Intro to Cryptography & Asymmetric Cryptography: Lecture 1

Question 1

How do we focus on what to secure?

Answer 1

Using CIA Triad:
1) Confidentiality
Prevention of unauthorised disclosure of information

2) Integrity
Prevention of unauthorised modification of information

3) Availability
Prevention of unauthorised withholding of services or resources

Question 2

Types of Cryptography

Answer 2

1) Symmetric Cryptography
+ Substitution ciphers: replaces each plaintext character with another according to a fixed pattern
- Monoalphabetic e.g. Caesar cipher
- Polyalphabetic e.g. Vigenere cipher

+ Transposition ciphers: plaintext characters are shifted in some regular pattern to different positions to form the ciphertext.
- Simple transposition cipher: reverse order of characters
- Rail fence cipher: eg. rail key of 3, pull down each character 1 level more than the previous until 3rd level then go back up.
- Columnar transposition cipher:
Two ways:
First, depending on key length, write the key then plaintext in the column length equal to the key and add padding when necessary. Next, arrange the columns in alphabetical sequence according to the first row (the key). Lastly, read the cipher text in columns without the first row (the key).
Second, when the key position information instead of key is given, read out according to key position information.

2) Asymmetric Cryptograpy
3) Hashing

Question 3

DES, AES, OTP

Answer 3

DES: Data Encryption standards
AES: Advanced Encryption Standard
OTP: One Time Pad

Question 4

Is there a safe way to generate and distribute a shared key?

Answer 4

Using Diffie-Hellman Key Exchange (Not a crypto protocol but a key exchange used specifically for symmetric key):
For example, Alice calculates her A and sends A over to bob:
A= G^a mod p

Then bob calculates his B and sends B over to Alice:
B= G^b mod p

The four variables A, B, p, G are then shared across the unsafe channel.

Alice receives B and calculates K with it:
K = B^a mod p

Bob receives A and calculates K with it:
K= A^b mod P

Question 5

Weaknesses of mono-alphabetic

Answer 5

• Main weakness of mono-alphabetic substitution/transpostion ciphers is that
  although the letters themselves change (position), their frequencies do not.

Question 6

Hashing for Digital Signature

Answer 6

• Hash of a document encrypted with the sender’s
private key (signed or authenticated).
• Sent to the recipient with the original document
(original doc can also be encrypted).
• They can decrypt the encrypted hash, and check it
against the hash of the (decrypted) document sent.
• A secure digital signature system can provide
Confidentiality and Integrity for data.
• Data security requires data protection as welleg.
access control, data backups etc.