| 167 | 169 | Versioning The most basic security-related feature of openEHR is its support for data integrity. This is mainly provided by the versioning model, specified in the change_control package in the Common Information Model, and in the Extract Information Model. Change-set based versioning of all information in the EHR and demographic services constitutes a basic integrity measure for information, since no content is ever physically modified, only new versions are created. All logical changes and deletions as well as additions are therefore physically implemented as new Versions rather than changes to existing information items. Clearly the integrity of the information will depend on the quality of the implementation; however, the simplest possible implementations (1 Version = 1 copy) can provide very good safety due to being write-once systems. The use of change-sets, known as Contributions in openEHR, provides a further unit of integrity corresponding to all items modified, created or deleted in a single unit of work by a user. The openEHR versioning model defines audit records for all changed items, which can be basic audits and/or any number of additional digitally signed attestations (e.g. by senior staff). This means that every write access of any kind to any part of an openEHR record is logged with the user identification, time, reason, and potentially other meta-data. Versioning is described in detail in section 8 on page 45. Digital Signature The possibility exists within an openEHR EHR to digitally sign each Version in a Versioned object (i.e. for each Version of any logical item, such as medications list, encounter note etc.). The signature is created as a private-key encryption (e.g. RSA-1) of a hash (e.g. MD5) of a canonical representation (such as in schema-based XML) of the Version being committed. A likely candidate for defining the signature and digest strings in openEHR is the openPGP message format (IETF RFC24402), due to being an open specification and self-describing. The use of RFC2440 for the format does not imply the use of the PGP distributed certificate infrastructure, or indeed any certification infrastructure; openEHR is agnostic on this point. If no public key or equivalent infrastructure is available, the encryption step might be omitted, resulting in a digest only of the content. The signature is stored within the Version object, allowing it to be conveniently carried within EHR Extracts. The process is shown in FIGURE 22. |
