# Abstract This document defines requirements for a baseline time-stamp policy for Time-Stamping Authorities (TSAs) issuing time-stamp tokens, supported by public key certificates, with an accuracy of one second or better. A TSA may define its own policy which enhances the policy defined in this document. Such a policy shall incorporate or further constrain the requirements identified in this document. # 1. Introduction The content of this RFC is based on [RFC 3628](https://datatracker.ietf.org/doc/html/rfc3628). The primary changes in this RFC are the removal of legal requirements, requirements to comply with the EU Directive, and requirements that specify the TSA will use on-premise hardware to manage the TSA primary key and service. In creating reliable and manageable digital evidence it is necessary to have an agreed upon method of associating time data to transaction so that they might be compared to each other at a later time. The quality of this evidence is based on creating and managing the data structure that represent the events and the quality of the parametric data points that anchor them to the real world. In this instance this being the time data and how it was applied. A typical transaction is a digitally signed document, where it is necessary to prove that the digital signature from the signer was applied when the signer's certificate was valid. A timestamp or a time mark (which is an audit record kept in a secure audit trail from a trusted third party) applied to a digital signature value proves that the digital signature was created before the date included in the time-stamp or time mark. To prove the digital signature was generated while the signer's certificate was valid, the digital signature must be verified and the following conditions satisfied: 1. the time-stamp (or time mark) was applied before the end of the validity period of the signer's certificate, 1. the time-stamp (or time mark) was applied either while the signer's certificate was not revoked or before the revocation date of the certificate. Thus a time-stamp (or time mark) applied in this manner proves that the digital signature was created while the signer's certificate was valid. This concept proves the validity of a digital signature over the whole of any certificate chain. Policy requirements to cover that case is the primary reason of this document. However, it should be observed that these policy requirements can be used to address other needs. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [BCP 14](https://datatracker.ietf.org/doc/html/bcp14), [RFC 2119](https://datatracker.ietf.org/doc/html/rfc2119). # 2. Overview These policy requirements are aimed at time-stamping services used in support of qualified electronic signatures but may be applied to any application requiring to prove that a datum existed before a particular time. These policy requirements are based on the use of public key cryptography, public key certificates and reliable time sources. The present document may be used by independent bodies as the basis for confirming that a TSA may be trusted for providing time-stamping services. This document addresses requirements for synchronizing TSAs issuing time-stamp tokens with Coordinated universal time (UTC) and digitally signed by TSUs. Subscriber and relying parties should consult the TSA's practice statement to obtain further details of precisely how this time-stamp policy is implemented by the particular TSA (e.g., protocols used in providing this service). This document does not specify: - protocols used to access the TSUs; NOTE 1: A time-stamping protocol is defined in [RFC 3161](https://datatracker.ietf.org/doc/html/rfc3161) and profiled in [TS 101 861](https://www.etsi.org/deliver/etsi_ts/101800_101899/101861/01.04.01_60/ts_101861v010401p.pdf). - how the requirements identified herein may be assessed by an independent body; - requirements for information to be made available to such independent bodies; - requirements on such independent bodies. # 3. Definitions and Abbreviations ## 3.1. Definitions For the purposes of the present document, the following terms and definitions apply: - relying party: recipient of a time-stamp token who relies on that time-stamp token. - subscriber: entity requiring the services provided by a TSA and which has explicitly or implicitly agreed to its terms and conditions. - time-stamp token: data object that binds a representation of a datum to a particular time, thus establishing evidence that the datum existed before that time. - time-stamping authority: authority which issues time-stamp tokens. - TSA Disclosure statement: set of statements about the policies and practices of a TSA that particularly require emphasis or disclosure to subscribers and relying parties, for example to meet regulatory requirements. - TSA practice statement: statement of the practices that a TSA employs in issuing time-stamp tokens. - TSA system: composition of IT products and components organized to support the provision of time-stamping services. - time-stamp policy: named set of rules that indicates the applicability of a time-stamp token to a particular community and/or class of application with common security requirements. - time-stamping unit: set of hardware and software which is managed as a unit and has a single time-stamp token signing key active at a time. - Coordinated Universal Time (UTC): Time scale based on the second as defined in ITU-R Recommendation TF.460-5. NOTE: For most practical purposes UTC is equivalent to mean solar time at the prime meridian. More specifically, UTC is a compromise between the highly stable atomic time (Temps Atomique International - TAI) and solar time derived from the irregular Earth rotation (related to the Greenwich mean sidereal time (GMST) by a conventional relationship). - UTC(k): Time-scale realized by the laboratory "k" and kept in close agreement with UTC, with the goal to reach plus or minus 100 ns. (See ITU-R Recommendation TF.536-1. NOTE: A list of UTC(k) laboratories is given in section 1 of Circular T disseminated by BIPM and available from the BIPM website (http://www.bipm.org/). ## 3.2. Abbreviations For the purposes of the present document, the following abbreviations apply: - TSA Time-Stamping Authority - TSU Time-Stamping Unit - TST Time-Stamp Token - UTC Coordinated Universal Time # 4. General Concepts ## 4.1. Time-Stamping Services The provision of time-stamping services is broken down into the following component services for the purposes of classifying requirements: - Time-stamping provision: This service component generates time-stamp tokens. - Time-stamping management: The service component that monitors and controls the operation of the time-stamping services to ensure that the service is provided as specified by the TSA. This service component is responsible for the installation and de-installation of the time-stamping provision service. For example, time-stamping management ensures that the clock used for time-stamping is correctly synchronized with UTC. This subdivision of services is only for the purposes of clarifying the requirements specified in the current document and places no restrictions on any subdivision of an implementation of time-stamping services. ## 4.2. Time-Stamping Authority The authority to issue time-stamp tokens, trusted by the users of the time-stamping services, i.e., subscribers and relying parties, is called the Time-Stamping Authority (TSA). TSA has overall responsibility for time-stamping services identified in clause 4.1. The TSA has responsibility for the operation of one or more TSU's which creates and signs on behalf of the TSA. The TSA responsible for issuing a time-stamp token is identifiable (see 7.3.1 h). The TSA may delegate the signing of timestamps to a cloud key management service. However, the TSA always maintains overall responsibility and ensures that the policy requirements identified in the present document are met. A TSA may operate several identifiable time-stamping units. Each unit has a different key. A TSA is a certification-service-provider which issues time-stamp tokens. ## 4.3.Subscriber The subscriber may be an organization comprising several end-users or an individual end-user. When the subscriber is an organization, some of the obligations that apply to that organization will have to apply as well to the end-users. In any case the organization will be held responsible if the obligations from the end-users are not correctly fulfilled and therefore the organization is expected to suitably inform its end users. When the subscriber is an end-user, the end-user will be held directly responsible if its obligations are not correctly fulfilled. ## 4.4. Time-Stamp Policy and TSA Practice Statement This section explains the relative roles of Time-stamp policy and TSA practice statement. It places no restriction on the form of a time- stamp policy or practice statement specification. ### 4.4.1. Purpose In general, the time-stamp policy states "what is to be adhered to," while a TSA practice statement states "how it is adhered to", i.e., the processes it will use in creating time-stamps and maintaining the accuracy of its clock. The relationship between the time-stamp policy and TSA practice statement is similar in nature to the relationship of other business policies which state the requirements of the business, while operational units define the practices and procedures of how these policies are to be carried out. The present document specifies a time-stamp policy to meet general requirements for trusted time-stamping services. TSAs specify in TSA practice statements how these requirements are met. ### 4.4.2. Level of Specificity The TSA practice statement is more specific than a time-stamp policy. A TSA practice statement is a more detailed description of the terms and conditions as well as business and operational practices of a TSA in issuing and otherwise managing time-stamping services. The TSA practice statement of a TSA enforces the rules established by a time-stamp policy. A TSA practice statement defines how a specific TSA meets the technical, organizational and procedural requirements identified in a time-stamp policy. NOTE: Even lower-level internal documentation may be appropriate for a TSA detailing the specific procedures necessary to complete the practices identified in the TSA practice statement. ### 4.4.3. Approach The approach of a time-stamp policy is significantly different from a TSA practice statement. A time-stamp policy is defined independently of the specific details of the specific operating environment of a TSA, whereas a TSA practice statement is tailored to the organizational structure, operating procedures, facilities, and computing environment of a TSA. A time-stamp policy may be defined by the user of times-stamp services, whereas the TSA practice statement is always defined by the provider. # 5. Time-Stamp Policies ## 5.1. Overview A time-stamp policy is a "named set of rules that indicates the applicability of a time-stamp token to a particular community and/or class of application with common security requirements" (see clauses [3.1](#31-definitions) and [4.4](#44-time-stamp-policy-and-tsa-practice-statement)). The present document defines requirements for a baseline time-stamp policy for TSAs issuing time-stamp tokens, supported by public key certificates, with an accuracy of 1 second or better. NOTE 1: Without additional measures the relying party may not be able to ensure the validity of a time-stamp token beyond the end of the validity period of the supporting certificate. See Annex A on verification of the validity of a time-stamp token beyond the validity period of the TSU's certificate. A TSA may define its own policy which enhances the policy defined in this document. Such a policy shall incorporate or further constrain the requirements identified in this document. NOTE 1: It is required that a time-stamp token includes an identifier for the applicable policy (see section [7.3.1](#731-time-stamp-token)). ## 5.2. Identification The object-identifier X.208 of this time-stamp policy is `1.3.6.1.4.1.57264.2`. In the TSA disclosure statement made available to subscribers and relying parties, a TSA shall also include the identifier for the time-stamp policy to indicate its conformance. ## 5.3. User Community and Applicability This service aims to provide binary transparency. This policy may be used for public time-stamping services or time-stamping services used within a closed community. ## 5.4. Conformance The TSA shall use the identifier for the timestamp policy in timestamp tokens as given in section [5.2](#52-identification), or define its own time-stamp policy that incorporates or further constrains the requirements identified in the present document: - If the TSA claims conformance to the identified timestamp policy and makes available to subscribers and relying parties on request the evidence to support the claim of conformance; or - If the TSA has been assessed to conform to the identified timestamp policy by an independent party. A conformant TSA must demonstrate that: - It meets its obligations as defined in section [6.1](#61-tsa-obligation); - It has implemented controls which meet the requirements specified in section [7](#7-requirements-on-tsa-practices). # 6. Obligations and Liability ## 6.1. TSA Obligations ### 6.1.1. General The TSA shall ensure that all requirements on TSA, as detailed in section [7](#7-requirements-on-tsa-practices), are implemented as applicable to the selected trusted time-stamp policy. The TSA shall ensure conformance with the procedures prescribed in this policy, even when the TSA functionality is undertaken by subcontractors. The TSA shall also ensure adherence to any additional obligations indicated in the time-stamp either directly or incorporated by reference. The TSA shall provide all its time-stamping services consistent with its practice statement. ### 6.1.2. TSA Obligations Towards Subscribers The TSA shall meet its claims as given in its terms and conditions including the availability and accuracy of its service. ## 6.2. Subscriber Obligations The current document places no specific obligations on the subscriber beyond any TSA specific requirements stated in the TSA's terms and condition. NOTE: It is advisable that, when obtaining a time-stamp token, the subscriber verifies that the time-stamp token has been correctly signed and that the private key used to sign the time-stamp token has not been compromised. ## 6.3. Relying Party Obligations The terms and conditions made available to relying parties shall include an obligation on the relying party that, when relying on a time-stamp token, it shall: 1. verify that the time-stamp token has been correctly signed and that the private key used to sign the time-stamp has not been compromised until the time of the verification; NOTE: During the TSU's certificate validity period, the validity of the signing key can be checked using current revocation status for the TSU's certificate. If the time of verification exceeds the end of the validity period of the corresponding certificate, see annex A for guidance. 1. take into account any limitations on the usage of the time-stamp indicated by the time-stamp policy; 1. take into account any other precautions prescribed in agreements or elsewhere. # 7. Requirements on TSA Practices The TSA shall implement the controls that meet the following requirements. These policy requirements are not meant to imply any restrictions on charging for TSA services. The requirements are indicated in terms of the security objectives, followed by more specific requirements for controls to meet those objectives where it is necessary to provide confidence that those objective will be met. NOTE: The details of controls required to meet an objective is a balance between achieving the necessary confidence whilst minimizing the restrictions on the techniques that a TSA may employ in issuing time-stamp tokens. In the case of section [7.4](#74-tsa-management-and-operation) (TSA management and operation), a reference is made to a source of more detailed control requirements. Due to these factors the specificity of the requirements given under a given topic may vary. The provision of a time-stamp token in response to a request is at the discretion of the TSA depending on any service level agreements with the subscriber. ## 7.1. Practice and Disclosure Statements ### 7.1.1. TSA Practice Statement The TSA shall ensure that it demonstrates the reliability necessary for providing time-stamping services. In particular: 1. The TSA shall have a risk assessment carried out in order to evaluate business assets and threats to those assets in order to determine the necessary security controls and operational procedures. 1. The TSA shall have a statement of the practices and procedures used to address all the requirements identified in this time-stamp policy. - NOTE 1: This policy makes no requirement as to the structure of the TSA practice statement. 1. The TSA's practice statement shall identify the obligations of all external organizations supporting the TSA services including the applicable policies and practices. 1. The TSA may make available to subscribers and relying parties its practice statement, and other relevant documentation, as necessary, to assess conformance to the time-stamp policy. - NOTE 2: The TSA is not generally required to make all the details of its practices public. 1. Maintainers of the TSA shall have final authority for approving the TSA practice statement and ensuring that the practices are properly implemented. Maintainers shall also review any changes to the TSA to confirm that they follow the approved practice statement. 1. The TSA shall give due notice of changes it intends to make in its practice statement and shall, following approval as in (5) above, make the revised TSA practice statement immediately available as required under (4) above. ## 7.2. Key Management Life Cycle ### 7.2.1. TSA Key Generation The TSA shall ensure that any cryptographic keys are generated in under controlled circumstances. In particular: 1. The generation of the TSU's signing key(s) shall be undertaken by personnel in trusted roles. The personnel authorized to carry out this function shall be limited to those requiring to do so under the TSA's practices. 1. The generation of the TSU's signing key(s) shall be carried out in a secure environment. It MAY be carried out in a cloud based environment that protects the key. 1. The TSU key generation algorithm, the resulting signing key length and signature algorithm used for signing time-stamp tokens key shall be recognized by TSA maintainers as being fit for the purposes of time-stamp tokens as issued by the TSA. ### 7.2.2. TSU Private Key Protection The TSA shall ensure that TSU private keys remain confidential and maintain their integrity. The TSU private signing key shall be securely stored in one of the following: - HSM - Cloud environment - On-prem environment with controlled access ### 7.2.3. TSU Public Key Distribution The TSA shall ensure that the integrity and authenticity of the TSU signature verification (public) keys and any associated parameters are maintained during its distribution to relying parties. In particular: 1. TSU signature verification (public) keys shall be made available to relying parties in a public key certificate. NOTE: For example, TSU's certificates may be issued by a certification authority operated by the same organization as the TSA, or issued by another authority. 1. The TSU's signature verification (public) key certificate shall be issued by a certification authority operating under a certificate policy which provides a level of security equivalent to, or higher than, this time-stamping policy. ### 7.2.4. Rekeying TSU's Key The life-time of TSU's certificate shall be not longer than the period of time that the chosen algorithm and key length is recognized as being fit for purpose (see section [7.2.1c](#721-tsa-key-generation))). NOTE 1: The following additional considerations apply when limiting that lifetime: - Should a TSU private key be compromised, then the longer the life-time, the more affected time-stamp tokens there will be. NOTE 2: TSU key compromise does not only depend on the characteristics of the storage system being used but also on the procedures being used at system initialization and key export (when that function is supported). ### 7.2.5. End of TSU Key Life Cycle The TSA shall ensure that TSU private signing keys are not used beyond the end of their life cycle. In particular: 1. Operational or technical procedures shall be in place to ensure that a new key is put in place when a TSU's key expires. 1. The TSU private signing keys, or any key part, including any copies shall be destroyed such that the private keys cannot be retrieved. 1. The TST generation system SHALL reject any attempt to issue TSTs if the signing private key has expired. ### 7.2.6. Life Cycle Management of the Cryptographic Module used to Sign Time-Stamps The TSA shall use one of the following to host the token signing software: - HSM - Cloud environment - On-prem environment with controlled access ## 7.3. Time-Stamping ### 7.3.1. Time-Stamp Token The TSA shall ensure that time-stamp tokens are issued securely and include the correct time. In particular: 1. The time-stamp token shall include an identifier for the time-stamp policy. 1. Each time-stamp token shall have a unique identifier. 1. The time values the TSU uses in the time-stamp token shall be traceable to at least one of the real time values distributed by a UTC(k) laboratory. 1. The time-stamp provider should periodically monitor its correctness of time with a set of trusted UTC sources. The recorded accuracy should be included in the returned time-stamp token. 1. The time-stamp provider SHOULD monitor for accuracy and alert if it's found to be out of sync. 1. The time-stamp token shall include a representation (e.g., hash value) of the datum being time-stamped as provided by the requestor. 1. The time-stamp token shall be signed using a key generated exclusively for this purpose. NOTE 1: A protocol for a time-stamp token is defined in RFC 3631 and profiled in TS 101 861. NOTE 2: In the case of a number of requests at approximately the same time, the ordering of the time within the accuracy of the TSU clock is not mandated. 1. The time-stamp token shall include: - where applicable, an identifier for the country in which the TSA is established; - an identifier for the TSA; - an identifier for the unit which issues the time-stamps. ### 7.3.2. Clock Synchronization with UTC The TSA shall ensure that its clock is synchronized with UTC within the declared accuracy. In particular: 1. The calibration of the TSU clocks shall be maintained such that the clocks shall not be expected to drift outside the declared accuracy. 1. The TSA shall ensure that, if the time that would be indicated in a time-stamp token drifts or jumps out of synchronization with UTC, this will be detected (see also [7.3.1e](#731-time-stamp-token))). NOTE 1: Relying parties are required to be informed of such events (see section [7.4.8](#748-compromise-of-tsa-services)). 1. The TSA shall ensure that clock synchronization is maintained when a leap second occurs as notified by the appropriate body. The change to take account of the leap second shall occur during the last minute of the day when the leap second is scheduled to occur. NOTE 1: A leap second is an adjustment to UTC by skipping or adding an extra second on the last second of a UTC month. First preference is given to the end of December and June, and second preference is given to the end of March and September. ## 7.4. TSA Management and Operation ### 7.4.1. Security Management The TSA shall ensure that the administrative and management procedures applied are adequate and correspond to recognized best practice. In particular: TSA General 1. The TSA shall retain responsibility for all aspects of the provision of time-stamping services within the scope of this time-stamp policy, whether or not functions are outsourced to subcontractors. Responsibilities of third parties shall be clearly defined by the TSA and appropriate arrangements made to ensure that third parties are bound to implement any controls required by the TSA. The TSA shall retain responsibility for the disclosure of relevant practices of all parties. 1. The TSA management shall provide direction on information security through a suitable high level steering forum that is responsible for defining the TSA's information security policy. The TSA shall ensure publication and communication of this policy to all employees who are impacted by it. 1. The information security infrastructure necessary to manage the security within the TSA shall be maintained at all times. Any changes that will impact on the level of security provided shall be approved by the TSA management forum. 1. The security controls and operating procedures for TSA systems and information assets providing the time-stamping services shall be documented, implemented and maintained. NOTE 1: The present documentation (commonly called a system security policy or manual) should identify all relevant targets, objects and potential threats related to the services provided and the safeguards required to avoid or limit the effects of those threats. It should describe the rules, directives and procedures regarding how the specified services and the associated security assurance are granted in addition to stating policy on incidents and disasters. - TSA shall ensure that the security of information is maintained when the responsibility for TSA functions has been outsourced to another organization or entity. ### 7.4.2. Asset Classification and Management The TSA shall ensure that its information and other assets receive an appropriate level of protection. In particular: - The TSA shall maintain an inventory of all assets and shall assign a classification for the protection requirements to those assets consistent with the risk analysis. ### 7.4.3. Personnel Security The TSA shall follow the principle of least privilege and ensure that those working on the TSA only have the minimal privilege needed to perform functions. ### 7.4.4. Physical and Environmental Security Ths TSA will host the timestamping authority service and store private keys with either on-prem hardware or a trusted cloud provider. If the TSA uses a cloud provider to host the service and private key, it shall ensure it uses a provider that has appropriate physical security settings. For both the time-stamping provision and the time-stamping management: - The TSA shall only use cloud providers that control physical access to facilities that will host the timestamping authority service and private key; - The TSA shall implement controls to avoid loss, damage or compromise of assets and interruption to business activities; - controls shall be implemented to avoid compromise or theft of information. - The following additional controls shall be applied to time-stamping management: - The TSA shall ensure it uses a cloud provider that keeps infrastructure in an environment which physically protects the services from compromise through unauthorized access to systems or data. ### 7.4.5. Operations Management The TSA shall ensure that the TSA system components are secure and correctly operated, with minimal risk of failure. In particular (general): 1. The integrity of TSA system components and information shall be protected against viruses, malicious and unauthorized software. 1. Incident reporting and response procedures shall be employed in such a way that damage from security incidents and malfunctions shall be minimized. 1. Media used within the TSA trustworthy systems shall be securely handled to protect media from damage, theft, unauthorized access and obsolescence. NOTE 1: Every member of personnel with management responsibilities is responsible for planning and effectively implementing the time-stamp policy and associated practices as documented in the TSA practice statement. 1. Procedures shall be established and implemented for all trusted and administrative roles that impact on the provision of time-stamping services. Media handling and security: 1. All media shall be handled securely in accordance with requirements of the information classification scheme (see section [7.4.2](#742-asset-classification-and-management)). Media containing sensitive data shall be securely disposed of when no longer required. System Planning: 1. Capacity demands shall be monitored and projections of future capacity requirements made to ensure that adequate processing power and storage are available. Incident reporting and response: 1. The TSA shall act in a timely and coordinated manner in order to respond quickly to incidents and to limit the impact of breaches of security. All incidents shall be reported as soon as possible after the incident. The following additional controls shall be applied to time-stamping management: Operations procedures and responsibilities 1. TSA security operations shall be separated from other operations. NOTE 1: TSA security operations' responsibilities include: - operational procedures and responsibilities; - secure systems planning and acceptance; - protection from malicious software; - housekeeping; - network management; - active monitoring of audit journals, event analysis and follow-up; - media handling and security; - data and software exchange. These operations shall be managed by TSA trusted personnel, as defined within the appropriate security policy, and, roles and responsibility documents. ### 7.4.6. System Access Management The TSA shall ensure that TSA system access is limited to properly authorized individuals. In particular (general): 1. Controls (e.g., firewalls) shall be implemented to protect the TSA's internal network domains from unauthorized access including access by subscribers and third parties. NOTE: Firewalls should also be configured to prevent all protocols and accesses not required for the operation of the TSA. 1. The TSA shall ensure effective administration of user (this includes operators, administrators and auditors) access to maintain system security, including user account management, auditing and timely modification or removal of access. 1. The TSA shall ensure that access to information and application system functions is restricted in accordance with the access control policy and that the TSA system provides sufficient computer security controls for the separation of trusted roles identified in TSA's practices, including the separation of security administrator and operation functions. Particularly, use of system utility programs is restricted and tightly controlled. 1. TSA personnel shall be accountable for their activities The following additional controls shall be applied to time-stamping management: 1. The TSA shall ensure that it uses a cloud provider that keeps local network components (e.g., routers) in a physically secure environment. ### 7.4.7. Trustworthy Systems Deployment and Maintenance The TSA shall use trustworthy systems and products that are protected against modification. NOTE: The risk analysis carried out on the TSA's services (see section [7.1.1](#711-tsa-practice-statement)) should identify its critical services requiring trustworthy systems and the levels of assurance required. In particular: - An analysis of security requirements shall be carried out at the design and requirements specification stage of any systems development project undertaken by the TSA or on behalf of the TSA to ensure that security is built into IT systems. - Change control procedures shall be applied for releases, modifications and emergency software fixes of any operational software. ### 7.4.8. Compromise of TSA Services The TSA shall ensure in the case of events which affect the security of the TSA's services, including compromise of TSU's private signing keys or detected loss of calibration, that relevant information is made available to subscribers and relying parties. In particular: 1. The TSA's disaster recovery plan shall address the compromise or suspected compromise of TSU's private signing keys or loss of calibration of a TSU clock, which may have affected time-stamp tokens which have been issued. 1. In the case of a compromise, or suspected compromise or loss of calibration the TSA shall make available to all subscribers and relying parties a description of compromise that occurred. 1. In the case of compromise to a TSU's operation (e.g., TSU key compromise), suspected compromise or loss of calibration the TSU shall not issue time-stamp tokens until steps are taken to recover from the compromise. - In case of major compromise of the TSA's operation or loss of calibration, wherever possible, the TSA shall make available to all subscribers and relying parties information which may be used to identify the time-stamp tokens which may have been affected, unless this breaches the privacy of the TSAs users or the security of the TSA services. NOTE: In case the private key does become compromised, an audit trail of all tokens generated by the TSA may provide a means to discriminate between genuine and false backdated tokens. Two time-stamp tokens from two different TSAs may be another way to address this issue. ### 7.4.9. TSA Termination The TSA shall ensure that potential disruptions to subscribers and relying parties are minimized as a result of the cessation of the TSA's time-stamping services, and in particular ensure continued maintenance of information required to verify the correctness of time-stamp tokens. In particular: 1. Before the TSA terminates its time-stamping services the following procedures shall be executed as a minimum: - the TSA shall make available to all subscribers and relying parties information concerning its termination; - TSA shall terminate authorization of any outside services performing key singing - the TSA shall maintain or transfer to a reliable party its obligations to make available its public key or its certificates to relying parties for a reasonable period; - TSU private keys, including backup copies, shall be destroyed in a manner such that the private keys cannot be retrieved. 1. The TSA shall state in its practices the provisions made for termination of service. This shall include: - notification of affected entities; - transferring the TSA obligations to other parties. 1. The TSA shall take steps to have the TSU's certificates revoked. # 8. Security Considerations When verifying time-stamp tokens it is necessary for the verifier to ensure that the TSU certificate is trusted and not revoked. This means that the security is dependent upon the security of the CA that has issued the TSU certificate for both issuing the certificate and providing accurate revocation status information for that certificate. When a time-stamp is verified as valid at a given point of time, this does not mean that it will necessarily remain valid later on. Every time, a time-stamp token is verified during the validity period of the TSU certificate, it must be verified again against the current revocation status information, since in case of compromise of a TSU private key, all the time-stamp tokens generated by that TSU become invalid. Annex A provides guidance about the long term verification of time-stamp tokens. In applying time-stamping to applications, consideration also needs to be given to the security of the application. In particular, when applying time-stamps it is necessary to ensure that the integrity of data is maintained before the time-stamp is applied. The requester ought to really make sure that the hash value included in the time-stamp token matches with the hash of the data. # Annex A (informative): Long Term Verification of Time-Stamp Tokens Usually, a time-stamp token becomes unverifiable beyond the end of the validity period of the certificate from the TSU, because the CA that has issued the certificate does not warrant any more that it will publish revocation data, including data about revocations due to key compromises. However, verification of a time-stamp token might still be performed beyond the end of the validity period of the certificate from the TSU, if, at the time of verification, it can be known that: - the TSU private key has not been compromised at any time up to the time that a relying part verifies a time-stamp token; - the hash algorithms used in the time-stamp token exhibits no collisions at the time of verification; - the signature algorithm and signature key size under which the time-stamp token has been signed is still beyond the reach of cryptographic attacks at the time of verification. If these conditions cannot be met, then the validity may be maintained by applying an additional time-stamp to protect the integrity of the previous one. The present document does not specify the details of how such protection may be obtained. For the time being, and until some enhancements are defined to support these features, the information may be obtained using-out-of bands means or alternatively in the context of closed environments. As an example, should a CA guaranty to maintain the revocation status of TSU certificates after the end of its validity period, this would fulfill the first requirement.