What is cryptology Greek krypto hide

What is cryptology? • Greek: “krypto” = hide • Cryptology – science of hiding = cryptography + cryptanalysis + steganography • Cryptography – secret writing • Cryptanalysis – analyzing (breaking) secrets Cryptanalysis is what attacker does Decipher or Decryption is what legitimate receiver does 30 Aug 2000 University of Virginia CS 551

Steganography • “Covered” messages • Technical Steganography – Invisible ink, shaved heads, microdots • Linguistic Steganography – “Open code” – secret message appears innocent • “East wind rain” = war with USA • Broken dolls in WWII – Hide message in low-order bits in GIF 30 Aug 2000 University of Virginia CS 551

Cryptology and Security Cryptology is a branch of mathematics. Security is about people. 30 Aug 2000 University of Virginia CS 551

Terminology Insecure Channel Plaintext Alice 30 Aug 2000 Encrypt Ciphertext Decrypt Eve C = E(P) P = D(C) E must be invertible University of Virginia CS 551 Plaintext Bob

Cryptography • Always involves 2 things: – Transformation – Secret 30 Aug 2000 University of Virginia CS 551

Kerckhoff’s Principle • Security should depend only on the key – Don’t assume enemy won’t know algorithm • Can capture machines, disassemble programs, etc. • Too expensive to invent new algorithm if it might have been compromised – Security through obscurity isn’t • Look at history of examples • Better to have scrutiny by open experts “The enemy knows the system being used. ” (Claude Shannon) 30 Aug 2000 University of Virginia CS 551

Alice and Bob Plaintext Encrypt Ciphertext Decrypt KE KD Alice C = E(KE, P) = EKE (P) P = D(KD, C) = DKD (C) If KE = KD it is symmetric encryption If KE KD it is asymmetric encryption 30 Aug 2000 Plaintext University of Virginia CS 551 Bob

Substitution Cipher • C = EK(p) Ci = K[pi] • Key is alphabet mapping: a J, b L, . . . • Suppose attacker knows algorithm but not key, how many keys to try? 26! If every person on earth tried one per second, it would take 5 B years. 30 Aug 2000 University of Virginia CS 551

Monoalphabetic Cipher “XBW HGQW XS ACFPSUWG FWPGWXF CF AWWKZV CDQGJCDWA CD BHYJD DJXHGW; WUWD XBW ZWJFX PHGCSHF YCDA CF GSHFWA LV XBW KGSYCFW SI FBJGCDQ RDSOZWAQW OCXBBWZA IGSY SXBWGF. ” 30 Aug 2000 University of Virginia CS 551

Frequency Analysis “XBW HGQW XS ACFPSUWG FWPGWXF CF AWWKZV CDQGJCDWA CD BHYJD DJXHGW; WUWD XBW ZWJFX PHGCSHF YCDA CF GSHFWA LV XBW KGSYCFW SI FBJGCDQ RDSOZWAQW OCXBBWZA IGSY SXBWGF. ” W: 20 C: 11 F: 11 G: 11 30 Aug 2000 “Normal” English: e 12% t 9% a 8% University of Virginia CS 551

Pattern Analysis “XBe HGQe XS ACFPSUe. G Fe. PGe. XF CF Aee. KZV CDQGJCDe. A CD BHYJD DJXHGe; e. Ue. D XBe Ze. JFX PHGCSHF YCDA CF GSHFe. A LV XBe KGSYCFe SI FBJGCDQ RDSOZe. AQe OCXBBe. ZA IGSY SXBe. GF. ” XBe = “the” Most common trigrams in English: the = 6. 4% and = 3. 4% 30 Aug 2000 University of Virginia CS 551

Guessing “the HGQe t. S ACFPSUe. G Fe. PGet. F CF Aee. KZV CDQGJCDe. A CD h. HYJD DJt. HGe; e. Ue. D the Ze. JFt PHGCSHF YCDA CF GSHFe. A LV the KGSYCFe SI Fh. JGCDQ RDSOZe. AQe OCthhe. ZA IGSY Sthe. GF. ” S = “o” 30 Aug 2000 University of Virginia CS 551

Guessing “the HGQe to ACFPo. Ue. G Fe. PGet. F CF Aee. KZV CDQGJCDe. A CD h. HYJD DJt. HGe; e. Ue. D the Ze. JFt PHGCo. HF YCDA CF Go. HFe. A LV the KGo. YCFe o. I Fh. JGCDQ RDo. OZe. AQe OCthhe. ZA IGo. Y othe. GF. ” othe. GF = “others” 30 Aug 2000 University of Virginia CS 551

Guessing “the Hr. Qe to ACs. Po. Uer se. Prets Cs Aee. KZV CDQr. JCDe. A CD h. HYJD DJt. Hre; e. Ue. D the Ze. Jst PHr. Co. Hs YCDA Cs ro. Hse. A LV the Kro. YCse o. I sh. Jr. CDQ RDo. OZe. AQe OCthhe. ZA Iro. Y others. ” “se. Prets” = “secrets” 30 Aug 2000 University of Virginia CS 551

Guessing “the Hr. Qe to ACsco. Uer secrets Cs Aee. KZV CDQr. JCDe. A CD h. HYJD DJt. Hre; e. Ue. D the Ze. Jst c. Hr. Co. Hs YCDA Cs ro. Hse. A LV the Kro. YCse o. I sh. Jr. CDQ RDo. OZe. AQe OCthhe. ZA Iro. Y others. ” “ACsco. Uer” = “discover” 30 Aug 2000 University of Virginia CS 551

Guessing “the Hr. Qe to discover secrets is dee. KZV i. DQr. Ji. Ded i. D h. HYJD DJt. Hre; eve. D the Ze. Jst c. Hrio. Hs Yi. Dd is ro. Hsed LV the Kro. Yise o. I sh. Jri. DQ RDo. OZed. Qe Oithhe. Zd Iro. Y others. ” 30 Aug 2000 University of Virginia CS 551

Monoalphabetic Cipher “The urge to discover secrets is deeply ingrained in human nature; even the least curious mind is roused by the promise of sharing knowledge withheld from others. ” - John Chadwick, The Decipherment of Linear B 30 Aug 2000 University of Virginia CS 551

Why was it so easy? • Doesn’t hide statistical properties of plaintext • Doesn’t hide relationships in plaintext (EE cannot match dg) • English (and all natural languages) are very redundant: about 1. 3 bits of information per letter – Compress English with gzip – about 1: 6 30 Aug 2000 University of Virginia CS 551

How to make it harder? • Cosmetic • Hide statistical properties: – Encrypt “e” with 12 different symbols, “t” with 9 different symbols, etc. – Add nulls, remove spaces • Polyalphbetic cipher – Use different substitutions • Transposition – Scramble order of letters 30 Aug 2000 University of Virginia CS 551

Types of Attacks • Ciphertext-only - How much Ciphertext? • Known Plaintext - often “Guessed Plaintext” • Chosen Plaintext (get ciphertext) – Not as uncommon as it sounds! • • Chosen Ciphertext (get plaintext) Not recommended in CS 551 Dumpster Diving Social Engineering “Rubber-hose cryptanalysis” – Cryptanalyst uses threats, blackmail, torture, bribery to get the key. 30 Aug 2000 University of Virginia CS 551

Really Brief History First 4000 years Vigenère Babbage breaks Vigenère; Kasiski (1863) publishes Cryptographers Alberti – first polyalphabetic cipher monoalphabetics Cryptanalysts 3000 BC 30 Aug 2000 al-Kindi - frequency analysis 900 1460 University of Virginia CS 551 1854

Really Brief History Last 100 years Quantum Crypto Mauborgne – one-time pad ? Linear, Differential Cryptanalysis Enigma adds rotors, stops repeated key Feistel block cipher, DES Turing’s loop attacks, Colossus Rejewski repeated message-key attack Cryptanalysts Mechanical ciphers - Enigma Cryptographers 1854 30 Aug 2000 1918 1939 1945 1973 University of Virginia CS 551 Public-Key

Themes 1 • Arms race between cryptographers and cryptanalysts – But, often disconnect between two (e. g. , Mary Queen of Scots uses monoalphabetic cipher long after known breakable) • Multi-disciplinary field – Linguists, classicists, mathematicians, computer scientists, physicists • Secrecy often means advances rediscovered and miscredited 30 Aug 2000 University of Virginia CS 551

Themes 2 • Dominated by needs of government: war is the great catalyst • Cryptanalysis advances led by most threatened countries: – France (1800 s), Poland (1930 s), England/US (WWII), Israel? (Today) 30 Aug 2000 University of Virginia CS 551

Security vs. Pragmatics • Trade-off between security and effort – one-time pad: perfect security, but requires distribution and secrecy of long key – DES: short key, fast algorithm, but breakable – quantum cryptography: perfect security, guaranteed secrecy of key, slow, requires expensive hardware • Don’t spend $10 M to protect $1 M. • Don’t protect $1 B with encryption that can be broken for $1 M. 30 Aug 2000 University of Virginia CS 551

Perfectly Secure Cipher: One-Time Pad • Mauborgne/Vernam [1917] • XOR ( ): 0 0=0 1 0=1 0 1=1 1 1=0 a a=0 a 0=a a b b=a • E(P, K) = P K D(C, K) = C K = (P K) K = P 30 Aug 2000 University of Virginia CS 551

Why perfectly secure? • For any given ciphertext, all plaintexts are equally possible. Ciphertext: 0100111110101 Key 1: 1100000100110 Plaintext 1: 1000111010011 = “CS” Key 2: 1100010100110 Plaintext 2: 1000101010011 = “BS” • More formal proof next time 30 Aug 2000 University of Virginia CS 551

Go to the beach? • Cannot reuse K – What if receiver has C 1 = P 1 K and C 2 = P 2 K C 1 C 2 = P 1 K P 2 K = P 1 P 2 • Need to generate truly random bit sequence as long as all messages • Need to securely distribute key 30 Aug 2000 University of Virginia CS 551

Summary • Fate of humanity depends on this course. • Meaning of: plaintext, ciphertext, key, encrypt, decrypt, cryptanalyze, steganography • Kinds of attacks on cryptosystems • Kerckhoff’s Principle • Monoalphabetic Cipher – How to cryptanalyze • One-Time Pad – Why its perfectly secure in theory – Why its not used often in practice 30 Aug 2000 University of Virginia CS 551