MD5 雜湊

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什麼是 MD5？

適用場景

• File integrity: compute MD5 after downloading to verify against official checksums
• Batch digest generation for config files or release notes
• HMAC-MD5 signing for legacy system integration
• Data deduplication via quick MD5 comparison
• Cache key generation as short unique identifiers
• Learning: understand hash algorithms by comparing MD5/SHA-1/SHA-256 outputs

使用方法

1. Select mode: Text, File, Batch, or HMAC-MD5
2. Set parameters: encoding, uppercase, 8-char grouping
3. Enter text, upload file, or paste batch content
4. Copy the hash result for verification or scripts

功能特點

• Four modes: Text, File, Batch line-by-line, and HMAC-MD5 covering common checksum scenarios
• UTF-8/Latin1 encoding selectable to match different system inputs
• Uppercase output and 8-char grouping for human readability
• Chunked file hashing: incremental reading for large files avoids memory issues
• Batch mode: each line hashed independently with original→hash output
• One-click copy of results

程式碼範例

// Note: Web Crypto API does NOT support MD5 (deprecated)
// Use SHA-256 instead for new projects
async function sha256(message) {
  const msgBuffer = new TextEncoder().encode(message);
  const hashBuffer = await crypto.subtle.digest('SHA-256', msgBuffer);
  return Array.from(new Uint8Array(hashBuffer)).map(b => b.toString(16).padStart(2,'0')).join('');
}
sha256('Hello World').then(console.log);

// MD5 in Node.js using crypto module
const crypto = require('crypto');
const md5 = crypto.createHash('md5').update('Hello World').digest('hex');
console.log(md5); // b10a8db164e0754105b7a99be72e3fe5

import hashlib
text = 'Hello World'
md5_hash = hashlib.md5(text.encode('utf-8')).hexdigest()
print(md5_hash)  # b10a8db164e0754105b7a99be72e3fe5

# File MD5 (chunked for large files)
def md5_file(filepath, chunk_size=8192):
    md5 = hashlib.md5()
    with open(filepath, 'rb') as f:
        while chunk := f.read(chunk_size):
            md5.update(chunk)
    return md5.hexdigest()

# HMAC-MD5
import hmac
hmac_md5 = hmac.new(b'secret-key', b'Hello World', hashlib.md5).hexdigest()
print(hmac_md5)

import java.security.MessageDigest;
import java.nio.charset.StandardCharsets;

public class MD5Example {
    public static String md5(String input) throws Exception {
        MessageDigest md = MessageDigest.getInstance("MD5");
        byte[] hash = md.digest(input.getBytes(StandardCharsets.UTF_8));
        StringBuilder sb = new StringBuilder();
        for (byte b : hash) sb.append(String.format("%02x", b));
        return sb.toString();
    }
    public static void main(String[] args) throws Exception {
        System.out.println(md5("Hello World"));
        // b10a8db164e0754105b7a99be72e3fe5
    }
}
// Note: Use SHA-256 for new code: MessageDigest.getInstance("SHA-256")

常見問題

術語表

Hash Function

A mathematical function mapping arbitrary-length input to fixed-length output. Good hash functions provide one-way-ness, collision resistance, and avalanche effect.

Collision Attack

Finding two distinct inputs that produce the same hash output. MD5 collisions were demonstrated by Wang et al. in 2004, enabling forged digital signatures.

Preimage Attack

Finding an input that produces a given hash output. Harder than collisions, but new 2024 research significantly weakened MD5 preimage resistance.

Rainbow Table

Precomputed hash→plaintext lookup tables for quickly cracking weak password hashes. Salting defeats rainbow tables.

HMAC

Hash-based Message Authentication Code using a secret key for message integrity/authentication. HMAC-MD5 and HMAC-SHA256 are common variants.HMAC Generator

bcrypt

A slow password-hashing algorithm with built-in salting and configurable cost factor, making brute-force attacks prohibitively expensive.

Checksum

A small fixed value used to verify data integrity. MD5, SHA-1, SHA-256 are commonly used as checksums to detect transmission corruption.

Avalanche Effect

A small input change (even 1 bit) causes a drastic, unpredictable change in output hash. MD5 still has good avalanche properties, contributing to its ongoing utility in non-security checks.

RFC 1321

The official MD5 standard submitted to IETF by Ronald Rivest in 1992, describing the algorithm's steps and test vectors.

Chunked Hashing

Processing large files in blocks rather than loading the entire file into memory, incrementally updating the hash state.

Common Hash Algorithm Comparison

MD5 Security Event Timeline

Authoritative References

• ReferenceRFC 1321 - The MD5 Message-Digest Algorithm
• ReferenceNIST SP 800-131A - Transitions: Recommendation for Transitioning the Use of Cryptographic Algorithms
• ReferenceWikipedia - MD5
• ReferenceMD5 Collision Attack (Wang et al., 2004)