Blowfish was designed in 1993 by Bruce Schneier as a fast, free, and public-domain alternative to existing proprietary encryption algorithms. A primary design goal was for it to be unpatented, license-free, and available for all uses without restriction. In contrast, RC2 and RC4 were developed by Ron Rivest for RSA Security and were initially maintained as proprietary trade secrets. Skipjack was developed by the U.S. National Security Agency (NSA) as a classified algorithm for use in the Clipper chip, making it a government-proprietary system until its declassification in 1998. Therefore, Blowfish is the only algorithm listed that was not designed to be proprietary.

A. RC2: Was designed by RSA Security as a proprietary trade secret, intended as a drop-in replacement for DES.
B. RC4: Was also designed for RSA Security as a proprietary trade secret until its source code was anonymously leaked in 1994.
D. Skipjack: Was a classified, government-proprietary algorithm developed by the U.S. NSA for its controversial Clipper chip initiative.

1. Schneier

B. (1994). Description of a New Variable-Length Key

64-Bit Block Cipher (Blowfish). In: Anderson

R. (eds) Fast Software Encryption. FSE 1993. Lecture Notes in Computer Science

vol 809. Springer

Berlin

Heidelberg. On page 191

the introduction states

"Blowfish is unpatented and license-free

and is available free for all uses." DOI: https://doi.org/10.1007/3-540-58108-124

2. National Institute of Standards and Technology (NIST). (1998

May 29). SKIPJACK and KEA Algorithm Specifications Version 2.0. Page 1

Section 1

"Introduction

" states

"The SKIPJACK algorithm was developed by the U.S. Government... The algorithm is classified..." This document marks its declassification for public evaluation.

3. Rivest

R. (1998). RFC 2268: A Description of the RC2(r) Encryption Algorithm. Internet Engineering Task Force (IETF). Section 1

"Introduction

" notes that RC2 is a proprietary algorithm of RSA Data Security

Inc.

4. Kaufman

C.

Perlman

R.

& Speciner

M. (2002). Network Security: Private Communication in a Public World (2nd ed.). Prentice Hall. In Chapter 14

"Algorithms

" the text discusses the history of RC4 as a trade secret of RSA Security until it was leaked. It also describes Skipjack's origin with the NSA and the Clipper chip. (Specific reference: Chapter 14

Section 14.3 "Stream Ciphers" for RC4; Section 14.2 "Block Ciphers" for Skipjack).