Cryptography and Network Security

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Cryptography and Network Security

• Third Edition
• by William Stallings
• Lecture slides by Lawrie Brown
• Modified by David Martin

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Chapter 13 Digital Signatures Authentication
Protocols

• To guard against the baneful influence exerted by
  strangers is therefore an elementary dictate of
  savage prudence. Hence before strangers are
  allowed to enter a district, or at least before
  they are permitted to mingle freely with the
  inhabitants, certain ceremonies are often
  performed by the natives of the country for the
  purpose of disarming the strangers of their
  magical powers, or of disinfecting, so to speak,
  the tainted atmosphere by which they are supposed
  to be surrounded.
• The Golden Bough, Sir James George Frazer

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Authentication Protocols

• used to convince parties of each other's identity
  and to exchange session keys
• based on what you know
• may be one-way or mutual
• key issues are
• confidentiality to protect session keys
• timeliness to prevent replay attacks

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Authentication Threats

• Impersonation by outsider
• Impersonation by insider
• How trusted are trusted components?
• Dictionary attacks when passwords present
• Eavesdropping
• For authentication protocols that use encryption
• Discovering session key may result in
  impersonation

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Types of Authentication

• What you know
• Password
• Secret (symmetric) key
• Private (asymmetric) key
• What you have
• Smart card
• Fingerprints, other biometrics
• Where you are
• Network location

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Where you are

• Network address authentication very common
• rlogin, rsh, rcp
• /.rhosts
• /etc/hosts.equiv
• Use of DNS
• NFS /etc/exports
• TCP wrappers
• Web servers
• /etc/hosts.allow, /etc/hosts.deny
• Firewalls
• Good use of defense in depth, but not enough

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HTTP Basic Authentication

• Plaintext login/password
• telnet, ftp
• HTTP Basic Authentication
• A ? S GET / HTTP/1.0
• A ? S 401 Unauthorized WWW-Authenticate Basic
  realmCS
• A ? S GET / HTTP/1.0 Authorization Basic
  QWxhZGRpbjpvcGVuIHNlc2FtZQ
• Criticize this "improvement"
• A ? S IDa, h(IDa,Kas), command
• A ? S response to command

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Replay Attacks

• where a valid signed message is copied and later
  resent
• simple replay
• repetition that can be detected (within
  acceptable time)
• repetition that cannot be detected
  (suppress-replay)
• reflection (to sender) without modification
• countermeasures include
• challenge/response (using unique nonce)
• use of sequence numbers (generally impractical)
• timestamps (needs synchronized clocks)

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Reflection attack

• Mutual authentication
• A ? B IDa, Na
• A ? B Nb , f(Kab, Na )
• A ? B f(Kab, Nb )
• Prescription break symmetry

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Digital Signatures

• have looked at MACs
• but does not address issues of trust mismatch
• digital signatures provide the ability to
• verify author, date time of signature
• authenticate message contents
• be verified by third parties to resolve disputes
• hence include authentication function with
  additional capabilities

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Digital Signature Properties

• 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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Using Symmetric Encryption

• as discussed previously can use a two-level
  hierarchy of keys
• usually with a trusted Key Distribution Center
  (KDC)
• each party shares own master key with KDC
• KDC generates session keys used for connections
  between parties
• master keys used to distribute these to them

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Needham-Schroeder Protocol

• original third-party key distribution protocol
• for session between A B mediated by KDC
• protocol overview is
• 1. A?KDC IDA IDB N1
• 2. A?KDC EKaKs IDB N1 EKbKsIDA
• 3. A?B EKbKsIDA
• 4. A?B EKsN2
• 5. A?B EKsf(N2)

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Needham-Schroeder, again
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Needham-Schroeder Protocol

• used to securely distribute a new session key for
  communications between A B
• session key used for enduring authentication
• but is vulnerable to a replay attack if an old
  session key has been compromised
• then message 3 can be resent convincing B that is
  communicating with A
• modifications to address this require
• timestamps (Denning 81)
• using an extra nonce (Neuman 93)

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Using Public-Key Encryption

• have a range of approaches based on the use of
  public-key encryption
• need to ensure you have correct public keys for
  other parties
• using a central Authentication Server (AS)
• various protocols exist using timestamps or nonces

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Denning AS Protocol

• Denning 81 presented the following
• 1. A?AS IDA IDB
• 2. A?AS EKRasIDAKUaT EKRasIDBKUbT
  
• 3. A?B EKRasIDAKUaT EKRasIDBKUbT
  EKUbEKRaKsT
• 4. A?B EKs message
• uses certificates
• note session key is chosen by A, hence AS need
  not be trusted to protect it
• timestamps prevent replay but require
  synchronized clocks

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Forward Secrecy

• Denning protocol can be summarized as
• Client chooses session key, signs, encrypts,
  transmits to server
• AS provides certificates in case they are needed
• Timestamps prevent replay
• Anyone knowing Bob's private key can decrypt
  multiple sessions
• Forward secrecy is the lack of this property
• In other words, a long-term key should not reveal
  short-term keys
• Important consideration for very sensitive data
• and mitigating key escrow threats

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Getting Forward Secrecy

• A general approach
• First authenticate yourself to peer
• Then invent and transmit a new key pair
• Peer responds by inventing and transmitting a new
  session key encrypted under new key pair
• Expensive
• Diffie-Hellman authentication
• The key is not presented in a form where knowing
  long-term authentication keys helps an adversary
• "Break backward" "forward secrecy"

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Woo-Lam '92 protocol

• A, B only know KUKDC uses nonces instead of
  timestamps
• 1. A ? KDC IDa, IDb
• 2. A ? KDC Cert b
• 3. A ? B EKUB Na, IDa
• 4. B ? KDC IDa, IDb, EKUKDC Na
• 5. B ? KDC Cert a, EKUB EKRKDC Na, Ks, IDb
  
• 6. A ? B EKUA EKRKDC Na, Ks, IDb , Nb
• 7. A ? B EKs Nb

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One-Way Authentication

• required when sender receiver are not in
  communications at same time (eg. email)
• have header in clear so can be delivered by email
  system
• may want contents of body protected sender
  authenticated

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Using Symmetric Encryption for One-way
authentication

• can refine use of KDC but cant have final
  exchange of nonces, namely
• 1. A?KDC IDA IDB N1
• 2. A?KDC EKaKs IDB N1 EKbKsIDA
• 3. A?B EKbKsIDA EKsM
• does not protect against replays to B
• could rely on timestamp in message, though email
  delays make this problematic
• use only for messages that do no harm when
  repeated (hard to enforce)

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Public-Key Approaches to One-way

• have seen some public-key approaches
• if confidentiality is major concern, can use
• A?B EKUbKs EKsM
• has encrypted session key, encrypted message
• used in PGP, PEM, S/MIME,
• if authentication needed use a signature with a
  certificate
• A?B M EKRaH(M) EKRasTIDAKUa
• with message, signature, certificate

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Authentication via Encryption

• Recall notion if it decrypts to something
  sensible then it must have been encrypted by a
  keyholder
• Detecting sensible is hard
• Encryption schemes do not enforce this (e.g.,
  DES-CBC)
• Use explicit auth mechanism instead
• So, instead of EKs(M), generate two session keys
  one for encryption, one for MAC

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Digital Signature Standard (DSS)

• US Govt approved signature scheme FIPS 186
• uses the SHA hash algorithm
• designed by NIST NSA in early 90's
• DSS is the standard, DSA is the algorithm
• a variant on ElGamal and Schnorr schemes
• creates a 320 bit signature, but comparable to
  RSA 512-1024 bit security
• security depends on difficulty of computing
  discrete logarithms

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Digital Signature Algorithm (DSA)
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DSA Key Generation

• have shared global public key values (p,q,g)
• a large prime p 2L
• where L 512 to 1024 bits and is a multiple of 64
  
• choose q, a 160 bit prime factor of p-1
• choose g h(p-1)/q
• where hltp-1, h(p-1)/q (mod p) gt 1
• users choose private compute public key
• choose xltq
• compute y gx (mod p)

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DSA Signature Creation

• to sign a message M the sender
• generates a random signature key k, kltq
• nb. k must be random, be destroyed after use, and
  never be reused
• then computes signature pair
• r (gk(mod p))(mod q)
• s (k-1.SHA(M) x.r)(mod q)
• sends signature (r,s) with message M

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DSA Signature Verification

• having received M signature (r,s)
• to verify a signature, recipient computes
• w s-1(mod q)
• u1 (SHA(M).w)(mod q)
• u2 (r.w)(mod q)
• v (gu1.yu2(mod p)) (mod q)
• if vr then signature is verified
• see book web site for details of proof why

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Summary

• have considered
• digital signatures
• authentication protocols (mutual one-way)
• digital signature standard