Long-term Archive And Notary R. Brandner Services (LTANS) InterComponentWare AG Internet-Draft T. Gondrom Expires: November 13, 2006 Open Text Corporation U. Pordesch Fraunhofer Gesellschaft May 12, 2006 Evidence Record Syntax (ERS) draft-ietf-ltans-ers-07 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on November 13, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract In many scenarios, users need to be able to ensure and prove the existence and integrity of data, especially digitally signed data, in a common and reproducible way over a long and possibly undetermined period of time. This document specifies the syntax and processing of an Evidence Record, designed for long-term non-repudiation of Brandner, et al. Expires November 13, 2006 [Page 1] Internet-Draft ERS May 2006 existence of data, which particularly can be used for conservation of evidence of digitally signed data. Conventions used in this document 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 [RFC2119]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. General Overview and Requirements . . . . . . . . . . . . 3 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. Evidence Record . . . . . . . . . . . . . . . . . . . . . . . 6 2.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2. Generation . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3. Verification . . . . . . . . . . . . . . . . . . . . . . . 8 3. Archive Time-Stamp . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2. Generation . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3. Verification . . . . . . . . . . . . . . . . . . . . . . . 12 4. Archive Time-Stamp Chain and Archive Time-Stamp Sequence . . . 12 4.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.2. Generation . . . . . . . . . . . . . . . . . . . . . . . . 13 4.3. Verification . . . . . . . . . . . . . . . . . . . . . . . 16 5. Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 17 6. ASN.1-Module . . . . . . . . . . . . . . . . . . . . . . . . . 18 7. Security Considerations . . . . . . . . . . . . . . . . . . . 19 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 8.1. Normative References . . . . . . . . . . . . . . . . . . . 20 8.2. Informative References . . . . . . . . . . . . . . . . . . 21 Appendix A. Evidence Record using CMS . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 Intellectual Property and Copyright Statements . . . . . . . . . . 25 Brandner, et al. Expires November 13, 2006 [Page 2] Internet-Draft ERS May 2006 1. Introduction 1.1. Motivation In many application areas of electronic data exchange a non- repudiation proof of existence of digital data has to be possible over long periods of time. An important example is digitally signed data, which sometimes has to be archived with digital signatures being verifiable over 30 years or more. During the archiving period hash algorithms and public key algorithms or their parameters can become weak or certificates can become invalid. To avoid the loss of probative value of a digital signature, it has to be provable that the digitally signed data already existed before such a critical event. This can be done by timely generating Time-Stamps for these data and by the renewal of these Time-Stamps during the archival period. It is necessary to standardize data formats and processing procedures for such Time-Stamps in order to be able to verify and communicate archived data preserving evidence. A first approach was made by IETF within [RFC3126], where an optional Archive Time-Stamp Attribute was specified for integration in signatures according to the Cryptographic Messages Syntax (CMS) [RFC3852]. Evidence Record Syntax (ERS) broadens and generalizes this approach for data of any format and takes Long-term archive service requirements [REQ2004] into account, in particular, the handling of huge numbers of data objects. ERS specifies a syntax for an Evidence Record, which contains Archive Time-Stamps and some additional data. This Evidence Record can be stored as an additional file to signed data (ER as file format) or integrated in signed data (ER as part of another syntax specification). ERS also specifies processes for generation and verification of Evidence Records and as an appendix integration and use in context of signed and enveloped messages according to CMS. ERS does not specify a protocol, instead, this is done in a complementary LTANS specification. 1.2. General Overview and Requirements ERS meets requirements for data structures set forth in [REQ2004]. The basis of the ERS are Archive Time-Stamps, which can cover a single data object (as an RFC3161 compliant time-stamp does) or can cover a group of data objects. An Archive Time-Stamp can be derived from hash-trees, first described by Merkle [Mer1980], combined with a time-stamp. The leaves of the hash-tree are hash values of the data objects. A time-stamp is requested only for the root hash of the hash-tree. The deletion of any referred data objects does not Brandner, et al. Expires November 13, 2006 [Page 3] Internet-Draft ERS May 2006 influence the provability of others. The hash-tree can be reduced to a few little sets of hash values, necessary to prove existence of a single data object or a data object group. These sets of hash values and the time-stamp yield the Archive Time-Stamp. For the generation of the Initial Archive Time-Stamp the data objects to be time-stamped have to be determined - depending on the context of ERS use, e.g. this could be a file, or a data object group consisting of multiple files, such as a document and its associated digital signature. Before cryptographic algorithms used within the Archive Time-Stamp become weak or time-stamp certificates become invalid, Archive Time- Stamps have to be renewed by generating a new Archive Time-Stamp. ERS distinguishes two ways for renewal of an Archive Time-Stamp, the simple Time-Stamp Renewal and the complex Hash-Tree Renewal. In the case of Time-Stamp Renewal the time-stamp of an Archive Time- Stamp has to be hashed and time-stamped by a new Archive Time-Stamp. It is not necessary to access the initially archived data objects itself. This simple form of renewal is sufficient, if only the hash algorithm or the public key-algorithm of the time-stamp of an Archive Time-Stamp is going to lose its security suitability or the time- stamp certificates get invalid. This is very efficient in particular, if Archive Time-Stamping is done by an archiving system or service, which implements a central management of Archive Time- Stamps. Time-Stamp renewal is not sufficient if the hash algorithm used to build the hash-tree of an Archive Time-Stamp becomes insecure. In the case of Hash-Tree Renewal not only the time-stamps but also the complete Archive Time-Stamps and the referred archived data objects have to be hashed and time-stamped again by a new Archive Time-Stamp. It is necessary to get the referred data objects and other Archive Time-Stamps. 1.3. Terminology (Archived) data object: Data unit that is archived and has to be preserved for a long time by the Long-term Archive Service. (Archived) data object group: A multitude of data objects, which for some reason belong together. E.g. a document file and a signature file could be a archived data object group, which represent signed data. Archive Time-Stamp: Is a time-stamp and lists of hash values, which allows to verify the existence of several data objects at a certain time. Brandner, et al. Expires November 13, 2006 [Page 4] Internet-Draft ERS May 2006 Archive Time-Stamp Chain: Part of a Archive Time-Stamp Sequence, it is a time-ordered sequence of Archive Time-Stamps, where each Archive Time-Stamp preserves non-repudiation of the previous Archive Time-Stamp, even after the previous Archive Time-Stamp becomes invalid. Overall non-repudiation is maintained until the new Archive Time-Stamp itself becomes invalid. The process of generating such an Archive Time-Stamp Chain is called Time-Stamp Renewal. Archive Time-Stamp Sequence: Part of the Evidence Record, it is a sequence of Archive Time-Stamp Chains, where each Archive Time- Stamp Chain preserves non-repudiation of the previous Archive Time-Stamp Chains, even after the hash algorithm used within the previous Archive Time-Stamps hash-tree became weak. Non- repudiation is preserved until the last Archive Time-Stamp of the last chain becomes invalid. The process of generating such an Archive Time-Stamp Sequence is called Hash-Tree Renewal. Evidence: Information that may be used to resolve a dispute about various aspects of authenticity of archived data objects. Evidence record: Collection of evidence compiled for one or more given archived data objects over time. An evidence record includes all Archive Time-Stamps (within structures of Archive Time-Stamp Chains and Archive Time-Stamp Sequences) and additional verification data, like certificates, revocation information, trust anchors, policy details, role information, etc. Reduced hash-tree: The process of reducing a Merkle hash-tree [MER1980] to a list of lists of hash values. This is the basis of storing the evidence for a single data object. Time-Stamp: A cryptographically secure confirmation generated by a Time Stamping Authority (TSA) [RFC3161] specifies a good structure for time-stamps and a protocol for communicating with a Time-stamp Authority (TSA). Further good structures and protocols for communicating with a Time-stamp Authority (TSA) may also, but not exclusively, be [I180141], [I180142], [I180143], and [ANSX995]. Trusted archive authority (TAA): A service responsible for preserving data for long periods of time, including generation and collection of evidence, storage of archived data objects and evidence, etc. A.K.A. Long-term archive service. An Archive Time-Stamp relates to a data object, if the hash value of this data object is part of the first hash value list of the Archive Time-Stamp. An Archive Time-Stamp relates to a data object group, if it relates to every data object of the group and no other data Brandner, et al. Expires November 13, 2006 [Page 5] Internet-Draft ERS May 2006 objects. An Archive Time-Stamp Chain relates to a data object / data object group, if its first Archive Time-Stamp relates to this data object/data object group. An Archive Time-Stamp Sequence relates to a data object / data object group, if its first Archive Time-Stamp Chain relates to this data object/data object group. 2. Evidence Record An Evidence Record is a unit of data, which is to be used to prove the existence of an archived data object or an archived data object group at a certain time. The Evidence Record contains Archive Time- Stamps, generated during a long period of archiving and possibly useful data for validation. It is possible to store this Evidence Record separately from the archived data objects or to integrate it into the data itself. For data types signed data and enveloped data of the CMS integration is specified in Appendix A. 2.1. Syntax Evidence Record has the following ASN.1 Syntax: EvidenceRecord ::= SEQUENCE { version INTEGER { v1(1) }, digestAlgorithms SEQUENCE OF AlgorithmIdentifier, cryptoInfos [0] CryptoInfos OPTIONAL, encryptionInfo [1] EncryptionInfo OPTIONAL, archiveTimeStampSequence ArchiveTimeStampSequence} CryptoInfos ::= SEQUENCE SIZE (1..MAX) OF CryptoInfo CryptoInfo ::= SEQUENCE { cryptoInfoType OBJECT IDENTIFIER cryptoInfoValue ANY DEFINED BY cryptoInfoType } EncryptionInfo ::= SEQUENCE { encryptionInfoType OBJECT IDENTIFIER, encryptionInfoValue [0] EXPLICIT ANY DEFINED BY encryptionInfoType } The fields have the following meanings: version is the syntax version number, for compatibility with future Brandner, et al. Expires November 13, 2006 [Page 6] Internet-Draft ERS May 2006 revisions of this specification. digestAlgorithms is a sequence of all the hash algorithms used to hash the data object over the archival period. cryptoInfos allows the storage of data useful in the validation of the archiveTimeStampSequence. This could include possible TrustAnchors, certificates, revocation information or the current definition of the suitability of cryptographic algorithms, past and present (e.g. RSA 768bit valid until 1998, RSA 1024bit valid until 2008, SHA1 valid until 2010). These items may be added based on the policy used. Since this data is not protected within any time-stamp, the data should be current and somehow verifiable. Such verification is out-of-scope of this document. encryptionInfo contains the necessary information in case encrypted content shall be handled. For discussion of syntax please refer to chapter 5.1 ArchiveTimeStampSequence is a sequence of ArchiveTimeStampChain, described in chapter 4. If the archive data objects were encrypted before generating Archive Time-stamps but a non-repudiation proof is needed for unencrypted data objects, the optional field encryption contains data necessary to re-encrypt data objects. If left out, it means that data objects are not encrypted. For further details see chapter 5. 2.2. Generation The generation of an EvidenceRecord overall can be described as follows: 1. Select archived data object or an archived group of data objects, which are documents or essential parts of it - depending on application. In the case that only essential parts of documents or objects shall be covered the application not defined in this draft MUST take care of the right extraction of binary data to be covered for generation EvidenceRecord and their verification. 2. Create Initial Archive Time-Stamp (see Archive Time-Stamp chapter 3). 3. Renew this Archive Time-Stamp when necessary, by Time-Stamp Renewal or Hash-Tree Renewal (see chapter 4). The process of generation depends on whether the Archive Time-Stamps are generated, stored and managed by a centralized instance or not. Brandner, et al. Expires November 13, 2006 [Page 7] Internet-Draft ERS May 2006 In case of central management it is possible to collect data objects from many documents, to build hash-trees, store them and reduce them later. In case of local generation it might be easier to generate a simple Archive Time-Stamp without building hash-trees and reducing them. Details of local generation procedure are not to be discussed in this specification. 2.3. Verification The Verification of an EvidenceRecord overall can be described as follows: 1. Select archived data object or a group of data objects, which were originally Archive Time-Stamped. 2. Re-encrypt data object/data object group, if encryption field is used (details see chapter 5) 3. Verify Archive Time-Stamp Sequence (details in chapter 3 and 4). 3. Archive Time-Stamp An Archive Time-Stamp is a time-stamp and some lists of hash values, which allow verification of the existence of a data object or a data object group at a certain time. The lists of hash values can be generated by reduction of an ordered Merkle hash-tree [Mer1980]. The leaves of this hash-tree are the hash values of the data objects to be time-stamped. Every inner node of the tree contains one hash value, which is generated by hashing the concatenation of the children nodes. The root hash value, which represents unambiguously all data objects, is time-stamped. 3.1. Syntax An Archive Time-Stamp has the following ASN.1 Syntax: ArchiveTimeStamp ::= SEQUENCE { digestAlgorithm AlgorithmIdentifier OPTIONAL, reducedHashtree [0] EXPLICIT SEQUENCE OF {SEQUENCE OF OCTET STRING} OPTIONAL, timeStamp ContentInfo} The fields of type ArchiveTimeStamp have the following meaning: digestAlgorithm identifies the digest algorithm and any associated parameters used within the reduced hash-tree. If the optional field digestAlgorithm is not present the digest algorithm of the time-stamp Brandner, et al. Expires November 13, 2006 [Page 8] Internet-Draft ERS May 2006 must be used. If time-stamps according to [RFC3161] are used, the content of this field must be identical to hashAlgorithm of messageImprint-Field of timeStampToken. reducedHashtree contains lists of hash values, which could be derived from a hash-tree by reduction to nodes necessary for verification of a single data object. Hash values are represented as octet strings. If the optional field reducedHashtree is not existent the Archive Time-Stamp is equal to the ordinary time-stamp. timeStamp should contain the time-stamp as defined in section 1.3 "Terminology". (e.g. as defined with timeStampToken in [RFC 3161]). Other types of time-stamp might be used, if they contain time data, time-stamped data and a cryptographically secure confirmation from the TSA of these data. 3.2. Generation The lists of hash values of an Archive Time-Stamp can be generated by the way of building and reducing a Merkle hash-tree [Mer1980]. Such a hash-tree can be built as follows: 1. Collect data objects to be time-stamped. 2. Choose secure hash algorithm H and generate hash values for the data objects, which will be the leaves of the hash-tree. 3. For each data group containing more than one document, its respective document hashes are binary sorted in ascending order, concatenated and hashed. 4. If there is more than one hash value, place them in groups and sort each group in binary ascending order. Concatenate these values and generate new hash values, which are inner nodes of this tree. (If additional hash values are needed, e.g. so that all nodes have the same number of children, any data may be hashed using H and used.) Repeat this step until there is only one hash value, which is the root node of the hash-tree. 5. Order a time-stamp for this root hash value. The hash algorithm in the time-stamp request must be the same as the hash algorithm of the hash-tree. An example of a constructed hash-tree for 3 data groups, where data group 1 and 3 only contain one document, and data group 2 contains 3 documents: Brandner, et al. Expires November 13, 2006 [Page 9] Internet-Draft ERS May 2006 +------+ | h123 | +------+ / \ / \ +----+ +----+ | h12| | h3 | +----+ +----+ / \ / \ +----+ +-------+ | h1 | | h2abc | +----+ +-------+ / | \ / | \ / | \ / | \ +----+ +----+ +----+ | h2a| | h2b| | h2c| +----+ +----+ +----+ Figure 1: Hash-tree h1 = H(d1) where d1 is the only data object in data group 1 h3 = H(d3) where d3 is the only data object in data group 3 h12 = H( binary sorted and concatenated (h1, h2abc)) h123 = H( binary sorted and concatenated (h12, h3)) h2a = H(first data object of data object group 2) h2b = H(second data object of data object group 2) h2c = H(third data object of data object group 2) h2abc = H( binary sorted and concatenated (h2a, h2b, h2c)) The hash-tree can be reduced to lists of hash values, necessary to have a proof of existence for a single data object: 1. Generate hash value h of the data object, using hash algorithm H of the hash-tree. 2. Select all hash values, which have the same father node as h. Generate the first list of hash values by arranging these hashes, in binary ascending order. Repeat this step for the father node of these hashes until the root hash is reached. The father nodes are not saved in the hash lists - they are computable. 3. Generate a reduced hash-tree by building the sequence of these hash value lists. Then add the time-stamp and the hash algorithm to get an Archive Time-Stamp. Brandner, et al. Expires November 13, 2006 [Page 10] Internet-Draft ERS May 2006 Assuming that the sorted binary ordering of the hashes in Figure 1 is: h2abc < h1 then the reduced hash-tree for data group 1 (d1) is: +--------------------------------+ | +----------------+ +--------+ | | | +------+ +----+ | | +----+ | | | | h2abc| | h1 | | | | h3 | | | | +------+ +----+ | | +----+ | | +----------------+ +--------+ | +--------------------------------+ Figure 2: Reduced hash-tree for data group 1 The pseudo ASN1 for this reduced hash-tree would look like: rht1 = SEQ( SEQ (h2abc, h1), SEQ (h3)) Assuming the same hash-tree as in figure 1 the reduced hash-tree for all data objects in data group 2 is identical. +-------------------------------------------------+ | +----------------------+ +--------+ +--------+ | | | +----+ +----+ +----+ | | +----+ | | +----+ | | | | | h2b| | h2c| | h2a| | | | h1 | | | | h3 | | | | | +----+ +----+ +----+ | | +----+ | | +----+ | | | +----------------------+ +--------+ +--------+ | +-------------------------------------------------+ Figure 3: Reduced hash-tree for data object group 2 The pseudo ASN1 for this reduced hash-tree would look like: rht2 = SEQ( SEQ (h2b, h2c, h2a), SEQ (h1), SEQ (h3)) Note, there are no restrictions to the quantity of hash value lists and of their length. Also note, that it is profitable but not required to build hash-trees and reduce them. An Archive Time-Stamp may consist only of one list of hash-values and a time-stamp or in the extreme case, only a time-stamp with no hash value lists. The certificates, CRLS or OCSP-Responses needed to verify the time- stamp SHOULD be stored in the time-stamp itself. A time-stamp according to [RFC 3161] is a CMS-object in which certificates can be stored in the certificates field and CRLs can be stored in the crls field of signed data. OCSP responses can be stored as unsigned attributes [RFC3126]. Note [ANSX995], [I180142] and [I180143] specify irrefutably verifiable time-stamps which do not depend on certificates, CRLS, or OCSP-Responses. Brandner, et al. Expires November 13, 2006 [Page 11] Internet-Draft ERS May 2006 3.3. Verification An Archive Time-Stamp shall prove that a data object existed at a certain time, given by time-stamp. This can be verified as follows: 1. Calculate hash value h of the data object with hash algorithm H given in field digestAlgorithm of the Archive Time-Stamp. 2. Search for hash value h in the first list of reducedHashtree. If not present, terminate verification process with negative result. 3. Concatenate the hash values of the actual list of hash values in binary ascending order and calculate the hash value h' with algorithm H. This hash value h' must become member of the next higher list of hash values. Continue step 3 until a root hash value is calculated. 4. Check time-stamp. In case of time-stamp according to [RFC 3161] the root hash value must correspond to hashedMessage and digestAlgorithm must correspond to hashAlgorithm field, both in messageImprint field of timeStampToken. If the proof is necessary for more than one data object, steps 1 and 2 have to be done for all data objects to be proved. If an additional proof is necessary that the Archive Time-Stamp relates to a data object group (e.g. a document and all its signatures) it can be verified additionally, that only the hash values of the given data objects are in the first hash value list. 4. Archive Time-Stamp Chain and Archive Time-Stamp Sequence Archive Time-Stamps are used for archive time-stamping. An Archive Time-Stamp proves the existence of single data objects or data object group at a certain time. However, this first Archive Time-Stamp in the first ArchiveTimeStampChain can become invalid, if hash algorithms or public key algorithms used in its hash-tree or time- stamp become weak or if the validity period of the time-stamp certificates expires or if the time-stamp certificates are revoked. If this is going to happen, the existence of the Archive Time-Stamp or archive time-stamped data has to be reassured, which can be done by creating new Archive Time-Stamps. Depending on whether the time- stamp becomes invalid or the hash algorithm of the hash-tree becomes weak, two kinds of Archive Time-Stamp renewals are possible: o Time-Stamp Renewal: A new Archive Time-Stamp is generated, which covers the time-stamp of the old one. One or more Archive Time- Stamps generated by Time-Stamp Renewal yield an Archive Time-Stamp Brandner, et al. Expires November 13, 2006 [Page 12] Internet-Draft ERS May 2006 Chain for a data object or data object group. o Hash-Tree Renewal: A new Archive Time-Stamp is generated, which covers all the old Archive Time-Stamps as well as the data objects. A new Archive Time-Stamp Chain is started. One or more Archive Time-Stamp Chains for a data object or data object group yield an Archive Time-Stamp Sequence. 4.1. Syntax ArchiveTimeStampChain and ArchiveTimeStampSequence have the following ASN.1 Syntax: ArchiveTimeStampChain ::= SEQUENCE OF ArchiveTimeStamp ArchiveTimeStampSequence ::= SEQUENCE OF ArchiveTimeStampChain ArchiveTimeStampChain and ArchiveTimeStampSequence MUST be ordered ascending by time of time-stamp. Within an ArchiveTimeStampChain all reducedHashtrees of the contained ArchiveTimeStamps MUST use the same Hash-Algorithm. 4.2. Generation A first Initial Archive Time-Stamp relates to a data object or a data object group. The application or the policy included in the LTANS Long term Archiving Protocol (to be specified) dictate when an Initial Archive Time-Stamp must be generated for each data object. Before cryptographic algorithms used within the Archive Time-Stamp become weak or time-stamp certificates are invalidated, Archive Time- Stamps have to be renewed by generating a new Archive Time-Stamp. In the case of Time-Stamp Renewal the content of the timeStamp field of the old Archive Time-Stamp has to be hashed and time-stamped by a new Archive Time-Stamp. For this procedure one could of course e.g. collect a number of old Archive Time-Stamps and build the new hash- tree with the hash values of the content of their timeStamp fields. This hash-tree of the new Archive Time-Stamp MUST use the same hash algorithm as the old one, which is specified in the digestAlgorithm field of the Archive Time-Stamp or if this value is not set (as it is optional) within the time-stamp itself. In the case of Hash-Tree Renewal not only the Archive Time-Stamp but also the data objects referred to by the initial Archive Time-Stamp have to be hashed and time-stamped again: 1. Select secure hash algorithm H. Brandner, et al. Expires November 13, 2006 [Page 13] Internet-Draft ERS May 2006 2. Select data objects d(i) referred to by initial Archive Time- Stamp (objects which are still present and not deleted). Generate hash values h(i) = H((d(i)). If data groups with more than one document are present, then one will have more than one hash for a group, i.e. h(i_a), h(i_b).., h(i_n) 3. atsc(i) is the encoded ArchiveTimeStampSequence, the concatenation of all previous Archive Time-Stamp Chains (in chronological order) related to data object d(i). Generate hash value ha(i) = H(atsc(i)). Note: The ArchiveTimeStampChains used are DER encoded, i.e. they contain sequence and length tags. 4. Concatenate each h(i) with ha(i) and generate hash values h(i)' = H (h(i)+ ha(i)). For multi-document groups, this is: h(i_a)' = H (h(i_a)+ ha(i)) h(i_b)' = H (h(i_b)+ ha(i)) etc. 5. Build a new Archive Time Stamp for each h(i)'. (hash-tree generation and reduction is defined in 3.2, note that each h(i)' will be treated in 3.2 as the document hash. The first hash value list in the reduced hash-tree should only contain h(i)'. For a multi-document group, the first hash value list will contain the new hashes for all the documents in this group, i.e. h(i_a)', h(i_b)'.., h(i_n)') 6. Create new ArchiveTimeStampChain and add this new Archive Time- Stamp. Brandner, et al. Expires November 13, 2006 [Page 14] Internet-Draft ERS May 2006 +-------+ | h123' | +-------+ / \ / \ +-----+ +----+ | h12'| | h3'| +-----+ +----+ / \ / \ +----+ +--------+ | h1'| | h2abc' | +----+ +--------+ / | \ / | \ / | \ / | \ +----+ +----+ +----+ |h2a'| |h2b'| |h2c'| +----+ +----+ +----+ Figure 4: Hash-tree from hash-tree renewal Let H be the new secure hash algorithm ha(1), ha(2), ha(3) are as defined in step 4 above h1' = H( binary sorted and concatenated (H(d1), ha(1))) d1 is the original document from data group 1 h3' = H( binary sorted and concatenated (H(d3), ha(3))) d3 is the original document from data group 3 h2a = H(first data object of data object group 2) ... h2c = H(third data object of data object group 2) h2a' = H( binary sorted and concatenated (h2a, ha(2))) ... h2c' = H( binary sorted and concatenated (h2c, ha(2))) h2abc = H( binary sorted and concatenated (h2a', h2b', h2c')) Before the Time-Stamp of an Archive Time-Stamp becomes invalid, the simple time-stamp renewal should be done. Only if the hash algorithm used within the hash-tree becomes weak, Hash-Tree Renewal must be done. In case of centralized Archive Time-Stamping, Archive Time- Stamps might be generated a long-time before other Archive Time- Stamps become invalid to be on the secure side. Nevertheless ArchiveTimeStamps, which are not necessary for verification, should Brandner, et al. Expires November 13, 2006 [Page 15] Internet-Draft ERS May 2006 not be added to ArchiveTimeStampChain or ArchiveTimeStampSequence. 4.3. Verification To get an non-repudiation proof that a data object existed at a certain time, the Archive Time-Stamp Chains and their relations to each other and to the data objects have to be proved: 1. Verify that the first Archive Time-Stamp of the first ArchiveTimestampChain (the Initial Archive Time-Stamp) contains the hash value of the data object. 2. Verify each ArchiveTimestampChain. The first hash value list of each ArchiveTimeStamp must contain the hash value of the time- stamp of the Archive Time-Stamp before. The Archive Time-Stamp has had to be valid at the time of the following Archive Time- Stamp (especially the time-stamp refer to section 3.3 verification step 4 has to be valid and secure at the moment the next Archive time-stamp has been applied. All Archive Time- Stamps within a chain MUST use the same hash algorithm and this algorithm MUST be secure at the time of the first Archive Time- Stamp of the following ArchiveTimeStampChain. 3. Verify that the first hash value list of the first Archive Time- Stamp of all other ArchiveTimeStampChains contains a hash value of the concatenation of the data object hash and the hash value of all older ArchiveTimeStampChain. Verify that this Archive Time-Stamp was generated before the last Archive Time-Stamp of the ArchiveTimeStampChain became invalid. In order to complete the non-repudiation proof for the data objects, the last Archive Time-Stamp has to be valid at the time the verification is performed. If the proof is necessary for more than one data object, steps 1 and 3 have to be done for all these data objects. If an additional proof is necessary that the Archive Time-Stamp Sequence relates to a data object group (e.g. a document and all its signatures) it can be verified additionally, that each first Archive Time-Stamp of each ArchiveTimeStampChain does not contain other hash values in its first hash value list. 5. Encryption If service providers are used to archive data and generate Archive Time-Stamps, it might be desirable or required that clients only send encrypted data to be archived. However, this means that evidence Brandner, et al. Expires November 13, 2006 [Page 16] Internet-Draft ERS May 2006 records refer to encrypted data objects and not to the unencrypted ones. ERS directly protects the integrity of the bit-stream and this some kind of freezes the bit structure at the time of archiving and later renewal or change of the encryption scheme, e.g. because the encryption is no longer secure is not possible without loosing the integrity proof of the ERS. In such cases the services of a data transformation (and by this also possible re-encryption) done by a notary service might be a possible solution. To avoid problems when using the evidence records in the future, additional special precautions have to be taken: o Encryption can affect the proof of existence of the unencrypted data. E.g. it could be possible to choose an algorithm or a key for decryption that is not the algorithm or key used for encryption. In this case, the evidence record would not be a non- repudiation proof for the unencrypted data. Therefore, only encryption methods may be used, which make it possible to prove that archive time-stamped encrypted data objects unambiguously represent unencrypted data objects. All data necessary to prove unambiguous representation has to be part of the archived data objects. (Note: Additionally the long term security of the used encryption schemes has to be analyzed on how it could be used to create collision attacks.) o When encrypted data objects and the evidence record are sent back, it may be desirable for clients to only store the unencrypted data objects and to delete the decrypted ones, in order to avoid duplicate storage. In order to use the evidence record, it must be then possible to re-encrypt the unencrypted data to get exactly the data that was originally archived. Therefore, additional data necessary to re-encrypt data objects should be inserted into the evidence record by the client (i.e. archive provider never sees these values). The syntax specifies an open structure to store the needed parameters of the used encryption methods and further information dependent on the used encryption methods. The use of the specified encryptionInfoType and encryptionInfoValue may be heavily dependent on the used mechanisms and has to be defined in other specifications. 5.1. Syntax EncryptionInfo-Field in EvidenceRecord has the following Syntax: EncryptionInfo ::= SEQUENCE { encryptionInfoType OBJECT IDENTIFIER, encryptionInfoValue [0] EXPLICIT ANY DEFINED BY encryptionInfoType Brandner, et al. Expires November 13, 2006 [Page 17] Internet-Draft ERS May 2006 } encryptionInfoType defines the type of information or structure that will be in the encryptionInfoValue field. encryptionInfoValue contains the specific information that is necessary to provide the proof that the unencrypted data is unambiguously represented by the encrypted data protected by the EvidenceRecord. 6. ASN.1-Module ERS {iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) ltans(11) id-mod(1) id-mod-ers(0) } DEFINITIONS IMPLICIT TAGS ::= BEGIN -- EXPORTS ALL -- IMPORTS TimeStampToken FROM PKIXTSP -- [RFC3161] {iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-tsp(13) } ContentInfo FROM CryptographicMessageSyntax2004 -- FROM [RFC3852] { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) } -- Imports from RFC 3280 [RFC3280], Appendix A.1 AlgorithmIdentifier FROM PKIX1Explicit88 { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) mod(0) pkix1-explicit(18) } -- LTANS specific idnetifiers id-ltans OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) ltans(11) } Brandner, et al. Expires November 13, 2006 [Page 18] Internet-Draft ERS May 2006 id-em OBJECT IDENTIFIER ::= { id-ltans 2 } -- ERS encryption methods ArchiveTimeStamp ::= SEQUENCE { digestAlgorithm AlgorithmIdentifier OPTIONAL, reducedHashtree [0] EXPLICIT SEQUENCE OF SEQUENCE OF OCTET STRING OPTIONAL, timeStamp ContentInfo} ArchiveTimeStampChain::= SEQUENCE SIZE (1..MAX) OF ArchiveTimeStamp ArchiveTimeStampSequence::= SEQUENCE SIZE (1..MAX) OF ArchiveTimeStampChain EvidenceRecord ::= SEQUENCE { version INTEGER { v1(1) }, digestAlgorithms SEQUENCE SIZE (1..MAX) OF AlgorithmIdentifier, cryptoInfos [0] CryptoInfos OPTIONAL, encryptionInfo [1] EncryptionInfo OPTIONAL, archiveTimeStampSequence ArchiveTimeStampSequence} CryptoInfos ::= SEQUENCE SIZE (1..MAX) OF CryptoInfo -- dynamically extensible information object set -- CryptoInfo ::= SEQUENCE { cryptoInfoType OBJECT IDENTIFIER cryptoInfoValue ANY DEFINED BY cryptoInfoType } EncryptionInfo ::= SEQUENCE { encryptionInfoType OBJECT IDENTIFIER, encryptionInfoValue [0] EXPLICIT ANY DEFINED BY encryptionInfoType } END 7. Security Considerations Secure Algorithms Brandner, et al. Expires November 13, 2006 [Page 19] Internet-Draft ERS May 2006 Cryptographic algorithms and parameters which are used within Archive Time-Stamps must be secure at the time of generation. This concerns the hash algorithm used in the hash lists of Archive Time-Stamp as well as hash algorithms and public key algorithms of the time-stamps. Publications regarding security suitability of cryptographic algorithms ([ETSI2003]) have to be considered by verifying components. A generic solution for automatic interpretation of security suitability policies in electronic form is desirable but not subject of this specification. Redundancy Algorithms can loose there security suitability untimely or Time Stamping Authorities may be considered as untrustworthy retrospectively. Therefore Archive Time-Stamps can lose their probative force. If Archive Time-Stamps are managed centrally several redundant ArchiveTimeStampSequences can be generated using different hash algorithms and different Time Stamping Authorities. Secure Time-Stamps Archive Time-Stamping is as secure as normal time stamping. Security requirements for Time Stamping Authorities stated in security policies have to be met. Renewed Archive Time-Stamps should have the same or higher quality as the Initial Archive Time-Stamp. Archive Time-Stamps used for signature renewal of signed data, should have the same or higher quality than maximum quality of the signatures. Secure Encryption For non-repudiation proof it does not matter, whether encryption has been broken or not. Nevertheless, users should keep secret their private keys and randoms used for encryption and disclose them only if needed (e.g. in a lawsuit to a judge or expert). They should use encryption algorithms and parameters which are prospected to be unbreakable as long as confidentiality of the archived data is important. 8. References 8.1. Normative References [ANSX995] American National Standard for Financial Services, "Trusted Timestamp Management and Security", ANSX X9.95- 2005, June 2005. [I180141] ISO/IEC JTC 1/SC 27, "Time stamping services - Part 1: Brandner, et al. Expires November 13, 2006 [Page 20] Internet-Draft ERS May 2006 Framework", ISO ISO-18014-1, February 2002. [I180142] ISO/IEC JTC 1/SC 27, "Time stamping services - Part 2: Mechanisms producing independent tokens", ISO ISO-18014-2, December 2002. [I180143] ISO/IEC JTC 1/SC 27, "Time stamping services - Part 3: Mechanisms producing linked tokens", ISO ISO-18014-3, February 2004. [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 3", RFC 2026, 1996. [RFC2119] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement Levels", RFC 2119, 1997. [RFC3126] Adams, C., Pinkas, D., Ross, J., and N. Pope, "Electronic Signature Formats for long term electronic signatures", RFC 3126, 2001. [RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato, "Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)", RFC 3161, August 2001. [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, August 2001. [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852, July 2004. 8.2. Informative References [ETSI2003] European Telecommunication Standards Institute (ETSI), Electronic Signatures and Infrastructures (ESI);, "Algorithms and Parameters for Secure Electronic Signatures", ETSI SR 002 176 V1.1.1, March 2003. [MER1980] Merkle, R., "Protocols for Public Key Cryptosystems, Proceedings of the 1980 IEEE Symposium on Security and Privacy (Oakland, CA, USA)", pages 122-134, April 1980. [REQ2004] Wallace, C., Brandner, R., and U. Pordesch, "Long-term Archive Service Requirements", I-D ???, 2005. Brandner, et al. Expires November 13, 2006 [Page 21] Internet-Draft ERS May 2006 Appendix A. Evidence Record using CMS An Evidence Record can be added to signed data or enveloped data in order to transfer them in a conclusive way. For CMS a sensible place to store such an Evidence Record is an unsigned attribute (signed message) or an unprotected attribute (enveloped message). The Evidence Record also contains information about the selection method which was used for the generation of the data objects to be time-stamped. In the case of CMS, two selection methods can be distinguished: 1. The CMS Object as a whole including contentInfo is selected as data object and archive time-stamped. This means that a hash value of the CMS object must be located in the first list of hash values of Archive Time-Stamps. 2. The CMS Object and the signed or encrypted content are included in the Archive Time-Stamp as separated objects. In this case the hash value of the CMS Object as well as the hash value of the content has to be stored in the first list of hash values as a group of data objects. However, other selection methods could also be applied like for instance in [RFC3126]. In the case of the two selection methods defined above, the Evidence Record has to be added to the first signature of the CMS Object of signed data. Depending on the selection method, the following Object Identifier is defined for the Evidence Record: Internal signature: id-EvidenceRecord ::= {id-em-env-data-Attribute 1} External signature: id-EvidenceRecord ::= {id-em-env-data-Attribute 2} The attributes should only occur once. If they appear several times, they have to be stored within the first signature in a chronological order. If the CMS object doesn't have the EvidenceRecord Attributes - which indicates that the EvidenceRecord has been provided externally - the archive time-stamped data object has to be generated over the complete CMS object within the existing coding. In case of verification, if only one EvidenceRecord is contained in the CMS object, the hash value must be generated over the CMS object without the one EvidenceRecord. This means that the attribute has to Brandner, et al. Expires November 13, 2006 [Page 22] Internet-Draft ERS May 2006 be removed before verification. The length of fields containing tags has to be adapted. Apart from that, the existing coding must not be modified. If several Archive Time-Stamps occur, the data object has to be generated as follows: o During verification of the first (in a chronological order) EvidenceRecord, all EvidenceRecord have to be removed in order to generate the data object. o During verification of the nth one EvidenceRecord, the first n-1 attributes should remain within the CMS object. o The verification of the nth one EvidenceRecord must result in a point of time when the document must have existed with the first n attributes. The verification of the n+1th attribute must prove that this requirement has been met. Brandner, et al. Expires November 13, 2006 [Page 23] Internet-Draft ERS May 2006 Authors' Addresses Ralf Brandner InterComponentWare AG Otto-Hahn-Str. 3 Walldorf D-69119 Germany Email: ralf.brandner@intercomponentware.com Tobias Gondrom Open Text Corporation Technopark 2 Werner-von-Siemens-Ring 20 Grasbrunn, Munich D-85630 Germany Phone: +49 (0) 89 4629-1816 Fax: +49 (0) 89 4629-33-1816 Email: tobias.gondrom@opentext.com Ulrich Pordesch Fraunhofer Gesellschaft Dolivostrasse 15 Darmstadt D-64293 Germany Email: ulrich.pordesch@zv.fraunhofer.de Brandner, et al. Expires November 13, 2006 [Page 24] Internet-Draft ERS May 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Brandner, et al. Expires November 13, 2006 [Page 25]