Computer Security: Crypto & Web Security
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1 CSE 484 / CSE M 584 Computer Security: Crypto & Web Security TA: Thomas Crosley tcrosley@cs Many slides by Franziska Roesner and Adrian Sham
2
3 Lab 1 Deadline Reminders Lab 1 Final due tomorrow! (4/29, 8pm). Upcoming office hours: Friday 2:00pm Kevin (CSE 021)
4 Today Crypto Summary RSA Summary Cer\ficate Authori\es Security Best Prac\ces
5 Cryptography Summary Goal: Privacy Symmetric keys: One-\me pad, Stream ciphers Block ciphers (e.g., DES, AES) à modes: EBC, CBC, CTR Public key crypto (e.g., Diffie-Hellman, RSA) Goal: Integrity MACs, oben using hash func\ons (e.g, MD5, SHA-256) Goal: Privacy and Integrity Encrypt-then-MAC Goal: Authen\city (and Integrity) Digital signatures (e.g., RSA, DSS)
6 RSA Summary Key genera\on Generate large primes p, q (and keep them private) Say, 1024 bits each (need primality tes\ng, too) Compute n = pq and ϕ(n) = (p-1)(q-1) Choose small e, rela\vely prime to ϕ(n) Compute unique d such that ed 1 mod ϕ(n) Public key = (e,n); private key = (d,n) Encryp\on of m: c m e mod n m must be, 0 <= m < n Modular exponen\a\on by repeated squaring Decryp\on of c: c d mod n = (m e ) d mod n = m
7 Sample RSA Decryp\on p=3, q=11, n=33, e=7, d=3 A-1 B-2 C-3 D-4 E-5 F-6 G-7 H-8 I-9 J-10 K-11 L-12 M-13 N-14 O-15 P-16 Q-17 R-18 S-19 T-20 U-21 V-22 W-23 X-24 Y-25 Z-26
8 Sample RSA Decryp\on How to compute d? Recall: ed 1 mod ϕ(n) (where ϕ(n) = (p-1)(q-1)) So d is inverse of e mod ϕ(n). How to compute modular inverse? Use extended Euclidean algorithm or Wolfram Alpha J Note that this is hard if you don t know ϕ(n) (i.e., can t factor n).
9 Cer\ficates
10 CA Ecosystem Source: hvp://conferences.sigcomm.org/imc/2013/papers/imc257-durumericaemb.pdf
11 [Sotirov et al. Rogue Certificates ] Colliding Cer\ficates set by the CA serial number validity period real cert domain name chosen prefix (difference) serial number validity period rogue cert domain name real cert RSA key Valid for both certificates! Hash to the same MD5 value! collision bits (computed)??? X.509 extensions signature identical bytes (copied from real cert) X.509 extensions signature 4/28/16 CSE 484 / CSE M Spring
12 Problem With Collisions Goal: Snape wants to trick Dumbledore into accept a document B from Harry that is different than document A that Harry actually signed Snape creates 2 documents A and B that have iden\cal hash value (collision!) Snape sends document A to Harry, who signs the hash and gives a signature to Snape Snape avaches that signature to document B and sends it to Dumbledore Dumbledore accepts it because the signatures match
13 More Rogue Certs In Jan 2013, a rogue *.google.com certificate was issued by an intermediate CA that gained its authority from the Turkish root CA TurkTrust TurkTrust accidentally issued intermediate CA certs to customers who requested regular certificates Ankara transit authority used its certificate to issue a fake *.google.com certificate in order to filter SSL traffic from its network This rogue *.google.com certificate was trusted by every browser in the world 4/28/16 CSE 484 / CSE M Spring
14 What is Prevy Good Privacy (PGP) hvp://lifehacker.com/how-to-encrypt-your- -and-keep-your-conversa\ons-p
15 Alterna\ve: Web of Trust Used in PGP (Prevy Good Privacy) Instead of a single root cer\ficate authority, each person has a set of keys they trust If public-key cer\ficate is signed by one of the trusted keys, the public key contained in it will be deemed valid Trust can be transi\ve Can use cer\fied keys for further cer\fica\on sig Alice ( Friend, Friend s key) sig Friend ( FoaF, FoaF s key) I trust Alice Alice Friend of Alice Friend of friend Bob
16 KeyBase Connect people s social media iden\\es to their public cryptographic keys. hvps://medium.com/@cdixon/keybase-bringing-public-key-cryptography-to-mainstreamusers-16a9379dddda#.klwu6rt36
17
18 Security Best Prac\ces
19 Ad and Social Media Blocking Benefits Can block malicious content from ads Faster loading pages Reduce bandwidth Privacy Cons Allows sobware to directly modify page False posi\ves Economic consequences for online businesses Social Widget Blocket: hvps://addons.mozilla.org/en-us/firefox/addon/sharemenot/
20 Password Managers Helps prevent reuse of passwords One ring master password to rule them all! Many op\ons available: LastPass: CloudBased password manager KeePass: Desktop applica\on Images : hvp://
21 Last Pass
22 Last Pass
23 KeePass
24 2 Factor Authen\ca\on Passwords may not be enough 2FA provices iden\fica\on of users by means of the combina\on of two different components (such as password and phone) List of sites that support 2FA: hvps://twofactorauth.org/
25 Using your phone
26 Hardware tokens
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Cryptography [Symmetric Encryption]
![Cryptography [Symmetric Encryption]](/thumbs/89/100564864.jpg "Cryptography [Symmetric Encryption]")
CSE 484 / CSE M 584: Computer Security and Privacy Cryptography [Symmetric Encryption] Spring 2017 Franziska (Franzi) Roesner franzi@cs.washington.edu Thanks to Dan Boneh, Dieter Gollmann, Dan Halperin,