Key exchange

Summary

Key exchange (also key establishment) is a method in cryptography by which cryptographic keys are exchanged between two parties, allowing use of a cryptographic algorithm. If the sender and receiver wish to exchange encrypted messages, each must be equipped to encrypt messages to be sent and decrypt messages received. The nature of the equipping they require depends on the encryption technique they might use. If they use a code, both will require a copy of the same codebook. If they use a cipher, they will need appropriate keys. If the cipher is a symmetric key cipher, both will need a copy of the same key. If it is an asymmetric key cipher with the public/private key property, both will need the other's public key. Channel of exchange Key exchange is done either in-band or out-of-band. The key exchange problem The key exchange problem describes ways to exchange whatever keys or other information are needed for establishing a secure communication channel so that n

Official source

https://en.wikipedia.org/wiki/Key_exchange

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Cryptanalysis of e-mail protocols providing perfect forward secrecy

Recently, two e-mail protocols were proposed claiming to provide perfect secrecy. These protocols use authentication and (Diffie-Hellman) key-exchange techniques, and as such, other standard security criteria besides perfect forward secrecy include key-replay resilience, known-key security, key freshness and unknown key-share resilience are expected too. In this paper, we show that the two protocols cannot resist replay attacks, and further that the first falls to unknown key-share attacks while the second fails to provide perfect forward secrecy, contrary to the designers' claims. Although the two protocols were intended by the designers to be more secure variants compared to the common e-mail protocol, our results show that being newer does not necessarily mean being more secure. (C) 2007 Elsevier B.V. All rights reserved.

2008

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To mitigate state exposure threats to long-lived instant messaging sessions, ratcheting was introduced, which is used in practice in protocols like Signal. However, existing ratcheting protocols generally come with a high cost. Recently, Caforio et al. proposed pragmatic constructions, which compose a weakly secure 'light' protocol and a strongly secure 'heavy' protocol, in order to achieve so-called ratcheting on-demand. The light protocol they proposed has still a high complexity. In this paper, we propose the lightest possible protocol we could imagine, which essentially encrypts and then hashes the secret key. We prove it secure in the standard model by introducing a new security notion, which relates symmetric encryption with key updates by hashing. Our protocol composes well with the generic transformation techniques by Caforio et al. to offer high security and performance at the same time. In a second step, we propose another protocol based on a newly defined integrated primitive, extending standard one-time authenticated encryption with an additional output block used as a secret key for the next message. We instantiate this primitive firstly from any authenticated encryption with associated data, and then we propose an efficient instantiation using advanced encryption standard (AES) encryption to update the key and AES-Galois/Counter mode of operation to encrypt and decrypt messages.

OXFORD UNIV PRESS2022

Secure Communication in Distributed Ada

Jörg Kienzle

This document describes an implementation of secure communication for distributed applications in Ada 95. It explains the basics of cryptography and introduces the problem of key exchange. After a brief description of distributed systems in Ada 95, the modular structure of the implementation and the way it handles key distribution is presented.

Springer Verlag1996