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OTR Mode of Operation for Authenticated Encryption

November 28, 2019

Introduction

The key idea of OTR
The key idea of OTR

Authenticated encryption (AE) is a symmetric-key cryptographic function achieving both confidentiality and integrity. OTR, which stands for Offset Two-Round, is an AE based on a blockcipher presented at Eurocrypt 2014.
OTR is parallelizable, rate-1, and on-line for both encryption and decryption, and requires only the blockcipher encryption function. OTR thus essentially achieves the minimum additional cost from the ordinal encryption-only modes, such as counter mode. The key idea of OTR is the use of two-round Feistel permutation with blockcipher-based round function.

OTR with AES blockcipher, called AES-OTR, is a candidate of new window CAESAR, a cryptographic competition on AE organized by leading professionals in the field and funded by National Institute of Standards and Technology, NIST.

Designer

Specifications

In principle OTR can be defined with n-bit blockcipher, or more generally any n-bit keyed (non-invertible) function, for any n.
AES-OTR specifies the case n=128 with the underlying blockcipher as AES-128 or AES-256.

Specification document of AES-OTR is available from new window the submission list of CAESAR.

Features

  • Encryption rate is 1 : one blockcipher call to process one-block input
  • On-line and parallelizable for both encryption and decryption
  • Uses blockcipher encryption function, no need for the inverse
  • Standard nonce-based provable security

Implementation Figures

(To be presented)

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