Chapter 9. Key management 이 상 일

Key management? Key management ? distribution of cryptogtaph keys Representing Identity  Ch.13 Authentication  Ch.11 Ch.9 : Propagate

NOTATION XY : {Z}K X send to Y Encipherd message Z Key K

1. Session and Interchange Keys Def 9.1 Session Key – cryptograpphic key associated with communication it self Interchage Key – cryptographic key associated with principal to a communication Comuncation <-> Principal to communcation

Session Key Specifically to exchange information with someone Reduce eavesdrop, replay attack, forward search Discard when the session end

Interchange key Associated with a principal A can use the Key shares with B To Convince B the sender is A A can use it to all session

9.2 Key Exchange To A to communicate secretly B To using a shared cryptographic key Criteria 1. to share cannot be transmitted in the clear 2. A and B decide to trust a third party 3. Protocals are public known

9.2.1 Classical Cryptographic Key Exchange and Authentication Simple protocol 1. Alice  Cathy: {request for session key to Bob}kalice 2. Cathy  Alice: {Ksesison }Kalice || {Ksession}Kbob 3. Alice  Bob: {Ksession}Kbob Problem Bob cannot know to Whom Attacker can exchange message intercept 3. Eve  Bob: {Ksession}Kbob

9.2.1 Classical Cryptographic Key Exchange and Authentication Needham – Schroeder protocol 1. Alice  Cathy : { Alice || Bob || rand1 } 2. Cathy  Alice : { Alice || Bob || rand1 || ksession || {Alice || ksession} kBob } kAlice 3. Alice  Bob : { Alice || ksession } kBob 4. Bob  Alice : { rand2 } ksession 5. Alice  Bob : { rand2 – 1 }ksession Nonce – two random number(rand1, rand2)

9.2.1 Classical Cryptographic Key Exchange and Authentication If Eve got record Replay Attack to Needham – Schroeder 1. Eve  Bob : { Alice || ksession } kBob 2. Bob  Alice : { rand3 } ksession [intercepted by Eve] 3. Eve  Bob : { rand3 – 1 }ksession

9.2.1 Classical Cryptographic Key Exchange and Authentication Needham – Schroeder protocol with timestamp 1. Alice  Cathy : { Alice || Bob || rand1 } 2. Cathy  Alice : { Alice || Bob || rand1 || ksession || {Alice || T || ksession} kBob } kAlice 3. Alice  Bob : { Alice || T || ksession } kBob 4. Bob  Alice : { rand2 } ksession 5. Alice Bob : { rand2 – 1 }ksession

9.2.1 Classical Cryptographic Key Exchange and Authentication Otway-Rees protocol 1. Alice  Bob : num || Alice || Bob || { rand1 || num || Alice || Bob }kAlice 2. Bob  Cathy : num || Alice || Bob || { rand1 || num || Alice || Bob }kAlice || {rand2 || num || Alice || Bob }kBob 3. Cathy  Bob : num || { rand1 || ksession }kAlice || { rand2 || ksession } kBob 4. Bob  Alice : num || { rand1 || ksession }kAlice

9.2.2 Kerberos Needham – Schroeder protocol modified Denning and Sacco Alice wants to use server S Authenticate herself to Kerberos system Obtain ticket to use S

9.2.2 Kerberos TAlice,Barnum = Barnum || {Alice || Alice address || valid time || kAlice,Barnum}kBarnum TAlice,Barnum : ticket kBarnum : key Barnum shares with the authentication server valid time : the time interval during which the ticket is valid AAlice,Barnum = {Alice || generation time || kt}kAlice,Barnum AAlice,Barnum : Authenticator

9.2.2 Kerberos - Alice want Guttenbergs file using - authentication server is Cerberus - ticket granting server is Barnum 1. Alice  Cerberus: Alice || Barnum 2. Cerberus  Alice : { kAlice,Barnum} kAlice || TAlice,Barnum 3. Alice  Barnum : Guttenberg || AAlice,Barnum || TAlice,Barnum 4. Barnum  Alice : Alice || {kAlice,Guttenberg} kAlice,Barnum || TAlice,Guttenberg 5. Alice  Guttenberg : AAlice,Guttenberg || TAlice,Guttenberg 6. Guttenberg  Alice : { t + 1} kAlice,Guttenberg

9.2.3 Public Key Cryptographic Exchange and Authentication Alice  Bob : { ksession } eBob eBob is Bob’s public key

9.2.3 Public Key Cryptographic Exchange and Authentication Problem : who the message comes from 1. Alice  Peter : { send me Bob's public key } [ intercepted by Eve ] 2. Eve  Peter : { send me Bob's public key } 3. Peter  Eve : eBob 4. Eve  Alice : eEve 5. Alice  Bob : { ksession } eEve [ intercepted by Eve ] 6. Eve  Bob : { ksession } eBob Alice  Bob : Alice || { { ksession } dAlice } eBob

9.3 Cryptographic Key Infrastructures A certificate is a token that bind an identity to a cryptographic key Certificate structure Calice = {ealice || Alice || T}dCathy

9.3.1 Certificate Signature Chains X.509 : Directory Authentication Framework Components 1. Version – each version 2. Serial Number – unique certificate 3. Signature algorithm identifier – identify algorithm, parameters, used to sign certificate 4. Issuer’s Distinguished Name– unique issuer’s name 5. Validity Interval – valid time 6. Subject’s Distinguished Name – uniquely identify subject

9.3.1 Certificate Signature Chains Components 7. Subject’s public key information – identify the algorithm, parameters, subject’s public key 8. Issuer’s unique idenfier (ver 2,3 only) 9. Subject’s unique idenfier (ver 2,3 only) 10. Extensions – (version 3 only) extensions in the a 11. Sugnature – algorithm and parameters used to sign the certificate

9.3.1 Certificate Signature Chains A certification authority(CA) is an entity that issues certificates X<<Y>> : certificate that X generated for the subject Y Signature chain : Cathy<<Dan>>Dan<<Bob>>

9.3.1 Certificate Signature Chains PGP Certificate Signature Chain OpenPGP public key structure 1. Version 2. Time of creation 3. Validity period 4. public key algorithm and parameters 5. public key

9.3.1 Certificate Signature Chains OpenPGP ver.3 1. Version 3 2. Signature type 3. Creation time 4. Key identifier of the signer 5. Public key algorithm 6. Hash algorithm 7. Part of signed hash value 8. Signature Ver.4 is more complex

9.3.1 Certificate Signature Chains Difference of X.509 and PGP PGP’s single key may have multiple signature PGP have a notion of trust – single key may have different levels of trust

9.4 Storing and Revoking keys Secret keys (for classical cryptosystems) and private keys (for public key cryptosystems) must protected Expiration date keys must revoke

9.4.1 Key storage Attacker can get key on a multiuser system or network system Solution – enciphering Key Feasible Solution – Physical device More physical device

9.4.2 Key Revocation Expiration date  compromise, change It must be revoked 1. To ensure that the revocation is correct 2. to ensure timeliness of the revocation throughout the infrastructure A certificate revocation list is a list of certificates that are no longer valid

9.5 Digital Signature Digital Signature : Construct that authenticates both the origin and contents of a message in a manner that is provable to a disinterested third party Classical Signature – Private Key Public Key Signature – Public Key

9.5.1 Classical Signature Cathy is third party, and have KAlice & KBob 1. Alice  Bob : {m} KAlice 2. Bob  Cathy : {m} KAlice 3. Cathy  Bob : {m} KBob

9.5.2 Public Key Signatures It use Public Key Ex) Based on RSA system

9.5.2 Public Key Signatures Alice knows Sign m1 and m2 F(05) 05^17mod77 = 3 R(17) 17^17mod77 = 19 5*17mod77 = 8 3*19mod77 = 57 So I(08) = 57

9.5.2 Public Key Signatures (06^53mod77)^11mod95=63 Bob want the contract to be N(13) 13^r mod77 = 6 then r = 59 rebobmodnbob = 59*53mod60 = 7 Replace public key (7,77) Reset private key 43 Then, (63^59mod95)^43mod77 = 13