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Also known as: Password Stretching
A technique that increases the cost of password guessing by applying repeated or memory-hard hashing to slow brute-force attacks.
DESCRIPTION
Key stretching strengthens low-entropy secrets such as passwords by increasing the computational (and often memory) cost of verification. Implementations include PBKDF2 (iterated hashing), bcrypt (adaptive cost), scrypt (memory-hard), and Argon2 (PHC winner). Salts are mandatory to prevent precomputation attacks, and peppers can add defense-in-depth. Choose parameters to match threat models and hardware (e.g., GPU-resistant memory hardness). Follow NIST and OWASP guidance and monitor for drift in attack capabilities.
ALTERNATIVE DEFINITIONS
OWASP
Key stretching increases the effort required to brute-force passwords by using algorithms like PBKDF2, bcrypt, scrypt, or Argon2 with appropriate parameters.
Emphasizes recommended algorithms and parameterization; paraphrased due to offline generation.
IETF RFC 8018
PKCS #5 (PBKDF2) specifies a password-based key derivation function that applies iteration to increase computational cost.
Defines PBKDF2 as a standard key-stretching method; paraphrased due to offline generation.
NIST SP 800-63B
NIST recommends salted, slow password hashing and, where possible, memory-hard functions to resist large-scale guessing attacks.
Regulatory guidance for verifiers; paraphrased due to offline generation.
RELATED TERMS
bcrypt Salt Pepper Rate Limiting Credential Stuffing Password Hashing Memory-Hard Function
COMPETING TERMS
Password-Based Key Derivation Function 2 scrypt Argon2
KNOWLEDGE AREAS
Data Security Software Engineering Data Privacy & Compliance
TARGET AUDIENCE
Security Architect Cybersecurity Engineer Software Developer DevOps Engineer IT Auditor Chief Information Security Officer Data Privacy Officer
EXTERNAL SOURCES
- Wikipedia - Key stretching
- OWASP - Best practices for password storage.
- IETF - PBKDF2 standard definition.
- PHC - Argon2 specification and resources.
- NIST - Digital identity guidance for verifiers.
TOP QUESTIONS
Why is key stretching essential for password security?
It raises the cost of each guess, slowing brute-force and credential stuffing, thereby improving resistance against offline attacks.
Category: Relevancy
How do I choose between PBKDF2, bcrypt, scrypt, and Argon2 for key stretching?
Argon2id is generally recommended for memory hardness and GPU resistance; PBKDF2 is widely standardized; scrypt offers memory hardness; bcrypt is mature but not memory-hard.
Category: Recommendation
What parameters are crucial in key stretching?
Iterations/time cost, memory cost, and parallelism should be tuned to balance security and performance, and periodically increased over time.
Category: Requirements
Do salts and peppers replace key stretching?
No. Salts prevent precomputation attacks; peppers add defense-in-depth. Key stretching is still necessary to slow guessing.
Category: Misunderstanding
What is a common mistake with key stretching?
Using raw fast hashes (e.g., SHA-256) for passwords without sufficient iteration or memory-hardening drastically weakens security.
Category: Mistakes
How often should parameters be updated?
Regularly reassess and increase costs as hardware improves; consider rehashing on login when parameters are outdated.
Category: Management
Is key stretching enough by itself?
No. Combine with MFA, rate limiting, breach monitoring, and secure secret handling for layered defense.
Category: Risks
What standards reference key stretching?
IETF RFC 8018 (PBKDF2) and NIST SP 800-63B provide requirements and guidance for verifiers.
Category: Standards
What hot topic surrounds key stretching today?
Adopting memory-hard Argon2id with balanced parameters against GPU/ASIC attackers is a key focus.
Category: Topic