Hashing

Introduction

• A hash value is a fixed-size string or characters that is computed by a hash function.
• A hash function takes an input of an arbitrary size and returns an output of fixed length, i.e., a hash value.
• Hashing is used to identify if a file is tampered by comparing the hash value with the original file hash value.
• Hashing also helps in storing passwords in database. The server then uses the hash function to ensure the correct value.

Hash Functions

• Hash functions are different from encryption.
• Hash functions are computationally impractical to go from output to input.
• A hash function takes some input data of any size and creates a summary or digest of that data.
• The output has a fixed size.
• Any slight change in the input data, even a single bit, should cause a significant change in the output.
• Most common hashing algorithms are
  • MD5 (Message-Digest Algorithm 5) - To generate MD5 hash use command md5sum *.txt.
  • SHA1 (Secure Hash Algorithm 1) - To generate SHA1 hash use command sha1sum *.txt.
  • SHA256 (Secure Hash Algorithm 256) - To generate SHA256 hash use command sha256sum *.txt.
• These hash function produce results in hexadecimal format which prints each raw byte as two hexadecimal digits.

Hash Collision

• Hash collision is when two inputs have the same hash value.
• As the output is limited and the input is practically limitless, the hash collision is likely to happen.
• A good hashing algorithm will keep this collision probability to a negligible value.
• To calculate the number of outputs a hash function can generate, use the formula 2^number_of_bits.
  • For a hash function that produces 4 bit hash value, the number of outputs is 2^4 = 16
• The pigeonhole effect states that the number of items (pigeons) is more than the number of containers (pigeonholes) and some pigeons has to share the pigeonholes.

Insecure Password Storage For Authentication

• Top 3 insecure password practices
  • Storing as plaintext. (Rockyou databreach)
  • Storing using deprecated encryption. (Adobe databreach)
  • Storing using insecure hashing algorithm. (LinkedIn databreach. SHA1 without salting.)

Using Hashing For Secure Password Storage

• A Rainbow Table is a lookup table of hashes to plaintexts, so you can quickly find out what password a user had just from the hash.
• Websites like https://crackstation.net/ and https://hashes.com/en/decrypt/hash use massive rainbow tables to provide fast password cracking for hashes without salts.
• To protect against a rainbow table attack, use a secure password with unique salting.

Recognizing Password Hashes

• Automated hash recognition tools often mix up hash types and are unreliable.
• Find different types of hashes in https://hashcat.net/wiki/doku.php?id=example_hashes

Linux Passwords

• Previously on linux, the password hashes were stored on /etc/passwd which was readable by everyone.

• Nowadays, the password hashes are stored in /etc/shadow which is readable by root only.

• Each line in the shadow file consists of nine fields separated by colons(:).

• The first two fields are login name and encrypted passwords. More information on other fields can be found by using the command man 5 shadow.

• The encrypted password field contains hashed passphrase with four components.

  • Prefix (Algorithm ID)
  • Options (Parameters)
  • Salt
  • Hash

• The prefix is used to find the hashing algorithm.

• Most common unix style password prefixes can be found in the decreasing strength using the command man 5 crypt.

  Prefix	Algorithm
  $y$	yescrypt is a scalable hashing scheme and is the default and recommended choice in new systems
  $gy$	gost-yescrypt uses the GOST R 34.11-2012 hash function and the yescrypt hashing method
  $7$	scrypt is a password-based key derivation function
  $2b$, $2y$, $2a$, $2x$	bcrypt is a hash based on the Blowfish block cipher originally developed for OpenBSD but supported on a recent version of FreeBSD, NetBSD, Solaris 10 and newer, and several Linux distributions
  $6$	sha512crypt is a hash based on SHA-2 with 512-bit output originally developed for GNU libc and commonly used on (older) Linux systems
  $md5	SunMD5 is a hash based on the MD5 algorithm originally developed for Solaris
  $1$	md5crypt is a hash based on the MD5 algorithm originally developed for FreeBSD

  MS Windows Password

  • MS Windows Password are hashed using New Technology LAN Manager (NLTM), a variant of MD4.
  • They are visually identical to MD4 and MD5. Use the context to determine the type of hash.
  • On windows, the password hashes are stored in Security Accounts Manager (SAM) and normal users are not allowed to access this.
  • Tools like mimikatz help to exist to circumvent MS Windows Security.

Password Cracking

• Password hashes cannot be decrypted as they are not encrypted.
• Password hashes should be cracked by generating and comparing the hashes of many different inputs such as rockyou.txt.
• Tools like Hashcat and John the Ripper are commonly used for these purposes.
• Graphics Processing Units (GPUs) have thousands of cores and they are very good at mathematical calculations involved in the hash functions.
• Some hashing algorithms, such as Bcrypt are designed so that hashing on a GPU does not provide any improvement over CPU to prevent cracking.
• It is generally a good practice to avoid VMs for the purpose of password cracking as the VMs do not have access to host's graphics card and will be relatively slower.

Cracking Hashes with Hashcat

• To crack the hashes using hashcat, use the below command

  hashcat -m <hash_type> -a <attack_mode> hashfile wordlist
  • -m <hash_type> specifies the hash-type in numeric format. For example, -m 1000 is for NTLM. Check the official documentation (man hashcat) and example page to find the hash type code to use
  • -a <attack_mode> specifies the attack-mode. For example, -a 0 is for straight, i.e., trying one password from the wordlist after the other.
  • hashfile is the file containing the hash you want to crack.
  • wordlist is the security word list you want to use in your attack.

Integrity Checking with Hashcat

• Hashing can be used to check that files haven’t been changed.
• Generate a hash for the file using functions such as sha256sum and compare it to ensure integrity.

HMACs

• HMAC - Keyed-Hash Message Authentication Code.
• HMAC uses a cryptographic hash function in combination with a secret key to verify the authenticity and integrity of data.
• The following steps give you a fair idea of how HMAC works.
  • The secret key is padded to the block size of the hash function.
  • The padded key is XORed with a constant (usually a block of zeros or ones).
  • The message is hashed using the hash function with the XORed key.
  • The result from Step 3 is then hashed again with the same hash function but using the padded key XORed with another constant.
  • The final output is the HMAC value, typically a fixed-size string.

Encoding

• Encoding converts data from one form to another to make it compatible with a specific system.
• Commonly used encoding formats when sending or saving data are Base32 and Base64.
• To encode a value to Base64 encoding use base64 <REGULAR_STRING>.
• To decode a value from Base64 encoding use base64 -d <ENCODED_STRING>.
• Encoding does not ensure confidentiality as it can be reversed.