22-NetSec2 - Network Security Issues, Part 2 EE 122-...

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Unformatted text preview: Network Security Issues, Part 2 EE 122- Networking Prof. Vern Paxson November 16, 2011 Game Plan • Address any pending questions regarding Monday’s lecture on network security threats • Introduce basic tools of cryptography – A lot of abstraction! • Will go light on a few details – Ask questions if properties not clear • Discuss building secure end-to-end channels – TLS/SSL ⇒ HTTPS Revisiting Types of Security Goals • Attacks can subvert each type of goal – Confidentiality: eavesdropping / theft of information – Integrity: altering data, manipulating execution (e.g., code injection) – Availability: denial-of-service • Attackers can also combine different types of attacks towards an overarching goal – E.g. use eavesdropping (confidentiality) to construct a spoofing attack (integrity) that tells a server to drop an important connection (denial-of-service) Building Secure End-to-End Channels • End-to-end = communication protections achieved all the way from originating client to intended server – With no need to trust network intermediaries • Dealing with threats: – Eavesdropping? • Encryption (symmetric key; public key) – Manipulation (injection, MITM )? • Integrity (use of a MAC ) – Impersonation? • Signatures What’s missing? Availability … ( ) Cryptographic Toolkit: Encryption • Encryption: algorithms that encode information in a form undecipherable unless one possesses the corresponding key • Suppose Alice wants to send a message M to Bob , such that an eavesdropper, Eve , can’t read it • Alice sends Bob a function E(M) computed from M • Bob then applies an inverse function D(E(M)) = M – Eve knows D, E but not key ⇒ can’t recover M from E(M) • Two basic types of such functions: – Symmetric key: same key K used to encrypt & decrypt. – Asymmetric or public-key : one key, K E , used to encrypt. A different (but related) key, K D , used to decrypt. Symmetric-Key Encryption • Symmetric key: the same key K used to encrypt and decrypt. – Alice sends E(M, K) to Bob . – Bob applies D(E(M, K), K) = M. – Eavesdropper Eve observes E(M, K), knows E & D, but can’t figure out how to get M from it unless she knows K • Requirements: – Alice and Bob have to agree on K in advance – K must be kept secret • Example algorithms: AES, DES , 3DES – Typical key lengths today: 128 or 256 bits – Encryption & decryption can be computed very efficiently Cryptographic Toolkit: Integrity • Suppose Alice wants to make sure her message M sent to Bob isn’t altered by a Man-in-the-Middle (MITM) along the way • Alice computes and also sends a Message Authentication Code (MAC) using a separate key shared between Alice & Bob – I.e., Alice sends both E(M, K 1 ) and MAC(M, K 2 ) • Upon receipt, Bob verifies that MAC(D(E(M, K 1 ), K 1 ), K 2 ) = MAC(M, K 2 ) • MITM doesn’t know K 2 , so can’t construct valid MAC to match their altered message – If MITM alters message, Bob detects this & discards Asymmetric /...
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This note was uploaded on 01/09/2012 for the course ELECTRICAL 122 taught by Professor Shenker during the Spring '11 term at Berkeley.

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22-NetSec2 - Network Security Issues, Part 2 EE 122-...

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