Internet Engineering Task Force (IETF) B. Haberman, Ed. Request for Comments: 5906 JHU/APL Category: Informational D. Mills ISSN: 2070-1721 U. Delaware June 2010 Network Time Protocol Version 4: Autokey Specification Abstract This memo describes the Autokey security model for authenticating servers to clients using the Network Time Protocol (NTP) and public key cryptography. Its design is based on the premise that IPsec schemes cannot be adopted intact, since that would preclude stateless servers and severely compromise timekeeping accuracy. In addition, Public Key Infrastructure (PKI) schemes presume authenticated time values are always available to enforce certificate lifetimes; however, cryptographically verified timestamps require interaction between the timekeeping and authentication functions. This memo includes the Autokey requirements analysis, design principles, and protocol specification. A detailed description of the protocol states, events, and transition functions is included. A prototype of the Autokey design based on this memo has been implemented, tested, and documented in the NTP version 4 (NTPv4) software distribution for the Unix, Windows, and Virtual Memory System (VMS) operating systems at http://www.ntp.org. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5906.
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Table of Contents 1. Introduction ....................................................4 2. NTP Security Model ..............................................4 3. Approach ........................................................7 4. Autokey Cryptography ............................................8 5. Autokey Protocol Overview ......................................12 6. NTP Secure Groups ..............................................14 7. Identity Schemes ...............................................19 8. Timestamps and Filestamps ......................................20 9. Autokey Operations .............................................22 10. Autokey Protocol Messages .....................................23 10.1. No-Operation .............................................26 10.2. Association Message (ASSOC) ..............................26 10.3. Certificate Message (CERT) ...............................26 10.4. Cookie Message (COOKIE) ..................................27 10.5. Autokey Message (AUTO) ...................................27 10.6. Leapseconds Values Message (LEAP) ........................27 10.7. Sign Message (SIGN) ......................................27 10.8. Identity Messages (IFF, GQ, MV) ..........................27 11. Autokey State Machine .........................................28 11.1. Status Word ..............................................28 11.2. Host State Variables .....................................30 11.3. Client State Variables (all modes) .......................33 11.4. Protocol State Transitions ...............................34 11.4.1. Server Dance ......................................34 11.4.2. Broadcast Dance ...................................35 11.4.3. Symmetric Dance ...................................36 11.5. Error Recovery ...........................................37 12. Security Considerations .......................................39 12.1. Protocol Vulnerability ...................................39 12.2. Clogging Vulnerability ...................................40 13. IANA Considerations ...........................................42 14. References ....................................................42 14.1. Normative References .....................................42 14.2. Informative References ...................................43 Appendix A. Timestamps, Filestamps, and Partial Ordering .........45 Appendix B. Identity Schemes .....................................46 Appendix C. Private Certificate (PC) Scheme ......................47 Appendix D. Trusted Certificate (TC) Scheme ......................47 Appendix E. Schnorr (IFF) Identity Scheme ........................48 Appendix F. Guillard-Quisquater (GQ) Identity Scheme .............49 Appendix G. Mu-Varadharajan (MV) Identity Scheme .................51 Appendix H. ASN.1 Encoding Rules .................................54 Appendix I. COOKIE Request, IFF Response, GQ Response, MV Response .............................................54 Appendix J. Certificates .........................................55
1. Introduction A distributed network service requires reliable, ubiquitous, and survivable provisions to prevent accidental or malicious attacks on the servers and clients in the network or the values they exchange. Reliability requires that clients can determine that received packets are authentic; that is, were actually sent by the intended server and not manufactured or modified by an intruder. Ubiquity requires that a client can verify the authenticity of a server using only public information. Survivability requires protection from faulty implementations, improper operation, and possibly malicious clogging and replay attacks. This memo describes a cryptographically sound and efficient methodology for use in the Network Time Protocol (NTP) [RFC5905]. The various key agreement schemes [RFC4306][RFC2412][RFC2522] proposed require per-association state variables, which contradicts the principles of the remote procedure call (RPC) paradigm in which servers keep no state for a possibly large client population. An evaluation of the PKI model and algorithms, e.g., as implemented in the OpenSSL library, leads to the conclusion that any scheme requiring every NTP packet to carry a PKI digital signature would result in unacceptably poor timekeeping performance. The Autokey protocol is based on a combination of PKI and a pseudo- random sequence generated by repeated hashes of a cryptographic value involving both public and private components. This scheme has been implemented, tested, and deployed in the Internet of today. A detailed description of the security model, design principles, and implementation is presented in this memo. This informational document describes the NTP extensions for Autokey as implemented in an NTPv4 software distribution available from http://www.ntp.org. This description is provided to offer a basis for future work and a reference for the software release. This document also describes the motivation for the extensions within the protocol. 2. NTP Security Model NTP security requirements are even more stringent than most other distributed services. First, the operation of the authentication mechanism and the time synchronization mechanism are inextricably intertwined. Reliable time synchronization requires cryptographic keys that are valid only over designated time intervals; but, time intervals can be enforced only when participating servers and clients are reliably synchronized to UTC. In addition, the NTP subnet is
hierarchical by nature, so time and trust flow from the primary servers at the root through secondary servers to the clients at the leaves. A client can claim authentic to dependent applications only if all servers on the path to the primary servers are bona fide authentic. In order to emphasize this requirement, in this memo, the notion of "authentic" is replaced by "proventic", an adjective new to English and derived from "provenance", as in the provenance of a painting. Having abused the language this far, the suffixes fixable to the various derivatives of authentic will be adopted for proventic as well. In NTP, each server authenticates the next-lower stratum servers and proventicates (authenticates by induction) the lowest stratum (primary) servers. Serious computer linguists would correctly interpret the proventic relation as the transitive closure of the authentic relation. It is important to note that the notion of proventic does not necessarily imply the time is correct. An NTP client mobilizes a number of concurrent associations with different servers and uses a crafted agreement algorithm to pluck truechimers from the population possibly including falsetickers. A particular association is proventic if the server certificate and identity have been verified by the means described in this memo. However, the statement "the client is synchronized to proventic sources" means that the system clock has been set using the time values of one or more proventic associations and according to the NTP mitigation algorithms. Over the last several years, the IETF has defined and evolved the IPsec infrastructure for privacy protection and source authentication in the Internet. The infrastructure includes the Encapsulating Security Payload (ESP) [RFC4303] and Authentication Header (AH) [RFC4302] for IPv4 and IPv6. Cryptographic algorithms that use these headers for various purposes include those developed for the PKI, including various message digest, digital signature, and key agreement algorithms. This memo takes no position on which message digest or digital signature algorithm is used. This is established by a profile for each community of users. It will facilitate the discussion in this memo to refer to the reference implementation available at http://www.ntp.org. It includes Autokey as described in this memo and is available to the general public; however, it is not part of the specification itself. The cryptographic means used by the reference implementation and its user community are based on the OpenSSL cryptographic software library available at http://www.openssl.org, but other libraries with equivalent functionality could be used as well. It is important for
distribution and export purposes that the way in which these algorithms are used precludes encryption of any data other than incidental to the construction of digital signatures. The fundamental assumption in NTP about the security model is that packets transmitted over the Internet can be intercepted by those other than the intended recipient, remanufactured in various ways, and replayed in whole or part. These packets can cause the client to believe or produce incorrect information, cause protocol operations to fail, interrupt network service, or consume precious network and processor resources. In the case of NTP, the assumed goal of the intruder is to inject false time values, disrupt the protocol or clog the network, servers, or clients with spurious packets that exhaust resources and deny service to legitimate applications. The mission of the algorithms and protocols described in this memo is to detect and discard spurious packets sent by someone other than the intended sender or sent by the intended sender, but modified or replayed by an intruder. There are a number of defense mechanisms already built in the NTP architecture, protocol, and algorithms. The on-wire timestamp exchange scheme is inherently resistant to spoofing, packet-loss, and replay attacks. The engineered clock filter, selection, and clustering algorithms are designed to defend against evil cliques of Byzantine traitors. While not necessarily designed to defeat determined intruders, these algorithms and accompanying sanity checks have functioned well over the years to deflect improperly operating but presumably friendly scenarios. However, these mechanisms do not securely identify and authenticate servers to clients. Without specific further protection, an intruder can inject any or all of the following attacks. 1. An intruder can intercept and archive packets forever, as well as all the public values ever generated and transmitted over the net. 2. An intruder can generate packets faster than the server, network, or client can process them, especially if they require expensive cryptographic computations. 3. In a wiretap attack, the intruder can intercept, modify, and replay a packet. However, it cannot permanently prevent onward transmission of the original packet; that is, it cannot break the wire, only tell lies and congest it. Except in the unlikely cases considered in Section 12, the modified packet cannot arrive at the victim before the original packet, nor does it have the server private keys or identity parameters.
4. In a man-in-the-middle or masquerade attack, the intruder is positioned between the server and client, so it can intercept, modify, and replay a packet and prevent onward transmission of the original packet. Except in unlikely cases considered in Section 12, the middleman does not have the server private keys. The NTP security model assumes the following possible limitations. 1. The running times for public key algorithms are relatively long and highly variable. In general, the performance of the time synchronization function is badly degraded if these algorithms must be used for every NTP packet. 2. In some modes of operation, it is not feasible for a server to retain state variables for every client. It is however feasible to regenerated them for a client upon arrival of a packet from that client. 3. The lifetime of cryptographic values must be enforced, which requires a reliable system clock. However, the sources that synchronize the system clock must be cryptographically proventicated. This circular interdependence of the timekeeping and proventication functions requires special handling. 4. Client security functions must involve only public values transmitted over the net. Private values must never be disclosed beyond the machine on which they were created, except in the case of a special trusted agent (TA) assigned for this purpose. Unlike the Secure Shell (SSH) security model, where the client must be securely authenticated to the server, in NTP, the server must be securely authenticated to the client. In SSH, each different interface address can be bound to a different name, as returned by a reverse-DNS query. In this design, separate public/private key pairs may be required for each interface address with a distinct name. A perceived advantage of this design is that the security compartment can be different for each interface. This allows a firewall, for instance, to require some interfaces to authenticate the client and others not. 3. Approach The Autokey protocol described in this memo is designed to meet the following objectives. In-depth discussions on these objectives is in the web briefings and will not be elaborated in this memo. Note that here, and elsewhere in this memo, mention of broadcast mode means multicast mode as well, with exceptions as noted in the NTP software documentation [RFC5905].
1. It must interoperate with the existing NTP architecture model and protocol design. In particular, it must support the symmetric key scheme described in [RFC1305]. As a practical matter, the reference implementation must use the same internal key management system, including the use of 32-bit key IDs and existing mechanisms to store, activate, and revoke keys. 2. It must provide for the independent collection of cryptographic values and time values. An NTP packet is accepted for processing only when the required cryptographic values have been obtained and verified and the packet has passed all header sanity checks. 3. It must not significantly degrade the potential accuracy of the NTP synchronization algorithms. In particular, it must not make unreasonable demands on the network or host processor and memory resources. 4. It must be resistant to cryptographic attacks, specifically those identified in the security model above. In particular, it must be tolerant of operational or implementation variances, such as packet loss or disorder, or suboptimal configurations. 5. It must build on a widely available suite of cryptographic algorithms, yet be independent of the particular choice. In particular, it must not require data encryption other than that which is incidental to signature and cookie encryption operations. 6. It must function in all the modes supported by NTP, including server, symmetric, and broadcast modes. 4. Autokey Cryptography Autokey cryptography is based on the PKI algorithms commonly used in the Secure Shell and Secure Sockets Layer (SSL) applications. As in these applications, Autokey uses message digests to detect packet modification, digital signatures to verify credentials, and public certificates to provide traceable authority. What makes Autokey cryptography unique is the way in which these algorithms are used to deflect intruder attacks while maintaining the integrity and accuracy of the time synchronization function. Autokey, like many other remote procedure call (RPC) protocols, depends on message digests for basic authentication; however, it is important to understand that message digests are also used by NTP when Autokey is not available or not configured. Selection of the digest algorithm is a function of NTP configuration and is transparent to Autokey.
The protocol design and reference implementation support both 128-bit and 160-bit message digest algorithms, each with a 32-bit key ID. In order to retain backwards compatibility with NTPv3, the NTPv4 key ID space is partitioned in two subspaces at a pivot point of 65536. Symmetric key IDs have values less than the pivot and indefinite lifetime. Autokey key IDs have pseudo-random values equal to or greater than the pivot and are expunged immediately after use. Both symmetric key and public key cryptography authenticate as shown in Figure 1. The server looks up the key associated with the key ID and calculates the message digest from the NTP header and extension fields together with the key value. The key ID and digest form the message authentication code (MAC) included with the message. The client does the same computation using its local copy of the key and compares the result with the digest in the MAC. If the values agree, the message is assumed authentic. +------------------+ | NTP Header and | | Extension Fields | +------------------+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Message Authentication Code | \|/ \|/ + (MAC) + ******************** | +-------------------------+ | * Compute Hash *<----| Key ID | Message Digest | + ******************** | +-------------------------+ | | +-+-+-+-+-+-+-|-+-+-+-+-+-+-+-+-+ \|/ \|/ +------------------+ +-------------+ | Message Digest |------>| Compare | +------------------+ +-------------+ Figure 1: Message Authentication Autokey uses specially contrived session keys, called autokeys, and a precomputed pseudo-random sequence of autokeys that are saved in the autokey list. The Autokey protocol operates separately for each association, so there may be several autokey sequences operating independently at the same time. +-------------+-------------+--------+--------+ | Src Address | Dst Address | Key ID | Cookie | +-------------+-------------+--------+--------+ Figure 2: NTPv4 Autokey
An autokey is computed from four fields in network byte order as shown in Figure 2. The four values are hashed using the MD5 algorithm to produce the 128-bit autokey value, which in the reference implementation is stored along with the key ID in a cache used for symmetric keys as well as autokeys. Keys are retrieved from the cache by key ID using hash tables and a fast lookup algorithm. For use with IPv4, the Src Address and Dst Address fields contain 32 bits; for use with IPv6, these fields contain 128 bits. In either case, the Key ID and Cookie fields contain 32 bits. Thus, an IPv4 autokey has four 32-bit words, while an IPv6 autokey has ten 32-bit words. The source and destination addresses and key ID are public values visible in the packet, while the cookie can be a public value or shared private value, depending on the NTP mode. The NTP packet format has been augmented to include one or more extension fields piggybacked between the original NTP header and the MAC. For packets without extension fields, the cookie is a shared private value. For packets with extension fields, the cookie has a default public value of zero, since these packets are validated independently using digital signatures. There are some scenarios where the use of endpoint IP addresses may be difficult or impossible. These include configurations where network address translation (NAT) devices are in use or when addresses are changed during an association lifetime due to mobility constraints. For Autokey, the only restriction is that the address fields that are visible in the transmitted packet must be the same as those used to construct the autokey list and that these fields be the same as those visible in the received packet. (The use of alternative means, such as Autokey host names (discussed later) or hashes of these names may be a topic for future study.)
+-----------+-----------+------+------+ +---------+ +-----+------+ |Src Address|Dst Address|Key ID|Cookie|-->| | |Final|Final | +-----------+-----------+------+------+ | Session | |Index|Key ID| | | | | | Key ID | +-----+------+ \|/ \|/ \|/ \|/ | List | | | ************************************* +---------+ \|/ \|/ * COMPUTE HASH * ******************* ************************************* *COMPUTE SIGNATURE* | Index n ******************* \|/ | +--------+ | | Next | \|/ | Key ID | +-----------+ +--------+ | Signature | Index n+1 +-----------+ Figure 3: Constructing the Key List Figure 3 shows how the autokey list and autokey values are computed. The key IDs used in the autokey list consist of a sequence starting with a random 32-bit nonce (autokey seed) greater than or equal to the pivot as the first key ID. The first autokey is computed as above using the given cookie and autokey seed and assigned index 0. The first 32 bits of the result in network byte order become the next key ID. The MD5 hash of the autokey is the key value saved in the key cache along with the key ID. The first 32 bits of the key become the key ID for the next autokey assigned index 1. Operations continue to generate the entire list. It may happen that a newly generated key ID is less than the pivot or collides with another one already generated (birthday event). When this happens, which occurs only rarely, the key list is terminated at that point. The lifetime of each key is set to expire one poll interval after its scheduled use. In the reference implementation, the list is terminated when the maximum key lifetime is about one hour, so for poll intervals above one hour, a new key list containing only a single entry is regenerated for every poll.
+------------------+ | NTP Header and | | Extension Fields | +------------------+ | | \|/ \|/ +---------+ **************** +--------+ | Session | * COMPUTE HASH *<---| Key ID |<---| Key ID | **************** +--------+ | List | | | +---------+ \|/ \|/ +-----------------------------------+ | Message Authentication Code (MAC) | +-----------------------------------+ Figure 4: Transmitting Messages The index of the last autokey in the list is saved along with the key ID for that entry, collectively called the autokey values. The autokey values are then signed for use later. The list is used in reverse order as shown in Figure 4, so that the first autokey used is the last one generated. The Autokey protocol includes a message to retrieve the autokey values and verify the signature, so that subsequent packets can be validated using one or more hashes that eventually match the last key ID (valid) or exceed the index (invalid). This is called the autokey test in the following and is done for every packet, including those with and without extension fields. In the reference implementation, the most recent key ID received is saved for comparison with the first 32 bits in network byte order of the next following key value. This minimizes the number of hash operations in case a single packet is lost. 5. Autokey Protocol Overview The Autokey protocol includes a number of request/response exchanges that must be completed in order. In each exchange, a client sends a request message with data and expects a server response message with data. Requests and responses are contained in extension fields, one request or response in each field, as described later. An NTP packet can contain one request message and one or more response messages. The following is a list of these messages. o Parameter exchange. The request includes the client host name and status word; the response includes the server host name and status word. The status word specifies the digest/signature scheme to use and the identity schemes supported.
o Certificate exchange. The request includes the subject name of a certificate; the response consists of a signed certificate with that subject name. If the issuer name is not the same as the subject name, it has been signed by a host one step closer to a trusted host, so certificate retrieval continues for the issuer name. If it is trusted and self-signed, the trail concludes at the trusted host. If nontrusted and self-signed, the host certificate has not yet been signed, so the trail temporarily loops. Completion of this exchange lights the VAL bit as described below. o Identity exchange. The certificate trail is generally not considered sufficient protection against man-in-the-middle attacks unless additional protection such as the proof-of-possession scheme described in [RFC2875] is available, but this is expensive and requires servers to retain state. Autokey can use one of the challenge/response identity schemes described in Appendix B. Completion of this exchange lights the IFF bit as described below. o Cookie exchange. The request includes the public key of the server. The response includes the server cookie encrypted with this key. The client uses this value when constructing the key list. Completion of this exchange lights the COOK bit as described below. o Autokey exchange. The request includes either no data or the autokey values in symmetric modes. The response includes the autokey values of the server. These values are used to verify the autokey sequence. Completion of this exchange lights the AUT bit as described below. o Sign exchange. This exchange is executed only when the client has synchronized to a proventic source. The request includes the self-signed client certificate. The server acting as certification authority (CA) interprets the certificate as a X.509v3 certificate request. It extracts the subject, issuer, and extension fields, builds a new certificate with these data along with its own serial number and expiration time, then signs it using its own private key and includes it in the response. The client uses the signed certificate in its own role as server for dependent clients. Completion of this exchange lights the SIGN bit as described below. o Leapseconds exchange. This exchange is executed only when the client has synchronized to a proventic source. This exchange occurs when the server has the leapseconds values, as indicated in the host status word. If so, the client requests the values and compares them with its own values, if available. If the server
values are newer than the client values, the client replaces its own with the server values. The client, acting as server, can now provide the most recent values to its dependent clients. In symmetric mode, this results in both peers having the newest values. Completion of this exchange lights the LPT bit as described below. Once the certificates and identity have been validated, subsequent packets are validated by digital signatures and the autokey sequence. The association is now proventic with respect to the downstratum trusted host, but is not yet selectable to discipline the system clock. The associations accumulate time values, and the mitigation algorithms continue in the usual way. When these algorithms have culled the falsetickers and cluster outliers and at least three survivors remain, the system clock has been synchronized to a proventic source. The time values for truechimer sources form a proventic partial ordering relative to the applicable signature timestamps. This raises the interesting issue of how to differentiate between the timestamps of different associations. It might happen, for instance, that the timestamp of some Autokey message is ahead of the system clock by some presumably small amount. For this reason, timestamp comparisons between different associations and between associations and the system clock are avoided, except in the NTP intersection and clustering algorithms and when determining whether a certificate has expired. 6. NTP Secure Groups NTP secure groups are used to define cryptographic compartments and security hierarchies. A secure group consists of a number of hosts dynamically assembled as a forest with roots the trusted hosts (THs) at the lowest stratum of the group. The THs do not have to be, but often are, primary (stratum 1) servers. A trusted authority (TA), not necessarily a group host, generates private identity keys for servers and public identity keys for clients at the leaves of the forest. The TA deploys the server keys to the THs and other designated servers using secure means and posts the client keys on a public web site. For Autokey purposes, all hosts belonging to a secure group have the same group name but different host names, not necessarily related to the DNS names. The group name is used in the subject and issuer fields of the TH certificates; the host name is used in these fields for other hosts. Thus, all host certificates are self-signed. During the use of the Autokey protocol, a client requests that the server sign its certificate and caches the result. A certificate
trail is constructed by each host, possibly via intermediate hosts and ending at a TH. Thus, each host along the trail retrieves the entire trail from its server(s) and provides this plus its own signed certificates to its clients. Secure groups can be configured as hierarchies where a TH of one group can be a client of one or more other groups operating at a lower stratum. In one scenario, THs for groups RED and GREEN can be cryptographically distinct, but both be clients of group BLUE operating at a lower stratum. In another scenario, THs for group CYAN can be clients of multiple groups YELLOW and MAGENTA, both operating at a lower stratum. There are many other scenarios, but all must be configured to include only acyclic certificate trails. In Figure 5, the Alice group consists of THs Alice, which is also the TA, and Carol. Dependent servers Brenda and Denise have configured Alice and Carol, respectively, as their time sources. Stratum 3 server Eileen has configured both Brenda and Denise as her time sources. Public certificates are identified by the subject and signed by the issuer. Note that the server group keys have been previously installed on Brenda and Denise and the client group keys installed on all machines.
+-------------+ +-------------+ +-------------+ | Alice Group | | Brenda | | Denise | | Alice | | | | | | +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ | Certificate | | Alice | | | | Brenda| | | | Denise| | +-+-+-+-+-+ | +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ | | Subject | | | Alice*| 1 | | | Alice | 4 | | | Carol | 4 | +-+-+-+-+-+ | +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ | | Issuer | S | | | | | | +-+-+-+-+-+ | +=======+ | | +-+-+-+-+ | | +-+-+-+-+ | | ||Alice|| 3 | | | Alice | | | | Carol | | Group Key | +=======+ | | +-+-+-+-+ | | +-+-+-+-+ | +=========+ +-------------+ | | Alice*| 2 | | | Carol*| 2 | || Group || S | Alice Group | | +-+-+-+-+ | | +-+-+-+-+ | +=========+ | Carol | | | | | | +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ | S = step | | Carol | | | | Brenda| | | | Denise| | * = trusted | +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ | | | Carol*| 1 | | | Brenda| 1 | | | Denise| 1 | | +-+-+-+-+ | | +-+-+-+-+ | | +-+-+-+-+ | | | | | | | | +=======+ | | +=======+ | | +=======+ | | ||Alice|| 3 | | ||Alice|| 3 | | ||Alice|| 3 | | +=======+ | | +=======+ | | +=======+ | +-------------+ +-------------+ +-------------+ Stratum 1 Stratum 2
+---------------------------------------------+ | Eileen | | | | +-+-+-+-+ +-+-+-+-+ | | | Eileen| | Eileen| | | +-+-+-+-+ +-+-+-+-+ | | | Brenda| 4 | Carol | 4 | | +-+-+-+-+ +-+-+-+-+ | | | | +-+-+-+-+ +-+-+-+-+ | | | Alice | | Carol | | | +-+-+-+-+ +-+-+-+-+ | | | Alice*| 2 | Carol*| 2 | | +-+-+-+-+ +-+-+-+-+ | | | | +-+-+-+-+ +-+-+-+-+ | | | Brenda| | Denise| | | +-+-+-+-+ +-+-+-+-+ | | | Alice | 2 | Carol | 2 | | +-+-+-+-+ +-+-+-+-+ | | | | +-+-+-+-+ | | | Eileen| | | +-+-+-+-+ | | | Eileen| 1 | | +-+-+-+-+ | | | | +=======+ | | ||Alice|| 3 | | +=======+ | +---------------------------------------------+ Stratum 3 Figure 5: NTP Secure Groups The steps in hiking the certificate trails and verifying identity are as follows. Note the step number in the description matches the step number in the figure. 1. The girls start by loading the host key, sign key, self-signed certificate, and group key. Each client and server acting as a client starts the Autokey protocol by retrieving the server host name and digest/signature. This is done using the ASSOC exchange described later. 2. They continue to load certificates recursively until a self- signed trusted certificate is found. Brenda and Denise immediately find trusted certificates for Alice and Carol,
respectively, but Eileen will loop because neither Brenda nor Denise have their own certificates signed by either Alice or Carol. This is done using the CERT exchange described later. 3. Brenda and Denise continue with the selected identity schemes to verify that Alice and Carol have the correct group key previously generated by Alice. This is done using one of the identity schemes IFF, GQ, or MV, described later. If this succeeds, each continues in step 4. 4. Brenda and Denise present their certificates for signature using the SIGN exchange described later. If this succeeds, either one of or both Brenda and Denise can now provide these signed certificates to Eileen, which may be looping in step 2. Eileen can now verify the trail via either Brenda or Denise to the trusted certificates for Alice and Carol. Once this is done, Eileen can complete the protocol just as Brenda and Denise did. For various reasons, it may be convenient for a server to have client keys for more than one group. For example, Figure 6 shows three secure groups Alice, Helen, and Carol arranged in a hierarchy. Hosts A, B, C, and D belong to Alice with A and B as her THs. Hosts R and S belong to Helen with R as her TH. Hosts X and Y belong to Carol with X as her TH. Note that the TH for a group is always the lowest stratum and that the hosts of the combined groups form an acyclic graph. Note also that the certificate trail for each group terminates on a TH for that group. ***** ***** @@@@@ Stratum 1 * A * * B * @ R @ ***** ***** @@@@@ \ / / \ / / ***** @@@@@ ********* 2 * C * @ S @ * Alice * ***** @@@@@ ********* / \ / / \ / @@@@@@@@@ ***** ##### @ Helen @ 3 * D * # X # @@@@@@@@@ ***** ##### / \ ######### / \ # Carol # ##### ##### ######### 4 # Y # # Z # ##### ##### Figure 6: Hierarchical Overlapping Groups
The intent of the scenario is to provide security separation, so that servers cannot masquerade as clients in other groups and clients cannot masquerade as servers. Assume, for example, that Alice and Helen belong to national standards laboratories and their server keys are used to confirm identity between members of each group. Carol is a prominent corporation receiving standards products and requiring cryptographic authentication. Perhaps under contract, host X belonging to Carol has client keys for both Alice and Helen and server keys for Carol. The Autokey protocol operates for each group separately while preserving security separation. Host X can prove identity in Carol to clients Y and Z, but cannot prove to anybody that it belongs to either Alice or Helen.