What is it?

• Confidentiality: only the sender and the receiver can understand the message.
• Authentication: needs to authenticate the sender and the receiver.
• Message Integrity: any alteration can be detected easily (inviolable)

Confidentiality

• m: plaintext message
• K_a: encryption key (string or number that is used in the encryption / decription algorithm)
• K_a(), K_a: encryption algorithm
• K_b: decryption key
• K_b(), K_b: decryption algorithm

• The two types of cryptography are: symmetric key cryptography and asymmetric key cryptography.

Symmetric Key Cryptography

• It is important to know how the receiver and the sender decided on which key to use.
  • TCP handshake? Then that should be secure as well!
  • Need to think of a way to securely communicate the key to use.

• Only 25 possible values of shifting.
• The key in this case is number of shifting (ex: 3)

• The key here is the mapping of alphabet to the permutation (permutation string).
• 26! possible permutations

• Monoalphabetic cipher can be deciphered using statistical analysis.
• Some consonants appear more frequently than others.
• Knowing that "bob" or "alice" appears in the text already gives out some letters.

• Ciphertext only attack: Trudy has only the ciphertext she can analyze using statistical analysis.
• Known-plaintext attack: Trudy knows the mapping for "alice" and "bob" and the permutation string.
• Chosen-plaintext attack: Trudy forces Alice to send a string of message, from which the mapping can be figured out.

• Use several mappings in cycle.
• The encryption key here is the cycle pattern, and substitution ciphers.

• You can see for the same b, it is now mapped to different alphabets.

• Number of permutations of mappings: (2^(k))!
  • where k is the number of bits in a block.

• DES and AES.
• AES > DES.
• DES can be solved easily now due to the technological advancement.

• The key issue with symmetric key cryptography is that both ends need to know the key.
  • This is problematic when the two have never met.
• Thus, the motivation for asymmetric cryptography.
  • The sender uses public key to encrypt,
  • The receiver uses private key to decrypt.

Asymmetric Key Cryptography

• Requirements:
  1. Need the encryption and decryption algorithms.
  2. Even if there is a coupling between the public and private keys, it should be impossible to even guess what the private key is from the public key.

• Message is just a bit pattern, and bit pattern can be translate to a number.
• RSA encrypts a number to produce another number.

• Choose 4 numbers to encrypt:
  • p and q (very large different prime numbers)
  • e (no common factor with z)
  • d (ed - 1 is divisible by z)
• (n, e) become public, (n, d) become private key.

• What the heck?

• What?

• swapping between public and private encryption algos still work.

• RSA on its own is too computationally-expensive.
• Use RSA on K_s. Decrypting the key to decrypt the DES.

Message Integrity

• We can use checksum, parity, and CRC.

• Checksum is not good for preventing attacks.

• Two input strings can have the same CRC checksum.

• The hash function should not have two input values that can generate the same hash outputs.

Hash Function: MD5

• Even though the file sizes are very different, they still produce 128 bits hash output.

• A small change in the input shows great difference in output.

• The sender cannot send the pair (m, H(m)) because if both of them are replaced with something correct, there is no way to detect the replacement.

• Send the hash code of message + authentication key.
  • Since the authentication key is only known between the sender and the receiver, there is no way an intruder can replace the H(m+s) since then it will be easily detected.

Authentication

• Digital siganture: cryptographic technique used to authenticate the sender and the receiver.

• In digital signature, we use the private key to encrypt and public key to decrypt.
• This way, if the public key can decrypt the message, then we know that it is send by Bob, because it is using Bob's private key to encrypt.

• It is quite expensive to encrypt the entire message, since the message can be long.
• The solution is to hash first so that a fixed size of hashed message can be encrypted.

• the message m is sent with the hased m.

• The issue right now is that there is no way to tell if the public key provided is by the intruder's or the sender's.

• We do not know if it is Bob's public key.
  • Ask Certification Authority (CA) - which keeps the data base for public keys - if it is indeed Bob's public key.
• But what if Trudy intercepts the communication between CA?:
  • CA signs the messages.
• Problem: We do not know CA's public key?
  • Store them in hardware.

• Use CA key to verify that it is indeed bob's public key.

• Firewall is used to block the external people from accessing the internal network.

1. prevents DoS attacks by an outsider.

• DoS attack sends countless TCP connection requests that any proper TCP request cannot be succeded.

• The packets go through the list of rules known as Access Control Lists to be checked if they can be passed.

• IP spoofing: when packets fake the IP address, there is no way to tell it by the firewall.