Roughtime
draft-ietf-ntp-roughtime-12
| Document | Type | Active Internet-Draft (ntp WG) | |
|---|---|---|---|
| Authors | Watson Ladd , Marcus Dansarie | ||
| Last updated | 2024-12-04 | ||
| Replaces | draft-roughtime-aanchal, draft-ietf-ntp-roughtime-ecosystem | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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draft-ietf-ntp-roughtime-12
Network Time Protocols W. Ladd
Internet-Draft Akamai Technologies
Intended status: Informational M. Dansarie
Expires: 7 June 2025 Netnod
4 December 2024
Roughtime
draft-ietf-ntp-roughtime-12
Abstract
This document describes Roughtime—a protocol that aims to achieve two
things: secure rough time synchronization even for clients without
any idea of what time it is, and giving clients a format by which to
report any inconsistencies they observe between time servers. This
document specifies the on-wire protocol required for these goals, and
discusses aspects of the ecosystem needed for it to work.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-ntp-roughtime/.
Source for this draft and an issue tracker can be found at
https://github.com/wbl/roughtime-draft.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on 7 June 2025.
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Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Single Server Mode . . . . . . . . . . . . . . . . . . . 4
3.2. Multi Server Mode . . . . . . . . . . . . . . . . . . . . 4
4. Message Format . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Data types . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.1. uint32 . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.2. uint64 . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.3. Tag . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.4. Timestamp . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Header . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Requests . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1.1. VER . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1.2. NONC . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1.3. SRV . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1.4. ZZZZ . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Responses . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2.1. SIG . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.2. NONC . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.3. PATH . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.4. SREP . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.5. CERT . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2.6. INDX . . . . . . . . . . . . . . . . . . . . . . . . 11
5.3. The Merkle Tree . . . . . . . . . . . . . . . . . . . . . 12
5.3.1. Root Value Validity Check Algorithm . . . . . . . . . 12
5.4. Validity of Response . . . . . . . . . . . . . . . . . . 13
6. Integration into NTP . . . . . . . . . . . . . . . . . . . . 13
7. Grease . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8. Roughtime Clients . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Necessary configuration . . . . . . . . . . . . . . . . . 14
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8.2. Measurement Sequence . . . . . . . . . . . . . . . . . . 14
8.3. Server Lists . . . . . . . . . . . . . . . . . . . . . . 15
8.4. Malfeasance Reporting . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9.1. Confidentiality . . . . . . . . . . . . . . . . . . . . . 17
9.2. Integrity and Authenticity . . . . . . . . . . . . . . . 17
9.3. Generating Private Keys . . . . . . . . . . . . . . . . . 17
9.4. Private Key Compromise . . . . . . . . . . . . . . . . . 17
9.5. Quantum Resistance . . . . . . . . . . . . . . . . . . . 17
9.6. Maintaining Lists of Servers . . . . . . . . . . . . . . 18
9.7. Amplification Attacks . . . . . . . . . . . . . . . . . . 18
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18
11. Operational Considerations . . . . . . . . . . . . . . . . . 18
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
12.1. Service Name and Transport Protocol Port Number
Registry . . . . . . . . . . . . . . . . . . . . . . . . 18
12.2. Roughtime Version Registry . . . . . . . . . . . . . . . 19
12.3. Roughtime Tag Registry . . . . . . . . . . . . . . . . . 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
13.1. Normative References . . . . . . . . . . . . . . . . . . 21
13.2. Informative References . . . . . . . . . . . . . . . . . 23
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
Time synchronization is essential to Internet security as many
security protocols and other applications require synchronization
[RFC738]. Unfortunately, widely deployed protocols such as the
Network Time Protocol (NTP) [RFC5905] lack essential security
features, and even newer protocols like Network Time Security (NTS)
[RFC8915] lack mechanisms to observe that the servers behave
correctly. Furthermore, clients may lack even a basic idea of the
time, creating bootstrapping problems.
The primary design goal of Roughtime is to permit devices to obtain a
rough idea of the current time from fairly static configuration
consisting of a key and a server, and to enable them to report any
inconsistencies they observe between time servers.
This memo is limited to describing the Roughtime on-wire protocol.
Apart from describing the server list and malfeasance report formats,
this memo does not describe the ecosystem required for maintaining
lists of trusted servers and processing malfeasance reports.
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2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Protocol Overview
Roughtime is a protocol for authenticated rough time synchronization
that enables clients to provide cryptographic proof of server
malfeasance. It does so by having responses from servers include a
signature over a value derived from the client's request, which
includes a nonce. This provides cryptographic proof that the
timestamp was issued after the server received the client's request.
The derived value included in the server's response is the root of a
Merkle tree [Merkle] which includes the hash value of the client's
request as the value of one of its leaf nodes. This enables the
server to amortize the relatively costly signing operation over a
number of client requests.
3.1. Single Server Mode
At its most basic level, Roughtime is a one round protocol in which a
completely fresh client requests the current time and the server
sends a signed response. The response includes a timestamp and a
radius used to indicate the server's certainty about the reported
time.
The client's request contains a nonce which the server incorporates
into its signed response. The client can verify the server's
signatures and—provided that the nonce has sufficient entropy—this
proves that the signed response could only have been generated after
the nonce.
3.2. Multi Server Mode
When using multiple servers, a client can detect, cryptographically
prove, and report inconsistencies between different servers.
A Roughtime server guarantees that the timestamp included in the
response to a query is generated after the reception of the query and
prior to the transmission of the associated response. If the time
response from a server is not consistent with time responses from
other servers, this indicates server error or intentional malfeasance
that can be reported and potentially used to impeach the server.
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Proofs of malfeasance are constructed by chaining requests to
different Roughtime servers. Details on proofs and malfeasance
reporting are provided in Section 8. For the reporting to result in
impeachment, an additional mechanism is required that provides a
review and impeachment process. Defining such a mechanism is beyond
the scope of this document. A simple option could be an online forum
where a court of human observers judge cases after reviewing input
reports.
4. Message Format
Roughtime messages are maps consisting of one or more (tag, value)
pairs. They start with a header, which contains the number of pairs,
the tags, and value offsets. The header is followed by a message
values section which contains the values associated with the tags in
the header. Messages MUST be formatted according to Figure 1 as
described in the following sections.
Messages MAY be recursive, i.e. the value of a tag can itself be a
Roughtime message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of pairs (uint32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. N-1 offsets (uint32) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. N tags (uint32) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. N Values .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Roughtime Message
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4.1. Data types
4.1.1. uint32
A uint32 is a 32 bit unsigned integer. It is serialized with the
least significant byte first.
4.1.2. uint64
A uint64 is a 64 bit unsigned integer. It is serialized with the
least significant byte first.
4.1.3. Tag
Tags are used to identify values in Roughtime messages. A tag is a
uint32 but can also be represented as a sequence of up to four ASCII
characters [RFC20] with the first character in the most significant
byte. ASCII strings shorter than four characters can be
unambiguously converted to tags by padding them with zero bytes.
Tags MUST NOT contain any other bytes than capital letters (A-Z) or
padding zero bytes. For example, the ASCII string "NONC" would
correspond to the tag 0x434e4f4e and "VER" would correspond to
0x00524556.
4.1.4. Timestamp
A timestamp is a representation of UTC time as a uint64 count of
seconds since 00:00:00 on 1 January 1970 (the Unix epoch), assuming
every day has 86400 seconds. This is a constant offset from the NTP
timestamp in seconds. Leap seconds do not have an unambiguous
representation in a timestamp, and this has implications for the
attainable accuracy and setting of the RADI tag.
4.2. Header
All Roughtime messages start with a header. The first four bytes of
the header is the uint32 number of tags N, and hence of (tag, value)
pairs.
The following 4*(N-1) bytes are offsets, each a uint32. The last 4*N
bytes in the header are tags. Offsets refer to the positions of the
values in the message values section. All offsets MUST be multiples
of four and placed in increasing order. The first post-header byte
is at offset 0. The offset array is considered to have a not
explicitly encoded value of 0 as its zeroth entry.
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The value associated with the ith tag begins at offset[i] and ends at
offset[i+1]-1, with the exception of the last value which ends at the
end of the message. Values may have zero length. All lengths and
offsets are in bytes.
Tags MUST be listed in the same order as the offsets of their values
and be sorted in ascending order by numeric value. A tag MUST NOT
appear more than once in a header.
5. Protocol Details
As described in Section 3, clients initiate time synchronization by
sending requests containing a nonce to servers who send signed time
responses in return. Roughtime packets can be sent between clients
and servers either as UDP datagrams or via TCP streams. Servers
SHOULD support both the UDP and TCP transport modes.
A Roughtime packet MUST be formatted according to Figure 2 and as
described here. The first field is a uint64 with the value
0x4d49544847554f52 ("ROUGHTIM" in ASCII). The second field is a
uint32 and contains the length of the third field. The third and
last field contains a Roughtime message as specified in Section 4.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x4d49544847554f52 (uint64) |
| ("ROUGHTIM") |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message length (uint32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Roughtime message .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Roughtime packet
Roughtime request and response packets MUST be transmitted in a
single datagram when the UDP transport mode is used. Setting the
packet's don't fragment bit [RFC791] is OPTIONAL in IPv4 networks.
Multiple requests and responses can be exchanged over an established
TCP connection. Clients MAY send multiple requests at once and
servers MAY send responses out of order. The connection SHOULD be
closed by the client when it has no more requests to send and has
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received all expected responses. Either side SHOULD close the
connection in response to synchronization, format, implementation-
defined timeouts, or other errors.
All requests and responses MUST contain the VER tag. It contains a
list of one or more uint32 version numbers. The version of Roughtime
specified by this memo has version number 1.
NOTE TO RFC EDITOR: remove this paragraph before publication. For
testing this draft of the memo, a version number of 0x8000000c is
used.
5.1. Requests
A request MUST contain the tags VER and NONC. It SHOULD include the
tag SRV. Other tags SHOULD be ignored by the server. A future
version of this protocol may mandate additional tags in the message
and assign them semantic meaning.
The size of the request message SHOULD be at least 1024 bytes when
the UDP transport mode is used. To attain this size the ZZZZ tag
SHOULD be added to the message. Responding to requests shorter than
1024 bytes is OPTIONAL and servers MUST NOT send responses larger
than the requests they are replying to, see Section 9.7.
5.1.1. VER
In a request, the VER tag contains a list of uint32 version numbers.
The VER tag MUST include at least one Roughtime version supported by
the client. The client MUST ensure that the version numbers and tags
included in the request are not incompatible with each other or the
packet contents.
The version numbers MUST NOT repeat and MUST be sorted in ascending
numerical order.
Servers SHOULD ignore any unknown version numbers in the list
supplied by the client. If the list contains no version numbers
supported by the server, it MAY respond with another version or
ignore the request entirely, see Section 5.2.4.
5.1.2. NONC
The value of the NONC tag is a 32-byte nonce. It SHOULD be generated
in a manner indistinguishable from random. BCP 106 [RFC4086]
contains specific guidelines regarding this.
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5.1.3. SRV
The SRV tag is used by the client to indicate which long-term public
key it expects to verify the response with. The value of the SRV tag
is H(0xff || public_key) where public_key is the server's long-term,
32-byte Ed25519 public key and H is SHA-512 truncated to the first 32
bytes.
5.1.4. ZZZZ
The ZZZZ tag is used to expand the response to the minimum required
length. Its value MUST be all zero bytes.
5.2. Responses
The server begins the request handling process with a set of long-
term keys. It resolves which long-term key to use with the following
procedure:
1. If the request contains a SRV tag, then the server looks up the
long-term key indicated by the SRV value. If no such key exists,
then the server MUST ignore the request.
2. If the request contains no SRV tag, but the server has just one
long-term key, it SHOULD select that key. Otherwise, if the
server has multiple long-term keys, then it MUST ignore the
request.
A response MUST contain the tags SIG, NONC, PATH, SREP, CERT, and
INDX. The structure of a response message is illustrated in
Figure 3.
|--SIG
|--NONC
|--PATH
|--SREP
| |--VER
| |--RADI
| |--MIDP
| |--VERS
| |--ROOT
|--CERT
| |--DELE
| | |--MINT
| | |--MAXT
| | |--PUBK
| |--SIG
|--INDX
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Figure 3: Roughtime response message structure.
5.2.1. SIG
In general, a SIG tag value is a 64-byte Ed25519 signature [RFC8032]
over a concatenation of a signature context ASCII string and the
entire value of a tag. All context strings MUST include a
terminating zero byte.
The SIG tag in the root of a response MUST be a signature over the
SREP value using the public key contained in CERT. The context
string MUST be "RoughTime v1 response signature".
5.2.2. NONC
The NONC tag MUST contain the nonce of the message being responded
to.
5.2.3. PATH
The PATH tag value MUST be a multiple of 32 bytes long and represent
a path of 32-byte hash values in the Merkle tree used to generate the
ROOT value as described in a Section 5.3. In the case where a
response is prepared for a single request and the Merkle tree
contains only the root node, the size of PATH MUST be zero.
The PATH MUST NOT contain more than 32 hash values. The maximum
length of PATH is normally limited by the maximum size of the
response message, see Section 5.1 and Section 9.7. Server
implementations SHOULD select a maximum Merkle tree height (see
Section 5.3) that ensures this.
5.2.4. SREP
The SREP tag contains a time response. Its value MUST be a Roughtime
message with the tags VER, RADI, MIDP, VERS, and ROOT.
The VER tag MUST contain a single version number. It SHOULD be one
of the version numbers supplied by the client in its request. The
server MUST ensure that the version number corresponds with the rest
of the packet contents.
The RADI tag value MUST be a uint32 representing the server's
estimate of the accuracy of MIDP in seconds. Servers MUST ensure
that the true time is within (MIDP-RADI, MIDP+RADI) at the moment of
processing. The value of RADI MUST NOT be zero. Since leap seconds
can not be unambiguously represented by Roughtime timestamps, servers
MUST take this into account when setting the RADI value during leap
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second events. Servers that do not have any leap second information
SHOULD set the value of RADI to at least 3. Failure to do so will
impact the observed correctness of Roughtime servers and can lead to
malfeasance reports.
The MIDP tag value MUST be the timestamp of the moment of processing.
The VERS tag value MUST contain a list of uint32 version numbers
supported by the server, sorted in ascending numerical order. It
MUST contain the version number specified in the VER tag.
The ROOT tag MUST contain a 32-byte value of a Merkle tree root as
described in Section 5.3.
5.2.5. CERT
The CERT tag contains a public-key certificate signed with the
server's private long-term key. Its value MUST be a Roughtime
message with the tags DELE and SIG, where SIG is a signature over the
DELE value. The context string used to generate SIG MUST be
"RoughTime v1 delegation signature".
The DELE tag contains a delegated public-key certificate used by the
server to sign the SREP tag. Its value MUST be a Roughtime message
with the tags MINT, MAXT, and PUBK. The purpose of the DELE tag is
to enable separation of a long-term public key from keys on devices
exposed to the public Internet.
The MINT tag is the minimum timestamp for which the key in PUBK is
trusted to sign responses. MIDP MUST be more than or equal to MINT
for a response to be considered valid.
The MAXT tag is the maximum timestamp for which the key in PUBK is
trusted to sign responses. MIDP MUST be less than or equal to MAXT
for a response to be considered valid.
The PUBK tag MUST contain a temporary 32-byte Ed25519 public key
which is used to sign the SREP tag.
5.2.6. INDX
The INDX tag value MUST be a uint32 determining the position of NONC
in the Merkle tree used to generate the ROOT value as described in
Section 5.3.
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5.3. The Merkle Tree
A Merkle tree [Merkle] is a binary tree where the value of each non-
leaf node is a hash value derived from its two children. The root of
the tree is thus dependent on all leaf nodes.
In Roughtime, each leaf node in the Merkle tree represents one
request. Leaf nodes are indexed left to right, beginning with zero.
The values of all nodes are calculated from the leaf nodes and up
towards the root node using the first 32 bytes of the output of the
SHA-512 hash algorithm [RFC6234]. For leaf nodes, the byte 0x00 is
prepended to the full value of the client's request packet, including
the "ROUGHTIM" header, before applying the hash function. For all
other nodes, the byte 0x01 is concatenated with first the left and
then the right child node value before applying the hash function.
The value of the Merkle tree's root node is included in the ROOT tag
of the response.
The index of a request leaf node is included in the INDX tag of the
response.
The values of all sibling nodes in the path between a request leaf
node and the root node are stored in the PATH tag so that the client
can reconstruct and validate the value in the ROOT tag using its
request packet. These values are each 32 bytes and are stored one
after the other with no additional padding or structure. The order
in which they are stored is described in the next section.
5.3.1. Root Value Validity Check Algorithm
This section describes how to compute the value of the root of the
Merkle tree from the values in the tags PATH, INDX, and NONC. The
bits of INDX are ordered from least to most significant. H(x)
denotes the first 32 bytes of the SHA-512 hash digest of x and ||
denotes concatenation.
The algorithm maintains a current value h. At initialization, h is
set to H(0x00 || request_packet). When no more entries remain in
PATH, h is the value of the root of the Merkle tree. All remaining
bits of INDX MUST be zero at that time. Otherwise, let node be the
next 32 bytes in PATH. If the current bit in INDX is 0 then h =
H(0x01 || node || hash), else h = H(0x01 || hash || node).
PATH is thus the siblings from the leaf to the root.
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5.4. Validity of Response
A client MUST check the following properties when it receives a
response. We assume the long-term server public key is known to the
client through other means.
The signature in CERT was made with the long-term key of the server.
The MIDP timestamp lies in the interval specified by the MINT and
MAXT timestamps.
The INDX and PATH values prove a hash value derived from the request
packet was included in the Merkle tree with value ROOT using the
algorithm in Section 5.3.1.
The signature of SREP in SIG validates with the public key in DELE.
A response that passes these checks is said to be valid. Validity of
a response does not prove that the timestamp's value in the response
is correct, but merely that the server guarantees that it signed the
timestamp and computed its signature during the time interval (MIDP-
RADI, MIDP+RADI).
6. Integration into NTP
We assume that there is a bound PHI on the frequency error in the
clock on the machine. Let delta be the time difference between the
clock on the client and the clock on the server, and let sigma
represent the error in the measured value of delta introduced by the
measurement process.
Given a measurement taken at a local time t, we know the true time is
in (t-delta-sigma, t-delta+sigma). After d seconds have elapsed we
know the true time is within (t-delta-sigma-d_PHI,
t-delta+sigma+d_PHI). A simple and effective way to mix with NTP or
Precision Time Protocol (PTP) discipline of the clock is to trim the
observed intervals in NTP to fit entirely within this window or
reject measurements that fall to far outside. This assumes time has
not been stepped. If the NTP process decides to step the time, it
MUST use Roughtime to ensure the new truetime estimate that will be
stepped to is consistent with the true time. Should this window
become too large, another Roughtime measurement is called for. The
definition of "too large" is implementation defined. Implementations
MAY use other, more sophisticated means of adjusting the clock
respecting Roughtime information. Other applications such as X.509
verification may wish to apply different rules.
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7. Grease
The primary purpose of grease is to prevent protocol ossification,
which could prohibit future protocol extensions and development
[RFC9170]. In Roughtime, grease is also intended to ensure that
clients validate signatures. To grease the Roughtime protocol,
servers SHOULD send back a fraction of responses with any of the
following: lack of mandatory tags, version numbers not in the
request, undefined tags, or invalid signatures together with
incorrect times. Clients MUST properly ignore undefined tags and
reject invalid responses. Servers MUST NOT send back responses with
incorrect times and valid signatures. Either signature MAY be
invalid for this application.
8. Roughtime Clients
8.1. Necessary configuration
To carry out a Roughtime measurement, a client SHOULD be equipped
with a list of servers, a minimum of three of which are operational
and not run by the same parties. Roughtime clients SHOULD regularly
update their view of which servers are trustworthy in order to
benefit from the detection of misbehavior. Clients SHOULD also have
a means of reporting to the provider of such a list, such as an
operating system or software vendor, a malfeasence report as
described below.
8.2. Measurement Sequence
The client randomly selects at least three servers from the list, and
sequentially queries them. The query sequence SHOULD be repeated
twice with the servers in the same order, to ensure that all possible
inconsistences can be detected.
The first probe uses a nonce that is randomly generated. The second
query uses H(resp || rand) where rand is a random 32-byte value and
resp is the entire response to the first probe, including the
"ROUGHTIM" header. Each subsequent query uses H(resp || rand) for
the previous response and a different 32-byte rand value. H(x)
and || are defined as in Section 5.3.1.
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For each pair of responses (i, j), where i was received before j, the
client MUST check that MIDP_i-RADI_i is less than or equal to
MIDP_j+RADI_j. If all checks pass, the times are consistent with
causal ordering. If at least one check fails, there has been a
malfeasance and the client SHOULD store a report for evaluation,
alert the user, and make another measurement. If the times reported
are consistent with the causal ordering, and the delay between
request and response is within an implementation-dependent maximum
value, the measurement succeeds.
8.3. Server Lists
To facilitate regular updates of lists of trusted servers, clients
SHOULD implement the server list format specified here. Server lists
MUST be formatted as JSON [RFC8259] objects and contain the key
"servers". Client lists MAY also contain the keys "sources" and
"reports".
The value of the "servers" key MUST be a list of server objects, each
containing the keys "name", "version", "publicKeyType", "publicKey",
and "addresses".
The value of "name" MUST be a string and SHOULD contain a server name
suitable for display to a user.
The value of "version" MUST be an integer that indicates the highest
Roughtime version number supported by the server.
NOTE TO RFC EDITOR: remove this paragraph before publication. To
indicate compatibility with drafts of this memo, a decimal
representation of the version number indicated in Section 5 SHOULD be
used. For indicating compatibility with pre-IETF specifications of
Roughtime, the version number 3000600613 SHOULD be used.
The value of "publicKeyType" MUST be a string indicating the
signature scheme used by the server. The value for servers
supporting version 1 of Roughtime MUST be "ed25519".
The value of "publicKey" MUST be a base64-encoded [RFC4648] string
representing the long-term public key of the server in a format
consistent with the value of "publicKeyType".
The value of "addresses" MUST be a list of address objects. An
address object MUST contain the keys "protocol" and "address". The
value of "protocol" MUST be either "tcp" or "udp", indicating the
transport mode to use. The value of "address" MUST be string
indicating a host and a port number, separated by a colon character,
for example "roughtime.example.com:2002". The host part SHALL be
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either an IPv4 address, an IPv6 address, or a fully qualified domain
name (FQDN). IPv4 addresses MUST be in dotted decimal notation.
IPv6 addresses MUST conform to the "Text Representation of Addresses"
[RFC4291] and MUST NOT include zone identifiers [RFC6874]. The port
part SHALL be a decimal integer representing a valid port number,
i.e. in the range 0-65535.
The value of "sources", if present, MUST be a list of strings
indicating where updated versions of the list may be aquired. Each
string MUST be a URL [RFC1738] pointing to a list in the format
specified here. The URL scheme MUST be HTTPS [RFC9110].
The value of "reports", if present, MUST be a string indicating a URL
[RFC1738] where malfeasance reports can be sent by clients using the
HTTP POST method [RFC9110]. The URL scheme MUST be HTTPS [RFC9110].
8.4. Malfeasance Reporting
A malfeasance report is cryptographic proof that a sequence of
responses arrived in that order. It can be used to demonstrate that
at least one server sent the wrong time.
A malfeasance report MUST be formatted as a JSON [RFC8259] object and
contain the key "responses". Its value MUST be an ordered list of
response objects. Each response object MUST contain the keys "rand",
"request", "response", and "publicKey". The values of all four keys
MUST be represented as base64-encoded [RFC4648] strings.
The "rand" key MAY be omitted from the first response object in the
list. In all other cases, its value MUST be the 32-byte value used
to generate the request nonce value from the previous response
packet.
The value of "request" MUST be the transmitted request packet,
including the "ROUGHTIM" header.
The value of "response" MUST be the received response packet,
including the "ROUGHTIM" header.
The value of "publicKey" MUST be the long-term key that the server
was expected to use for deriving the response signature.
When the client's list of servers has an associated URL for
malfeasance reports, it SHOULD send a report whenever it has
performed a measurement sequence in accordance with Section 8.2 and
detected that at least one of the responses is inconsistent with
causal ordering. Since the failure of a popular Roughtime server can
cause numerous clients to send malfeasance reports at the same time,
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clients MUST use a reporting mechanism that avoids overloading the
server receiving the reports. Clients SHOULD use exponential backoff
for this purpose, with an initial and minimum retry interval of at
least 10 seconds.
Clients MUST NOT send malfeasance reports in response to signature
verification failures or any other protocol errors.
9. Security Considerations
9.1. Confidentiality
This protocol does not provide any confidentiality. Given the nature
of timestamps, such impact is minor.
9.2. Integrity and Authenticity
The Roughtime protocol only provides integrity and authenticity
protection for data contained in the SREP tag. Accordingly, new tags
SHOULD be added to the SREP tag whenever possible.
9.3. Generating Private Keys
Although any random 256-bit string can be used as a private Ed25519
key, it has a high risk of being vulnerable to small-subgroup attacks
and timing side-channel leaks. For this reason, all private keys
used in Roughtime MUST be generated following the procedure described
in Section 5.1.5 of RFC 8032 [RFC8032].
9.4. Private Key Compromise
The compromise of a PUBK's private key, even past MAXT, is a problem
as the private key can be used to sign invalid times that are in the
range MINT to MAXT, and thus violate the good-behavior guarantee of
the server. To protect against this, it is necessary for clients to
query multiple servers in accordance with the procedure described in
Section 8.2.
9.5. Quantum Resistance
Since the only supported signature scheme, Ed25519, is not quantum
resistant, the Roughtime version described in this memo will not
survive the advent of quantum computers.
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9.6. Maintaining Lists of Servers
The infrastructure and procedures for maintaining a list of trusted
servers and adjudicating violations of the rules by servers is not
discussed in this document and is essential for security.
9.7. Amplification Attacks
UDP protocols that send responses significantly larger than requests,
such as NTP, have previously been leveraged for amplification
attacks. To prevent Roughtime from being used for such attacks,
servers MUST NOT send response packets larger than the request
packets sent by clients.
10. Privacy Considerations
This protocol is designed to obscure all client identifiers. Servers
necessarily have persistent long-term identities essential to
enforcing correct behavior. Generating nonces in a nonrandom manner
can cause leaks of private data or enable tracking of clients as they
move between networks.
11. Operational Considerations
It is expected that clients identify a server by its long-term public
key. In multi-tenancy environments, where multiple servers may be
listening on the same IP or port space, the protocol is designed so
that the client indicates which server it expects to respond. This
is done with the SRV tag.
12. IANA Considerations
12.1. Service Name and Transport Protocol Port Number Registry
IANA is requested to allocate the following entry in the Service Name
and Transport Protocol Port Number Registry:
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Service Name: Roughtime
Transport Protocol: tcp,udp
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Description: Roughtime time synchronization
Reference: [[this memo]]
Port Number: [[TBD1]], selected by IANA from the User Port range
12.2. Roughtime Version Registry
IANA is requested to create a new registry entitled "Roughtime
Version Registry". Entries shall have the following fields:
Version ID (REQUIRED): a 32-bit unsigned integer
Version name (REQUIRED): A short text string naming the version being
identified.
Reference (REQUIRED): A reference to a relevant specification
document.
The policy for allocation of new entries SHOULD be: IETF Review.
The initial contents of this registry shall be as follows:
+=======================+======================+===============+
| Version ID | Version name | Reference |
+=======================+======================+===============+
| 0x0 | Reserved | [[this memo]] |
+-----------------------+----------------------+---------------+
| 0x1 | Roughtime version 1 | [[this memo]] |
+-----------------------+----------------------+---------------+
| 0x2-0x7fffffff | Unassigned | |
+-----------------------+----------------------+---------------+
| 0x80000000-0xffffffff | Reserved for Private | [[this memo]] |
+-----------------------+----------------------+---------------+
| | or Experimental use | |
+-----------------------+----------------------+---------------+
Table 1
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12.3. Roughtime Tag Registry
IANA is requested to create a new registry entitled "Roughtime Tag
Registry". Entries SHALL have the following fields:
Tag (REQUIRED): A 32-bit unsigned integer in hexadecimal format.
ASCII Representation (REQUIRED): The ASCII representation of the tag
in accordance with Section 4.1.3 of this memo.
Reference (REQUIRED): A reference to a relevant specification
document.
The policy for allocation of new entries in this registry SHOULD be:
Specification Required.
The initial contents of this registry SHALL be as follows:
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+============+======================+===============+
| Tag | ASCII Representation | Reference |
+============+======================+===============+
| 0x00474953 | SIG | [[this memo]] |
+------------+----------------------+---------------+
| 0x00524556 | VER | [[this memo]] |
+------------+----------------------+---------------+
| 0x00565253 | SRV | [[this memo]] |
+------------+----------------------+---------------+
| 0x434e4f4e | NONC | [[this memo]] |
+------------+----------------------+---------------+
| 0x454c4544 | DELE | [[this memo]] |
+------------+----------------------+---------------+
| 0x48544150 | PATH | [[this memo]] |
+------------+----------------------+---------------+
| 0x49444152 | RADI | [[this memo]] |
+------------+----------------------+---------------+
| 0x4b425550 | PUBK | [[this memo]] |
+------------+----------------------+---------------+
| 0x5044494d | MIDP | [[this memo]] |
+------------+----------------------+---------------+
| 0x50455253 | SREP | [[this memo]] |
+------------+----------------------+---------------+
| 0x53524556 | VERS | [[this memo]] |
+------------+----------------------+---------------+
| 0x544e494d | MINT | [[this memo]] |
+------------+----------------------+---------------+
| 0x544f4f52 | ROOT | [[this memo]] |
+------------+----------------------+---------------+
| 0x54524543 | CERT | [[this memo]] |
+------------+----------------------+---------------+
| 0x5458414d | MAXT | [[this memo]] |
+------------+----------------------+---------------+
| 0x58444e49 | INDX | [[this memo]] |
+------------+----------------------+---------------+
| 0x5a5a5a5a | ZZZZ | [[this memo]] |
+------------+----------------------+---------------+
Table 2
13. References
13.1. Normative References
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, DOI 10.17487/RFC1738,
December 1994, <https://www.rfc-editor.org/rfc/rfc1738>.
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[RFC20] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/rfc/rfc20>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/rfc/rfc4086>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/rfc/rfc4291>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/rfc/rfc6234>.
[RFC6874] Carpenter, B., Cheshire, S., and R. Hinden, "Representing
IPv6 Zone Identifiers in Address Literals and Uniform
Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
February 2013, <https://www.rfc-editor.org/rfc/rfc6874>.
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/rfc/rfc791>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/rfc/rfc8032>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
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[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/rfc/rfc8259>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/rfc/rfc9110>.
[RFC9170] Thomson, M. and T. Pauly, "Long-Term Viability of Protocol
Extension Mechanisms", RFC 9170, DOI 10.17487/RFC9170,
December 2021, <https://www.rfc-editor.org/rfc/rfc9170>.
13.2. Informative References
[Merkle] Merkle, R. C., "A Digital Signature Based on a
Conventional Encryption Function", in Pomerance, C. (eds)
Advances in Cryptology, Lecture Notes in Computer
Science vol 293, DOI 10.1007/3-540-47184-2_32, 1988,
<https://doi.org/10.1007/3-540-48184-2_32>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/rfc/rfc5905>.
[RFC738] Harrenstien, K., "Time server", RFC 738,
DOI 10.17487/RFC0738, October 1977,
<https://www.rfc-editor.org/rfc/rfc738>.
[RFC8915] Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
Sundblad, "Network Time Security for the Network Time
Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,
<https://www.rfc-editor.org/rfc/rfc8915>.
Acknowledgments
Aanchal Malhotra and Adam Langley authored early drafts of this memo.
Daniel Franke, Sarah Grant, Martin Langer, Ben Laurie, Peter
Löthberg, Hal Murray, Tal Mizrahi, Ruben Nijveld, Christopher Patton,
Thomas Peterson, Rich Salz, Dieter Sibold, Ragnar Sundblad, Kristof
Teichel, David Venhoek, Ulrich Windl, and the other members of the
NTP working group contributed comments and suggestions as well as
pointed out errors.
Authors' Addresses
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Watson Ladd
Akamai Technologies
Email: watsonbladd@gmail.com
Marcus Dansarie
Netnod
Email: marcus@dansarie.se
URI: https://orcid.org/0000-0001-9246-0263
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