Lecture 16: Security CDK4: Chapter 7 CDK5: Chapter 11 TvS: Chapter 9

15-Nov-15COMP28112 Lecture 162 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 ….

15-Nov-15COMP28112 Lecture 163 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

15-Nov-15COMP28112 Lecture 164 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

15-Nov-15COMP28112 Lecture 165 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.

15-Nov-15COMP28112 Lecture 166 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

15-Nov-15COMP28112 Lecture 167 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.

15-Nov-15COMP28112 Lecture 168 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!

15-Nov-15COMP28112 Lecture 169 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

15-Nov-15COMP28112 Lecture 1610 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.

15-Nov-15COMP28112 Lecture 1611 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

15-Nov-15COMP28112 Lecture 1612 Pragmatics Better not to rely on secrecy of encryption algorithms – in practice clients and servers need to know, or even negotiate, these – rely only 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

15-Nov-15COMP28112 Lecture 1613 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

15-Nov-15COMP28112 Lecture 1614 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!

15-Nov-15COMP28112 Lecture 1615 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

15-Nov-15COMP28112 Lecture 1616 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

15-Nov-15COMP28112 Lecture 1617 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)

15-Nov-15COMP28112 Lecture 1618 Cryptography notations KAKA Alice’s secret key KBKB Bob’s secret key K AB Secret key shared between Alice and Bob K Apriv Alice’s private key (known only to Alice) K Apub Alice’s public key (published by Alice for all to read) {M} K MessageM encrypted with keyK [M]K]K MessageM signed with key K

15-Nov-15COMP28112 Lecture 1619 The Needham–Schroeder secret-key authentication protocol (7.14 in CDK) HeaderMessageNotes 1. A->S: A, B, N A A requests S to supply a key for communication with B. 2. S->A:{N A, B, K AB, {K AB, A} K B } K A S returns a message encrypted in A’s secret key, containing a newly generated key K AB and a ‘ticket’ encrypted in B’s secret key. The nonce N A 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. 3. A->B: A sends the ‘ticket’ to B. 4. B->A: B decrypts the ticket and uses the new key K AB to encrypt another nonce N B. 5. A->B: A demonstrates to B that it was the sender of the previous message by returning an agreed transformation of N B. {K AB, A} K B {N B } K AB {N B - 1} K AB

15-Nov-15COMP28112 Lecture 1620 The same also from TvS (Fig 9.17)

15-Nov-15COMP28112 Lecture 1621 Summary Whole 3 rd year course on using Cryptography in Networks …. This lecture has only scratched the surface – read Chapter 7 of CDK4 or Chapter 11 of CDK5 or Chapter 9 of TvS if you are interested.