Department of Information Engineering1 Major Concerns in Electronic Commerce Authentication –there must be proof of identity of the parties in an electronic transaction; Non-repudiation –there must be proof of agreement to the terms of transaction and prevention of denial of commitment; Integrity –there must be protection that the content of a message has not been tampered with or modified;

Department of Information Engineering2 Major Concerns in Electronic Commerce Confidentiality –there must be protection that the content and information of a transaction is kept private and secret from unauthorised third party; and Legal commitment –functions and actions of an electronic transaction must have legal backing. A digital signature must be as legally binding as a handwritten signature. The submissibility of electronic records to Law Courts must be established.

Department of Information Engineering3 E-commerce Communication on Internet is insecure –like postal service, sometimes your letter may go through places where bandits abound, and these bandits may read your confidential information modified your confidential information What can you do? –locked the letter in a box –modern day cryptography works in a similar but more interesting way

Department of Information Engineering4 Symmetric keys How to communicate in secrecy? Sender –lock the confidential message in a box Receiver –the box can only be opened by the receiver using the same key Symmetric key –the key to lock and to unlock are the same –a key is a string of numbers

Department of Information Engineering5 Encryption and Decryption What is encryption? –turns a readable text into something that is unreadable –locking What is decryption? –The reverse process of encryption –turns a unreadable text into a readable one –unlocking

Department of Information Engineering6 How By substitution (used by the Romans in Julius Caesar time) By transposition (changing the position) a b c d e x y z a b c d e x y z a b P O S I T I O N O P I S I T N O

Department of Information Engineering7 Encryption and Decryption Example encryption –a) substitution (shift by 13 letters) michael --> zvpunry –b) transposition zvpunry -->vzuprny decryption –shift 13 letters and tranpose

Department of Information Engineering8 Key distribution problem OK, you can send the messages securely, but how the problem is –you must find a secure way to send the key to the receiver in the first place !! –key distribution problem

Department of Information Engineering9 Public key cryptography Public key –a very important idea first reported in the 70s –asymmetric key one key to lock - public key one key to unlock - private key –anybody can have your public key but they can only lock only you can do the unlocking with the private key –solve the key distribution problem !!

Department of Information Engineering10 Comparison Symmetric key algorithm –fast –has key distribution problem public key algorithm –slow –solve key distribution problem in practice, secure communication is achieved by –using public key to distribute the symmetric key –then use the symmetric key to communicate

Department of Information Engineering11 Additional property of public key We have a pair of keys –public key and private key you can either use –public key to lock, private key to unlock, or –private key to lock, public key to unlock

Department of Information Engineering12 Digital signature In business world, important documents must be signed. How to sign an electronic document? A sends an e-document to B B signs (encrypts) it using his private key and return the document to A Why? Because if later B repudiates that he has signed the document, A can show the court by decrypting the encrypted document using B’s public key !!

Department of Information Engineering13 Digital signature The logic –If a document can be decrypted using B`s public key, then who encrypted the document? –MUST be B, because if a document can be decrypted a public key, then the document can only be encrypted using a private key –only B has the private key !! –So in the court, B cannot repudiate he has not signed the document

Department of Information Engineering14 Digital certificate Authentication problem –suppose you receive the public key of B on Internet, how can you be so sure that the public key is really from B? –e.g. you find a web page called –the web site claims it is a sister company of microsoft –the web has a public which you can use to secure the communication –should you use the key to send your payment ?

Department of Information Engineering15 Digital certificate You open a internet bank and receive a message from a person who called himself Li Ka Shing –Mr. Li sends you his public key, request you to transfer 100 million dollars e-cash to his account –should you follow his instruction? The type of problem is called authentication –how to certify the identity of the person/company

Department of Information Engineering16 Digital certificate digital certificate –B publicizes his address and his public key on Internet, and let this document be signed by a trusted authority - the certificate authority (CA) –CA checks B’s real identify in the traditional way (paper certificate, ID card number, …) –CA is trustworthy, and you can check whether the document is signed by CA by using CA’s public key –but how can you be sure that the public key of CA is really from CA? –The key can be certified by yet another CA, or can be published on newspaper or from other trusted source

Department of Information Engineering17 PKI What is PKI (public key infrastructure)? –PKI is an infrastructure to support e-commerce and is based on public key –to ensure the trustworthiness of public key obtained from Internet, we need the key to be certified by a CA –in Hong Kong, the Post Office is going to play the role of a CA –so if you run a company and you want people on Internet to have confidence on your public key, you should publish your key information on Internet, and let the Hong Kong Post Office to digitally sign it

Department of Information Engineering18 Integrity Although your secret code cannot be decrypted, your enemy can intercept it and change a few bits after you decrypt the code, you may get a totally different message how to prevent this to happen? –Send a message digest together with the message What is a message digest? –It is a hashed message and has very few bits (typically between bits)

Department of Information Engineering19 Message digest The simplest hashing function - remainder e.g. Let a = 1, b = 2,..., z = 26 to hash `apple` into a number between 0-9 –add up the letters (a=1, p=16, l=12, e=5) –apple = = 50 –divide it by 10, the remainder must be between 0-9 –hashed value = remainder of 50/10 = 0 –send the message `apple` together with the hash value 0 –if someone changes the a in apple to o (i.e. opple), you can detect it because the hash code is different o=15, o+p+p+l+e = 64, hash code = 4

Department of Information Engineering20 Message digest The hash function used in real-life is of course more complicated –compressed a document (no size limitation) into digits –any small change in the document (even a space) will give a different hashed value –this hashed value is called the message digest

Department of Information Engineering21 Message digest Document Agreement to purchase... message digest Document Agreement to purchase... compute the message digest senderreceiver message digest Same?

Department of Information Engineering22 Message digest Because the message digest is a unique identifier of the original document –can digitally signed the message digest (which is shorter) instead of the entire document (which is longer) for authentication –a shorter message is easier to compare visually

Department of Information Engineering23 Message digest on your name card Use of message digest on name card –public key is usually over 1000 bits long –cannot print it on name card, better get it from Internet –print the message digest of public key (call finger print) on name card instead (only 16 characters long) –download the public key, and compute the fingerprint if the computed one is same as the one on name card, then the public key is authentic

Department of Information Engineering24 Summary Issues and solutions of e-commerce –How to send your confidential data securely? use public and symmetric keys –How can you trust a public key? make sure the public key is certified by a CA –How to sign document on Internet? use digital signature –how to know whether the document has been modified? use message digest