Cryptography Introduction

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Clyde Kelley
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1 Cryptgraphy Intrductin Last Updated: Sep 7, 2017

2 Cryptgraphy Literal definitin means hidden writing Until mdern times, cryptgraphy was synnymus with encryptin, but the field has expanded This lecture reviews a high-level descriptin f fur cryptgraphic primitives we will learn abut this semester Symmetric Encryptin (AES) Public-Key Cryptgraphy (RSA) Secure One-Way Hash (SHA-256) Message Authenticatin Cde (MAC)

3 What is Encryptin? Transfrming infrmatin s that its true meaning is hidden Requires special knwledge t retrieve Mdern encryptin algrithms use transpsitin and substitutin in cmplex ways that are hard t reverse Examples frm histry that are easy t break ROT-13 (aka Caesar Cipher) is easy t break, simple substitutin cipher Vigenere cipher plyalphabetic substitutin cipher Examples f strng encryptin AES 3DES RC4 RSA

4 Types f Encryptin Schemes Cipher Classical Rtr Machines Mdern Substitutin Stegangraphy Transpsitin Public Key Secret Key GOOD DOG PLLX XLP PLSX TWF GOOD DOG DGOGDOO Hierarchy & Examples based n:

5 Perfect Encryptin Scheme? One-Time Pad (XOR message with key) Example*: Message: ONETIMEPAD Key: TBFRGFARFM Ciphertext: IPKLPSFHGQ The key TBFRGFARFM decrypts the message t ONETIMEPAD The key POYYAEAAZX decrypts the message t SALMONEGGS The key BXFGBMTMXM decrypts the message t GREENFLUID *Frm Applied Cryptgraphy (Schneier)

6 Symmetric Encryptin Als knwn as Cnventinal encryptin Secret-key encryptin Single-key encryptin

7 Symmetric Encryptin Mdel Alice Bb Key Key Plaintext Ciphertext Plaintext Encryptin Algrithm Decryptin Algrithm

8 Use Cases Web brwsing using HTTPS Encrypted chat (WhatsApp and Signal) Encrypted (PGP)

9 Cryptgraphic Hash Functin Surce:

10 Hashing Use Cases Digital signature File integrity verificatin (TripWire) Passwrd hashing Rtkit detectin

11 Message Authenticatin Cde (MAC) Surce: Netwrk Security Essentials (Stallings)

12 HMAC Use Cases Web brwser message integrity (HTTPS) Integrity f messages in authenticatin prtcls Ckie integrity Web applicatin remte prcedure calls

13 Mdel fr Encryptin with Public Key Cryptgraphy Alice Bb s Public Key Bb Bb s Private Key Plaintext Ciphertext Plaintext Encryptin Algrithm Decryptin Algrithm

14 Mdel fr Digital Signature with Public Key Cryptgraphy Alice Bb Alice s Private Key Alice s Public Key Plaintext Signing Algrithm Verificatin Algrithm? Plaintext

15 Symmetric Encryptin

16 Requirements 1. Strng algrithm (cipher) Attacker is unable t decrypt ciphertext r discver the key even if attacker has samples f ciphertext/plaintext created using the secret key 2. Fast Assumptin: Sender and receiver must securely btain and stre the secret key

17 Kerckhffs Principle The security f the symmetric encryptin depends n the secrecy f the key, nt the secrecy f the algrithm Dr. Auguste Kerckhffs ( ) Dutch linguist and cryptgrapher

18 Types f Ciphers Blck cipher (3DES, AES) Plaintext is brken up int fixed-size blcks Typical blck size (64, 128 bits) Stream cipher (RC4) Prcess plaintext cntinuusly Usually ne byte at a time

19 What can g wrng? Algrithm Relying n the secrecy f the algrithm Example: Substitutin ciphers Using an algrithm incrrectly Example: WEP used RC4 incrrectly Key T big Slw Strage T small Vulnerable t brute frce attack try all pssible keys

20 Big Numbers Cryptgraphy uses REALLY big numbers 1 in 2 61 dds f winning the ltt and being hit by lightning n the same day 2 92 atms in the average human bdy pssible keys in a 128-bit key atms in the planet atms in the sun atms in the galaxy pssible keys in a 256-bit key

Thermdynamic Limitatins* Physics: T set r clear a bit requires n less than kt k is the Bltzman cnstant (1.38*10-16 erg/ºk) T is the abslute temperature f the system Assuming T = 3.

21 Thermdynamic Limitatins* Physics: T set r clear a bit requires n less than kt k is the Bltzman cnstant (1.38*10-16 erg/ºk) T is the abslute temperature f the system Assuming T = 3.2ºK (ambient temperature f universe) kt = 4.4*10-16 ergs Annual energy utput f the sun 1.21*10 41 ergs Enugh t cycle thrugh a 187-bit cunter Build a Dysn sphere arund the sun and cllect all energy fr 32 years Enugh energy t cycle thrugh a 192-bit cunter. Supernva prduces in the neighbrhd f ergs Enugh t cycle thrugh a 219-bit cunter *Frm Applied Cryptgraphy (Schneier)

22 Public Key Cryptgraphy Terminlgy Public Key Private Key Digital Signature Cnfidentiality Yu encrypt with a public key, and yu decrypt with a private key Integrity/Authenticatin Yu sign with a private key, and yu verify the signature with the crrespnding public key Examples Diffie-Hellman RSA Elliptic Curve Cryptgraphy (ECC) Identity-based Encryptin (IBE)

23 Assignment Review this slide deck regularly t learn the high level abstractins fr these primitives. I ll expect yu t describe them t me n an exam withut any ntes. Yu dn t really knw smething until yu can teach it t smene else Study the AES NIST spec and watch the flash dem

Cryptography Introduction

Cryptography Introduction Last Updated: Aug 20, 2013 Terminology Access Control o Authentication Assurance that entities are who they claim to be o Authorization Assurance that entities have permission