Internet DRAFT - draft-dansarie-nts
draft-dansarie-nts
NTP Working Group D. Franke
Internet-Draft
Intended status: Standards Track D. Sibold
Expires: January 3, 2019 K. Teichel
PTB
M. Dansarie
R. Sundblad
Netnod
July 02, 2018
Network Time Security for the Network Time Protocol
draft-dansarie-nts-00
Abstract
This memo specifies Network Time Security (NTS), a mechanism for
using Transport Layer Security (TLS) and Authenticated Encryption
with Associated Data (AEAD) to provide cryptographic security for the
client-server mode of the Network Time Protocol (NTP).
NTS is structured as a suite of two loosely coupled sub-protocols:
the NTS Key Establishment Protocol (NTS-KE) and the NTS Extension
Fields for NTPv4. NTS-KE handles NTS service authentication, initial
handshaking, and key extraction over TLS. Encryption and
authentication during NTP time synchronization is performed through
the NTS Extension Fields in otherwise standard NTP packets. Except
for during the initial NTS-KE process, all state required by the
protocol is held by the client in opaque cookies.
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 January 3, 2019.
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Copyright Notice
Copyright (c) 2018 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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Protocol Overview . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 6
3. TLS Profile for Network Time Security . . . . . . . . . . . . 6
4. The NTS Key Establishment Protocol . . . . . . . . . . . . . 7
4.1. NTS-KE Record Types . . . . . . . . . . . . . . . . . . . 9
4.1.1. End of Message . . . . . . . . . . . . . . . . . . . 9
4.1.2. NTS Next Protocol Negotiation . . . . . . . . . . . . 10
4.1.3. Error . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1.4. Warning . . . . . . . . . . . . . . . . . . . . . . . 10
4.1.5. AEAD Algorithm Negotiation . . . . . . . . . . . . . 11
4.1.6. New Cookie for NTPv4 . . . . . . . . . . . . . . . . 11
4.1.7. NTP Server Negotiation . . . . . . . . . . . . . . . 12
4.2. Key Extraction (generally) . . . . . . . . . . . . . . . 12
4.3. Key Extraction (for NTPv4) . . . . . . . . . . . . . . . 13
5. NTS Extension Fields for NTPv4 . . . . . . . . . . . . . . . 13
5.1. Packet Structure Overview . . . . . . . . . . . . . . . . 13
5.2. The Unique Identifier Extension Field . . . . . . . . . . 14
5.3. The NTS Cookie Extension Field . . . . . . . . . . . . . 14
5.4. The NTS Cookie Placeholder Extension Field . . . . . . . 14
5.5. The NTS Authenticator and Encrypted Extension Fields
Extension Field . . . . . . . . . . . . . . . . . . . . . 15
6. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 17
7. Suggested Format for NTS Cookies . . . . . . . . . . . . . . 21
8. Usage of NTP pools . . . . . . . . . . . . . . . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
9.1. Service Name and Transport Protocol Port Number Registry 23
9.2. TLS Application-Layer Protocol Negotiation (ALPN)
Protocol IDs Registry . . . . . . . . . . . . . . . . . . 23
9.3. TLS Exporter Labels Registry . . . . . . . . . . . . . . 24
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9.4. NTP Kiss-o'-Death Codes Registry . . . . . . . . . . . . 24
9.5. NTP Extension Field Types Registry . . . . . . . . . . . 24
9.6. Network Time Security Key Establishment Record Types
Registry . . . . . . . . . . . . . . . . . . . . . . . . 25
9.7. Network Time Security Next Protocols Registry . . . . . . 26
9.8. Network Time Security Error and Warning Codes Registries 27
10. Security Considerations . . . . . . . . . . . . . . . . . . . 28
10.1. Sensitivity to DDoS attacks . . . . . . . . . . . . . . 28
10.2. Avoiding DDoS Amplification . . . . . . . . . . . . . . 28
10.3. Initial Verification of Server Certificates . . . . . . 29
10.4. Delay Attacks . . . . . . . . . . . . . . . . . . . . . 30
10.5. Random Number Generation . . . . . . . . . . . . . . . . 30
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 30
11.1. Unlinkability . . . . . . . . . . . . . . . . . . . . . 31
11.2. Confidentiality . . . . . . . . . . . . . . . . . . . . 31
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
13.1. Normative References . . . . . . . . . . . . . . . . . . 32
13.2. Informative References . . . . . . . . . . . . . . . . . 33
Appendix A. Terms and Abbreviations . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction
This memo specifies Network Time Security (NTS), a cryptographic
security mechanism for network time synchronization. A complete
specification is provided for application of NTS to the client-server
mode of the Network Time Protocol (NTP) [RFC5905].
1.1. Objectives
The objectives of NTS are as follows:
o Identity: Through the use of the X.509 public key infrastructure,
implementations may cryptographically establish the identity of
the parties they are communicating with.
o Authentication: Implementations may cryptographically verify that
any time synchronization packets are authentic, i.e., that they
were produced by an identified party and have not been modified in
transit.
o Confidentiality: Although basic time synchronization data is
considered non-confidential and sent in the clear, NTS includes
support for encrypting NTP extension fields.
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o Replay prevention: Client implementations may detect when a
received time synchronization packet is a replay of a previous
packet.
o Request-response consistency: Client implementations may verify
that a time synchronization packet received from a server was sent
in response to a particular request from the client.
o Unlinkability: For mobile clients, NTS will not leak any
information additional to NTP which would permit a passive
adversary to determine that two packets sent over different
networks came from the same client.
o Non-amplification: Implementations (especially server
implementations) may avoid acting as distributed denial-of-service
(DDoS) amplifiers by never responding to a request with a packet
larger than the request packet.
o Scalability: Server implementations may serve large numbers of
clients without having to retain any client-specific state.
o Resilience: Attacks on or faults in parts of the NTS
infrastructure should not completely prohibit clients from
performing time synchronization.
1.2. Protocol Overview
The Network Time Protocol includes many different operating modes to
support various network topologies. In addition to its best-known
and most-widely-used client-server mode, it also includes modes for
synchronization between symmetric peers, a control mode for server
monitoring and administration, and a broadcast mode. These various
modes have differing and partly contradictory requirements for
security and performance. Symmetric and control modes demand mutual
authentication and mutual replay protection. Additionally, for
certain message types control mode may require confidentiality as
well as authentication. Client-server mode places more stringent
requirements on resource utilization than other modes, because
servers may have vast number of clients and be unable to afford to
maintain per-client state. However, client-server mode also has more
relaxed security needs, because only the client requires replay
protection: it is harmless for stateless servers to process replayed
packets. The security demands of symmetric and control modes, on the
other hand, are in conflict with the resource-utilization demands of
client-server mode: any scheme which provides replay protection
inherently involves maintaining some state to keep track of what
messages have already been seen.
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This memo specifies NTS exclusively for the client-server mode of
NTP. To this end, NTS is structured as a suite of two protocols:
The "NTS Extension Fields for NTPv4" are a collection of NTP
extension fields for cryptographically securing NTPv4 using
previously-established key material. They are suitable for
securing client-server mode because the server can implement them
without retaining per-client state. All state is kept by the
client and provided to the server in the form of an encrypted
cookie supplied with each request. On the other hand, the NTS
Extension Fields are suitable *only* for client-server mode
because only the client, and not the server, is protected from
replay.
The "NTS Key Establishment" protocol (NTS-KE) is a mechanism for
establishing key material for use with the NTS Extension Fields
for NTPv4. It uses TLS to exchange keys, provide the client with
an initial supply of cookies, and negotiate some additional
protocol options. After this exchange, the TLS channel is closed
with no per-client state remaining on the server side.
The typical protocol flow is as follows: The client connects to an
NTS-KE server on the NTS TCP port and the two parties perform a TLS
handshake. Via the TLS channel, the parties negotiate some
additional protocol parameters and the server sends the client a
supply of cookies along with a list of one or more IP addresses to
NTP servers for which the cookies are valid. The parties use TLS key
export [RFC5705] to extract key material which will be used in the
next phase of the protocol. This negotiation takes only a single
round trip, after which the server closes the connection and discards
all associated state. At this point the NTS-KE phase of the protocol
is complete. Ideally, the client never needs to connect to the NTS-
KE server again.
Time synchronization proceeds with one of the indicated NTP servers
over the NTP UDP port. The client sends the server an NTP client
packet which includes several extension fields. Included among these
fields are a cookie (previously provided by the key exchange server)
and an authentication tag, computed using key material extracted from
the NTS-KE handshake. The NTP server uses the cookie to recover this
key material and send back an authenticated response. The response
includes a fresh, encrypted cookie which the client then sends back
in the clear in a subsequent request. (This constant refreshing of
cookies is necessary in order to achieve NTS's unlinkability goal.)
Figure 1 provides an overview of the high-level interaction between
the client, the NTS-KE server, and the NTP server. Note that the
cookies' data format and the exchange of secrets between NTS-KE and
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NTP servers are not part of this specification and are implementation
dependent. However, a suggested format for NTS cookies is provided
in Section 7.
+--------------+
| |
+-> | NTP Server 1 |
| | |
Shared cookie | +--------------+
+---------------+ encryption parameters | +--------------+
| | (Implementation dependent) | | |
| NTS-KE Server | <------------------------------+-> | NTP Server 2 |
| | | | |
+---------------+ | +--------------+
^ | .
| | .
| 1. Negotiate parameters, | .
| receive initial cookie | +--------------+
| supply, generate AEAD keys, | | |
| and receive NTP server IP +-> | NTP Server N |
| addresses using "NTS Key | |
| Establishment" protocol. +--------------+
| ^
| |
| +----------+ |
| | | |
+-----------> | Client | <-------------------------+
| | 2. Perform authenticated
+----------+ time synchronization
and generate new
cookies using "NTS
Extension Fields for
NTPv4".
Figure 1: Overview of High-Level Interactions in NTS
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. TLS Profile for Network Time Security
Network Time Security makes use of TLS [RFC8446] for NTS key
establishment.
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Since securing time protocols is (as of 2018) a novel application of
TLS, no backward-compatibility concerns exist to justify using
obsolete, insecure, or otherwise broken TLS features or versions. We
therefore put forward the following requirements and guidelines,
roughly representing 2018's best practices:
Implementations MUST NOT negotiate TLS versions earlier than 1.3.
Implementations willing to negotiate more than one possible version
of TLS SHOULD NOT respond to handshake failures by retrying with a
downgraded protocol version. If they do, they MUST implement TLS
Fallback SCSV [RFC7507].
Use of the Application-Layer Protocol Negotiation Extension [RFC7301]
is integral to NTS and support for it is REQUIRED for
interoperability.
4. The NTS Key Establishment Protocol
The NTS key establishment protocol is conducted via TCP port
[[TBD1]]. The two endpoints carry out a TLS handshake in conformance
with Section 3, with the client offering (via an ALPN [RFC7301]
extension), and the server accepting, an application-layer protocol
of "ntske/1". Immediately following a successful handshake, the
client SHALL send a single request as Application Data encapsulated
in the TLS-protected channel. Then, the server SHALL send a single
response followed by a TLS "Close notify" alert and then discard the
channel state.
The client's request and the server's response each SHALL consist of
a sequence of records formatted according to Figure 2. Requests and
non-error responses each SHALL include exactly one NTS Next Protocol
Negotiation record. The sequence SHALL be terminated by a "End of
Message" record. The requirement that all NTS-KE messages be
terminated by an End of Message record makes them self-delimiting.
Clients and servers MAY enforce length limits on requests and
responses, however, servers MUST accept requests of at least 1024
octets and clients SHOULD accept responses of at least 65536 octets.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| Record Type | Body Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Record Body .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: NTS-KE Record Format
The fields of an NTS-KE record are defined as follows:
C (Critical Bit): Determines the disposition of unrecognized
Record Types. Implementations which receive a record with an
unrecognized Record Type MUST ignore the record if the Critical
Bit is 0 and MUST treat it as an error if the Critical Bit is 1.
Record Type Number: A 15-bit integer in network byte order. The
semantics of record types 0-6 are specified in this memo.
Additional type numbers SHALL be tracked through the IANA Network
Time Security Key Establishment Record Types registry.
Body Length: The length of the Record Body field, in octets, as a
16-bit integer in network byte order. Record bodies MAY have any
representable length and need not be aligned to a word boundary.
Record Body: The syntax and semantics of this field SHALL be
determined by the Record Type.
For clarity regarding bit-endianness: the Critical Bit is the most-
significant bit of the first octet. In C, given a network buffer
`unsigned char b[]` containing an NTS-KE record, the critical bit is
`b[0] >> 7` while the record type is `((b[0] & 0x7f) << 8) + b[1]`.
Figure 3 provides a schematic overview of the key exchange. It
displays the protocol steps to be performed by the NTS client and
server and record types to be exchanged.
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+---------------------------------------+
| - Verify client request message. |
| - Extract TLS key material. |
| - Generate KE response message. |
| - Include Record Types: |
| o NTS Next Protocol Negotiation |
| o AEAD Algorithm Negotiation |
| o NTP Server Negotiation |
| o New Cookie for NTPv4 |
| o <New Cookie for NTPv4> |
| o End of Message |
+-----------------+---------------------+
|
|
Server -----------+---------------+-----+----------------------->
^ \
/ \
/ TLS application \
/ data \
/ \
/ V
Client -----+---------------------------------+---------------->
| |
| |
| |
+-----------+----------------------+ +------+-----------------+
|- Generate KE request message. | |- Verify server response|
| - Include Record Types: | | message. |
| o NTS Next Protocol Negotiation | |- Extract cookie(s). |
| o AEAD Algorithm Negotiation | | |
| o <NTP Server Negotiation> | | |
| o End of Message | | |
+----------------------------------+ +------------------------+
Figure 3: NTS Key Exchange Messages
4.1. NTS-KE Record Types
The following NTS-KE Record Types are defined:
4.1.1. End of Message
The End of Message record has a Record Type number of 0 and a zero-
length body. It MUST occur exactly once as the final record of every
NTS-KE request and response. The Critical Bit MUST be set.
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4.1.2. NTS Next Protocol Negotiation
The NTS Next Protocol Negotiation record has a Record Type number of
1. It MUST occur exactly once in every NTS-KE request and response.
Its body consists of a sequence of 16-bit unsigned integers in
network byte order. Each integer represents a Protocol ID from the
IANA Network Time Security Next Protocols registry. The Critical Bit
MUST be set.
The Protocol IDs listed in the client's NTS Next Protocol Negotiation
record denote those protocols which the client wishes to speak using
the key material established through this NTS-KE session. The
Protocol IDs listed in the server's response MUST comprise a subset
of those listed in the request and denote those protocols which the
server is willing and able to speak using the key material
established through this NTS-KE session. The client MAY proceed with
one or more of them. The request MUST list at least one protocol,
but the response MAY be empty.
4.1.3. Error
The Error record has a Record Type number of 2. Its body is exactly
two octets long, consisting of an unsigned 16-bit integer in network
byte order, denoting an error code. The Critical Bit MUST be set.
Clients MUST NOT include Error records in their request. If clients
receive a server response which includes an Error record, they MUST
discard any negotiated key material and MUST NOT proceed to the Next
Protocol.
The following error codes are defined:
Error code 0 means "Unrecognized Critical Record". The server
MUST respond with this error code if the request included a record
which the server did not understand and which had its Critical Bit
set. The client SHOULD NOT retry its request without
modification.
Error code 1 means "Bad Request". The server MUST respond with
this error if, upon the expiration of an implementation-defined
timeout, it has not yet received a complete and syntactically
well-formed request from the client.
4.1.4. Warning
The Warning record has a Record Type number of 3. Its body is
exactly two octets long, consisting of an unsigned 16-bit integer in
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network byte order, denoting a warning code. The Critical Bit MUST
be set.
Clients MUST NOT include Warning records in their request. If
clients receive a server response which includes a Warning record,
they MAY discard any negotiated key material and abort without
proceeding to the Next Protocol. Unrecognized warning codes MUST be
treated as errors.
This memo defines no warning codes.
4.1.5. AEAD Algorithm Negotiation
The AEAD Algorithm Negotiation record has a Record Type number of 4.
Its body consists of a sequence of unsigned 16-bit integers in
network byte order, denoting Numeric Identifiers from the IANA AEAD
registry [RFC5116]. The Critical Bit MAY be set.
If the NTS Next Protocol Negotiation record offers Protocol ID 0 (for
NTPv4), then this record MUST be included exactly once. Other
protocols MAY require it as well.
When included in a request, this record denotes which AEAD algorithms
the client is willing to use to secure the Next Protocol, in
decreasing preference order. When included in a response, this
record denotes which algorithm the server chooses to use. It is
empty if the server supports none of the algorithms offered. In
requests, the list MUST include at least one algorithm. In
responses, it MUST include at most one. Honoring the client's
preference order is OPTIONAL: servers may select among any of the
client's offered choices, even if they are able to support some other
algorithm which the client prefers more.
Server implementations of NTS extension fields for NTPv4 (Section 5)
MUST support AEAD_AES_SIV_CMAC_256 [RFC5297] (Numeric Identifier 15).
That is, if the client includes AEAD_AES_SIV_CMAC_256 in its AEAD
Algorithm Negotiation record and the server accepts Protocol ID 0
(NTPv4) in its NTS Next Protocol Negotiation record, then the
server's AEAD Algorithm Negotiation record MUST NOT be empty.
4.1.6. New Cookie for NTPv4
The New Cookie for NTPv4 record has a Record Type number of 5. The
contents of its body SHALL be implementation-defined and clients MUST
NOT attempt to interpret them. See Section 7 for a suggested
construction.
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Clients MUST NOT send records of this type. Servers MUST send at
least one record of this type, and SHOULD send eight of them, if the
Next Protocol Negotiation response record contains Protocol ID 0
(NTPv4) and the AEAD Algorithm Negotiation response record is not
empty. The Critical Bit SHOULD NOT be set.
4.1.7. NTP Server Negotiation
The NTP Server Negotiation record has a Record Type number of 6. The
record MAY be sent by a client in a request and SHOULD be sent by a
server as part of a reply. Its body consists of a sequence of IPv4
and/or IPv6 addresses. Both address types are represented by 16
octets in network byte order. To achieve this, IPv4 addresses are
represented as "IPv4-mapped IPv6 addresses" in the format specified
in RFC 4291, Section 2.5.5.2 [RFC4291]. For example: The IPv4
address 192.0.2.1 would be mapped to the IPv6 address space as
::ffff:192.0.2.1. The Critical Bit SHOULD be set.
When used in a request, the NTP Server Negotiation record is the
client's way of indicating to the KE server which NTP servers it
wishes to receive cookies for. Honoring the client's NTP server
preferences is OPTIONAL. When used in a response, this record
informs the client about which NTP servers the received cookies can
be used with in the next phase of the protocol. The client SHOULD
NOT attempt to use the received cookies with any other NTP servers
than those indicated by the KE server.
If a response does not include this record, the client SHOULD assume
that the received cookies are valid for use with an NTP server at the
same network address as the key exchange server.
4.2. Key Extraction (generally)
Following a successful run of the NTS-KE protocol, key material SHALL
be extracted according to RFC 5705 [RFC5705]. Inputs to the exporter
function are to be constructed in a manner specific to the negotiated
Next Protocol. However, all protocols which utilize NTS-KE MUST
conform to the following two rules:
The disambiguating label string MUST be "EXPORTER-network-time-
security/1".
The per-association context value MUST be provided and MUST begin
with the two-octet Protocol ID which was negotiated as a Next
Protocol.
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4.3. Key Extraction (for NTPv4)
Following a successful run of the NTS-KE protocol wherein Protocol ID
0 (NTPv4) is selected as a Next Protocol, two AEAD keys SHALL be
extracted: a client-to-server (C2S) key and a server-to-client (S2C)
key. These keys SHALL be computed according to RFC 5705 [RFC5705],
using the following inputs.
The disambiguating label string SHALL be "EXPORTER-network-time-
security/1".
The per-association context value SHALL consist of the following
five octets:
The first two octets SHALL be zero (the Protocol ID for NTPv4).
The next two octets SHALL be the Numeric Identifier of the
negotiated AEAD Algorithm in network byte order.
The final octet SHALL be 0x00 for the C2S key and 0x01 for the
S2C key.
Implementations wishing to derive additional keys for private or
experimental use MUST NOT do so by extending the above-specified
syntax for per-association context values. Instead, they SHOULD use
their own disambiguating label string. Note that RFC 5705 [RFC5705]
provides that disambiguating label strings beginning with
"EXPERIMENTAL" MAY be used without IANA registration.
5. NTS Extension Fields for NTPv4
5.1. Packet Structure Overview
In general, an NTS-protected NTPv4 packet consists of:
The usual 48-octet NTP header which is authenticated but not
encrypted.
Some extension fields which are authenticated but not encrypted.
An extension field which contains AEAD output (i.e., an
authentication tag and possible ciphertext). The corresponding
plaintext, if non-empty, consists of some extension fields which
benefit from both encryption and authentication.
Possibly, some additional extension fields which are neither
encrypted nor authenticated. These are discarded by the receiver.
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Always included among the authenticated or authenticated-and-
encrypted extension fields are a cookie extension field and a unique
identifier extension field. The purpose of the cookie extension
field is to enable the server to offload storage of session state
onto the client. The purpose of the unique identifier extension
field is to protect the client from replay attacks.
5.2. The Unique Identifier Extension Field
The Unique Identifier extension field provides the client with a
cryptographically strong means of detecting replayed packets. It has
a Field Type of [[TBD2]]. When the extension field is included in a
client packet (mode 3), its body SHALL consist of a string of octets
generated uniformly at random. The string MUST be at least 32 octets
long. When the extension field is included in a server packet (mode
4), its body SHALL contain the same octet string as was provided in
the client packet to which the server is responding. All server
packets generated by NTS-implementing servers in response to client
packets containing this extension field MUST also contain this field
with the same content as in the client's request. The field's use in
modes other than client-server is not defined.
This extension field MAY also be used standalone, without NTS, in
which case it provides the client with a means of detecting spoofed
packets from off-path attackers. Historically, NTP's origin
timestamp field has played both these roles, but for cryptographic
purposes this is suboptimal because it is only 64 bits long and,
depending on implementation details, most of those bits may be
predictable. In contrast, the Unique Identifier extension field
enables a degree of unpredictability and collision resistance more
consistent with cryptographic best practice.
5.3. The NTS Cookie Extension Field
The NTS Cookie extension field has a Field Type of [[TBD3]]. Its
purpose is to carry information which enables the server to recompute
keys and other session state without having to store any per-client
state. The contents of its body SHALL be implementation-defined and
clients MUST NOT attempt to interpret them. See Section 7 for a
suggested construction. The NTS Cookie extension field MUST NOT be
included in NTP packets whose mode is other than 3 (client) or 4
(server).
5.4. The NTS Cookie Placeholder Extension Field
The NTS Cookie Placeholder extension field has a Field Type of
[[TBD4]]. When this extension field is included in a client packet
(mode 3), it communicates to the server that the client wishes it to
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send additional cookies in its response. This extension field MUST
NOT be included in NTP packets whose mode is other than 3.
Whenever an NTS Cookie Placeholder extension field is present, it
MUST be accompanied by an NTS Cookie extension field. The body
length of the NTS Cookie Placeholder extension field MUST be the same
as the body length of the NTS Cookie extension field. This length
requirement serves to ensure that the response will not be larger
than the request, in order to improve timekeeping precision and
prevent DDoS amplification. The contents of the NTS Cookie
Placeholder extension field's body are undefined and, aside from
checking its length, MUST be ignored by the server.
5.5. The NTS Authenticator and Encrypted Extension Fields Extension
Field
The NTS Authenticator and Encrypted Extension Fields extension field
is the central cryptographic element of an NTS-protected NTP packet.
Its Field Type is [[TBD5]]. It SHALL be formatted according to
Figure 4 and include the following fields:
Nonce length: Two octets in network byte order, giving the length
of the Nonce field, excluding any padding, interpreted as an
unsigned integer.
Ciphertext Length: Two octets in network byte order, giving the
length of the Ciphertext field, excluding any padding, interpreted
as an unsigned integer.
Nonce: A nonce as required by the negotiated AEAD Algorithm. The
field is zero-padded to a word (four octets) boundary.
Ciphertext: The output of the negotiated AEAD Algorithm. The
structure of this field is determined by the negotiated algorithm,
but it typically contains an authentication tag in addition to the
actual ciphertext. The field is zero-padded to a word (four
octets) boundary.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce Length | Ciphertext Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Nonce, including up to 3 bytes padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Ciphertext, including up to 3 bytes padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: NTS Authenticator and Encrypted Extension Fields Extension
Field Format
The Ciphertext field SHALL be formed by providing the following
inputs to the negotiated AEAD Algorithm:
K: For packets sent from the client to the server, the C2S key
SHALL be used. For packets sent from the server to the client,
the S2C key SHALL be used.
A: The associated data SHALL consist of the portion of the NTP
packet beginning from the start of the NTP header and ending at
the end of the last extension field which precedes the NTS
Authenticator and Encrypted Extension Fields extension field.
P: The plaintext SHALL consist of all (if any) NTP extension
fields to be encrypted. The format of any such fields SHALL be in
accordance with RFC 7822 [RFC7822]. If multiple extension fields
are present they SHALL be joined by concatenation.
N: The nonce SHALL be formed however required by the negotiated
AEAD Algorithm.
The NTS Authenticator and Encrypted Extension Fields extension field
MUST NOT be included in NTP packets whose mode is other than 3
(client) or 4 (server).
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6. Protocol Details
A client sending an NTS-protected request SHALL include the following
extension fields as displayed in Figure 5:
Exactly one Unique Identifier extension field which MUST be
authenticated, MUST NOT be encrypted, and whose contents MUST NOT
duplicate those of any previous request.
Exactly one NTS Cookie extension field which MUST be authenticated
and MUST NOT be encrypted. The cookie MUST be one which has been
previously provided to the client; either from the key exchange
server during the NTS-KE handshake or from the NTP server in
response to a previous NTS-protected NTP request. To protect the
client's privacy, the same cookie SHOULD NOT be included in
multiple requests. If the client does not have any cookies that
it has not already sent, it SHOULD initiate a re-run the NTS-KE
protocol.
Exactly one NTS Authenticator and Encrypted Extension Fields
extension field, generated using an AEAD Algorithm and C2S key
established through NTS-KE.
The client MAY include one or more NTS Cookie Placeholder extension
fields which MUST be authenticated and MAY be encrypted. The number
of NTS Cookie Placeholder extension fields that the client includes
SHOULD be such that if the client includes N placeholders and the
server sends back N+1 cookies, the number of unused cookies stored by
the client will come to eight. The client SHOULD NOT include more
than seven NTS Cookie Placeholder extension fields in a request.
When both the client and server adhere to all cookie-management
guidance provided in this memo, the number of placeholder extension
fields will equal the number of dropped packets since the last
successful volley.
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+---------------------------------------+
| - Verify time request message. |
| - Generate time response message. |
| - Include NTPv4 extension fields: |
| o Unique Identifier EF |
| o NTS Cookie EF |
| o <NTS Cookie EF> |
| |
| - Generate AEAD tag of NTP message. |
| - Add NTS Authentication and |
| Encrypted Extension Fields EF. |
| - Transmit time response packet. |
+-----------------+---------------------+
|
|
Server -----------+---------------+-----+----------------------->
^ \
/ \
Time request / \ Time response
(mode 3) / \ (mode 4)
/ \
/ V
Client -----+---------------------------------+---------------->
| |
| |
| |
+-----------+-----------------------+ +-----+------------------+
|- Generate time request message. | |- Verify time response |
| - Include NTPv4 extension fields: | | message. |
| o Unique Identifier EF | |- Extract cookie(s). |
| o NTS Cookie EF | |- Time synchronization |
| o <NTS Cookie Placeholder EF> | | processing. |
| | +------------------------+
|- Generate AEAD tag of NTP message.|
|- Add NTS Authentication and |
| Encrypted Extension Fields EF. |
|- Transmit time request packet. |
+-----------------------------------+
Figure 5: NTS Time Synchronization Messages
The client MAY include additional (non-NTS-related) extension fields
which MAY appear prior to the NTS Authenticator and Encrypted
Extension Fields extension fields (therefore authenticated but not
encrypted), within it (therefore encrypted and authenticated), or
after it (therefore neither encrypted nor authenticated). In
general, however, the server MUST discard any unauthenticated
extension fields and process the packet as though they were not
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present. Servers MAY implement exceptions to this requirement for
particular extension fields if their specification explicitly
provides for such.
Upon receiving an NTS-protected request, the server SHALL (through
some implementation-defined mechanism) use the cookie to recover the
AEAD Algorithm, C2S key, and S2C key associated with the request, and
then use the C2S key to authenticate the packet and decrypt the
ciphertext. If the cookie is valid and authentication and decryption
succeed, the server SHALL include the following extension fields in
its response:
Exactly one Unique Identifier extension field which MUST be
authenticated, MUST NOT be encrypted, and whose contents SHALL
echo those provided by the client.
Exactly one NTS Authenticator and Encrypted Extension Fields
extension field, generated using the AEAD algorithm and S2C key
recovered from the cookie provided by the client.
One or more NTS Cookie extension fields which MUST be
authenticated and encrypted. The number of NTS Cookie extension
fields included SHOULD be equal to, and MUST NOT exceed, one plus
the number of valid NTS Cookie Placeholder extension fields
included in the request. The cookies returned in those fields
MUST be valid for use with the NTP server that sent them. They
MAY be valid for other NTP servers as well, but there is no way
for the server to indicate this.
We emphasize the contrast that NTS Cookie extension fields MUST NOT
be encrypted when sent from client to server, but MUST be encrypted
from sent from server to client. The former is necessary in order
for the server to be able to recover the C2S and S2C keys, while the
latter is necessary to satisfy the unlinkability goals discussed in
Section 11.1. We emphasize also that "encrypted" means encapsulated
within the the NTS Authenticator and Encrypted Extensions extension
field. While the body of an NTS Cookie extension field will
generally consist of some sort of AEAD output (regardless of whether
the recommendations of Section 7 are precisely followed), this is not
sufficient to make the extension field "encrypted".
The server MAY include additional (non-NTS-related) extension fields
which MAY appear prior to the NTS Authenticator and Encrypted
Extension Fields extension field (therefore authenticated but not
encrypted), within it (therefore encrypted and authenticated), or
after it (therefore neither encrypted nor authenticated). In
general, however, the client MUST discard any unauthenticated
extension fields and process the packet as though they were not
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present. Clients MAY implement exceptions to this requirement for
particular extension fields if their specification explicitly
provides for such.
Upon receiving an NTS-protected response, the client MUST verify that
the Unique Identifier matches that of an outstanding request, and
that the packet is authentic under the S2C key associated with that
request. If either of these checks fails, the packet MUST be
discarded without further processing.
If the server is unable to validate the cookie or authenticate the
request, it SHOULD respond with a Kiss-o'-Death (KoD) packet (see RFC
5905, Section 7.4 [RFC5905]) with kiss code "NTSN", meaning "NTS
negative-acknowledgment (NAK)". It MUST NOT include any NTS Cookie
or NTS Authenticator and Encrypted Extension Fields extension fields.
If the NTP server has previously responded with authentic NTS-
protected NTP packets (i.e., packets containing the NTS Authenticator
and Encrypted Extension Fields extension field), the client MUST
verify that any KoD packets received from the server contain the
Unique Identifier extension field and that the Unique Identifier
matches that of an outstanding request. If this check fails, the
packet MUST be discarded without further processing. If this check
passes, the client MUST comply with RFC 5095, Section 7.4 [RFC5905]
where required. A client MAY automatically re-run the NTS-KE
protocol upon forced disassociation from an NTP server. In that
case, it MUST be able to detect and stop looping between the NTS-KE
and NTP servers.
Upon reception of the NTS NAK kiss code, the client SHOULD wait until
the next poll for a valid NTS-protected response and if none is
received, initiate a fresh NTS-KE handshake to try to renegotiate new
cookies, AEAD keys, and parameters. If the NTS-KE handshake
succeeds, the client MUST discard all old cookies and parameters and
use the new ones instead. As long as the NTS-KE handshake has not
succeeded, the client SHOULD continue polling the NTP server using
the cookies and parameters it has.
The client MAY reuse cookies in order to prioritize resilience over
unlinkability. Which of the two that should be prioritized in any
particular case is dependent on the application and the user's
preference. Section 11.1 describes the privacy considerations of
this in further detail.
To allow for NTP session restart when the NTS-KE server is
unavailable and to reduce NTS-KE server load, the client SHOULD keep
at least one unused but recent cookie, AEAD keys, negotiated AEAD
algorithm, and other necessary parameters on persistent storage.
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This way, the client is able to resume the NTP session without
performing renewed NTS-KE negotiation.
7. Suggested Format for NTS Cookies
This section is non-normative. It gives a suggested way for servers
to construct NTS cookies. All normative requirements are stated in
Section 4.1.6 and Section 5.3.
The role of cookies in NTS is closely analogous to that of session
cookies in TLS. Accordingly, the thematic resemblance of this
section to RFC 5077 [RFC5077] is deliberate and the reader should
likewise take heed of its security considerations.
Servers should select an AEAD algorithm which they will use to
encrypt and authenticate cookies. The chosen algorithm should be one
such as AEAD_AES_SIV_CMAC_256 [RFC5297] which resists accidental
nonce reuse. It need not be the same as the one that was negotiated
with the client. Servers should randomly generate and store a master
AEAD key `K`. Servers should additionally choose a non-secret, unique
value `I` as key-identifier for `K`.
Servers should periodically (e.g., once daily) generate a new pair
(I,K) and immediately switch to using these values for all newly-
generated cookies. Immediately following each such key rotation,
servers should securely erase any keys generated two or more rotation
periods prior. Servers should continue to accept any cookie
generated using keys that they have not yet erased, even if those
keys are no longer current. Erasing old keys provides for forward
secrecy, limiting the scope of what old information can be stolen if
a master key is somehow compromised. Holding on to a limited number
of old keys allows clients to seamlessly transition from one
generation to the next without having to perform a new NTS-KE
handshake.
The need to keep keys synchronized between NTS-KE and NTP servers as
well as across load-balanced clusters can make automatic key rotation
challenging. However, the task can be accomplished without the need
for central key-management infrastructure by using a ratchet, i.e.,
making each new key a deterministic, cryptographically pseudo-random
function of its predecessor. A recommended concrete implementation
of this approach is to use HKDF [RFC5869] to derive new keys, using
the key's predecessor as Input Keying Material and its key identifier
as a salt.
To form a cookie, servers should first form a plaintext `P`
consisting of the following fields:
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The AEAD algorithm negotiated during NTS-KE.
The S2C key.
The C2S key.
Servers should then generate a nonce `N` uniformly at random, and
form AEAD output `C` by encrypting `P` under key `K` with nonce `N`
and no associated data.
The cookie should consist of the tuple `(I,N,C)`.
To verify and decrypt a cookie provided by the client, first parse it
into its components `I`, `N`, and `C`. Use `I` to look up its
decryption key `K`. If the key whose identifier is `I` has been
erased or never existed, decryption fails; reply with an NTS NAK.
Otherwise, attempt to decrypt and verify ciphertext `C` using key `K`
and nonce `N` with no associated data. If decryption or verification
fails, reply with an NTS NAK. Otherwise, parse out the contents of
the resulting plaintext `P` to obtain the negotiated AEAD algorithm,
S2C key, and C2S key.
8. Usage of NTP pools
Many NTP server pools exist. Some of them have thousands of
individual servers spread out over several continents. Due to their
size and prevalence, it can be expected that a significant portion of
NTP users are users of NTP pools.
The separation of the initial NTS key exchange from the authenticated
NTP protocol simplifies the implementation of NTS on pool
infrastructures. Since NTS key exchange over TLS is expected to be a
rare occurrence in comparison with the normal authenticated NTP
request and response traffic, even large pools should require a
relatively small number of NTS-KE servers. This eliminates the need
for complex certificate infrastructures. The lower timing and
hardware requirements on NTS-KE servers also provide for load-
balancing solutions that aren't suitable for NTP servers, such as
virtual machine implementations that are started and stopped as
needed.
The ability for NTS-KE servers to freely choose what NTP servers they
will issue cookies for means that each pool can implement whatever
secret-sharing system between NTS-KE and NTP servers it deems
suitable. For example, in a large pool where the trust in the
individual NTP server administrators is relatively low, it may be
necessary to have separate shared secrets for each possible pair of
NTS-KE and NTP servers. It should also be noted that not all NTS-KE
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servers in a pool must have the ability to issue cookies for all NTP
servers in that pool.
Due to their freedom to choose what servers to issue cookies for,
NTS-KE servers can perform a number of functions in addition to
authenticating themselves to clients and issuing cookies. This
includes load-balancing and geographic assignment of clients to NTP
servers.
9. IANA Considerations
9.1. Service Name and Transport Protocol Port Number Registry
IANA is requested to allocate two entries, identical except for the
Transport Protocol, in the Service Name and Transport Protocol Port
Number Registry [RFC6335] as follows:
Service Name: nts
Transport Protocol: tcp, udp
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Description: Network Time Security
Reference: [[this memo]]
Port Number: [[TBD1]], selected by IANA from the system port range
9.2. TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs
Registry
IANA is requested to allocate the following entry in the TLS
Application-Layer Protocol Negotiation (ALPN) Protocol IDs registry
[RFC7301]:
Protocol: Network Time Security Key Establishment, version 1
Identification Sequence:
0x6E 0x74 0x73 0x6B 0x65 0x2F 0x31 ("ntske/1")
Reference: [[this memo]], Section 4
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9.3. TLS Exporter Labels Registry
IANA is requested to allocate the following entry in the TLS Exporter
Labels Registry [RFC5705]:
+--------------------+---------+-------------+---------------+------+
| Value | DTLS-OK | Recommended | Reference | Note |
+--------------------+---------+-------------+---------------+------+
| EXPORTER-network- | Y | Y | [[this | |
| time-security/1 | | | memo]], | |
| | | | Section 4.2 | |
+--------------------+---------+-------------+---------------+------+
9.4. NTP Kiss-o'-Death Codes Registry
IANA is requested to allocate the following entry in the registry of
NTP Kiss-o'-Death Codes [RFC5905]:
+------+----------------------------------------+-------------------+
| Code | Meaning | Reference |
+------+----------------------------------------+-------------------+
| NTSN | Network Time Security (NTS) negative- | [[this memo]], |
| | acknowledgment (NAK) | Section 6 |
+------+----------------------------------------+-------------------+
9.5. NTP Extension Field Types Registry
IANA is requested to allocate the following entries in the NTP
Extension Field Types registry [RFC5905]:
+----------+----------------------------------+---------------------+
| Field | Meaning | Reference |
| Type | | |
+----------+----------------------------------+---------------------+
| [[TBD2]] | Unique Identifier | [[this memo]], |
| | | Section 5.2 |
| [[TBD3]] | NTS Cookie | [[this memo]], |
| | | Section 5.3 |
| [[TBD4]] | NTS Cookie Placeholder | [[this memo]], |
| | | Section 5.4 |
| [[TBD5]] | NTS Authenticator and Encrypted | [[this memo]], |
| | Extension Fields | Section 5.5 |
+----------+----------------------------------+---------------------+
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9.6. Network Time Security Key Establishment Record Types Registry
IANA is requested to create a new registry entitled "Network Time
Security Key Establishment Record Types". Entries SHALL have the
following fields:
Record Type Number (REQUIRED): An integer in the range 0-32767
inclusive.
Description (REQUIRED): A short text description of the purpose of
the field.
Set Critical Bit (REQUIRED): One of "MUST", "SHOULD", "MAY",
"SHOULD NOT", or "MUST NOT".
Reference (REQUIRED): A reference to a document specifying the
semantics of the record.
The policy for allocation of new entries in this registry SHALL vary
by the Record Type Number, as follows:
0-1023: IETF Review.
1024-16383: Specification Required.
16384-32767: Private and Experimental Use.
Applications for new entries SHALL specify the contents of the
Description, Set Critical Bit, and Reference fields as well as which
of the above ranges the Record Type Number should be allocated from.
Applicants MAY request a specific Record Type Number and such
requests MAY be granted at the registrar's discretion.
The initial contents of this registry SHALL be as follows:
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+-------------+----------------------+-----------+------------------+
| Record Type | Description | Set | Reference |
| Number | | Critical | |
| | | Bit | |
+-------------+----------------------+-----------+------------------+
| 0 | End of Message | MUST | [[this memo]], |
| | | | Section 4.1.1 |
| 1 | NTS Next Protocol | MUST | [[this memo]], |
| | Negotiation | | Section 4.1.2 |
| 2 | Error | MUST | [[this memo]], |
| | | | Section 4.1.3 |
| 3 | Warning | MUST | [[this memo]], |
| | | | Section 4.1.4 |
| 4 | AEAD Algorithm | MAY | [[this memo]], |
| | Negotiation | | Section 4.1.5 |
| 5 | New Cookie for NTPv4 | SHOULD | [[this memo]], |
| | | NOT | Section 4.1.6 |
| 6 | NTP Server | SHOULD | [[this memo]], |
| | Negotiation | | Section 4.1.7 |
| 16384-32767 | Reserved for Private | MAY | [[this memo]] |
| | & Experimental Use | | |
+-------------+----------------------+-----------+------------------+
9.7. Network Time Security Next Protocols Registry
IANA is requested to create a new registry entitled "Network Time
Security Next Protocols". Entries SHALL have the following fields:
Protocol ID (REQUIRED): An integer in the range 0-65535 inclusive,
functioning as an identifier.
Protocol Name (REQUIRED): A short text string naming the protocol
being identified.
Reference (RECOMMENDED): A reference to a relevant specification
document. If no relevant document exists, a point-of-contact for
questions regarding the entry SHOULD be listed here in lieu.
Applications for new entries in this registry SHALL specify all
desired fields and SHALL be granted upon approval by a Designated
Expert. Protocol IDs 32768-65535 SHALL be reserved for Private or
Experimental Use and SHALL NOT be registered.
The initial contents of this registry SHALL be as follows:
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+-------------+-------------------------------+---------------------+
| Protocol ID | Protocol Name | Reference |
+-------------+-------------------------------+---------------------+
| 0 | Network Time Protocol version | [[this memo]] |
| | 4 (NTPv4) | |
| 32768-65535 | Reserved for Private or | Reserved by [[this |
| | Experimental Use | memo]] |
+-------------+-------------------------------+---------------------+
9.8. Network Time Security Error and Warning Codes Registries
IANA is requested to create two new registries entitled "Network Time
Security Error Codes" and "Network Time Security Warning Codes".
Entries in each SHALL have the following fields:
Number (REQUIRED): An integer in the range 0-65535 inclusive
Description (REQUIRED): A short text description of the condition.
Reference (REQUIRED): A reference to a relevant specification
document.
The policy for allocation of new entries in these registries SHALL
vary by their Number, as follows:
0-1023: IETF Review.
1024-32767: Specification Required.
32768-65535: Private and Experimental Use.
The initial contents of the Network Time Security Error Codes
Registry SHALL be as follows:
+--------+-----------------------------+----------------------------+
| Number | Description | Reference |
+--------+-----------------------------+----------------------------+
| 0 | Unrecognized Critical | [[this memo]], Section |
| | Extension | 4.1.3 |
| 1 | Bad Request | [[this memo]], Section |
| | | 4.1.3 |
+--------+-----------------------------+----------------------------+
The Network Time Security Warning Codes Registry SHALL initially be
empty.
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10. Security Considerations
10.1. Sensitivity to DDoS attacks
The introduction of NTS brings with it the introduction of asymmetric
cryptography to NTP. Asymmetric cryptography is necessary for
initial server authentication and AEAD key extraction. Asymmetric
cryptosystems are generally orders of magnitude slower than their
symmetric counterparts. This makes it much harder to build systems
that can serve requests at a rate corresponding to the full line
speed of the network connection. This, in turn, opens up a new
possibility for DDoS attacks on NTP services.
The main protection against these attacks in NTS lies in that the use
of asymmetric cryptosystems is only necessary in the initial NTS-KE
phase of the protocol. Since the protocol design enables separation
of the NTS-KE and NTP servers, a successful DDoS attack on an NTS-KE
server separated from the NTP service it supports will not affect NTP
users that have already performed initial authentication, AEAD key
extraction, and cookie exchange. Furthermore, as noted in Section 8,
NTP-KE capacity is easier to scale up and down than NTP server
capacity.
NTS users should also consider that they are not fully protected
against DDoS attacks by on-path adversaries. In addition to dropping
packets and attacks such as those described in Section 10.4, an on-
path attacker can send spoofed kiss-o'-death replies, which are not
authenticated, in response to NTP requests. This could result in
significantly increased load on the NTS-KE server. Implementers have
to weigh the user's need for unlinkability against the added
resilience that comes with cookie reuse in cases of NTS-KE server
unavailability.
10.2. Avoiding DDoS Amplification
Certain non-standard and/or deprecated features of the Network Time
Protocol enable clients to send a request to a server which causes
the server to send a response much larger than the request. Servers
which enable these features can be abused in order to amplify traffic
volume in DDoS attacks by sending them a request with a spoofed
source IP. In recent years, attacks of this nature have become an
endemic nuisance.
NTS is designed to avoid contributing any further to this problem by
ensuring that NTS-related extension fields included in server
responses will be the same size as the NTS-related extension fields
sent by the client. In particular, this is why the client is
required to send a separate and appropriately padded-out NTS Cookie
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Placeholder extension field for every cookie it wants to get back,
rather than being permitted simply to specify a desired quantity.
Due to the RFC 7822 [RFC7822] requirement that extensions be padded
and aligned to four-octet boundaries, response size may still in some
cases exceed request size by up to three octets. This is
sufficiently inconsequential that we have declined to address it.
10.3. Initial Verification of Server Certificates
NTS's security goals are undermined if the client fails to verify
that the X.509 certificate chain presented by the NTS-KE server is
valid and rooted in a trusted certificate authority. RFC 5280
[RFC5280] and RFC 6125 [RFC6125] specify how such verification is to
be performed in general. However, the expectation that the client
does not yet have a correctly-set system clock at the time of
certificate verification presents difficulties with verifying that
the certificate is within its validity period, i.e., that the current
time lies between the times specified in the certificate's notBefore
and notAfter fields. It may be operationally necessary in some cases
for a client to accept a certificate which appears to be expired or
not yet valid. While there is no perfect solution to this problem,
there are several mitigations the client can implement to make it
more difficult for an adversary to successfully present an expired
certificate:
Check whether the system time is in fact unreliable. If the
system clock has previously been synchronized since last boot,
then on operating systems which implement a kernel-based phase-
locked-loop API, a call to ntp_gettime() should show a maximum
error less than NTP_PHASE_MAX. In this case, the clock SHOULD be
considered reliable and certificates can be strictly validated.
Allow the system administrator to specify that certificates should
*always* be strictly validated. Such a configuration is
appropriate on systems which have a battery-backed clock and which
can reasonably prompt the user to manually set an approximately-
correct time if it appears to be needed.
Once the clock has been synchronized, periodically write the
current system time to persistent storage. Do not accept any
certificate whose notAfter field is earlier than the last recorded
time.
Do not process time packets from servers if the time computed from
them falls outside the validity period of the server's
certificate.
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Use multiple time sources. The ability to pass off an expired
certificate is only useful to an adversary who has compromised the
corresponding private key. If the adversary has compromised only
a minority of servers, NTP's selection algorithm (RFC 5905 section
11.2.1 [RFC5905]) will protect the client from accepting bad time
from the adversary-controlled servers.
10.4. Delay Attacks
In a packet delay attack, an adversary with the ability to act as a
man-in-the-middle delays time synchronization packets between client
and server asymmetrically [RFC7384]. Since NTP's formula for
computing time offset relies on the assumption that network latency
is roughly symmetrical, this leads to the client to compute an
inaccurate value [Mizrahi]. The delay attack does not reorder or
modify the content of the exchanged synchronization packets.
Therefore, cryptographic means do not provide a feasible way to
mitigate this attack. However, the maximum error that an adversary
can introduce is bounded by half of the round trip delay.
RFC 5905 [RFC5905] specifies a parameter called MAXDIST which denotes
the maximum round-trip latency (including not only the immediate
round trip between client and server, but the whole distance back to
the reference clock as reported in the Root Delay field) that a
client will tolerate before concluding that the server is unsuitable
for synchronization. The standard value for MAXDIST is one second,
although some implementations use larger values. Whatever value a
client chooses, the maximum error which can be introduced by a delay
attack is MAXDIST/2.
Usage of multiple time sources, or multiple network paths to a given
time source [Shpiner], may also serve to mitigate delay attacks if
the adversary is in control of only some of the paths.
10.5. Random Number Generation
At various points in NTS, the generation of cryptographically secure
random numbers is required. Whenever this draft specifies the use of
random numbers, cryptographically secure random number generation
MUST be used. RFC 4086 [RFC4086] contains guidelines concerning this
topic.
11. Privacy Considerations
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11.1. Unlinkability
Unlinkability prevents a device from being tracked when it changes
network addresses (e.g., because said device moved between different
networks). In other words, unlinkability thwarts an attacker that
seeks to link a new network address used by a device with a network
address that it was formerly using through recognizable data that the
device persistently sends as part of an NTS-secured NTP association.
This is the justification for continually supplying the client with
fresh cookies, so that a cookie never represents recognizable data in
the sense outlined above.
NTS's unlinkability objective is merely to not leak any additional
data that could be used to link a device's network address. NTS does
not rectify legacy linkability issues that are already present in
NTP. Thus, a client that requires unlinkability must also minimize
information transmitted in a client query (mode 3) packet as
described in the NTP Client Data Minimization Internet-Draft
[I-D.ietf-ntp-data-minimization].
The unlinkability objective only holds for time synchronization
traffic, as opposed to key exchange traffic. This implies that it
cannot be guaranteed for devices that function not only as time
clients, but also as time servers (because the latter can be
externally triggered to send authentication data).
It should also be noted that it could be possible to link devices
that operate as time servers from their time synchronization traffic,
using information exposed in (mode 4) server response packets (e.g.,
reference ID, reference time, stratum, poll). Also, devices that
respond to NTP control queries could be linked using the information
revealed by control queries.
11.2. Confidentiality
NTS does not protect the confidentiality of information in NTP's
header fields. When clients implement NTP Client Data Minimization
[I-D.ietf-ntp-data-minimization], client packet headers do not
contain any information which the client could conceivably wish to
keep secret: one field is random and all others are fixed.
Information in server packet headers is likewise public: the origin
timestamp is copied from the client's (random) transmit timestamp and
all other fields are set the same regardless of the identity of the
client making the request.
Future extension fields could hypothetically contain sensitive
information, in which case NTS provides a mechanism for encrypting
them.
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12. Acknowledgements
The authors would like to thank Richard Barnes, Steven Bellovin,
Scott Fluhrer, Sharon Goldberg, Russ Housley, Martin Langer, Miroslav
Lichvar, Aanchal Malhotra, Dave Mills, Danny Mayer, Karen O'Donoghue,
Eric K. Rescorla, Stephen Roettger, Kurt Roeckx, Kyle Rose, Rich
Salz, Brian Sniffen, Susan Sons, Douglas Stebila, Harlan Stenn,
Martin Thomson, and Richard Welty for contributions to this document
and comments on the design of NTS.
The idea of separation of the NTS-KE server from the NTP server was
added by Marcus Dansarie and Ragnar Sundblad. Thanks for this work
goes to Patrik Faeltstroem (Faltstrom) and Joachim Stroembergsson
(Strombergsson) for review and ideas.
13. References
13.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[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>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[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>.
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[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[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>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
<https://www.rfc-editor.org/info/rfc7507>.
[RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
(NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
March 2016, <https://www.rfc-editor.org/info/rfc7822>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, July 2018,
<https://www.rfc-editor.org/info/rfc8446>.
13.2. Informative References
[I-D.ietf-ntp-data-minimization]
Franke, D. and A. Malhotra, "NTP Client Data
Minimization", draft-ietf-ntp-data-minimization-02 (work
in progress), July 2018.
[Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks
against time synchronization protocols", in Proceedings
of Precision Clock Synchronization for Measurement Control
and Communication, ISPCS 2012, pp. 1-6, September 2012.
[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/info/rfc4086>.
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[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5297] Harkins, D., "Synthetic Initialization Vector (SIV)
Authenticated Encryption Using the Advanced Encryption
Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
2008, <https://www.rfc-editor.org/info/rfc5297>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[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>.
[Shpiner] "Multi-path Time Protocols", in Proceedings of IEEE
International Symposium on Precision Clock Synchronization
for Measurement, Control and Communication (ISPCS),
September 2013.
Appendix A. Terms and Abbreviations
AEAD Authenticated Encryption with Associated Data [RFC5116]
ALPN Application-Layer Protocol Negotiation [RFC7301]
C2S Client-to-server
DDoS Distributed Denial-of-Service
EF Extension Field [RFC5905]
HKDF Hashed Message Authentication Code-based Key Derivation
Function [RFC5869]
IANA Internet Assigned Numbers Authority
IP Internet Protocol
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KoD Kiss-o'-Death [RFC5905]
NTP Network Time Protocol [RFC5905]
NTS Network Time Security
NTS-KE Network Time Security Key Exchange
S2C Server-to-client
SCSV Signaling Cipher Suite Value [RFC7507]
TCP Transmission Control Protocol [RFC0793]
TLS Transport Layer Security [RFC8446]
UDP User Datagram Protocol [RFC0768]
Authors' Addresses
Daniel Fox Franke
Email: dfoxfranke@gmail.com
URI: https://www.dfranke.us
Dieter Sibold
Physikalisch-Technische Bundesanstalt
Bundesallee 100
Braunschweig D-38116
Germany
Phone: +49-(0)531-592-8420
Fax: +49-531-592-698420
Email: dieter.sibold@ptb.de
Kristof Teichel
Physikalisch-Technische Bundesanstalt
Bundesallee 100
Braunschweig D-38116
Germany
Phone: +49-(0)531-592-4471
Email: kristof.teichel@ptb.de
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Marcus Dansarie
Email: marcus@dansarie.se
Ragnar Sundblad
Netnod
Email: ragge@netnod.se
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