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Algorithms for Internet Applications > Bis Ende > Flashcards

Bis Ende Flashcards

1
Q

TRIE

A
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2
Q

Unterscheid PATRICIA und PAT-tree

A

PAT-Trees preprocess a text for searching patterns.

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3
Q

Sisstring

A
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4
Q

Page Rank

A
  • between 0 and 1
  • likelihood to be visited by a person randomly surfing the web an following links
  • Weight x
  • 1/N: probability to visit A when being selected randomly out of all documents
  • sum of ranks of all documents which link to A, divided by their number of outgoing links
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5
Q

Page Rank ausrechnen

A

putting the ranks of the latest iteration into the formula until the values converge

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6
Q

Data Encryption Standard

A
  • 64 bit key in 54 by ignoring every 8th bit
  • permutation table: alt in Tabelle -> neue Postition in Tabelle
  • round keys: dividing 54 into 28 bit halves and left shifting in q cyclic way
  • different bits of the key are used in each of the 16 rounds of the DES, which is one of the strengths of this algorithm
  • number of weak keys however is quite small (about 64)
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7
Q

Hexadecimal

A
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8
Q

DES

S-box

A
  • nonlinear
  • safety of DES
  • each S-box is a 4x16 table
  • In DES there are 8 fixed S-Boxes
  • S-box reduces the amount of bits Input: 6 bits -> output: 4 bits
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9
Q

S-box nonlinear Beweis

A
    1. und letzte = Zeile
  • dazwischen = Spalte
  • bei 0 anfangen zu zählen

linearity would allow for attacks with easy linear algebra.

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10
Q

RSA

A
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11
Q

Explain how a search engine would proceed in order to return the relevant documents for our query and write down the mathematical formulas.

A
  • eliminate all documents which do not contain any of the query terms
  • number of times each query term occurs in each document, the so-called term frequency tfij (with i being the term and j being the document)
  • inverse document frequency factor idfi: weight of terms that occur very frequently in the collection and increases the weight of terms that occur rarely.
  • dividing the number of all documents (here: n) by the number of documents containing the term (document frequency dfi)
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12
Q

Take into consideration that some documents might be (a lot) larger than others. What problem might occur regarding our query terms and how can we prevent this problem.

A
  • we need to normalize this count in order to prevent a bias towards longer documents to give a measure of the importance
  • Nenner: to the maximum term frequency of any term occurring in the document dj
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13
Q

What could you do in order to improve the relevance of one of your documents? Which actions are considered manipulation?

A

search engine optimization (SEO)

on-/off-page factors

punish manipulations by giving the document a low relevance

  • Term-/Keyword-frequency: use certain keywords very often (clever writing or spamming/doorway page)
  • Link structure: Documents linking to your document improve its rating (link farms)
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14
Q

cryptology

  • Confusion
  • Diffusion
  • One-Time Pad
A
  • obscures the relationship between plaintext and ciphertext characters (e.g. substitution)
  • dissipates the redundancy of the plaintext (e.g. permutation); diffusion alone is easily cracked
  • only provably safe encryption technique: random key of the same length as the plaintext is generated and only used once
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15
Q

What are the requirements for a public key cryptosystem?

A
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16
Q

Data Encryption Algorithms

A

only differ at 22 positions of the 64-bit positions in total.
If this problem occurs for many plaintexts, then the avalanche effect is not working well enough.

Without the avalanche effect (especially provided by the S-boxes), the differential cryptanalysis would be facilitated.

17
Q

RSA

Requirements

A
18
Q

RSA

Choice of N, c und d

A
19
Q

RSA

Encryption

A
20
Q

CFM & OFB

A
21
Q

Assume that, due to a transmission error, one bit is changed in a cipher block somewhere in C. Explain the advantages and disadvantages of the error propagation in OFB!

A
22
Q

Consider the following example: After Eve has inserted the 0 in plaintext P′, she observes the ciphertext C′. Try to decrypt the suffix of P.

A
23
Q

four requirements of good cryptographic hash functions

A
24
Q

Diffie-Hellmann

A
  • if p is a prime of at least 300 digits, and a and b are at least 100 digits: discrete logarithm problem (can not be computed in realistic time using public information)
  • The problem here: authenticity of Alice and Bob is not validated
25
Q

Explain why the RSA algorithm can currently be regarded as a very safe encryption method.

Which future developments can decrease the security of RSA in a significant way?

A
  • strength of RSA lies in the factorisation problem
  • exponential time complexity
  • currently practically infeasible to compute p and q
  • quantum computer
26
Q

advantages and disadvantages of RSA compared to a symmetric-key method

A

Ad

  • secure key exchange: RSA the secret key (private key) is not transmitted, only the public key.
  • public key system allows everybody to send encrypted messages without prior com- munication with the receiver.
  • In RSA, communication between k partners requires only k key pairs.

DisAd

  • High computational cost for encryption and decryption: too large to be used on large amounts of data. This is why RSA is often used in combination with other symmetric methods
  • easy to break if there are only few possible plaintext messages.
27
Q

A hacker makes copies of the Ecash money files you have stored on your computer. Can he spend the money? Explain your answer!

A
  • digital bank notes are not associated with a person, but with the emitting bank
  • respective bank keeps track of all serial numbers of the spent and signed bank notes, so that these notes can only be used once
28
Q

What is the purpose of the dual signature used in the SET-protocol?

A
  • accountability and traceability are restricted
  • The purpose is to have a combination of order and payment information in the signature without disclosing the order to the bank and the payment information to the merchant.
29
Q

Why does it make sense to apply a hash function on the secure passphrase?

A

compress passphrases of arbitrary length to fixed length keys, does not increase the security

entropy is in fact reduced

hashing the hash itself for a large number of times increases the time an attacker spends

30
Q

Explain the reasons which justify the use of a separate data key.

A

when you have a separate data key, you just have to de- and reencrypt the single key, which can be done in an instance

With a separate data key, you can generate a copy of this key and let your colleague encrypt it with his personal passphrase

31
Q

Why are asymmetric-key methods like RSA better suited for digital signatures compared to symmetric-key methods?

A
  • signature needs to be associated with one person
  • private key is the unique thing that clearly identifies a person
  • exists in asymmetric-key methods and not in symmetric-key methods
32
Q

Explain what certification authorities are and why they are necessary.

A
  • officially institution which guarantees the validity and the correctness of public keys with
  • a key certificate
  • -> verify the signature is the public key of the correct person
  • prevents: man- in-the-middle attack

Algorithms for Internet Applications (4 decks)

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