Salting and Key Stretching

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• values used for private key must be selected at random
• low entropy is a concern when a cryptographic system uses user-generated data
  • e.g., password
  • users tend to select low entropy passwords
• salting and key stretching help protect password-derived cryptographic secrets

Salting

• Cryptographic hash functions are often used for password storage and transmission
  • hashes are one way
    • cannot be decrypted back to the plaintext password
  • but are vulnerable to brute-force and dictionary attacks
• can slow down these attacks by adding a salt value to the hash

A salt is a security countermeasure that mitigates the impact of precomputed hash table attacks by adding a pseudorandom value to (“salting”) each plaintext input.

• computed as:
• unique, random salt value should be generated for each user account
  • mitigates the risk that
    • if users choose identical plaintext passwords,
    • there would be identical hash values in the password file
• salt is not kept secret
  • any system verifying the hash must know the value of the salt
  • simply means an attacker cannot use precomputed hash tables
  • hash values must be recompiled with specific salt for each password

Key Stretching

Key stretching is a technique that strengthens potentially weak input for cryptographic key generation, such as passwords or passphrases created by people, against brute force attacks.

• process
  • takes a key that’s generated from a user password
  • plus a random salt value
  • and repeatedly converts it to a longer and more disordered key
• initial key may be put through thousand rounds of hashing
  • may not be difficult for the attacker to replicate
  • doesn’t make the key stronger
  • it does slow the attack down
    • attacker has to do extra processing for each possible key value
• can be performed by using a particular software library to hash and save passwords when they are created
  • Password-Based Key Derivation Function 2 (PBKDF2) is very widely used for this purpose
    • part of Wi-Fi Protected Access (WPA)