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A cryptanalytic technique exploiting the birthday paradox to find hash collisions in about 2^(n/2) work.
DESCRIPTION
A birthday attack leverages collision probabilities to find two distinct inputs hashing to the same output faster than brute force for specific outputs. Against an n-bit hash, it typically requires about 2^(n/2) operations to find any collision. This bound dictates necessary hash output sizes to achieve desired collision security. Modern designs and security levels consider birthday attacks fundamental. Practical impacts include signatures on hashed messages and deduplication integrity.
ALTERNATIVE DEFINITIONS
Wikipedia
In cryptography, a birthday attack is a type of cryptographic attack that exploits the mathematics behind the birthday problem in probability theory.
Connects attack strategy directly to the birthday problem.
IETF RFC 4949
Birthday attack: An attack that uses the mathematics of the birthday problem to find collisions in hash functions.
Glossary-level definition oriented to security terminology.
NIST SP 800-107
Due to the birthday paradox, the expected work to find a collision in an n-bit hash function is approximately 2^{n/2} hash operations.
Quantifies the birthday bound for practical security assessment.
RELATED TERMS
Hash Function Collision Resistance Preimage Attack Second-Preimage Attack Birthday Paradox Merkle–Damgård SHA-256 Random Oracle Model Entropy Cryptanalysis
COMPETING TERMS
Collision-Resistant Hash Function
RELATIONS
- Broader: Cryptanalysis
- Derived from: Birthday Paradox
KNOWLEDGE AREAS
TARGET AUDIENCE
Cybersecurity Engineer Security Architect Chief Information Security Officer Software Developer
EXTERNAL SOURCES
- Wikipedia - Birthday attack
- NIST - Guidance on hash security and birthday bound
- IETF - Overview of attacks on cryptographic hashes
TOP QUESTIONS
How does a Birthday Attack work?
It searches for any two inputs with the same hash, exploiting quadratic pair growth to find a collision in about 2^(n/2) operations.
Category: Definition
How to defend against a Birthday Attack?
Use sufficiently long hash outputs (e.g., 256-bit), collision-resistant designs, and protocols that bind to unique contexts.
Category: HowTo
What is the biggest risk of Birthday Attacks?
They can undermine digital signatures and deduplication by producing conflicting messages with identical hashes.
Category: Risk
How do Birthday Attacks compare to preimage attacks?
Birthday attacks find any collision; preimage attacks target a specific output and typically require 2^n work for n-bit hashes.
Category: Alternatives
Do salting and domain separation help?
Yes, they reduce cross-domain collisions and thwart multi-target or reuse-based collision strategies.
Category: BestPractice
Are modern hashes safe from Birthday Attacks?
Appropriately sized hashes like SHA-256 provide strong collision resistance; shorter hashes are vulnerable.
Category: Opinion
What is the storage cost in Birthday Attacks?
Memory for storing candidates is significant; distinguished point methods can trade memory for time.
Category: Requirements
Is there a quantum speedup for Birthday Attacks?
Quantum algorithms provide limited speedups; collision search lower bounds persist near 2^(n/3)–2^(n/2) depending on model.
Category: Opinion
Where to learn more about Birthday Attacks?
Read NIST SP 800-107 for birthday-bound implications and RFC 4270 for hash attack overviews.
Category: GettingStarted