OCB For Block Ciphers Without 128-Bit Blocks
draft-krovetz-ocb-wideblock-00

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Internet Engineering Task Force                               T. Krovetz
Internet-Draft                                          Sacramento State
Updates: 7523 (if approved)                                 May 11, 2018
Intended status: Informational
Expires: November 12, 2018

              OCB For Block Ciphers Without 128-Bit Blocks
                     draft-krovetz-ocb-wideblock-00

Abstract

   The OCB authenticated-encryption algorithm is specified in RFC 7523,
   but only for blockciphers with 128-bit blocks such as AES.  This
   document extends RFC 7523 by specifying how OCB is used with
   blockciphers of any blocklength.  When the blocklength is 128 bits,
   this specification and that in RFC 7523 are identical.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 12, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (https://trustee.ietf.org/license-info) in effect on the date of
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   include Simplified BSD License text as described in Section 4.e of

Krovetz                 Expires November 12, 2018               [Page 1]
Internet-Draft      OCB for non-128-bit cipher blocks           May 2018

   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Notation and Basic Operations . . . . . . . . . . . . . . . .   3
   3.  OCB Global Parameters . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Constants Derived From BLOCKLEN . . . . . . . . . . . . .   5
   4.  OCB Algorithms  . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Associated-Data Processing: HASH  . . . . . . . . . . . .   5
     4.2.  Encryption: OCB-ENCRYPT . . . . . . . . . . . . . . . . .   8
     4.3.  Decryption: OCB-DECRYPT . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
     5.1.  Nonce Requirements  . . . . . . . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Sample Results . . . . . . . . . . . . . . . . . . .  14
     A.1.  Example 1: 64-bit Blockcipher, 0-byte Input . . . . . . .  15
     A.2.  Example 2: 64-bit Blockcipher, 4-byte Input . . . . . . .  15
     A.3.  Example 3: 64-bit Blockcipher, 8-byte Input . . . . . . .  16
     A.4.  Example 4: 64-bit Blockcipher, 20-byte Input  . . . . . .  17
     A.5.  Example 5: 256-bit Blockcipher, 80-byte Input . . . . . .  19
     A.6.  Example 6: Extended Tests . . . . . . . . . . . . . . . .  21
   Appendix B.  Generating RESIDUE, SHIFT, MASKLEN, TAGREP Constants  22
     B.1.  Sage Script For Generating MASKLEN and SHIFT  . . . . . .  23
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   OCB is a shared-key, blockcipher-based authentication scheme
   specified in [RFC7523].  It was designed with the AES blockcipher in
   mind and thus envisioned only being used with 128-bit blockciphers.
   The resulting RFC 7523 does not allow blockciphers with larger or
   smaller blocklengths.  This document respecifies OCB in a more
   general manner, eliminating the expectation that a 128-bit
   blockcipher is used.  This update is consistent with RFC 7523 and
   does not contradict it in any way.  For applications using 128-bit
   blockciphers, RFC 7523 should be preferred because it is simpler,
   self-contained, and has more applicable test vectors.

   Changing the blocklength used in OCB is not a simple matter.  There
   are non-trivially defined constants used in OCB that must be
   recalculated for each different blocklength.  What follows is largely
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