- Cryptography - Encryption of information
- Cryptanalysis - research into cryptographic methods
- Cryptolinguistics - Examination of language features
- Steganography - hiding information in information
- Alphabet - Finite amount of characters available for composing a message
- Plain message - message to be encrypted m
- Ciphertext - The encrypted message C
- Encryption Function E= (M, Ke) with Key E
- Decryption Function - D=(C, Kd) with Key D
- What does Kerkhoff’s principle state ?
The security of an encryption method is based solely on the secrecy of the key and not the secrecy of the encryption algorithm
Explain the substitution procedure .
Each element in a message is replaced by another element but is not moved in the text
What is codification
When whole words are replaced
What is ciphering and what are the 2 types of ciphering ?
If one replaces individual characters of an alphabet, it is a
matter of ciphering.
Types:
- caesar cipher
- Atbash cipher
What is the mono alphabetic solution?
Example: Caesar shift/shift cipher
Each character of the plaintext alphabet is mapped to a character of the ciphertext alphabet
What are the two major groups of ciphers - hint from previous flashcard **
- Monoalphabetic ciphers
- the whole text is encrypted with a
fixed assignment table - Polyalphabetic ciphers - several assignment tables in alteration
Transposition method
Example - Column Transposition: rows and columns are swapped
Each element of a message is moved to a different position within the text.
The element itself is not changed
Polyalphabetic substitution
Substitution by means of several ciphertext alphabets
– Plaintext character is not always replaced with the same ciphertext character
– Blurring of frequencies in plain text
Block ciphers - Explain the steps.
▪ Divide the plaintext into blocks of a certain size before encryption
▪ Each block has identical sizes in plaintext and ciphertext
▪ Blocks are encrypted with a secret key
▪ Decryption is done with the same key (symmetric encryption).
▪ If each block is encrypted independently of the others, this is called ECB mode (Electronic Codebook).
▪ ECB Mode is rarely used
Cipher Block Chaining (CBC Mode)
- uses feedback (XOR linking of an encrypted block with the next block to be encrypted)
▪ This prevents that same plaintext block from always resulting in the same encrypted block
▪ The 1st block to be encrypted is linked to an initialization vector (IV), which must also be known to the recipient
What are Hash functions?
Mathematical one-way function - maps a file or message of arbitrary (random) length to a fixed-length value.
Characteristics of hash functions…
- Non-injective - (= each element of the target set is assumed to be a function value at most once)
- Even the smallest changes (bit) of the source file change the hash value (= avalanche effect).
2 Typical Hash algorithms
▪ MD5 - Message Digest 5
▪ SHA - Secure Hash Algorithm
Classification of cryptanalytic attacks
– Attack with known plain text
– Attack with freely selected plain text
– Attack with selected ciphertext
▪ Brute force
– systematically try out all possible keys
▪ Dictionary Attack
– Dictionary attack, password collections
What is confusion and diffusion in encryption procedures
- Diffusion - Statistical structure of the plaintext M scatter in ciphertext C
- Confusion - The relationship of the key K to the Ciphertext C must be maximally complex
Requirements for Diffusion
▪ Dispersion of the structure (e.g. by transposition or permutation)
▪ small change in plaintext => big effect in ciphertext
▪ statistical analysis is made more difficult
Requirements for confusion
▪ Complexity between key and ciphertext as large as possible
▪ It must not be possible to derive the key simply from ciphertext
▪ e.g. by means of substitution mechanisms
