Internet Engineering Task Force H. Stenn
Internet-Draft D. Mayer
Intended status: Standards Track Network Time Foundation
Expires: September 26, 2019 March 25, 2019
Network Time Protocol MAC/Last Extension Fields
draft-stenn-ntp-mac-last-ef-04
Abstract
NTP packets can be authenticated by a Message Authentication Code
(MAC) if a MAC is present at the end of an NTP packet. The legacy
format for this MAC is not formatted as an NTP Extension Field, and
its presence may cause some implementations a parsing ambiguity.
This proposal introduces two ways to resolve this problem. One is to
provide a MAC Extension Field. The other is an extension field that
unambiguously declares itself to be the last extension field in an
NTP packet (so any additional data MUST be a legacy MAC).
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH BEFORE PUBLISHING:
The source code and issues list for this draft can be found in
https://github.com/hstenn/ietf-ntp-mac-last-ef
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 26, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. The Last Extension Field Extension Field - LAST-EF . . . . . 3
3. MAC Extension Field . . . . . . . . . . . . . . . . . . . . . 4
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Normative References . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
NTPv4 is defined by RFC 5905 [RFC5905], and it and earlier versions
of the NTP Protocol have supported symmetric private key Message
Authentication Code (MAC) authentication. MACs were first described
in Appendix C of RFC 1305 [RFC1305] and are further described in RFC
5905 [RFC5905]. As the number of Extension Fields grows there is an
increasing chance some implementations will find a parsing ambiguity
when deciding if the "next" set of data is an Extension Field or a
legacy MAC. This proposal defines two new Extension Fields to avoid
this potential ambiguity. One, LAST-EF, is used to signify that it
is the last Extension Field in the packet. If the LAST-EF is
present, any subsequent data MUST be considered to be a legacy MAC,
or if you prefer, any subsequent data MUST NOT be considered to be an
EF. The other, MAC-EF, allows one or more MACs to be encapsulated in
an Extension Field. If all parties in an association support MAC-EF,
the use of a legacy MAC may be avoided.
1.1. 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].
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2. The Last Extension Field Extension Field - LAST-EF
Now that multiple extension fields are a possibility, additional
packet data could be either an Extension Field or a legacy MAC.
Having a means to indicate that there are no more Extension Fields in
an NTP packet and any subsequent data MUST be something else, almost
certainly a legacy MAC, is a valuable facility.
The format of a LAST-EF is an Extension Field comprised of an
identified Field Type and an appropriate Field Length.
In the example below the Field Length in the LAST-EF is 4, because
there is clearly no need in this case for the 28 octets required by
RFC 7822 [RFC7822]. But the LAST-EF could have any supported length,
as any payload is ignored.
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
+---------------+---------------+-------------------------------+
| Field Type | Field Length |
+-------------------------------+-------------------------------+
NTP Extension Field: Last Extension Field - LAST-EF
Field Type: TBD (Recommendation for IANA: 0x0008 (Last Extension
Field))
Field Length: 4 (minimum)
Payload: Ignored if present - none needed. SHOULD be zeroes.
Example:
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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
+---------------+---------------+-------------------------------+
| Field Type (0x0008) | Field Length (0x0004) |
+-------------------------------+-------------------------------+
| MAC Key ID |
+-------------------------------+-------------------------------+
| Sixteen |
+-------------------------------+-------------------------------+
| Octets |
+-------------------------------+-------------------------------+
| of |
+-------------------------------+-------------------------------+
| MAC |
+-------------------------------+-------------------------------+
Example: NTP Extension Field: Last Extension Field, followed by a
Legacy MAC
3. MAC Extension Field
Now that multiple extension fields are a possibility, there is a
chance that additional packet data could be either an Extension Field
or a legacy MAC. There is benefit to encapsulating the MAC in an
extension field. By encapsulating the MAC in an EF, we also have the
option to include multiple MACs in a packet, which may be of use in
broadcast scenarios, for example.
There are two forms of this extension field. The first supports a
single MAC, requiring 4 octets' overhead for the EF header. The
second form supports one or more MACs in the EF payload, and requires
at least 8 octets.
The format of a MAC-EF is an Extension Field comprised of an
identified Field Type and an appropriate Field Length.
A Field Type value of TBD (0x0003 is suggested) identifies this
extension field as a MAC Extension field for a single MAC. In this
case, the payload consists of the four octet MAC Key ID followed by
the MAC digest, and any desired (possibly random data) padding.
A Field Type value of TBD (0x0103 is suggested) identifies this
extension field as a MAC extension field for one or more MACs. In
this case, the payload consists of an unsigned 16-bit MAC Count (N)
followed by N unsigned 16-bit MAC length fields. If there are an
even number of MACs specified there is an unused 16-bit field which
SHOULD be 0x0000 at the end of the set of MAC length values so that
the subsequent MAC data is longword (4-octet) aligned. Each MAC
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SHALL be padded so that any subsequent MAC starts on a 4-octet
boundary. Optional (possibly random data) padding is allowed.
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
+---------------+---------------+-------------------------------+
| Field Type (0x0003) | Field Length |
+-------------------------------+-------------------------------+
. MAC 1 Key ID .
. +-+-+-+-+-+-+-+-+-+-+-+-.
. MAC 1 Key Data | Random Data Padding .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NTP Extension Field: MAC EF Format (Single MAC)
Field Type: TBD (Recommendation for IANA: 0x0003 (MAC-EF: Single
MAC))
Field Length: As needed.
Payload: As described.
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
+---------------+---------------+-------------------------------+
| Field Type (0x0103) | Field Length |
+-------------------------------+-------------------------------+
| MAC Count | MAC 1 Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC 2 Length | MAC 3 Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. MAC 1 Key ID .
. +-+-+-+-+-+-+-+-+-+-+-+-.
. MAC 1 Key Data | Random Data Padding .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. MAC 2 Key ID .
. +-+-+-+-+-+-+-+-+-+-+-+-+-.
. MAC 2 Key Data | Random Data Padding .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. MAC 3 Key ID .
. +-+-+-+-+-+-+-+-+-+-.
. MAC 3 Key Data |Random Data Padding.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding (as needed) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NTP Extension Field: MAC EF Format (1 or more MACs)
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Field Type: TBD (Recommendation for IANA: 0x0103 (MAC-EF: 1 or more
MACs))
Field Length: As needed.
Payload: As described.
A MAC consisting of 4 octets of zeros means the MAC is a crypto-NAK,
as defined by RFC5905 [RFC5905].
Additional MACs SHOULD NOT be present if there is a crypto-NAK
present in the packet.
Each MAC within the extension field consists of a 32-bit key
identifier which SHOULD be unique to the set of key identifiers in
this MAC extension field followed by ((MAC Length) - 4) octets of
data, optionally followed by random octets to pad the key data to the
length specified earlier in the extension field. That key identifier
is a shared secret which defines the algorithm to be used and a
cookie or secret to be used in generating the digest. The MAC digest
is produced by hashing the data from the beginning of the NTP packet
up to but not including the start of the MAC extension field. The
calculation of the digest SHOULD be a hash of this data concatenated
with the 32-bit keyid (in network-order), and the key. When sending
or receiving a key identifier each side needs to agree on the key
identifier, algorithm and the cookie or secret used to produce the
digest along with the digest lengths. Note that the sender may send
more bytes than are required by the digest algorithm. This would be
done to make it more difficult for a casual observer to identify the
algorithm being used based on the length of the data. The digest
data begins immediately after the key ID, and any padding octets
SHOULD be random.
4. Acknowledgements
MAC-EF: The authors gratefully acknowledge Dave Mills for his
insightful comments. Hal Murray asked if there was a way for the
MAC-EF to require only 4 octets of overhead if there was only a
single MAC in the payload.
5. IANA Considerations
This memo requests IANA to allocate NTP Extension Field Types:
0x0003 MAC-EF (Single MAC)
0x0103 MAC-EF (1 or more MACs)
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0x0008 LAST-EF
6. Security Considerations
The security considerations of time protocols in general are
discussed in RFC7384 [RFC7384], and the security considerations of
NTP are discussed in RFC5905 [RFC5905].
Digests MD5, DES and SHA-1 are considered compromised and should not
be used [COMP].
[DISCUSS] Each MAC length should be at least 20 octets long to allow
for 4 octets of key ID and at least 16 octets of digest and random
padding. For a 128-bit digest, there would be 4 octets of key ID, 16
octets of digest, plus any desired octets of random padding. For
SHA-256 digests there are 4 octets of key ID, 32 octets digest, plus
any desired octets of random padding. Using MAC lengths that include
random padding may make it more difficult for an attacker to know
which digest algorithms are used.
7. Normative References
[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation and Analysis", RFC 1305,
DOI 10.17487/RFC1305, March 1992,
<https://www.rfc-editor.org/info/rfc1305>.
[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>.
[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>.
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Authors' Addresses
Harlan Stenn
Network Time Foundation
P.O. Box 918
Talent, OR 97540
US
Email: stenn@nwtime.org
Danny Mayer
Network Time Foundation
P.O. Box 918
Talent, OR 97540
US
Email: mayer@ntp.org
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