Strong Cryptographic Infrastructure and its Applications Dr Lucas Hui Center for Information Security & Cryptography Department of Computer Science & Information Systems The University of Hong Kong Tel: Fax: Workshop on Strong Cryptographic Infrastructure December 17, 1998

Content 1. Use of Cryptography (in electronic commerce activities, Internet services) 2. Cryptographic Library 3. Public Key Infrastructure 4. Applications 5. Cryptographic Infrastructure 6. Relations of 2,3,4,5 7. The SCI project in HKUCSIS

Use of Cryptography (in E.C. etc) Hash functions (SHA, MD5) Symmetric key crypto-system (DES, DES3) Public Key Crypto-system –Digital signature –Data Encryption –Advanced usage : double hashing, group signature In real usage, techniques are combined

Public Key Crypto-system A has public key Apub, & private key Aprv From Apub, almost impossible to find Aprv Apub is known to all; Aprv is secret to A

Digital Signature using Public Key Crypto-systems A sends a signed message M to B –A sends Aprv(M) to B, B decrypts with Apub

Data Encryption using Public Key Crypto-systems A sends a confidential message M to B –A sends Bpub(M) to B, B decrypts with Bprv

Cryptographic Library Provide cryptographic algorithms such as RSA Provide interface to add new cryptographic algorithms easily Provide other functions Q : How to set up/manage the private/public keys? A : Using a Public Key Infrastructure

Problem with Pub Key distribution A talks to B, Hacker H attacks as follows: –To A, H pretends B. To B, H pretends A –H sees secrets between A and B, and can modify the messages

Solution to Pub Key distribution Need : when B gives Bpub to A, a trusted third party (Certification Authority, CA) is needed to endorse Bpub is correct

Certification Authority A wants to get B’s public key Bpub. How? Method 1 : use a repository Method 2 : B gives Bpub to A, which is endorsed by a trusted-third-party, the CA (Certification Authority). This is B’s public key certificate BCert, which is Bpub signed by CA’s private key CAprv CA’s public key, CApub, is known to all A use CApub to verify that BCert is correct

X.509 Public Key Infrastructure (PKIX) Set up, manage, and terminate usage of keys (private/public), & public key certificates –Registration & Initialization –Certification (signing of a certificate) –Key pair recovery –Key Generation, Update –Cross-certification –Key Revocation (managing CRLs) Note: Make use of Cryptographic functions

Cross Certification

Cross-Certification 2 CAs: CA1, CA2, & 2 persons: A, B. CA2 issues a public key Cert BCert to B (Bpub signed by CA2’s private key) CA1 issues a Cross-Cert, XCert, to CA2 (CA2’s public key signed by CA1’s private key) A trusts CA1 (A knows CA1’s public key) B sends BCert and XCert to A A can now verify B’s public key in 2 steps.

Applications Examples: E. Commerce, E. Banking, Secure , Secure Workflow (in schools, etc) Using a transaction protocol which makes use of cryptographic algorithms from a Cryptographic Library Use PKIX for subject (customers, etc) identity and encryption key management

Cryptographic Infrastructure

Strong Cryptographic Infrastructure Project (SCI) in HKUCSIS What does Strong mean? –Algorithms are “Strong” (e.g. RSA-1024) –“Strong” in implementation (e.g. Random No.) –New encryption paradigm (elliptic curves) supported by ISF available to users in HK Start with Strong Cryptographic Library (SCL) Beta version expected in March 1999

SCI project team Dr Lucas Hui (Chief Designer) Dr K.P. Chow (Project Manager) Dr W.W. Tsang, Prof Francis Chin, Prof G. Marsaglia Dr C.F. Chong, Dr H.W. Chan Ms Vivien Chan, Mr Marcus Lee, Mr K.M. Chan Mr Doug Kwan, Mr Luke Lam, Mr Henry Fung, Ms Taellus Lo