# CUNSHENG DING HKUST, Hong Kong. Computer Security. Computer Security. Cunsheng DING, HKUST COMP4631

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1 Cunsheng DING, HKUST

2 Lecture 08: Key Management for One-key Ciphers Topics of this Lecture 1. The generation and distribution of secret keys. 2. A key distribution protocol with a key distribution center. 3. The Diffie-Hellman key exchange protocol. Page 1

3 Secret Key Generation Question: How to generate a secret key for a one-key cipher? Answer: It depends on the specific cryptosystem. Case I: The secret key k is a binary string k 1 k 2 k n, where k i are independent of each other. Solution 1: If n is not long, say 128, flipping a coin n times. Solution 2: Use a pseudorandom number generator. Case II: Key bits must satisfy certain relations. In this case, no general approach exits. It differs from system to system. Page 2

4 Key Distribution: Necessity For conventional encryption, the two parties must share the same key. The key must be protected from access by others. The key should be changed regularly (an adversary or enemy may learn the key in some way). Key distribution: delivering a key to both parties, without allowing others to see the key. Key agreement: agreeing on a key by parties involved, without allowing others to see the key. Page 3

5 Key Distribution: some General Approaches A selects a key, and physically delivers it to B. A third party can select the key and and physically deliver it to both A and B. If A and B have previously and recently used a key, one party can transmit the new key to the other, encrypted using the old key. If A and B each has an encrypted connection to a third party C, C can deliver a key on the encrypted links to A and B. Page 4

6 Key Distribution: more General Approaches Secret key distribution using a public key cipher. (It will be introduced later.) Other key distribution protocols. Remark: As an example of protocols for key distribution, we introduce a key distribution protocol using a key distribution center. Page 5

7 A Key Distribution Protocol Parties involved: A key distribution center (KDC), a group of people to communicate with each other. Requirements: Whenever A wants to communicate with B, the KDC should generate a temporary key (called session key) and distribute it to A and B. Both confidentiality and authenticity must be achieved. Remark: The session key (temporary key) is established only for this communications between A and B. Page 6

8 A Key Distribution Protocol Continued Building blocks needed: The KDC and all parties involved in this communication system use a one-key block cipher. The KDC and each party A share a secret key k a, which is called a master key. Remark: The master keys are used to protect the sessions keys when they are distributed. Page 7

9 A Key Distribution Protocol Continued Pictorial description of use of the key hierarchy: master keys non-cryptographic protection session keys cryptographic protection data cryptographic protection Page 8

10 A Key Distribution Protocol (1) request N 1 key distribution center (KDC) (2) E ka [k request N 1 E kb (k ID A )] Initiator A (3) E kb [k ID A ] (4) E k [N 2 ID B ] Responder B (5) E k [N 2 ] Page 9

11 Parameters in the Key Distribution Protocol N i is a nonce, used as identifier for that transaction. k a, k b master keys, k secret key. Question: Which steps are for authentication? Question: Does it provide mutual authentication or authentication in one direction? Page 10

12 Explaining the Key Distribution Protocol (1) The nonce may be a timestamp, a counter, or a random number. The minimum requirement is that it differs with each request. Also it should be hard for an opponent to guess it. So random number is a good choice. When A receives (2), A can verify that its original request was not altered before reception by the KDC. Because of the nonce, that is not a reply of some previous request. The message (2) also includes two items intended for B: the one-time session key k, and an identifier of A (i.e., its network address), ID A. Page 11

13 Explaining the Key Distribution Protocol (2) After Step (3), a session key has been securely delivered to A and B. They may begin their protected exchange. Steps (4) and (5) assure B that the original message received in Step (3) was not a replay of an earlier one by a third person. Question: Why? Steps (4), (5) and (3) are for authentication. Page 12

14 Discrete logarithms Primitive roots: Let p be a prime. An integer α is called a primitive root of p if each nonzero element a F p can be uniquely expressed as a = α i mod p for some integer i, where 0 i p 2. Discrete logarithm: The exponent i is referred to as the discrete logarithm, or index, of a for the base α, and is denoted log α a or ind α (a). Discrete logarithm problem: Given p, α, and a, find log α a. This is in general very hard. Brute force solution: compute b = α i mod p for all i, 0 i p 2 and check if b = a. Page 13

15 Primitive roots Example: 2 is a primitive root of the prime 11. Also we have log 2 (6) = 9. i i mod Theorem: Every prime p has at least one primitive root. Page 14

16 To find primitive roots Rule of tumb: Most primes p have a small primitive root. For example, for the primes less than , approximately 37.5% have 2 as a primitive root, and approximately 87.4% have a primitive root of value 7 or less. For primes of reasonable size, many programming languages for mathematics have commands for finding primitive roots. Page 15

17 Diffie-Hellman Key Exchange Protocol User A User B Generate random X A < p calculate Y A = α X A mod p Y A Generate random X B < p Calculate Y B = α X B mod p Calculate k = (Y B ) X A mod p Y B Calculate k = (Y A ) X B mod p Page 16

18 Diffie-Hellman Key Exchange Protocol It is for two users to exchange a key securely that can then be used for subsequent encryption of message. k = α X AX B mod p. Also p and α are publicly known. But X A and X B must be kept secret. The security with respect to passive attacks is based on the belief that solving the discrete logarithm problem is hard in general. It is vulnerable to an active attack if an adversary has control over the communication cannel. Page 17

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