schnorr Digital Signature

• Schnorr signature is a digital signature produced
  by the Schnorr signature algorithm.
• Its security is also based on the intractability
  of certain discrete algorithms.
• The Schnorr signature is considered the
  simplest digital signature scheme to be provably
  secure in Oracle model. 
• It is efficient and generates short signatures.
  It was covered by U.S. Patent 4,995,082 which
  expired in February 2008. (explore wikipedia)

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DSS vs RSA Signatures
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Exercise

• What are weak points in digital signatures?
• Which type of digital signature is currently
  used?
• What types of attacks are associated with digital
  signature?
• Difference b/w RSA DSS
• Create an auto removable file
• Practical demonstration