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CNS2010lecture 3 :: cyphers1 ELEC5616 computer and network security matt barrie

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1 CNS2010lecture 3 :: cyphers1 ELEC5616 computer and network security matt barrie

2 CNS2010lecture 3 :: cyphers2 cryptography Cryptography is the study of mathematical techniques related to the design of cyphers. It is one example of many mechanisms that make up security: –Cryptography –Signature / Pattern Matching –Access Control –Statistical Profiling –Traffic Security –Countermeasures –Software Security –Operating System Security –Tamper Resistance

3 CNS2010lecture 3 :: cyphers3 cryptography The fundamental application of cryptography is in facilitating secure communications over an insecure channel. How can Alice send a message to Bob over an insecure channel with Eve listening in? Eve is an active attacker and may tap, insert or modify messages in transit. How does one use cryptography to provide security such as: –Authentication ? –Confidentiality ? –Integrity ? –Non-repudiation ? AliceBob Eve

4 CNS2010lecture 3 :: cyphers4 symmetric cyphers The traditional way of achieving this is through private key encryption. This is also known as symmetric encryption as the key used to encrypt and decrypt messages is the same. A symmetric cypher is one defined by the rule: D k (E k (m)) = m E Message m Secret Key k C = E k (m) D Secret Key k Original Message m EncryptionDecryption

5 CNS2010lecture 3 :: cyphers5 communication with symmetric cyphers Alice and Bob share an encryption algorithm E k, a decryption algorithm D k, and a secret key, k. Alice wants to send Bob a message, m. The unencrypted message m is known as either the plaintext or the cleartext. Alice encrypts m by computing the cyphertext c = E k (m) and sends it to Bob. Bob decrypts c by computing D k (c) = m to retrieve the original plaintext message m.

6 CNS2010lecture 3 :: cyphers6 It is computationally hard to decrypt c without the secret key, k. The secret key k is usually a large number (≥ 64 bits). The range of possible values of k is called the key space K –For 64 bit keys the keyspace would be ( ) The range of possible messages is the message space M. A cryptosystem is a system consisting of an algorithm, plus all possible plaintexts, cyphertexts and keys. symmetric cryptosystem AliceBob c = E k (m) Secret Key k Eve Doesn’t know k Can’t decrypt!

7 CNS2010lecture 3 :: cyphers7 types of symmetric cyphers Stream cyphers operating on a single bit (or byte) at a time. Block cyphers operating on blocks (numbers of bits) of plaintext at a time. ED PlaintextCyphertext Original Plaintext key

8 CNS2010lecture 3 :: cyphers8 cryptanalysis We always assume that attackers have –Complete access to the communications channel –Complete knowledge about the cryptosystem Secrecy must exist completely within the key There are five major attack models: Cyphertext-only attack (COA) –Attacker only has access to the cyphertext –Given : c 1 = E k (m 1 ), c 2 = E k (m 2 ), …, c n = E n (m n ) –Find any of :m 1, m 2, m n, k, or an algorithm to infer m n+1 from c n+1

9 CNS2010lecture 3 :: cyphers9 cryptanalysis - attack models Known-plaintext attack (KPA) –Attacker intercepts a random plaintext / cyphertext pair –Given : m 1, c 1 = E k (m 1 ), …, c n = E n (m n ) –Find any of :Either k or an algorithm to infer m n+1 from c n+1 Chosen-plaintext attack (CPA) –Attacker chooses a message, m 1, and gets the cyphertext. –Stronger than KPA (some cyphers resistant to KPA are not resistant to CPA) –Given : m 1, c 1 = E k (m 1 ), …, c n = E n (m n ) where attacker chooses m 1 –Find any of :Either k or an algorithm to infer m n+1 from c n+1

10 CNS2010lecture 3 :: cyphers10 cryptanalysis - attack models Chosen-cyphertext attack (CCA) –Attacker specifies a cyphertext, C, and gets the plaintext. –Given : c 1, m 1 = D k (c 1 ), …, m n = D n (c n ) where attacker chooses c 1 –Find any of :k Rubber-hose attack (RHA) –The cryptanalyst breaks knuckles, blackmails, threatens or tortures someone until they cough up the key. –Sometimes known as a purchase-key attack. –Extremely powerful. –Usually the easiest way to break a cryptosystem. –Not permissible for Wargames.

11 CNS2010lecture 3 :: cyphers11 attack examples Known-plaintext attack: –An attacker knowing that source code is being encrypted (the first bytes most likely to be #include, copyright notices etc.) –Famous break of the Japanese PURPLE cypher in WWII A complex cypher used to protect high level communications The allies had already broken several of the Japanese diplomatic cyphers PURPLE was used to protect communications, but the Japanese could only afford to build and deploy 12 cypher machines They sent these to the twelve most important embassies Some messages needed to be broadcast to all embassies So some messages had to be sent using old cyphers the US had already broken! Chosen-plaintext attack: –Feed intelligence to an ambassador with goal that it is encrypted and sent home

12 CNS2010lecture 3 :: cyphers12 classes of break Worst to least severe: Total break –Attacker finds secret key k and hence can compute all D k (c) Global deduction –Attacker finds alternate algorithm A equivalent to all D k (c), without finding k Local deduction (or instance deduction) –Attacker finds the plaintext of one intercepted cyphertext Information deduction –Attacker gains some information about the key or plaintext, e.g. a few bits or the meaning of a message.

13 CNS2010lecture 3 :: cyphers13 attack metrics An algorithm is unconditionally secure, if no matter how much cyphertext an attacker has, there is not enough information to deduce the plaintext. Information security is a resource game; attacks are measured in terms of: –Data requirements how much data is necessary to succeed? –Processing requirements (or work factor) how much time is needed to perform the attack? –Memory requirements how much storage space is required? –Computational cost Dollar-seconds

14 CNS2010lecture 3 :: cyphers14 substitution cyphers Substitution cyphers are the oldest form of cypher The secret key consists of a table which maps letter substitutions between plaintext and cyphertext. Most famous is the Caesar cypher where each letter is shifted by 3 (modulo 26): abcdefghijklmnopqrstuvwxyz DEFGHIJKLMNOPQRSTUVWXYZABC –“A” becomes “D” –“B” becomes “E” Similar to this is ROT13 which shifts the plaintext 13 places, so encrypting twice results in the plaintext: ROT13(ROT13(m)) = m

15 CNS2010lecture 3 :: cyphers15 substitution cyphers There are 26! (factorial) possible keys (~4 x large!) Monoalphabetic (single character) substitution cypher: Substitution cyphers are easy to break using frequency analysis of the letters (a cyphertext-only attack) –single letters –digraphs (groups of two letters) –trigraphs (three letters) abcdefghijklmnopqrstuvwxyz keyXNYAHPOGZQWBTSFLRCVMUEKJDI plaintextTHISCOURSEROCKSTHEBLOCK cyphertextMGZVYFUCVHCFYWVMGHNBFYW

16 CNS2010lecture 3 :: cyphers16 substitution cyphers Substitution cyphers are easy to break using frequency analysis of the letters (a cyphertext-only attack) Letter countLetter Count English DictionaryCyphertext a 18924n a …?n b 3311 o bo c 7852 p 5150 cp d 9491 q 179 dq e r er f 5176 s fs g 4480 t gt h u 5815 hu i v 2613 iv j 255 w 3899 jw k 1193 x 512 kx l 9990 y 3642 ly m 6152 z 188 mz

17 CNS2010lecture 3 :: cyphers17 permutation cyphers Otherwise known as a transposition cypher The secret key is a random permutation, π Given a message m = m 1 m 2 m 3... m n One can compute its encryption by E π (m) = m π (1) m π (2) … m π (n) Suppose π = Then E π (“crypto”) = “PYCTRO”

18 CNS2010lecture 3 :: cyphers18 vignere cypher Originates in Rome in the sixteenth century A vignere cypher is a polyalphabetic substitution cypher (made up of multiple monoalphabetic substitution cyphers) The secret key is a word. Encryption is performed by adding the key modulo 26 in blocks: plaintextlaunchmissilesatlosangeles keycryptocryptocryptocryptocr =========================================== cyphertextnrscvvozqhbzgjyiecurlvwzgj Note that punctuation and white space is removed (this would make the code easy to break)

19 CNS2010lecture 3 :: cyphers19 breaking vignere cyphers The index of coincidence is a statistical measure of text, useful in distinguishing simple substitution ciphers from vignere cyphers. Intuitively it is a measure of the probability of collision of a symbol if a string is compared against a random shifted version of itself. The index of coincidence is measured by the following formula: IC = ∑ F = Frequency of symbol in text, N = Length of text F (F - 1) N (N - 1) althoughthecipherisinscrutableandoftenunforgeabletoanyonewithoutthis oftenunforgeabletoanyonewithoutthisAlthoughthecipherisinscrutableand x x x x IOC = 4/68 = 6%

20 CNS2010lecture 3 :: cyphers20 breaking vignere cyphers Using the standard frequencies with which individual letters appear in English text, the probability that a coincidence will occur is κ p = If the text is random, and the different letters are chosen with equal probability, the probability of a coincidence is much smaller (κ r = ). The most important property of the index of coincidence is that it is the same for text encrypted with a simple substitution cipher as for plain text (Why?).

21 CNS2010lecture 3 :: cyphers21 index of coincidence by language Malay Dutch Japanese Hebrew German Spanish Arabic French Portuguese Finnish Italian Danish Norwegian Greek English Swedish Serbo-Croatian Russian Random

22 CNS2010lecture 3 :: cyphers22 example :: breaking vignere cyphers Cyphertext TPCTY LVEOO GBVRC BTWXS IHDKD QIRVQ QUKWL TMNQO EKMLP AURKL VHIUX YJRNV QWJEK UEQVD IXPLU RKLVT QSLKI LWAZI JWXPL QRKIO PWFME XLLCP KDIKV EUXYX EAAQV MEKVN AVZRQ JGEMX LQUPM PCRLO IZPZZ FPONI AYPVQ RMVHC QZFLV IKGUK LRXER MDWVG RVMPQ PWLWT TIYEQ JAYMK XBPUK PZJBF WIRKS QJMSV FYKFP QLRXE OIPID IQQLI ICPFP LMVBR BUAMW KLLUM FLRXE CDQFV IKNWZ KUIXF BTIII CQZQM JQVUX UVZXL XMDAY IIOMP AZXEK VYIDC EGIDX NMQJQ ZQVMP FMESC EQGQD IDIJD MDXYI ACGES WSIFQ YIUMQ CBQSE EINBT CNSOM AUQLW BQVFL VALTS AJKLV JIZHJ MPVZQ XTLCQ ZFLDC ECVPW LRQQB TIVQV UWGPK LFTAF IKLXH BQVKL BGIEE KLFTA FCCEK FAQPR LEGID QVWMG MPMCC LNWDH DCPRQ DMKJX KTQXY LFFMZ SKXEA NMGVJ OQUYI CIPVQ NICMH GCZXF XEGUF LRXDQ LAAEM KVWFL VTFVK MYJIJ GBALV EOVPK PFZFP OWMEH KGAEM EXEGU AVEMK INAVZ RQJMQ HFMQT CEXTE RUMYI KSHPW IXYIT CGILV VBKVU WYSRN LIECO CQZUP ZJQWX YCJSR NCZXF XEGMP ICMSG ZYIFP LTLRV FQJKV QIEIJ KMEMW PBGCZ XFXEG MFSYM AGUQX VEZJU QXFHL VPKAZ PIHWD XYSRC ZFQPK LFBTC JTFTQ FMJKL QLXIR HJGQZ XFXEG TMRUS CWXDM XLQPM EWHYF ESQRD ILNWD HWSOV PKRRQ BUAMO VJLTB TCIMD JBQSL WKGAE WROBD ZURXQ VUWGP FYQQN FVFYY NMMRU SCVPK QVVZA KGXFJ COQZI VRBOQ QWRRA FMEXI SVCTX XYIJV PMXRJ CNQOX DCPQC XJFVF CUFLP WBTDM RK Shift Index <<< the key length is most likely 7 (closest to 6.6%) 8.032

23 CNS2010lecture 3 :: cyphers23 breaking vignere cyphers Once the key length N is known, we attack the N subtexts of the message independently by frequency analysis. Taking every N-th symbol gives a monoalphabetic substitution cypher (0, N, 2N, …; 1, N+1, 2N+1; … etc.). Note the index of coincidence varies by language, and can be domain-specific.

24 CNS2010lecture 3 :: cyphers24 rotor machines Rotor machines are mechanical devices that rotate varying sized disks in different ratios using gears. Popular during WWII e.g. German ENIGMA. Enigma used 3 or 4 replaceable rotors. On each rotor was arranged the alphabet in random order. The "key” is the choice and initial positions of the rotors.

25 CNS2010lecture 3 :: cyphers25 rotor machines Suppose you wished to encrypt “B”. After pressing “B" the first rotor would select a permutation for “B” say “C”. The second rotor would match “C” with its own permutation, say “G”, the third “G” to “H” etc. After determining “B” to represent “C” the first rotor would be advanced a notch, so that if “B” were to be typed again, a different encryption would result (here “G”). Similarly for rotor 2, after a full rotation of rotor 1 in this manner, etc possible permutations for each letter.

26 CNS2010lecture 3 :: cyphers26 xor XOR (addition modulo 2) is commonly used to provide security in software programs (although extremely weak!). The message m is xor’d bitwise with a secret key: c = m  k m = c  k XOR is a Vignere cypher and easy to break: –Determine the key length N from index of coincidence –Shift cyphertext by N and XOR with itself –This removes the key (c  c’ = m  k  m’  k = m  m’) –Results in message XORd with a shifted version of itself –Language is extremely redundant (English ~1.3 bits / byte) –Easy to then decrypt

27 CNS2010lecture 3 :: cyphers27 one time pad A one time pad is where we use a different substitution cypher for each letter of the plaintext. Encryption is xor (for bits) or addition modulo-26. Provided the secret key is truly random, the plaintext does not repeat and the pad is never used again, a one time pad is perfectly secure. Failure in any these requirements results in no security. Strength comes from the fact that a truly random key added to plaintext results in truly random cyphertext. No amount of computing power can break a OTP, since every possible plaintext is equally likely. Problems: key distribution, key destruction, synchronisation. Used for ultra-secure low bandwidth communications.

28 CNS2010lecture 3 :: cyphers28 perfect secrecy Goal of cryptography is that cyphertext tells absolutely nothing about the plaintext. A cypher has perfect secrecy if For all m є M, c є C the plaintext and cyphertext are statistically independent: Pr [ m 1 = m 2 | c 1 = c 2 ] = Pr [ m 1 = m 2 ] Assuming each transmitted message is equally likely, the probability that the transmitted message is m is Pr [ m 1 = m 2 ] = |M| -1 Now the probability that the transmitted message is m given that the observed cyphertext is c is : Pr [ m 1 = m ] = | {k: E k (m) = c, k є K} | |K|

29 CNS2010lecture 3 :: cyphers29 perfect secrecy of OTP The key space must be at least as large as the set of plaintexts –i.e. |K| ≥ |M| For M = C = {0,1} n : –Any cypher with perfect secrecy satisfies |K| ≥ 2 n The one time pad has perfect secrecy since M = C = K = {0,1} n Thus Pr [ m 1 = m 2 ] = 1 / 2 n Pr [ m 1 = m 2 | c 1 = c 2 ] = 1 / 2 n Note: we require k є K to be as long as the message –The conundrum: we have to securely communicate a key as long as the message in advance.

30 CNS2010lecture 3 :: cyphers30 attacks on the OTP A two-time pad has perfect insecurity! –If c 1 = m 1  k and c 2 = m 2  k –then c 1  c 2 = m 1  m 2 The OTP is highly malleable. –An attacker can easily create a new cyphertext with a known relationship to the plaintext without decryption. –Stream cyphers suffer from the same problem.

31 CNS2010lecture 3 :: cyphers31 example :: OTP voting scheme Suppose plaintext is a one bit vote v є {0,1} –Where v = 0 is a vote for Labor, v = 1 is a vote for Liberal Alice encrypts her vote using a OTP and sends to Bob c = b  k(k є {0,1} randomly chosen) Eve intercepts the cyphertext and sends with bits flipped c’ = c  1 Bob receives c’ and decrypts vote = c’  k = c  1  k = b  k  1  k = !b(the vote is reversed!)

32 CNS2010lecture 3 :: cyphers32 references Handbook of Applied Cryptography –§1.4 - §1.5 –§7.1 - §7.3 Stallings (3 rd Ed) –§2


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