wdiff rfc8548v1.txt sample

Internet Engineering Task Force (IETF) A. Bittau
Request for Comments: 8548 Google
Category: Experimental D. Giffin
ISSN: 2070-1721 Stanford University
M. Handley
University College London
D. Mazieres
Stanford University
Q. Slack
Sourcegraph
E. Smith
Kestrel Institute
March 2019

Cryptographic Protection of TCP Streams (tcpcrypt)

Abstract

This document specifies "tcpcrypt", a TCP encryption protocol
designed for use in conjunction with the TCP Encryption Negotiation
Option (TCP-ENO). 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 can be transmitted. However, the extra latency can
be avoided between two hosts that have recently established a
previous tcpcrypt connection.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are candidates for any level of
Internet Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8548.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://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. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Encryption Protocol . . . . . . . . . . . . . . . . . . . . . 3
3.1. Cryptographic Algorithms . . . . . . . . . . . . . . . . 4
3.2. Protocol Negotiation . . . . . . . . . . . . . . . . . . 5
3.3. Key Exchange . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Session ID . . . . . . . . . . . . . . . . . . . . . . . 9
3.5. Session Resumption . . . . . . . . . . . . . . . . . . . 9
3.6. Data Encryption and Authentication . . . . . . . . . . . 13
3.7. TCP Header Protection . . . . . . . . . . . . . . . . . . 15 14
3.8. Rekeying . . . . . . . . . . . . . . . . . . . . . . . . 15
3.9. Keep-Alive . . . . . . . . . . . . . . . . . . . . . . . 16
4. Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . 17 16
4.1. Key-Exchange Messages . . . . . . . . . . . . . . . . . . 17
4.2. Encryption Frames . . . . . . . . . . . . . . . . . . . . 19
4.2.1. Plaintext . . . . . . . . . . . . . . . . . . . . . . 19
4.2.2. Associated Data . . . . . . . . . . . . . . . . . . . 20
4.2.3. Frame ID . . . . . . . . . . . . . . . . . . . . . . 20
4.3. Constant Values . . . . . . . . . . . . . . . . . . . . . 21
5. Key-Agreement Schemes . . . . . . . . . . . . . . . . . . . . 21
6. AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . . . 23
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8.1. Asymmetric Roles . . . . . . . . . . . . . . . . . . . . 26
8.2. Verified Liveness . . . . . . . . . . . . . . . . . . . . 26
8.3. Mandatory Key-Agreement Schemes . . . . . . . . . . . . . 26
9. Experiments . . . . . . . . . . . . . . . . . . . . . . . . . 27
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
10.1. Normative References . . . . . . . . . . . . . . . . . . 28
10.2. Informative References . . . . . . . . . . . . . . . . . 29
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 30
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30

1. Introduction

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

o Meet the requirements of the TCP Encryption Negotiation Option
(TCP-ENO) [RFC8547] for protecting connection data.

o Be amenable to small, self-contained implementations inside TCP
stacks.

o Minimize additional latency at connection startup.

o As much as possible, prevent connection failure in the presence of
NATs and other middleboxes that might normalize traffic or
otherwise manipulate TCP segments.

o Operate independently of IP addresses, making it possible to
authenticate resumed sessions efficiently even when either end
changes IP address.

A companion document [TCPINC-API] describes recommended interfaces
for configuring certain parameters of this protocol.

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Encryption Protocol

This section describes the operation of the tcpcrypt protocol. The
wire format of all messages is specified in Section 4.

3.1. Cryptographic Algorithms

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

o A _key key agreement scheme_ scheme is used with a short-lived public key to
agree upon a shared secret.

o An _extract function_ extract function is used to generate a pseudo-random key (PRK)
from some initial keying material produced by the key agreement
scheme. The notation Extract(S, IKM) denotes the output of the
extract function with salt S and initial keying material IKM.

o A _collision-resistant collision-resistant pseudo-random function (CPRF)_ (CPRF) is used to
generate multiple cryptographic keys from a pseudo-random key,
typically the output of the extract function. The CPRF produces
an arbitrary amount of Output Keying Material (OKM), and we use
the notation CPRF(K, CONST, L) to designate the first L bytes of
the OKM produced by the CPRF when parameterized by key K and the
constant CONST.

The Extract and CPRF functions used by the tcpcrypt variants defined
in this document are the Extract and Expand functions of the HMAC-
based Key Derivation Function (HKDF) [RFC5869], which is built on
Keyed-Hashing for Message Authentication (HMAC) [RFC2104]. These are
defined as follows in terms of the function "HMAC-Hash(key, value)" HMAC-Hash(key, value) for
a negotiated "Hash" Hash function such as SHA-256; the symbol "|" denotes
concatenation, and the counter concatenated to the right of CONST
occupies a single octet.

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

OKM = first L octets of T(1) | T(2) | T(3) | ...
where L <= 255*OutputLength(Hash)

Figure 1: HKDF Functions Used for Key Derivation

Lastly, once tcpcrypt has been successfully set up and encryption
keys have been derived, an algorithm for Authenticated Encryption
with Associated Data (AEAD) is used to protect the confidentiality
and integrity of all transmitted application data. AEAD algorithms
use a single key to encrypt their input data and also to generate a
cryptographic tag to accompany the resulting ciphertext; when
decryption is performed, the tag allows authentication of the
encrypted data and of optional associated plaintext data.

3.2. Protocol Negotiation

Tcpcrypt depends on TCP-ENO [RFC8547] to negotiate whether encryption
will be enabled for a connection as well as which key-agreement
scheme to use. TCP-ENO negotiates the use of a particular TCP
encryption protocol (TEP) by including protocol identifiers in ENO
suboptions. This document associates four TEP identifiers with the
tcpcrypt protocol as listed in Table 4 of Section 7. Each identifier
indicates the use of a particular key-agreement scheme, with an
associated CPRF and length parameter. Future standards can associate
additional TEP identifiers with tcpcrypt following the assignment
policy specified by TCP-ENO.

An active opener that wishes to negotiate the use of tcpcrypt
includes an ENO option in its SYN segment. That option includes
suboptions with tcpcrypt TEP identifiers indicating 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 global suboptions as specified by TCP-ENO.

If a passive opener receives an ENO option including tcpcrypt TEPs
that it supports, it MAY then attach an ENO option to its SYN-ACK
segment, including solely the TEP 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. As
one result 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, but in the case of simultaneous
open, an additional mechanism breaks the symmetry and assigns a
distinct role to each host. TCP-ENO uses the terms "host A" and
"host B" to identify each end of a connection uniquely; this document
employs those terms in the same way.

An ENO suboption includes a flag "v" v which indicates the presence of
associated variable-length data. In order to propose fresh key
agreement with a particular tcpcrypt TEP, a host sends a one-byte
suboption containing the TEP identifier and "v v = 0". 0. In order to
propose session resumption (described further below) with a
particular TEP, a host sends a variable-length suboption containing
the TEP identifier, the flag "v v = 1", 1, an identifier derived from a
session secret previously negotiated with the same host and the same
TEP, and a nonce.

Once two hosts have exchanged SYN segments, TCP-ENO defines the
_negotiated TEP_
negotiated TEP to be the last valid TEP 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 TEP,
hosts MUST disable TCP-ENO and tcpcrypt.

If the negotiated TEP was sent by host B with "v v = 0", 0, it means that
fresh key agreement will be performed as described in Section 3.3.
If, on the other hand, host B sent the TEP with "v v = 1" 1 and both hosts
sent appropriate resumption identifiers in their suboption data, then
the key-exchange messages will be omitted in favor of determining
keys via session resumption as described in Section 3.5. With
session resumption, protected application data MAY be sent
immediately as detailed in Section 3.6.

Note that the negotiated TEP is determined without reference to the
"v" v
bits in ENO suboptions, so if host A offers resumption with a
particular TEP and host B replies with a non-resumption suboption
with the same TEP, that could become the negotiated TEP, in which
case fresh key agreement will be performed. That is, sending a
resumption suboption also implies willingness to perform fresh key
agreement with the indicated TEP.

As REQUIRED by TCP-ENO, once a host has both sent and received an ACK
segment containing a valid ENO option, encryption MUST be enabled and
plaintext application data MUST NOT ever be exchanged on the
connection. If the negotiated TEP is among those listed in Table 4,
a host MUST follow the protocol described in this document.

3.3. Key Exchange

Following successful negotiation of a tcpcrypt TEP, all further
signaling is performed in the Data portion of TCP segments. Except
when resumption was negotiated (described in Section 3.5), the two
hosts perform key exchange through two messages, "Init1" Init1 and "Init2", Init2, at
the start of the data streams of host A and host B, 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" Init1 or "Init2" Init2 message MUST have TCP's push flag (PSH) set.

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

A -> B: Init1 = { INIT1_MAGIC, sym_cipher_list, N_A, Pub_A }
B -> A: Init2 = { INIT2_MAGIC, sym_cipher, N_B, Pub_B }
The concrete format of these messages is specified in Section 4.1.

The parameters are defined as follows:

o "INIT1_MAGIC", "INIT2_MAGIC": constants INIT1_MAGIC, INIT2_MAGIC: Constants defined as the values
"0x15101a0e" and "0x097105e0", respectively. See in Section 4.3.

o "sym_cipher_list": a sym_cipher_list: A list of identifiers of symmetric ciphers (AEAD
algorithms) acceptable to host A. These are specified in Table 5
of Section 7.

o "sym_cipher": the sym_cipher: The symmetric cipher selected by host B from the
"sym_cipher_list"
sym_cipher_list sent by host A.

o "N_A", "N_B": nonces N_A, N_B: Nonces chosen at random by hosts A and B, respectively.

o "Pub_A", "Pub_B": Pub_A, Pub_B: Ephemeral public keys for hosts A and B,
respectively. These, as well as their corresponding private keys,
are short-lived values that MUST be refreshed frequently. The
private keys SHOULD NOT ever be written to persistent storage.
The security risks associated with the storage of these keys are
discussed in Section 8.

If a host receives an ephemeral public key from its peer and a key-
validation step fails (see Section 5), it MUST abort the connection
and raise an error condition distinct from the end-of-file condition.

The ephemeral secret "ES" ES is the result of the key-agreement algorithm
(see Section 5) indicated by the negotiated TEP. The inputs to the
algorithm are the local host's ephemeral private key and the remote
host's ephemeral public key. For example, host A would compute "ES" ES
using its own private key (not transmitted) and host B's public key, "Pub_B".
Pub_B.

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

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

Above, "|" denotes concatenation, "eno-transcript" eno-transcript is the protocol-
negotiation transcript defined in Section 4.8 of [RFC8547], and
"Init1" Init1
and "Init2" Init2 are the transmitted encodings of the messages described in
Section 4.1.

A series of "session secrets" are computed from "PRK" PRK as follows:

ss[0] = PRK
ss[i] = CPRF(ss[i-1], CONST_NEXTK, K_LEN)
The value "ss[0]" ss[0] is used to generate all key material for the current
connection. The values "ss[i]" ss[i] for "i i > 0" 0 are used by session
resumption to avoid public key cryptography when establishing
subsequent connections between the same two hosts as described in
Section 3.5. The "CONST_*" CONST_* values are constants defined in
Section 4.3. The length "K_LEN" K_LEN depends on the tcpcrypt TEP in use,
and is specified in Section 5.

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

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

The process of advancing through the series of master keys is
described in Section 3.8. The values represented by "sn[i]" sn[i] are
"session nonces". For the initial session with "i i = 0", 0, the session
nonce is zero bytes long. The values for subsequent sessions are
derived from fresh connection data as described in Section 3.5.

Finally, each master key "mk[j]" mk[j] is used to generate traffic keys for
protecting application data using authenticated encryption:

k_ab[j] = CPRF(mk[j], CONST_KEY_A, ae_key_len + ae_nonce_len)
k_ba[j] = CPRF(mk[j], CONST_KEY_B, ae_key_len + ae_nonce_len)

In the first session derived from fresh key agreement, traffic keys
"k_ab[j]"
k_ab[j] are used by host A to encrypt and host B to decrypt, while
keys "k_ba[j]" k_ba[j] are used by host B to encrypt and host A to decrypt. In
a resumed session, as described more thoroughly in Section 3.5, each
host uses the keys in the same way as it did in the original session,
regardless of its role in the current session, e.g., session; for example, if a host
played role "A" in the first session, it will use keys "k_ab[j]" k_ab[j] to
encrypt in each derived session.

The values "ae_key_len" ae_key_len and "ae_nonce_len" ae_nonce_len depend on the
authenticated-encryption authenticated-
encryption algorithm selected and are given in Table 3 of Section 6.
The algorithm uses the first "ae_key_len" ae_key_len bytes of each traffic key as
an authenticated-encryption key, and it uses the following "ae_nonce_len"
ae_nonce_len bytes as a "nonce randomizer".

Implementations SHOULD provide an interface allowing the user to
specify, for a particular connection, the set of AEAD algorithms to
advertise in "sym_cipher_list" sym_cipher_list (when playing role "A") and also the
order of preference to use when selecting an algorithm from those
offered (when playing role "B"). A companion document [TCPINC-API]
describes recommended interfaces for this purpose.

After host B sends "Init2" Init2 or host A receives it, that host MAY
immediately begin transmitting protected application data as
described in Section 3.6.

If host A receives "Init2" Init2 with a "sym_cipher" sym_cipher value that was not present
in the "sym_cipher_list" sym_cipher_list it previously transmitted in
"Init1", Init1, it MUST
abort the connection and raise an error condition distinct from the
end-of-file condition.

Throughout this document, to "abort the connection" means to issue
the "Abort" command as described in Section 3.8 of [RFC0793]. That
is, the TCP connection is destroyed, RESET is transmitted, and the
local user is alerted to the abort event.

3.4. Session ID

TCP-ENO requires each TEP to define a _session ID_ session ID value that uniquely
identifies each encrypted connection.

A tcpcrypt session ID begins with the byte transmitted by host B that
contains the negotiated TEP identifier along with the "v" v bit. The
remainder of the ID is derived from the session secret and session
nonce, as follows:

session_id[i] = TEP-byte | CPRF(ss[i], CONST_SESSID | sn[i], K_LEN)

Again, the length "K_LEN" K_LEN depends on the TEP and is specified in
Section 5.

3.5. Session Resumption

If two hosts have previously negotiated a session with secret
"ss[i-1]",
ss[i-1], they can establish a new connection without public-key
operations using "ss[i]", ss[i], the next session secret in the sequence
derived from the original PRK.

A host signals their its willingness to resume with a particular session
secret by sending a SYN segment with a resumption suboption, i.e., an
ENO suboption containing the negotiated TEP identifier of the
previous session, half of the "resumption identifier" for the new
session, and a "resumption nonce".

The resumption nonce MUST have a minimum length of zero bytes and
maximum length of eight bytes. The value MUST be chosen randomly or
using a mechanism that guarantees uniqueness even in the face of
virtual-machine cloning or other re-execution of the same session.
An attacker who can force either side of a connection to reuse a
session secret with the same nonce will completely break the security
of tcpcrypt. Reuse of session secrets is possible in the event of
virtual-machine cloning or reuse of system-level hibernation state.
Implementations SHOULD provide an API through which to set the
resumption nonce length and MUST default to eight bytes if they
cannot prohibit the reuse of session secrets.

The resumption identifier is calculated from a session secret "ss[i]" ss[i]
as follows:

resume[i] = CPRF(ss[i], CONST_RESUME, 18)

To name a session for resumption, a host sends either the first or
second half of the resumption identifier according to the role it
played in the original session with secret "ss[0]". ss[0].

A host that originally played role "A" and wishes to resume from a
cached session sends a suboption with the first half of the
resumption identifier:

byte 0 1 9 10
+------+------+--...--+------+------+--...--+------+
| TEP- | resume[i]{0..8} | nonce_a |
| byte | | |
+------+------+--...--+------+------+--...--+------+

Figure 2: Resumption suboption sent when original role was "A".

Note: The TEP-byte contains a tcpcrypt TEP identifier and "v v = 1". 1. The
nonce value MUST have length between 0 and 8 bytes.

Similarly, a host that originally played role "B" sends a suboption
with the second half of the resumption identifier:

byte 0 1 9 10
+------+------+--...--+------+------+--...--+------+
| TEP- | resume[i]{9..17} | nonce_b |
| byte | | |
+------+------+--...--+------+------+--...--+------+

Figure 3: Resumption suboption sent when original role was "B".

Note: The TEP-byte contains a tcpcrypt TEP identifier and v = 1. The
nonce value MUST have length between 0 and 8 bytes.

If a passive opener receives a resumption suboption containing an
identifier-half that names a session secret that it has cached, and
the subobtion's TEP matches the TEP used in the previous session, it
SHOULD (with exceptions specified below) agree to resume from the
cached session by sending its own resumption suboption, which will
contain the other half of the identifier. Otherwise, it MUST NOT
agree to resumption.

If a passive opener does not agree to resumption with a particular
TEP, it MAY either request fresh key exchange by responding with a
non-resumption suboption using the same TEP or else respond to any
other received TEP suboption.

If a passive opener receives an ENO suboption with a TEP identifier
and "v v = 1", 1, but the suboption data is less than 9 bytes in length, it
MUST behave as if the same TEP had been sent with "v v = 0". 0. That is,
the suboption MUST be interpreted as an offer to negotiate fresh key
exchange with that TEP.

If an active opener sends a resumption suboption with a particular
TEP and the appropriate half of a resumption identifier, and then, in
the same TCP handshake, it receives a resumption suboption with the
same TEP and an identifier-half that does not match that resumption
identifier, it MUST ignore that suboption. In the typical case that
this was the only ENO suboption received, this means the host MUST
disable TCP-ENO and tcpcrypt; it MUST NOT send any more ENO options
and MUST NOT encrypt the connection.

When a host concludes that TCP-ENO negotiation has succeeded for some
TEP that was received in a resumption suboption, it MUST then enable
encryption with that TEP using the cached session secret. To do
this, it first constructs "sn[i]" sn[i] as follows:

sn[i] = nonce_a | nonce_b

Master keys are then computed from "s[i]" s[i] and "sn[i]" sn[i] as described in
Section 3.3 as well as from application data encrypted as described
in Section 3.6.

The session ID (Section 3.4) is constructed in the same way for
resumed sessions as it is for fresh ones. In this case, the first
byte will always have "v v = 1". 1. The remainder of the ID is derived from
the cached session secret and the session nonce that was generated
during resumption.

In the case of simultaneous open where TCP-ENO is able to establish
asymmetric roles, two hosts that simultaneously send SYN segments
with compatible resumption suboptions MAY resume the associated
session.

In a particular SYN segment, a host SHOULD NOT send more than one
resumption suboption (because this consumes TCP option space and is
unlikely to be a useful practice), and it MUST NOT send more than one
resumption suboption with the same TEP identifier. But in addition
to any resumption suboptions, an active opener MAY include non-
resumption suboptions describing other TEPs it supports (in addition
to the TEP in the resumption suboption).

After using the session secret "ss[i]" ss[i] to compute "mk[0]", mk[0],
implementations SHOULD compute and cache "ss[i+1]" ss[i+1] for possible use by
a later session, then erase "ss[i]" ss[i] from memory. Hosts MAY retain
"ss[i+1]"
ss[i+1] until it is used or the memory needs to be reclaimed. Hosts
SHOULD NOT write any session secrets to non-volatile storage.

When proposing resumption, the active opener MUST use the lowest
value of "i" i that has not already been used (successfully or not) to
negotiate resumption with the same host and for the same original
session secret "ss[0]". ss[0].

A given session secret "ss[i]" ss[i] MUST NOT be used to secure more than one
TCP connection. To prevent this, a host MUST NOT resume with a
session secret if it has ever enabled encryption in the past with the
same secret, in either role. In the event that two hosts
simultaneously send SYN segments to each other that propose
resumption with the same session secret but with both segments not
part of a simultaneous open, both connections would need to revert to
fresh key exchange. To avoid this limitation, implementations MAY
choose to implement session resumption such that all session secrets
derived from a given "ss[0]" ss[0] are used for either passive or active
opens at the same host, not both.

If two hosts have previously negotiated a tcpcrypt session, either
host MAY later 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 keys "k_ab[j]" k_ab[j] for all
sessions derived from the same original session secret "ss[0]", ss[0], or with
keys "k_ba[j]". k_ba[j]. 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 cache session secrets MUST provide a means for
applications to control that caching. In particular, when an
application requests a new TCP connection, it MUST have a way to
specify two policies for the duration of the connection: 1) that
resumption requests will be ignored, and thus fresh key exchange will
be necessary; and 2) that no session secrets will be cached. (These
policies can be specified independently or as a unit.) And for an
established connection, an application MUST have a means to cause any
cache state that was used in or resulted from establishing the
connection to be flushed. A companion document [TCPINC-API]
describes recommended interfaces for this purpose.

3.6. Data Encryption and Authentication

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

This protection is provided via algorithms for Authenticated
Encryption with Associated Data (AEAD). The permitted algorithms are
listed in Table 5 in of Section 7. Additional algorithms can be
specified in the future according to the policy in that section. One
algorithm is selected during the negotiation described in
Section 3.3. The lengths "ae_key_len" ae_key_len and "ae_nonce_len" ae_nonce_len associated with
each algorithm are found in Table 3 in of Section 6 along with
requirements for which algorithms MUST be implemented.

The format of an encryption 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" data portion of a "plaintext"
value, which is then encrypted to yield a frame's "ciphertext" 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" control field and the length of the
ciphertext.

A "frame ID" value (see Section 4.2.3) is also constructed for the
frame, but not explicitly transmitted. It contains a 64-bit "offset" offset
field whose integer value is the zero-indexed byte offset of the
beginning of the current encryption frame in the underlying TCP
datastream. (That is, the offset in the framing stream, not the
plaintext application stream.) The offset is then left-padded with
zero-valued bytes to form a value of length "ae_nonce_len". ae_nonce_len. Because it
is strictly necessary for the security of the AEAD algorithms
specified in this document, an implementation MUST NOT ever transmit
distinct frames with the same frame ID 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" offset field to wrap) before
rekeying as described in Section 3.8.

With reference to the "AEAD Interface" described in Section 2 of
[RFC5116], tcpcrypt invokes the AEAD algorithm with values taken from
the traffic key "k_ab[j]" k_ab[j] or "k_ba[j]" k_ba[j] for some "j", j, according to the
host's role as described in Section 3.3.

First, the traffic key is divided into two parts:

ae_key_len + ae_nonce_len - 1
|
byte 0 ae_key_len |
| | |
v v v
+----+----+--...--+----+----+----+--...--+----+
| K | NR |
+----+----+--...--+----+----+----+--...--+----+

\_____________________________________________/
traffic key

Figure 4: Format of Traffic Key

The first "ae_key_len" ae_key_len bytes of the traffic key provide the AEAD key
"K", K,
while the remaining "ae_nonce_len" ae_nonce_len bytes provide a "nonce randomizer"
value "NR". NR. The frame ID is then combined via bitwise exclusive-or
with the nonce randomizer to yield "N", N, the AEAD nonce for the frame:

N = frame_ID XOR NR

The plaintext value serves as "P", P, and the associated data as "A". A. The
output of the encryption operation, "C", C, is transmitted in the frame's "ciphertext"
ciphertext field.

When a frame is received, tcpcrypt reconstructs the associated data
and frame ID values (the former contains only data sent in the clear,
and the latter is implicit in the TCP stream), computes the nonce "N" N
as above, and provides these and the ciphertext value to the AEAD
decryption operation. The output of this operation is either a
plaintext value "P" P or the special symbol FAIL. In the latter case,
the implementation SHOULD abort the connection and raise an error
condition distinct from the end-of-file condition. But if none of
the TCP segment(s) containing the frame have been acknowledged and
retransmission could potentially result in a valid frame, an
implementation MAY instead drop these segments (and "renege" if they
have been selectively acknowledged (SACKed), according to Section 8
of [RFC2018]).

3.7. TCP Header Protection

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

When the "URGp" URGp bit is set, the "urgent" 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.

A sender MUST set the "FINp" FINp bit on the last frame it sends in the
connection (unless it aborts the connection) and MUST NOT set "FINp" FINp on
any other frame.

TCP sets the FIN flag when a sender has no more data, which with
tcpcrypt means setting FIN on the segment containing the last byte of
the last frame. However, a receiver MUST report the end-of-file
condition to the connection's local user when and only when it
receives a frame with the "FINp" FINp bit set. If a host receives a segment
with the TCP FIN flag set but the received datastream including this
segment does not contain a frame with "FINp" FINp set, the host SHOULD abort
the connection and raise an error condition distinct from the end-of-file end-of-
file condition. But if there are unacknowledged segments whose
retransmission could potentially result in a valid frame, the host
MAY instead drop the segment with the TCP FIN flag set (and "renege"
if it has been SACKed, according to Section 8 of [RFC2018]).

3.8. Rekeying

Rekeying 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.

As described in Section 3.3, a master key "mk[j]" mk[j] is used to generate
two encryption keys "k_ab[j]" k_ab[j] and "k_ba[j]". k_ba[j]. We refer to these as a
_key set_ key
set with _generation number_ "j". generation number j. Each host maintains both a
_local local
generation number_ number that determines which key set it uses to encrypt
outgoing frames and a _remote remote generation number_ number equal to the highest
generation used in frames received from its peer. Initially, these
two generation numbers are set to zero.

A host MAY increment its local generation number beyond the remote
generation number it has recorded. We call this action _initiating
rekeying_. "initiating
rekeying".

When a host has incremented its local generation number and uses the
new key set for the first time to encrypt an outgoing frame, it MUST
set "rekey rekey = 1" 1 for that frame. It MUST set "rekey rekey = 0" 0 in all other
cases.

When a host receives a frame with "rekey rekey = 1", 1, it increments its record
of the remote generation number. If the remote generation number is
now greater than the local generation number, the receiver MUST
immediately increment its local generation number to match.
Moreover, if the receiver has not yet transmitted a segment with the
FIN flag set, it MUST immediately send a frame (with empty
application data if necessary) with "rekey rekey = 1". 1.

A host MUST NOT initiate more than one concurrent rekey operation if
it has no data to send; that is, it MUST NOT initiate rekeying with
an empty encryption frame more than once while its record of the
remote generation number is less than its own.

Note that when parts of the datastream are retransmitted, TCP
requires that implementations always send the same data bytes for the
same TCP sequence numbers. Thus, frame data in retransmitted
segments MUST be encrypted with the same key as when it was first
transmitted, regardless of the current local generation number.

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.

3.9. Keep-Alive

Instead of using TCP keep-alives to verify that the remote endpoint
is still responsive, tcpcrypt implementations SHOULD employ the
rekeying mechanism for this purpose, as follows. When necessary, a
host SHOULD probe the liveness of its peer by initiating rekeying and
transmitting a new frame immediately (with empty application data if
necessary).

As described in Section 3.8, a host receiving a frame encrypted under
a generation number greater than its own MUST increment its own
generation number and (if it has not already transmitted a segment
with FIN set) 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 8.2.

4. Encodings

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

4.1. Key-Exchange Messages

The "Init1" Init1 message has the following encoding:

byte 0 1 2 3
+-------+-------+-------+-------+
| INIT1_MAGIC |
| |
+-------+-------+-------+-------+

4 5 6 7
+-------+-------+-------+-------+
| message_len |
| = M |
+-------+-------+-------+-------+

8
+--------+-----+----+-----+----+---...---+-----+-----+
|nciphers|sym_ |sym_ | |sym_ |
| = K |cipher[0] |cipher[1] | |cipher[K-1]|
+--------+-----+----+-----+----+---...---+-----+-----+

2*K + 9 2*K + 9 + N_A_LEN
| |
v v
+-------+---...---+-------+-------+---...---+-------+
| N_A | Pub_A |
| | |
+-------+---...---+-------+-------+---...---+-------+

M - 1
+-------+---...---+-------+
| ignored |
| |
+-------+---...---+-------+

The constant "INIT1_MAGIC" INIT1_MAGIC is defined in Section 4.3. The four-byte
field "message_len" message_len gives the length of the entire "Init1" Init1 message,
encoded as a big-endian integer. The "nciphers" nciphers field contains an
integer value that specifies the number of two-byte symmetric-cipher
identifiers that follow. The "sym_cipher[i]" sym_cipher[i] identifiers indicate
cryptographic algorithms in Table 5 in Section 7. The length
"N_A_LEN" N_A_LEN
and the length of "Pub_A" Pub_A are both determined by the negotiated TEP as
described in Section 5.

Implementations of this protocol MUST construct "Init1" Init1 such that the
field "ignored" ignored has zero length; that is, they MUST construct the
message such that its end, as determined by "message_len", message_len, coincides
with the end of the field "Pub_A". Pub_A. When receiving "Init1", Init1, however,
implementations MUST permit and ignore any bytes following "Pub_A". Pub_A.

The "Init2" Init2 message has the following encoding:

byte 0 1 2 3
+-------+-------+-------+-------+
| INIT2_MAGIC |
| |
+-------+-------+-------+-------+

4 5 6 7 8 9
+-------+-------+-------+-------+-------+-------+
| message_len | sym_cipher |
| = M | |
+-------+-------+-------+-------+-------+-------+

10 10 + N_B_LEN
| |
v v
+-------+---...---+-------+-------+---...---+-------+
| N_B | Pub_B |
| | |
+-------+---...---+-------+-------+---...---+-------+

M - 1
+-------+---...---+-------+
| ignored |
| |
+-------+---...---+-------+

The constant "INIT2_MAGIC" INIT2_MAGIC is defined in Section 4.3. The four-byte
field "message_len" message_len gives the length of the entire "Init2" Init2 message,
encoded as a big-endian integer. The "sym_cipher" sym_cipher value is a selection
from the symmetric-cipher identifiers in the previously-
received "Init1" previously-received
Init1 message. The length "N_B_LEN" N_B_LEN and the length of
"Pub_B" Pub_B are both
determined by the negotiated TEP as described in Section 5.

Implementations of this protocol MUST construct "Init2" Init2 such that the
field "ignored" ignored has zero length; that is, they MUST construct the
message such that its end, as determined by "message_len", message_len, coincides
with the end of the "Pub_B" Pub_B field. When receiving "Init2", Init2, however,
implementations MUST permit and ignore any bytes following "Pub_B". Pub_B.

4.2. Encryption Frames

An _encryption frame_ encryption frame comprises a control byte and a length-prefixed
ciphertext value:

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

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

The byte "control" control has this structure:

bit 7 1 0
+-------+---...---+-------+-------+
| cres | rekey |
+-------+---...---+-------+-------+

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

The use of the "rekey" rekey field is described in Section 3.8.

4.2.1. Plaintext

The "ciphertext" 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" clen in the previous section will generally be greater
than "plen", plen, as the ciphertext produced by the authenticated-
encryption authenticated-encryption
scheme both encrypts the application data and provides redundancy
with which to verify its integrity.)

The "flags" flags byte has this structure:

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

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

When the "URGp" URGp bit is set, it indicates that the "urgent" 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" urgent and of the flag bits is described in
Section 3.7.

4.2.2. Associated Data

An encryption 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" control and "clen" clen fields.

4.2.3. Frame ID

Lastly, a "frame ID" (used to construct the nonce for the AEAD
algorithm) has this format:

byte 0 ae_nonce_len - 8 ae_nonce_len - 1
| | |
v v v
+-----+--...--+-----+-----+--...--+-----+
| 0 | | 0 | offset |
+-----+--...--+-----+-----+--...--+-----+

The 8-byte "offset" offset field contains an integer in big-endian format.
Its value is specified in Section 3.6. Zero-valued bytes are
prepended to the "offset" offset field to form a structure of length
"ae_nonce_len".
ae_nonce_len.

4.3. Constant Values

The table below defines values for the constants used in the
protocol.

+------------+--------------+
| Value | Name |
+------------+--------------+
| 0x01 | CONST_NEXTK |
| 0x02 | CONST_SESSID |
| 0x03 | CONST_REKEY |
| 0x04 | CONST_KEY_A |
| 0x05 | CONST_KEY_B |
| 0x06 | CONST_RESUME |
| 0x15101a0e | INIT1_MAGIC |
| 0x097105e0 | INIT2_MAGIC |
+------------+--------------+

Table 1: Constant Values Used in the Protocol

5. Key-Agreement Schemes

The TEP negotiated via TCP-ENO indicates the use of one of the key-
agreement schemes named in Table 4 in Section 7. For example,
"TCPCRYPT_ECDHE_P256"
TCPCRYPT_ECDHE_P256 names the tcpcrypt protocol using ECDHE-P256
together with the CPRF and length parameters specified below.

All the TEPs specified in this document require the use of HKDF-
Expand-SHA256 as the CPRF, and these lengths for nonces and session
secrets:

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

Future documents assigning additional TEPs for use with tcpcrypt
might specify different values for the lengths above. Note that the
minimum session ID length specified by TCP-ENO, together with the way
tcpcrypt constructs session IDs, implies that "K_LEN" K_LEN MUST have length
at least 32 bytes.

Key-agreement schemes ECDHE-P256 and ECDHE-P521 employ the Elliptic
Curve Secret Value Derivation Primitive, Diffie-Hellman version
(ECSVDP-DH) defined in [IEEE-1363]. The named curves are defined in
[NIST-DSS]. When the public-key values "Pub_A" Pub_A and "Pub_B" Pub_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 [IEEE-1363] and are prefixed by a two-byte length
in big-endian format:

Implementations MUST encode these "pubkey" pubkey values in "compressed
format". Implementations MUST validate these "pubkey" pubkey values according
to the algorithm in Section A.16.10 of [IEEE-1363].

Key-agreement schemes ECDHE-Curve25519 and ECDHE-Curve448 perform the
Diffie-Hellman protocol using the functions X25519 and X448,
respectively. Implementations SHOULD compute these functions using
the algorithms described in [RFC7748]. When they do so,
implementations MUST check whether the computed Diffie-Hellman shared
secret is the all-zero value and abort if so, as described in
Section 6 of [RFC7748]. Alternative implementations of these
functions SHOULD abort when either input forces the shared secret to
one of a small set of values as discussed in Section 7 of [RFC7748].

For these schemes, public-key values "Pub_A" Pub_A and "Pub_B" Pub_B are transmitted
directly with no length prefix: 32 bytes for ECDHE-
Curve25519 ECDHE-Curve25519 and 56
bytes for ECDHE-Curve448.

Table 2 below specifies the requirement levels of the four TEPs
specified in this document. In particular, all implementations of
tcpcrypt MUST support TCPCRYPT_ECDHE_Curve25519. However, system
administrators MAY configure which TEPs a host will negotiate
independent of these implementation requirements.

Table 2: Requirements for Implementation of TEPs

6. AEAD Algorithms

This document uses "sym-cipher" sym-cipher identifiers in the messages "Init1" Init1 and "Init2"
Init2 (see Section 3.3) to negotiate the use of AEAD algorithms; the
values of these identifiers are given in Table 5 in Section 7. The
algorithms "AEAD_AES_128_GCM" AEAD_AES_128_GCM and "AEAD_AES_256_GCM" AEAD_AES_256_GCM are specified in
[RFC5116]. The algorithm "AEAD_CHACHA20_POLY1305" AEAD_CHACHA20_POLY1305 is specified in
[RFC8439].

Implementations MUST support certain AEAD algorithms according to
Table 3. Note that system administrators MAY configure which
algorithms a host will negotiate independent of these requirements.

Lastly, this document uses the lengths "ae_key_len" ae_key_len and
"ae_nonce_len" ae_nonce_len to
specify aspects of encryption and data formats. These values depend
on the negotiated AEAD algorithm, also according to the table below.

Table 3: Requirement and Lengths for Each AEAD Algorithm

7. IANA Considerations

For use with TCP-ENO's negotiation mechanism, tcpcrypt's TEP
identifiers have been incorporated in IANA's "TCP Encryption Protocol
Identifiers" registry under the "Transmission Control Protocol (TCP)
Parameters" registry, as in Table 4. The various key-agreement
schemes used by these tcpcrypt variants are defined in Section 5.

+-------+---------------------------+-----------+
| Value | Meaning | Reference |
+-------+---------------------------+-----------+
| 0x21 | TCPCRYPT_ECDHE_P256 | [RFC8548] |
| 0x22 | TCPCRYPT_ECDHE_P521 | [RFC8548] |
| 0x23 | TCPCRYPT_ECDHE_Curve25519 | [RFC8548] |
| 0x24 | TCPCRYPT_ECDHE_Curve448 | [RFC8548] |
+-------+---------------------------+-----------+

Table 4: TEP Identifiers for Use with tcpcrypt

In Section 6, this document defines the use of several AEAD
algorithms for encrypting application data. To name these
algorithms, the tcpcrypt protocol uses two-byte identifiers in the
range 0x0001 to 0xFFFF, inclusively, for which IANA maintains a new
"tcpcrypt AEAD Algorithms" registry under the "Transmission Control
Protocol (TCP) Parameters" registry. The initial values for this
registry are given in Table 5. Future assignments are to be made
upon satisfying either of two policies defined in [RFC8126]: "IETF
Review" or (for non-IETF stream specifications) "Expert Review with
RFC Required." IANA will furthermore provide early allocation
[RFC7120] to facilitate testing before RFCs are finalized.

Table 5: Authenticated-Encryption Algorithms for Use with tcpcrypt

8. Security Considerations

All of the security considerations of TCP-ENO apply to tcpcrypt. In
particular, tcpcrypt does not protect against active network
attackers 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., Elliptic Curve Diffie-Hellman (ECDH).
A proof of this property for an earlier version of the protocol has
been published [tcpcrypt].

To ensure middlebox compatibility, tcpcrypt does not protect TCP
headers. Therefore, the protocol is vulnerable to denial-of-service
from off-path attackers just as plain TCP is. Possible attacks
include desynchronizing the underlying TCP stream, injecting RST or
FIN segments, and forging rekey bits. These attacks will cause a
tcpcrypt connection to hang or fail with an error, but not in any
circumstance where plain TCP could continue uncorrupted.
Implementations MUST give higher-level software a way to distinguish
such errors from a clean end-of-stream (indicated by an authenticated
"FINp"
FINp bit) so that applications can avoid semantic truncation attacks.

There is no "key confirmation" step in tcpcrypt. This is not needed
because tcpcrypt's threat model includes the possibility of a
connection to an adversary. If key negotiation is compromised and
yields two different keys, failed integrity checks on every
subsequent frame will cause the connection either to hang or to
abort. This is not a new threat as an active attacker can achieve
the same results against a plain TCP connection by injecting RST
segments or modifying sequence and acknowledgement numbers.

Tcpcrypt uses short-lived public keys to provide forward secrecy;
once an implementation removes these keys from memory, a compromise
of the system will not provide any means to derive the session
secrets for past connections. All currently-specified key agreement
schemes involve key agreement based on Ephemeral Elliptic Curve
Diffie-Hellman (ECDHE), meaning a new key pair can be efficiently
computed for each connection. If implementations reuse these
parameters, they MUST limit the lifetime of the private parameters as
far as is practical in order to minimize the number of past
connections that are vulnerable. Of course, placing private keys in
persistent storage introduces severe risks that they will not be
destroyed reliably and in a timely fashion, and it SHOULD be avoided
whenever possible.

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

The cipher-suites specified in this document all use HMAC-SHA256 to
implement the collision-resistant pseudo-random function denoted by
"CPRF".
CPRF. A collision-resistant function is one for 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.

Lastly, 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.
[RFC4086] provides recommendations on how to achieve this.

Most implementations will rely on a device's pseudo-random generator,
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.

8.1. Asymmetric Roles

Tcpcrypt transforms a shared pseudo-random key (PRK) into
cryptographic traffic 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).

8.2. 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. Therefore, 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.)

To counter this threat, tcpcrypt specifies a way to stimulate the
remote host to send verifiably fresh and authentic data, described in
Section 3.9.

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.

8.3. Mandatory Key-Agreement Schemes

This document mandates that tcpcrypt implementations provide support
for at least one key-agreement scheme: ECDHE using Curve25519. This
choice of a single mandatory algorithm is the result of a difficult
tradeoff between cryptographic diversity and the ease and security of
actual deployment.

The IETF's appraisal of best current practice on this matter
[RFC7696] says, "Ideally, two independent sets of mandatory-to-
implement algorithms will be specified, allowing for a primary suite
and a secondary suite. This approach ensures that the secondary
suite is widely deployed if a flaw is found in the primary one."

To meet that ideal, it might appear natural to also mandate ECDHE
using P-256. However, implementing the Diffie-Hellman function using
NIST elliptic curves (including those specified for use with
tcpcrypt, P-256 and P-521) appears to be very difficult to achieve
without introducing vulnerability to side-channel attacks
[NIST-fail]. Although well-trusted implementations are available as
part of large cryptographic libraries, these can be difficult to
extract for use in operating-system kernels where tcpcrypt is usually
best implemented. In contrast, the characteristics of Curve25519
together with its recent popularity has led to many safe and
efficient implementations, including some that fit naturally into the
kernel environment.

[RFC7696] insists that, "The selected algorithms need to be resistant
to side-channel attacks and also meet the performance, power, and
code size requirements on a wide variety of platforms." On this
principle, tcpcrypt excludes the NIST curves from the set of
mandatory-to-implement key-agreement algorithms.

Lastly, this document encourages support for key agreement with
Curve448, categorizing it as RECOMMENDED. Curve448 appears likely to
admit safe and efficient implementations. However, support is not
REQUIRED because existing implementations might not yet be
sufficiently well proven.

9. Experiments

Some experience will be required to determine whether the tcpcrypt
protocol can be deployed safely and successfully across the diverse
environments of the global internet.

Safety means that TCP implementations that support tcpcrypt are able
to communicate reliably in all the same settings as they would
without tcpcrypt. As described in Section 9 of [RFC8547], this
property can be subverted if middleboxes strip ENO options from non-
SYN segments after allowing them in SYN segments, or if the
particular communication patterns of tcpcrypt offend the policies of
middleboxes doing deep-packet inspection.

Success, in addition to safety, means hosts that implement tcpcrypt
actually enable encryption when connecting to one another. This
property depends on the network's treatment of the TCP-ENO handshake
and can be subverted if middleboxes merely strip unknown TCP options
or terminate TCP connections and relay data back and forth
unencrypted.

Ease of implementation will be a further challenge to deployment.
Because tcpcrypt requires encryption operations on frames that may
span TCP segments, kernel implementations are forced to buffer
segments in different ways than are necessary for plain TCP. More
implementation experience will show how much additional code
complexity is required in various operating systems and what kind of
performance effects can be expected.

10. References

10.1. Normative References

[IEEE-1363]
IEEE, "IEEE Standard Specifications for Public-Key
Cryptography", IEEE Standard 1363-2000,
DOI 10.1109/IEEESTD.2000.92292.

[NIST-DSS]
National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS)", FIPS PUB 186-4,
DOI 10.6028/NIST.FIPS.186-4, July 2013.

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.

[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<https://www.rfc-editor.org/info/rfc2018>.

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.

[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.

[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://www.rfc-editor.org/info/rfc7120>.

[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8439] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,
<https://www.rfc-editor.org/info/rfc8439>.

[RFC8547] Bittau, A., Giffin, D., Handley, M., Mazieres, D., and E.
Smith, "TCP-ENO: Encryption Negotiation Option", RFC 8547,
DOI 10.17487/RFC8547, March 2019,
<https://www.rfc-editor.org/info/rfc8547>.

10.2. Informative References

[NIST-fail]
Bernstein, D. and T. Lange, "Failures in NIST's ECC
Standards", January 2016,
<https://cr.yp.to/newelliptic/nistecc-20160106.pdf>.

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.

[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.

[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
<https://www.rfc-editor.org/info/rfc7696>.

[tcpcrypt]
Bittau, A., Hamburg, M., Handley, M., Mazieres, D., and D.
Boneh, "The case for ubiquitous transport-level
encryption", USENIX Security Symposium, August 2010.

[TCPINC-API]
Bittau, A., Boneh, D., Giffin, D., Handley, M., Mazieres,
D., and E. Smith, "Interface Extensions for TCP-ENO and
tcpcrypt", Work in Progress, draft-ietf-tcpinc-api-06,
June 2018.

Acknowledgments

We are grateful for contributions, help, discussions, and feedback
from the TCPINC Working Group and from other IETF reviewers,
including Marcelo Bagnulo, David Black, Bob Briscoe, Jana Iyengar,
Stephen Kent, Tero Kivinen, Mirja Kuhlewind, Yoav Nir, Christoph
Paasch, Eric Rescorla, Kyle Rose, and Dale Worley.

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.

Contributors

Dan Boneh and Michael Hamburg were coauthors of the draft that became
this document.

Authors' Addresses

Andrea Bittau
Google
345 Spear Street
San Francisco, CA 94105
United States of America

Email: bittau@google.com
Daniel B. Giffin
Stanford University
353 Serra Mall, Room 288
Stanford, CA 94305
United States of America

Email: dbg@scs.stanford.edu daniel@beech-grove.net

Mark Handley
University College London
Gower St.
London WC1E 6BT
United Kingdom

Email: M.Handley@cs.ucl.ac.uk

David Mazieres
Stanford University
353 Serra Mall, Room 290
Stanford, CA 94305
United States of America

Email: dm@uun.org

Quinn Slack
Sourcegraph
121 2nd St Ste 200
San Francisco, CA 94105
United States of America

Email: sqs@sourcegraph.com

Eric W. Smith
Kestrel Institute
3260 Hillview Avenue
Palo Alto, CA 94304
United States of America

Email: eric.smith@kestrel.edu