Password based cryptography


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  • The reasons for its wide use are easy to understand: it is mainly its user-friendliness that makes it an attractive choice. Users must remember just a password of their choice and store no other complicated data like long random keys or certificates.
  • “rainbow” technique which employs precomputation to speed upthe process of cracking individual passwords.Dictionary attacks work on the assumption that most passwords consist of whole words, dates, or numbers taken from a dictionary.(1)On-line password guessing attacks: An attacker tries to use a guessed password to pass the verification of theauthentication server in an on-line manner. Generally, the authentication server can detect such an attack bynoticing continuous authentication failures.(2) Off-line password guessing attacks: An attacker eavesdrops communication messages during a protocol andstores them locally. Then he/she tries to find out the weak password by repeatedly guessing a possible passwordand verifying the correctness of the guess via the captured information in an off-line manner. In general,such an attack can be prevented only by carefully designing the protocol such that no verifiable informationcan be used by the attack to verify the correctness of one guess on password.Replay attacks:In this attack, an adversary tries to replay messages partially or completely obtained in previous communications.If he can impersonate other users or expose other secret that is sensitive and useful for further deceptions, byguessing attacks, known-plaintext attacks or other cryptographic analysis methods, then the protocol is said to bevulnerable to replay attacks.Syllable attack is combination of both brute force and dictionary attack. This cracking technique is used when the password is not an existing word.Rule Based Attack:This type of attack is used when attacker gets some information about the password. This is the most powerful attack because the cracker knows about the type of password. This technique involves use of brute force, dictionary and syllable attacks
  • If they don’t compute those hashes on-the-fly, but keep a dictionary of precomputed hashes, then nothing is gained.
  • Psedue random function
  • If you don’t keep it a secret, at the very least you force the attacker to compute hashes on-the-fly rather than keeping a dictionary of precomputed hashes, which even for cheap hash functions will severely increase the amount of time required to get at your encrypted data.
  • DOS attacks, customer services and server can handle a lot of parallel access.(Completely Automated Public Turing Test to tell Computers and Humans Apartrequires the party that makes the attempt to send a proofthat it invested some non-trivial computation time in constructingits request.As a specific examplein the context of preventing dictionary attacks, theserver could require that a login attempt is accompanied bya value x that satisfies the requirement, say, that the last20 bits of H(x,username,password,time-of-day) are all 0,where H is a hash function such as SHA. If we assume thatSHA behaves as a random function, then the attacker wouldneed to check on the average 219 values for x before it findsa value that satisfies the test. that The computation of xadds a relatively negligible overhead to a single login attempt,but can significantly slow down the operation of adictionary attack.
  • Password based cryptography

    1. 1. Password-based Cryptography 1 PRESENTED BY ISHRAQ FATAFTAH
    2. 2. Agenda 2 Introduction. Security attacks. Password-based cryptography. Common countermeasures against dictionary attacks. Conclusion.
    3. 3. Introduction 3 Passwords are the most common method of authentication. Consists of a string of characters to gain access to resources. Usually, passwords are human memorable that considered as a vulnerability in security. Passwords are derived from a small domain.
    4. 4. Introduction 4 Password creation rules have been enforced to increase the quality of passwords like:  Letters and numeric.  Non-alphanumeric characters.  Passphrases.  Symbols.  Increased password length.
    5. 5. Well Known Passwords attacks 5 Guessing attacks.  Brute force attack (Rainbow).  Dictionary attacks.  Online dictionary attacks.  Offline dictionary attacks. Resetting attacks. Replay attacks. Syllable attacks. Social engineering and shoulder surfing.
    6. 6. Password based Cryptography 6 Attempt to derive security key directly from passwords. Some processing are needed to turn passwords into security keys.  Password based authentication techniques.  The use of iteration count.  Construct key derivation function.
    7. 7. Password based Authentication 7 System and user agree on a list of passwords.
    8. 8. Password based Authentication using Hashes 8 A hash function is any well-defined procedure or mathematical function that converts a large, possibly variable-sized amount of data into a small datum. Hash functions should be:  Easy to compute the hash value for any given message.  Infeasible to find a message that has a given hash.  Infeasible to modify a message without changing its hash.  Infeasible to find two different messages with the same hash.
    9. 9. Password based Authentication using Hashes 9 System hashes user password.
    10. 10. Password based Authentication using Hashes 10 Using Dictionary attacks that uses hashes of dictionary words. Attacker might not know the exact hash function used, which means they must attempt each dictionary word for each hash function they’re considering.
    11. 11. Password based Authentication using Salts 11 8 Byte random number. DK = KDF (P, S) Producing a large set of keys corresponding to a given password. Benefits:  Difficult to pre compute all keys corresponding to a dictionary of password by attacker.  It is unlikely to select the same key twice.
    12. 12. Password based Authentication using Salts 12 System salts user password.
    13. 13. Password based Authentication using Salts 13 What if passwords+salt was input to a hash function?  Not one hash for a given dictionary word.  There are as many different hashes as there are possible values for the salt.
    14. 14. Password based cryptography using Iteration count 14 Increasing the cost of producing keys from a password. Using fixed number C with Password Random Function (PRF). As number of iteration increases, as the cost of exhaustive search for passwords increases. Minimum of 1000 iteration is recommended.
    15. 15. Password-based key derivation 15 A key derivation function produces a derived key from a base key and other parameters. The base key is a password and the other parameters are a salt value and an iteration count.
    16. 16. Password-based key derivation 16 Key derivation algorithm:  Select a salt S and an iteration count c.  Select a length in octets for the derived key.  Apply the key derivation function to the password, the salt, the iteration count and the key length to produce a derived key.  Output the derived key.  y = F(p, s, c)
    17. 17. Common countermeasures against online dictionary attacks 17 Delayed response.  Prevent attacker from checking many passwords in a short time. Account locking. Both insufficient in network environment. Pricing via processing. Use of Captcha.
    18. 18. Common countermeasures against offline dictionary attacks 18 Can be easily prevented using Public key cryptography. First password based authentication protocol secure against offline dictionary attacks, called EKE. Encrypted Key Exchange , one party encrypts a (one- time) public key using a password, and sends it to a second party, who decrypts it and uses it to negotiate a shared key with the first party.
    19. 19. Common countermeasures against offline dictionary attacks 19 Password authenticated key exchange (PAKE) is where two or more parties, based only on their knowledge of a password, establish a cryptographic key using an exchange of messages, such that an unauthorized party cannot participate in the method and is constrained as much as possible from guessing the password. Zero-Knowledge Concepts.
    20. 20. Conclusion 20 Data has nowadays become our most valuable asset which needs to be protected at any cost. Most common authentication techniques are passwords. Human-memorable passwords are vulnerable to attacks. Authentication techniques requires substantial change in their infrastructure. There is no satisfactory means to counter password attacks.