Archive Time-Stamps-Syntax <draft-brandner-etal-ats-00.txt>

Archive Time-Stamps-Syntax<draft-brandner-etal-ats-00.txt> Dr. Ulrich Pordesch pordesch@sit.fhg.de

Archive Time Stamping Archiving Signed Documents • Long-term Problems • algorithms get weak, certificates expire • verification data no longer available • changes of formats and media • ArchiSig-Project 2001 - 2003 • requirements, concepts,implementation, evaluation • ATS one of the results

Long-term non-repudiation • Signatures: Proof of integrity and authenticity • Need to archive documents for 30 or more years • Signature and hashalgorithms / parameters can get weak, certificates expire or revoked • Long-term non repudiation prove of existence of signed documents and verification data needed • also very useful for unsigned documents

Requirements practical, effective, privacy protecting and law conformant: • timestamps with digital signatures needed • take weakness of pk- and hash-algorithms into account • minimalize quantity of time-stamps • avoid access to archived data as far as possible • independency of formats of documents or signatures • time-stamp groups of data objects together • no side effects of deletion of documents • optional encryption must preserve evidence value • use existing qualified time-stamp-services and protocols • no new trusted third parties

Electronic Signature Formats (RFC 3126) • Approach • adding (archive) time-stamps as unsigned attributes for each signature • Problems: • a great many time-stamps • need to access data and signatures(document format dependent) • not applicable to encrypted data,non standard signature formats,seperatly stored verification data,unsigned documents • not conformant to german signature law:new signature does not includeall previous signatures • Alternative (second) solution is needed

Approach client • Select data objects (document, ..) • Optional: Encrypt data objects service / archive system • Initial Archive Time-Stamp • Renewal: Time-Stamp Renewal, Hashtree Renewal • Reduce hashtrees, generate Archive Timestamps Element client • Optional: Decrypt data objects • Optional: Integrate as an attribute if wanted • Verify Archive Time-Stamps Element anddocument

Archive Time Stamp Syntax • Syntax and Processing (particularly verification)of an Archive Time Stamp Element • to verify existance of any data objects over an undetermined period of time, useable for signature renewal • optimized (but not restricted to) centralized Archive Time Stamping by Archive Systems or Services • including optional encryption • addendum: integration into signed documents • Not specified here: • Service protocol: possible but not necessary for internal use • Architectures of archive systems

Archive Time-Stamp • Archive Time Stamp • hash-tree (Merkle) • qualif. time-stamp containing digital signature • single time-stamp for many data objects • Initial Stamp • event: after document is archived • collect hash values and build tree,request time-stamp • store archive time-stamp • renew if necesary • Reduction to Enhanced Time-Stamp • necessary hash values for verification +time-stamp • {SEQUENCE of SEQUENCE of OCTET STRINGtime-stamp}

Time-Stamp-Renewal • Event: Any algorithm in time-stamp gets weak(or time-stamp-certificate expires) • Method • hash time-stamp with old hash algorithm • and include it in new archive time-stamp • Properties • no access to data objects • only few (at minimum 1) time-stamp for an whole archive • Reduction: ArchiveTimeStampChain • SEQUENCE of Enhanced Time Stamp

Hashtree-Renewal • Event: Hash-Algorithm of chain gets weak • Method (for each data object) • build Archive Time-Stamp chain • include hash of (hash of chain + hash of data object) in new Archive Time-Stamp • Properties • need to access data objects • but: avoidable by easy to implement redundancy • Reduction: ArchiveTimeStampSequence • SEQUENCE of ArchiveTimeStampChain

Optional Encryption • Caution: Encryption must be unambigious! • Method: • encrypt data object before archiving using CMS-Encryption (Algorithms: RSA, DES-CBC) • normal archive time-stamping • decrypt encrypted data object when got bak • add CMS-cover to Archive-Time-Stamp-Element store content seperately • verification: reconstruct of archive time-stamped data object by decryption of content-encryption key, reencrypt content, insert content

Appendices • Optional Integration • CMS: signed data • Archive Time-Stamps-Element as an unsigned signature attribute for signature • Optional use of Enhanced Time-Stamp • CMS: signed data • Attribute for multiple signature verification

Implementation and Evaluation • Implementation • Patient documentation system • Archivesystem + Archive Time Stamping • Verification tool • Evaluation • Test within hospital • simulation study with lawyers, judges, technical experts

Summary • Syntax + Processing of Archive Time-Stamp Element • optimized for centralized time-stamping • effective for large document volumes • applicable for any data objects and groups of data objects • normally no need to access data • redundancy easy to realize • on base of existing services

Archive Time-Stamps-Syntax <draft-brandner-etal-ats-00.txt>