Software Security Seminar - 1 Chapter 2. Protocol Building Blocks 발표자 : 최두호 Applied Cryptography

Software Security Seminar - 2 Contents 2.1 Introduction to Protocols 2.2 Communications using Symmetric Cryptography 2.3 On-Way Functions 2.4 One-Way Hash Functions 2.5 Communications using Public-Key Cryptography

Software Security Seminar - 3 What is Protocols? A protocol is a series of steps involving two or more parties : -series of steps : has a sequence, from start to finish -two or more parties : required at least two people to complete the protocol -each step involves at least one of two things : computation bu one or more of the parties messages sent among the parties A cryptographic protocol is a protocol that uses cryptography.

Software Security Seminar - 4 The purpose of protocols 전화로 물건 구매 포커 게임 투표 컴퓨터상에서 구현 Anybody on computer network may be dishonest! Daily life

Software Security Seminar - 5 Alice : First participant in all the protocols Bob : Second participant in all the protocols Carol : Participant in the three- and four-party protocols Dave : Participant in the four-party protocols Eve : Eavesdropper Mallory : Malicious active attacker Trent : Trusted arbitrator Walter : Warden, he’ll be guarding Alice and Bob in some protocols Peggy : Prover Victor : Verifier The Players

Software Security Seminar - 6 An arbitrator : third party trusted to complete a protocol(ex. lawyer) Example (1) Alice gives the title to the lawyer. (2) Bob gives the check to Alice. (3) Alice deposits the check. (4) After waiting a specified time period for the check to clear, the lawyer gives the title to Bob. If the check does not clear within the specified time period, Alice shows proof of this to the lawyer and the lawyer returns the title to Alice. Arbitrated Protocols

Software Security Seminar - 7 Problems with computer arbitrators : - faceless arbitrator - the cost of maintaining an arbitrator - delay in any arbitrated protocol - bottleneck : needs the number of arbitrators -> increase the cost - vulnerable point for an attacker, since everyone on the network must trust the arbitrator Arbitrated Protocols(conti.)

Software Security Seminar - 8 Arbitrated protocols can be subdivided into two lower- level subprotocols : - a nonarbitrated subprotocol - an arbitrated subprotocol : excuted only when there is a dispute Adjudicated Protocols

Software Security Seminar - 9 The best type of protocol The protocol itself guarantees fairness : no arbitrator, no adjudicator Self-Enforcing Protocols

Software Security Seminar - 10 Against cryptographic algorithms used in protocols Against cryptographic techniques used to implement the algorithms and protocols Against the protocols themselves -Passive Attack : eavesdrop, the attacker does not affect the protocol ( player Eve) -Active Attack : introduce new messages in the protocol, delete existing message, interrupt a communication channel, alter stored information(player Mallory) Attacks against Protocols

Software Security Seminar - 11 Cheater : an attacker who is one of the parties involved in the protocol -Passive cheaters follow the protocol but try to obtain more information -Active cheaters disrupt the protocol in progress in an attempt to cheat Attacks against Protocols(conti.)

Software Security Seminar Introduction to Protocols 2.2 Communications using Symmetric Cryptography 2.3 On-Way Functions 2.4 One-Way Hash Functions 2.5 Communications using Public-Key Cryptography

Software Security Seminar - 13 (1) Alice and Bob agree on a cryptosystem. (2) Alice and Bob agree on a key. (3) Alice takes her plaintext message and encrypts it using the encryption algorithm and the key. This creates a ciphertext message. (4) Alice sends the ciphertext message to Bob. (5) Bob decrypts the ciphertext message with the same algorithm and key and reads it. Basic Scenario

Software Security Seminar - 14 Keys must be distributed in secret If a key is compromised, Eve can decrypt all message traffic and she can pretend to be one of the parties Key management : A network n users requires n(n-1)/2 Problems

Software Security Seminar Introduction to Protocols 2.2 Communications using Symmetric Cryptography 2.3 On-Way Functions 2.4 One-Way Hash Functions 2.5 Communications using Public-Key Cryptography

Software Security Seminar - 16 One-way function is relatively easy to compute but significantly harder to revere Given x, it is easy to compute f(x) but given f(x), it is hard to compute x Example : smashing a plate It can not be used for encryption Definition

Software Security Seminar - 17 A special type of one-way functioin It is easy to compute f(x) for given x, hard to compute x for given f(x) but if you know the secret y, you can easily compute x for given f(x), y Example : A watch It can be used public-key cryptography A trapdoor one-way function

Software Security Seminar Introduction to Protocols 2.2 Communications using Symmetric Cryptography 2.3 On-Way Functions 2.4 One-Way Hash Functions 2.5 Communications using Public-Key Cryptography

Software Security Seminar - 19 A one-way hash function : compression function, contraction function, message digest, fingerprint, cryptographic checksum, message integrity check(MIC), manipulation detection code(MDC) Input : string with arbitrary length, output : string with fixed length -> many to one function Example : byte XOR Collision-free : It is hard to find x,x’ such that f(x)=f(x’) One-way Hash functions

Software Security Seminar - 20 Data Authentication Code(DAC) A one-way hash function with the addition of a secret key : f(x,k) = y, k : a key MACs can make by using a hash function or a block encryption algorithm Message Authentication Codes(MACs)

Software Security Seminar Introduction to Protocols 2.2 Communications using Symmetric Cryptography 2.3 On-Way Functions 2.4 One-Way Hash Functions 2.5 Communications using Public-Key Cryptography

Software Security Seminar , Diffie-Hellman (1) Alice and Bob agree on a public-key cryptosystem. (2) Bob sends Alice his public key. (3) Alice encrypts her message using Bob’s public key and sends it to Bob. (4) Bob decrypts Alice’s message using his private key. It solves the key-management problem. Concept

Software Security Seminar - 23 Content Encryption : Symmetric-Key Cryptosystem Sending Secret Key : Public-Key Cryptosystem Problem -Public-key algorithms are slow -Vulnerable to chosen plaintext attacks Hybrid Cryptosystems