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- 1. Cryptography II Domain 5 Pages 790-821 Official CISSP CBK Third Edition Jem Jensen StaridLabs
- 2. A Quick Recap ● Converting plaintext into ciphertext through transpositions or substitutions ● Spartan Scytale – Wrap message (written on a belt/strap) around a stick to decipher ● Caesar cipher – shift the whole alphabet ● Nazi Enigma machines ● Key – used to encrypt/decrypt the plaintext
- 3. Substitution Ciphers ● Replaces a letter for a letter ● Like in the newspaper! ● Examples: – Caesar cipher – Decoder ring
- 4. Playfair Cipher ● Used through WW2 ● Pre-shared keyword (Ex: CRYPTO) ● Keyword is fed into a table, followed by the remaining alphabet – Merge I&J C R Y P T O A B D E F G H I/J K L M N Q S U V W X Z
- 5. Playfair Cipher ● Break apart plaintext into pairs – Sprinkle filler characters (Ex: K) WE AR EK ST AR IK DL AK BS
- 6. Playfair Cipher ● Draw a box between the 2 letters in the pair – If the box is larger than 1xn, swap letters with the other 2 corners (Ex: WE = BZ) – If the box is only 1 row thick, use the next letter in line (Ex: AR=RV) C R Y P T O A B D E F G H I/J K L M N Q S U V W X Z
- 7. Transposition Cipher ● Change the order of letters ● Like pig latin! ● Move first letter of a word to the end ● Add an “a” sound at the end ● Et-gay ti-ay?
- 8. Rail Fence ● Like writing the message on a fence then tilting the posts to the side ● Draw a box with 2+ columns thick ● Enter the text in a zig-zag fashion ● Read the text across the columns for encrypted version (Ex: gvecslirmaieahodeael) ● Susceptible to frequency analysis
- 9. A quick side note ● Frequency analysis ● E, T, A, & O are the most common letters in English words ● Z, Q, & X are the least used ● If you know a message is in English and you see a lot of E, T, A & Os or very few Z, Q, or X – you know it's a transposition cipher and can probably start guessing some letters correctly
- 10. Rectangular Substitution Table ● Sender and receiver agree on table dimensions and the order to read the message (Ex: 4x4 table, read top to bottom, left to right) = WERAESIBATDSRAL Also susceptible to frequency analysis W E A R E S T A R I D L A B S
- 11. Monoalphabetic Cipher ● Like a Caesar cipher except we scramble the alphabet instead of shifting it CAESAR MONO Ex: In the above Caesar, FEED=IHHG In the above Mono, FEED=IWWU A B C D E F ... ... Y Z M G P U W I ... ... T K A B C D E F ... ... Y Z D E F G H I ... ... B C
- 12. Polyalphabetic Cipher ● Like a monoalphabetic except we add more rows of scrambled alphabets POLY Ex: In the Mono, FEED=IWWU In the above Poly, FEED=IXWC A B C D E F ... ... Y Z M G P U W I ... ... T K N B V C X Z ... ... F W
- 13. Blaise De Vigenére ● Transposition table
- 14. Blaise De Vigenére ● Sender/receiver have a pre-shared keyword ● Locate the intersection of the keyword with the plaintext for the cipher equivalent Example Keyword: CABBAGE Plaintext: FEEDBOB Ciphertext: HEFEBUF
- 15. Modular Mathematics ● Modulus/Modulo Operation ● The remainder after dividing a number by n Example 5 mod 2 : 5/2 : 3/2 = 1 4 mod 2 : 4/2 : 4/2 = 0
- 16. Running Key Cipher ● Since there are 26 letters in the alphabet, we'll use mod 26 Ciphertext = (plaintext + key) mod 26 C = P + K (mod 26) ● Example: Plaintext: E (or 4) C = (4 + 22) mod 26 = 0 Key: W (or 22) Ciphertext: A A B C D E F ... ... Y Z 0 1 2 3 4 5 ... ... 24 25
- 17. Running Key Cipher ● To translate CHEEK using the key of FEED: ● The longer the key, the better, since there's less repetition Ptxt C H E E K Key F E E D F n(P) 2 7 4 4 10 n(K) 5 4 4 3 5 n(C) 7 11 8 7 15 Ctxt H L I H P
- 18. One-time Pads ● Gilbert Vernam asserted that a cipher key as long as the plaintext (that doesn't repeat) should be unbreakable ● Requires a sender and receiver to exchange the long key beforehand (ex: book ciphers) ● Ideally the key is randomly chosen values like letters, numbers, Geiger counter readings
- 19. One-time Pads ● Key of ksosdfshepwlqz ● The key could be longer than the plaintext Ptxt D O N O T Key K S O S D n(P) 3 14 13 14 19 n(K) 10 18 14 18 3 n(C) 13 32 27 6 22 Ctxt N B G W S
- 20. A step back ● So what have we learned? ● Crypto can provide integrity controls ● If the message was altered by a modification or error, it won't decrypt ● Some degree of authentication ● Only the people who have the key should be able to use it. If we only share the key with one person, we can know it's them
- 21. Symmetric Cryptography ● The sender and receiver use a single shared key for encryption/decryption ● Key management is the biggest problem ● We can't send the keys over the same channel we're trying to encrypt! ● Send the key via out-of-band distribution (ex: fax, mailing a CD, calling on the phone)
- 22. Symmetric Cryptography ● Fast, secure cheap ● Doesn't provide non-reputiation ● Limited message integrity and access control ● We can tell the message changed and it requires the key in order to be read ● Physical example: ● 10 people have a key to the server room. Who entered at 11pm? We only know it was one of the 10
- 23. Examples of Symmetric Algorithms ● Caesar Cipher ● Spartan Scytale ● Enigma Machines
- 24. DES ● Data Encryption Standard ● Harst Feistal ● Take input block, divide it in half, and XOR several times – Each XOR is called a round ● Became gov't standard in 1977 ● Repaced by AES
- 25. Block Cipher Modes ● ECB – Electronic Codebook Mode ● Essentially a digital representation of a codebook. Feed in plaintext and it outputs the ciphertext using a table ● Useful for very short messages to reduce repeating (64-bit) ● CBC – Cipher Block Chaining Mode ● Uses an IV and chaining function so that subsequent output will be different even if the input was identical ● Initial input block is XORed with the randomly chosen IV ● Output if then XORed with next input
- 26. Block Cipher Modes ● CFB – Cipher Feedback Mode ● Input broken into segments (usually 8 bit) ● Segments are XORed with a random IV ● Previous segment is XORed with the next segment ● Drawback – if a bit is corrupted, everything after will be too ● OFB – Output Feedback Mode ● Like CFB but uses encrypted keystream instead of ciphertext so that one corruption won't affect future encryption ● Can also generate keystream ahead of time for future use ● CTR – Counter Mode ● Like OFB but increments a counter for keystream ● Allows for out-of-order processing
- 27. Pros/Cons of DES ● Fast ● Breaking DES was unrealistic back in the day ● Susceptible to brute-forcing since key is only 56 bits long
- 28. Double DES ● Since brute forcing if our main method of attack... let's just double the key! ● Done by running DES over DES with diff keys Yo dawg! I heard you like DES so I DESed your DES so you can DES while you DES!
- 29. Double DES ● Meet in the middle attack ● Double DES had hoped that their “key doubling” would result in an attack taking exponentially longer ● Can still brute force ● Just takes twice as long ● Step 1: Encrypt plaintext with all possible keys ● Step 2: Decrypt using all possible keys
- 30. Triple DES (3DES) ● Third time's the charm! ● Eliminates meet in the middle attack I don't know many more times can I DES the DES with a DES...
- 31. AES ● Beat out 3DES in terms of: ● Security ● Speed ● Larger block size ● Rijndael algorithm
- 32. CCMP ● Counter mode cipher block chaining message authentication code protocol ● AES with 128 bit key ● 48 bit IV – Prevents replay attacks ● CTR counter – Provides data privacy ● MIC – message integrity check code ● Used in 802.11i standard
- 33. Algorithms ● Rijndael ● Substitute bytes (s-box substitution) ● shift rows (transposition) ● mix columns (substitution) ● add round key (XOR with this round's key) ● IDEA (1991) ● 8 rounds of transposition and substitution ● CAST (1996) ● 48 rounds
- 34. Algorithms ● SAFER ● Patent-free, 64 or 128 bit ● Used in bluetooth ● Blowfish ● Extremely fast and memory efficient ● Slow to regenerate keys ● Currently unreakable ● Twofish
- 35. Algorithms ● RC5 ● RSA algorithm ● Very adaptable – 4 modes of operation ● RC4 (1987) ● Stream cipher ● Varying length keys
- 36. Pros/Cons of Symmetric Algorithms ● Very fast, secure ● Can be implemented in hardware and software ● Key management can be difficult ● Can be challenges with distributing the keys ● Manage additional out of bound channel ● Can't provide non-reputiation
- 37. Next Week: Asymmetric Algorithms!

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