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Public Key Infrastructure (PKI) Jen-Chang Liu, 2004 Ref1: Ch.10, “ Cryptography and Network Security ”, Stalling, 2003. Ref2: Ch.5, “ Cryptography and.

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Presentation on theme: "Public Key Infrastructure (PKI) Jen-Chang Liu, 2004 Ref1: Ch.10, “ Cryptography and Network Security ”, Stalling, 2003. Ref2: Ch.5, “ Cryptography and."— Presentation transcript:

1 Public Key Infrastructure (PKI) Jen-Chang Liu, 2004 Ref1: Ch.10, “ Cryptography and Network Security ”, Stalling, Ref2: Ch.5, “ Cryptography and Network Security ”, A. Kahate, McGraw Hill, Ref3: Ch. 6, “ RSA Security ’ s Official Guide to Cryptography ”, 2001

2 Outline Key management in public-key cryptosystem Public Key Certificate (PKC) X.509 standard Public Key Infrastructure (PKI)

3 Key management (Ref1) Two issues for public-key cryptosystem Distribution of public keys The use of public-key encryption to distribute secret keys (keys for symmetric cipher) Distribution of public keys Public announcement Public available directory Public-key authority Public-key certificates

4 1. Public announcement Drawback: the opponent can pretend to be another user Ex. post public keys to public forums, such as USENET newsgroup and Internet mailing list

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6 2. Public available directory Some trusted entity maintains a publicly available dynamic directory of public keys Register the public key Register the public key {A, KU a } {B, KU b } … Attack: an opponent invades the public-key directory, and counterfeit public keys

7 3. Public-key authority (Fig 10.3) Central authority: 1. Maintain directory of public keys 2. Each participant knows the public key for the authority A can confirm the message from the authority N 1 : 認證 B 的身份 N 2 : 認證 A 的身份

8 Outline Key management in public-key cryptosystem Q: How to authenticate the association between the public key with the owner ? Public Key Certificate (PKC) X.509 standard Public Key Infrastructure (PKI)

9 Public key certificate (PKC 公開金 鑰憑證 ) A public key certificate signifies the association between my public key and me Ex. Like a driver license or passport Q: Who can approve the association ? A: A trusted entity – Certificate Authority (CA) Q: What is the content of a digital certificate? A: X.509 standard

10 Example: Digital certificate

11 X.509 certificate format 1988, ITU X.509 version 1

12 X.509 V2 extensions: unique identifier V2 extensions: Issuer unique identifier Subject unique identifier Motivation: Deal with the possibility that the issuer (CA’s name) name and the subject name (certificate holder’s name) might be duplicated over time RFC2459 specifies that these two names should never be reused, so V2 extensions are made optional

13 X. 509 V3 certificate extensions Certificate Revocation List (CRL 憑證廢止列 ) Certificate policies Authority key identifier: CA may have multiple private-public key pairs. This field defines which of these key pairs is used to sign the certificate Key usage: 1. digital signature 2. Certificate signing 3. CRL signing 4. Key enciphering 5. Data enciphering 6. Diffie-Hellman key exchange

14 Certificate Authority (CA 憑證簽發機構 ) CA is a trusted agency that can issue digital certificate. Ex. VeriSign, Entrust, …

15 Outline Key management in public-key cryptosystem Public Key Certificate (PKC) X.509 standard Public Key Infrastructure (PKI) PKI components Certification creation steps Certificate hierarchies Certificate revocation * Distribution of public keys is non-trivial

16 PKI components The interaction between PKI components End user Registration authority (RA) CA Key recovery server X.500 directory

17 Registration authority (RA) RA: an intermediate entity between the end users and the CA Share the workload of CA Accept and verify registration info about new users Generate keys on behalf of the end users Accept and authorize requests for key backups and recovery Accept and authorize requests for certificate revocation RA does not generate certificate CA becomes an isolated entity, which makes it less susceptible to security attacks

18 Key recovery server Q: End users lose their private keys? A: CA must revoke the corresponding PKC, a new key pair must be generated, a new corresponding PKC must be created A2: provide a key recovery server CA backs up private keys at the time of creation

19 Certificate directory Q: where to store the certificates? A1: end user stores on his local machine A2: CAs use a certificate directory (or a central storage location) Provide a single point for certificate administration and distribution (ex. for later certificate revocation) Certificate directories need not to be trusted

20 Outline Key management in public-key cryptosystem Public Key Certificate (PKC) X.509 standard Public Key Infrastructure (PKI) PKI components Certification creation steps (Ref2) Certificate hierarchies Certificate revocation

21 Certificate creation steps Key generation Registration Verification Certificate creation

22 1. Subject generating his own key pair 2. RA generating a key pair for subject 1.RA knows private key! 2.How to transmit it to user?

23 Registration ( 註冊、登錄 ) Certificate signing request (CSR) (PKCS#10, part of the Public Key Cryptography Standard)

24 On-line registration example

25 Verification 1. RA verify the user ’ s credentials 2. Check the Proof of Possession of the private key Q: What if a user claims that she never possessed the private key, when a document signed with her private key causes legal problems? Sol 1: RA demands user to sign her CSR Sol 2: RA generates a random number, encrypt it with the user’s public key, then challenge the user …

26 Certificate creation CA creates a digital certificate for the user Certificates in X.509 standard format Q: Why should we trust digital certificates? Certificate goes to RA (or user) Certificate directory Backup user private key (if necessary)

27 Questions about certificate Why should we trust digital certificate? Similar to: how do we verify a passport? How does the CA sign a digital certificate? How can we verify a digital certificate?

28 X.509 certificate format

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30 Question about CA ’ s public key How do we get CA’s public key of some certificate ? Get CA’s certificate – which approve the association between the public key with CA Who signs CA’s certificate? The organization of CAs CA hierarchies and self-signed certificate Cross-certification

31 CA hierarchy Purpose: root CA can delegate job to lower CAs Chain of trust

32 Self-signed certificate for root CA Who signs for root CA? 1.Root CA is automatic considered as trusted CA 2. Software contains a pre-programmed, hard coded certificated of the root CA 3. The root CA signs its own certificate (self-signed certificate)

33 Example: Self-signed root certificate

34 Cross-certification Root CAs in different countries

35 Outline Key management in public-key cryptosystem Public Key Certificate (PKC) X.509 standard Public Key Infrastructure (PKI) PKI components Certification creation steps (Ref2) Certificate hierarchies Certificate revocation

36 Certificate revocation 憑證廢止 Ex. lost of credit card, driver license, … Reasons for certification revocation: The private key is compromised The CA made mistakes while issuing a certificate The certificate holder leaves a job, … Before using a certificate, we check Does the certificate belong to the owner? (check certificate signature) Is the certificate valid, or is it revoked?

37 How to revoke a certificate? Certificate has been issued, how to revoke it?

38 Certificate revocation list (CRL) CRL is a list of revoked certificates published regularly by CA

39 Validating a certificate using CRL

40 Problems with CRL 1. CRL can be a large file -> long transmission time Sol: delta CRL 2. CRL are published Periodically => can not check online status Sol: online certificate status check

41 Online Certificate Status Protocol (HTTP) CA setup this server

42 Key management (Ref1) Two issues for public-key cryptosystem Distribution of public keys The use of public-key encryption to distribute secret keys (keys for symmetric cipher) Distribution of public keys Public announcement Public available directory Public-key authority Public-key certificates

43 4. Public-key certificates 憑證 (Fig 10.4) Certificate: contain public key and other information, generate from the certificate authority Application must be in person or by secure channel 1. Anyone can read, verify 2. Only CA can create Time: verify currency of certificate

44 Simple secret key distribution Public-key scheme has slow data rate  use public key to distribute secret key  use secret key scheme for data encryption session key (secret key) E KU e || ID A intercept KU e [ K s ] E KU a [ K s ] KsKs

45 Secret key distribution with confidentiality and authentication Against active and passive attacks Authenticate B Authenticate A Confidentiality (only B can read) authentication (only A can create it)

46 A hybrid and hierarchical scheme KDC A B C Use public-key scheme to distribute master key MK A MK B Use master keys with KDC to distribute session key KsKs KsKs Advantage: 1.Use master key to distribute session keys, instead of using public-key scheme => faster ! 2.Backward compatible to old KDC scheme (master + session key)


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