CISSP Domain 3 Security Engineering – Part 2 – Cryptographic Concepts Cheat Sheet


History of Cryptography

Egyptian Hieroglyphics

Hieroglyphics are stylized pictorial writing used in ancient Egypt. Some Hieroglyphics contained small puzzles, meant to attract the attention of the reader who would solve the simple pictorial challenge.

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. Applying the function one more time results in rot-26 which decrypts the message.

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.

vigenere-highlight

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. Arrange the first 36 letters of plaintext along one line of letters on the disks then pick any other line of “jumbled” letters: this is the cipher text. Continue this process for the next 36 characters of the plaintext.

Book Cipher and running-key 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.

Ex. As , and ..” etc. Translates to “As life and fortune are risked by serving his majesty…”

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.

running-key-cipher

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 identical 100page 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.

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About the Author

Alfred Tong
Author and owner of this blog. A Networking enthusiast, full time networking and systems Engineer. Generally curious about all things IT.Certifications: GIAC GSEC, CCNP-S, CCNP, CCSP, CCDP, CCNA, RHCE, JNCIA - FWV