CNT4406/5412 Network Security
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1 CNT4406/5412 Network Security Introduction to Cryptography Zhi Wang Florida State University Fall 2014 Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
2 Introduction What is Cryptography Mangling data into unintelligible form in a manner allowing lossless unmangling usually one-to-one in size (unlike compression) RSA is different other services: integrity check and authentication encryption decryption plaintext ciphertext plaintext key key Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
3 Introduction Cryptography Caveats We normally cannot prove a cipher is secure, instead we assume it is secure until unproven arms race of cryptographers and cryptanalysts (Fred) improves it cryptography systems usually have an algorithm and a key publish the algorithm while keeping the key secret Fundamental Tenet of Cryptography If lots of smart people have failed to solve a problem, then it probably won t be solved (soon). Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
4 Introduction Computational Difficulty Algorithm should be efficient to compute but significantly difficult for a brute-force cryptanalysis Brute-force cryptanalysis: try all keys until looks like plaintext a longer key means more work for brute-force cryptanalysis encryption: O(N+1), brute-force: O(2 N+1 ) Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
5 Introduction Computational Difficulty Algorithm should be efficient to compute but significantly difficult for a brute-force cryptanalysis Brute-force cryptanalysis: try all keys until looks like plaintext a longer key means more work for brute-force cryptanalysis encryption: O(N+1), brute-force: O(2 N+1 ) Advances in computing benefit cryptographer more, but make old uses of cryptography easier to break DES (56 bit key) was standardized in It took 56 hours to break it in 1998, less than 1 day in 2008 Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
6 Introduction Breaking an Encryption Scheme Ciphertext only Fred has access to enough ciphertext only brute-force search until finding a recognizable plaintext Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
7 Introduction Breaking an Encryption Scheme Ciphertext only Fred has access to enough ciphertext only brute-force search until finding a recognizable plaintext Known plaintext Fred can obtain some < plaintext, ciphertext > pairs he has no control over what the plaintext is Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
8 Introduction Breaking an Encryption Scheme Ciphertext only Fred has access to enough ciphertext only brute-force search until finding a recognizable plaintext Known plaintext Fred can obtain some < plaintext, ciphertext > pairs he has no control over what the plaintext is Chosen plaintext Fred can choose a plaintext and have its ciphertext computed Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
9 Introduction Notation Symbol Meaning XOR, exclusive or concatenation (e.g., ab cd = abcd) K{message} encrypted with secret key K {message} Bob encrypted with Bob s public key [message] Bob signed by Bob with its private key Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
10 Trivial Ciphers Trivial Ciphers Caesar cipher: shift each letter by 3 A D, B E Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
11 Trivial Ciphers Trivial Ciphers Caesar cipher: shift each letter by 3 A D, B E Captain midnight secret decoder ring: shift each letter by n IBM HAL (n = 1) 26 possibilities Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
12 Trivial Ciphers Trivial Ciphers Caesar cipher: shift each letter by 3 A D, B E Captain midnight secret decoder ring: shift each letter by n IBM HAL (n = 1) 26 possibilities Monoalphabetic cipher: arbitrary mapping of one letter to another 26! (about ) possibilities letter frequencies is preserved, making it vulnerable to letter frequency analysis Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
13 Trivial Ciphers Breaking Monoalphabetic Cipher Match high/low-frequency n-grams in the language to the ciphertext until a recognizable plaintext (n = 1, 2, 3) English: e, t, a, o, I, n, s, h, r... Example: Si spy net work, big fedjaw iog link kyxogy Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
14 Trivial Ciphers Breaking Monoalphabetic Cipher Match high/low-frequency n-grams in the language to the ciphertext until a recognizable plaintext (n = 1, 2, 3) English: e, t, a, o, I, n, s, h, r... Example: Si spy net work, big fedjaw iog link kyxogy enxtybwpjld.fai..gkso...r. abcdefghijklmnopqrstuvwxyz Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
15 Trivial Ciphers Breaking Monoalphabetic Cipher Match high/low-frequency n-grams in the language to the ciphertext until a recognizable plaintext (n = 1, 2, 3) English: e, t, a, o, I, n, s, h, r... Example: Si spy net work, big fedjaw iog link kyxogy enxtybwpjld.fai..gkso...r. abcdefghijklmnopqrstuvwxyz To the bad guys, for making our jobs secure Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
16 Trivial Ciphers Fun Fact about Words Scrambled words can still be parsed by the human brain Aoccdrnig to rscheearch at Cmabrigde uinervtisy, it deosn t mttaer waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteres are at the rghit pclae. The rset can be a tatol mses and you cansitll raed it wouthit a porbelm. Tihs is bcuseae we do not raed ervey lteter by itslef but the wrod as a wlohe. Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
17 Types of Hash function zero keys Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
18 Types of Hash function zero keys Secret key cryptography (symmetric cryptography) one shared key Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
19 Types of Hash function zero keys Secret key cryptography (symmetric cryptography) one shared key Public key cryptography (asymmetric cryptography) two keys (public and private) Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
20 Secret Key Cryptography One key shared by both participators one key, two operations (encryption and decryption) how to securely agree on the key? examples: DES, IDEA, AES... Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
21 Applications of SKC Transmitting data over insecure channel eavesdropping, modification, deletion r A K AB {r A } Alice r B Bob K AB {r B } Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
22 Applications of SKC Transmitting data over insecure channel eavesdropping, modification, deletion Secure storage on insecure media Google deletes your (replicated) data by discarding the key r A K AB {r A } Alice r B Bob K AB {r B } Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
23 Applications of SKC Transmitting data over insecure channel eavesdropping, modification, deletion Secure storage on insecure media Google deletes your (replicated) data by discarding the key Data integrity check to prevent forgery of data checksum r A K AB {r A } Alice r B Bob K AB {r B } Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
24 Applications of SKC Transmitting data over insecure channel eavesdropping, modification, deletion Secure storage on insecure media Google deletes your (replicated) data by discarding the key Data integrity check to prevent forgery of data checksum Authentication: prove knowledge of the key w/o revealing it Trudy can impersonate Alice to Bob by reflection attack r A K AB {r A } Alice r B Bob K AB {r B } Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
25 Public Key Cryptography Public key for encryption, and private key for decryption one operation (encryption), two keys (inverse of each other) anybody can encrypt a message, but only the owner of the private key can decrypt it Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
26 Applications of PKC Transmitting data over insecure channel A sends {m A } eb, B decrypts with d B Alice B sends {m B } ea, A decrypts with d A Bob Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
27 Applications of PKC Transmitting data over insecure channel A sends {m A } eb, B decrypts with d B Alice B sends {m B } ea, A decrypts with d A Bob Secure storage on insecure media generate a secret key for data encryption, then encrypt the key with PKC because PKC is slow Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
28 Applications of PKC Authentication A sends {r} eb, B decrypts with d B Alice B sends r, A verifies it Bob Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
29 Applications of PKC Authentication A sends {r} eb, B decrypts with d B Alice B sends r, A verifies it Bob Digital signature provides integrity and non-repudiation Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
30 Hash Function Hash function computes a fixed-length (short) number from a message of arbitrary length given m, easy to compute h(m) given h(m), no easy way to find a m 1 that hashes to h(m) computationally infeasible to find h(m 1 ) = h(m 2 ) Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
31 Applications of Hash Function Password hash store a password hash instead of the password itself use salt to mitigate the rainbow table attack h(p s) Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
32 Applications of Hash Function Password hash store a password hash instead of the password itself use salt to mitigate the rainbow table attack h(p s) Message integrity use keyed hash to protect message integrity (h(m key)) Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
33 Applications of Hash Function Password hash store a password hash instead of the password itself use salt to mitigate the rainbow table attack h(p s) Message integrity use keyed hash to protect message integrity (h(m key)) Message fingerprint and downline load security publish with the data its md5 hash (download a copy of firefox) Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
34 Applications of Hash Function Password hash store a password hash instead of the password itself use salt to mitigate the rainbow table attack h(p s) Message integrity use keyed hash to protect message integrity (h(m key)) Message fingerprint and downline load security publish with the data its md5 hash (download a copy of firefox) Digital signature efficiency sign a hash of the message instead of the message itself (Public key encryption is inefficient) Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
35 Summary What is cryptograph Three ways to break cryptography Trivial ciphers Cryptographic functions and their applications Next lecture: secret key cryptography Zhi Wang (FSU) CNT4406/5412 Network Security Fall / 18
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