COSC366 Final Study Guide

• Secure communication


  • Confidentiality: Keep others from reading Alice's messages/data
  • Integrity: Keep others from undetectably tampering with Alice's messages/data
  • Authenticity: Keep others from undetectably impersonating Alice
• Plaintext


  The actual message we want to send
• Encryption algorithm


  Takes the plaintext, a key, (and randomness) and generates a ciphertext
• Ciphertext


  The data that actually gets written to the insecure infrastructure

  
• Decryption algorithm


  Takes ciphertext and a key, returns plaintext
• Symmetric Key systems


  Both parties have the same key (shared secret)
• Asymmetric Key systems


  One person holds a private key, everyone else knows their public key

  
• Symmetric Key systems are very fast, but require sharing a secret key

  
• Asymmetric Key systems are very slow, but don't require sharing a secret key

  
• Block Ciphers


  Based on a pseudorandom function, essentially a keyed permutation, takes in a fixed number of bytes, spits out a permutation of those bytes of the same size
• ECB (Electronic Codebook) Mode


  Block ciphers are deterministic, for a given m and K, E(K,m) always returns the same c, an eavesdropper could determine when messages are re-sent

  
• NEVER use ECB mode (but over 50% of Android apps do)
• One-way Hash Function


  • Difference from Hash Function: one-way, collision resistant
  • Common One-way Hash Functions: MD series, SHA series
• MD2, MD4 - severely broken (obsolete), MD5 - collision resistance property broken, one-way property not broken, MD6 - developed in response to proposal by NIST
• Secure communication


  • Confidentiality: Keep others from reading Alice's messages/data
  • Integrity: Keep others from undetectably tampering with Alice's messages/data
  • Authenticity: Keep others from undetectably impersonating Alice
• Symmetric Key vs Asymmetric Key

  • Symmetric Key: requires secure channel for key distribution, impossible for two parties having no prior relationship, needs many keys for n parties to communicate
  • Asymmetric Key: keys are mathematically linked, very slow systems

  
• Plaintext


  The actual message we want to send
• Public Key Encryption


  Each party has a pair (K, K-1) of keys: K is the public key, and used for encryption, K-1 is the private key, and used for decryption, satisfies DK-1[EK[M]] = M, knowing the public-key K, it is computationally infeasible to compute the private key K-1

  
• Digital Signatures


  Provide Integrity: Generate the signature with my private key, anyone can validate it with my public key, key use is inverse of encryption

  
• Encryption algorithm


  Takes the plaintext, a key, (and randomness) and generates a ciphertext
• Fundamental problem with public key cryptography: Bob has no way to tell whether the public key he has received belongs to Alice or not

  
• Ciphertext


  The data that actually gets written to the insecure infrastructure

  
• Decryption algorithm


  Takes ciphertext and a key, returns plaintext
• Public Key Infrastructure (PKI)

  Certificate Authority (CA): a trusted party, responsible for verifying the identity of users, and then bind the verified identity to a public keys, Digital Certificates: A document certifying that the public key included inside does belong to the identity described in the document, X.509 standard

  
• Symmetric Key systems


  Both parties have the same key (shared secret)
• Core Functionalities of CA

  Verify the subject, Signing digital certificates
• Asymmetric Key systems


  One person holds a private key, everyone else knows their public key
• PKI defeats the Man-in-the-Middle (MITM) attack by verifying the authenticity of the public key
• Symmetric Key systems are very fast but require sharing a secret
• Attacks on CA's verification process (Comodo Breach) and signing process (DigiNotar Breach) can compromise the PKI system
• Asymmetric Key systems are very slow but don't require sharing a secret
• Attacks on the algorithms used in digital certificates, such as collision attacks on one-way hash functions (MD5, SHA-1), can also compromise the PKI system
• Attacks on user confirmation, where the software does not properly compare the information provided by the user with the information in the server's certificate, can also lead to security flaws
• Block Ciphers


  • Based on a pseudorandom function
  • Essentially a keyed permutation
  • Takes in a fixed number of bytes, spits out a permutation of those bytes of the same size
  • Divide the data to be encrypted up into blocks of fixed size, padding at the end
  • Blocks are fed into a PRF, and the ciphertext is based on output
  • Have a mode, a way of combining the blocks of a message and (often) randomness
• SQL Injection


  Mixing data and code together is the cause of several types of vulnerabilities and attacks including SQL Injection attack, XSS attack, attacks on the system() function and format string attacks

  
• SQL Injection Countermeasures


  Blacklisting: Delete the characters you don't want, Downside: "Peter O'Connor"
• ECB Mode


  • Block ciphers are deterministic
  • For a given m and K, E(K,m) always returns the same c
  • An eavesdropper could determine when messages are re-sent
  • The same data block gets encrypted the same way, revealing patterns of data when a data block repeats
  • When the same key is used, the same message is encrypted the same way

  
• NEVER use ECB (but over 50% of Android apps do)
• Hash Function


  Maps arbitrary size data to data of fixed size
• Certificate


  Valid and authentic