Internet Engineering Task Force A. Malhotra
Internet-Draft Boston University
Intended status: Informational A. Langley
Expires: December 3, 2020 Google
W. Ladd
Cloudflare
June 01, 2020
Roughtime
draft-ietf-ntp-roughtime-02
Abstract
This document specifies Roughtime - a protocol that aims to achieve
rough time synchronization while detecting servers that provide
inaccurate time and providing cryptographic proof of their
malfeasance.
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
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This Internet-Draft will expire on December 3, 2020.
Copyright Notice
Copyright (c) 2020 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
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4
4. The Guarantee . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Message Format . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Data Types . . . . . . . . . . . . . . . . . . . . . . . 6
5.1.1. int32 . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1.2. uint32 . . . . . . . . . . . . . . . . . . . . . . . 6
5.1.3. uint64 . . . . . . . . . . . . . . . . . . . . . . . 6
5.1.4. Tag . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1.5. Timestamp . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Header . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Requests . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Responses . . . . . . . . . . . . . . . . . . . . . . . . 9
6.3. The Merkle Tree . . . . . . . . . . . . . . . . . . . . . 11
6.3.1. Root Value Validity Check Algorithm . . . . . . . . . 12
6.4. Validity of Response . . . . . . . . . . . . . . . . . . 12
7. Integration into NTP . . . . . . . . . . . . . . . . . . . . 12
8. Cheater Detection . . . . . . . . . . . . . . . . . . . . . . 13
9. Grease . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10. Roughtime Servers . . . . . . . . . . . . . . . . . . . . . . 14
11. Trust Anchors and Policies . . . . . . . . . . . . . . . . . 14
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
13.1. Service Name and Transport Protocol Port Number Registry 15
13.2. Roughtime Version Registry . . . . . . . . . . . . . . . 15
13.3. Roughtime Tag Registry . . . . . . . . . . . . . . . . . 15
14. Security Considerations . . . . . . . . . . . . . . . . . . . 16
15. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
16.1. Normative References . . . . . . . . . . . . . . . . . . 17
16.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Terms and Abbreviations . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
Time synchronization is essential to Internet security as many
security protocols and other applications require synchronization
[RFC7384] [MCBG]. 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)
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[I-D.ietf-ntp-using-nts-for-ntp] fail to ensure that the servers
behave correctly. Authenticating time servers prevents network
adversaries from modifying time packets, but an authenticated time
server still has full control over the contents of the time packet
and may go rogue. The Roughtime protocol provides cryptographic
proof of malfeasance, enabling clients to detect and prove to a third
party a server's attempts to influence the time a client computes.
+--------------+----------------------+-----------------------------+
| Protocol | Authenticated Server | Server Malfeasance Evidence |
+--------------+----------------------+-----------------------------+
| NTP, Chronos | N | N |
| NTP-MD5 | Y* | N |
| NTP-Autokey | Y** | N |
| NTS | Y | N |
| Roughtime | Y | Y |
+--------------+----------------------+-----------------------------+
Security Properties of current protocols
Table 1
Y* For security issues with symmetric-key based NTP-MD5
authentication, please refer to RFC 8573 [RFC8573].
Y** For security issues with Autokey Public Key Authentication, refer
to [Autokey].
More specifically,
o If a server's timestamps do not fit into the time context of other
servers' responses, then a Roughtime client can cryptographically
prove this misbehavior to third parties. This helps detect "bad"
servers.
o A Roughtime client can roughly detect (with no absolute guarantee)
a delay attack [DelayAttacks] but can not cryptographically prove
this to a third party. However, the absence of proof of
malfeasance should not be considered a proof of absence of
malfeasance. So Roughtime should not be used as a witness that a
server is overall "good".
o Note that delay attacks cannot be detected/stopped by any
protocol. Delay attacks can not, however, undermine the security
guarantees provided by Roughtime.
o Although delay attacks cannot be prevented, they can be limited to
a predetermined upper bound. This can be done by defining a
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maximal tolerable Round Trip Time (RTT) value, MAX-RTT, that a
Roughtime client is willing to accept. A Roughtime client can
measure the RTT of every request-response handshake and compare it
to MAX-RTT. If the RTT exceeds MAX-RTT, the corresponding server
is assumed to be a falseticker. When this approach is used the
maximal time error that can be caused by a delay attack is MAX-
RTT/2. It should be noted that this approach assumes that the
nature of the system is known to the client, including reasonable
upper bounds on the RTT value.
2. Requirements Language
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 rough time synchronization that enables
clients to provide cryptographic proof of server malfeasance. It
does so by having responses from servers include a signature with a
certificate rooted in a long-term public/private key pair over a
value derived from a nonce provided by the client in its request.
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 which includes
the hash of the client's nonce 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.
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. For example, a radius of 1,000,000 microseconds
means the server is absolutely confident that the true time is within
one second of the reported time.
The server proves freshness of its response as follows: The client's
request contains a nonce. The server incorporates the nonce into its
signed response so that the client can verify the server's signatures
covering the nonce issued by the client. Provided that the nonce has
sufficient entropy, this proves that the signed response could only
have been generated after the nonce.
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Chaining multiple servers: For subsequent requests, the client
generates a new nonce by hashing the reply from the previous server
with a random value (a blind). This proves that the nonce was
created after the reply from the previous server. It sends the new
nonce in a request to the next server and receives a response that
includes a signature covering the nonce.
Cryptographic proof of misbehavior: If the time from the second
server is before the first, then the client has proof that at least
one of the servers is misbehaving; the reply from the second server
implicitly shows that it was created later because of the way that
the client constructed the nonce. If the time from the second server
is too far in the future, the client can contact the first server
again with a new nonce generated from the second server's response
and get a signature that was provably created afterwards, but with an
earlier timestamp.
With only two servers, the client can end up with proof that
something is wrong, but no idea what the correct time is. But with
half a dozen or more independent servers, the client will end up with
chain of proof of any server's misbehavior, signed by several others,
and (presumably) enough accurate replies to establish what the
correct time is. Furthermore, this proof may be validated by third
parties ultimately leading to a revocation of trust in the
misbehaving server.
4. The Guarantee
A Roughtime server guarantees that a response to a query sent at t_1,
received at t_2, and with timestamp t_3 has been created between the
transmission of the query and its reception. If t_3 is not within
that interval, a server inconsistency may be detected and used to
impeach the server. The propagation of such a guarantee and its use
of type synchronization is discussed in Section 7. No delay attacker
may affect this: they may only expand the interval between t_1 and
t_2, or of course stop the measurement in the first place.
5. 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.
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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) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Values .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Roughtime Message Format
5.1. Data Types
5.1.1. int32
An int32 is a 32 bit signed integer. It is serialized in sign-
magitude representation with the sign bit in the most significant
bit. It is serialized least significant byte first. The negative
zero value (0x80000000) MUST NOT be used.
5.1.2. uint32
A uint32 is a 32 bit unsigned integer. It is serialized with the
least significant byte first.
5.1.3. uint64
A uint64 is a 64 bit unsigned integer. It is serialized with the
least significant byte first.
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5.1.4. Tag
Tags are used to identify values in Roughtime messages. A tag is a
uint32 but may also be listed as a sequence of up to four ASCII
characters [RFC0020]. ASCII strings shorter than four characters can
be unambiguously converted to tags by padding them with zero bytes.
For example, the ASCII string "NONC" would correspond to the tag
0x434e4f4e and "PAD" would correspond to 0x00444150.
5.1.5. Timestamp
A timestamp is a uint64 interpreted in the following way. The most
significant 3 bytes contain the integer part of a Modified Julian
Date (MJD). The least significant 5 bytes is a count of the number
of Coordinated Universal Time (UTC) microseconds [ITU-R_TF.460-6]
since midnight on that day.
The MJD is the number of UTC days since 17 November 1858
[ITU-R_TF.457-2]. It is useful to note that 1 January 1970 is 40,587
days after 17 November 1858.
Note that, unlike NTP, this representation does not use the full
number of bits in the fractional part and that days with leap seconds
will have more or fewer than the nominal 86,400,000,000 microseconds.
5.2. 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. 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.
Tags MUST be listed in the same order as the offsets of their values.
A tag MUST NOT appear more than once in a header. Tags MUST also be
sorted by numeric value.
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6. Protocol
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 the UDP transport mode, while TCP transport is
OPTIONAL.
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 5.1.
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 Format
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 [RFC0791] 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
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.
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For testing drafts of this memo, a version number of 0x80000000 plus
the draft number is used.
6.1. Requests
A request is a Roughtime message with the tags NONC and VER. Over a
UDP connection the size of the request message SHOULD be at least
1024 bytes. To attain this size the PAD tag SHOULD be added to the
message. Tags other than NONC and VER SHOULD be ignored by the
server. Responding to requests shorter than 1024 bytes is OPTIONAL
and servers MUST NOT send responses larger than the requests they are
replying to.
The value of the NONC tag is a 64 byte nonce. It SHOULD be generated
by hashing a previous Roughtime response message together with a
blind as described in Section 8. If no previous responses are
avaiable to the client, the nonce SHOULD be generated at random.
In a request, the VER tag contains a list of versions. 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 PAD tag SHOULD be used by clients to ensure their request
messages are at least 1024 bytes in size. Its value SHOULD be all
zeros.
6.2. Responses
A response MUST contain the tags SREP, SIG, CERT, INDX, PATH and VER.
The SIG tag is a signature over the SREP value using the public key
contained in CERT, as explained below.
The SREP tag contains a time response. Its value is a Roughtime
message with the tags ROOT, MIDP, and RADI, and NONC. The server MAY
include any of the tags DUT1, DTAI and LEAP in the contents of the
SREP tag.
The ROOT tag contains a 32 byte value of a Merkle tree root as
described in Section 6.3.
The MIDP tag value is a timestamp of the moment of processing.
The RADI tag value is a uint32 representing the server's estimate of
the accuracy of MIDP in microseconds. Servers MUST ensure that the
true time is within (MIDP-RADI, MIDP+RADI) at the time they compose
the response message.
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The NONC tag contains the nonce of the message being responded to.
The DUT1 tag value is an int32 indicating the predicted difference
between UT1 and UTC (UT1 - UTC) in milliseconds as given by the
International Earth Rotation and Reference Systems Service (IERS).
The DTAI tag value is an int32 indicating the current difference
between International Atomic Time (TAI) and UTC (TAI - UTC) in
milliseconds as published in the International Bureau of Weights and
Measures' (BIPM) Circular T.
The LEAP tag contains zero or more int32 values. Each value
represents the MJD of a past or future leap second event. Positive
values represent the addition of a second at the indicated date and
negative values represent the removal of a second at the indicated
(negative) date. The first item in the list MUST be the last (past
or future) leap second event that the server knows about. The leap
second events MUST be sorted in reverse chronological order. A leap
tag with zero int32 values indicates that the server does not hold
any updated leap second information.
The SIG tag value is a 64 byte Ed25519 signature [RFC8032] over a
signature context concatenated with the entire value of a DELE or
SREP tag. Signatures of DELE tags MUST use the ASCII string
"RoughTime v1 delegation signature--" and signatures of SREP tags
MUST use the ASCII string "RoughTime v1 response signature" as
signature context. Both strings MUST include a terminating zero
byte.
The CERT tag contains a public-key certificate signed with the
server's long-term key. Its value is a Roughtime message with the
tags DELE and SIG, where SIG is a signature over the DELE value.
The DELE tag contains a delegated public-key certificate used by the
server to sign the SREP tag. Its value is 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.
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The PUBK tag contains a temporary 32 byte Ed25519 public key which is
used to sign the SREP tag.
The INDX tag value is a uint32 determining the position of NONC in
the Merkle tree used to generate the ROOT value as described in
Section 6.3.
The PATH tag value is a multiple of 32 bytes long and represents a
path of 32 byte hash values in the Merkle tree used to generate the
ROOT value as described in Section 6.3. 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 is zero.
In a response, 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.
6.3. The Merkle Tree
A Merkle tree 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 the nonce
of one request that a response message is sent in reply to. 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-512/256 hash algorithm [SHS]. For leaf nodes, the byte 0x00 is
prepended to the nonce 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's nonce node is included in the INDX tag of
the response.
The values of all sibling nodes in the path between a request's nonce
node and the root node is stored in the PATH tag so that the client
can reconstruct and validate the value in the ROOT tag using its
nonce.
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6.3.1. Root Value Validity Check Algorithm
One starts by computing the hash of the NONC value from the request,
with 0x00 prepended. Then one walks from the least significant bit
of INDX to the most significant bit, and also walks towards the end
of PATH.
If PATH ends then the remaining bits of the INDX MUST be all zero.
This indicates the termination of the walk, and the current value
MUST equal ROOT if the response is valid.
If the current bit is 0, one hashes 0x01, the current hash, and the
value from PATH to derive the next current value.
If the current bit is 1 one hashes 0x01, the value from PATH, and the
current hash to derive the next current value.
6.4. 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.
o The signature in CERT was made with the long-term key of the
server.
o The DELE timestamps and the MIDP value are consistent.
o The INDX and PATH values prove NONC was included in the Merkle
tree with value ROOT using the algorithm in Section 6.3.1.
o 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 the time is correct, but merely that the
server signed it, and thus guarantees that it began to compute the
signature at a time in the interval (MIDP-RADI, MIDP+RADI).
7. Integration into NTP
We assume that there is a bound PHI on the frequency error in the
clock on the machine. Given a measurement taken at a local time t1,
we know the true time is in [ t1-delta-sigma, t1-delta+sigma ].
After d seconds have elapsed we know the true time is within [ t1-
delta-sigma-d*PHI, t1-delta+sigma+d*PHI]. A simple and effective way
to mix with NTP or PTP discipline of the clock is to trim the
observed intervals in NTP to fit entirely within this window or
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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.
8. Cheater Detection
A chain of responses is a series of responses where the SHA-512/256
hash of the preceding response H, is concatenated with a 64 byte
blind X, and then SHA-512/256(H, X) is the nonce used in the
subsequent response. These may be represented as an array of objects
in JavaScript Object Notation (JSON) format [RFC8259] where each
object may have keys "blind" and "response_packet". Packet has the
Base64 [RFC4648] encoded bytes of the packet and blind is the Base64
encoded blind used for the next nonce. The last packet needs no
blind.
A pair of responses (r_1, r_2) is invalid if MIDP_1-RADI_1 >
MIDP_2+RADI_2. A chain of longer length is invalid if for any i, j
such that i < j, (r_i, r_j) is an invalid pair.
Invalidity of a chain is proof that causality has been violated if
all servers were reporting correct time. An invalid chain where all
individual responses are valid is cryptographic proof of malfeasance
of at least one server: if all servers had the correct time in the
chain, causality would imply that MIDP_1-RADI_1 < MIDP_2+RADI_2.
In conducting the comparison of timestamps one must know the length
of a day and hence have historical leap second data for the days in
question. However if violations are greater then a second the loss
of leap second data doesn't impede their detection.
9. Grease
Servers MAY send back a fraction of responses that are syntactically
invalid or contain invalid signatures as well as incorrect times.
Clients MUST properly reject such responses. Servers MUST NOT send
back responses with incorrect times and valid signatures. Either
signature MAY be invalid for this application.
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10. Roughtime Servers
The below list contains a list of servers with their public keys in
Base64 format. These servers may implement older versions of this
specification.
address: roughtime.cloudflare.com
port: 2002
long-term key: gD63hSj3ScS+wuOeGrubXlq35N1c5Lby/S+T7MNTjxo=
address: roughtime.int08h.com
port: 2002
long-term key: AW5uAoTSTDfG5NfY1bTh08GUnOqlRb+HVhbJ3ODJvsE=
address: roughtime.sandbox.google.com
port: 2002
long-term key: etPaaIxcBMY1oUeGpwvPMCJMwlRVNxv51KK/tktoJTQ=
address: roughtime.se
port: 2002
long-term key: S3AzfZJ5CjSdkJ21ZJGbxqdYP/SoE8fXKY0+aicsehI=
11. Trust Anchors and Policies
A trust anchor is any distributor of a list of trusted servers. It
is RECOMMENDED that trust anchors subscribe to a common public forum
where evidence of malfeasance may be shared and discussed. Trust
anchors SHOULD subscribe to a zero-tolerance policy: any generation
of incorrect timestamps will result in removal. To enable this trust
anchors SHOULD list a wide variety of servers so the removal of a
server does not result in operational issues for clients. Clients
SHOULD attempt to detect malfeasance and have a way to report it to
trust anchors.
Because only a single Roughtime server is required for successful
synchronization, Roughtime does not have the incentive problems that
have prevented effective enforcement of discipline on the web PKI.
We expect that some clients will aggressively monitor server
behavior.
12. Acknowledgements
Marcus Dansarie contributed many fruitful ideas. Thomas Peterson
corrected multiple nits. Peter Loethberg (Lothberg), Tal Mizrahi,
Ragnar Sundblad, Kristof Teichel, and the other members of the NTP
working group contributed comments and suggestions.
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13. IANA Considerations
13.1. 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 [RFC6335]:
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
13.2. Roughtime Version Registry
IANA is requested to create a new registry entitled " Roughtime
Version Registry " Entries shall have the following fields:
Version (REQUIRED): a 32-bit unsigned integer
Reference (REQUIRED): the description of the version
The policy for allocation of new entries SHOULD be IETF consensus.
Versions with the high bit set are reserved.
The initial contents of this registry shall be as follows:
+---------+---------------+
| Version | Reference |
+---------+---------------+
| 1 | [[this memo]] |
+---------+---------------+
13.3. 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.
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ASCII Representation (OPTIONAL): The ASCII representation of the
tag in accordance with Section 5.1.4 of this memo, if applicable.
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:
+------------+----------------------+---------------+
| Tag | ASCII Representation | Reference |
+------------+----------------------+---------------+
| 0x00444150 | PAD | [[this memo]] |
| 0x00474953 | SIG | [[this memo]] |
| 0x00524556 | VER | [[this memo]] |
| 0x31545544 | DUT1 | [[this memo]] |
| 0x434e4f48 | NONC | [[this memo]] |
| 0x454c4544 | DELE | [[this memo]] |
| 0x48544150 | PATH | [[this memo]] |
| 0x49415444 | DTAI | [[this memo]] |
| 0x49444152 | RADI | [[this memo]] |
| 0x4b425550 | PUBK | [[this memo]] |
| 0x5041454c | LEAP | [[this memo]] |
| 0x5044494d | MIDP | [[this memo]] |
| 0x50455253 | SREP | [[this memo]] |
| 0x544e494d | MINT | [[this memo]] |
| 0x544f4f52 | ROOT | [[this memo]] |
| 0x54524543 | CERT | [[this memo]] |
| 0x5458414d | MAXT | [[this memo]] |
| 0x58444e49 | INDX | [[this memo]] |
+------------+----------------------+---------------+
14. Security Considerations
Since the only supported signature scheme, Ed25519, is not quantum
resistant, this protocol will not survive the advent of quantum
computers.
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. Roughtime clients MUST update their view of
which servers are trustworthy in order to benefit from the detection
of misbehavior.
Validating timestamps made on different dates requires knowledge of
leap seconds in order to calculate time intervals correctly.
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Servers carry out a significant amount of computation in response to
clients, and thus may experience vulnerability to denial of service
attacks.
This protocol does not provide any confidentiality, and given the
nature of timestamps such impact is minor.
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.
Servers MUST NOT send response packets larger than the request
packets sent by clients, in order to prevent amplification attacks.
15. 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 from previous
responses without using a blind can enable tracking of clients as
they move between networks.
16. References
16.1. Normative References
[ITU-R_TF.457-2]
ITU-R, "Use of the Modified Julian Date by the Standard-
Frequency and Time-Signal Services", ITU-R
Recommendation TF.457-2, October 1997.
[ITU-R_TF.460-6]
ITU-R, "Standard-Frequency and Time-Signal Emissions",
ITU-R Recommendation TF.460-6, February 2002.
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/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/info/rfc6234>.
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[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[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/info/rfc8032>.
[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/info/rfc8259>.
[SHS] NIST, "Secure Hash Standard", FIPS 180-4, August 2015.
16.2. Informative References
[Autokey] Rottger, S., "Analysis of the NTP Autokey Procedures",
2012, <https://zero-entropy.de/autokey_analysis.pdf>.
[DelayAttacks]
Mizrahi, T., "A Game Theoretic Analysis of Delay Attacks
Against Time Synchronization Protocols",
DOI 10.1109/ISPCS.2012.6336612, 2012,
<https://ieeexplore.ieee.org/document/6336612>.
[I-D.ietf-ntp-using-nts-for-ntp]
Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
Sundblad, "Network Time Security for the Network Time
Protocol", draft-ietf-ntp-using-nts-for-ntp-25 (work in
progress), March 2020.
[MCBG] Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg,
"Attacking the Network Time Protocol", 2015,
<https://eprint.iacr.org/2015/1020>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
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[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[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/info/rfc2119>.
[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/info/rfc5905>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <https://www.rfc-editor.org/info/rfc7384>.
[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/info/rfc8174>.
[RFC8573] Malhotra, A. and S. Goldberg, "Message Authentication Code
for the Network Time Protocol", RFC 8573,
DOI 10.17487/RFC8573, June 2019,
<https://www.rfc-editor.org/info/rfc8573>.
Appendix A. Terms and Abbreviations
ASCII American Standard Code for Information Interchange
IANA Internet Assigned Numbers Authority
JSON JavaScript Object Notation [RFC8259]
MJD Modified Julian Date
NTP Network Time Protocol [RFC5905]
NTS Network Time Security [I-D.ietf-ntp-using-nts-for-ntp]
TAI International Atomic Time (Temps Atomique International)
[ITU-R_TF.460-6]
TCP Transmission Control Protocol [RFC0793]
UDP User Datagram Protocol [RFC0768]
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UT Universal Time [ITU-R_TF.460-6]
UTC Coordinated Universal Time [ITU-R_TF.460-6]
Authors' Addresses
Aanchal Malhotra
Boston University
111 Cummington Mall
Boston 02215
USA
Email: aanchal4@bu.edu
Adam Langley
Google
Email:
agl@google.com
Watson Ladd
Cloudflare
101 Townsend St
San Francisco
USA
Email: watsonbladd@gmail.com
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