Introduction to PKI, Certificates & Public Key Cryptography

1. Introduction to PKI,Certificates& Public Key Cryptography Erwan Lemonnier

2. Introduction to PKI, Certificates & Public Key Cryptography – erwan@defcom.com Role of Computer Security • CIA • Confidentiality: protection against data disclosure • Integrity: protection against data modification • Availability: protection against data disponibility • Identification & Authentication (I&A) • Provide a way of identifying entities, and controlling this identity • Non-repudiability • Bind an entity to its actions

3. Introduction to PKI, Certificates & Public Key Cryptography – erwan@defcom.com How to implement CIA, I&A, N-R ? With Cryptography ! • Main cryptographic tools: • Hash Functions • Secret Key Cryptography • Public Key Cryptography • And their combinations: • Certificates • PKI

4. Introduction to PKI, Certificates & Public Key Cryptography – erwan@defcom.com Main cryptographic tools • Hash Functions: • Bind one entity with a unique ID => Signature • Hash + Encryption => trusted signature • Symmetric Key Cryptography • 2 users share a secret key S and • an algorithm. • S(S(M)) = M • Problem: • how to exchange secret keys ? • =>Secret Key Server(ex: kerberos)

5. Introduction to PKI, Certificates & Public Key Cryptography – erwan@defcom.com Main cryptographic tools • Public Key Cryptography: • Each user has a public key P and a private key S, and an algorithm A. • P(S(M)) = S(P(M)) = M • No shared secret ! Encryption with Public Key Crypto Authentication with Public Key Crypto

6. Introduction to PKI, Certificates & Public Key Cryptography – erwan@defcom.com Main cryptographic tools, PKI • How to distribute public keys ? • Public Key Server (PKS), key exchange protocols Public Key Infrastructure (PKI): PKI = N x (Entities with private keys) + public key exchange system REM: Public Key algorithms are slow • Need to use both Public & Secret Key Cryptography • Public Key Protocols work in 3 phases • Authentication via Public Key Cryptography (challenge) • Exchange of a session Secret Key, encrypted with Public Key Crypto • Session encrypted with Symmetric Cryptography

7. an entity’s description (name, etc.) + entity’s public key + expiration date, serial number, etc. + CA’s name + a signature issued by a CA Certificate = Introduction to PKI, Certificates & Public Key Cryptography – erwan@defcom.com Certificate • A certificate binds an entity with its public key. • It’s just a digitally signed piece of data. • digital ID card The certificate is issued and signed by a trusted Certificate Authority (CA) • Digital signature: • CA signature = certificate hash, • encrypted with CA’s private key

8. Introduction to PKI, Certificates & Public Key Cryptography – erwan@defcom.com Certificate • The certificate’s CA is the only entity able to create/modify the certificate • the CA has to be trusted • Certificates enable: • Clients to authenticate servers • Servers to authenticate clients • Public key exchange without Public Key Server • No disclosure of private/secret keys. Certificates are usually stored encrypted. • Special features: • chains of CAs, to distribute the task of issuing Certificates • Certificate Revocation List, to disable certificates

9. Introduction to PKI, Certificates & Public Key Cryptography – erwan@defcom.com Usual cryptographic algorithms & infrastructures Hash: MD4, MD5, SHA-1 Symmetric Key: DES, 3DES, AES (Rijnael), IDEA, RC4 Public/Private Key: RSA, Diffie-Hellman Certificat: X509 PKI: IPSec, SSL, (kerberos)

10. Introduction to PKI, Certificates & Public Key Cryptography – erwan@defcom.com example: IPSec • IPSec works at IP level. • Provide authentication and encryption. Used to build VPNs. • Configuration: • 2 transfert modes: tunnel or transport • 2 transfert protocols: • AH (Authentication Header) => authenticated traffic • ESP (Encapsulating Security Payload) => encrypted traffic • Key exchange protocols: • Internet Key Exchange (IKE), • Internet Security Association and Key Management Protocol (ISAKMP), • etc.

11. Introduction to PKI, Certificates & Public Key Cryptography – erwan@defcom.com Weaknesses of PKI and Certificates • PKI: • unsecured server: hackable Public Key/Certificate servers • unsecured client: private keys/passwords can be stolen/spied • weak algorithm: short keys, implementation or design breach • Certificate: • unsecured computer: certificates can be stolen, password spied • certificate password: certificates are stored encrypted, with weak password • untrustable CA: easy to be issued a certificate from a CA • users: they seldom check if CA can be trusted before accepting certificates (netscape GUI) • Attack example: • hack client’s computer, steal certificate & password • man in the middle

12. Introduction to PKI, Certificates & Public Key Cryptography – erwan@defcom.com Links Book: Applied cryptography, Bruce Schneier URLs: theory.lcs.mit.edu/~rivest/crypto-security.html www.counterpane.com/pki-risks.html www.csc.gatech.edu/~copeland/8813/slides/ www.iplanet.com/developer/docs/articles/security/pki.html web.mit.edu/6.857/OldStuff/Fall96/www/main.html