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Cryptography & Ciphers

RC4 Drop Cipher Processor

Reproduce legacy RC4-drop streams with explicit key encoding and dword counts. Run an RFC 6229 sample at byte offset 768.

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Paste legacy RC4-drop data. Match its key encoding, input format, and dropped dwords. Hex output preserves every byte.

Options

Enter 1–256 key bytes. Passphrase means raw encoded key bytes here; no password derivation runs.

One dword is four bytes. The default 192 skips 768 keystream bytes.

Your result appears here

Reproduce legacy RC4-drop streams with explicit key encoding and dword counts. Run an RFC 6229 sample at byte offset 768.

This tool runs open-source code from CyberChef by GCHQ (Apache License 2.0). See open-source credits.

The operation discards the selected number of keystream dwords before transforming the input. One dword contains four bytes.

Hex and Base64 outputs preserve arbitrary transformed bytes. UTF8 output requires valid text bytes.

  • Check whether a legacy implementation measures its drop parameter in bytes or four-byte words.

Sources: RFC 6229: independent RC4 keystream vectors; RFC 7465: prohibition of RC4 cipher suites in TLS

Legacy use and missing authentication

  • Each run restarts the stream; it cannot continue a previous packet’s state.
  • The operation does not derive a key from a password.
  • RC4-drop does not authenticate data and is unsuitable for new encryption.

Set the key bytes and dropped dwords

  1. Enter the key and choose Passphrase format.
  2. Choose Input format and Output format.
  3. Match Number of dwords to drop with the source system.

Reproduce a keystream segment with zero bytes

Input: 00000000000000000000000000000000

Settings: Passphrase format Hex; key 0102030405; Input format Hex; Output format Hex; drop 192 dwords.

Result: eb62638d4f0ba1fe9fca20e05bf8ff2b

Sixteen zero bytes expose the RFC 6229 keystream at byte offset 768. The published vector supplies the expected output.

Questions

How do I decrypt RC4-drop output?

Apply the same operation with the same key and drop count. Stream encryption and decryption use the same transformation.