Chapter 3 - Cryptography Only Flashcards
Cryptology
Science of secure communications. Encompasses both cryptography and cryptoanalysis
Cryptoanalysis
Science of breaking encrypted messages
Cryptography
Creates messages whose meaning is hidden
Permutation
Another word for Transposition. Provides diffusion by rearranging the characters of the plaintext, anagram-style.
Work Factor
Describes how much effort it takes to break encryption
Good Encryption
Not based on secret formulas but on good math.
Monoalphabetic cipher
uses one alphabet: a specific letter is substituted for another. Susceptible to frequency analysis.
Polyalphabetic cipher
uses multiple alphabets, a specific letter maybe used in one round and another the next round – addresses frequency analysis
Modular Math
Modular math lies behind much of cryptography: simply put, modular math shows you what remains (the remainder) after division. It is sometimes called clock math because we use it to tell time: assuming 12 hour clock, 6 hours past 9:00pm is 3:00am. In other words, 9:6 is 15, divided 12 leaves remainder 3.
XOr
Combining a key with plaintext via XOR creates a ciphertext. XOR-ing the same key to the ciphertext restores the original plaintext.
Rules: If two bits are different the answer is “1.” If two bits are the same the answer is “0.”
Data at rest and Data in motion
Cryptography is able to protect data at rest and data in motion. Full disk encryption can protect data at rest and SSL or IPSEC VPN can protect data in motion.
Protocol Governance
Describes the process of selecting the right method (cipher) and implementation for the right job, typically at an organization-wide scale.
Egyptian Hieroglyphics
Hieroglyphics are stylized pictorial writing used in ancient Egypt.
Spartan Scytale
The Scytale was used in ancient Sparta around 400BC. A strip of parchment was wrapped around a rod. The plaintext was encrypted by writing lengthwise down the rod. The message was then unwound and sent. When unwound the words appeared as a meaningless jumble. The receiver possessing a rod of the same diameter, wrapped the parchment across the rod, reassembling the message.
Caeser Cipher and other rotational ciphers
The Ceasar cipher is am monoalphabetic rotation Cipher. Caeser rotated each letter of the plaintext forward three times to encrypt. Another common rotation cipher is rot-13, move the letter forward 13 times to encrypt.
Vigenere Cipher
The Vignere Cipher is a polyalphabetic Cipher. A matrix is formed with the alphabet and lookups are done with the key (repeated up to the length of the plaintext) to form the cipher text.
Cipher Disk
Cipher disks have two concentric disks, each with a alphabet around the periphery. They allow both monoalphabetic and polyalphabetic encryption. For monoalphabetic encryption two parties agree on a fixed starting offset and then turn the wheel once every X characters. For polyalphabetic encryption, do the same as before but turn where once every X characters and then turn the inner disk 1 character to the right after every 10 characters
Jefferson Disks
Invention of Thomas Jefferson. It is also known has “Wheel Cipher”. The device contains 36 wooden disks in which each disk has random characters of the 26 characters of the alphabet printed along each disk in random order. The other party must posses the identical set of disks.
Order the disks into saying something, then pass the jumbled line to the other person. To decrypt they make the same jumble and then look for lines that make sense.
Book Cipher
A book cipher uses whole words from a well-known text such as a dictionary. To encode, agree on a text source and note the page number, line and word offset of each word you would like to encode.
Looks something like . . . . As 12.4.5 I am 12.8.17, which translates to “As vengeance I am god!” 12 is the page, 4 is the row, 5 is 5 words from the left, etc.
Running-key cipher
Running-key ciphers also use well-known texts as the basis for their keys: instead of using whole words, they use modulus math to “add” letters to each other.
Find a picture of this, pretty cool.
Codebooks
Codebooks assign a code for important people, locations, and terms. Each word in the code-book has two codenames, Ex. the President was “Adam” or “Asia”, the secretary of state was “Abel” or “Austria”
One-time Pad
A one-time pad uses identical paired pads of random characters, with a set amount of characters per page. Assume a pair of identical 100 page one time pads with 1000 random characters per page. Once created and securely distributed, to communicate use modular addition to encrypt and modular subtraction to decrypt. Discard page of pad once used. This is mathematically proven to be secure as long as pads are kept secure, and pages are never reused.
Vernam Cipher
Named after Gilbert Vernam, created a teletypewriter capable of encypting and decrypting using paper rolls containing the encryption key (One time pad). The Vernam Cipher used bits, the one time pad bits were XORed to the plaintext bits.
Hebern Machines and Purple
Hebern machines are a class of cryptograhic devices known as rotor machines named after Edward Hebern. They look like manual typewriters with electrified rotors.
