Cryptography

Question 1

What is the law regarding passwords?

Answer 1

There’s now a law which makes it an offence to not reveal your password when a law enforcer asks you to.

Question 2

What is a cryptosystem?

Disguises

Answer 2

Disguises messages, allowing only selected people to see through the disguise.

Question 3

What is cyphertext?

Jumbled

Answer 3

Where all the information is jumbled up so it can be interpreted

Question 4

What are the two basic methods for disguising messages?

Answer 4

• Transposition
• Substitution

Question 5

What is transposition?

Answer 5

Letters rearranged into a different order

Question 6

What is substitution?

Answer 6

Letters replaced by other letters or symbols

Question 7

What is the method for simple transposition encryption?

Answer 7

• Decide a keyword
• Write the key horizontally as the heading for columns.
• Assign numerical values to each letter based on the letter’s order of appearance in the alphabet.
• Align plaintext message across each column.
• Read down each column according to the ordinal value (col A1 first followed by col C2 etc).

Question 8

What is the method for simple transposition decryption?

Answer 8

• Write the key horizontally as the heading for columns.
• Assign numerical values to each letter based on the letter’s order of appearance in the alphabet.
• Place the cyphertext segments under each column.
• Read across each column to obtain the plaintext.

Question 9

What is simple substitution encryption also known as?

Answer 9

Caesar cypher

Question 10

What is the method for simple substitution encryption?

Answer 10

rewatch lecture?

Question 11

What is the problem with the shift cypher?

Answer 11

The number of keys is too small (26)

Question 12

What is an alternative approach to the shift cypher?

Permutation

Answer 12

Use a random key permutation for each letter in the alphabet

Question 13

What is symmetric key cryptography?

Answer 13

Same key used to encrypt and decrypt the message.

Question 14

What is symmetric key cryptography?

Answer 14

Same key used to encrypt and decrypt the message.

Question 15

What is asymmetric key cryptography?

Answer 15

• The key for encryption is not the same as the key for decryption.
• Two keys are needed.
• Send public key to intended recipients.
• Private key always remains with the sender.
• Assures that messages are from their advertised source.

Question 16

What is a hash?

Transformation of data

Answer 16

A hash is a transformation of data into a distilled form (message digest) that is unique to the data – the process is not reversible (one-way-hash).

Question 17

What is a cryptographic hash used for?

Integrity

Answer 17

A (cryptographic) hash is used to verify the integrity of data – check if it has been altered in any way.

Question 18

What is hashing used for?

Integrity

Answer 18

• Verify the integrity
• Authenticate that the document is from who it claims to be

Question 19

What is a PPK?

Answer 19

Public-private key

Question 20

What is the procedure for digital signing?

Intended recipient, message digest, senders public key

Answer 20

• Arrange for intended recipient to obtain a copy of your public key.
• Compute message digest for data.
• Encrypt (PPK) digest using private key and append it to the original message before sending it to the intended recipient.
• Recipient uses sender’s public key to check that the message has really come from the stated sender and that it hasn’t been altered in any way.

Question 21

How does the recipient confirm the message is authentic?

Senders public key, digest

Answer 21

• The digest is decrypted by the recipient using the sender’s public key.
• The recipient then computes their own digest of the message and compares this with the decrypted value.
• If the digests are identical then assume the message is who it claims to be from (authenticate) AND that it hasn’t been altered in transit (validate integrity).

Question 22

What does a digital certificate do?

Public key

Answer 22

Verifies that a public key belongs to the owner of the certificate.

Question 23

What is the encryption method for the vigenere cypher?

Answer 23

• Decide a keyword
• Assign each letter in the keyword a numerical value depending on where they are in the alphabet (A=0, Z=25)
• Write each value from the keyword under the plaintext
• Shift the letter by the number of spaces corresponding by the keyword

Question 24

What is the decryption method for the vigenere cypher?

Answer 24

• Assign each letter in the keyword a numerical value depending on where they are in the alphabet (A=0, Z=25)
• Write each value from the keyword under the plaintext
• Shift the letter backwards by the number of spaces corresponding by the keyword

Question 25

Cryptosystem

Disguises

Answer 25

Disguises messages, allowing only selected people to see through the disguise.

Question 26

Cryptography

Designing

Answer 26

Is the science of designing, building, and using cryptosystems.

Question 27

Cryptanalysis

Breaking

Answer 27

Is the science of breaking a cryptosystem.

Question 28

Cryptology

Study

Answer 28

Is the study of cryptography and cryptanalysis

Question 29

Digital Envelope

Answer 29

• Sender obtains a copy of the receivers public key (from receivers digital certificate)
• Sender uses receivers public key to encrypt the digitally signed message creating a digital envelope
• Sealed envelope is sent to receiver via internet or other untrusted communication channel