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Also known as: Secure Hash Algorithm 1
A deprecated cryptographic hash function producing 160-bit digests due to collision vulnerabilities.
DESCRIPTION
SHA-1 is a NIST-standardized cryptographic hash function producing 160-bit message digests. Collision attacks demonstrated practical weaknesses, leading major platforms to deprecate SHA-1 for signatures and certificates. It persists in legacy systems but should be replaced by SHA-256 or stronger algorithms. Many libraries disable SHA-1 in default configurations to reduce risk. Outdated and not recommended; modern alternatives include SHA-2 and SHA-3.
ALTERNATIVE DEFINITIONS
Wikipedia
SHA-1 is a cryptographic hash function which takes an input and produces a 160-bit (20-byte) hash value known as a message digest.
States 160-bit output and common name ‘message digest.’
IETF RFC 3174
The SHA-1 produces a 160-bit output called a message digest.
Formal, standard RFC describing output size and function.
NIST FIPS 180-4
The Secure Hash Standard (SHS) specifies secure hash algorithms—SHA-1, SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, and SHA-512/256.
Places SHA-1 within the broader SHS suite and specifies family members.
RELATED TERMS
Cryptographic Hash Function HMAC Collision Attack Digital Signature TLS NIST FIPS 180 Git
COMPETING TERMS
KNOWLEDGE AREAS
TARGET AUDIENCE
Cybersecurity Engineer Security Architect Chief Information Security Officer Software Developer Cloud Architect
EXTERNAL SOURCES
- Wikipedia - SHA-1
- NIST - Secure Hash Standard specification
- IETF - SHA-1 RFC specification
- OWASP - Hash collision attack overview
TOP QUESTIONS
Why is SHA-1 considered insecure today?
Practical collision attacks have been demonstrated, undermining integrity guarantees. Standards bodies and vendors have deprecated SHA-1 in signatures and certificates.
Category: Risk
Where is SHA-1 still found?
Legacy systems, older archives, and some non-security contexts may still use SHA-1. Migration plans should prioritize critical cryptographic uses.
Category: Management
What should replace SHA-1?
Use SHA-256 or stronger algorithms from SHA-2 or SHA-3, depending on performance and compliance requirements.
Category: Alternatives
Does SHA-1’s weakness affect HMAC-SHA1?
HMAC construction mitigates collision-based attacks; however, many policies still require stronger hashes for uniformity and future safety.
Category: Opinion
How did the SHA-1 collision get demonstrated?
Academic and industry teams generated two distinct documents with the same SHA-1 digest, proving practical collision feasibility.
Category: Example
What standards govern SHA-1?
NIST FIPS 180 series and IETF RFC 3174 specify SHA-1 and related hash functions.
Category: Standard
Is SHA-1 acceptable for checksums?
Prefer stronger hashes even for checksums; integrity needs can evolve and accidental reuse encourages unsafe practices.
Category: Mistakes
How do I assess migration impact from SHA-1?
Inventory cryptographic uses, update libraries, re-issue certificates, and test compatibility across dependent systems.
Category: HowTo
What regulatory guidance exists on SHA-1?
NIST guidance and industry baselines advise discontinuing SHA-1 for cryptographic assurance.
Category: Regulation