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

Cryptographic protection of TCP Streams (tcpcrypt)


This document specifies tcpcrypt, a cryptographic protocol that protects TCP payload data. Use of the protocol is negotiated by means of the TCP Encryption Negotiation Option (TCP-ENO) [I-D.ietf-tcpinc-tcpeno]. Tcpcrypt coexists with middleboxes by tolerating resegmentation, NATs, and other manipulations of the TCP header. The protocol is self-contained and specifically tailored to TCP implementations, which often reside in kernels or other environments in which large external software dependencies can be undesirable. Because the size of TCP options is limited, the protocol requires one additional one-way message latency to perform key exchange before application data may be transmited. However, this cost can be avoided between two hosts that have recently established a previous tcpcrypt connection.

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

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 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.

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

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

This document describes tcpcrypt, an extension to TCP for cryptographic protection of session data. Tcpcrypt was designed to meet the following goals:

3. Encryption protocol

This section describes the tcpcrypt protocol at an abstract level. The concrete format of all messages is specified in Section 4.

3.1. Cryptographic algorithms

Setting up a tcpcrypt connection employs three types of cryptographic algorithms:

The Extract and CPRF functions used by default are the Extract and Expand functions of HKDF [RFC5869]. These are defined as follows in terms of the PRF HMAC-Hash(key, value) for a negotiated Hash function:

HKDF-Extract(salt, IKM) -> PRK
   PRK = HMAC-Hash(salt, IKM)

   T(0) = empty string (zero length)
   T(1) = HMAC-Hash(PRK, T(0) | CONST | 0x01)
   T(2) = HMAC-Hash(PRK, T(1) | CONST | 0x02)
   T(3) = HMAC-Hash(PRK, T(2) | CONST | 0x03)

   OKM  = first L octets of T(1) | T(2) | T(3) | ...

Figure 1: The symbol | denotes concatenation, and the counter concatenated after CONST is a single octet.

Lastly, once tcpcrypt has been successfully set up, an authenticated encryption mode is used to protect the confidentiality and integrity of all transmitted application data.

3.2. Protocol negotiation

Tcpcrypt depends on TCP-ENO [I-D.ietf-tcpinc-tcpeno] to negotiate whether encryption will be enabled for a connection, and also which key agreement scheme to use. TCP-ENO negotiates an encryption spec by means of suboptions embedded in SYN segments. Each suboption is identified by a byte consisting of a seven-bit encryption spec identifier value, cs, and a one-bit additional-data indicator, v. This document reserves and associates four cs values with tcpcrypt, as listed in Table 1; future standards may associate additional values with tcpcrypt.

An active opener that wishes to negotiate the use of tcpcrypt will include an ENO option in its SYN segment, and that option will include tcpcrypt suboptions corresponding to the key-agreement schemes it is willing to enable. The active opener MAY additionally include suboptions indicating support for encryption protocols other than tcpcrypt, as well as other general options as specified by TCP-ENO.

If a passive opener receives an ENO option including tcpcrypt suboptions it supports, it MAY then attach an ENO option to its SYN-ACK segment, including solely the suboption it wishes to enable.

To establish distinct roles for the two hosts in each connection, tcpcrypt depends on the role-negotiation mechanism of TCP-ENO [I-D.ietf-tcpinc-tcpeno]. As part of the negotiation process, TCP-ENO assigns hosts unique roles abstractly called "A" at one end of the connection and "B" at the other. Generally, an active opener plays the "A" role and a passive opener plays the "B" role, though an additional mechanism breaks the symmetry of simultaneous open. This document adopts the terms "host A" and "host B" to identify each end of a connection uniquely, following TCP-ENO's designation.

Once two hosts have exchanged SYN segments, the negotiated spec is the last spec identifier in the SYN segment of host B (that is, the passive opener in the absence of simultaneous open) that also occurs in that of host A. If there is no such spec, hosts MUST disable TCP-ENO and tcpcrypt.

The negotiated suboption is the ENO suboption from the SYN segment of host B that contains the negotiated spec, if it exists.

As required by TCP-ENO, once a host has both sent and received an ACK segment containing an ENO option, encryption MUST be enabled and plaintext application data MUST NOT ever be exchanged on the connection. If the negotiated spec is a cs value associated with tcpcrypt, a host MUST follow the protocol described in this document. In particular, if the negotiated suboption contains v = 0, a fresh key agreement will be perfomed as described below in Section 3.3; if it contains v = 1, the key-exchange messages are omitted in favor of determining keys via session-caching as described in Section 3.4, and protected application data may immediately be sent as detailed in Section 3.5.

3.3. Key exchange

Following successful negotiation of a tcpcrypt spec, all further signaling is performed in the Data portion of TCP segments. Except when resumption was negotiated (described below in Section 3.4), the two hosts perform key exchange through two messages, Init1 and Init2, at the start of host A's and host B's data streams, respectively. These messages may span multiple TCP segments and need not end at a segment boundary. However, the segment containing the last byte of an Init1 or Init2 message SHOULD have TCP's PSH bit set.

The key exchange protocol, in abstract, proceeds as follows:

A -> B:  Init1 = { INIT1_MAGIC, sym-cipher-list, N_A, PK_A }
B -> A:  Init2 = { INIT2_MAGIC, sym-cipher, N_B, PK_B }

The concrete format of these messages is specified in further detail in Section 4.1.

The parameters are defined as follows:

The ephemeral secret (ES) is defined to be the result of the key-agreement algorithm whose inputs are the local host's ephemeral private key and the remote host's ephemeral public key. For example, host A would compute ES using its own private key (not transmitted) and host B's public key, PK_B.

The two sides then compute a pseudo-random key (PRK), from which all session keys are derived, as follows:

PRK = Extract (N_A, eno-transcript | Init1 | Init2 | ES)

Above, | denotes concatenation; eno-transcript is the protocol-negotiation transcript defined in TCP-ENO; and Init1 and Init2 are the transmitted encodings of the messages described in Section 4.1.

A series of "session secrets" and corresponding session identifiers are then computed from PRK as follows:

 ss[0] = PRK
 ss[i] = CPRF (ss[i-1], CONST_NEXTK, K_LEN)


The value ss[0] is used to generate all key material for the current connection. SID[0] is the bare session ID for the current connection, and will with overwhelming probability be unique for each individual TCP connection.

The values of ss[i] for i > 0 can be used to avoid public key cryptography when establishing subsequent connections between the same two hosts, as described in Section 3.4. The CONST_* values are constants defined in Table 3. The length K_LEN depends on the tcpcrypt spec in use, and is specified in Section 5.

To yield the session ID required by TCP-ENO [I-D.ietf-tcpinc-tcpeno], tcpcrypt concatenates the first byte of the negotiated suboption (that is, including the v bit as transmitted by host B) with the bare session ID for a particular connection:

session ID = subopt-byte | SID

Given a session secret ss, the two sides compute a series of master keys as follows:

mk[0] = CPRF (ss, CONST_REKEY, K_LEN)
mk[i] = CPRF (mk[i-1], CONST_REKEY, K_LEN)

Finally, each master key mk is used to generate keys for authenticated encryption for the "A" and "B" roles. Key k_ab is used by host A to encrypt and host B to decrypt, while k_ba is used by host B to encrypt and host A to decrypt.

k_ab = CPRF (mk, CONST_KEY_A, ae_keylen)
k_ba = CPRF (mk, CONST_KEY_B, ae_keylen)

The value ae_keylen depends on the authenticated-encryption algorithm selected, and is given under "Key Length" in Table 2.

After host B sends Init2 or host A receives it, that host may immediately begin transmitting protected application data as described in Section 3.5.

3.4. Session caching

When two hosts have already negotiated session secret ss[i-1], they can establish a new connection without public-key operations using ss[i]. Willingness to employ this facility is signalled by sending a SYN segment with a resumption suboption: an ENO suboption containing the negotiated spec identifier from the original session and the flag v = 1 (indicating variable-length data).

An active opener wishing to resume from a cached session may send a resumption suboption whose content is the nine-byte prefix of the associated bare session ID:

byte     0        1                  9      (10 bytes total)
     | spec-  |       SID[i]{0..8}        |
     |  byte  |                           |

Figure 2: ENO suboption used to initiate session resumption. The spec-byte contains a tcpcrypt cs value and v = 1.

The active opener MUST use the lowest value of i that has not already been used to successfully negotiate resumption with the same host and for the same pre-session key ss[0].

A host SHOULD also include ENO suboptions describing the key-agreement schemes it supports in addition to the resume suboption, so as to fall back to full key exchange in the event that resumption fails. Implementations MUST NOT send more than one resumption suboption for the same cs value in the same SYN segment.

If the passive opener recognizes the prefix of SID[i] and knows ss[i], it SHOULD (with exceptions specified below) respond with an ENO option containing an empty resumption suboption with matching spec identifier; that is, a suboption whose initial byte gives the cs value from host A's resumption suboption and sets v = 1, but whose contents are empty. (The only way to encode this is as the last ENO suboption.)

Otherwise, the passive opener SHOULD inspect any other ENO suboptions in hopes of negotiating a fresh key exchange as described in Section 3.3.

A host MUST ignore a resumption suboption if has successfully negotiated resumption in the past, in either role, with the same SID[i]. In the event that two hosts simultaneously send SYN segments to each other with the same SID[i], but the two segments are not part of a simultaneous open, both connections will have to revert to public key cryptography. To avoid this limitation, implementations MAY choose to implement session caching such that a given pre-session key ss[0] is only used for either passive or active opens at the same host, not both.

In the case of simultaneous open where TCP-ENO is able to establish asymmetric roles, two hosts that simultaneously send SYN segments with resumption suboptions containing the same SID[i] may resume the associated session.

Hosts MUST NOT send, and upon receipt MUST ignore, an empty resumption suboption in a SYN-only segment.

After using ss[i] to compute mk[0], implementations SHOULD compute and cache ss[i+1] for possible use by a later session, then erase ss[i] from memory. Hosts SHOULD keep ss[i+1] around for a period of time until it is used or the memory needs to be reclaimed. Hosts SHOULD NOT write a cached ss[i+1] value to non-volatile storage.

When two hosts have previously negotiated a tcpcrypt session, either host may initiate session resumption regardless of which host was the active opener or played the "A" role in the previous session.

However, a given host must either encrypt with k_ab for all sessions derived from the same pre-session key ss[0], or with k_ba. Thus, which keys a host uses to send segments is not affected by the role it plays in the current connection: it depends only on whether the host played the "A" or "B" role in the initial session.

Implementations that perform session caching MUST provide a means for applications to control session caching, including flushing cached session secrets associated with an ESTABLISHED connection or disabling the use of caching for a particular connection. A recommended interface is described in Section 10.

The session ID required by TCP-ENO and exposed to applications is constructed in the same way for resumed sessions as it is for fresh ones, as described above in Section 3.3. In particular, the first byte of the session ID is the first byte of the current connection's negotiated suboption, which means the byte will contain v = 1; and the remainder is SID[i], the bare session ID for the resumed session.

3.5. Data encryption and authentication

Following key exchange (or its omission via session caching), all further communication in a tcpcrypt-enabled connection is carried out within delimited application frames that are encrypted and authenticated using the agreed keys.

This protection is provided via algorithms for Authenticated Encryption with Associated Data (AEAD). The particular algorithms that may be used are listed in Table 2. One algorithm is selected during the negotiation described in Section 3.3.

The format of an application frame is specified in Section 4.2. A sending host breaks its stream of application data into a series of chunks. Each chunk is placed in the data portion of a "plaintext" value, which is then encrypted to yield a frame's ciphertext field. Chunks must be small enough that the ciphertext (whose length depends on the AEAD cipher used, and is generally slightly longer than the plaintext) has length less than 2^16 bytes.

An "associated data" value (see Section 4.2.2) is constructed for the frame. It contains the frame's control field and the length of the ciphertext.

A "frame nonce" value (see Section 4.2.3) is also constructed for the frame (but not explicitly transmitted), containing an offset field whose integer value is the zero-indexed byte offset of the beginning of the current application frame in the underlying TCP datastream. (That is, the offset in the framing stream, not the plaintext application stream.) Because it is strictly necessary for the security of the AEAD algorithm, an implementation MUST NOT ever transmit distinct frames with the same nonce value under the same encryption key. In particular, a retransmitted TCP segment MUST contain the same payload bytes for the same TCP sequence numbers, and a host MUST NOT transmit more than 2^64 bytes in the underlying TCP datastream (which would cause the offset field to wrap) before re-keying.

With reference to the "AEAD Interface" described in Section 2 of [RFC5116], tcpcrypt invokes the AEAD algorithm with the secret key K set to k_ab or k_ba, according to the host's role as described in Section 3.3. The plaintext value serves as P, the associated data as A, and the frame nonce as N. The output of the encryption operation, C, is transmitted in the frame's ciphertext field.

When a frame is received, tcpcrypt reconstructs the associated data and frame nonce values (the former contains only data sent in the clear, and the latter is implicit in the TCP stream), and provides these and the ciphertext value to the the AEAD decryption operation. The output of this operation is either P, a plaintext value, or the special symbol FAIL. In the latter case, the implementation MUST either ignore the frame or terminate the connection.

3.6. TCP header protection

The ciphertext field of the application frame contains protected versions of certain TCP header values.

When URGp is set, the urgent value indicates an offset from the current frame's beginning offset; the sum of these offsets gives the index of the last byte of urgent data in the application datastream.

When FINp is set, it indicates that the sender will send no more application data after this frame. A receiver MUST ignore the TCP FIN flag and instead wait for FINp to signal to the local application that the stream is complete.

3.7. Re-keying

Re-keying allows hosts to wipe from memory keys that could decrypt previously transmitted segments. It also allows the use of AEAD ciphers that can securely encrypt only a bounded number of messages under a given key.

We refer to the two encryption keys (k_ab, k_ba) as a key-set. We refer to the key-set generated by mk[i] as the key-set with generation number i within a session. Each host maintains a current generation number that it uses to encrypt outgoing frames. Initially, the two hosts have current generation number 0.

When a host has just incremented its current generation number and has used the new key-set for the first time to encrypt an outgoing frame, it MUST set that frame's rekey field (see Section 4.2) to 1. It MUST set this field to zero in all other cases.

A host MAY increment its generation number beyond the highest generation it knows the other side to be using. We call this action initiating re-keying.

A host SHOULD NOT initiate more than one concurrent re-key operation if it has no data to send; that is, it should not initiate re-keying with an empty application frame more than once while its record of the remote host's current generation number is less than its own.

On receipt, a host increments its record of the remote host's current generation number if and only if the rekey field is set to 1.

If a received frame's generation number is greater than the receiver's current generation number, the receiver MUST immediately increment its current generation number to match. After incrementing its generation number, if the receiver does not have any application data to send, it MUST send an empty application frame with the rekey field set to 1.

When retransmitting, implementations must always transmit the same bytes for the same TCP sequence numbers. Thus, a frame in a retransmitted segment MUST always be encrypted with the same key as when it was originally transmitted.

Implementations SHOULD delete older-generation keys from memory once they have received all frames they will need to decrypt with the old keys and have encrypted all outgoing frames under the old keys.

3.8. Keep-alive

Instead of using TCP Keep-Alives to verify that the remote endpoint is still alive, tcpcrypt implementations SHOULD employ the re-keying mechanism, as follows. When necessary, a host SHOULD probe the liveness of its peer by initiating re-keying as described in Section 3.7, and then transmitting a new frame (with zero-length application data if necessary). A host receiving a frame whose key generation number is greater than its current generation number MUST increment its current generation number and MUST immediately transmit a new frame (with zero-length application data, if necessary).

Implementations MAY use TCP Keep-Alives for purposes that do not require endpoint authentication, as discussed in Section 9.2.

4. Encodings

This section provides byte-level encodings for values transmitted or computed by the protocol.

4.1. Key exchange messages

The Init1 message has the following encoding:

byte   0       1       2       3
   |          INIT1_MAGIC          |
   |                               |

           4        5      6       7
       |          message_len          |
       |              = M              |

       |nciphers|sym-   |sym-   |         |sym-   |
       | =K+1   |cipher0|cipher1|         |cipherK|

          K + 10                    K + 10 + N_A_LEN
           |                         |
           v                         v
       |           N_A           |          PK_A           |
       |                         |                         |

                           M - 1
       |          ignored        |
       |                         |

The constant INIT1_MAGIC is defined in Table 3. The four-byte field message_len gives the length of the entire Init1 message, encoded as a big-endian integer. The nciphers field contains an integer value that specifies the number of one-byte symmetric-cipher identifiers that follow. The sym-cipher bytes identify cryptographic algorithms in Table 2. The length N_A_LEN and the length of PK_A are both determined by the negotiated key-agreement scheme, as described in Section 5.

When sending Init1, implementations of this protocol MUST omit the field ignored; that is, they must construct the message such that its end, as determined by message_len, coincides with the end of the field PK_A. When receiving Init1, however, implementations MUST permit and ignore any bytes following PK_A.

The Init2 message has the following encoding:

byte   0       1       2       3
   |          INIT2_MAGIC          |
   |                               |

           4        5      6       7       8
       |          message_len          |sym-   |
       |              = M              |cipher |

           9                        9 + N_B_LEN
           |                         |
           v                         v
       |           N_B           |          PK_B           |
       |                         |                         |

                           M - 1
       |          ignored        |
       |                         |

The constant INIT2_MAGIC is defined in Table 3. The four-byte field message_len gives the length of the entire Init2 message, encoded as a big-endian integer. The sym-cipher value is a selection from the symmetric-cipher identifiers in the previously-received Init1 message. The length N_B_LEN and the length of PK_B are both determined by the negotiated key-agreement scheme, as described in Section 5.

When sending Init2, implementations of this protocol MUST omit the field ignored; that is, they must construct the message such that its end, as determined by message_len, coincides with the end of the PK_B field. When receiving Init2, however, implementations MUST permit and ignore any bytes following PK_B.

4.2. Application frames

An application frame comprises a control byte and a length-prefixed ciphertext value:

byte   0       1       2       3               clen+2
   |control|      clen     |        ciphertext       |

The field clen is an integer in big-endian format and gives the length of the ciphertext field.

The byte control has this structure:

bit     7                 1       0
    |          cres           | rekey |

The seven-bit field cres is reserved; implementations MUST set these bits to zero when sending, and MUST ignore them when receiving.

The use of the rekey field is described in Section 3.7.

4.2.1. Plaintext

The ciphertext field is the result of applying the negotiated authenticated-encryption algorithm to a "plaintext" value, which has one of these two formats:

byte   0       1               plen-1
   | flags |           data          |

byte   0       1       2       3              plen-1
   | flags |    urgent     |          data           |

(Note that clen in the previous section will generally be greater than plen, as the ciphertext produced by the authenticated-encryption scheme must both encrypt the application data and provide a way to verify its integrity.)

The flags byte has this structure:

bit    7    6    5    4    3    2    1    0
    |            fres             |URGp|FINp|

The six-bit value fres is reserved; implementations MUST set these six bits to zero when sending, and MUST ignore them when receiving.

When the URGp bit is set, it indicates that the urgent field is present, and thus that the plaintext value has the second structure variant above; otherwise the first variant is used.

The meaning of urgent and of the flag bits is described in Section 3.6.

4.2.2. Associated data

An application frame's "associated data" (which is supplied to the AEAD algorithm when decrypting the ciphertext and verifying the frame's integrity) has this format:

byte   0       1       2
   |control|     clen      |

It contains the same values as the frame's control and clen fields.

4.2.3. Frame nonce

Lastly, a "frame nonce" (provided as input to the AEAD algorithm) has this format:

 0 |     FRAME_NONCE_MAGIC     |
 4 |                           |
   +           offset          +
 8 |                           |

The 4-byte magic constant is defined in Table 3. The 8-byte offset field contains an integer in big-endian format. Its value is specified in Section 3.5.

5. Key agreement schemes

The encryption spec negotiated via TCP-ENO may indicate the use of one of the key-agreement schemes named in Table 1. For example, TCPCRYPT_ECDHE_P256 names the tcpcrypt protocol with key-agreement scheme ECDHE-P256.

All schemes listed there use HKDF-Expand-SHA256 as the CPRF, and these lengths for nonces and session keys:

N_A_LEN: 32 bytes
N_B_LEN: 32 bytes
K_LEN:   32 bytes

Key-agreement schemes ECDHE-P256 and ECDHE-P521 employ the ECSVDP-DH secret value derivation primitive defined in [ieee1363]. The named curves are defined in [nist-dss]. When the public-key values PK_A and PK_B are transmitted as described in Section 4.1, they are encoded with the "Elliptic Curve Point to Octet String Conversion Primitive" described in Section E.2.3 of [ieee1363], and are prefixed by a two-byte length in big-endian format:

byte   0       1       2               L - 1
   |   pubkey_len  |          pubkey         |
   |      = L      |                         |

Implementations SHOULD encode these pubkey values in "compressed format", and MUST accept values encoded in "compressed", "uncompressed" or "hybrid" formats.

Key-agreement schemes ECDHE-Curve25519 and ECDHE-Curve448 use the functions X25519 and X448, respectively, to perform the Diffie-Helman protocol as described in [RFC7748]. When using these ciphers, public-key values PK_A and PK_B are transmitted directly with no length prefix: 32 bytes for Curve25519, and 56 bytes for Curve448.

A tcpcrypt implementation MUST support at least the schemes ECDHE-P256 and ECDHE-P521, although system administrators need not enable them.

6. AEAD algorithms

Specifiers and key-lengths for AEAD algorithms are given in Table 2. The algorithms AEAD_AES_128_GCM and AEAD_AES_256_GCM are specified in [RFC5116]. The algorithm AEAD_CHACHA20_POLY1305 is specified in [RFC7539].

7. IANA considerations

Tcpcrypt's spec identifiers (cs values) will need to be added to IANA's ENO suboption registry, as follows:

cs values for use with tcpcrypt
cs Spec name
0x23 TCPCRYPT_ECDHE_Curve25519
0x24 TCPCRYPT_ECDHE_Curve448

A "tcpcrypt AEAD parameter" registry needs to be maintained by IANA as in the following table. The use of encryption is described in Section 3.5.

Authenticated-encryption algorithms corresponding to sym-cipher specifiers in Init1 and Init2 messages.
AEAD Algorithm Key Length sym-cipher
AEAD_AES_128_GCM 16 bytes 0x01
AEAD_AES_256_GCM 32 bytes 0x02
AEAD_CHACHA20_POLY1305 32 bytes 0x10

8. Security considerations

Public-key generation, public-key encryption, and shared-secret generation all require randomness. Other tcpcrypt functions may also require randomness, depending on the algorithms and modes of operation selected. A weak pseudo-random generator at either host will compromise tcpcrypt's security. Many of tcpcrypt's cryptographic functions require random input, and thus any host implementing tcpcrypt MUST have access to a cryptographically-secure source of randomness or pseudo-randomness.

Most implementations will rely on system-wide pseudo-random generators seeded from hardware events and a seed carried over from the previous boot. Once a pseudo-random generator has been properly seeded, it can generate effectively arbitrary amounts of pseudo-random data. However, until a pseudo-random generator has been seeded with sufficient entropy, not only will tcpcrypt be insecure, it will reveal information that further weakens the security of the pseudo-random generator, potentially harming other applications. As required by TCP-ENO, implementations MUST NOT send ENO options unless they have access to an adequate source of randomness.

The cipher-suites specified in this document all use HMAC-SHA256 to implement the collision-resistant pseudo-random function denoted by CPRF. A collision-resistant function is one on which, for sufficiently large L, an attacker cannot find two distinct inputs K_1, CONST_1 and K_2, CONST_2 such that CPRF(K_1, CONST_1, L) = CPRF(K_2, CONST_2, L). Collision resistance is important to assure the uniqueness of session IDs, which are generated using the CPRF.

All of the security considerations of TCP-ENO apply to tcpcrypt. In particular, tcpcrypt does not protect against active eavesdroppers unless applications authenticate the session ID. If it can be established that the session IDs computed at each end of the connection match, then tcpcrypt guarantees that no man-in-the-middle attacks occurred unless the attacker has broken the underlying cryptographic primitives (e.g., ECDH). A proof of this property for an earlier version of the protocol has been published [tcpcrypt].

To gain middlebox compatibility, tcpcrypt does not protect TCP headers. Hence, the protocol is vulnerable to denial-of-service from off-path attackers. Possible attacks include desynchronizing the underlying TCP stream, injecting RST packets, and forging or suppressing rekey bits. These attacks will cause a tcpcrypt connection to hang or fail with an error. Implementations MUST give higher-level software a way to distinguish such errors from a clean end-of-stream (indicated by an authenticated FINp bit) so that applications can avoid semantic truncation attacks.

There is no "key confirmation" step in tcpcrypt. This is not required because tcpcrypt's threat model includes the possibility of a connection to an adversary. If key negotiation is compromised and yields two different keys, all subsequent frames will be ignored due failed integrity checks, causing the application's connection to hang. This is not a new threat because in plain TCP, an active attacker could have modified sequence and acknowledgement numbers to hang the connection anyway.

Tcpcrypt uses short-lived public keys to provide forward secrecy. All currently specified key agreement schemes involve ECDHE-based key agreement, meaning a new key can be efficiently computed for each connection. If implementations reuse these parameters, they SHOULD limit the lifetime of the private parameters, ideally to no more than two minutes.

Attackers cannot force passive openers to move forward in their session caching chain without guessing the content of the resumption suboption, which will be hard without key knowledge.

9. Design notes

9.1. Asymmetric roles

Tcpcrypt transforms a shared pseudo-random key (PRK) into cryptographic session keys for each direction. Doing so requires an asymmetry in the protocol, as the key derivation function must be perturbed differently to generate different keys in each direction. Tcpcrypt includes other asymmetries in the roles of the two hosts, such as the process of negotiating algorithms (e.g., proposing vs. selecting cipher suites).

9.2. Verified liveness

Many hosts implement TCP Keep-Alives [RFC1122] as an option for applications to ensure that the other end of a TCP connection still exists even when there is no data to be sent. A TCP Keep-Alive segment carries a sequence number one prior to the beginning of the send window, and may carry one byte of "garbage" data. Such a segment causes the remote side to send an acknowledgment.

Unfortunately, tcpcrypt cannot cryptographically verify Keep-Alive acknowledgments. Hence, an attacker could prolong the existence of a session at one host after the other end of the connection no longer exists. (Such an attack might prevent a process with sensitive data from exiting, giving an attacker more time to compromise a host and extract the sensitive data.)

Thus, tcpcrypt specifies a way to stimulate the remote host to send verifiably fresh and authentic data, described in Section 3.8.

The TCP keep-alive mechanism has also been used for its effects on intermediate nodes in the network, such as preventing flow state from expiring at NAT boxes or firewalls. As these purposes do not require the authentication of endpoints, implementations may safely accomplish them using either the existing TCP keep-alive mechanism or tcpcrypt's verified keep-alive mechanism.

10. API extensions

Applications aware of tcpcrypt will need an API for interacting with the protocol. They can do so if implementations provide the recommended API for TCP-ENO. This section recommends several additions to that API, described in the style of socket options. However, these recommendations are non-normative:

The following option is read-only:


Returns the one-byte authenticated encryption algorithm in use by the connection (as specified in Table 2).

The following option is write-only:


Setting this option to non-zero wipes cached session keys as specified in Section 3.4. Useful if application-level authentication discovers a man in the middle attack, to prevent the next connection from using session caching.

The following options should be readable and writable:


Set of allowed symmetric ciphers and message authentication codes this host advertises in Init1 messages.

Order of preference of symmetric ciphers.

Finally, system administrators must be able to set the following system-wide parameters:

  • Default TCP_CRYPT_ACONF value
  • Default TCP_CRYPT_BCONF value
  • Types, key lengths, and regeneration intervals of local host's short-lived public keys for implementations that do not use fresh ECDH parameters for each connection.

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 gifts from Intel (to Brad Karp) and from Google; by NSF award CNS-0716806 (A Clean-Slate Infrastructure for Information Flow Control); by DARPA CRASH under contract #N66001-10-2-4088; and by the Stanford Secure Internet of Things Project.

12. References

12.1. Normative References

, ", "
[I-D.ietf-tcpinc-tcpeno] Bittau, A., Boneh, D., Giffin, D., Handley, M., Mazieres, D. and E. Smith, "TCP-ENO: Encryption Negotiation Option", Internet-Draft draft-ietf-tcpinc-tcpeno-03, July 2016.
[ieee1363]IEEE Standard Specifications for Public-Key Cryptography (IEEE Std 1363-2000)", 2000.
[nist-dss]Digital Signature Standard, FIPS 186-2", 2000.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010.
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015.
[RFC7748] Langley, A., Hamburg, M. and S. Turner, Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016.

12.2. Informative References

[RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989.
[tcpcrypt] Bittau, A., Hamburg, M., Handley, M., Mazieres, D. and D. Boneh, "The case for ubiquitous transport-level encryption", USENIX Security , 2010.

Appendix A. Protocol constant values

Protocol constants
Value Name
0x15101a0e INIT1_MAGIC
0x097105e0 INIT2_MAGIC

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:
Mike Hamburg Stanford University 353 Serra Mall, Room 475 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:
Quinn Slack Stanford University 353 Serra Mall, Room 288 Stanford, CA, 94305 US EMail:
Eric W. Smith Kestrel Institute 3260 Hillview Avenue Palo Alto, CA, 94304 US EMail: