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Overview

Hashing transforms plaintext into a fixed-size digest using a one-way function, making it irreversible by design. It is used to protect data integrity, support confidentiality, and securely store passwords. Unlike encryption, hashed data cannot be decrypted — only compared against a new hash of the same input.

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Terminology

Term	Definition
Hash Function	Takes input of arbitrary size and produces a fixed-size output (digest)
Hash Value / Digest	Fixed-size string output produced by a hash function
Hash Collision	When two different inputs produce the same hash output
Salt	Random value added to a password before hashing; unique per user
Password Salting	Adding salt to prevent precomputed attacks such as rainbow tables
HMAC	Keyed-Hash Message Authentication Code; combines a secret key with a hash function
NThash	Modern Windows password hash format; MD4-based
Rainbow Table	Precomputed table of hash values used to reverse hash lookups

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Core Concepts

Hashing Algorithms

Algorithm	Command	Output Length
MD5	md5sum	32 hex characters
SHA1	sha1sum	40 hex characters
SHA256	sha256sum	64 hex characters
SHA512	sha512sum	128 hex characters

Salting

• A salt is a random value added to a password before hashing
• Unique per user — prevents two users with the same password from having the same hash
• Defeats precomputed rainbow table attacks
• Salt is stored alongside the hash so verification can reproduce it

HMAC (Keyed-Hash Message Authentication Code)

Combines a secret key with a hash function to verify both integrity and authenticity.

Process:

1. Key is padded to block size
2. Key is XORed with a constant
3. Message is hashed with the modified key
4. Result is hashed again with a second modified key
5. Output is a fixed-size HMAC value

Linux Password Storage

Passwords stored in /etc/shadow — see Linux Fundamentals for file structure detail.

Format: $prefix$options$salt$hash Fields separated by colons. View algorithm info: man 5 crypt

Algorithm	Prefix
yescrypt	$y$
gost-yescrypt	$gy$
scrypt	$7$
bcrypt	$2b$
sha512crypt	$6$
md5crypt	$1$
SunMD5	$md5

Windows Password Storage

• Stored in SAM (Security Accounts Manager) — see Windows Fundamentals
• Uses NTLM (MD4-based) hashing
• Modern format: NThash

Hash Identification Quick Reference

Hash Type	Length	Notes
MD5	32 hex chars	Legacy, fast — avoid for passwords
SHA1	40 hex chars	Deprecated for security use
SHA256	64 hex chars	Common integrity verification
SHA512	128 hex chars	Strong password hashing

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Related Concepts

• Cryptography Basics
• Linux Fundamentals
• Windows Fundamentals

Related Techniques

• Password Cracking

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References / Images

• Hash function diagrams
• HMAC workflow visualization
• /etc/shadow structure examples
• CrackStation: https://crackstation.net↗
• Hashes.com: https://hashes.com↗