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A historic stream cipher once widely used, now deprecated due to significant statistical biases and practical attacks.
DESCRIPTION
RC4, designed in 1987, is a simple byte-oriented stream cipher that became pervasive in WEP, early TLS, and other protocols. Research revealed serious biases and vulnerabilities, enabling practical plaintext recovery and key attacks. Standards bodies have prohibited its use in TLS and other contexts. Modern systems should use AEADs like AES-GCM or ChaCha20-Poly1305 instead of RC4.
ALTERNATIVE DEFINITIONS
IETF RFC 7465
This document prohibits the use of RC4 cipher suites in TLS; RC4 is found to have weaknesses that make it vulnerable in TLS.
Authoritative deprecation in TLS due to weaknesses.
NIST SP 800-52r2
RC4 is not allowed in TLS configurations due to its known vulnerabilities and should not be used.
US government guidance banning RC4 in TLS configurations.
Wikipedia
RC4 is a stream cipher that is simple and fast in software but has serious vulnerabilities and is considered insecure.
Summarizes algorithm nature and insecurity concisely.
RELATED TERMS
Stream Cipher ARCFOUR Wired Equivalent Privacy TLS Systematic Error Key Scheduling Algorithm Temporal Key Integrity Protocol Poly1305
COMPETING TERMS
KNOWLEDGE AREAS
Data Security Software Engineering
TARGET AUDIENCE
Security Architect Cybersecurity Engineer Software Developer Chief Information Security Officer
EXTERNAL SOURCES
- Wikipedia - RC4
- IETF - Prohibits RC4 in TLS
- IETF - ARCFOUR modes for SSH (context)
- NIST - TLS guidance deprecating RC4
TOP QUESTIONS
Why is RC4 considered insecure today?
Statistical biases and key scheduling weaknesses enable practical plaintext recovery and attacks in protocols like TLS and WEP.
Category: Understanding
Where was RC4 historically used?
It appeared in WEP/WPA-TKIP, SSL/TLS cipher suites, and some SSH configurations, before being phased out.
Category: Example
What are secure alternatives to RC4?
Use AEAD ciphers such as AES-GCM or ChaCha20-Poly1305, which provide confidentiality and integrity.
Category: BestPractices
Is RC4 ever acceptable for non-security checksums?
No. Even non-security contexts risk misuse; prefer modern, safe primitives tailored to the purpose.
Category: Risks
What standard deprecated RC4 for TLS?
IETF RFC 7465 explicitly prohibits RC4 cipher suites in TLS due to vulnerabilities.
Category: Regulation
Can RC4 be safely used with additional integrity checks?
No. Integrity checks don’t address RC4’s keystream biases and leakage risks. Replace the cipher entirely.
Category: Opinion
Was RC4 patented or proprietary?
RC4 was proprietary, with the algorithm leaked in 1994 as “ARCFOUR,” leading to widespread unofficial implementations.
Category: Origin
What attacks famously broke RC4 in WEP?
The Fluhrer–Mantin–Shamir attack exploited weak IV usage and biases to recover WEP keys.
Category: Example
Is RC4 supported by modern TLS libraries?
No. Modern libraries and configurations disable RC4, complying with standards and best practices.
Category: Relevancy