COEN 351 Non-Repudiation

A non-repudiation service provides assurance of the origin or delivery of data in order to protect the sender against false denial by the recipient that the data has been received, or to protect the recipient against false denial by the sender that the data has been sent. Thus, a non- repudiation service provides evidence to prevent a person from unilaterally modifying or terminating legal obligations arising out of a transaction effected by computer-based means. American Bar Association

Non-Repudiation Legal Need: Enough evidence to meet the legal requirement for proof that signature / communication occured. Not exactly equivalent to the cryptography standard.

Non-Repudiation Traditional written signature delivered by mail Can be forged. But forging is difficult. Name under Easily forged. Digitally signed Almost impossible to forge Attack needs key theft Or needs successful crypto-attack on verified and trusted scheme.

Non-Repudiation Types of non-repudiation: Non-repudiation of origin Non-repudiation of submission Non-repudiation of delivery

Non-Repudiation Non-repudiation request Parties need to agree on non-repudiation services. Non-repudiation service involves 5 different activities: Non-repudiation request Record generation Record distribution Record verification Record retention

Non-Repudiation Phases Non-repudiation request Often implicit. Request for attestation (additional signatures) Record generation Needs to involve the potential repudiator Autonomous generation or by trusted third party Record distribution To trusted third party or to person requesting non- repudiation. Record verification Record retention

Non-Repudiation of Origin: Mechanisms Originator’s Digital Signature with Certificate Message Digital Signature: Hash of Message encrypted with private key of originator. Certificate of Digital Signature: Certificate by trusted party containing public key of originator. Key-Revocation Possibility: Time of key revocation is crucial, message needs to contain a time stamp. Stored at recipient.

Non-Repudiation of Origin: Mechanisms Digital Signature of a Trusted Third Party Originator sends message to trusted third party and authenticates her/him-self. Trusted third party digitally signs the message and returns it to the originator. Originator sends it to recipient. Recipient stores it.

Non-Repudiation of Origin: Mechanisms Digital Signature of a Trusted Third Party Originator sends message to trusted third party and authenticates her/him-self. Trusted third party digitally signs the message and returns it to the originator. Originator sends it to recipient. Recipient stores it.

Non-Repudiation of Origin: Mechanisms Digital Signature of Message Hash Digital signatures are compute-expensive. Calculate a secure hash of the message. Hash is easy / fast to calculate Impossible to find other message with the same hash. WARNING: Analysis of secure hash functions is about a decade behind analysis of encryption security. MD5, SHA1 are not yet broken But less secure than thought SHA-2 family seems to be O.K. (2006) Then use the various schemes to sign the hash.

Non-Repudiation of Origin Transactional Certificate Only used for one transaction. Originator provides: Message Hash of message, encrypted by private key of originator. Signature of trusted party on originator signature

Non-Repudiation of Origin Inline trusted third party (Evidence stored) Originator sends message to trusted third party. Trusted third party stores evidence of transaction (signed message digest, time stamp) Trusted third party forwards message.

Non-Repudiation of Delivery Recipient Acknowledgment with Signature Recipient signs digest of received message and sends it back to the sender. “Reluctant recipient problem”

Non-Repudiation of Delivery Trusted Delivery Agent Akin to process server Police officer, deputy delivering summons or subpoena. Delivery agent is trusted when attesting to handing message to recipient Delivery agent signs digest of message and returns it to sender after handing it to the receiver.

Non-Repudiation of Delivery Progressive Delivery Reports Mail transfer protocol hands messages from one mail server to the next. Possible to send reports from each mail server. header has a record of those hand-offs Unfortunately, these might be faked, too.

Protocols: SMTP Server used to fak To: From: This is a spoofed message. From Tue Dec 23 17:25: Return-Path: Received: from Xavier (dhcp engr.scu.edu [ ]) by server4.engr.scu.edu ( / ) with ESMTP id hBO1Plpv for ; Tue, 23 Dec :25: Received: from mail pickup service by Xavier with Microsoft SMTPSVC; Tue, 23 Dec :25: To: From: Message-ID: X-OriginalArrivalTime: 24 Dec :25: (UTC) FILETIME=[D3B56160:01C3C9 BC] Date: 23 Dec :25: X-Spam-Checker-Version: SpamAssassin 2.60-rc3 ( exp) on server4.engr.scu.edu X-Spam-Level: X-Spam-Status: No, hits=0.3 required=5.0 tests=NO_REAL_NAME autolearn=no version=2.60-rc3 This is a spoofed message.

Non-Repudiation of Submission Messages are handled by a delivery system Not under control of sender Reasonably efficient in sending messages Delivery system can send receipt to sender.

Non-Repudiation Trusted Third Party Role Public-key certification Identity and authority validation By (co)signing Time stamping service Records retention Delivery intermediation Dispute resolution