Kiran Subramanyam ps2471@nyu.edu Password Cracking 1

Introduction Password authentication is everywhere, but vulnerable. In 1979, Robert Morris and Ken Thompson published “Password Security: A Case History”* which estimated that 86% of all passwords can be cracked. Two types of attacks: Online and Offline *http://cm.bell-labs.com/who/dmr/passwd.ps 2

Online Attacks Try to login to a live system by trying various ID/password combinations. This method is not very efficient as it is time consuming. Most systems lock the account/ID after a certain number of unsuccessful attempts, a strategy known as “rate limiting”. 3

Offline Attacks Obtain objects (e.g. files) that store passwords. But passwords are not stored in clear text. Most modern systems store either hashes of passwords or in an encrypted form. Therefore it is necessary to “crack” the passwords using various tools and techniques. 4

What is Hashing Hash functions map data of arbitrary size to data of fixed size (hash value or digest) with the following important properties: it is easy to compute the hash value for any given input text. it is infeasible to generate the input text from a given hash value (one way function). it is infeasible to find two different inputs with the same hash. Examples are MD5, SHA-1, SHA-256 etc. 5

Offline Attack Types Dictionary Attack – Build a list of candidate passwords and hash/encrypt each one to see if there is a match. Brute Force Attack – Systematically try all possible combinations of characters. Hybrid Attack – Try variations of each candidate based on mangling rules. Rainbow Table Lookups. 6

What are Rainbow Tables A pre-computed table of hash values of all possible inputs (usually up to a given length). Such tables require large amounts of storage. Searching for values in a table is faster than computing large number of hashes. This represents a space/time trade-off in comparison with wordlist based attacks. Hash chains are a further optimization of Rainbow Tables to decrease their storage requirement. 7

Evolution of Cracking Traditional methods employ tools such as Cain and Able, John The Ripper etc. using CPU core power. Advanced tools use graphical processing units (GPUs) on video cards and load rainbow tables onto very fast solid state drives (SSDs). Tools like Hashcat, Rainbow Crack, Cryptohaze Multiforcer, etc., are GPU-supported tools that utilize the GPU cores for cracking the hashes. 8

CPU versus GPU Architecturally, the CPU is composed of a few cores with lots of cache memory that can handle a few software threads at a time. In contrast, a GPU is composed of hundreds of cores that can handle thousands of threads simultaneously. For e.g. a CPU core can execute four 32-bit instructions per clock, whereas a GPU like the Radeon HD 5970 can execute 3200 32-bit instructions per clock. 9

Time required to Crack 10 characters: 3.76 quadrillion possible combinations Cracking online using web app hitting a target site with one thousand guesses per second: 3.7 weeks. Cracking offline using high‐powered servers or desktops (one hundred billion guesses/second): 10.45 hours Cracking offline, using massively parallel multiprocessing clusters or grid (one hundred trillion guesses per second): 37.61 seconds. 10

Defense: Salts Salt is random data that is used as an additional input to the hashing function and different for each password I,e. Hash value = hash(salt, password) Salts combat the use of rainbow tables because the size of the table increases exponentially with the length of the password. When cracking lists, use of salts negates any potential efficiency due to repeated passwords since salts make each hash value unique. 11

Defense: Expensive Hash Typically hash functions are fast, but it also makes cracking them faster. Solution is to use expensive hash functions. bcrypt – Besides incorporating a salt to protect against rainbow table attacks, bcrypt is an adaptive function: over time, the iteration count can be increased to make it slower. scrypt – The algorithm was specifically designed to make it costly to perform large-scale custom hardware attacks by requiring large amounts of memory. 12