What is the most commonly used stream cipher?

The most commonly used stream cipher is RC4, which is a symmetric key stream cipher designed by Ron Rivest in 1987. Its popularity stems from its simplicity and speed in software, making it a preferred choice for many applications, such as securing web traffic and other communications. RC4 generates a pseudo-random keystream that is combined with the plaintext to produce ciphertext, which is then decrypted using the same technique with the same key.

RC4's widespread adoption can be seen in protocols such as SSL/TLS and WEP, although its security has come under scrutiny over the years due to vulnerabilities that have been identified. Despite these weaknesses, its historical significance as a stream cipher makes it the most well-known among the options presented.

In contrast, the other options listed serve different purposes. A5/1 is a stream cipher used in GSM encryption, SHA-1 is a hashing algorithm not used for encryption, and MD5 is also a hashing algorithm that produces a fixed-size hash from variable-length input but does not serve as a stream cipher. Each of these alternatives has its own specialized use in cryptography but does not match the role and prevalence of RC4 as a stream cipher.