Internet Security Hashing Algorithms Lecture Notes PDF

Internet Security Hashing Algorithms Nu r t u r ing Cu r io u s M ind s ,P r o d u c ing P as s io nat e Engineer s  Internet Security Describe and apply hashing algorithms MD5. Describe and apply hashing algorithms SHA1. Compare and Co...

Internet Security Hashing Algorithms Nu r t u r ing Cu r io u s M ind s ,P r o d u c ing P as s io nat e Engineer s  Internet Security Describe and apply hashing algorithms MD5. Describe and apply hashing algorithms SHA1. Compare and Contrast hashing algorithms. Analysis of hashing applications. 2  Hash Used for data integrity assurance Data of Arbitrary Length Based on one-way mathematical function Input data produce a fixed-length digest (fingerprint) of output data Hash Digest is cryptographically strong, Function impossible to recover input data from its digest e888r4555tggieqd Fixed Length Hash Well-known hash functions are Message Digest 5 (MD5) – 128 bits digest Secure Hash Algorithm 1 (SHA-1) – 160 bits digest 3  Hashing Hash Value md5 "Hello World" b10a8db164e0754105b7a99be72e3fe5 md5 "Hello" 8b1a9953c4611296a827abf8c47804d7 md5 "Hard Cheese" 3591c82675e50481e3b427c353666ab6 Sha1 "Hello World" 0a4d55a8d778e5022fab701977c5d840bbc486d0 Sha256 "Hello World" a591a6d40bf420404a011733cfb7b190d62c65bf0bcda32 b57b277d9ad9f146e 4  Message Digest 5 (MD5) One way function Easy to compute the hash from the given data, but unfeasible to compute input data given only a hash Collision resistant, i.e. two messages with the same hash are very unlikely to occur Popular for Internet application  SHA-1 Secure Hash Algorithm Published in 1994 Produce a 160-bit message digest The algorithm is slightly slower than MD5  Password Hashing Account creation and password hashing The user creates an account. Password is hashed and stored in the database. (cleartext password is never stored anywhere) User attempts to login Attempted password is hashed. Hash is checked against hash of original password in password file. User gains access when hash matches Never tell the user if it was the username or password they got wrong. Always display a generic message like "Invalid username or password." This prevents attackers from enumerating valid usernames without knowing their passwords. 7  How to crack Hashed passwords Dictionary attacks Uses a file containing words, phrases, common passwords, and other strings that are likely to be used as a password. Each word in the file is hashed, and its hash is compared to the password hash Brute force attack Tries every possible combination of characters up to a given length 8  How to crack Hashed passwords Hash table lookup Pre-compute the hashes of the passwords in a password dictionary and store them with their corresponding password. Requires more storage but fast compared to rainbow table. Reverse-lookup table Obtain a user's password and hash, determine algorithm and apply lookup to all other accounts Rainbow table Pre-computed table for reversing cryptographic hash functions, usually for cracking password hashes. Efficient way to store pre-compute hashes but is slower than hash table lookup. 9  Adding Salt… Lookup tables and rainbow tables only work because each password is hashed the exact same way. Randomize the hashes by appending or prepending a random string, called a salt, to the password before hashing E.g. md5sum – "Hello there"+"01234567890" http://crackstation.net/hashing-security.htm  Hash Collision Hash functions map arbitrary amounts of data to fixed-length strings, there must be some inputs that hash into the same string. Cryptographic hash functions are designed to make these collisions incredibly difficult to find. MD5 Collision https://www.mscs.dal.ca/~selinger/md5collision/ 11 Encryption Versus Hashing Encryption Hashing Key is usually added Uses a key as an to text; the two are Use of Key input to an combined, and the encryption method combination is hashed Output is of a Length of Output is similar in fixed short length, Result length to input regardless of input Reversible; One-way function; hash Reversibility ciphertext can be cannot be “de-hashed” decrypted back to back to the original string plaintext  Purpose of Hashing Non-repudiation Cannot deny what has been sent. Detect doctored message Create a “digest” on the original message using hash function(s) Message and “digest” are sent together to recipient for verification. Add salt to hash to decrease risk of hash collision  Terminology Message encryption Ciphertext of entire message serves as authenticator Message authentication code (MAC) A Public function of message and secret key that produces a fixed length value that serves as authenticator. Hash function A public function that maps a message of any length into fixed length hash value that serves as authenticator  Terminology Hash-based Message Authentication Code (HMAC) Combination of Hash function and MAC using shared secret Hashing Algorithms and Security – Computerphile Video: https://www.youtube.com/watch?v=b4b8ktEV4Bg  Message Integrity Allows communicating parties to verify that received messages are authentic. Content of message has not been altered Source of message is who/what you think it is Message has not been artificially delayed (playback attack) Sequence of messages is maintained Formal Requirements of Hash function H() can be applied to any block of data of any size H() produces a fixed length output H(x) is relatively easy to compute for any given x. One way property Weak collision resistance Given an input X and a hashing function H(), it is very difficult to find another input X’ on which H(X) = H(X’). Strong collision resistance Given a hashing function H() and two arbitrary inputs X and Y, there exists an absolute minimum chance of H(X) being equal to H(Y).  Hashing Algorithms MD5 (Message Digest 5) Computes 128-bit hash value Widely used for file integrity checking MD-5 is based on MD-4 and was created to address vulnerabilities found in MD-4. D5 generates 128-bit hash values over 512-bit blocks in 4 rounds of 16 steps each. SHA-1 (Secure Hash Algorithm -1) Computes 160-bit hash value NIST approved message digest algorithm SHA-1 also operates on 512-bit blocks, but produces a 160-bit hash value in 4 rounds of 20 steps each. Other implementations of SHA-256, SHA-384, SHA-512  Popular Hashing Implementations MD5 Designed by Ron Rivest in 1991 128 bits output (too short for current use) developed in 2005 Can be broken on a single notebook within minutes SHA Can be broken on a single notebook within minutes Designed by the NSA, published in 1993 Family of functions: SHA-1, SHA-224,-256,-384,-512 Current recommendation: use SHA-256 (newer)  optional Birthday Attack Collisions Two messages with the same hash value Based on the “birthday paradox” http://en.wikipedia.org/wiki/Birthday_problem Suppose N people in a room and randomly choosen, the probability of 2 people having the same birthday. Hash algorithms should be resistant to this attack Unique digest for each input.  Message Authentication Code (MAC) s = shared secret key message s Internet H( ) message s H( ) compare message Authenticates sender with shared secret key Verifies message integrity No encryption to the message! Also called “keyed hash” Notation: h = H(s||m) ; send m||h  HMAC Popular MAC standard Addresses some subtle security flaws 1. Concatenates secret to front of message. 2. Hashes concatenated message 3. Concatenates the secret to front of digest 4. Hashes the combination again. Securing Stream Ciphers (HMAC) - Computerphile Video: https://youtu.be/wlSG3pEiQdc  End-point authentication Want to be sure of the originator of the message – end-point authentication. Assuming Alice and Bob have a shared secret, will MAC provide message authentication. We do know that Alice created the message. But did she send it? 29  Application of Hash in Digital Signatures Cryptographic technique analogous to hand- written signatures. sender (Bob) digitally signs document, establishing he is document owner/creator. Goal is similar to that of a MAC, except now use public- key cryptography verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document  Official (Open) Questions? HASHING ALGORITHMS v2.0/2012 31