RC4 (Rivest Cipher 4)
RC4 is used in many applications, including TLS (Transport Layer Security), WEP (Wired Equivalent Privacy), WPA (Wi-Fi Protected Access), TKIP (Temporal Key Integrity Protocol), Microsoft XBOX, Oracle SQL, Microsoft PPTP, Microsoft Office, and Adobe Acrobat.
RC4 is not patented, but it is a trade secret of RSA Security. An alleged copy of the RC4 source code was published anonymously in 1994. This code behaves identically to the official RC4. RC4 has been extensively analyzed since that time and has thus far proven to be secure against all reasonable attacks.
The exportable variant of RC4 utilizes a 40-bit key. This key size is vulnerable to a brute force attack using current technology. Many RC4 implementations use a 40-bit key size.
RC5 and RC6 are block ciphers which are not closely related to RC4.