Network Working Group A. Bittau
Internet-Draft D. Boneh
Intended status: Experimental D. Giffin
Expires: January 9, 2017 Stanford University
M. Handley
University College London
D. Mazieres
Stanford University
E. Smith
Kestrel Institute
July 8, 2016

TCP-ENO: Encryption Negotiation Option


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

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This Internet-Draft will expire on January 9, 2017.

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 ( 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

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.
  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 initiates a connection by sending 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, but responds to one with a SYN-ACK segment. With the BSD socket API, passive openers call listen and accept, rather than connect. 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:

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= |                    |
     | TBD | 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.

byte    0     1     2     3                     ... N+1
     |Kind=|Len= |Opt_0|Opt_1|       |Opt_i|   Opt_i   |
     | TBD | 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   |
    | TBD | 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:

  • Global configuration data (discussed in Section 4.2),
  • Suboption data length for the next suboption (discussed in Section 4.4), or
  • An offer to use a particular encryption spec detailed in a separate document.

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 fields governing the length of the next suboption. In the absence of a length field, a spec identifier suboption with v = 1 has suboption data extending to the end of the TCP option.

Initial suboption byte values
cs v Meaning
0x00-0x1f 0 General suboption (Section 4.2)
0x00-0x1f 1 Length field (Section 4.4)
0x20-0x7f 0 Encryption spec without suboption data
0x20-0x7f 1 Encryption spec followed by suboption data

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 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:


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.)

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.

The middleware bit m functions similarly to the application-aware bit a, but is available to middleware shared by multiple applications, some of which might have an independent use for the a bit. When set, the bit indicates a desire to engage in some endpoint authentication protocol before turning the connection over to the application. 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 bit a, no mandatory mode is needed for the middleware bit. Middleware using the m bit SHOULD employ length fields and unique identifiers to allow the m bit to be multiplexed across authentication schemes, but the mechanism for doing so is outside the scope of this document.

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 inferred from 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 field. There are two kinds of length field: length bytes specifying up to 32 bytes of suboption data, and length words specifying up to 256 bytes.

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 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 or word 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 an octet of the form shown in Figure 6 (with the high three bits 100) is followed by an octet in which the high bit is clear (meaning v = 0), then the two octets together form 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, the octet following a length word MUST be a spec identifier suboption and 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

The zzzz bits in a length word MUST be set to 0 by a sender.

A receiver MUST ignore an ENO option in a SYN segment and MUST disable encryption if any of the following holds of the ENO option:

  1. A length field indicates that a suboption would extend beyond the end of the ENO TCP option,
  2. The zzzz bits in a length word are not 0,
  3. A length byte is immediately followed by an octet in the range 0x80-0x9f (indicating another length field with no intervening spec identifier suboption), or
  4. A length word is immediately followed by an octet in the range 0x00-0x9f.

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 definition 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 a ENO option (or contains an ill-formed SYN-form ENO option),
  2. The SYN segment it receives does not contain a valid spec identifier, or
  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 NOT contain plaintext application data, but rather ciphertext, key negotiation parameters, or other messages as determined by the negotiated spec.

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 with high probability fail to reach key agreement between two honest endpoints if the spec's selection resulted from tampering with the contents of SYN-form ENO options. (Of course, in the absence of endpoint authentication, two honest endpoints can still each end up talking to a man-in-the-middle attacker rather than to each other.)

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.

5. Requirements for encryption specs

TCP-ENO affords spec authors a large amount of design flexibility. However, 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.

  • Specs MUST protect TCP data streams with authenticated encryption.
  • 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.
  • Specs MUST NOT permit the negotiation of any encryption algorithms with significantly less than 128-bit security.
  • 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.)
  • 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.
  • 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.
  • 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 and that is computable by both endpoints of the connection. Implementations SHOULD expose the session ID to applications via an API extension. Applications that are aware of TCP-ENO SHOULD authenticate the TCP endpoints by incorporating the values of the session ID and TCP-ENO role (A or B) into higher-layer authentication mechanisms.

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 the following normative list of requirements:

  • The session ID MUST be at least 33 bytes (including the one-byte suboption), though specs may choose longer session IDs.
  • The session ID MUST depend in a collision-resistant way on all of the following (meaning it is computationally infeasible to produce collisions of the session ID derivation function unless all of the following quantities are identical):
    • Fresh data contributed by both sides of the connection,
    • 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, and
    • The negotiation transcript specified in Section 4.7.
  • 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.
  • 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).

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.

(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-tcpm-tcp-edo][I-D.briscoe-tcpm-inspace-mode-tcpbis][I-D.touch-tcpm-tcp-syn-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 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 allows such authentication to be slipped underneath legacy applications--if both sides set the m bit, then before turning the socket over to the application, the two endpoints engage in a server authentication protocol. Over time, the consequences of failed or missing authentication can gradually be increased from issuing log messages to aborting the connection if some as yet unspecified DNS record indicates authentication is mandatory. Through shared library updates, such authentication can potentially be added transparently to legacy applications without recompilation.

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 a network path may 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 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 TBD (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:

  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 TCP connections and proxy them, or simply downgrade connections to unencrypted). Moreover, opportunistic encryption 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 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 document defines a new TCP option kind for TCP-ENO, assigned a value of TBD from the TCP option space. This value is defined as:

TCP Option Kind Numbers
Kind Length Meaning Reference
TBD N Encryption Negotiation (TCP-ENO) [RFC-TBD]

Early implementations of TCP-ENO made unauthorized use of TCP option kind 69. However, implementations MUST NOT make use of option kind numbers not assigned by IANA. More recent implementations used experimental option 253 per [RFC6994] with 16-bit ExID 0x454E, and SHOULD migrate to option TBD by default.

This document defines a 7-bit cs field in the range of 0x20-0x7f for which IANA shall maintain a new sub-registry entitled "TCP-ENO encryption spec identifiers" under the "Transmission Control Protocol (TCP) Parameters" registry. The description of this registry should be interpreted with respect to the terminology defined in [RFC5226].

The intention is for IANA to grant registration requests for spec identifiers in anticipation of a published RFC. Hence, a Specification is Required. However, to allow for implementation experience, identifiers should be allocated prior to the RFC being approved for publication. A Designated Expert appointed by the IESG area director shall approve allocations once it seems more likely than not that an RFC will eventually be published. The Designated Expert shall post a request to the TCPINC WG mailing list (or a successor designated by the Area Director) for comment and review, including an Internet-Draft. Before a period of 30 days has passed, the Designated Expert will either approve or deny the registration request and publish a notice of the decision to the TCPINC WG mailing list or its successor, as well as informing IANA. A denial notice must be justified by an explanation, and in the cases where it is possible, concrete suggestions on how the request can be modified so as to become acceptable should be provided.

The initial values of the TCP-ENO encryption spec identifier registry are shown in Table 2.

Value Meaning Reference
0x20 Experimental Use
0x21 TCPCRYPT_ECDHE_P256 [I-D.ietf-tcpinc-tcpcrypt]
0x22 TCPCRYPT_ECDHE_P521 [I-D.ietf-tcpinc-tcpcrypt]
0x23 TCPCRYPT_ECDHE_Curve25519 [I-D.ietf-tcpinc-tcpcrypt]
0x24 TCPCRYPT_ECDHE_Curve448 [I-D.ietf-tcpinc-tcpcrypt]
0x30 TCP-Use-TLS [I-D.ietf-tcpinc-use-tls]

Figure: TCP-ENO Spec identifiers

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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4086] Eastlake 3rd, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options", RFC 6994, DOI 10.17487/RFC6994, August 2013.

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)", Internet-Draft draft-briscoe-tcpm-inspace-mode-tcpbis-00, March 2015.
[I-D.ietf-tcpinc-tcpcrypt] Bittau, A., Boneh, D., Giffin, D., Hamburg, M., Handley, M., Mazieres, D., Slack, Q. and E. Smith, "Cryptographic protection of TCP Streams (tcpcrypt)", Internet-Draft draft-ietf-tcpinc-tcpcrypt-01, February 2016.
[I-D.ietf-tcpinc-use-tls] Rescorla, E., "Using TLS to Protect TCP Streams", Internet-Draft draft-ietf-tcpinc-use-tls-01, May 2016.
[I-D.ietf-tcpm-tcp-edo] Touch, J. and W. Eddy, "TCP Extended Data Offset Option", Internet-Draft draft-ietf-tcpm-tcp-edo-06, 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", Internet-Draft draft-touch-tcpm-tcp-syn-ext-opt-04, 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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S. and P. Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, RFC 5382, DOI 10.17487/RFC5382, October 2008.
[RFC6394] Barnes, R., "Use Cases and Requirements for DNS-Based Authentication of Named Entities (DANE)", RFC 6394, DOI 10.17487/RFC6394, October 2011.
[RFC7616] Shekh-Yusef, R., Ahrens, D. and S. Bremer, "HTTP Digest Access Authentication", RFC 7616, DOI 10.17487/RFC7616, September 2015.

Authors' Addresses

Andrea Bittau Stanford University 353 Serra Mall, Room 288 Stanford, CA, 94305 US EMail:
Dan Boneh Stanford University 353 Serra Mall, Room 475 Stanford, CA, 94305 US EMail:
Daniel B. Giffin Stanford University 353 Serra Mall, Room 288 Stanford, CA, 94305 US EMail:
Mark Handley University College London Gower St. London, WC1E 6BT UK EMail:
David Mazieres Stanford University 353 Serra Mall, Room 290 Stanford, CA, 94305 US EMail:
Eric W. Smith Kestrel Institute 3260 Hillview Avenue Palo Alto, CA, 94304 US EMail: