Title: Theory / Unimportant / CS / Algo / Crypto / Schneier
Tags: récupéré plusieurs questionExample
On every note of this deck, unless a note says otherwise:
Chapter: 1
Section: 1

Question: Two kinds of cryptography
Answer: cryptography that will stop your kid sister from reading your files
Page: 9
Chapter: Preface
Answer2: cryptography that will stop major governments from reading your files

Name: Sender
Definition: Someone emitting a message
Page: 12

Name: Receiver
Definition: The intended person who should receive a message
Page: 12

Name: Plaintext
Definition: A message without any modification
Page: 13
Notation: \(M\) or \(P\)
Example: stream of bits, a text file, a bitmap, a stream of digitized voice, a digital video image
Name2: cleartext

Name: encryption
Definition: The process of disguising a message in such a way as to hide its substance
Page: 13
Notation: \(E\)
Name2: encipher

Name: ciphertext
Definition: An encrypted message
Page: 13
Typ: Binary data
Denoted by: \(C\)

Name: decryption
Definition: turning encrypted message to plain text
Page: 13
Notation: \(D\)
Name2: deciphering

Name: cryptography
Definition: confidentiality
Page: 13-14
Definition2: authentication
Conjdef: A subset of
Definition3: integrity
Definition4: nonrepudiation
Applied to3: The art and science of keeping messages secure

Name: cryptographer
Definition: A practitionner of cryptography
Page: 13

Name: cryptanalyst
Definition: A practitionner of cryptanalysis
Page: 13

Name: cryptanalysis
Definition: the art and science of breaking ciphertext
Page: 13

Name: cryptology
Definition: The branch of mathematics encompassing both cryptography and cryptanalysis
Page: 13

Name: cryptologist
Definition: practitionners of cryptology
Page: 13
Typ: People trained in theoretical mathematics

Question: \(D(E(M))\) equals
Answer: \(M\)
Page: 13
Chapter: 1
Variables: \(M\) a message
\(D\) a decryption function
\(E\) an encryption function

Name: Authentication
Definition: It should be possible for the receiver of a message to ascertain its origin; an intruder should not be able to masquerade as someone else.
Page: 13

Name: Integrity
Definition: It should be possible for the receiver of a message to verify that it has not been modified in transit; an intruder should not be able to substitute a false message for a legitimate one.
Page: 13

Name: Nonrepudiation
Definition: A sender should not be able to falsely deny later that he sent a message.
Page: 14

Name: Confidentiality
Definition: Only the receiver can read the message
Page: 14

Name: A cipher
Definition: the mathematical function used for encryption and decryption
Page: 14
Name2: A cryptographic algorithm

Name: A restricted algorithm
Definition: An algo whose security comes from the fact that it is unknown
Page: 14

Question: Why not to use restricted algorithm
Answer: If someone leave the groups, algo should be changed
Page: 14
Chapter: 1
Context: Crypto
Answer2: If algo leak, it should be changed
Answer3: It should be created to be different from off-the-shelf product
Answer4: it allows no quality control or standardization

Question: Why people use restricted algorithm
Answer: They don't know that it's unsecure
Page: 14
Chapter: 1
Answer2: They don't care that it's unsecure

Name: The keyspace
Definition: The set of possible keys
Page: 14
Variables: \(C\) a cypher

Name: A key
Definition: A parameter for encryption and decription, shared by sender and receiver
Page: 14
Variables: \(C\) a cypher

Name: An encryption key (private key?)
Definition: A parameter for encryption, owned only by sender
Page: 14
Variables: \(C\) a cypher

Name: A decryption key
Definition: A parameter for decryption, owned only by sender
Page: 14
Variables: \(C\) a cypher

Question: Why to use key-based cypher ?
Answer: Algo can be analyzed
Page: 15
Chapter: 1
Answer2: If eavesdropper know the key, it suffices to change it
Answer3: algo can be mass produced

Name: A cryptosystem
Definition: Algo
Page: 15
Definition2: all possible plaintext
Conjdef: All of
Definition3: all possible cyphertext
Definition4: all possible keys

Name: Symmetric algorithm
Definition: Algo where the encryption key can be calculated from the decryption key and vice versa
Page: 15
Context: key-based algorithm
Name2: conventional algorithm
Example2: single-
key algorithms, or one-key algorithms
Intuition: secret-key algorithms,

Name: stream algorithm
Definition: An algorithm working on bit or byte, one after the other
Page: 15

Name: block algorithm
Definition: An algorithm encrypting block by block
Page: 15
Context: cryptography

Name: A block
Page: 15
Context: cryptography
Example: 64 bits
Typ: A group of bit

Name: Public-key algorithm
Definition: An algorithm designed so that the key used for encryption is different from the key used for decryption.
Page: 15
Context: cryptography
Definition2: Furthermore, the decryption key cannot (at least in any reasonable amount of time) be calculated from the encryption key.
Name2: asymmetric algorithms

Name: Public-key
Definition: The encryption key
Page: 15
Context: Public-key algorithm

Name: Private-key
Definition: The decryption key
Page: 16
Context: Public-key algorithm
Name2: secret key

Question: eavesdroppers have access to
Answer: algorithm
Page: 16
Chapter: 1
Answer2: communication channel
Answer3: public-key if there is one
Answer4: cyphertext
Answer5: implementation

Name: eavesdroppers
Definition: Someone trying to have access to the message, given everything else
Page: 16
Context: no notation, but 8 names
Notation: adversaries
Name2: attackers
Notation2: interceptors
Name3: interlopers
Name4: opponents
Notation3: intruders
Notation4: the enemy

Name: Cryptanalysis
Definition: the science of recovering the plaintext of a message without access to the key.
Page: 16
Definition2: finding weaknesses in a cryptosystem that eventually lead to the previous results.
Conjdef: All of

Name: compromise
Definition: The loss of a key through noncryptanalytic means
Page: 16

Name: An attack
Definition: An attempted cryptanalysis
Page: 16

Question: The general types of cryptanalytic attacks
Answer: Ciphertext-only attack.
Page: 16
Chapter: 1
Answer2: Known-plaintext attack.
Answer3: Chosen-plaintext attack
Answer4: Adaptive-chosen-plaintext attack
Answer5: Chosen-ciphertext attack
Answer6: Chosen-key attack
Answer7: Rubber-hose cryptanalysis.

Name: Ciphertext-only attack
Definition: The cryptanalyst’s job is to recover the plaintext of as many messages as possible, or better yet to deduce the key (or keys) used to encrypt the messages, in order to decrypt other messages encrypted with the same keys.
Page: 16
Definition2: The cryptanalyst has the ciphertext of several messages, all of which have been encrypted using the same encryption algorithm.
Conjdef: An attack such that

Name: Known-plaintext attack
Definition: His job is to deduce the key (or keys) used to encrypt the messages or an algorithm to decrypt any new messages encrypted with the same key (or keys)
Page: 16
Index: 2
Definition2: The cryptanalyst has access not only to the ciphertext of several
messages, but also to the plaintext of those messages.
Conjdef: An attack such that

Name: Chosen-plaintext attack
Definition: His job is to deduce the key (or keys) used to encrypt the messages or an algorithm to decrypt any new messages encrypted with the same key (or keys).
Page: 16
Index: 3
Definition2: The cryptanalyst not only has access to the ciphertext and associated plaintext for several messages, but he also chooses the plaintext that gets encrypted.
Conjdef: An attack such that

Name: Adaptive-chosen-plaintext attack.
Definition: His job is to deduce the key (or keys) used to encrypt the messages or an algorithm to decrypt any new messages encrypted with the same key (or keys).
Page: 16
Index: 4
Definition2: The cryptanalyst choose the plaintext that is encrypted, but he can also modify his choice based on the results of previous encryption.
Conjdef: An attack such that

Name: Chosen-ciphertext attack.
Definition: His job is to deduce the key (or keys) used to encrypt the messages or an algorithm to decrypt any new messages encrypted with the same key (or keys).
Page: 17
Index: 5
Definition2: The cryptanalyst can choose different ciphertexts to be decrypted and has access to the decrypted plaintext.
Conjdef: An attack such that

Name: Chosen-key attack
Definition: An attack such that the cryptanalist has some knowledge about the relationship between different keys
Page: 17
Index: 6

Name: Rubber-hose cryptanalysis
Definition: The cryptanalyst threatens, blackmails, or tortures
someone until they give him the key.
Page: 17
Index: 7

Name: Purchase-key cryptanalysis
Definition: Attack by Bribery
Page: 17
Index: 7

Name: Chosen-text attack
Definition: chosen-plaintext attack
Page: 17
Index: 5
Definition2: a chosen-ciphertext attack
Conjdef: Both

Question: Best way to break an algorithm
Answer: Rubber-hose cryptanalysis
Page: 17
Chapter: 1
Index: 5

Question: How to do chosen plaintext attack ?
Answer: Give some text to someone so that it transmit it securely
Page: 17
Chapter: 1
Index: 5

Question: Examples of "known plaintext" attack ?
Answer: Source code, program. With a lot of keyword repeated
Page: 17
Chapter: 1
Index: 5

Question: Those who claim to have an unbreakable cipher simply because they can’t break it are
Answer: geniuses
Page: 18
Chapter: 1
Answer2: fools
Index: 5

Question: Good cryptographers rely
Answer: on peer review to separate the good algorithms from the bad.
Page: 18
Chapter: 1
Index: 5

Smaller0: the cost required to break an algorithm is greater than the value of the encrypted data
Greater0: you’re probably safe
Page: 18
Chapter: 1
Index: 5
Smaller1: the time required to break an algorithm is longer than the time the encrypted data must remain secret
Context: cryptography
Connect Smaller0: or
Smaller2: the amount of data encrypted with a single key is less than the amount of data necessary to break the algorithm

Question: categories of breaking an algorithm in decreasing order of severity
Answer: Total break.
Page: 18
Chapter: 1
Answer2: Global deduction.
Index: 5
Answer3: Instance (or local) deduction.
Answer4: Information deduction.

Name: Total break.
Definition: A cryptanalyst finds the key, \(K\), such that \(D_K (C)=P\).
Page: 18
Index: 5

Name: Global deduction
Definition: A cryptanalyst finds an alternate algorithm, \(A\), equivalent to \(D_K (C)\), without knowing \(K\).
Page: 18

Name: Instance deduction
Definition: A cryptanalyst finds the plaintext of an intercepted ciphertext
Page: 18
Name2: Local deduction

Name: Information deduction
Definition: A cryptanalyst gains some information about the key or plaintext. This information could be a few bits of the key, some information about the form of the plaintext, and so forth.
Page: 18

Name: An unconditionnaly secure algo
Definition: An algo such that, no matter how much ciphertext a cryptanalyst has, there is not enough information to recover the plaintext.
Page: 18
Example: one-time pad

Name: Brute-force attack
Definition: Attack by trying every single key and check for output which makes sens
Page: 19

Name: A computationally secure algorithm
Definition: An algorithm which can not be broken with available resources, either ucrrent or future
Page: 19
Name2: a strong algorithm

Name: Data complexity
Definition: The amount of data needed as input to the attack.
Page: 19
Context: Attack

Name: Processing complexity
Definition: he time needed to perform the attack. This is often called the work factor.
Page: 19
Context: Attack

Name: Storage requirements.
Definition: The amount of memory needed to do the attack.
Page: 19
Context: Attack
Index: 3

Name: The complexity of an attack
Definition: data complexity
Page: 19
Context: Attack
Definition2: processing complexity
Conjdef: The minimum of
Definition3: storage requirements