CSE/EE 461 Lecture 23

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1 CSE/EE 461 Lecture 23 Network Security David Wetherall Last Time Naming Application Presentation How do we name hosts etc.? Session Transport Network Domain Name System (DNS) Data Link /URLs Physical Focus Topics djw // CSE/EE 461, Winter 2003 L23.2 1

2 This Time Network security Application Presentation How do we secure distributed systems? Session Transport Network Privacy, integrity, authenticity Data Link Cryptography Physical Focus Topics djw // CSE/EE 461, Winter 2003 L23.3 What do we mean by Security? Networks are fundamentally shared Need means to protect messages sent by legitimate participants from others with access to the network Privacy: messages can t be eavesdropped Integrity: messages can t be tampered with Authenticity: messages were sent by the right party These are in addition to the need to protect networked systems from intrusions and compromise by attackers djw // CSE/EE 461, Winter 2003 L23.4 2

3 Approaches at 10,000 ft Physical security Tackle the problem of sharing directly Security through obscurity Hope no-one will find out what you re doing! Throw math at the problem Cryptography Why is security difficult? It s a negative goal: can you be sure there are no flaws? Often assumptions turn out to be invalid, esp. randomness djw // CSE/EE 461, Winter 2003 L23.5 Basic Encryption for Privacy Sender (M) Receiver (M) Encrypt E(M,K E ) Ciphertext (C) Decrypt D(C, K D ) Cryptographer chooses functions E, D and keys K E, K D Mathematical basis Cryptanalyst try to break the system Depends on what is known: E and D, M and C? djw // CSE/EE 461, Winter 2003 L23.6 3

4 Secret Key Functions (DES, IDEA) Encrypt with secret key Ciphertext Decrypt with secret key Single key (symmetric) is shared between parties Often chosen randomly, but must be communicated djw // CSE/EE 461, Winter 2003 L23.7 Basics of DES Initial permutation Each Round: L i 1 R i 1 Round 1 F K i Round 2 56-bit key L i R i Round 16 Final permutation DES uses a 64 bit key (56 8) Message encrypted 64 bits at a time 16 rounds in the encryption Each round scrambles 64 bits djw // CSE/EE 461, Winter 2003 L23.8 4

5 DES (cont.) Block 1 Block 2 Block 3 Block 4 IV DES DES DES DES Cipher 1 Cipher 2 Cipher 3 Cipher 4 Repeat process for larger messages with chaining djw // CSE/EE 461, Winter 2003 L23.9 Public Key Functions (RSA) Encrypt with public key Ciphertext Decrypt with private key Public and private key related mathematically Public key can be published; private is a secret djw // CSE/EE 461, Winter 2003 L

6 Authentication Protocols Three-way handshake for mutual authentication Client and server share secrets, e.g., login password Client Server Client authenticates server here Session key exchanged ClientId, E(x, CHK) E(x 1, SHK), E(y, SHK) E(y 1, CHK) E(SK, SHK) Server authenticates client here djw // CSE/EE 461, Winter 2003 L23.11 Authenticity and Integrity Sometimes we care about knowing messages authentic, but don t care about privacy. If only sender and receiver knew the keys we would be done but that s often not the case A pair of keys for each pair of communicating parties? In public key (RSA) systems the encryption key is potentially known by everyone anyone could have sent us a confidential message by encrypting with our public key djw // CSE/EE 461, Winter 2003 L

7 RSA Digital Signature Encrypt with PRIVATE key Ciphertext Decrypt with PUBLIC key Notice that we reversed the role of the keys (and the math just works out) so only one party can send the message but anyone can check it s authenticity djw // CSE/EE 461, Winter 2003 L23.13 A Faster RSA Signature Encryption can be expensive, e.g., RSA 1Kbps To speed up, let s sign just the checksum instead! Check that the encrypted bit is a signature of the checksum Problem: Easy to alter data without altering checksum Answer: Cryptographically strong checksums called message digests where it s computationally difficult to choose data with a given checksum But they still run much more quickly than encryption MD5 (128 bits) is the most common example djw // CSE/EE 461, Winter 2003 L

8 Message Digests (MD5, SHA) Act as a cryptographic checksum or hash Typically small compared to message (MD5 128 bits) One-way : infeasible to find two messages with same digest Initial digest Transform Message (padded) 512 bits 512 bits 512 bits Transform Transform Message digest djw // CSE/EE 461, Winter 2003 L23.15 Cryptography in Protocols These techniques can be applied at different levels: IP packets (IPSEC) Web transfers or other transports (SSL/TLS, Secure HTTP) (PGP) Next time.. djw // CSE/EE 461, Winter 2003 L

9 Key Concepts Privacy, integrity, and authenticity Cryptographic mechanisms are used to support these properties: private key, public key and digests djw // CSE/EE 461, Winter 2003 L

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