Question 1

What is a cryptographic hash function?

Accepted Answer

A cryptographic hash function is a mathematical algorithm that takes input data of any size and produces a fixed-size output (the hash or digest). Key properties include: (1) Deterministic - the same input always produces the same hash, (2) Fast computation - hashes can be generated quickly, (3) Pre-image resistance - it's computationally infeasible to reverse the hash to get the original input, (4) Small changes in input produce drastically different hashes (avalanche effect), and (5) Collision resistance - it's extremely difficult to find two different inputs that produce the same hash. These properties make hash functions essential for data integrity verification, password storage, digital signatures, and blockchain technology.

Question 2

What are the main differences between MD5, SHA-1, SHA-256, and SHA-512?

Accepted Answer

These hash functions differ in security and output size: **MD5** (128-bit/32 hex chars) - Fast but cryptographically broken since 2004. Never use for security. Only acceptable for non-security checksums. **SHA-1** (160-bit/40 hex chars) - Deprecated since 2017 due to collision attacks. Google demonstrated practical collisions. **SHA-256** (256-bit/64 hex chars) - Part of SHA-2 family. Current industry standard. Used in Bitcoin, TLS certificates, and most modern applications. **SHA-512** (512-bit/128 hex chars) - More secure than SHA-256 with larger output. Slower on 32-bit systems but faster on 64-bit systems. Use SHA-256 or SHA-512 for all security-sensitive applications. Avoid MD5 and SHA-1 unless required for legacy compatibility.

Question 3

Should I use MD5 or SHA-1 for password hashing?

Accepted Answer

**Absolutely not!** Never use MD5 or SHA-1 for password hashing - this is a critical security mistake. Both are cryptographically broken and vulnerable to collision attacks and rainbow table lookups. For password hashing, you MUST use specialized password hashing algorithms designed to be slow and resistant to brute force attacks: **bcrypt** - Industry standard, adjustable work factor, resistant to GPU attacks. **scrypt** - Memory-hard function, resistant to hardware attacks. **Argon2** - Modern winner of Password Hashing Competition, best overall security. **PBKDF2-HMAC-SHA256** - Acceptable alternative, widely supported. These algorithms include automatic salting and configurable computational cost to slow down brute force attacks. General-purpose hash functions like MD5/SHA-1/SHA-256 are designed to be fast, which makes them terrible for passwords.

Question 4

What is HMAC and when should I use it?

Accepted Answer

HMAC (Hash-based Message Authentication Code) combines a cryptographic hash function with a secret key to provide both data integrity and authentication. Unlike regular hashing, HMAC proves that: (1) The message hasn't been tampered with (integrity), and (2) The message came from someone who knows the secret key (authenticity). HMAC works by hashing the message twice with the key incorporated. Use cases include: API request signing (AWS, webhooks), JWT signatures, Cookie integrity verification, Message authentication in encrypted communications, and Challenge-response authentication. HMAC prevents tampering even if the attacker can see the message. Common implementations: HMAC-SHA256 (most popular), HMAC-SHA512 (more secure), HMAC-MD5 (legacy only). Both sender and receiver must share the same secret key.

Question 5

How can I verify file integrity using hashes?

Accepted Answer

Hash verification ensures files haven't been corrupted or tampered with during download: (1) **Obtain the official hash** - Software publishers provide SHA-256 hashes on their download pages, (2) **Generate hash of your downloaded file** - Use this tool or command-line tools (sha256sum, certutil), (3) **Compare hashes** - They must match exactly. Even one character difference means corruption or tampering. This protects against: Incomplete downloads, File corruption during transfer, Man-in-the-middle attacks, Malware injection, and Compromised download mirrors. Always download the hash from an official source (HTTPS, PGP-signed). SHA-256 is standard for file verification. Linux ISO images, Windows updates, and software downloads commonly provide SHA-256 checksums. Never trust a hash provided by the same untrusted source as the file.

Question 6

What are rainbow tables and how do salts protect against them?

Accepted Answer

Rainbow tables are pre-computed databases of password hashes used to crack passwords instantly. Instead of trying millions of password guesses, attackers look up the hash in the rainbow table to find the original password. Example: Hash "password123" with MD5 → lookup MD5 hash in rainbow table → instantly recover password. **Salts** prevent rainbow tables by adding unique random data to each password before hashing. With salts: (1) Each password has a different hash even if passwords are identical, (2) Attackers must compute hashes for every possible password+salt combination, (3) Rainbow tables become useless because the salt space is too large. Modern password hashing algorithms (bcrypt, scrypt, Argon2) include automatic salting. This is why you must use proper password hashing - never plain SHA-256 or MD5, which are vulnerable to rainbow table attacks even without salts.

Question 7

Can I reverse a hash to get the original data?

Accepted Answer

No - cryptographic hash functions are designed to be one-way (pre-image resistant). You cannot mathematically reverse SHA-256 hash back to the original input. However, attackers can attempt: **Brute force** - Try all possible inputs until finding a match (extremely slow for strong passwords with proper hashing). **Dictionary attacks** - Try common passwords and phrases (very effective against weak passwords). **Rainbow tables** - Look up pre-computed hashes (defeated by salts). **Collision attacks** - Find any input that produces the same hash (doesn't recover original input, but can bypass integrity checks). This is why weak passwords are vulnerable even with strong hashing - attackers don't reverse the hash, they try common passwords until they find a match. Strong passwords (16+ characters, mixed types) make brute force attacks computationally infeasible even with fast hash functions.

Question 8

What is SHA-3 and should I use it instead of SHA-2?

Accepted Answer

SHA-3 is the latest cryptographic hash function standardized by NIST in 2015, designed as an alternative to SHA-2, not a replacement. Key differences: **SHA-2** (SHA-256, SHA-512) - Based on Merkle-Damgård construction, battle-tested since 2001, industry standard, supported everywhere. **SHA-3** - Based on Keccak sponge construction, different internal design, provides similar security level, less widespread adoption. **Should you use SHA-3?** Currently, stick with SHA-256/SHA-512 unless you have specific requirements. SHA-2 remains secure with no practical attacks. SHA-3 provides: Alternative construction if SHA-2 is compromised, Potential performance benefits in hardware, Different security properties for specialized applications. Use SHA-256 for general purposes. SHA-3 is future-proofing but offers no immediate security benefit over SHA-256. Both are quantum-resistant against current quantum computers.

Free Hash Generator - MD5, SHA256, bcrypt & More

What will you use these hashes for?

Password Storage

Data Integrity Verification

Digital Signatures

HMAC / API Authentication

File Checksums

Legacy System Compatibility

General Purpose / Not Sure

Hash Algorithms

Text Input

Hash Comparison

Hash Identifier

Need Professional Security Services?

References & Citations

Key Security Terms

Cryptographic Hash Function

JSON Web Token (JWT)

Subresource Integrity (SRI)

File Signatures (Magic Numbers)

Frequently Asked Questions

⚠️ Security Notice