Lecture 18 Security CDK 4 Chapter 7 CDK

Lecture 18: Security CDK 4: Chapter 7 CDK 5: Chapter 11 Tv. S: Chapter 9 COMP 28112 Lecture 18

Security in 1 machine Within 1 machine, the OS is responsible for • Verifying users’ identities • Checking access rights to shared objects Not too difficult! But a network brings new problems: • Different machines – identity mapping • Network weaknesses …. 3/8/2021 COMP 28112 Lecture 18 2

Security Problems of the Network Messages can be: • Read => loss of secrecy/privacy • Altered • Created containing forged information • Copied and replayed Can also have denial of service attacks 3/8/2021 COMP 28112 Lecture 18 3

Cryptography All the problems with messages find solutions based on • The inclusion (and checking) of extra information (e. g. sequence nos. ) and • Encryption – Using some algorithm, plaintext is converted to encrypted text 3/8/2021 COMP 28112 Lecture 18 4

Secure Channels • How to make client-server communication secure? – 1. Authentication of the communicating parties. We may need to ensure message integrity, confidentiality, etc. – 2. Authorization. Is the client authorized to have that request carried out? Relates to controlling access to the resources (granting access rights). • A secure channel (Voydock and Kent, 1983) protects senders and receivers against interception, modification and fabrication of messages. 3/8/2021 COMP 28112 Lecture 18 5

Properties of Secure Channels A secure channel connects two processes and: • Ensures each process knows reliably the identity of the principal on whose behalf the other process is acting • Each message includes a physical or logical time stamp to prevent replay or reordering 3/8/2021 COMP 28112 Lecture 18 6

Digital Signatures • Need an electronic version of a signature to authenticate a message • Unlike paper signatures, we need to stop a signature being attached to other messages by cutting and pasting! So a signature will include the message or a secure digest derived from the signed message. 3/8/2021 COMP 28112 Lecture 18 7

Secure digests • A digest is also called a secure hash function. • Need to be sure that, given the digest value, the receiver cannot invent another message which yields the same digest value! 3/8/2021 COMP 28112 Lecture 18 8

Uses of signatures There are two aspects: • Authentication – receiver knows that the message is from the signer, because it is assumed impossible to forge a signature • Non-repudiation – the sender can’t deny sending the message 3/8/2021 COMP 28112 Lecture 18 9

Keys in Cryptography Two kinds: • Shared secret keys – the major problem is how to establish a shared secret key between principals who never meet! • Public/private key pairs – every one has such a pair; everyone knows all the public keys, only the principal knows the private. Need other key to decrypt message. 3/8/2021 COMP 28112 Lecture 18 10

Public key encryption • Requires 100 – 1000 times as much processing power as secret-key algorithms • Rely on the “impossibility” of deriving the private key from the public one 3/8/2021 COMP 28112 Lecture 18 11

Pragmatics • Better not to rely on secrecy of encryption algorithms – in practice clients and servers need to know, or even negotiate, these – rely on keys • Bigger keys take longer to crack – but it is often just a matter of time, and the timescale gets shorter as attackers get more powerful computers 3/8/2021 COMP 28112 Lecture 18 12

Sending a confidential message • A -> B: Message encrypted with B’s public key • Only B can decrypt OR (more practically) • A -> B: 1 -off session key, K, encrypted with B’s public key • A -> B: Message encrypted with K 3/8/2021 COMP 28112 Lecture 18 13

Digital Signature with Public Keys • A -> B: A’s identity, and the message encrypted with A’s private key • B can decrypt using A’s public key – and the fact that this works proves that A encrypted it – as only A knows that key. • To keep the above confidential, A could encrypt the whole with B’s public key! 3/8/2021 COMP 28112 Lecture 18 14

Key distribution • Need a way to get the public keys • This needs to be secure – e. g. if an enemy intercepts the request for a key, it can reply with one it invented • On an intranet, we can design a simple protocol for an authentication server 3/8/2021 COMP 28112 Lecture 18 15

Digital Certificates • In the wider world, we need to be able to be sure that servers (and clients) are who they claim to be (and give their public keys). • A certificate is an unforgeable document issued by a more trustworthy source • Thus a certification chain is established • Revocation – hard, use expiry date 3/8/2021 COMP 28112 Lecture 18 16

Needham-Schroeder Authentication Protocol • Designed for use with an authentication server, S, which has secret keys for all principals • How to get two processes, A and B, communicating securely without revealing secret keys to each other • Used in Kerberos (within Intranets) 3/8/2021 COMP 28112 Lecture 18 17

Cryptography notations 3/8/2021 KA Alice’s secret key KB Bob’s secret key KAB Secret key shared between Alice and Bob KApriv Alice’s private key (known only to Alice) KApub Alice’s public key (published by Alice for all to read) {M}K Message M encrypted with key K [M]K Message M signed with key K COMP 28112 Lecture 18 18

The Needham–Schroeder secret-key authentication protocol (7. 14 in CDK) Header Message Notes 1. A->S: A, B, NA A requests S to supply a key for communication with B. S returns a message encrypted in A’s secret key, containing a newly generated key KAB and a ‘ticket’ encrypted in B’s secret key. The nonce NA demonstrates that the message was sent in response to the preceding one. A believes that S sent the message because only S knows A’s secret key. A sends the ‘ticket’ to B. 2. S->A: {NA , B, KAB, {KAB, A}KB}KA 3. A->B: {KAB, A}KB 4. B->A: {NB}KAB 5. A->B: {NB - 1}KAB 3/8/2021 B decrypts the ticket and uses the new key KAB to encrypt another nonce NB. A demonstrates to B that it was the sender of the previous message by returning an agreed transformation of NB. COMP 28112 Lecture 18 19

The same also from Tv. S (Fig 9. 17) 3/8/2021 COMP 28112 Lecture 18 20

Summary • Whole 3 rd year course on using Cryptography in Networks …. • This lecture has only scratched the surface – read Chapter 7 of CDK 4 or Chapter 11 of CDK 5 or Chapter 9 of Tv. S if you are interested. 3/8/2021 COMP 28112 Lecture 18 21