Protecting Network Time Security Messages with the Cryptographic Message Syntax (CMS)

This document provides details on how to construct NTS messages in practice. NTS provides secure time synchronization with time servers using Network Time Protocol (NTP) or Precision Time Protocol (PTP) . Among other things, this document describes a convention for using the Cryptographic Message Syntax (CMS) to protect messages in the Network Time Security (NTS) protocol. Encryption is used to provide confidentiality of secrets, and digital signatures are used to provide authentication and integrity of content. Sometimes CMS is used in an exclusively ASN.1 environment. In this case, the NTS message may use any syntax that facilitates easy implementation.

Regarding the usage of CMS, we differentiate between four archetypes according to which the NTS message types can be structured. They are presented below. Note that the NTS Message Object that is at the core of each structure does not necessarily contain all the data needed for the particular message type, but may contain only that data which needs to be secured directly with cryptographic operations using the CMS. Specific information about what is included can be found in . This archetype is used for actual time synchronization messages (explicitly, the following message types: time_request, time_response, server_broad, see , Section 6) as well as for the very first messages of a unicast or a broadcast exchange (client_assoc or client_bpar, respectively) and the broadcast keycheck exchange (client_keycheck and server_keycheck). This archetype does not make use of any CMS structures at all. Figure 1 illustrates this structure.

This archetype is used for secure transmission of the cookie (only for the server_cook message type, see , Section 6). For this, the following CMS structure is used: First, the NTS message MUST be encrypted using the EnvelopedData content type. EnvelopedData supports nearly any form of key management. In the NTS protocol the client provides a certificate in an unprotected message, and the public key from this certificate, if it is valid, will be used to establish a pairwise symmetric key for the encryption of the protected NTS message. Second, the EnvelopedData content MUST be digitally signed using the SignedData content type. SignedData supports nearly any form of digital signature, and in the NTS protocol the server will include its certificate within the SignedData content type. Third, the SignedData content type MUST be encapsulated in a ContentInfo content type. Figure 2 illustrates this structure.

This archetype is used for server_assoc and server_bpar message types. It uses the following CMS structure: First, the NTS message object MUST be wrapped in a SignedData content type. The messages MUST be digitally signed, and certificates included. SignedData supports nearly any form of digital signature, and in the NTS protocol the server will include its certificate within the SignedData content type. Second, the SignedData content type MUST be encapsulated in a ContentInfo content type. Figure 3 illustrates this structure.

This archetype is used for the client_cook message type. It uses a CMS structure much like the NTS-Signed archetype (see Figure 3), with the only difference being that messages SHOULD NOT be digitally signed. This archetype employs the CMS structure merely in order to transport certificates.

Overall, three CMS content types are used for NTS messages by the archetypes above. Explicitly, those content types are ContentInfo, SignedData and EnvelopedData. The following is a description of how the fields of those content types are used in detail.

The ContentInfo content type is used in all archetypes except NTS-Plain. The fields of the ContentInfo content type are used as follows: contentType -- indicates the type of the associated content. For all archetypes which use ContentInfo (these are NTS-Certified, NTS-Signed and NTS-Encrypted-and-Signed), it MUST contain the object identifier for the SignedData content type:

content -- is the associated content. For all archetypes using ContentInfo, it MUST contain the DER encoded SignedData content type.

The SignedData content type is used in the NTS-Certified, NTS-Signed and NTS-Encrypted-and-Signed archetypes, but not in the NTS-Plain archetype. The fields of the SignedData content type are used as follows: version -- the appropriate value depends on the optional items that are included. In the NTS protocol, the signer certificate MUST be included and other items MAY be included. The instructions in Section 5.1 MUST be followed to set the correct value. digestAlgorithms -- is a collection of message digest algorithm identifiers. In the NTS protocol, there MUST be exactly one algorithm identifier present. The instructions in Section 5.4 of MUST be followed. encapContentInfo -- this structure is always present. In the NTS protocol, it MUST follow these conventions: eContentType -- is an object identifier. In the NTS protocol, for the NTS-Certified and NTS-Signed archetypes, it MUST identify the type of the NTS message that was encapsulated. For the NTS-Encrypted-and-Signed archetype, it MUST contain the object identifier for the EnvelopedData content type:

eContent is the content itself, carried as an octet string. For the NTS-Certified and NTS-Signed archetypes, it MUST contain the DER encoded encapsulated NTS message object. The instructions in Section 6.3 of [RFC5652] MUST be followed. For the NTS-Encrypted-and-Signed archetype, it MUST contain the DER encoded EnvelopedData content type. certificates -- is a collection of certificates. In the NTS protocol, it MUST contain the DER encoded certificate [RFC5280] of the sender. It is intended that the collection of certificates be sufficient for the recipient to construct a certification path from a recognized "root" or "top-level certification authority" to the certificate used by the sender. crls -- is a collection of revocation status information. In the NTS protocol, it MAY contain one or more DER encoded CRLs [RFC5280]. It is intended that the collection contain information sufficient to determine whether the certificates in the certificates field are valid. signerInfos -- is a collection of per-signer information. In the NTS protocol, for the NTS-Certified archetype, this SHOULD be left out. For both the NTS-Signed and the NTS-Encrypted-and-Signed archetypes, there MUST be exactly one SignerInfo structure present. The details of the SignerInfo type are discussed in Section 5.3 of . In the NTS protocol, it MUST follow these conventions: version -- is the syntax version number. In the NTS protocol, the SignerIdentifier is subjectKeyIdentifier, therefore the version MUST be 3. sid -- identifies the signer's certificate. In the NTS protocol, the "sid" field contains the subjectKeyIdentifier from the signer's certificate. digestAlgorithm -- identifies the message digest algorithm and any associated parameters used by the signer. In the NTS protocol, the identifier MUST match the single algorithm identifier present in the digestAlgorithms. signedAttrs -- is a collection of attributes that are signed. In the NTS protocol, it MUST be present, and it MUST contain the following attributes: Content Type -- see Section 11.1 of . Message Digest -- see Section 11.2 of . In addition, it MAY contain the following attributes: Signing Time -- see Section 11.3 of . Binary Signing Time -- see Section 3 of . signatureAlgorithm -- identifies the signature algorithm and any associated parameters used by the signer to generate the digital signature. signature is the result of digital signature generation using the message digest and the signer's private key. The instructions in Section 5.5 of MUST be followed. unsignedAttrs -- is an optional collection of attributes that are not signed. In the NTS protocol, it MUST be absent.

The EnvelopedData content type is used only in the NTS-Encrypted-and-Signed archetype. The fields of the EnvelopedData content type are used as follows: version -- the appropriate value depends on the type of key management that is used. The instructions in Section 6.1 MUST be followed to set the correct value. originatorInfo -- this structure is present only if required by the key management algorithm. In the NTS protocol, it MUST be present when a key agreement algorithm is used, and it MUST be absent when a key transport algorithm is used. The instructions in Section 6.1 of MUST be followed. recipientInfos -- this structure is always present. In the NTS protocol, it MUST contain exactly one entry that allows the client to determine the key used to encrypt the NTS message. The instructions in Section 6.2 of MUST be followed. encryptedContentInfo -- this structure is always present. In the NTS protocol, it MUST follow these conventions: contentType -- indicates the type of content. In the NTS protocol, it MUST identify the type of the NTS message that was encrypted. contentEncryptionAlgorithm -- identifies the content-encryption algorithm and any associated parameters used to encrypt the content. encryptedContent -- is the encrypted content. In the NTS protocol, it MUST contain the encrypted NTS message. The instructions in Section 6.3 of [RFC5652] MUST be followed. unprotectedAttrs -- this structure is optional. In the NTS protocol, it MUST be absent.

This section presents some hints about the structures of the NTS message objects for the different message types when one wishes to implement the security mechanisms.

The following ASN.1 coded data type "NTSNonce" is needed for other types used below for NTS messages. It specifies a 128 bit nonce as required in several message types:

The following ASN.1 coded data types are also necessary for other types.

This message is structured according to the NTS-Plain archetype. There is no data necessary besides that which is transported in the NTS message object, which is an ASN.1 object of type "ClientAssocData" and structured as follows: