Digital Signatures - E-Commerce - Lecture Slides, Slides of Fundamentals of E-Commerce

Download Digital Signatures - E-Commerce - Lecture Slides and more Slides Fundamentals of E-Commerce in PDF only on Docsity! 4-1Electronic Commerce (WS-02/03) 4. B2C,B2E Systems: Concepts and Architectures 4.1 Business-to-Consumer Systems Architectures and Components Shop Functionalities, Selected Components 4.2 Electronic Fulfillment & Payment Secure Communication, Security and Trust Encryption: Standards, Authentication: Digital Signatures, Certification Authorities Electronic Payment Models, Standards and Systems 4.3 Mobile E-Commerce and Location-Based Services Docsity.com 4-2Electronic Commerce (WS-02/03) ECommerce Reference Model Base Technologies (Internet-, Communication-, Security-, DB-, Software-Technology) Forms of Payment Security, Trust Transact. Control Agent Technlgy Mediation, Negotiation EDI Political and Legal Aspects of EC Business Process Reengineering (BPR) Tools Electronic Trading Systems (Shop Systems) Virtual Organizations Kinds of Cooperation Applications for horizontal and vertical sectors Technical issues Organi- zational issues Docsity.com 4-5Electronic Commerce (WS-02/03) Digital Signatures with Secret Keys M Signing Verifying H(M+K) h h'H(M+K) h h = h'? K M signed doc M K MACs: Message Authentication Codes digest See: MD5, SHA Docsity.com 4-6Electronic Commerce (WS-02/03) Performance of encryption and secure digest algorithms Key size/hash size (bits) Extrapolated speed (kbytes/sec.) PRB optimized (kbytes/s) TEA 128 700 - DES 56 350 7746 Triple-DES 112 120 2842 IDEA 128 700 4469 RSA 512 7 - RSA 2048 1 - MD5 128 1740 62425 SHA 160 750 25162 Docsity.com 4-7Electronic Commerce (WS-02/03) Problem: How to distribute keys? Key distribution by non-networked means is impractical for distributed object systems Key distribution is a problem for both secret and public keys oSecret keys: Obvious oPublic keys: How do we know the principal that gives us a public key is who we assume the principal is? Use trusted key distribution service and secure key distribution protocol! Docsity.com 4-10Electronic Commerce (WS-02/03) Needham/Schroeder Protocol Key Distribution Server AS Client C Server S 2: {NC,S,KCS,{KCS,C}KS}KC 3: {KCS,C}KS 4: {NS}KCS 5: {NS-1}KCS 1: C, S, NC For Secret Keys: Docsity.com 4-11Electronic Commerce (WS-02/03) Needham/Schroeder Protocol For Public Keys: Key Distribution Server AS Client C Server S 2: {KPubS, S}KCAS 3: {NC,C}KPubS 6: {NC,NS}KPubC 7: {NS}KPubS 1: C, S 4: S,C 5: {KPubC, C}KSAS Docsity.com 4-12Electronic Commerce (WS-02/03) Secure Socket Layer (SSL) Secure Transport between Browser and Web-Server Also used for object-oriented middleware Based on RSA public key technology Client generates secret session key Client uses public key of server to encrypt session key and transmit it to the server Session key is used to encrypt any communication between client and server Docsity.com 4-15Electronic Commerce (WS-02/03) DigiCash Application of Blind Signatures (1) Application of blind signatures in DigiCash (ECash): Blinding process example: o The bank client (information creator role) wants to have 1 € as digital cash. o He creates n (10.000.000) digital cash units of the value 1 € with random serial numbers. He encodes them and sends them to the bank. The bank (authenticator role) picks n-1 (9.999.999) at random, verifies the denomination and deletes them. o One cash unit remains (Note: the bank does not know its serial number). o Note: Checking the denomination reveals the serial number. It requires a decryption step by the user, thus the user knows which cash units are being verified. o The probability is very low that the remaining cash unit is not a 1 €, but a 1.000.000.000 € cash unit (if the client tried to commit fraud). o The bank signs this cash unit, and thus validates it to make it a digital coin. The coin is returned to the user who decodes it. The user has 1 € (very high probability) or 1.000.000.000 € (very low probability). See figure on next slide. Docsity.com 4-16Electronic Commerce (WS-02/03) DigiCash Application of Blind Signatures (2) Internet 1 encode decode fX6 1 fX611111111 fX6fX6fX6fX6fX6fX6fX6fX6fX6 verify n-1 for correct denomination sign 1 fi make it valid Bank customer Bank Docsity.com 4-17Electronic Commerce (WS-02/03) Authentication: Fair Blind Signature Problem: The intraceability of anonymous electronic cash has problems: For example, criminals could obtain a ransom for a kidnapping or launder money without yielding a trace of identity. Solution: Unblinding the blind signature when needed. Blind signatures that can reveal the identity of the holder of the signature are known as fair blind signatures. Link-and-Recovery Fair Blind Signatures (obtain information about who spent the money) o Involves three roles: cash user, signer (bank) and a third trusted entity (judge or certification authority). o When needed, the cash signing protocol enables the trusted entity to reveal the sender of electronic cash (details omitted, see literature). Trustee-Based Fair Blind Signatures (obtain information about user spending habits) o The user provides trustees with information that allow the trustees to recognise the electronic notes of the user. Each of the trustees receives a part of this information that makes the electronic spending of the user traceable. o When needed, the spending pattern of the user can be revealed by putting together the trustees’ individual information. Problem: If trustees collude (get together), they can build up a user’s spending profile. Docsity.com