Cryptography Flashcards
What is the law regarding passwords?
There’s now a law which makes it an offence to not reveal your password when a law enforcer asks you to.
What is a cryptosystem?
Disguises
Disguises messages, allowing only selected people to see through the disguise.
What is cyphertext?
Jumbled
Where all the information is jumbled up so it can be interpreted
What are the two basic methods for disguising messages?
- Transposition
- Substitution
What is transposition?
Letters rearranged into a different order
What is substitution?
Letters replaced by other letters or symbols
What is the method for simple transposition encryption?
- 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).
What is the method for simple transposition decryption?
- 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.
What is simple substitution encryption also known as?
Caesar cypher
What is the method for simple substitution encryption?
rewatch lecture?
What is the problem with the shift cypher?
The number of keys is too small (26)
What is an alternative approach to the shift cypher?
Permutation
Use a random key permutation for each letter in the alphabet
What is symmetric key cryptography?
Same key used to encrypt and decrypt the message.
What is symmetric key cryptography?
Same key used to encrypt and decrypt the message.
What is asymmetric key cryptography?
- 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.
What is a hash?
Transformation of data
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).
What is a cryptographic hash used for?
Integrity
A (cryptographic) hash is used to verify the integrity of data – check if it has been altered in any way.
What is hashing used for?
Integrity
- Verify the integrity
- Authenticate that the document is from who it claims to be
What is a PPK?
Public-private key
What is the procedure for digital signing?
Intended recipient, message digest, senders public key
- 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.
How does the recipient confirm the message is authentic?
Senders public key, digest
- 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).
What does a digital certificate do?
Public key
Verifies that a public key belongs to the owner of the certificate.
What is the encryption method for the vigenere cypher?
- 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
What is the decryption method for the vigenere cypher?
- 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
