3.5.6.8 Encryption. Flashcards
Define encryption.
Process of scrambling data so it cannot be understood by others, unless they know the method used to encrypt and the key.
Define decryption.
Process of turning the scrambled data back into plaintext.
Define plaintext.
Data in human-readable form.
Define cyphertext.
Data that has been encrypted.
Outline a Caesar cipher.
Caesar ciphers encrypt information by replacing characters. One character is always
replaced by the same character
Outline the two types of Caesar ciphers.
Shift ciphers and substitution ciphers.
Outline the process of a shift cipher.
When encrypting using a shift cipher, all of the letters in the alphabet are shifted by the
same amount. The amount by which characters are shifted forms the key.
Outline a substitution cipher.
Substitution ciphers are a type of Caesar cipher in which letters are randomly replaced.
Why are Caesar ciphers so easily cracked?
The frequency at which each character occurs can
provide a clue as to which letter has been replaced with which. The most commonly
occurring letter in English is likely to be an E. Once you discover just one character, a shift cipher can be completely cracked, as the key can be found.
This is known as frequency analysis.
Substitution ciphers are a little
better but are still relatively easy to crack.
Outline the brute force approach.
The shift cipher (Caesar) is insecure as there are only 26 possible choices for the key, meaning that each number could be tried, one by one, until the encryption (aka brute force).
Define a vernam cipher.
The Vernam cipher is an example of a one-time pad cipher. Additionally, the Vernam cipher requires the key to be
random and at least as long as the plaintext that is to be encrypted.
Define a one-time pad cipher.
The key is used once before being discarded.
Outline the process of the vernam cipher.
- Aligning the characters of the plaintext and the key.
- Converting each character to binary (using an
information coding system). - Applying a logical XOR operation to the two bit
patterns. - Converting the result back to a character.
Outline decoding a vernam cipher.
Since the key is random, the ciphertext is random and so the cipher is considered completely secure. Vernam is the only cipher mathematically proven to be completely secure.
Compare Vernam cipher with ciphers that depend on computational security.
All ciphers other than the Vernam cipher are, in theory, crackable, but not within a
reasonable timeframe given current computing power. Ciphers that use this form of
security are said to rely on computational security.
