Cryptography Flashcards
Define Cryptograhpy
Cryptography is the (art and) science of keeping information secure This is usually done by encoding it

What is Cryptanalysis?
Cryptanalysis is the (art and) science of breaking a code
What is Cryptology?
Cryptology is the branch of math needed for cryptography and cryptanalysis
Which 4 areas of IS can Cryptography help with?
Confidentiality: only authorized persons are allowed to decode a message Authentication: receiver of a message (e.g. a password) should be able to ascertain its origin Integrity: receiver of a message should be able to verify that it hasn’t been modified Non-repudiation: a sender shouldn’t be able to falsely deny that they sent a message -We’re talking about messages here, but the principles can be applied to any information
What is plaintext?
The original message
what is encryption?
Disguising the content of a message
What is ciphertext?
the encrypted message
What is Decryption?
Turning ciphertext back into plaintext is called decryption
What is a cryptographic algorithm?
the mathematical function used for encryption and decryption
What is a restricted algorithm?
If security is based on keeping the algorithm secret, then it’s a restricted algorithm
Why are restricted algorithms a bad idea?
Every time a user leaves a group, the algorithm has to be changed A group must develop their own algorithm; if they don’t have the expertise, then it will be subpar
What is a cryptographic key?
taken from a large range of possible values, the keyspace used as additional input for the en-/decryption function to do the en-/decrypting
What is the benifit of having the security based in the key, not in the details of the algorithm?
The algorithms can be published and analyzed by experts for possible flaws Software for the algorithm can be mass-produced Even if an eavesdropper knows the algorithm, without the key messages cannot be read
What is a cryptosystem?
An algorithm together with all possible plaintexts, ciphertexts, and keys is called a cryptosystem
What are symmetic algorithms aka conventional algorithms?
that encryption and decryption keys are identical Anyone who has the key can decrypt a message However, before two persons/systems can communicate, they need to agree on a key
What are Asymmetric algorithms aka public-key algorithms?
The key used for encrypting messages is the public key The key used for decrypting messages is the private key
How do public-key algos work?
The public key can be published, so that anyone can encrypt messages to a person/system The private key is only known to the receiver This only works if the private key cannot be calculated from the public key (in any reasonable time)
Name 4 important encryption techniques
Substitution ciphers Transposition ciphers Stream ciphers Block ciphers
What is a substitution cypher?
In a substitution cipher each character in the plaintext is substituted for another character
What is a Ceasar cypher?
One of the earliest techniques to be used in the famous Caesar cipher In the Caesar cipher each character is shifted (by three characters in the original cipher)
What is a Multiplicative Inverse?
Multiplicative inverse: a multiplicative inverse of x is the number we need to multiply to x to get 1.
What is the problem with Monoalphabetic ciphers?
Monoalphabetic ciphers are not very secure and can be easily broken by statistical means: Different characters have typical frequencies in languages
Homophonic substitution ciphers?
Homophonic substitution ciphers try to obscure the frequencies by mapping a character to more than one code For example, “A” could correspond to 5, 13, 25, or 56; while for “B” this could be 7, 19, 32, or 42
What is the problem with homophonic substitution ciphers?
While this makes analysis a bit harder, it doesn’t hide all statistical properties With the help of a computer can usually be broken in a few seconds

