Blog|Security

What is RC4 Encryption? Is RC4 Secure in 2026?

Stepn AlienBy Stepn Alien
|Jan 13, 2026|6 min read

You've probably heard of RC4 encryption. It powered SSL, TLS, and Wi-Fi security for years. But is RC4 still secure to use in 2026?

Short answer: No, RC4 is considered insecure and should be disabled.

Long answer: RC4 was revolutionary in its time, but multiple vulnerabilities have been discovered that make it unsuitable for modern security requirements. NIST officially recommends against using RC4.

Let's dive deep into what RC4 is, how it works, and why you should avoid it.

What is RC4 Encryption?

RC4 (Rivest Cipher 4) is a symmetric key stream cipher designed by Ron Rivest in 1987 for RSA Security. Unlike block ciphers that encrypt fixed-size chunks, RC4 operates on data byte by byte, making it a stream cipher.

Key characteristics of RC4:

  • Variable key size: 1 to 256 bytes (8 to 2048 bits)
  • Typical key size: 40-bit, 64-bit, or 128-bit
  • Type: Symmetric stream cipher
  • Speed: Extremely fast and simple to implement

    • RC4 was widely used in applications like SSL/TLS protocols, IEEE 802.11 wireless LAN standards, and the WEP (Wired Equivalent Privacy) security protocol.

    RC4 Brief History

    Ron Rivest designed RC4 in 1987 while working at RSA Security. The algorithm was initially a trade secret, but in 1994, someone anonymously posted the source code to an internet mailing list. While RSA Security never officially confirmed the leak, the algorithm became widely known.

    Interestingly, Rivest didn't officially describe the algorithm until 2014 when he documented its history on Wikipedia.

    How RC4 Works

    RC4 operates in two phases: Key Scheduling Algorithm (KSA) and Pseudo-Random Generation Algorithm (PRGA).

    Step 1: State Array Initialization

    First, create a 256-byte state array where each element equals its index:

    S[256] = {0, 1, 2, 3, ..., 254, 255}

    Step 2: Key Scheduling Algorithm (KSA)

    KSA uses the secret key to scramble the state array:

    for i = 0 to 255:
    j = (j + S[i] + Key[i mod keyLength]) mod 256
    swap(S[i], S[j])

    This creates a permutation of the state array based on your secret key.

    Step 3: Pseudo-Random Generation Algorithm (PRGA)

    PRGA generates the actual keystream for encryption:

    i = j = 0
while generating:
    i = (i + 1) mod 256
    j = (j + S[i]) mod 256
    swap(S[i], S[j])
    t = (S[i] + S[j]) mod 256
    output S[t]  // This is the keystream byte

    Encryption and Decryption

    The keystream is XORed with plaintext to produce ciphertext. Decryption uses the same process. XOR the ciphertext with the same keystream to recover plaintext.

    Ciphertext = Plaintext XOR Keystream
Plaintext = Ciphertext XOR Keystream

    Why RC4 Was Popular

    RC4 dominated for nearly two decades because of several advantages:

  • Simplicity: Easy to implement in both software and hardware
  • Speed: Extremely fast compared to other ciphers of its era
  • Low memory: Requires minimal RAM to operate
  • Handles large streams: Efficient for encrypting continuous data streams

    • RC4 Vulnerabilities: Why It's No Longer Secure

    Despite its popularity, RC4 has critical security flaws that make it unsuitable for modern use:

    Key Biases

    RC4's key scheduling algorithm produces statistical biases in the keystream. Attackers can exploit these biases to deduce information about the encryption key.

    Weak Initial Keystream Bytes

    The first few bytes of RC4's output exhibit predictable patterns. This weakness is particularly dangerous and has been exploited in multiple attacks.

    The FMS Attack (2001)

    The Fluhrer, Mantin, and Shamir attack demonstrated that certain weak keys leak information about the secret key. This attack famously broke WEP encryption.

    The Bar Mitzvah Attack (2015)

    This attack exploits biases in RC4 to recover portions of plaintext, particularly affecting TLS connections using RC4.

    NOMORE Attack (2015)

    Researchers demonstrated practical attacks against RC4 in TLS that could decrypt cookies within 75 hours.

    No Authentication

    RC4 provides no built-in authentication, making it vulnerable to bit-flipping attacks where attackers can modify ciphertext to produce predictable plaintext changes.

    Advantages vs. Disadvantages

    AdvantagesDisadvantages
    Simple implementationBiased initial keystream bytes
    Fast encryption speedVulnerable to key recovery attacks
    Low memory requirementsNo built-in authentication
    Works on large data streamsBanned by security standards (NIST)
    Easy hardware implementationSusceptible to bit-flipping attacks

    How to Disable RC4

    If you're running servers that might still support RC4, here's how to disable it:

    On Windows (Registry)

    [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\
SecurityProviders\SCHANNEL\Ciphers\RC4 128/128]
"Enabled"=dword:00000000

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\
SecurityProviders\SCHANNEL\Ciphers\RC4 40/128]
"Enabled"=dword:00000000

    On Apache

    Add to your SSL configuration:

    SSLCipherSuite HIGH:!RC4:!aNULL:!MD5

    On Nginx

    ssl_ciphers 'HIGH:!RC4:!aNULL:!MD5';

    Modern Alternatives to RC4

    Instead of RC4, use these secure alternatives:

  • AES-GCM: The gold standard for authenticated encryption
  • ChaCha20-Poly1305: Modern stream cipher, excellent for mobile devices
  • AES-256: Strong block cipher for sensitive data

    • The Bottom Line

    RC4 served the internet well for two decades, but its time has passed. Multiple devastating attacks, official deprecation by NIST, and removal from major browsers mean RC4 should never be used in new systems.

    If you're still using RC4 anywhere, disable it now. Switch to AES-GCM or ChaCha20-Poly1305 for modern, secure encryption. Your data security depends on it.

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