Internet DRAFT - draft-bittau-tcpinc-tcpeno
draft-bittau-tcpinc-tcpeno
Network Working Group A. Bittau
Internet-Draft D. Boneh
Intended status: Experimental D. Giffin
Expires: March 13, 2016 Stanford University
M. Handley
University College London
D. Mazieres
Stanford University
E. Smith
Kestrel Institute
September 10, 2015
TCP-ENO: Encryption Negotiation Option
draft-bittau-tcpinc-tcpeno-02
Abstract
Despite growing adoption of TLS [RFC5246], a significant fraction of
TCP traffic on the Internet remains unencrypted. The persistence of
unencrypted traffic can be attributed to at least two factors.
First, some legacy protocols lack a signaling mechanism (such as a
"STARTTLS" command) by which to convey support for encryption, making
incremental deployment impossible. Second, legacy applications
themselves cannot always be upgraded, requiring a way to implement
encryption transparently entirely within the transport layer. The
TCP Encryption Negotiation Option (TCP-ENO) addresses both of these
problems through a new TCP option kind providing out-of-band, fully
backward-compatible negotiation of encryption.
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 http://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 March 13, 2016.
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Copyright Notice
Copyright (c) 2015 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Requirements language . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The TCP-ENO option . . . . . . . . . . . . . . . . . . . . . 4
3.1. TCP-ENO roles . . . . . . . . . . . . . . . . . . . . . . 6
3.2. TCP-ENO handshake . . . . . . . . . . . . . . . . . . . . 7
3.2.1. Handshake examples . . . . . . . . . . . . . . . . . 9
3.3. General suboptions . . . . . . . . . . . . . . . . . . . 10
3.4. Negotiation transcript . . . . . . . . . . . . . . . . . 11
4. Requirements for encryption specs . . . . . . . . . . . . . . 12
4.1. Session IDs . . . . . . . . . . . . . . . . . . . . . . . 13
4.2. Option kind sharing . . . . . . . . . . . . . . . . . . . 14
5. API extensions . . . . . . . . . . . . . . . . . . . . . . . 15
6. Open issues . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Experiments . . . . . . . . . . . . . . . . . . . . . . . 15
6.2. Simultaneous open . . . . . . . . . . . . . . . . . . . . 15
6.3. Multiple Session IDs . . . . . . . . . . . . . . . . . . 16
6.4. Suboption data . . . . . . . . . . . . . . . . . . . . . 17
7. Security considerations . . . . . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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 [RFC2119].
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2. Introduction
Many applications and protocols running on top of TCP today do not
encrypt traffic. This failure to encrypt lowers the bar for certain
attacks, harming both user privacy and system security.
Counteracting the problem demands a minimally intrusive, backward-
compatible mechanism for incrementally deploying encryption. The TCP
Encryption Negotiation Option (TCP-ENO) specified in this document
provides such a mechanism.
While the need for encryption is immediate, future developments could
alter trade-offs and change the best approach to TCP-level encryption
(beyond introducing new cipher suites). For example:
o Increased option space in TCP [I-D.ietf-tcpm-tcp-edo][I-D.briscoe-
tcpm-inspace-mode-tcpbis][I-D.touch-tcpm-tcp-syn-ext-opt] could
reduce round trip times and simplify protocols.
o API revisions to socket interfaces [RFC3493] could benefit from
integration with TCP-level encryption, particularly if combined
with technologies such as DANE [RFC6394].
o The forthcoming TLS 1.3 [I-D.ietf-tls-tls13] standard could reach
more applications given an out-of-band, backward-compatible
mechanism for enabling encryption.
o TCP fast open [RFC7413], as it gains more widespread adoption and
middlebox acceptance, could potentially benefit from tailored
encryption support.
o Cryptographic developments that either shorten or lengthen the
minimal key exchange messages required could affect how such
messages are best encoded in TCP segments.
Introducing TCP options, extending operating system interfaces to
support TCP-level encryption, and extending applications to take
advantage of TCP-level encryption will all require effort. To the
greatest extent possible, this effort ought to remain applicable if
the need arises to change encryption strategies. To this end, it is
useful to consider two questions separately:
1. How to negotiate the use of encryption at the TCP layer, and
2. How to perform encryption at the TCP layer.
This document addresses question 1 with a new option called TCP-ENO.
TCP-ENO provides a framework in which two endpoints can agree on one
among multiple possible TCP encryption _specs_. For future
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compatibility, encryption specs can vary widely in terms of wire
format, use of TCP option space, and integration with the TCP header
and segmentation. A companion document, the TCPINC encryption spec,
addresses question 2. TCPINC enables TCP-level traffic encryption
today. TCP-ENO ensures that the effort invested to deploy TCPINC can
benefit future encryption specs should a different approach at some
point be preferable.
At a lower level, TCP-ENO was designed to achieve the following
goals:
1. Enable endpoints to negotiate the use of a separately specified
encryption _spec_.
2. Transparently fall back to unencrypted TCP when not supported by
both endpoints.
3. Provide signaling through which applications can better take
advantage of TCP-level encryption (for instance by improving
authentication mechanisms in the presence of TCP-level
encryption).
4. Provide a standard negotiation transcript through which specs can
defend against tampering with TCP-ENO.
5. Make parsimonious use of TCP option space.
6. Define roles for the two ends of a TCP connection, so as to name
each end of a connection for encryption or authentication
purposes even following a symmetric simultaneous open.
3. The TCP-ENO option
TCP-ENO is a TCP option used during connection establishment to
negotiate how to encrypt traffic. As an option, TCP-ENO can be
deployed incrementally. Legacy hosts unaware of the option simply
ignore it and never send it, causing traffic to fall back to
unencrypted TCP. Similarly, middleboxes that strip out unknown
options including TCP-ENO will downgrade connections to plaintext
without breaking them. Of course, downgrading makes TCP-ENO
vulnerable to active attackers, but appropriately modified
applications can protect themselves by considering the state of TCP-
level encryption during authentication, as discussed in Section 7.
The ENO option takes two forms. In TCP segments with the SYN flag
set, it acts as a container for a series of one or more suboptions,
labeled "Opt_0", "Opt_1", ... in Figure 1. In non-SYN segments, ENO
conveys only a single bit of information, namely an acknowledgment
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that the sender received an ENO option in the other host's SYN
segment. (Such acknowledgments enable graceful fallback to
unencrypted TCP in the event that a middlebox strips ENO options in
one direction.) Figure 2 illustrates the non-SYN form of the ENO
option. Encryption specs MAY include extra bytes in a non-SYN ENO
option, but TCP-ENO itself MUST ignore them. In accordance with TCP
[RFC0793], the first two bytes of the ENO option always consist of
the kind (ENO) and the total length of the option.
byte 0 1 2 3 2+i 3+i ... N-1
+-----+-----+-----+-----+--...--+-----+----...----+
|Kind=|Len= |Opt_0|Opt_1| |Opt_i| Opt_i |
| ENO | N | | | | | data |
+-----+-----+-----+-----+--...--+-----+----...----+
Figure 1: TCP-ENO option in SYN segment (MUST contain at least one
suboption)
byte 0 1 0 1 2 N-1
+-----+-----+ +-----+-----+-----...----+
|Kind=|Len= | |Kind=|Len= | ignored |
| ENO | 2 | or | ENO | N | by TCP-ENO |
+-----+-----+ +-----+-----+-----...----+
Figure 2: non-SYN TCP-ENO option in segment without SYN flag
Every suboption starts with a byte of the form illustrated in
Figure 3. The seven-bit value "cs" specifies the meaning of the
suboption. Each value of "cs" either specifies general parameters
(discussed in Section 3.3) or indicates the willingness to use a
specific encryption spec detailed in a separate document.
bit 7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+
| v | cs |
+---+---+---+---+---+---+---+---+
v - 1 when suboption followed by variable-length data
cs - global configuration option or encryption spec identifier
Figure 3: Format of suboption byte
The high bit "v" in a suboption's first byte specifies whether or not
the suboption is followed by variable-length data. If "v" is 0, the
suboption consists of only the one byte shown in Figure 3. If "v" is
1, then the suboption is followed by variable-length data. Suboption
data MAY be used for session caching, cipher suite negotiation, key
exchange, or other purposes, as determined by the value of "cs".
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Every suboption but the last in an ENO option MUST be a one-byte
suboption (with "v" = 0). The last suboption MAY be a variable-
length suboption. Its length is determined by the total length of
the TCP option. In Figure 1, "Opt_i" is the variable-length option;
its total size is N-(2+i) bytes--one byte for "Opt_i" itself and
N-(3+i) bytes for additional data. Multiple suboptions with data may
be included in a single TCP SYN segment by repeating the ENO option.
Table 1 summarizes the allocation of values of "cs". Values under
0x10 are assigned to _general suboptions_ whose meaning applies
across encryption specs, as discussed in Section 3.3. Values greater
than or equal to 0x20 will be assigned to _spec identifiers_. Values
in the range 0x10-0x1f are reserved for possible future general
options. Implementations MUST ignore all unknown suboptions.
+-----------+-----------------------------------------------------+
| cs | Meaning |
+-----------+-----------------------------------------------------+
| 0x00-0x0f | General options (see Section 3.3) |
| 0x10-0x1f | Reserved for possible use by future general options |
| 0x20-0x7f | Used to designate encryption specs |
+-----------+-----------------------------------------------------+
Table 1: Allocation of cs bits in TCP-ENO suboptions
3.1. TCP-ENO roles
TCP-ENO uses abstract roles to distinguish the two ends of a TCP
connection: One host plays the "A" role, while the other host plays
the "B" role. Following a normal three-way handshake, the active
opener plays the A role and the passive opener plays the B role. An
active opener is a host that sends a SYN segment without the ACK flag
set (after a "connect" system call on socket-based systems). A
passive opener's SYN segment always contains the ACK flag (and
follows a "listen" call on socket-based systems).
Roles are abstracted from the active/passive opener distinction to
deal with simultaneous open, in which both hosts are active openers.
For simultaneous open, the general suboptions discussed in
Section 3.3 define a tie-breaker bit "b", where the host with "b = 1"
plays the B role, and the host with "b = 0" plays the A role. If two
active openers have the same "b" bit, TCP-ENO fails and reverts to
unencrypted TCP.
More precisely, the above role assignment can be reduced to comparing
a two-bit role _priority_ for each host, shown in Figure 4. The most
significant bit, "p", is 1 for a passive opener and 0 for an active
opener. The least-significant bit "b" is the tie-breaker bit. The
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host with the lower priority assumes the A role; the host with the
higher priority assumes the B role. In the event of a tie, TCP-ENO
fails and MUST continue with unencrypted TCP as if the ENO options
had not been present in SYN segments.
bit 1 0
+---+---+
| p b |
+---+---+
p - 0 for active opener, 1 for passive opener
b - b bit from general suboptions sent by host
Figure 4: Role priority of an endpoint
Encryption specs SHOULD refer to TCP-ENO's A and B roles to specify
asymmetric behavior by the two hosts. For the remainder of this
document, we will use the terms "host A" and "host B" to designate
the hosts with role A and B respectively in a connection.
3.2. TCP-ENO handshake
The TCP-ENO option is intended for use during TCP connection
establishment. To enable incremental deployment, a host needs to
ensure both that the other host supports TCP-ENO and that no
middlebox has stripped the ENO option from its own TCP segments. In
the event that either of these conditions does not hold,
implementations MUST immediately cease sending TCP-ENO options and
MUST continue with unencrypted TCP as if the ENO option had not been
present.
More precisely, for negotiation to succeed, the TCP-ENO option MUST
be present in the SYN segment sent by each host, so as to indicate
support for TCP-ENO. Additionally, the ENO option MUST be present in
the first ACK segment sent by each host, so as to indicate that no
middlebox stripped the ENO option from the ACKed SYN. Depending on
whether a host is an active or a passive opener, the first ACK
segment may or may not be the same as the SYN segment. Specifically:
o An active opener begins with a SYN-only segment, and hence must
send two segments containing ENO options. The initial SYN-only
segment MUST contain an ENO option with at least one suboption, as
pictured in Figure 1. If ENO succeeds, the active opener's first
ACK segment MUST subsequently contain a non-SYN ENO option, as
pictured in Figure 2.
o A passive opener's first transmitted segment has both the SYN and
ACK flags set. Therefore, a passive opener sends an ENO option of
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the type shown in Figure 1 in its single SYN-ACK segment and does
not send a non-SYN ENO option.
A spec identifier in one host's SYN segment is _valid_ if it is
compatible with a suboption in the other host's SYN segment. Two
suboptions are _compatible_ when they have the same "cs" value (>=
0x20) and when the particular combination of "v" bits and suboption
data in suboptions of the two SYN segments is well-defined by the
corresponding encryption spec. Specs MAY allow or disallow any
combination of values of "v" in the two SYN segments.
Once the two sides have exchanged SYN segments, the _negotiated spec_
is the last valid spec identifier in the SYN segment of host B (that
is, the passive opener in the absence of simultaneous open). In
other words, the order of suboptions in host B's SYN segment
determines spec priority, while the order of suboptions in host A's
SYN segment has no effect. Hosts must disable TCP-ENO if there is no
valid spec in host B's SYN segment. Note that negotiation
prioritizes the last rather than the first valid suboption so as to
favor the spec with suboption data, if there is one.
When possible, host B SHOULD send only one spec identifier (suboption
in the range 0x20-0xff), and SHOULD ensure this option is valid.
However, sending a single valid spec identifier is not required, as
doing so could be impractical in some cases, such as simultaneous
open or library-level implementations that can only provide a static
TCP-ENO option to the kernel.
A host MUST disable ENO if any of the following conditions holds:
1. The host receives a SYN segment without an ENO option,
2. The host receives a SYN segment that contains no valid encryption
specs when paired with the SYN segment that the host has already
sent or would otherwise have sent,
3. The host receives a SYN segment containing general suboptions
that are incompatible with the SYN segment that it has already
sent or would otherwise have sent, or
4. The first ACK segment received by a host does not contain an ENO
option.
After disabling ENO, a host MUST NOT transmit any further ENO options
and MUST fall back to unencrypted TCP.
Conversely, if a host receives an ACK segment containing an ENO
option, then encryption MUST be enabled. From this point the host
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MUST follow the encryption protocol of the negotiated spec and MUST
NOT present raw TCP payload data to the application. In particular,
data segments MUST contain ciphertext or key agreement messages as
determined by the negotiated spec, and MUST NOT contain plaintext
application data.
3.2.1. Handshake examples
(1) A -> B: SYN ENO<X,Y>
(2) B -> A: SYN-ACK ENO<Y>
(3) A -> B: ACK ENO<>
[rest of connection encrypted according to spec for Y]
Figure 5: Three-way handshake with successful TCP-ENO negotiation
Figure 5 shows a three-way handshake with a successful TCP-ENO
negotiation. The two sides agree to follow the encryption spec
identified by suboption Y.
(1) A -> B: SYN ENO<X,Y>
(2) B -> A: SYN-ACK
(3) A -> B: ACK
[rest of connection unencrypted legacy TCP]
Figure 6: Three-way handshake with failed TCP-ENO negotiation
Figure 6 shows a failed TCP-ENO negotiation. The active opener (A)
indicates support for specs corresponding to suboptions X and Y.
Unfortunately, at this point one of thee things occurs:
1. The passive opener (B) does not support TCP-ENO,
2. B supports TCP-ENO, but supports neither of specs X and Y, and so
does not reply with an ENO option, or
3. The network stripped the ENO option out of A's SYN segment, so B
did not receive it.
Whichever of the above applies, the connection transparently falls
back to unencrypted TCP.
(1) A -> B: SYN ENO<X,Y>
(2) B -> A: SYN-ACK ENO<X> [ENO stripped by middlebox]
(3) A -> B: ACK
[rest of connection unencrypted legacy TCP]
Figure 7: Failed TCP-ENO negotiation because of network filtering
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Figure 7 Shows another handshake with a failed encryption
negotiation. In this case, the passive opener B receives an ENO
option from A and replies. However, the reverse network path from B
to A strips ENO options. Hence, A does not receive an ENO option
from B, disables ENO, and does not include the required non-SYN ENO
option when ACKing the other host's SYN segment. The lack of ENO in
A's ACK segment signals to B that the connection will not be
encrypted. At this point, the two hosts proceed with an unencrypted
TCP connection.
(1) A -> B: SYN ENO<Y,X>
(2) B -> A: SYN ENO<0x01,X,Y,Z>
(3) A -> B: SYN-ACK ENO<Y,X>
(4) B -> A: SYN-ACK ENO<0x01,X,Y,Z>
[rest of connection encrypted according to spec for Y]
Figure 8: Simultaneous open with successful TCP-ENO negotiation
Figure 8 shows a successful TCP-ENO negotiation with simultaneous
open. Here the first four segments MUST contain an ENO option, as
each side sends both a SYN-only and a SYN-ACK segment. The ENO
option in each hosts's SYN-ACK is identical to the ENO option in its
SYN-only segment, as otherwise connection establishment could not
recover from the loss of a SYN segment. Note the use of the tie-
breaker bit in general suboption 0x01 assigns B its role, as
discussed in Section 3.3. The last valid spec in B's ENO option is
Y, so Y is the negotiated spec.
3.3. General suboptions
Suboptions 0x00-0x0f are used for general conditions that apply
regardless of the negotiated encryption spec. A TCP segment MUST
include at most one ENO suboption whose high nibble is 0. The value
of the low nibble is interpreted as a bitmask, illustrated in
Figure 9.
bit 7 6 5 4 3 2 1 0
+---+---+---+---+---+-------+---+
| 0 0 0 0 z aa b |
+---+---+---+---+---+-------+---+
z - Zero bit (reserved for future use)
aa - Application-aware bits
b - Tie-breaker bit for simultaneous open
Figure 9: Format of the general option byte
The fields of the bitmask are interpreted as follows:
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z The "z" bit is reserved for future revisions of TCP-ENO. Its
value MUST be set to zero in sent segments and ignored in received
segments.
aa The two application-aware bits indicate that the application on
the sending host is aware of TCP-ENO and has been extended to
alter its behavior in the presence of encrypted TCP. There are
four possible values, as shown in Table 2. The default, when
applications have not been modified to take advantage of TCP-ENO,
MUST be 00. However, implementations SHOULD provide an API
through which applications can set the bits to other values and
query for the other host's application-aware bits. The value 01
indicates that the application is aware of TCP-ENO. The value 10
(binary) is reserved for future use. It MUST be interpreted as
the application being aware of TCP-ENO, but MUST never be sent.
Value 11 (binary) indicates that an application is aware of TCP-
ENO and requires application awareness from the other side. If
one host sends value 00 and the other host sends 11, then TCP-ENO
MUST be disabled and fall back to unencrypted TCP. Any other
combination of values (including the reserved 10) is compatible
with enabling encryption. A possible use of value 11 is for
applications that perform legacy encryption and wish to disable
TCP-ENO unless higher-layer encryption can be disabled.
+-------+-------------------------------------------------------+
| Value | Meaning |
+-------+-------------------------------------------------------+
| 00 | Application is not aware of TCP-ENO |
| 01 | Application is aware of TCP-ENO |
| 10 | Reserved but interpreted as ENO-aware |
| 11 | Application awareness is mandatory for use of TCP-ENO |
+-------+-------------------------------------------------------+
Table 2: Meaning of the two application-aware bits
b This is the tie-breaker bit in role priority, discussed in
Section 3.1.
A SYN segment without an explicit general suboption has an implicit
general suboption of 0x00.
3.4. Negotiation transcript
To defend against attacks on encryption negotiation itself,
encryption specs need a way to reference a transcript of TCP-ENO's
negotiation. In particular, an encryption spec MUST fail with high
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probability if its selection resulted from tampering with or forging
initial SYN segments.
TCP-ENO defines its negotiation transcript as a packed data structure
consisting of a series of TCP-ENO options (each including the ENO and
length bytes, as they appeared in the TCP header). Specifically, the
transcript is constructed from the following, in order:
1. Every TCP-ENO option in host A's SYN segment, including the kind
and length bytes, in the order the options appeared in that SYN
segment.
2. A minimal two-byte ENO option, as shown on the left in Figure 2.
3. Every TCP-ENO option in host B's SYN segment, including the kind
and length bytes, in the order the options appeared in that SYN
segment.
4. A minimal two-byte ENO option, as shown on the left in Figure 2.
Note that 2 and 4 merely serve as delimiters to separate the two
hosts' options from each other and from any data that follows the
transcript. Note further that any ignored data in non-SYN ENO
options does not appear in the transcript. Because parts 2 and 4 are
always exactly two bytes and SYN segments MUST NOT contain two-byte
ENO options, this encoding is unambiguous.
For the transcript to be well defined, hosts MUST NOT alter ENO
options in retransmitted segments, or between the SYN and SYN-ACK
segments of a simultaneous open, except that an active opener MAY
remove the ENO option altogether from a retransmitted SYN-only
segment and disable TCP-ENO. Such removal could be useful if
middleboxes are dropping segments with the ENO option.
4. Requirements for encryption specs
TCP-ENO was designed to afford encryption spec authors a large amount
of design flexibility. Nonetheless, to fit all encryption specs into
a coherent framework and abstract most of the differences away for
application writers, all encryption specs claiming ENO "cs" numbers
MUST satisfy the following properties.
o Specs MUST protect TCP data streams with authenticated encryption.
o Specs MUST define a session ID whose value identifies the TCP
connection and, with overwhelming probability, is unique over all
time if either host correctly obeys the spec. Section 4.1
describes the requirements of the session ID in more detail.
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o Specs MUST NOT permit the negotiation of any encryption algorithms
with significantly less than 128-bit security.
o Specs MUST NOT allow the negotiation of null cipher suites, even
for debugging purposes. (Implementations MAY support debugging
modes that allow applications to extract their own session keys.)
o Specs MUST NOT allow the negotiation of encryption modes that do
not provide forward secrecy some bounded, short time after the
close of a TCP connection.
o Specs MUST protect and authenticate the end-of-file marker
traditionally conveyed by TCP's FIN flag when the remote
application calls "close" or "shutdown". However, end-of-file MAY
be conveyed though a mechanism other than TCP FIN. Moreover,
specs MAY permit attacks that cause TCP connections to abort, but
such an abort MUST raise an error that is distinct from an end-of-
file condition.
o Specs MAY disallow the use of TCP urgent data by applications, but
MUST NOT allow attackers to manipulate the URG flag and urgent
pointer in ways that are visible to applications.
4.1. Session IDs
Each spec MUST define a session ID that uniquely identifies each
encrypted TCP connection. Implementations SHOULD expose the session
ID to applications via an API extension. Applications that are aware
of TCP-ENO SHOULD incorporate the session ID value and TCP-ENO role
(A or B) into any authentication mechanisms layered over TCP
encryption so as to authenticate actual TCP endpoints.
In order to avoid replay attacks and prevent authenticated session
IDs from being used out of context, session IDs MUST be unique over
all time with high probability. This uniqueness property MUST hold
even if one end of a connection maliciously manipulates the protocol
in an effort to create duplicate session IDs. In other words, it
MUST be infeasible for a host, even by deviating from the encryption
spec, to establish two TCP connections with the same session ID to
remote hosts obeying the spec.
To prevent session IDs from being confused across specs, all session
IDs begin with the negotiated spec identifier--that is, the last
valid spec identifier in host B's SYN segment. If the "v" bit was 1
in host B's SYN segment, then it is also 1 in the session ID.
However, only the first byte is included, not the suboption data.
Figure 10 shows the resulting format. This format is designed for
spec authors to compute unique identifiers; it is not intended for
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applications authors to pick apart session IDs. Applications SHOULD
treat session IDs as monolithic opaque values and SHOULD NOT discard
the first byte to shorten identifiers.
byte 0 1 2 N-1 N
+-----+------------...------------+
| sub-| collision-resistant hash |
| opt | of connection information |
+-----+------------...------------+
Figure 10: Format of a session ID
Though specs retain considerable flexibility in their definitions of
the session ID, all session IDs MUST meet certain minimum
requirements. In particular:
o The session ID MUST be at least 33 bytes (including the one-byte
suboption), though specs may choose longer session IDs.
o The session ID MUST depend in a collision-resistant way on fresh
data contributed by both sides of the connection.
o The session ID MUST depend in a collision-resistant way on any
public keys, public Diffie-Hellman parameters, or other public
asymmetric cryptographic parameters that are employed by the
encryption spec and have corresponding private data that is known
by only one side of the connection.
o Unless and until applications disclose information about the
session ID, all but the first byte MUST be computationally
indistinguishable from random bytes to a network eavesdropper.
o Applications MAY chose to make session IDs public. Therefore,
specs MUST NOT place any confidential data in the session ID (such
as data permitting the derivation of session keys).
o The session ID MUST depend on the negotiation transcript specified
in Section 3.4 in a collision-resistant way.
4.2. Option kind sharing
This draft does not specify the use of ENO options in any segments
other than the initial SYN and ACK segments of a connection.
Moreover, it does not specify the content of ENO options in an
initial ACK segment that has the SYN flag clear. As a result, any
use of the ENO option kind after the SYN exchange will not conflict
with TCP-ENO. Therefore, encryption specs that require TCP option
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space MAY re-purpose the ENO option kind for use in segments after
the initial SYN.
5. API extensions
Implementations SHOULD provide API extensions through which
applications can query and configure the behavior of TCP-ENO,
including retrieving session IDs, setting and reading application-
aware bits, and specifying which specs to negotiate. The specifics
of such an API are outside the scope of this document.
6. Open issues
This document has experimental status because of several open issues.
Some questions about TCP-ENO's viability depend on middlebox behavior
that can only be determined a posteriori. Hence, initial deployment
of ENO will be an experiment. In addition, a few design questions
exists on which consensus is not clear, and hence for which greater
discussion and justification of TCP-ENO's design may be helpful.
6.1. Experiments
One of the primary open questions is to what extent middleboxes will
permit the use of TCP-ENO. Once TCP-ENO is deployed, we will be in a
better position to gather data on two types of failure:
1. Middleboxes downgrading TCP-ENO connections to unencrypted TCP.
This can happen if middleboxes strip unknown TCP options or if
they terminate TCP connections and relay data back and forth.
2. Middleboxes causing TCP-ENO connections to fail completely. This
can happen if applications perform deep packet inspection and
start dropping segments that unexpectedly contain ciphertext.
The first type of failure is tolerable since TCP-ENO is designed for
incremental deployment anyway. The second type of failure is more
problematic, and, if prevalent, will require the development of
techniques to avoid and recover from such failures.
6.2. Simultaneous open
Simultaneous open is the only way to establish a TCP connection
between TCP hosts in certain NAT configurations [RFC5382]. The
principle challenge in simultaneous open is breaking TCP's symmetry
for both sides to agree on the assignment of the A and B roles.
Relying on TCP/IP header fields such as the IP address, port number,
and initial sequence number is problematic as these values may be
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modified by middleboxes, meaning a sender does not know what values
the recipient will see for these fields.
The authors lack data on how prevalent simultaneous open is in the
wild. The use of simultaneous open has been specified for ICE
[RFC6544], but the highest profile implementation (the firefox
browser) currently prefers UDP over TCP when permitted by firewalls.
Moreover, applications of ICE typically already encrypt data and
would disable TCP-ENO to avoid double encryption. It is therefore
unclear what level of support TCP-ENO should provide for simultaneous
open, or at what cost such support is justified. The working group
has discussed four levels of support with no clear consensus:
1. Require applications to break the tie out of band and assign
themselves A and B roles. If applications do not assign the
roles properly, the TCP connection fails entirely.
2. As above, require applications to specify roles, but if they do
so incorrectly fall back to unencrypted TCP.
3. Require applications to declare that they are using simultaneous
open, but do not require them to negotiate roles. Leave it to
TCP-ENO break the tie and negotiate roles.
4. Design TCP-ENO so that it works completely transparently in
conjunction with simultaneous open, with no application
involvement required.
This simplest and cheapest solution is obviously #1. This document
currently embraces design point #2, at the cost of an extra bit (the
"b" bit in the general suboption) for hosts to check whether roles
were properly assigned. Solution #3 would likely consume 4-8
additional bytes of option space in the case of a simultaneous open,
so as to include a random tie-breaker value. Solution #4 would
consume 4-8 additional bytes of option space in every SYN segment, as
current APIs make it impossible to distinguish a "connect" call
intended for a simultaneous open from one intended for a three-way
handshake.
6.3. Multiple Session IDs
Though currently specs must output a single session ID, it might
alternatively be useful to define multiple identifiers per
connection. As an example, a public session ID might be used to
authenticate a connection, while a private session ID could be used
as an authentication key to link out-of-band data (such as another
TCP connection) to the original connection.
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6.4. Suboption data
TCP-ENO currently optimizes for the case that a single suboption per
SYN segment contains suboption data. This design was chosen in
expectation that the following two use cases will be the most common:
o An active opener advertises support for multiple specs using one-
byte suboptions. The passive opener picks one of the advertised
specs and replies with a single suboption, possibly using
suboption data for options within the negotiated spec. Such spec-
specific options might convey supported elliptic curves or public
key ciphers.
o An active opener advertises support for multiple specs as above,
but also includes a single longer suboption containing a session
caching cookie with which the hosts may be able to avoid the cost
of public key cryptography. In this case, the server either
accepts the cookie or reverts to picking one of the other specs as
in the previous case.
Both of these use cases require at most one multi-byte suboption per
SYN segment. To optimize for this case, TCP-ENO relies on the TCP
option length byte to specify the length of the multi-byte suboption
implicitly. Segments with more than one multi-byte suboption must
repeat the ENO kind byte, losing one byte of precious TCP SYN option
space.
An alternative would be for each multi-byte suboption to be followed
by its own length field. This would cost an extra byte of SYN option
space in the two cases above, but save one byte for each additional
multi-byte suboption.
As an example, in the current ENO design, a SYN segment with ENO
suboption containing 2 bytes of data consumes 5 bytes (the ENO kind,
the TCP option length, the spec identifier, and 2 bytes of suboption
data). An ENO option with two 2-byte suboptions requires double
this, or 10 bytes. By contrast, in a design with a suboption length
byte, one 2-byte suboption would cost 6 bytes (ENO kind, TCP option
length, suboption, suboption length, and 2 bytes of option data), but
two 2-byte suboptions could be packed together, without repeating the
ENO kind byte, in only 9 bytes of option space.
In the event that the above two use cases are not the most prevalent,
it may be worth revisiting ENO's choice of optimized case.
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7. Security considerations
An obvious use case for TCP-ENO is opportunistic encryption.
However, if applications do not check and verify the session ID, they
will be open to man-in-the-middle attacks as well as simple downgrade
attacks in which an attacker strips off the TCP-ENO option. Hence,
where possible, applications SHOULD be modified to fold the session
ID into authentication mechanisms, and SHOULD employ the application-
aware bits as needed to enable such negotiation in a backward-
compatible way.
Because TCP-ENO enables multiple different encryption specs to
coexist, security could potentially be only as strong as the weakest
available encryption spec. For this reason, it is crucial for
session IDs to depend on the TCP-ENO transcript in a strong way.
Hence, encryption specs SHOULD compute session IDs using only well-
studied and conservative hash functions. Thus, even if an encryption
spec is broken, and even if people deprecate it instead of disabling
it, and even if an attacker tampers with ENO options to force
negotiation of the broken spec, it should still be intractable for
the attacker to induce identical session IDs at both hosts.
Implementations MUST not send ENO options unless encryption specs
have access to a strong source of randomness or pseudo-randomness.
Without secret unpredictable data at both ends of a connection, it is
impossible for encryption specs to satisfy the confidentiality and
forward secrecy properties required by this document.
8. IANA Considerations
A new TCP option kind number needs to be assigned to ENO by IANA.
In addition, IANA will need to maintain an ENO suboption registry
mapping suboption "cs" values to encryption specs.
9. Acknowledgments
This work was funded by DARPA CRASH under contract #N66001-10-2-4088.
10. References
10.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
10.2. Informative References
[I-D.briscoe-tcpm-inspace-mode-tcpbis]
Briscoe, B., "Inner Space for all TCP Options (Kitchen
Sink Draft - to be Split Up)", draft-briscoe-tcpm-inspace-
mode-tcpbis-00 (work in progress), March 2015.
[I-D.ietf-tcpm-tcp-edo]
Touch, J. and W. Eddy, "TCP Extended Data Offset Option",
draft-ietf-tcpm-tcp-edo-03 (work in progress), April 2015.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-07 (work in progress),
July 2015.
[I-D.touch-tcpm-tcp-syn-ext-opt]
Touch, J. and T. Faber, "TCP SYN Extended Option Space
Using an Out-of-Band Segment", draft-touch-tcpm-tcp-syn-
ext-opt-02 (work in progress), April 2015.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6", RFC
3493, DOI 10.17487/RFC3493, February 2003,
<http://www.rfc-editor.org/info/rfc3493>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, DOI 10.17487/RFC5382, October 2008,
<http://www.rfc-editor.org/info/rfc5382>.
[RFC6394] Barnes, R., "Use Cases and Requirements for DNS-Based
Authentication of Named Entities (DANE)", RFC 6394, DOI
10.17487/RFC6394, October 2011,
<http://www.rfc-editor.org/info/rfc6394>.
[RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
"TCP Candidates with Interactive Connectivity
Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
March 2012, <http://www.rfc-editor.org/info/rfc6544>.
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[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<http://www.rfc-editor.org/info/rfc7413>.
Authors' Addresses
Andrea Bittau
Stanford University
353 Serra Mall, Room 288
Stanford, CA 94305
US
Email: bittau@cs.stanford.edu
Dan Boneh
Stanford University
353 Serra Mall, Room 475
Stanford, CA 94305
US
Email: dabo@cs.stanford.edu
Daniel B. Giffin
Stanford University
353 Serra Mall, Room 288
Stanford, CA 94305
US
Email: dbg@scs.stanford.edu
Mark Handley
University College London
Gower St.
London WC1E 6BT
UK
Email: M.Handley@cs.ucl.ac.uk
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David Mazieres
Stanford University
353 Serra Mall, Room 290
Stanford, CA 94305
US
Email: dm@uun.org
Eric W. Smith
Kestrel Institute
3260 Hillview Avenue
Palo Alto, CA 94304
US
Email: eric.smith@kestrel.edu
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