CN-6.ppt

Cryptography and

Cryptography and

Network Security

Network Security

Chapter 13

Chapter 13

Fourth Edition Fourth Edition by William Stallings by William Stallings

Chapter 13 –

Chapter 13 –

Digital Signatures &

Digital Signatures &

Authentication Protocols

Authentication Protocols

To guard against the baneful influence exerted by strangers To guard against the baneful influence exerted by strangers

is therefore an elementary dictate of savage prudence. is therefore an elementary dictate of savage prudence.

Hence before strangers are allowed to enter a district, or Hence before strangers are allowed to enter a district, or

at least before they are permitted to mingle freely with at least before they are permitted to mingle freely with

the inhabitants, certain ceremonies are often performed the inhabitants, certain ceremonies are often performed

by the natives of the country for the purpose of disarming by the natives of the country for the purpose of disarming

the strangers of their magical powers, or of disinfecting, the strangers of their magical powers, or of disinfecting,

so to speak, the tainted atmosphere by which they are so to speak, the tainted atmosphere by which they are

supposed to be surrounded. supposed to be surrounded.

—

Digital Signatures

Digital Signatures

 have looked at _{have looked at} message authentication _{message authentication}  but does not address issues of lack of trustbut does not address issues of lack of trust  digital signatures provide the ability to: _{digital signatures provide the ability to:}

 verify author, date & time of signatureverify author, date & time of signature  authenticate message contents authenticate message contents

 be verified by third parties to resolve disputesbe verified by third parties to resolve disputes  hence include authentication function with _{hence include authentication function with}

Digital Signature Properties

Digital Signature Properties

 _{must depend on the message signedmust depend on the message signed}

 must use information unique to sender_{must use information unique to sender}

 to prevent both forgery and denialto prevent both forgery and denial

 must be relatively easy to produce_{must be relatively easy to produce}

 must be relatively easy to recognize & verify_{must be relatively easy to recognize & verify}  _{be computationally infeasible to forge be computationally infeasible to forge}

 with new message for existing digital signaturewith new message for existing digital signature

Direct Digital Signatures

Direct Digital Signatures

 involve only sender & receiver_{involve only sender & receiver}

 assumed receiver has sender’s public-key_{assumed receiver has sender’s public-key}  digital signature made by sender signing _{digital signature made by sender signing}

entire message or hash with private-key entire message or hash with private-key

 can encrypt using receivers public-key_{can encrypt using receivers public-key}  important that sign first then encrypt _{important that sign first then encrypt}

message & signature message & signature

Arbitrated Digital Signatures

Arbitrated Digital Signatures

 involves use of arbiter A_{involves use of arbiter A}

 validates any signed messagevalidates any signed message

 then dated and sent to recipientthen dated and sent to recipient

 requires suitable level of trust in arbiter_{requires suitable level of trust in arbiter}

 can be implemented with either private or _{can be implemented with either private or}

public-key algorithms public-key algorithms

Authentication Protocols

Authentication Protocols

 used to convince parties of each others _{used to convince parties of each others}

identity and to exchange session keys identity and to exchange session keys

 may be one-way or mutual_{may be one-way or mutual}

 key issues are_{key issues are}

 confidentiality – to protect session keysconfidentiality – to protect session keys

 timeliness – to prevent replay attackstimeliness – to prevent replay attacks

 published protocols are often found to _{published protocols are often found to}

Replay Attacks

Replay Attacks

 where a valid signed message is copied and _{where a valid signed message is copied and}

later resent later resent

 simple replaysimple replay

 repetition that can be loggedrepetition that can be logged

 repetition that cannot be detectedrepetition that cannot be detected

 backward replay without modificationbackward replay without modification  countermeasures include_{countermeasures include}

 use of sequence numbers (generally impractical)use of sequence numbers (generally impractical)  timestamps (needs synchronized clocks)timestamps (needs synchronized clocks)

Using Symmetric Encryption

Using Symmetric Encryption

 as discussed previously can use a two-_{as discussed previously can use a}

two-level hierarchy of keys level hierarchy of keys

 usually with a trusted Key Distribution _{usually with a trusted Key Distribution}

Center (KDC) Center (KDC)

 each party shares own master key with KDCeach party shares own master key with KDC

 KDC generates session keys used for KDC generates session keys used for

connections between parties connections between parties

Needham-Schroeder Protocol

Needham-Schroeder Protocol

 original third-party key distribution protocol_{original third-party key distribution protocol}  for session between A B mediated by KDC_{for session between A B mediated by KDC}  protocol overview is:_{protocol overview is:}

1.

1. A->KDC: A->KDC: IDID_AA || || IDID_BB || || NN₁₁

2

2. KDC ->. KDC -> A: EA: E_KaKa[Ks[Ks || || IDID_BB || || NN₁₁ || E || E_KbKb[[KsKs||||IDID_AA] ]] ]

3.

3. A ->A -> B: B: EE_KbKb[[KsKs||||IDID_AA]]

4.

4. B ->B -> A: A: EE_KsKs[[NN₂₂]]

5.

Needham-Schroeder Protocol

Needham-Schroeder Protocol

 used to securely distribute a new session _{used to securely distribute a new session}

key for communications between A & B key for communications between A & B

 but is vulnerable to a replay attack if an old _{but is vulnerable to a replay attack if an old}

session key has been compromised session key has been compromised

 then message 3 can be resent convincing B then message 3 can be resent convincing B

that is communicating with A that is communicating with A

 modifications to address this require:_{modifications to address this require:}  timestamps (Denning 81)timestamps (Denning 81)

Using Public-Key Encryption

Using Public-Key Encryption

 have a range of approaches based on the _{have a range of approaches based on the}

use of public-key encryption use of public-key encryption

 need to ensure have correct public keys _{need to ensure have correct public keys}

for other parties for other parties

 _{using a central Authentication Server (AS)using a central Authentication Server (AS)}

 various protocols exist using timestamps _{various protocols exist using timestamps}

Denning AS Protocol

Denning AS Protocol

 Denning 81 presented the following:_{Denning 81 presented the following:}

1.

1. A ->A -> AS: AS: IDID_AA || || IDID_BB

2.

2. AS ->AS -> A: EA: E_PRasPRas[[IDID_AA||PU||PU_aa||T] || E||T] || E_PRasPRas[[IDID_BB||PU||PU_bb||T] ||T]

3.

3. A ->A -> B: EB: E_PRasPRas[[IDID_AA||PU||PU_aa||T] || E||T] || E_PRasPRas[[IDID_BB||PU||PU_bb||T] || ||T] || E

E_PUbPUb[E[E_PRasPRas[K[K_ss||T]] ||T]]

 _{note session key is chosen by A, hence AS need note session key is chosen by A, hence AS need}

not be trusted to protect it not be trusted to protect it

 _{timestamps prevent replay but require timestamps prevent replay but require}

One-Way Authentication

One-Way Authentication

 required when sender & receiver are not in _{required when sender & receiver are not in}

communications at same time (eg. email) communications at same time (eg. email)

 have header in clear so can be delivered _{have header in clear so can be delivered}

by email system by email system

 _{may want contents of body protected & may want contents of body protected &}

Using Symmetric Encryption

Using Symmetric Encryption

 can refine use of KDC but can’t have final _{can refine use of KDC but can’t have final}

exchange of nonces, vis: exchange of nonces, vis:

1.

1. AA->->KDC: KDC: IDID_AA || || IDID_BB || || NN₁₁

2

2. KDC . KDC -> -> A: EA: E_KaKa[Ks[Ks || || IDID_BB || || NN₁₁ || E || E_KbKb[[KsKs||||IDID_AA] ]] ]

3.

3. A A -> -> B: B: EE_KbKb[[KsKs||||IDID_AA] || E] || E_KsKs[M][M]

 does not protect against replays_{does not protect against replays}

 could rely on timestamp in message, though could rely on timestamp in message, though

Public-Key Approaches

Public-Key Approaches

 have seen some public-key approaches_{have seen some public-key approaches}

 if confidentiality is major concern, can use:_{if confidentiality is major concern, can use:}

A

A->->B: EB: E_PUbPUb[Ks] || E[Ks] || E_KsKs[M][M]

 has encrypted session key, encrypted messagehas encrypted session key, encrypted message

 if authentication needed use a digital _{if authentication needed use a digital}

signature with a digital certificate: signature with a digital certificate:

A

A->->B: M || EB: M || E_PRaPRa[H(M)] || E[H(M)] || E_PRasPRas[T||ID[T||ID_AA||PU||PU_aa] ]

Digital Signature

Digital Signature

Standard

Standard

(DSS)

(DSS)

 US Govt approved signature scheme_{US Govt approved signature scheme}  designed by NIST & NSA in early 90's _{designed by NIST & NSA in early 90's}  published as FIPS-186 in 1991_{published as FIPS-186 in 1991}

 revised in 1993, 1996 & then 2000_{revised in 1993, 1996 & then 2000}  uses the SHA hash algorithm _{uses the SHA hash algorithm}

 DSS is the standard, DSA is the algorithm_{DSS is the standard, DSA is the algorithm}

 FIPS 186-2 (2000) includes alternative RSA & _{FIPS 186-2 (2000) includes alternative RSA &}

Digital Signature

Digital Signature

Algorithm

Algorithm

(DSA)

(DSA)

 creates a 320 bit signature_{creates a 320 bit signature}

 with 512-1024 bit security_{with 512-1024 bit security}

 smaller and faster than RSA_{smaller and faster than RSA}

 a digital signature scheme only_{a digital signature scheme only}

 _{security depends on difficulty of computing security depends on difficulty of computing}

discrete logarithms discrete logarithms

Digital Signature

Digital Signature

Algorithm

Algorithm

(DSA)

DSA Key Generation

DSA Key Generation

 have shared global public key values (p,q,g): _{have shared global public key values (p,q,g):}

 choose q, a 160 bit choose q, a 160 bit

 choose a large prime choose a large prime p = 2p = 2LL

• where L= 512 to 1024 bits and is a multiple of 64_{where L= 512 to 1024 bits and is a multiple of 64}

• and q is a prime factor of _{and q is a prime factor of} (p-1)_(p-1)

 choose choose g = hg = h(p-1)/q(p-1)/q

• where _where h<p-1, h_{h<p-1, h}(p-1)/q (p-1)/q _{(mod p) > 1(mod p) > 1}

 users choose private & compute public key: _{users choose private & compute public key:}

 choose choose x<qx<q

DSA Signature Creation

DSA Signature Creation

 to _to sign_sign a message _{a message MM} the sender: _{the sender:}

 generates a random signature key generates a random signature key k, k<qk, k<q

 nb. nb. kk must be random, be destroyed after must be random, be destroyed after

use, and never be reused use, and never be reused

 then computes signature pair: _{then computes signature pair:}

r = (g

r = (gkk_{(mod p))(mod q) (mod p))(mod q)}

s = (k

s = (k-1-1_{.H(M)+ x.r)(mod q).H(M)+ x.r)(mod q)}

DSA Signature Verification

DSA Signature Verification

 having received M & _{having received M &} signature _{signature (r,s)(r,s)}

 to _to verify_verify a signature, recipient computes: _{a signature, recipient computes:}

w = s

w = s-1-1_{(mod q) (mod q)}

u1= (H(M).w)(mod q) u1= (H(M).w)(mod q)

u2= (r.w)(mod q) u2= (r.w)(mod q)

v = (g

v = (gu1u1.y_.yu2u2(mod p)) (mod q) _{(mod p)) (mod q)}  if _{if v=rv=r} then signature is verified _{then signature is verified}

Summary

Summary

 have discussed:_{have discussed:}

 digital signaturesdigital signatures

 authentication protocols (mutual & one-way)authentication protocols (mutual & one-way)