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Network Working Group                                        K. Kaukonen
                                                               R. Thayer

Internet Draft                                                 July 1997


                  A Stream Cipher Encryption Algorithm "Arcfour"
                       <draft-kaukonen-cipher-arcfour-01.txt>


Status of this Memo

          This document is an Internet-Draft.  Internet-Drafts are
          working documents of the Internet Engineering Task Force
          (IETF), its areas, and its working groups.  Note that other
          groups may also distribute working documents as
          Internet-Drafts.

          Internet-Drafts are draft documents valid for a maximum of six
          months and may be updated, replaced, or obsoleted by other
          documents at any time.  It is inappropriate to use Internet-
          Drafts as reference material or to cite them other than as
          ``work in progress.''

          To learn the current status of any Internet-Draft, please
          check the ``1id-abstracts.txt'' listing contained in the
          Internet- Drafts Shadow Directories on ftp.is.co.za (Africa),
          nic.nordu.net (Europe), munnari.oz.au (Pacific Rim),
          ds.internic.net (US East Coast), or ftp.isi.edu (US West
          Coast).


Abstract

          This document describes an algorithm here called Arcfour that
          is believed to be fully interoperable with the RC4 algoritm.
          RC4 is trademark of RSA Data Security, Inc.  There is a need
          in the Internet community for an encryption algorithm that
          provides interoperable operation with existing deployed
          commercial cryptographic applications.  This interoperability
          will allow for a smoother transition to protocols that have
          been developed through the IETF standards process.













Kaukonen, Thayer                                                [Page 1]


Internet Draft        An Encryption Algorithm                  July 1997




TABLE OF CONTENTS


          STATUS OF THIS MEMO..........................................1


          ABSTRACT.....................................................1


          1. INTRODUCTION..............................................3


          2. REQUIREMENTS FOR THIS ENCRYPTION ALGORITHM................3


          3. DESCRIPTION OF ALGORITHM..................................4

          3.1. KEY SETUP...............................................4
          3.2. STREAM GENERATION.......................................4


          4. INTELLECTUAL PROPERTY CONSIDERATIONS......................5


          5. ACKNOWLEDGEMENTS..........................................5


          6. SECURITY CONSIDERATIONS...................................5


          7. REFERENCES................................................6


          8. AUTHORS' ADDRESSES........................................7


          APPENDIX.....................................................7

          A. TEST VECTORS..............................................7
          B. SAMPLE CODE..............................................10










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Internet Draft        An Encryption Algorithm                  July 1997



1. Introduction

          There is a need in the Internet community for an encryption
          algorithm that provides interoperable operation with existing
          deployed commercial cryptographic applications.  This
          interoperability allows for a smoother transition to protocols
          that have been developed through the IETF standards process.
          This document describes an existing algorithm that satisifies
          this requirement.

          There is a large body of experience in developing and
          deploying encryption applications, especially in the HTTP/HTML
          browser/server markets.  These browsers typically implement
          the RC4 encryption algorithm provided by [RSA].  It would be
          beneficial for the IETF standards processes to produce
          protocols that can be deployed into existing Internet
          environments.  This would allow graceful addition of new
          (IETF-developed) protocols. It would allow less disruption of
          existing users, since there would be more interoperability
          between pre-exisiting protocols and IETF- based protocols.


2. Requirements for this Encryption Algorithm

          The algorithm described here is called Arcfour, and it has
          been chosen because it is compatible with the RC4(TM) algoritm
          that is one of the most popular encryption algorithms in the
          browser market.  (See chapter Intellectual Property
          Considerations.)  Arcfour is potentially useful in several
          environments, including IPSEC [IPSEC], SSH [SSH], and TLS
          [TLS].  There are existing Internet Drafts that describe how
          it can be applied, see e.g. [Caronni], [SSH], and [TLS].

          The algorithm can be used with a variety of key lengths.  It
          specifically can be operated with 40-bit keys and with 128-bit
          keys.  See the Security Considerations section for comments on
          the use of 40-bit keys.

          Compatibility of the algorithm with commercial algorithms can
          be tested by comparing the encrypted data that is produced by
          the test vectors listed in the appendix to this document.










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3. Description of Algorithm

          The algorithm itself is documented in [Schneier], pages
          397-398, in the chapter titled "Other Stream Ciphers and Real
          Random Sequence Generators".

3.1 Key Setup

          1. Allocate an 256 elemment array of 8 bit bytes to be used as
          an S-box, label it
              S [0] .. S [255].

          2. Initialize the S-box.  Fill each entry first with it's
          index:

              S [0] = 0; S [1] = 1; etc. up to S [255] = 255;

          3. Fill another array of the same size (256) with the key,
          repeating bytes as necessary.

              for (i = 0; i < 256; i = i + 1)
                S2 [i] = key [i % keylen];

          4. Set j to zero and initialize the S-box like this:

              for (i = 0; i < 256; i = i + 1)
              {
                j = (j + S [i] + S2 [i]) % 256;
                temp = S [i];
                S [i] = S [j];
                S [j] = temp;
              }

          5. Initialize i and j to zero.  If superuser priviledged
          program sniffing is feared (that is, always) set also the S2
          array and the key array to zero.  That gives a slightly better
          proctection since the key is believed to be not feasible to
          calculate after it has been zeroed and thus forgotten.

3.2 Stream Generation

          For either encryption or decryption, the input text is
          processed one byte at a time.  A pseudorandom byte K is
          generated:

              i = (i+1) % 256;
              j = (j + S[i]) % 256;
              temp = S [i];
              S [i] = S [j];


Kaukonen, Thayer                                                [Page 4]


Internet Draft        An Encryption Algorithm                  July 1997


              S [j] = temp;
              t = (S [i] + S [j]) % 256;
              K = S [t];

          To encrypt, XOR the value K with the next byte of the
          plaintext.  To decrypt, XOR the value K with the next byte of
          the ciphertext.

4. Intellectual Property Considerations

          This document does not address Intellectual Property issues.
          No claim is made as to who owns this algorithm, of the
          performance of the algoritm, its cryptographic security or any
          other liability issues related to the algoritm itself, its
          implementation or use.

          The Arcfour algorithm is believed to be fully interoperable
          with the RC4 algorithm.  "RC4" is believed to be trademark of
          RSA Data Security, Inc.  Contact [RSA] if RC4(TM) algorithm is
          needed.

5. Acknowledgements

          This work was based on conversations with several collegues
          within the IETF.

6. Security Considerations

          This algorithm can be operated with several different key
          sizes.  If the key is 128 bits in length then this algorithm
          is believed to be secure.  If the key length is significantly
          shorter, specifically 40 bits, then there are known attacts
          that have been successfully applied.  For this algorithm to be
          operated in a cryptographically sound manner it is believed
          that a key length of 128 bits or more should be used.

          On the other hand, the 40-bit version of this algorithm is
          specifically regulated by the U.S. Government.  This means
          that deployment of 40-bit implementations may be easier to
          export than alternative algorithms.

          It must be strongly recommended that no two plaintexts are
          encrypted with the same key. Otherwise the plaintext can
          usually be broken, and often even quite easily. If the two
          encrypted messages are XORed together, the result is XOR of
          the original plaintexts. Given the encrypted messages are text
          strings, credit card numbers, or other byte streams with some
          known properties, the plaintexts can be estimated with great
          accuracy. See the [DAWSON AND NIELSEN] for more details.



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          Initial cryptanalysis results are favorable, but the current
          literature should be consulted to assess the security of this
          cipher.  A good starting point for a citation search would be
          [GOLIC].  For Internet news group posting, start with
          [FINNEY], [JENKINS] and [ROOS].

7. References

          [Caronni] Caronni, G., Waldvogel, M.  "The ESP Stream
          Transform", ftp://ds.internic.net/internet-drafts/
          draft-caronni-esp-stream-01.txt, September, 1996.

          [COMMERCE] Test vectors issued by United States Department of
          Commerce, Bureau of Export Administration, Office of Strategic
          Trade and Foreign Policy, Strategic Trade Controls Division.

          [CRYPTLIB] Gutmann, P, Young, E., Plumb, C.  "Cryptlib, A
          Portable Encryption Library", Version 2.00.
          http://www.cs.auckland.ac.nz/~pgut001/cryptlib.html, 1996.

          [DAWSON AND NIELSEN] Dawson E. and Nielsen L.: Automated
          Cryptoanalysis of XOR Plaintext Strings, Cryptologia, April
          1996, Volume XX, Number 2.

          [FINNEY] Finney, H. Internet message posted to sci.crypt 21
          September, 1994.

          [GOLIC] Golic, J. "Linear Statistical Weakness of Alleged RC4
          Keystream Generator." In, W. Fumy (ed.), Proceedings of
          Eurocrypt '97, 226-238, Springer-Verlag, 1997.

          [IPSEC] Atkinson, R, "Security Architecture for the Internet
          Protocol", ftp://ds.internic.net/rfc/rfc1825.txt, August 1995.

          [JENKINS] Jenkins, B. Internet message posted to sci.crypt 22
          September, 1994.

          [ROOS] Roos, A. Internet message posted to sci.crypt 28
          September, 1995.

          [RSA] RSA Data Security, Inc., http://www.rsa.com, Address:
          RSA Data Security, Inc.  100 Marine Parkway, Suite 500,
          Redwood City, CA 94065-1031.

          [SCHNEIER] Schneier, B. "Applied Cryptography", Second
          Edition, http://www.counterpane.com.  Published by John Wiley
          & Sons, Inc.  ISBN 0-471-11709-9, 1996.

          [SSH] Ylonen, T., "SSH Transport Layer Protocol",
          ftp://ietf.org/internet-drafts/draft-ietf-secsh-transport-00.txt,
          March, 1997.

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          [SSH ARCFOUR] Kaukonen, K. Long test vectors for Arcfour and
          RC4 algorithms issued by Kalle Kaukonen, SSH Communications
          Security, Ltd, July, 1997.

          [TLS] Freier, A., Karlton, P., Kocher, P., Dierks, T., " The
          TLS Protocol", ftp://ds.internic.net/internet-drafts/
          draft-ietf-tls-protocol-00.txt, December, 1996.

8. Authors' Addresses

          Kalle Kaukonen
          SSH Communications Security
          Tekniikantie 12
          02150 Espoo
          Finland
          kalle@ssh.fi
          +358 40 526 0364
          Fax +358 9 4354 3206

          Rodney Thayer
          Sable Technology Corporation
          246 Walnut Street
          Newton Massachusetts 02160
          rodney@sabletech.com
          +1 617 332 7292
          Fax +1 617 332 7970




Appendix

A. Test Vectors

          1. Test Vectors from [CRYPTLIB]:
               Plain Text:
                    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
               Key:
                    0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF

               Cipher Text:
                    0x74, 0x94, 0xC2, 0xE7, 0x10, 0x4B, 0x08, 0x79

          2. Test Vectors from [COMMERCE]:
               Plain Text:
                    0xdc, 0xee, 0x4c, 0xf9, 0x2c
               Key:
                    0x61, 0x8a, 0x63, 0xd2, 0xfb
               Cipher Text:
                    0xf1, 0x38, 0x29, 0xc9, 0xde


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Internet Draft        An Encryption Algorithm                  July 1997


          3. Test Vectors from [SSH ARCFOUR]:
               Plain Text:
                    0x52, 0x75, 0x69, 0x73, 0x6c, 0x69, 0x6e, 0x6e,
                    0x75, 0x6e, 0x20, 0x6c, 0x61, 0x75, 0x6c, 0x75,
                    0x20, 0x6b, 0x6f, 0x72, 0x76, 0x69, 0x73, 0x73,
                    0x73, 0x61, 0x6e, 0x69, 0x2c, 0x20, 0x74, 0xe4,
                    0x68, 0x6b, 0xe4, 0x70, 0xe4, 0x69, 0x64, 0x65,
                    0x6e, 0x20, 0x70, 0xe4, 0xe4, 0x6c, 0x6c, 0xe4,
                    0x20, 0x74, 0xe4, 0x79, 0x73, 0x69, 0x6b, 0x75,
                    0x75, 0x2e, 0x20, 0x4b, 0x65, 0x73, 0xe4, 0x79,
                    0xf6, 0x6e, 0x20, 0x6f, 0x6e, 0x20, 0x6f, 0x6e,
                    0x6e, 0x69, 0x20, 0x6f, 0x6d, 0x61, 0x6e, 0x61,
                    0x6e, 0x69, 0x2c, 0x20, 0x6b, 0x61, 0x73, 0x6b,
                    0x69, 0x73, 0x61, 0x76, 0x75, 0x75, 0x6e, 0x20,
                    0x6c, 0x61, 0x61, 0x6b, 0x73, 0x6f, 0x74, 0x20,
                    0x76, 0x65, 0x72, 0x68, 0x6f, 0x75, 0x75, 0x2e,
                    0x20, 0x45, 0x6e, 0x20, 0x6d, 0x61, 0x20, 0x69,
                    0x6c, 0x6f, 0x69, 0x74, 0x73, 0x65, 0x2c, 0x20,
                    0x73, 0x75, 0x72, 0x65, 0x20, 0x68, 0x75, 0x6f,
                    0x6b, 0x61, 0x61, 0x2c, 0x20, 0x6d, 0x75, 0x74,
                    0x74, 0x61, 0x20, 0x6d, 0x65, 0x74, 0x73, 0xe4,
                    0x6e, 0x20, 0x74, 0x75, 0x6d, 0x6d, 0x75, 0x75,
                    0x73, 0x20, 0x6d, 0x75, 0x6c, 0x6c, 0x65, 0x20,
                    0x74, 0x75, 0x6f, 0x6b, 0x61, 0x61, 0x2e, 0x20,
                    0x50, 0x75, 0x75, 0x6e, 0x74, 0x6f, 0x20, 0x70,
                    0x69, 0x6c, 0x76, 0x65, 0x6e, 0x2c, 0x20, 0x6d,
                    0x69, 0x20, 0x68, 0x75, 0x6b, 0x6b, 0x75, 0x75,
                    0x2c, 0x20, 0x73, 0x69, 0x69, 0x6e, 0x74, 0x6f,
                    0x20, 0x76, 0x61, 0x72, 0x61, 0x6e, 0x20, 0x74,
                    0x75, 0x75, 0x6c, 0x69, 0x73, 0x65, 0x6e, 0x2c,
                    0x20, 0x6d, 0x69, 0x20, 0x6e, 0x75, 0x6b, 0x6b,
                    0x75, 0x75, 0x2e, 0x20, 0x54, 0x75, 0x6f, 0x6b,
                    0x73, 0x75, 0x74, 0x20, 0x76, 0x61, 0x6e, 0x61,
                    0x6d, 0x6f, 0x6e, 0x20, 0x6a, 0x61, 0x20, 0x76,
                    0x61, 0x72, 0x6a, 0x6f, 0x74, 0x20, 0x76, 0x65,
                    0x65, 0x6e, 0x2c, 0x20, 0x6e, 0x69, 0x69, 0x73,
                    0x74, 0xe4, 0x20, 0x73, 0x79, 0x64, 0xe4, 0x6d,
                    0x65, 0x6e, 0x69, 0x20, 0x6c, 0x61, 0x75, 0x6c,
                    0x75, 0x6e, 0x20, 0x74, 0x65, 0x65, 0x6e, 0x2e,
                    0x20, 0x2d, 0x20, 0x45, 0x69, 0x6e, 0x6f, 0x20,
                    0x4c, 0x65, 0x69, 0x6e, 0x6f

              Key:
                    0x29, 0x04, 0x19, 0x72, 0xfb, 0x42, 0xba, 0x5f,
                    0xc7, 0x12, 0x77, 0x12, 0xf1, 0x38, 0x29, 0xc9







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              Cipher Text:
                    0x35, 0x81, 0x86, 0x99, 0x90, 0x01, 0xe6, 0xb5,
                    0xda, 0xf0, 0x5e, 0xce, 0xeb, 0x7e, 0xee, 0x21,
                    0xe0, 0x68, 0x9c, 0x1f, 0x00, 0xee, 0xa8, 0x1f,
                    0x7d, 0xd2, 0xca, 0xae, 0xe1, 0xd2, 0x76, 0x3e,
                    0x68, 0xaf, 0x0e, 0xad, 0x33, 0xd6, 0x6c, 0x26,
                    0x8b, 0xc9, 0x46, 0xc4, 0x84, 0xfb, 0xe9, 0x4c,
                    0x5f, 0x5e, 0x0b, 0x86, 0xa5, 0x92, 0x79, 0xe4,
                    0xf8, 0x24, 0xe7, 0xa6, 0x40, 0xbd, 0x22, 0x32,
                    0x10, 0xb0, 0xa6, 0x11, 0x60, 0xb7, 0xbc, 0xe9,
                    0x86, 0xea, 0x65, 0x68, 0x80, 0x03, 0x59, 0x6b,
                    0x63, 0x0a, 0x6b, 0x90, 0xf8, 0xe0, 0xca, 0xf6,
                    0x91, 0x2a, 0x98, 0xeb, 0x87, 0x21, 0x76, 0xe8,
                    0x3c, 0x20, 0x2c, 0xaa, 0x64, 0x16, 0x6d, 0x2c,
                    0xce, 0x57, 0xff, 0x1b, 0xca, 0x57, 0xb2, 0x13,
                    0xf0, 0xed, 0x1a, 0xa7, 0x2f, 0xb8, 0xea, 0x52,
                    0xb0, 0xbe, 0x01, 0xcd, 0x1e, 0x41, 0x28, 0x67,
                    0x72, 0x0b, 0x32, 0x6e, 0xb3, 0x89, 0xd0, 0x11,
                    0xbd, 0x70, 0xd8, 0xaf, 0x03, 0x5f, 0xb0, 0xd8,
                    0x58, 0x9d, 0xbc, 0xe3, 0xc6, 0x66, 0xf5, 0xea,
                    0x8d, 0x4c, 0x79, 0x54, 0xc5, 0x0c, 0x3f, 0x34,
                    0x0b, 0x04, 0x67, 0xf8, 0x1b, 0x42, 0x59, 0x61,
                    0xc1, 0x18, 0x43, 0x07, 0x4d, 0xf6, 0x20, 0xf2,
                    0x08, 0x40, 0x4b, 0x39, 0x4c, 0xf9, 0xd3, 0x7f,
                    0xf5, 0x4b, 0x5f, 0x1a, 0xd8, 0xf6, 0xea, 0x7d,
                    0xa3, 0xc5, 0x61, 0xdf, 0xa7, 0x28, 0x1f, 0x96,
                    0x44, 0x63, 0xd2, 0xcc, 0x35, 0xa4, 0xd1, 0xb0,
                    0x34, 0x90, 0xde, 0xc5, 0x1b, 0x07, 0x11, 0xfb,
                    0xd6, 0xf5, 0x5f, 0x79, 0x23, 0x4d, 0x5b, 0x7c,
                    0x76, 0x66, 0x22, 0xa6, 0x6d, 0xe9, 0x2b, 0xe9,
                    0x96, 0x46, 0x1d, 0x5e, 0x4d, 0xc8, 0x78, 0xef,
                    0x9b, 0xca, 0x03, 0x05, 0x21, 0xe8, 0x35, 0x1e,
                    0x4b, 0xae, 0xd2, 0xfd, 0x04, 0xf9, 0x46, 0x73,
                    0x68, 0xc4, 0xad, 0x6a, 0xc1, 0x86, 0xd0, 0x82,
                    0x45, 0xb2, 0x63, 0xa2, 0x66, 0x6d, 0x1f, 0x6c,
                    0x54, 0x20, 0xf1, 0x59, 0x9d, 0xfd, 0x9f, 0x43,
                    0x89, 0x21, 0xc2, 0xf5, 0xa4, 0x63, 0x93, 0x8c,
                    0xe0, 0x98, 0x22, 0x65, 0xee, 0xf7, 0x01, 0x79,
                    0xbc, 0x55, 0x3f, 0x33, 0x9e, 0xb1, 0xa4, 0xc1,
                    0xaf, 0x5f, 0x6a, 0x54, 0x7f












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B. Sample Code


     /*        This code illustrates a sample implementation
     *                 of the Arcfour algorithm
     *         Copyright (c) April 29, 1997 Kalle Kaukonen.
     *                    All Rights Reserved.
     *
     * Redistribution and use in source and binary forms, with or
     * without modification, are permitted provided that this copyright
     * notice and disclaimer are retained.
     *
     * THIS SOFTWARE IS PROVIDED BY KALLE KAUKONEN AND CONTRIBUTORS ``AS
     * IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
     * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
     * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL KALLE
     * KAUKONEN OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
     * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
     * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
     * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
     * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
     * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
     * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     */


     typedef struct
     {
        unsigned int x;
        unsigned int y;
        unsigned char state[256];
     } ArcfourContext;

     void arcfour_init(ArcfourContext *ctx, const unsigned char *key,
                   unsigned int key_len);
     unsigned int arcfour_byte(ArcfourContext *ctx);
     void arcfour_encrypt(ArcfourContext *ctx, unsigned char *dest,
                      const unsigned char *src, unsigned int len);



     int main(int argc, char **argv)
     {
       unsigned char dest[500];
       unsigned char mykey[] = {0x29, 0x04, 0x19, 0x72, 0xfb, 0x42,
                                0xba, 0x5f, 0xc7, 0x12, 0x77, 0x12,
                                0xf1, 0x38, 0x29, 0xc9};
       unsigned char src[]   = "Know thyself";

       ArcfourContext mycontext;

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       /* Initialize the algoritm */
       arcfour_init(&mycontext, mykey, 16);

       /* Encrypt 13 bytes of the src string */
       arcfour_encrypt(&mycontext, dest, src, 13);

       /* Now "dest" contains the encrypted string. Do whatever
          you please with it... */

       return 0;
     }



     void arcfour_init(ArcfourContext *ctx, const unsigned char *key,
                   unsigned int key_len)
     {
       unsigned int t, u;
       unsigned int keyindex;
       unsigned int stateindex;
       unsigned char *state;
       unsigned int counter;

       state = ctx->state;
       ctx->x = 0;
       ctx->y = 0;
       for (counter = 0; counter < 256; counter++)
         state[counter] = counter;
       keyindex = 0;
       stateindex = 0;
       for (counter = 0; counter < 256; counter++)
         {
           t = state[counter];
           stateindex = (stateindex + key[keyindex] + t) & 0xff;
           u = state[stateindex];
           state[stateindex] = t;
           state[counter] = u;
           if (++keyindex >= key_len)
             keyindex = 0;
         }
     }











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     unsigned int arcfour_byte(ArcfourContext *ctx)
     {
       unsigned int x;
       unsigned int y;
       unsigned int sx, sy;
       unsigned char *state;

       state = ctx->state;
       x = (ctx->x + 1) & 0xff;
       sx = state[x];
       y = (sx + ctx->y) & 0xff;
       sy = state[y];
       ctx->x = x;
       ctx->y = y;
       state[y] = sx;
       state[x] = sy;
       return state[(sx + sy) & 0xff];
     }


     void arcfour_encrypt(ArcfourContext *ctx, unsigned char *dest,
                      const unsigned char *src, unsigned int len)
     {
       unsigned int i;
       for (i = 0; i < len; i++)
         dest[i] = src[i] ^ arcfour_byte(ctx);
     }

























Kaukonen, Thayer                                               [Page 11]


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