Internet DRAFT - draft-touch-tcp-ao-encrypt
draft-touch-tcp-ao-encrypt
TCPM WG J. Touch
Internet Draft
Intended status: Standards Track July 19, 2018
Expires: January 2019
A TCP Authentication Option Extension for Payload Encryption
draft-touch-tcp-ao-encrypt-09.txt
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Abstract
This document describes an extension to the TCP Authentication
Option (TCP-AO) to encrypt the TCP segment payload in addition to
providing TCP-AO's authentication of the payload, TCP header, and IP
pseudoheader. This extension augments how the packet contents and
headers are processed and which keys are derived, and adds a
capability for in-band coordination of unauthenticated Diffie-
Hellman key exchange at connection establishment. The extension
preserves key rollover coordination and protection of long-lived
connections.
Table of Contents
1. Introduction...................................................2
2. Conventions used in this document..............................3
3. Background.....................................................3
4. Authenticated and Unauthenticated Modes........................4
5. Extension for Payload Encryption...............................4
5.1. Additional Master Key Tuple components....................4
5.2. Additional traffic keys...................................5
5.3. Per-Connection TCP-AO Parameters..........................5
5.4. Traffic Encryption Key Derivation Functions...............6
6. TCP-AO-ENC Interaction with TCP................................6
6.1. Sending TCP Segments......................................6
6.2. Receiving TCP Segments....................................7
6.3. Other TCP Impact..........................................7
7. Security Considerations........................................7
8. Keying Algorithms..............................................8
9. IANA Considerations............................................8
10. References....................................................8
10.1. Normative References.....................................8
10.2. Informative References...................................8
11. Acknowledgments...............................................9
1. Introduction
This document describes an extension to the TCP Authentication
Option (TCP-AO) [RFC5925] called TCP-AO-ENC to support its use to
encrypt TCP segment payload contents in addition to authenticating
the segment. TCP-AO-ENC is intended for use where TCP user data
privacy is required and where TCP control protocol protection is
also needed.
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TCP-AO-ENC supports two different modes: authenticated encryption
and unauthenticated (BTNS-style) encryption [RFC5387]. Authenticated
mode (ENC-AUTH) relies on out-of-band coordination of master key
tuples in the same way as TCP-AO, and protects all segments of a
connection. Unauthenticated (ENC-BTNS) mode supports in-band
unsigned Diffie-Hellman key exchange during the initial SYN, and
protects connections from all except man-in-the-middle attacks
during connection establishment.
This document assumes detailed familiarity with TCP-AO [RFC5925].
TCP-AO-ENC extends how TCP-AO generates traffic keys and how those
keys are used to process TCP segment headers and payloads, but does
not otherwise alter other aspects of the TCP-AO mechanism [RFC5926].
2. Conventions used in this document
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].
When used in lower case, these words have their conventional meaning
and do not convey the interpretations in RFC-2119.
3. Background
The premise of TCP-AO-ENC is that it might be useful to allow TCP-AO
to encrypt TCP segment payloads, in addition to authenticating the
entire segment.
This is accomplished by the following additions, as a preview:
o An encryption flag to indicate when segment payload encryption is
used.
o Traffic encryption key, in addition to the TCP-AO traffic
(authentication) key. TCP-AO-ENC can be used with only symmetric
ciphers that avoiding the need for padding (stream ciphers).
o Augment input and output processing to include
encryption/decryption.
TCP-AO-ENC does not change any other aspects of TCP-AO [RFC5925],
and is compatible with TCP-AO-NAT [RFC6978]. TCP-AO-NAT is intended
for use only where coordinated between endpoints for connections
that match the shared Master Key Tuple (MKT) parameters, as with all
other MKT parameters.
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4. Authenticated and Unauthenticated Modes
TCP-AO-ENC includes two modes: authenticated (ENC-AUTH) and
unauthenticated (ENC-BTNS). The latter is consistent with the
"Better Than Nothing Security" approach to protect communication in
the absence of public key infrastructure (PKI) or pre-shared keys
[RFC5387].
ENC-AUTH mode operates in the same way as original TCP-AO
authentication, by relying on out-of-band Master Key Tuples (MKTs)
that are deployed in advance of new connections. All segments of
connections covered by ENC-AUTH are encrypted and authenticated
using keying material derived from those MKTs.
ENC-BTNS mode can be used without an out-of-band key exchange
protocol. It exchanges unauthenticated, unsigned Diffie-Hellman
nonces during connection establishment in-band, and uses that
information to derive keys used to protect the remainder of the
connection. ENC-BTNS mode is susceptible to man-in-the-middle
attacks in which the adversary both participates in the initial
nonce exchange and processes subsequent segments; this protection
increases the effort of the attacker and can help avoid low-effort
DDOS attacks that disrupt established connections [RFC4953].
Because ENC-BTNS uses in-band nonce exchange only during the initial
SYN, TCP-AO key rollover is not used in that mode. The KeyIDs used
during the nonce exchange are recorded and used throughout the
connection.
5. Extension for Payload Encryption
The following describe the additions to TCP-AO needed to support
TCP-AO-ENC.
5.1. Additional Master Key Tuple components
TCP-AO-ENC augments the MKT as follows; as with other MKT
components, these MUST NOT change during a connection:
o TCP encryption mode. This indicates the use and mode (ENC-AUTH
or ENC-BTNS) of segment payload encryption, or is clear when
encryption is not used (e.g., for conventional TCP-AO).
o Encryption Key Derivation Function (E-KDF). Indicates the key
derivation function and its parameters, as used to generate
traffic encryption keys from master keys in the same way that the
TCP-AO KDG generates traffic (authentication) keys.
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o Encryption algorithm. Indicates the encryption algorithm and its
parameters as used for encrypted connections.
PTCP-AO-ENC processes TCP packets in the same way as TCP-AO, except
that it replaces the authentication input and output processing as
follows:
5.2. Additional traffic keys
TCP-AO-ENC uses the E-KDF to derive four additional keys used for
traffic encryption:
o Send_SYN_traffic_encryption_key
o Receive_SYN_traffic_encryption_key
o Send_other_traffic_encryption_key
o Receive_other_traffic_encryption_key
5.3. Per-Connection TCP-AO Parameters
The per-connection TCP-AO parameters are not affected by the use of
TCP-AO-ENC-AUTH, except that MKTs indicated by Current_key and
Rnext_key would indicate the use of payload encryption.
The per-connection TCP-AO parameters for TCP-AO-ENC-BTNS are
augmented by the addition of the following Diffie-Hellman nonces:
o Send_nonce. The locally-generated Diffie-Hellman nonce.
o Receive_nonce. The Diffie-Hellman nonce generated by the remote
end of the connection.
These nonces are exchanged during the initial SYN exchange in ENC-
BTNS mode; for ENC-AUTH mode, similar information is exchanged out-
of-band and is used to derive the encryption keys. KeyIDs used with
these nonces are recorded during nonce exchange and used for the
remainder of the connection.
The use of payload encryption as specified in these MKTs SHOULD NOT
change during a TCP connection.
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5.4. Traffic Encryption Key Derivation Functions
Traffic encryption keys are derived from the MKTs using the E-KDF,
in the same way and used on the same segments as their corresponding
authentication keys, e.g.:
o Send_SYN_traffic_encryption_key / Send_SYN_traffic_key
o Receive_SYN_traffic_encryption_key / Receive_SYN_traffic__key
o Send_other_traffic_encryption_key / Send_other_traffic__key
o Receive_other_traffic_encryption_key / Receive_other_traffic_key
6. TCP-AO-ENC Interaction with TCP
TCP-AO-ENC augments TCP segment send and receive processing to
include encryption/decryption. Note that the encryption
initialization vector MAY depend on TCP header state, but MUST NOT
depend on the processing of previous segments because segments may
arrive (and need to be decrypted) out of order.
6.1. Sending TCP Segments
For ENC-BTNS, initial SYN and SYN-ACK are used to establish the
Diffie-Hellman nonces as follows:
o Initial SYN and SYN-ACK. The initial SYN (SYN and not ACK) and
SYN-ACK segments are not encrypted or authenticated. Instead,
their HMAC field contains the Diffie-Hellman nonce in network-
standard byte order.
The size of the Diffie-Hellman nonce determines the strength of the
resulting association. For TCP, such nonces are limited to the
available option space in the SYN and SYN-ACK segments. This
currently limits the nonce to 128 bits (16 bytes). Larger nonces can
be supported using extensions to expand the TCP SYN option space
[Bo14][Br14][To18].
Because these segments are not authenticated or encrypted, they
SHOULD NOT contain user data. In a typical client-server system,
user data usually commences in other segments anyway.
All other TCP segments are processed as follows:
1. The segment payload is encrypted in-place using the traffic
encryption key.
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2. The segment is authenticated using TCP-AO as per [RFC5925].
6.2. Receiving TCP Segments
For ENC-BTNS, initial SYN and SYN-ACK are used to establish the
Diffie-Hellman nonces as follows:
o Initial SYN and SYN-ACK. The unauthenticated Diffie-Hellman nonce
is extracted from the HMAC field, and used to construct the
encryption and traffic keys for the connection. Because these
segments are not encrypted or authenticated, no further
processing is required.
All other incoming TCP segments are processed as follows:
1. TCP-AO authenticates the segment, including discarding it if
authentication fails, as per [RFC5925].
2. The segment payload is decrypted in-place using the traffic
encryption key.
6.3. Other TCP Impact
TCP-AO-ENC has no impact on TCP beyond that of TCP-AO, including
impact on TCP header size, connectionless resets, and ICMP handling.
TCP-AO-ENC is compatible with the use of TCP-AO-NAT if traversal of
NAT boxes is desired.
7. Security Considerations
TCP-AO-ENC augments TCP-AO to provide segment payload privacy.
TCP-AO-ENC relies on TCP-AO's authentication to avoid replay attacks
and to ensure that the segments originate from the intended source.
TCP-AO-ENC supports only stream ciphers because the TCP segment must
be encrypted and decrypted in-situ. Support for padding would
require additional option space to indicate the original message
length, and this complication does not seem necessary.
The design of TCP-AO-ENC can support either symmetric or asymmetric
keys. However, because TCP-AO derives traffic (authentication) keys
from MKTs using KDFs, it was deemed sufficient that TCP-AO-ENC
derive traffic encryption keys from MKTs using E-KDFs in a similar
manner, and both endpoints would thus derive the same traffic
encryption keys just as they derive the same traffic
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(authentication) keys. Extensions of TCP-AO-ENC to support
asymmetric keying are possible if traffic keys are managed using an
out-of-band mechanism, but not if they are derived from MKTs.
ENC-AUTH has no additional security considerations. ENC-BTNS cannot
authenticate or encrypt the segments used for nonce exchange, i.e.,
the initial SYN and SYN-ACK. As a result, ENC-BTNS is susceptible to
man-in-the-middle attacks during connection establishment, but
remains useful to ensure that established connections are protected.
8. Keying Algorithms
TCP-AO-ENC algorithms are specified in a separate document, as was
the custom for TCP-AO.
NOTES [for that doc]:
o E-KDF - also, can a MKT use the same alg for KDF and E-KDF?
o Encryption algorithm - possibilities include AES CTR (CTR initial
value can be the ESN) or AES CBC and Camellia CBC as per TLS 1.2.
9. IANA Considerations
There are no IANA considerations for this document. This section can
be removed upon publication as an RFC.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5925] Touch, J., A. Mankin, R. Bonica, "The TCP Authentication
Option", RFC 5925, Jun. 2010.
[RFC5926] Lebovitz, G., E. Rescorla, "Cryptographic Algorithms for
the TCP Authentication Option (TCP-AO)", RFC 5926, June
2010.
10.2. Informative References
[Bo14] Borman, D., "TCP Four-Way Handshake", draft-borman-
tcp4way-00 (work in progress), Oct. 2014.
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[Br14] Briscoe, B., "Inner Space for TCP Options", draft-briscoe-
tcpm-inner-space-01 (work in progress), Oct. 2014.
[RFC4953] J. Touch, "Defending TCP Against Spoofing Attacks", RFC
4953, July 2007.
[RFC5387] Touch, J., Black, D., and Y. Wang, "Problem and
Applicability Statement for Better-Than-Nothing Security
(BTNS)", RFC 5387, November 2008.
[RFC6978] Touch, J., "A TCP Authentication Option Extension for NAT
Traversal", RFC 6978, July 2013.
[To18] Touch, J., and T. Faber, "TCP SYN Extended Option Space
Using an Out-of-Band Segment", draft-touch-tcpm-tcp-syn-
ext-opt (work in progress), Jan. 2018.
11. Acknowledgments
This extension was informed by discussions with Gene Tsudik, and
various members of the TCPM and TCPCRYPT mailing lists, including
Christian von Roques.
This work is partly supported by USC/ISI's Postel Center.
This document was prepared using 2-Word-v2.0.template.dot.
Author's Address
Joe Touch
Manhattan Beach, CA 90266 USA
Phone: +1 (310) 560-0334
Email: touch@strayalpha.com
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