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1 CS591/Crypto chow C. Edward Chow Basic Cryptography Chapters 5&2 of “Security Engineering” Ross Anderson Chapter 8 of “Computer Networks” Tanenbaum CS691 – Chapter 9 of “Computer Security” Matt Bishopz

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2 CS591/Crypto chow Topics to be Covered Introduction to Cryptography Substitution Ciphers Transposition Ciphers One-Time Pads: Quantum Cryptography Two Fundamental Cryptographic Principles Symmetric-Key Algorithms Public-Key Algorithms Key Management: Digital Signatures Introduction to Cryptography Substitution Ciphers Transposition Ciphers One-Time Pads: Quantum Cryptography Two Fundamental Cryptographic Principles Symmetric-Key Algorithms Public-Key Algorithms Key Management: Digital Signatures

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3 CS591/Crypto chow Cryptography Greek words for “Secret Writing”. Cipher vs. Code 1. Cipher: a character-to-character or bit-to-bit transformation without regard to linguistic sturcture of the msg. 2. Code: replace one word woth another word or symbol. e.g., Navajo code WWII (chay-da-gahi-nail- tsaidi(“tortoise killer). The art of devising ciphers is called cryptography The art of breaking ciphers is called cryptanalysis Cryptography and cryptanalysis is collectively known as Cryptology. Greek words for “Secret Writing”. Cipher vs. Code 1. Cipher: a character-to-character or bit-to-bit transformation without regard to linguistic sturcture of the msg. 2. Code: replace one word woth another word or symbol. e.g., Navajo code WWII (chay-da-gahi-nail- tsaidi(“tortoise killer). The art of devising ciphers is called cryptography The art of breaking ciphers is called cryptanalysis Cryptography and cryptanalysis is collectively known as Cryptology.

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4 CS591/Crypto chow Cryptosystem A cryptosystem is a 5-tuple (E,D,M,K,C), where M —a set of plaintexts (some use P as symbol); K —the set of keys; C —the set of ciphertexts; E: M x K C —the set of enciphering functions; D: C x K M —the set of deciphering functions; Caesar cipher example: M = { all sequences of Roman letters } K = { i | i an integer such that 0 i 25 } E = { E k | k K and m M, E k (m)=(m+k) mod 26 } D = { D k | k K and c C, D k (c)=(26+c-k) mod 26 } E.g., the word m=“HELLO” c=“KHOOR” what is k? A cryptosystem is a 5-tuple (E,D,M,K,C), where M —a set of plaintexts (some use P as symbol); K —the set of keys; C —the set of ciphertexts; E: M x K C —the set of enciphering functions; D: C x K M —the set of deciphering functions; Caesar cipher example: M = { all sequences of Roman letters } K = { i | i an integer such that 0 i 25 } E = { E k | k K and m M, E k (m)=(m+k) mod 26 } D = { D k | k K and c C, D k (c)=(26+c-k) mod 26 } E.g., the word m=“HELLO” c=“KHOOR” what is k?

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5 CS591/Crypto chow Encryption Model (Symmetric-Key Cipher) Adapted from Tanenbaum Computer Networks Figure 8.2 Here P=M, and key is moved as subscript of functions.

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6 CS591/Crypto chow Basic Encryption Methods Two categories: Substitution Ciphers: each letter or group of letters is replaced by another letter or another group of letters. It preserves the order of the plaintext symbol but disguise them. –E.g., Caesar cipher; mono-alphabetic substitution (each letter map onto another letter; 26!=4x10^26 possible keys) –ZHQM ZMGM ZMFM --G. JULIUS CAESAR Transposition Ciphers: Reorder the letters but do not disguise them. –E.g., Rail Fence cipher m=“HELLOWORLD” distribute the letter up and down between two rows from left to right; then output row-wise. HLOOL c=“HLOOLELWRD” ELWRD Two categories: Substitution Ciphers: each letter or group of letters is replaced by another letter or another group of letters. It preserves the order of the plaintext symbol but disguise them. –E.g., Caesar cipher; mono-alphabetic substitution (each letter map onto another letter; 26!=4x10^26 possible keys) –ZHQM ZMGM ZMFM --G. JULIUS CAESAR Transposition Ciphers: Reorder the letters but do not disguise them. –E.g., Rail Fence cipher m=“HELLOWORLD” distribute the letter up and down between two rows from left to right; then output row-wise. HLOOL c=“HLOOLELWRD” ELWRD

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7 CS591/Crypto chow Kerckhof’s Principle “All crypto algorithms must be public; only the keys are secret” --- “La Cryptographie Militaire,” J. des Sciences Militaires, vol. 9, pp.5-38, Jan and pp , Feb Trying to keep the algorithm secret (security by obscurity principle?) never works. Reasons: Logistic issue: Too much effort to invent, test, and install new algorithm “Publish the algorithm and let academic cryptologists to break the system. If no one succeeded in 5 years, it must be pretty solid.” Real secrecy in the key, its length a major design issue “All crypto algorithms must be public; only the keys are secret” --- “La Cryptographie Militaire,” J. des Sciences Militaires, vol. 9, pp.5-38, Jan and pp , Feb Trying to keep the algorithm secret (security by obscurity principle?) never works. Reasons: Logistic issue: Too much effort to invent, test, and install new algorithm “Publish the algorithm and let academic cryptologists to break the system. If no one succeeded in 5 years, it must be pretty solid.” Real secrecy in the key, its length a major design issue

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8 CS591/Crypto chow Key Length and Work Factor 2 digit key 100 combinations. 6 digit key 1 million combinations. 64 bit keys to prevent kid brother from reading your bit keys for routine commercial use >256 bits keep major governments at bay. 2 digit key 100 combinations. 6 digit key 1 million combinations. 64 bit keys to prevent kid brother from reading your bit keys for routine commercial use >256 bits keep major governments at bay.

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9 CS591/Crypto chow 3 Variations of Cryptoanalysis or Cipher Attacks Ciphertext-only: cryptoanalyst has some ciphertext; no (does not know the corresponding) plaintext; no key; no E or D functions. Known plaintext: some matched ciphertext and plaintext. (no key; may/may not know the E or D functions) Chosen plaintext: has the ability to encrypt pieces of plaintext of his own choosing. (know the E or D function; does not know the keys used). Ciphertext-only: cryptoanalyst has some ciphertext; no (does not know the corresponding) plaintext; no key; no E or D functions. Known plaintext: some matched ciphertext and plaintext. (no key; may/may not know the E or D functions) Chosen plaintext: has the ability to encrypt pieces of plaintext of his own choosing. (know the E or D function; does not know the keys used).

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10 CS591/Crypto chow Basic Cipher Attacks for Mono-alphetic Substitution Use statistical properties of natural languages. Rank of Frequency of appearance of Unigrams (single letter): e, t, o, a, n, i etc Digrams (two-letter): th, in, er, re, an, etc Trigrams (three-letter): the, ing, and, ion, etc Counting the frequencies of letters in ciphertext. Tentatively assign the most common one to e; next to t... Then look at the trigrams of decipher text, if “tXe” appears, it should be “the”! Original deciphering function should be changed to map from X to h. Similarly, what should we do if we see “thYt”? How about “aZW”? Use statistical properties of natural languages. Rank of Frequency of appearance of Unigrams (single letter): e, t, o, a, n, i etc Digrams (two-letter): th, in, er, re, an, etc Trigrams (three-letter): the, ing, and, ion, etc Counting the frequencies of letters in ciphertext. Tentatively assign the most common one to e; next to t... Then look at the trigrams of decipher text, if “tXe” appears, it should be “the”! Original deciphering function should be changed to map from X to h. Similarly, what should we do if we see “thYt”? How about “aZW”?

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11 CS591/Crypto chow Cipher Attack: Guess a Probable Word Ciphertext from accounting firm CTBMN BYCTC BTJDS QXBNS GSTJC BTSWX CTQTZ CQVUJ QJSGS TJQZZ MNQJS VLNSX VSZJU JDSTS JQUUS JUBXJ DSKSU JSNTK BGAQJ ZBGYQ TLCTZ BNYBN QJSW A likely word is financial. Based on two i are separated by four letters, we look for same pattern in ciphertext. There are 12 hits, position 6, 15, 27, 31, 42, 48, , 70, 71, 76, and 82. Only 31 and 42 has the next letter n (ana) separated by one letter. i C and J; n T and S respectively. Ciphertext from accounting firm CTBMN BYCTC BTJDS QXBNS GSTJC BTSWX CTQTZ CQVUJ QJSGS TJQZZ MNQJS VLNSX VSZJU JDSTS JQUUS JUBXJ DSKSU JSNTK BGAQJ ZBGYQ TLCTZ BNYBN QJSW A likely word is financial. Based on two i are separated by four letters, we look for same pattern in ciphertext. There are 12 hits, position 6, 15, 27, 31, 42, 48, , 70, 71, 76, and 82. Only 31 and 42 has the next letter n (ana) separated by one letter. i C and J; n T and S respectively.

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12 CS591/Crypto chow Mono-Alphabetic Substitution Exercises/Tools Chapter 5 page 74. Solve this puzzle. CYAN RWSGKFR AN AH RHTFANY MSOYRM OYSH SMSEAC NCMAKO How about JFK quote? XYAWO GAOOA GPEMO HPQCW IPNLG RPIXL TXLOA NNYCS YXBOY MNBIN YOBTY QYNAI Chapter 5 page 74. Solve this puzzle. CYAN RWSGKFR AN AH RHTFANY MSOYRM OYSH SMSEAC NCMAKO How about JFK quote? XYAWO GAOOA GPEMO HPQCW IPNLG RPIXL TXLOA NNYCS YXBOY MNBIN YOBTY QYNAI

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13 CS591/Crypto chow Columnar Transposition Ciphers A transposition cipher. Keyed by a phrase such as “MEGABUCK”. The letter in the key indicated the order of columns to be output. Plaintext horizontally read in, ciphertext read out column by column.

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14 CS591/Crypto chow Breaking Transposition Cipher Check if the frequency distribution of unigrams are the same in the ciphertext. If it is, then this is a transposition cipher. Guess the number of columns. Assume we suspect “milliondollars” appear in ciphertext Analyze keylength: Observe digrams MO, IL, LL, LA, IR, and OS in ciphertext as wrapping around effect keylength = 8. Analyze order of columns. If keylength is small, column pairs (k(k-1) of them) of ciphertext can be examined and see if the patterns of digrams in the deciphered text match those of the language frequency distribution. Check if the frequency distribution of unigrams are the same in the ciphertext. If it is, then this is a transposition cipher. Guess the number of columns. Assume we suspect “milliondollars” appear in ciphertext Analyze keylength: Observe digrams MO, IL, LL, LA, IR, and OS in ciphertext as wrapping around effect keylength = 8. Analyze order of columns. If keylength is small, column pairs (k(k-1) of them) of ciphertext can be examined and see if the patterns of digrams in the deciphered text match those of the language frequency distribution.

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15 CS591/Crypto chow One-Time Pads The use of a one-time pad for encryption and the possibility of getting any possible plaintext from the ciphertext by the use of some other pad. Unbreakable Cipher. Choose long random bit string as key (same length of the text?) Use Bit XOR as E and D. Problem: How to distribute and protect the key.

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16 CS591/Crypto chow Quantum Cryptography Can be used to transfer one-time pad over networks. Here Fiber channel is assumed. Light comes as little packets called Photons Photons can be polarized using filter such as sunglass. The photons after the polarized filters will be polarized in the direction of the filter’s axis. If the beam goes through the 2 nd filter and 1. the two filters are perpendicular, no photons get through. 2. The light intensity after the 2n filter is proportion to the square of the cosine of the angle between the two filter axes. Can be used to transfer one-time pad over networks. Here Fiber channel is assumed. Light comes as little packets called Photons Photons can be polarized using filter such as sunglass. The photons after the polarized filters will be polarized in the direction of the filter’s axis. If the beam goes through the 2 nd filter and 1. the two filters are perpendicular, no photons get through. 2. The light intensity after the 2n filter is proportion to the square of the cosine of the angle between the two filter axes.

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17 CS591/Crypto chow How one-time pad is sent using Quantum cryptography Alice and Bob each has two set of filters. One with vertical and horizontal filters called rectilinear basis. The other set rotate 45 degree called diagonal basis. Alice assigns vertical as 0 and horizontal as 1; lower left to upper right as 0 and upper left to lower right as 1. Send this assignment in plain text to Bob. Alice pick a one-time pad Transfer them bit by bit to Bob using one of the two bases at random. See Figure 8-5(a) Bob does not know which bases to use, he randomly pick one. See Figure 8-5(b). If he picks right, he gets correct bits. If not, he gets random bit. A photon hit a filter with 45 degree to photon’s polarization, will randomly jump to the axis of the filter or perpendicular to that. Figure 8-5(c). Bob tells Alice the axes he used in plaintext. Alice tells him which are right/wrong in plaintext. Figure 8-5(d). Now both have a correct bit string. Figure 8-5(e). Trudy’s choices of bases Figure 8-5(f). Trudy’s bit pattern with Unknown bits. Alice and Bob each has two set of filters. One with vertical and horizontal filters called rectilinear basis. The other set rotate 45 degree called diagonal basis. Alice assigns vertical as 0 and horizontal as 1; lower left to upper right as 0 and upper left to lower right as 1. Send this assignment in plain text to Bob. Alice pick a one-time pad Transfer them bit by bit to Bob using one of the two bases at random. See Figure 8-5(a) Bob does not know which bases to use, he randomly pick one. See Figure 8-5(b). If he picks right, he gets correct bits. If not, he gets random bit. A photon hit a filter with 45 degree to photon’s polarization, will randomly jump to the axis of the filter or perpendicular to that. Figure 8-5(c). Bob tells Alice the axes he used in plaintext. Alice tells him which are right/wrong in plaintext. Figure 8-5(d). Now both have a correct bit string. Figure 8-5(e). Trudy’s choices of bases Figure 8-5(f). Trudy’s bit pattern with Unknown bits.

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18 CS591/Crypto chow Quantum Cryptography

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19 CS591/Crypto chow Two Fundamental Cryptographic Principles Redundancy: Messages must contains some redundancy. E.g., Last three bytes of encrypted packet content are product # and quantity. Recent fired employee can capture the packet replace the last three byte quantity field with a random number. How redundancy can help? Freshness Some method is needed to foil replay attacks. How to defense reply attacks? Timestamp alone? Redundancy: Messages must contains some redundancy. E.g., Last three bytes of encrypted packet content are product # and quantity. Recent fired employee can capture the packet replace the last three byte quantity field with a random number. How redundancy can help? Freshness Some method is needed to foil replay attacks. How to defense reply attacks? Timestamp alone?

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20 CS591/Crypto chow Symmetric-Key Algorithms DES – The Data Encryption Standard AES – The Advanced Encryption Standard Cipher Modes Other Ciphers Cryptanalysis DES – The Data Encryption Standard AES – The Advanced Encryption Standard Cipher Modes Other Ciphers Cryptanalysis

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21 CS591/Crypto chow Product Ciphers Basic elements of product ciphers. (a) P-box. (b) S-box. (c) Product. Hardware implementation. P: permutation (transpose). S: Substitution; in, out. Software implementation: through iterations called rounds Basic elements of product ciphers. (a) P-box. (b) S-box. (c) Product. Hardware implementation. P: permutation (transpose). S: Substitution; in, out. Software implementation: through iterations called rounds

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22 CS591/Crypto chow Data Encryption Standard The data encryption standard 1977/IBM (128 56 bits key) for unclassified info. (a) General outline. (b) Detail of one iteration. The circled + means exclusive OR. Ri-1 expand to 48 bit; exclusive or with Ki Divide into 8 groups 6bit each Each group goes through S-Box; output is 4 bits.

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23 CS591/Crypto chow Triple DES IS8732; two keys (a) Triple encryption using DES. (b) Decryption. Compatible with old DES. If we use the same key instead a different K bit key 2^128=3*10^38keys; evaluate one key per picoseconds it takes 10^10 years. IS8732; two keys (a) Triple encryption using DES. (b) Decryption. Compatible with old DES. If we use the same key instead a different K bit key 2^128=3*10^38keys; evaluate one key per picoseconds it takes 10^10 years.

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24 CS591/Crypto chow AES – The Advanced Encryption Standard Rules for AES proposals 1. The algorithm must be a symmetric block cipher. 2. The full design must be public. 3. Key lengths of 128, 192, and 256 bits supported. 4. Both software and hardware implementations required 5. The algorithm must be public or licensed on nondiscriminatory terms. Rules for AES proposals 1. The algorithm must be a symmetric block cipher. 2. The full design must be public. 3. Key lengths of 128, 192, and 256 bits supported. 4. Both software and hardware implementations required 5. The algorithm must be public or licensed on nondiscriminatory terms.

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25 CS591/Crypto chow AES (2) An outline of Rijndael, by Joan Daemen and Vincent Rijmen 86 votes

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26 CS591/Crypto chow AES (3) Creating of the state and rk arrays.

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27 CS591/Crypto chow Electronic Code Book Mode The plaintext of a file encrypted as 16 DES blocks. AES/DES are monoalphabetic substitution cipher. Same plaintext with same key in, same ciphertext out. Cipher Attack: Just substitute 12 th block with 4 th block and Leslie has a Merry Christmas. The plaintext of a file encrypted as 16 DES blocks. AES/DES are monoalphabetic substitution cipher. Same plaintext with same key in, same ciphertext out. Cipher Attack: Just substitute 12 th block with 4 th block and Leslie has a Merry Christmas.

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28 CS591/Crypto chow Cipher Block Chaining Mode Use the chaining mode to defeat the above attack. C1 depends on C0. Cipher block chaining. (a) Encryption. (b) Decryption. Use the chaining mode to defeat the above attack. C1 depends on C0. Cipher block chaining. (a) Encryption. (b) Decryption.

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29 CS591/Crypto chow Cipher Feedback Mode For terminal type application, wait for 8 characters before sending the ciphertext is not an option. Use Character Chaining mode with a shift register. (a) Encryption. (b) Decryption. For terminal type application, wait for 8 characters before sending the ciphertext is not an option. Use Character Chaining mode with a shift register. (a) Encryption. (b) Decryption.

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30 CS591/Crypto chow Stream Cipher Mode In previous modes, one bit Tx error messing whole block. In Stream cipher mode, early ciphertext does not involve with later encryption and one bit Tx error one bit plaintext error A stream cipher. (a) Encryption. (b) Decryption. In previous modes, one bit Tx error messing whole block. In Stream cipher mode, early ciphertext does not involve with later encryption and one bit Tx error one bit plaintext error A stream cipher. (a) Encryption. (b) Decryption.

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31 CS591/Crypto chow Counter Mode Except electronic code book mode, previous modes requires first decrypting all the blocks ahead the current block. Make it difficult to do random access of encrypted files. (problem is reuse attack, when same key is used. Encryption using counter mode. Except electronic code book mode, previous modes requires first decrypting all the blocks ahead the current block. Make it difficult to do random access of encrypted files. (problem is reuse attack, when same key is used. Encryption using counter mode.

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32 CS591/Crypto chow Cryptanalysis Rijndael got adopted as AES. Serpent(cambridge) 2 nd. Some common symmetric-key cryptographic algorithms. Rijndael got adopted as AES. Serpent(cambridge) 2 nd. Some common symmetric-key cryptographic algorithms.

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33 CS591/Crypto chow Public-Key Algorithms 1976 Diffie and Hellman proposed crypto scheme with two keys; public key and private key. Requirement: Must be computationally easy to encipher/decipher msg using these keys. Must be computationally infeasible to derive the private key from public key. Must be computationally infeasible to determine the private key from a chosen plaintext attack. Symmetric key exchange protocol RSA Other Public-Key Algorithms 1976 Diffie and Hellman proposed crypto scheme with two keys; public key and private key. Requirement: Must be computationally easy to encipher/decipher msg using these keys. Must be computationally infeasible to derive the private key from public key. Must be computationally infeasible to determine the private key from a chosen plaintext attack. Symmetric key exchange protocol RSA Other Public-Key Algorithms

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34 CS591/Crypto chow Diffie-Hellman’s Symmetric Key Exchange Protocol It is based on discrete logarithm problem. Alice and Bob chooses a prime p=53 and g=17 which is not 0, 1, or p-1=52. Alice chooses private key=5, public key=17 5 mod 53 =40. Bob choose private key=7, public key=17 7 mod 53=6. Bob would like to send msg to alice. Bob compute the shared secret key by enciphering Alice’s public key using his private key: 40 7 mod 53 = 38 Encipher the msg with key=38. Alice computes the shared secret key as 6 5 mod 53=38. Then decipher the msg with key=38. It is based on discrete logarithm problem. Alice and Bob chooses a prime p=53 and g=17 which is not 0, 1, or p-1=52. Alice chooses private key=5, public key=17 5 mod 53 =40. Bob choose private key=7, public key=17 7 mod 53=6. Bob would like to send msg to alice. Bob compute the shared secret key by enciphering Alice’s public key using his private key: 40 7 mod 53 = 38 Encipher the msg with key=38. Alice computes the shared secret key as 6 5 mod 53=38. Then decipher the msg with key=38.

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35 CS591/Crypto chow Modular Arithmetic

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36 CS591/Crypto chow Properties of Modular Arithmetic

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38 CS591/Crypto chow Fermat Theorem Example

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40 CS591/Crypto chow Totient Function Properties

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42 CS591/Crypto chow Euler’s Theorem and Examples

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43 CS591/Crypto chow RSA A response to Diffie Helleman challege by Ron Rivest, Adi Sharmir, and Len Adleman at MIT. An exponentiation cipher utilizing Euler’s Theorem. Choose two prime numbers p and q. Let n = p*q. The totient (n) of n is the number of numbers less than n with no factors in common with n. (n)=(p-1)(q-1) E.g., (10) =4; since 1,3, 7, 9 are relative prime of 10. Choose e

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45 CS591/Crypto chow RSA: Confidentiality Example Encrypted using Alice’s Public key Let p=7 and q = 11. n=77 and (n) =60. Alice choose e=17, a relative prime to 60 private key is d=53 where e*d mod (n) =1; 17*53 mod 60 = 1 If we represent 07 as and 25 as Z, 26 as blank, then HELLO WORLD will be Using Alice public key the cipher text is: 07^17 mod 77 = 28 04^17 mod 77 = 16 11^17 mod 77 = 44 … 03^17 mod 77 = 75. Only Alice can decipher with private key 53. Let p=7 and q = 11. n=77 and (n) =60. Alice choose e=17, a relative prime to 60 private key is d=53 where e*d mod (n) =1; 17*53 mod 60 = 1 If we represent 07 as and 25 as Z, 26 as blank, then HELLO WORLD will be Using Alice public key the cipher text is: 07^17 mod 77 = 28 04^17 mod 77 = 16 11^17 mod 77 = 44 … 03^17 mod 77 = 75. Only Alice can decipher with private key 53.

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46 CS591/Crypto chow RSA: Origin Authentication Example Encrypted using Alice’s Private key Let p=7 and q = 11. n=77 and (n) =60. Alice choose e=17, private key is d=53. Public key is (17, 77). 07^53 mod 77 = 35 04^53 mod 77 = 09 11^53 mod 77 = 44 … 03^53 mod 77 = 05 Let p=7 and q = 11. n=77 and (n) =60. Alice choose e=17, private key is d=53. Public key is (17, 77). 07^53 mod 77 = 35 04^53 mod 77 = 09 11^53 mod 77 = 44 … 03^53 mod 77 = 05

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47 CS591/Crypto chow RSA: Confidentiality and Authentication Example Encrypted using Sender’s Private key and the recipient’s public key Bob’s public key 37, private key 13. Alice’s public key 17, private key 53. Encipherment: (07^53 mod 77)^37 mod 77 = 07 (04^53 mod 77)^37 mod 77 = 37 (11^53 mod 77)^37 mod 77 = 44 … (03^53 mod 77)^37 mod 77 = 47 Decipherment: decipher with recipient’s private key and authenticate with sender’s public key. (07^13 mod 77)^17 mod 77 = 07 (37^13 mod 77)^17 mod 77 = 04 (44^13 mod 77)^17 mod 77 = 11 … (47^13 mod 77)^17 mod 77 = 03 Encrypted using Sender’s Private key and the recipient’s public key Bob’s public key 37, private key 13. Alice’s public key 17, private key 53. Encipherment: (07^53 mod 77)^37 mod 77 = 07 (04^53 mod 77)^37 mod 77 = 37 (11^53 mod 77)^37 mod 77 = 44 … (03^53 mod 77)^37 mod 77 = 47 Decipherment: decipher with recipient’s private key and authenticate with sender’s public key. (07^13 mod 77)^17 mod 77 = 07 (37^13 mod 77)^17 mod 77 = 04 (44^13 mod 77)^17 mod 77 = 11 … (47^13 mod 77)^17 mod 77 = 03

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48 CS591/Crypto chow RSA Cipher Attack/Defense Example Simplified example, character as block. NO can be swapped and interpreted as ON. (Attack is on or not?) Defense: 6 bits for block # followed by 8 bit character. The rest 1010 bits contains random data. Rearrange of blocks can be detected by checking the block # field. This is an application of the Redundancy principle. Simplified example, character as block. NO can be swapped and interpreted as ON. (Attack is on or not?) Defense: 6 bits for block # followed by 8 bit character. The rest 1010 bits contains random data. Rearrange of blocks can be detected by checking the block # field. This is an application of the Redundancy principle.

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49 CS591/Crypto chow RSA Actual RSA primes should be at least 512 bits modulus at least 1024 bits. RSA combined with hash to prevent reordering attack. An example of the RSA algorithm. Here p=3, q=11, n=33, z=(p-1)*(q-1)=20, choose d=7, which is relative prime of z. choose e=3 where e*d mod 20 = 1. Here (3, 20) is public key. (7,20) is private key. C=P e mod n; P=C d mod n; Actual RSA primes should be at least 512 bits modulus at least 1024 bits. RSA combined with hash to prevent reordering attack. An example of the RSA algorithm. Here p=3, q=11, n=33, z=(p-1)*(q-1)=20, choose d=7, which is relative prime of z. choose e=3 where e*d mod 20 = 1. Here (3, 20) is public key. (7,20) is private key. C=P e mod n; P=C d mod n;

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50 CS591/Crypto chow Digital Signatures Symmetric-Key Signatures Public-Key Signatures Message Digests The Birthday Attack Symmetric-Key Signatures Public-Key Signatures Message Digests The Birthday Attack

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51 CS591/Crypto chow Symmetric-Key Signatures Digital signatures with Big Brother (BB: a central authority trust by everyone; knows everything!). Everyone (i) carries secret key Ki to BB’s office. Alice enciphers the following data with her secret key K A (B:Bob’s ID, RA: a random number chosen by Alice, t: timestamp, P: plaintext msg.); send it to BB. BB deciphers with K A ; encrypted the data together with a signed msg K BB (A, t, P) using B’s secret key. Digital signatures with Big Brother (BB: a central authority trust by everyone; knows everything!). Everyone (i) carries secret key Ki to BB’s office. Alice enciphers the following data with her secret key K A (B:Bob’s ID, RA: a random number chosen by Alice, t: timestamp, P: plaintext msg.); send it to BB. BB deciphers with K A ; encrypted the data together with a signed msg K BB (A, t, P) using B’s secret key.

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52 CS591/Crypto chow Symmetric-Key Signatures: Nonrepudiation and Against Relay Attack BB will not accept a msg from Alice unless it is encrypted with KA. K BB (A, t, P) contains the proof that A sent P at time t. Against replay attack: Against instant replay: Within some time period, Bob can check if RA has been reuse; drop those packet. Against old replay: Based on the timestamp, Bob can discard old messages. Disadvantage: BB needs to be trusted by everyone. BB gets to read all signed msgs. BB will not accept a msg from Alice unless it is encrypted with KA. K BB (A, t, P) contains the proof that A sent P at time t. Against replay attack: Against instant replay: Within some time period, Bob can check if RA has been reuse; drop those packet. Against old replay: Based on the timestamp, Bob can discard old messages. Disadvantage: BB needs to be trusted by everyone. BB gets to read all signed msgs.

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53 CS591/Crypto chow Public-Key Signatures Digital signatures using public-key cryptography. 1991, NIST proposed Digital Signature Standard (DSS) using variant of the EL Gamal public key algorithm (discrete logarithm). But it is too secret (NSA designed); too slow (10-40 times slower than RSA for checking signature); too new (not yet thoroughly analyzed); too insecure (fixed 512bits; later changed to 1024 bits) Digital signatures using public-key cryptography. 1991, NIST proposed Digital Signature Standard (DSS) using variant of the EL Gamal public key algorithm (discrete logarithm). But it is too secret (NSA designed); too slow (10-40 times slower than RSA for checking signature); too new (not yet thoroughly analyzed); too insecure (fixed 512bits; later changed to 1024 bits)

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54 CS591/Crypto chow Message Digests Authentication without encrypting the entire msg. 4 properties of Message Digest (MD hash function: arbitrarily long plaintext fixed-length bit string. Given P, it is easy to compute MD(P). Given MD(P), it is effectively impossible to find P. Given P no one can find P’ such that MD(P’)=MD(P). A change to the input of even 1 bit produces a very different output. For example, Instead of K BB (A, t, P), we have K BB (A, t, MD(P)) Digital signatures using message digests. Authentication without encrypting the entire msg. 4 properties of Message Digest (MD hash function: arbitrarily long plaintext fixed-length bit string. Given P, it is easy to compute MD(P). Given MD(P), it is effectively impossible to find P. Given P no one can find P’ such that MD(P’)=MD(P). A change to the input of even 1 bit produces a very different output. For example, Instead of K BB (A, t, P), we have K BB (A, t, MD(P)) Digital signatures using message digests.

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55 CS591/Crypto chow MD5 Designed by Ronald Rivest th of a series of MD. Pre-computation Step: First pad the msg a length of 448 bits (mod 512). Original msg is then appended with 64 bit integer; make it a multiple of 512 bits. Initialize 128-bit buffer to a fixed value. Computation step: Take 512 bit block of input, perform 4 round of mixing, thoroughly with 128 bit buffer (with sine function and table look up) Process continues until all input consume. The content of 128-bit buffer form the message digest. You can access it on our CS Unix machines using OpenSSL pacckage “openssl md5” For example, the following yield a pretty good random key: bash-2.05a# (date; ps auxg) | openssl md ddf1371aefc921d504fa51e6a bash-2.05a# (date; ps auxg) | openssl md5 7c56744bb2440abcc2de7a492ae32d06 Designed by Ronald Rivest th of a series of MD. Pre-computation Step: First pad the msg a length of 448 bits (mod 512). Original msg is then appended with 64 bit integer; make it a multiple of 512 bits. Initialize 128-bit buffer to a fixed value. Computation step: Take 512 bit block of input, perform 4 round of mixing, thoroughly with 128 bit buffer (with sine function and table look up) Process continues until all input consume. The content of 128-bit buffer form the message digest. You can access it on our CS Unix machines using OpenSSL pacckage “openssl md5” For example, the following yield a pretty good random key: bash-2.05a# (date; ps auxg) | openssl md ddf1371aefc921d504fa51e6a bash-2.05a# (date; ps auxg) | openssl md5 7c56744bb2440abcc2de7a492ae32d06

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56 CS591/Crypto chow SHA-1 Developed by NSA and blessed by NIST in FIP It generates 160 bit message digest. Use of SHA-1 and RSA for signing nonsecret messages. bash-2.05a# (date; ps auxg) | openssl sha1 44c702745bdeced27d8c01b8bcda28bb311e51f4 Developed by NSA and blessed by NIST in FIP It generates 160 bit message digest. Use of SHA-1 and RSA for signing nonsecret messages. bash-2.05a# (date; ps auxg) | openssl sha1 44c702745bdeced27d8c01b8bcda28bb311e51f4

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57 CS591/Crypto chow SHA-1 (2) (a) A message padded out to a multiple of 512 bits. (b) The output 32 bit variables. (c) The word array. (a) A message padded out to a multiple of 512 bits. (b) The output 32 bit variables. (c) The word array.

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58 CS591/Crypto chow SHA1 Copy 16 work block input to w0 to w15. Scramble them to w16 to w79 with Wi= S 1 (Wi-3 XOR Wi-8 XOR Wi-14 XOR Wi-16) (16 <=i<= 79) S b (W) represent the left circular rotation of 32-bit word W by b bits. Actual step in pseudo-C code: for (i = 0; i < 80; i++) { temp = S5(A) + fi (B, C, D) + E + Wi + Ki; E=D; D=C; C = S30 (B); B=A; A =temp;} f (B, C, D) = (B AND C) OR (NOT B AND D) ( 0<= i <=19) f (B, C, D) = B XOR C XOR D (20 <= i <= 39) f (B, C, D) = (B AND C) OR (B AND D) OR (C AND D) (40 <= i <= 59) f (B, C, D) = B XOR C XOR D(60<= i <= 79) At the end of 80 iterations, A-E added to H0-H4 respectively. Continue the rest of the input blocks. Work ongoing for 256, 384, 512bit hashes. Copy 16 work block input to w0 to w15. Scramble them to w16 to w79 with Wi= S 1 (Wi-3 XOR Wi-8 XOR Wi-14 XOR Wi-16) (16 <=i<= 79) S b (W) represent the left circular rotation of 32-bit word W by b bits. Actual step in pseudo-C code: for (i = 0; i < 80; i++) { temp = S5(A) + fi (B, C, D) + E + Wi + Ki; E=D; D=C; C = S30 (B); B=A; A =temp;} f (B, C, D) = (B AND C) OR (NOT B AND D) ( 0<= i <=19) f (B, C, D) = B XOR C XOR D (20 <= i <= 39) f (B, C, D) = (B AND C) OR (B AND D) OR (C AND D) (40 <= i <= 59) f (B, C, D) = B XOR C XOR D(60<= i <= 79) At the end of 80 iterations, A-E added to H0-H4 respectively. Continue the rest of the input blocks. Work ongoing for 256, 384, 512bit hashes.

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59 CS591/Crypto chow The Birth Day Attack It takes 2 m operations to attack m-bit MD. But it takes 2 m/2 operations using birthday attack. Yuval 1979 paper on “how to swindle Rabin” Example: two tenure faculty up for promotion: Tom and Dick. Tom earlier by two years. Tom asks Dept. Chair, Marilyn to write recommendation letter: Secretary Ellen Loves Dick. She prepares two letters. It takes 2 m operations to attack m-bit MD. But it takes 2 m/2 operations using birthday attack. Yuval 1979 paper on “how to swindle Rabin” Example: two tenure faculty up for promotion: Tom and Dick. Tom earlier by two years. Tom asks Dept. Chair, Marilyn to write recommendation letter: Secretary Ellen Loves Dick. She prepares two letters.

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60 CS591/Crypto chow Official Letter 1 Dear Dean Smith, This [letter I message] is to give my [honest I frank] opinion of Prof. Tom Wilson, who is [a candidate I up] for tenure [now I this year]. I have [known I worked with] Prof. Wilson for [about I almost] six years. He is an [outstanding I excellent] researcher of great [talent I ability] known [worldwide I internationally] for his [brilliant I creative] insights into [many I a wide variety of] [difficult I challenging] problems. He is also a [highly I greatly] [respected I admired] [teacher I educator]. His students give his [classes I courses] [rave I spectacular] reviews. He is [our I the Department's] [most popular I best-loved] [teacher I instructor]. [In addition I Additionally] Prof. Wilson is a [gifted I effective] fund raiser. His [grants I contracts] have brought a [large I substantial] amount of money into [the I our] Department. [This money has I These funds have] [enabled I permitted] us to [pursue I carry out] many [special I important] programs, [such as I for example] your State 2000 program. Without these funds we would [be unable I not be able] to continue this program. which is so [important I essential] to both of us. I strongly urge you to grant him tenure. Dear Dean Smith, This [letter I message] is to give my [honest I frank] opinion of Prof. Tom Wilson, who is [a candidate I up] for tenure [now I this year]. I have [known I worked with] Prof. Wilson for [about I almost] six years. He is an [outstanding I excellent] researcher of great [talent I ability] known [worldwide I internationally] for his [brilliant I creative] insights into [many I a wide variety of] [difficult I challenging] problems. He is also a [highly I greatly] [respected I admired] [teacher I educator]. His students give his [classes I courses] [rave I spectacular] reviews. He is [our I the Department's] [most popular I best-loved] [teacher I instructor]. [In addition I Additionally] Prof. Wilson is a [gifted I effective] fund raiser. His [grants I contracts] have brought a [large I substantial] amount of money into [the I our] Department. [This money has I These funds have] [enabled I permitted] us to [pursue I carry out] many [special I important] programs, [such as I for example] your State 2000 program. Without these funds we would [be unable I not be able] to continue this program. which is so [important I essential] to both of us. I strongly urge you to grant him tenure.

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61 CS591/Crypto chow Fake Letter Dear Dean Smith. This [letter I message] is to give my [honest I frank] opinion of Prof. Tom Wilson, who is [a candidate I up] for tenure [now I this year]. I have [known I worked with] Tom for [about I almost] six years. He is a [poor I weak] researcher not well known in his [field I area]. His research [hardly ever I rarely] shows [insight in I understanding of] the [key I major] problems of [the I our] day. Furthermore, he is not a [respected I admired] [teacher I educator]. His students give his [classes I courses] [poor I bad ] reviews. He is [our I the Department's] least popular [teacher I instructor], known [mostly I primarily] within [the I our] Department for his [tendency I propensity] to [ridicule I embarrass] students [foolish I imprudent] enough to ask questions in his classes. [In addition I Additionally] Tom is a [poor I marginal] fund raiser. His [grants I contracts] have brought only a [meager I insignificant] amount of money into [the I our] Department. Unless new [money is I funds are] quickly located, we may have to cancel some essential programs, such as your State 2000 program. Unfortunately, under these [conditions I circumstances] I cannot in good [conscience I faith] recommend him to you for [tenure I a permanent position]. Dear Dean Smith. This [letter I message] is to give my [honest I frank] opinion of Prof. Tom Wilson, who is [a candidate I up] for tenure [now I this year]. I have [known I worked with] Tom for [about I almost] six years. He is a [poor I weak] researcher not well known in his [field I area]. His research [hardly ever I rarely] shows [insight in I understanding of] the [key I major] problems of [the I our] day. Furthermore, he is not a [respected I admired] [teacher I educator]. His students give his [classes I courses] [poor I bad ] reviews. He is [our I the Department's] least popular [teacher I instructor], known [mostly I primarily] within [the I our] Department for his [tendency I propensity] to [ridicule I embarrass] students [foolish I imprudent] enough to ask questions in his classes. [In addition I Additionally] Tom is a [poor I marginal] fund raiser. His [grants I contracts] have brought only a [meager I insignificant] amount of money into [the I our] Department. Unless new [money is I funds are] quickly located, we may have to cancel some essential programs, such as your State 2000 program. Unfortunately, under these [conditions I circumstances] I cannot in good [conscience I faith] recommend him to you for [tenure I a permanent position].

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62 CS591/Crypto chow The Rest of Story Now Ellen programs her computer to compute the 232 message digests of each letter overnight. Chances are. one digest of the first letter will match one digest of the second letter. If not. she can add a few more options and try again during the weekend. Suppose that she finds a match. Call the "good" letter A and the "bad" one B. Ellen now s letter A to Marilyn for her approval. Letter B she keeps completely secret, showing it to no one. Marilyn, of course. approves, computes her 64-bit message digest. signs the digest. and s the signed digest off to Dean Smith. Independently, Ellen s letter B to the Dean (not letter A, as she is supposed to). After getting the letter and signed message digest. the Dean runs the message digest algorithm on letter B, sees that it agrees with what Marilyn sent him, and fires Tom. The Dean does not realize that Ellen managed to generate two letters with the same message digest and sent her a different one than Marilyn saw and approved. (Optional ending: Ellen tells Dick what she did. Dick is appalled and breaks off with her. Ellen is furious and confesses to Marilyn. Marilyn calls the Dean. Tom gets tenure after all.) With MD5 the birthday attack is difficult because even at 1 billion digests per second, it would take over 500 years to compute all 264 digests of two letters with 64 variants each, and even then a match is not guaranteed. Of course, with 5000 computers working in parallel, 500 years becomes 5 weeks. SHA-1 is better (because it is longer). Now Ellen programs her computer to compute the 232 message digests of each letter overnight. Chances are. one digest of the first letter will match one digest of the second letter. If not. she can add a few more options and try again during the weekend. Suppose that she finds a match. Call the "good" letter A and the "bad" one B. Ellen now s letter A to Marilyn for her approval. Letter B she keeps completely secret, showing it to no one. Marilyn, of course. approves, computes her 64-bit message digest. signs the digest. and s the signed digest off to Dean Smith. Independently, Ellen s letter B to the Dean (not letter A, as she is supposed to). After getting the letter and signed message digest. the Dean runs the message digest algorithm on letter B, sees that it agrees with what Marilyn sent him, and fires Tom. The Dean does not realize that Ellen managed to generate two letters with the same message digest and sent her a different one than Marilyn saw and approved. (Optional ending: Ellen tells Dick what she did. Dick is appalled and breaks off with her. Ellen is furious and confesses to Marilyn. Marilyn calls the Dean. Tom gets tenure after all.) With MD5 the birthday attack is difficult because even at 1 billion digests per second, it would take over 500 years to compute all 264 digests of two letters with 64 variants each, and even then a match is not guaranteed. Of course, with 5000 computers working in parallel, 500 years becomes 5 weeks. SHA-1 is better (because it is longer).

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63 CS591/Crypto chow Management of Public Keys Certificates X.509 Public Key Infrastructures Certificates X.509 Public Key Infrastructures

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64 CS591/Crypto chow Problems with Public-Key Encryption A way for Trudy to subvert public-key encryption.

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65 CS591/Crypto chow Certificates A possible certificate and its signed hash.

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66 CS591/Crypto chow X.509 The basic fields of an X.509 certificate.

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67 CS591/Crypto chow Public-Key Infrastructures (a) A hierarchical PKI. (b) A chain of certificates.

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68 CS591/Crypto chow Communication Security IPsec Firewalls Virtual Private Networks Wireless Security IPsec Firewalls Virtual Private Networks Wireless Security

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69 CS591/Crypto chow IPsec The IPsec authentication header in transport mode for IPv4.

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70 CS591/Crypto chow IPsec (2) (a) ESP in transport mode. (b) ESP in tunnel mode.

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71 CS591/Crypto chow Firewalls A firewall consisting of two packet filters and an application gateway.

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72 CS591/Crypto chow Virtual Private Networks (a) A leased-line private network. (b) A virtual private network.

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73 CS591/Crypto chow Security Packet encryption using WEP.

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74 CS591/Crypto chow Authentication Protocols Authentication Based on a Shared Secret Key Establishing a Shared Key: Diffie-Hellman Authentication Using a Key Distribution Center Authentication Using Kerberos Authentication Using Public-Key Cryptography Authentication Based on a Shared Secret Key Establishing a Shared Key: Diffie-Hellman Authentication Using a Key Distribution Center Authentication Using Kerberos Authentication Using Public-Key Cryptography

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75 CS591/Crypto chow Authentication Based on a Shared Secret Key Two-way authentication using a challenge-response protocol.

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76 CS591/Crypto chow Authentication Based on a Shared Secret Key (2) A shortened two-way authentication protocol.

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77 CS591/Crypto chow Authentication Based on a Shared Secret Key (3) The reflection attack.

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78 CS591/Crypto chow Authentication Based on a Shared Secret Key (4) A reflection attack on the protocol of Fig A reflection attack on the protocol of Fig

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79 CS591/Crypto chow Authentication Based on a Shared Secret Key (5) Authentication using HMACs.

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80 CS591/Crypto chow Establishing a Shared Key: The Diffie-Hellman Key Exchange The Diffie-Hellman key exchange.

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81 CS591/Crypto chow Establishing a Shared Key: The Diffie-Hellman Key Exchange The bucket brigade or man-in-the-middle attack.

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82 CS591/Crypto chow Authentication Using a Key Distribution Center A first attempt at an authentication protocol using a KDC.

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83 CS591/Crypto chow Authentication Using a Key Distribution Center (2) The Needham-Schroeder authentication protocol.

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84 CS591/Crypto chow Authentication Using a Key Distribution Center (3) The Otway-Rees authentication protocol (slightly simplified).

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85 CS591/Crypto chow Authentication Using Kerberos The operation of Kerberos V4.

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86 CS591/Crypto chow Authentication Using Public-Key Cryptography Mutual authentication using public-key cryptography.

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87 CS591/Crypto chow Security PGP – Pretty Good Privacy PEM – Privacy Enhanced Mail S/MIME PGP – Pretty Good Privacy PEM – Privacy Enhanced Mail S/MIME

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88 CS591/Crypto chow PGP – Pretty Good Privacy PGP in operation for sending a message.

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89 CS591/Crypto chow PGP – Pretty Good Privacy (2) A PGP message.

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90 CS591/Crypto chow Web Security Threats Secure Naming SSL – The Secure Sockets Layer Mobile Code Security Threats Secure Naming SSL – The Secure Sockets Layer Mobile Code Security

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91 CS591/Crypto chow Secure Naming (a) Normal situation. (b) An attack based on breaking into DNS and modifying Bob's record.

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92 CS591/Crypto chow Secure Naming (2) How Trudy spoofs Alice's ISP.

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93 CS591/Crypto chow Secure DNS An example RRSet for bob.com. The KEY record is Bob's public key. The SIG record is the top-level com server's signed has of the A and KEY records to verify their authenticity.

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94 CS591/Crypto chow Self-Certifying Names A self-certifying URL containing a hash of server's name and public key.

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95 CS591/Crypto chow SSL—The Secure Sockets Layer Layers (and protocols) for a home user browsing with SSL.

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96 CS591/Crypto chow SSL (2) A simplified version of the SSL connection establishment subprotocol.

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97 CS591/Crypto chow SSL (3) Data transmission using SSL.

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98 CS591/Crypto chow Java Applet Security Applets inserted into a Java Virtual Machine interpreter inside the browser.

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99 CS591/Crypto chow Social Issues Privacy Freedom of Speech Copyright Privacy Freedom of Speech Copyright

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100 CS591/Crypto chow Anonymous R ers Users who wish anonymity chain requests through multiple anonymous r ers.

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101 CS591/Crypto chow Freedom of Speech Possibly banned material: 1. Material inappropriate for children or teenagers. 2. Hate aimed at various ethnic, religious, sexual, or other groups. 3. Information about democracy and democratic values. 4. Accounts of historical events contradicting the government's version. 5. Manuals for picking locks, building weapons, encrypting messages, etc. Possibly banned material: 1. Material inappropriate for children or teenagers. 2. Hate aimed at various ethnic, religious, sexual, or other groups. 3. Information about democracy and democratic values. 4. Accounts of historical events contradicting the government's version. 5. Manuals for picking locks, building weapons, encrypting messages, etc.

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102 CS591/Crypto chow Steganography (a) Three zebras and a tree. (b) Three zebras, a tree, and the complete text of five plays by William Shakespeare.

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