TCP-ENO: Encryption Negotiation Option
draft-ietf-tcpinc-tcpeno-02
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8547.
|
|
|---|---|---|---|
| Authors | Andrea Bittau, Dan Boneh , Daniel B. Giffin , Mark J. Handley , David Mazieres , Eric W. Smith | ||
| Last updated | 2016-06-29 | ||
| Replaces | draft-bittau-tcpinc-tcpeno | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
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| IESG | IESG state | Became RFC 8547 (Experimental) | |
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| Responsible AD | Mirja Kühlewind | ||
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draft-ietf-tcpinc-tcpeno-02
Network Working Group A. Bittau
Internet-Draft D. Boneh
Intended status: Experimental D. Giffin
Expires: January 1, 2017 Stanford University
M. Handley
University College London
D. Mazieres
Stanford University
E. Smith
Kestrel Institute
June 30, 2016
TCP-ENO: Encryption Negotiation Option
draft-ietf-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 January 1, 2017.
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Copyright Notice
Copyright (c) 2016 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 . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Design goals . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. TCP-ENO specification . . . . . . . . . . . . . . . . . . . . 5
4.1. ENO option . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. General suboptions . . . . . . . . . . . . . . . . . . . 8
4.3. TCP-ENO roles . . . . . . . . . . . . . . . . . . . . . . 10
4.4. Specifying suboption data length . . . . . . . . . . . . 10
4.5. The negotiated spec . . . . . . . . . . . . . . . . . . . 12
4.6. TCP-ENO handshake . . . . . . . . . . . . . . . . . . . . 12
4.7. Negotiation transcript . . . . . . . . . . . . . . . . . 13
5. Requirements for encryption specs . . . . . . . . . . . . . . 14
5.1. Session IDs . . . . . . . . . . . . . . . . . . . . . . . 14
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Design rationale . . . . . . . . . . . . . . . . . . . . . . 17
7.1. Future developments . . . . . . . . . . . . . . . . . . . 17
7.2. Handshake robustness . . . . . . . . . . . . . . . . . . 18
7.3. Suboption data . . . . . . . . . . . . . . . . . . . . . 18
7.4. Passive role bit . . . . . . . . . . . . . . . . . . . . 19
7.5. Option kind sharing . . . . . . . . . . . . . . . . . . . 19
8. Experiments . . . . . . . . . . . . . . . . . . . . . . . . . 19
9. Security considerations . . . . . . . . . . . . . . . . . . . 20
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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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].
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.
Introducing TCP options, extending operating system interfaces to
support TCP-level encryption, and extending applications to take
advantage of TCP-level encryption all require effort. To the
greatest extent possible, the effort invested in realizing TCP-level
encryption today needs to remain applicable in the future should the
need arise 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 TCP option, ENO. TCP-
ENO provides a framework in which two endpoints can agree on one
among multiple possible TCP encryption _specs_. For future
compatibility, encryption specs can vary widely in terms of wire
format, use of TCP option space, and integration with the TCP header
and segmentation. However, ENO abstracts these differences to ensure
the introduction of new encryption specs can be transparent to
applications taking advantage of TCP-level encryption.
Question 2 is addressed by one or more companion documents describing
encryption specs. While current specs enable TCP-level traffic
encryption today, TCP-ENO ensures that the effort invested to deploy
today's specs will additionally benefit future specs.
2.1. Design goals
TCP-ENO was designed to achieve the following goals:
1. Enable endpoints to negotiate the use of a separately specified
TCP encryption _spec_.
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2. Transparently fall back to unencrypted TCP when not supported by
both endpoints.
3. Provide out-of-band 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. Terminology
We define the following terms, which are used throughout this
document:
SYN segment
A TCP segment in which the SYN flag is set
ACK segment
A TCP segment in which the ACK flag is set (which includes most
segments other than an initial SYN segment)
non-SYN segment
A TCP segment in which the SYN flag is clear
SYN-only segment
A TCP segment in which the SYN flag is set but the ACK flag is
clear
SYN-ACK segment
A TCP segment in which the SYN and ACK flags are both set
Active opener
A host that sends a SYN-only segment. With the BSD socket API,
this occurs when an application calls "connect". In client-server
configurations, active openers are typically clients.
Passive opener
A host that does not send a SYN-only segment (only a SYN-ACK
segment). With the BSD socket API, this occurs following a call
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to "listen". In client-server configurations, passive openers are
typically servers.
Simultaneous open
The act of symmetrically establishing a TCP connection between two
active openers (both of which call "connect" with BSD sockets).
Each host of a simultaneous open sends both a SYN-only and a SYN-
ACK segment. Simultaneous open is less common than asymmetric
open, but can be used for NAT traversal by peer-to-peer
applications [RFC5382].
Encryption spec
A separate document specifying an approach to encrypting TCP
traffic in conjunction with TCP-ENO.
Spec identifier
A unique 7-bit value in the range 0x20-0x7f that IANA has assigned
to an encryption spec.
Negotiated [encryption] spec
The single encryption spec governing a TCP connection, as
determined by the protocol specified in this document.
4. TCP-ENO specification
TCP-ENO extends TCP connection establishment to enable encryption
opportunistically. It uses a new TCP option kind to negotiate one
among multiple possible encryption specs--separate documents
describing how to do actual traffic encryption. The negotiation
involves hosts exchanging sets of supported specs, where each spec is
represented by a _suboption_ within a larger TCP option in the
offering host's SYN segment.
If TCP-ENO succeeds, it yields the following information:
o A negotiated encryption spec, represented by a unique 7-bit spec
identifier,
o A few extra bytes of suboption data from each host, if needed by
the spec,
o A negotiation transcript that the negotiated spec must
cryptograhpically authenticate,
o Role assignments designating one endpoint "host A" and the other
endpoint "host B", and
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o A few bits indicating whether or not the application at each end
knows it is using TCP-ENO.
If TCP-ENO fails, encryption is disabled and the connection falls
back to traditional unencrypted TCP.
The remainder of this section provides the normative description of
the TCP ENO option and handshake protocol.
4.1. ENO option
TCP-ENO employs an option in the TCP header [RFC0793]. There are two
equivalent kinds of ENO option, shown in Figure 1. Section 10
specifies which of the two kinds is permissible and/or preferred.
byte 0 1 2 N+1 (N+2 bytes total)
+-----+-----+-----+--....--+-----+
|Kind=|Len= | |
| 69 | N+2 | contents (N bytes) |
+-----+-----+-----+--....--+-----+
byte 0 1 2 3 4 N+3 (N+4 bytes total)
+-----+-----+-----+-----+-----+--....--+-----+
|Kind=|Len= | ExID | |
| 253 | N+4 | 69 | 78 | contents (N bytes) |
+-----+-----+-----+-----+-----+--....--+-----+
Figure 1: Two equivalent kinds of TCP-ENO option
The contents of an ENO option can take one of two forms. A SYN form,
illustrated in Figure 2, appears only in SYN segments. A non-SYN
form, illustrated in Figure 3, appears only in non-SYN segments. The
SYN form of ENO acts as a container for one or more suboptions,
labeled "Opt_0", "Opt_1", ... in Figure 2. The non-SYN form, by its
presence, acts as a one-bit acknowledgment, with the actual contents
ignored by ENO. Particular encryption specs MAY assign additional
meaning to the contents of non-SYN ENO options. When a negotiated
spec does not assign such meaning, the contents of a non-SYN ENO
option SHOULD be zero bytes.
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byte 0 1 2 3 ... N+1
+-----+-----+-----+-----+--...--+-----+----...----+
|Kind=|Len= |Opt_0|Opt_1| |Opt_i| Opt_i |
| 69 | N+2 | | | | | data |
+-----+-----+-----+-----+--...--+-----+----...----+
byte 0 1 2 3 4 5 ... N+3
+-----+-----+-----+-----+-----+-----+--...--+-----+----...----+
|Kind=|Len= | ExID |Opt_0|Opt_1| |Opt_i| Opt_i |
| 253 | N+4 | 69 | 78 | | | | | data |
+-----+-----+-----+-----+-----+-----+--...--+-----+----...----+
Figure 2: SYN form of ENO
byte 0 1 2 N+1
+-----+-----+-----...----+
|Kind=|Len= | ignored |
| 69 | N+2 | by TCP-ENO |
+-----+-----+-----...----+
byte 0 1 2 3 4 N+3
+-----+-----+-----+-----+-----...----+
|Kind=|Len= | ExID | ignored |
| 253 | N+4 | 69 | 78 | by TCP-ENO |
+-----+-----+-----+-----+-----...----+
Figure 3: Non-SYN form of ENO, where N MAY be 0
Every suboption starts with a byte of the form illustrated in
Figure 4. The high bit "v", when set, introduces suboptions with
variable-length data. When "v = 0", the byte itself constitutes the
entirety of the suboption. The 7-bit value "cs" expresses one of:
o Global configuration data (discussed in Section 4.2),
o Suboption data length for the next suboption (discussed in
Section 4.4), or
o An offer to use a particular encryption spec detailed in a
separate document.
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bit 7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+
| v | cs |
+---+---+---+---+---+---+---+---+
v - non-zero for use with variable-length suboption data
cs - global configuration option or encryption spec identifier
Figure 4: Format of initial suboption byte
Table 1 summarizes the meaning of initial suboption bytes. Values of
"cs" greater than or equal to 0x20 are spec identifiers, while those
below 0x20 are shared between general suboptions and length bytes.
When "v = 0", the initial suboption byte constitutes the entirety of
the suboption and all information is expressed by the 7-bit value
"cs", which can be a spec identifier or general suboption. When "v =
1", it indicates a suboption with one or more bytes of suboption
data. Only spec identifiers may have suboption data, not general
suboptions. Hence, bytes with "v = 1" and "cs < 0x20" are not
general suboptions but rather length bytes governing the length of
the next suboption. In the absence of a length byte, a spec
identifier suboption with "v = 1" has suboption data extending to the
end of the TCP option.
+-----------+---+--------------------------------------------+
| cs | v | Meaning |
+-----------+---+--------------------------------------------+
| 0x00-0x1f | 0 | General suboption (Section 4.2) |
| 0x00-0x1f | 1 | Length byte (Section 4.4) |
| 0x20-0x7f | 0 | Encryption spec without suboption data |
| 0x20-0x7f | 1 | Encryption spec followed by suboption data |
+-----------+---+--------------------------------------------+
Table 1: Initial suboption byte values
A SYN segment MUST contain at most one ENO TCP option. If a SYN
segment contains more than one ENO option, the receiver MUST behave
as though the segment contained no ENO options and disable
encryption. An encryption spec MAY define the use of multiple ENO
options in a non-SYN segment. For non-SYN segments, ENO itself only
distinguishes between the presence or absence of ENO options;
multiple ENO options are interpreted the same as one.
4.2. General suboptions
Suboptions 0x00-0x1f are used for general conditions that apply
regardless of the negotiated encryption spec. A TCP SYN segment MUST
include at most one ENO suboption in this range. A receiver MUST
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ignore all but the first suboption in this range so as to anticipate
future revisions of ENO that assign new meaning to bits in subsequent
general suboptions. The value of a general suboption byte is
interpreted as a bitmask, illustrated in Figure 5.
bit 7 6 5 4 3 2 1 0
+---+---+---+-------+---+---+---+
| 0 | 0 | 0 | zz | m | a | b |
+---+---+---+-------+---+---+---+
b - Passive role bit
a - Application-aware bit
m - Middleware signaling bit
zz - Zero bits (reserved for future use)
Figure 5: Format of the general option byte
The fields of the bitmask are interpreted as follows:
b
The passive role bit MUST be 1 for all passive openers. For
active openers, it MUST default to 0, but implementations SHOULD
provide an API through which an application can set "b = 1" before
calling "connect". (Manual configuration of "b" is necessary for
simultaneous open.)
a
The application-aware bit "a" is an out-of-band signal indicating
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. Implementations MUST set this bit to 0 by default,
and SHOULD provide an API through which applications can change
the value of the bit as well as examine the value of the bit sent
by the remote host. Implementations SHOULD furthermore support a
_mandatory_ application-aware mode in which TCP-ENO is
automatically disabled if the remote host does not set "a = 1".
m
The middleware bit "m" functions similarly to the application-
aware bit "a". It is available for middleware such as shared
libraries needing out-of-band signaling to improve the security of
legacy applications. Implementations MUST set this bit to 0 by
default and SHOULD provide an API through which software can
change the value. Unlike the application-aware "a" bit, no
mandatory mode is needed for the middleware bit.
zz
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The "zz" bits are reserved for future revisions of TCP-ENO. They
MUST be set to zero in sent segments and MUST be ignored in
received segments.
A SYN segment without an explicit general suboption has an implicit
general suboption of 0x00. Because passive openers MUST always set
"b = 1", they cannot rely on this implicit 0x00 byte and MUST include
an explicit general suboption in the ENO options of their SYN-ACK
segments.
4.3. TCP-ENO roles
TCP-ENO uses abstract roles to distinguish the two ends of a TCP
connection. These roles are determined by the "b" bit in the general
suboption. The host that sent an implicit or explicit suboption with
"b = 0" plays the "A" role. The host that sent "b = 1" plays the "B"
role.
If both sides of a connection set "b = 1" (which can happen if the
active opener misconfigures "b" before calling "connect"), or both
sides set "b = 0" (which can happen with simultaneous open), then
TCP-ENO MUST be disabled and the connection MUST fall back to
unencrypted TCP.
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 A and B roles, respectively, in a connection.
4.4. Specifying suboption data length
An encryption spec MAY optionally specify the use of one or more
bytes of suboption data. The presence of such data is indicated by
setting "v = 1" in the initial suboption byte (see Figure 4). By
default, suboption data extends to the end of the TCP option. Hence,
if only one suboption requires data, the most compact way to encode
it is to place it last in the ENO option, after all one-byte
suboptions. As an example, in Figure 2, the last suboption, "Opt_i",
has suboption data and thus requires "v = 1"; however, the suboption
data length can be implicit in the total length of the TCP option.
When a suboption with data is not last in an ENO option, the sender
MUST explicitly specify the suboption data length for the receiver to
know where the next suboption starts. The sender does so by
preceding the suboption with a length byte.
Figure 6 shows the format of a length byte. It encodes a 5-bit value
"nnnnn". Adding one to "nnnnn" yields the length of the suboption
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data not including the length byte and initial spec identifier byte.
Hence, a length byte can designate a suboption carrying anywhere from
1 to 32 bytes of suboption data (inclusive).
bit 7 6 5 4 3 2 1 0
+---+---+---+-------------------+
| 1 0 0 nnnnn |
+---+---+---+-------------------+
nnnnn - 5-bit value encoding (length - 1)
Figure 6: Format of a length byte
A suboption preceded by a length byte MUST be a spec identifier ("cs
>= 0x20") and MUST have "v = 1". Figure 7 shows an example of such a
suboption.
byte 0 1 2 nnnnn+2 (nnnnn+3 bytes total)
+------+------+-------...-------+
|length| spec | suboption data |
| byte |ident.| (nnnnn+1 bytes) |
+------+------+-------...-------+
length byte - specifies nnnnn
spec identifier - MUST have v = 1 and cs >= 0x20
suboption data - length specified by nnnnn+1
Figure 7: Suboption with length byte
If the octet following a length byte has the high bit clear (meaning
"v = 0"), then the length byte and following octet together are
interpreted as a length word, as shown in Figure 8. The length word
encodes an 8-bit value corresponding to one less than the suboption
data length. As with length bytes, spec identifiers MUST have "v =
1".
bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 1 0 0 zzzz | m | 0 | nnnnnnn |
+---+---+---+-------------------+---+---------------------------+
nnnnnnn - 7 least significant bits of 8-bit value (length - 1)
m - Most significant bit of 8-bit value (length - 1)
zzzz - Bits that MUST be zero (reserved for future use)
Figure 8: Format of a length word
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The "zzzz" bits in a length word MUST be set to 0 by a sender. If a
host receives a length word in which the "zzzz" bits are not all 0,
it MUST ignore the entire received ENO option and disable encryption.
Similarly, if a length byte or word in a received SYN segment
indicates that a TCP-ENO suboption would extend beyond the end of the
ENO TCP option, the receiver MUST behave as though the received SYN
segment contained no TCP-ENO option and disable encryption.
4.5. The negotiated spec
A spec identifier "cs" is _valid_ for a connection when:
1. Each side has sent a suboption for "cs" in its SYN-form ENO
option,
2. Any suboption data in these "cs" suboptions is valid according to
the spec and satisfies any runtime constraints, and
3. If one host sends multiple suboptions with "cs", then such
repetition is well-defined by the encryption spec.
The _negotiated encryption spec_ is the last valid spec identifier in
host B's SYN-form ENO option. This means host B specifies suboptions
in order of increasing priority, while host A does not influence spec
priority.
A passive opener (which is always host B) sees the remote host's SYN
segment before constructing its own SYN-ACK. Hence, a passive opener
SHOULD include only one spec identifier in SYN-ACK segments and
SHOULD ensure this spec identifier is valid. However, simultaneous
open or implementation considerations can prevent host B from
offering only one encryption spec.
4.6. TCP-ENO handshake
A host employing TCP-ENO for a connection MUST include an ENO option
in every TCP segment sent until either encryption is disabled or the
host receives a non-SYN segment.
A host MUST disable encryption, refrain from sending any further ENO
options, and fall back to unencrypted TCP if any of the following
occurs:
1. Any segment it receives up to and including the first received
ACK segment does not contain an ENO option,
2. The SYN segment it receives does not contain a valid spec
identifier, or
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3. It receives a SYN segment with an incompatible general suboption.
(Specifically, incompatible means the two hosts set the same "b"
value or the connection is in mandatory application-aware mode
and the remote host set "a = 0".)
Hosts MUST NOT alter SYN-form ENO options in retransmitted segments,
or between the SYN and SYN-ACK segments of a simultaneous open, with
two exceptions for an active opener. First, an active opener MAY
unilaterally disable ENO (and thus remove the ENO option) between
retransmissions of a SYN-only segment. (Such removal could be useful
if middleboxes are dropping segments with the ENO option.) Second,
an active opener performing simultaneous open MAY include no TCP-ENO
option in its SYN-ACK if the received SYN caused it to disable
encryption according to the above rules (for instance because role
negotiation failed).
Once a host has both sent and received an ACK segment containing an
ENO option, encryption MUST be enabled. Once encryption is enabled,
hosts 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.
4.7. 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
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 two TCP-ENO options exactly as they appeared in the TCP
header (including the TCP option kind, TCP option length byte, and,
for option kind 253, the bytes 69 and 78 as illustrated in Figure 1).
The transcript is constructed from the following, in order:
1. The TCP-ENO option in host A's SYN segment, including the kind
and length bytes.
2. The TCP-ENO option in host B's SYN segment, including the kind
and length bytes.
Note that because the ENO options in the transcript contain length
bytes as specified by TCP, the transcript unambiguously delimits A's
and B's ENO options.
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5. Requirements for encryption specs
Though TCP-ENO affords spec authors a large amount of design
flexibility, to abstract spec differences away from applications
requires fitting them all into a coherent framework. As such, any
encryption spec claiming an ENO spec identifier MUST satisfy the
following normative list of 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 5.1
describes the requirements of the session ID in more detail.
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 depend on long-lived secrets for data
confidentiality, as implementations SHOULD 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.
5.1. 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.
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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 9 shows the resulting format. This format is designed for
spec authors to compute unique identifiers; it is not intended for
application 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 9: 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.
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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 4.7 in a collision-resistant way.
6. Examples
This subsection illustrates the TCP-ENO handshake with a few non-
normative examples.
(1) A -> B: SYN ENO<X,Y>
(2) B -> A: SYN-ACK ENO<b=1,Y>
(3) A -> B: ACK ENO<>
[rest of connection encrypted according to spec for Y]
Figure 10: Three-way handshake with successful TCP-ENO negotiation
Figure 10 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 11: Three-way handshake with failed TCP-ENO negotiation
Figure 11 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 several 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,
3. B supports TCP-ENO, but has the connection configured in
mandatory application-aware mode and thus disables ENO because
A's SYN segment does not set the application-aware bit, or
4. 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.
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(1) A -> B: SYN ENO<X,Y>
(2) B -> A: SYN-ACK ENO<b=1,X> [ENO stripped by middlebox]
(3) A -> B: ACK
[rest of connection unencrypted legacy TCP]
Figure 12: Failed TCP-ENO negotiation because of network filtering
Figure 12 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 a non-SYN form 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<b=1,X,Y,Z>
(3) A -> B: SYN-ACK ENO<Y,X>
(4) B -> A: SYN-ACK ENO<b=1,X,Y,Z>
[rest of connection encrypted according to spec for Y]
Figure 13: Simultaneous open with successful TCP-ENO negotiation
Figure 13 shows a successful TCP-ENO negotiation with simultaneous
open. Here the first four segments MUST contain a SYN-form ENO
option, as each side sends both a SYN-only and a SYN-ACK segment.
The ENO option in each host'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. The last valid spec in
host B's ENO option is Y, so Y is the negotiated spec.
7. Design rationale
This section describes some of the design rationale behind TCP-ENO.
7.1. Future developments
TCP-ENO is designed to capitalize on future developments that could
alter trade-offs and change the best approach to TCP-level encryption
(beyond introducing new cipher suites). By way of example, we
discuss a few such possible developments.
Various proposals exist to increase option space in TCP [I-D.ietf-tcp
m-tcp-edo][I-D.briscoe-tcpm-inspace-mode-tcpbis][I-D.touch-tcpm-tcp-s
yn-ext-opt]. If SYN segments gain large options, it becomes possible
to fit public keys or Diffie-Hellman parameters into SYN segments.
Future encryption specs can take advantage of this by performing key
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agreement directly within suboption data, both simplifying protocols
and reducing the number of round trips required for connection setup.
New revisions to socket interfaces [RFC3493] could involve library
calls that simultaneously have access to hostname information and an
underlying TCP connection. Such an API enables the possibility of
authenticating servers transparently to the application, particularly
in conjunction with technologies such as DANE [RFC6394]. The
middleware bit "m" in general suboptions enables a middleware library
to indicate to the peer that it wishes to engage in an authentication
protocol before turning the TCP connection over to the application.
Different middleware authentication protocols can employ unique
identifiers to multiplex the "m" bit.
TLS can currently only be added to legacy applications whose
protocols accommodate a STARTTLS command or equivalent. TCP-ENO,
because it provides out-of-band signaling, opens the possibility of
future TLS revisions being generically applicable to any TCP
application.
7.2. Handshake robustness
Incremental deployment of TCP-ENO depends critically on failure cases
devolving to unencrypted TCP rather than causing the entire TCP
connection to fail.
Because some network paths drop ENO options in one direction only, a
host must know not just that the peer supports encryption, but that
the peer has received an ENO option. To this end, ENO disables
encryption unless it receives an ACK segment bearing an ENO option.
To stay robust in the face of dropped segments, hosts must continue
to include non-SYN form ENO options in segments until such point as
they have received a non-SYN segment from the other side.
One particularly pernicious middlebox behavior found in the wild is
load balancers that echo unknown TCP options found in SYN segments
back to an active opener. The passive role bit "b" in general
suboptions ensures encryption will always be disabled under such
circumstances, as sending back a verbatim copy of an active opener's
SYN-form ENO option always causes role negotiation to fail.
7.3. Suboption data
Encryption specs can employ suboption data for session caching,
cipher suite negotiation, or other purposes. However, TCP currently
limits total option space consumed by all options to only 40 bytes,
making it impractical to have many suboptions with data. For this
reason, ENO optimizes the case of a single suboption with data by
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inferring the length of the last suboption from the TCP option
length. Doing so saves one byte.
7.4. Passive role bit
TCP-ENO, associated encryption specs, and applications all have
asymmetries that require an unambiguous way to identify one of the
two connection endpoints. As an example, Section 4.7 specifies that
host A's ENO option comes before host B's in the negotiation
transcript. As another example, an application might need to
authenticate one end of a TCP connection with a digital signature.
To ensure the signed message cannot not be interpreted out of context
to authenticate the other end, the signed message would need to
include both the session ID and the local role, A or B.
A normal TCP three-way handshake involves one active and one passive
opener. This asymmetry is captured by the default configuration of
the "b" bit in the general suboption. With simultaneous open, both
hosts are active openers, so TCP-ENO requires that one host manually
configure "b = 1". An alternate design might automatically break the
symmetry to avoid this need for manual configuration. However, all
such designs we considered either lacked robustness or consumed
precious bytes of SYN option space even in the absence of
simultaneous open. (One complicating factor is that TCP does not
know it is participating in a simultaneous open until after it has
sent a SYN segment. Moreover, with packet loss, one host might never
learn it has participated in a simultaneous open.)
7.5. Option kind sharing
This draft does not specify the use of ENO options beyond the first
few segments of a connection. Moreover, it does not specify the
content of ENO options in non-SYN segments, only their presence. As
a result, any use of option kind 69 (or option kind 253 with ExID
0x454E) after the SYN exchange does not conflict with this document.
Because in addition ENO guarantees at most one negotiated spec per
connection, encryption specs will not conflict with one another or
ENO if they use ENO's option kind for out-of-band signaling in non-
SYN segments.
8. Experiments
This document has experimental status because TCP-ENO's viability
depends on middlebox behavior that can only be determined _a
posteriori_. Specifically, we must determine 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:
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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.
9. Security considerations
An obvious use case for TCP-ENO is opportunistic encryption--that is,
encrypting some connections, but only where supported and without any
kind of endpoint authentication. Opportunistic encryption protects
against undetectable large-scale eavesdropping. However, it does not
protect against detectable large-scale eavesdropping (for instance,
if ISPs terminate and proxy TCP connections or simply downgrade them
to unencrypted). Moreover, it emphatically does not protect against
targeted attacks that employ trivial spoofing to redirect a specific
high-value connection to a man-in-the-middle attacker.
Achieving stronger security with TCP-ENO requires verifying session
IDs. Any application relying on ENO for communications security MUST
incorporate session IDs into its endpoint authentication. By way of
example, an authentication mechanism based on keyed digests (such
Digest Access Authentication [RFC7616]) can be extended to include
the role and session ID in the input of the keyed digest. Where
necessary for backwards compatibility, applications SHOULD use the
application-aware bit to negotiate the inclusion of session IDs in
authentication.
Because TCP-ENO enables multiple different encryption specs to
coexist, security could potentially be only as strong as the weakest
available spec. In particular, if session IDs do not depend on the
TCP-ENO transcript in a strong way, an attacker can undetectably
tamper with ENO options to force negotiation of a deprecated and
vulnerable spec. To avoid such problems, specs SHOULD compute
session IDs using only well-studied and conservative hash functions.
That way, even if other parts of a spec are vulnerable, it is still
intractable for an attacker to induce identical session IDs at both
ends after tampering with ENO contents in SYN segments.
Implementations MUST NOT send ENO options unless they have access to
an adequate source of randomness [RFC4086]. Without secret
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unpredictable data at both ends of a connection, it is impossible for
encryption specs to achieve confidentiality and forward secrecy.
Because systems typically have very little entropy on bootup,
implementations might need to disable TCP-ENO until after system
initialization.
With a regular three-way handshake (meaning no simultaneous open),
the non-SYN form ENO option in an active opener's first ACK segment
MAY contain N > 0 bytes of spec-specific data, as shown in Figure 3.
Such data is not part of the TCP-ENO negotiation transcript, and
hence MUST be separately authenticated by the encryption spec.
10. IANA Considerations
[This section is a placeholder, as David Black has offered to help
rewrite it.]
Implementations MUST NOT use option kind 69 unless and until it is
assigned to TCP-ENO by IANA. In the meantime, implementations MUST
use experimental option 253 [RFC6994], to which IANA has assigned
ExID 0x454E (encoded by decimal bytes 69, 78 in Figure 1).
Conversely, after IANA assigns a dedicated option kind to TCP-ENO,
the use of option 253 is deprecated.
IANA will need to maintain an ENO suboption registry mapping
suboption "cs" values to encryption specs.
11. Acknowledgments
We are grateful for contributions, help, discussions, and feedback
from the TCPINC working group, including Marcelo Bagnulo, David
Black, Bob Briscoe, Jana Iyengar, Tero Kivinen, Mirja Kuhlewind, Yoav
Nir, Christoph Paasch, Eric Rescorla, and Kyle Rose. This work was
funded by DARPA CRASH and the Stanford Secure Internet of Things
Project.
12. References
12.1. 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>.
[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>.
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[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options",
RFC 6994, DOI 10.17487/RFC6994, August 2013,
<http://www.rfc-editor.org/info/rfc6994>.
12.2. 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-06 (work in progress), June 2016.
[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-04 (work in progress), April 2016.
[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,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[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>.
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[RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
Digest Access Authentication", RFC 7616,
DOI 10.17487/RFC7616, September 2015,
<http://www.rfc-editor.org/info/rfc7616>.
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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