Garantía y Seguridad en Sistemas y Redes
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1 Garantía y Seguridad en Sistemas y Redes Tema 2. Cryptographic Tools Esteban Stafford Departamento de Ingeniería Informá2ca y Electrónica Este tema se publica bajo Licencia: Crea2ve Commons BY- NC- SA 4.0
2 Contents Basics of Cryptography Confidentiality with Symmetric Encryption Message Authentication and Hash Functions Public-ey Cryptography Digital Signatures and ey Management Practical Application: SSL/TLS 1
3 What can cryptography do? Confidentiality Integrity Availability Authenticity Accountability key key Plaintext Input X E Y=E(,X) D X=D(,Y) Plaintext Output 2
4 Caesar Cypher Named after Julius Caesar, although used long before. Representative of the substitution cypher algorithms. The encryption process shifts each letter a number of places. (key) Easily attackable by brute force. 3
5 Substitution Cypher The key is a substitution table. The encryption changes each letter by the corresponding in the table. Infeasible to break by brute force. Breakable by cryptanalysis. 4 Probability Substitution Cypher a b c d e f g h i j k l mn o p q r s t u vwx y z Working examples of old cyphers simonsingh.net/the_black_chamber/chamberguide.html 5
6 Substitution Cypher The key is a substitution table. The encryption changes each letter by the corresponding in the table. Infeasible to break by brute force. Breakable by cryptanalysis. 4 Probability Substitution Cypher ab c de f gh i j k l mno pq r s t u vwx y z Working examples of old cyphers 5
7 Vernam cipher Commonly known as One-time-pad. Proven to be unbreakable (Go Shannon!) ey must be as long as plain text and shared by peers. ey must me truly random and can be used only once. 6
8 Randomness Cryptography relies heavily on random numbers. Quality of random numbers Uniform distribution Independence Pseudo-random numbers Mathematical modular expression derives number from the last. Good distribution and price, but bad independence. True-random numbers Monitor random or chaotic physical process. Good distribution and independence, but expensive. 7
9 Symmetric Encryption key key Plaintext Input X Y=E(,X) X=D(,Y) E D Plaintext Output How robust is this? Strong encryption algorithm. Cryptanalysis: Mathematically derive X and/or from Y. Or simplify brute-force. eys subject to brute-force attacks Sender and receiver must obtain keys in secure fashion. 8
10 Data Encryption Standard (DES) 64 bit blocks, 56 bit keys. Adopted NIST 1977 Cryptanalysis not yet successful. 1997: call for replacement proposals. July 1998: $250,000 machine cracks DES in 56h. Advanced Encryption Standard (AES) 128 bit blocks, 128,192,256 bit keys. Adopted 2001 Years to break (2700 checks/µs) AES256 AES196 3DES168 AES128 3DES112 DES ey length(bits) 9
11 Message Authentication with Symmetric Encryption No confidentiality. Protection against falsification, alteration, delay or replay. Message must contain metadata: error-detection code, sequence number, timestamp. Message Message Transmit Message M = M MAC Message Authentication Code (MAC) Last bits of DES encrypted message. 10
12 One-way Hash Functions Easy x H H(x) Infeasible x: unlimited size, H(x): fixed length. Preimage resistant: guess x from H(x). 2 nd preimage resistant: find y x such that H(x)=H(y). Collision resistant: find any x and y such that H(x)=H(y). Hash function crisis: RIPMED-160, SHA-256 and SHA
13 Message Authentication with One-way Hash Functions One-way Hash Functions are faster than Symmetric Encryption Using shared secret value (). Message Message Transmit Message H H = Secure Hash Algorithm(SHA): SHA bit long, used but weaker than expected. Currently 256, 384, 512 bit long. Use in password storage and intrusion detection. 12
14 Public-ey Encryption Proposed by Diffie and Hellman in eys are created in pairs. One reverses the other. Bob sends confidential message to Alice. PU a PR a Plaintext Input X Y=E(PU a,x) X=D(PR a,y) E D Plaintext Output Bob sends authentic message to Alice. PR b PU b Plaintext Input X Y=E(PU b,x) X=D(PR b,y) E D Plaintext Output 13
15 Public-ey Cryptography Myths: Public-key encryption is more secure than Symmetric-key encryption. Public-key encryption makes symmetric obsolete. Public-key distribution is easier than Symmetric-key distributions. Implementations: Rivest, Shamir and Adleman (RSA) Proposed in Digital Signature Algorithm (DSA) Proposed in Stronger than RSA. Faster signing, but slower verifying. Elliptic Curve Cryptography (ECC) Stronger than RSA. Allows shorter keys. 14
16 Digital Signature Messages can be signed by encrypting MAC with private key. Message Message Transmit Message H PR b PU b = H E D 15
17 Distribution of public keys Certificate Authority(CA) signs public keys (Certificate) Bob s ID Bob s Pub. ey H H PR CA CA info PU CA = E Certificate D Who certifies the CA? Chicken-and-egg problem. Root CAs distribute their public keys self-signed. Applications and OS ship with common root certificates. 16
18 Certificate Chain of Trust CA CRL Root CA Document Bob s Cert CA Info Bob s ID CA Cert Root Info CA s ID Root Cert Users can recursively validate signatures to the root CA. Certificate Revocation Lists(CRL) enumerate invalid certs. 17
19 SSL/TLS Handshake Supported algorithms Random number Verify certificate Extract server public key Encrypt pre-master key with server public key Compute symmetric master key Send checksum of handshake messages with master key Verify checksum Selected algorithm Random number Server certificate Decrypt pre-master key with server private key Compute symmetric master key Verify checksum Send checksum of handshake messages with master key 18
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