RFC 8548
Cryptographic Protection of TCP Streams (tcpcrypt)
Pages: 32
Experimental
Experimental
Part 1 of 2 – Pages 1 to 17
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 May 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. Copyright Notice Copyright (c) 2019 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 (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 . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4 3. Encryption Protocol . . . . . . . . . . . . . . . . . . . . . 4 3.1. Cryptographic Algorithms . . . . . . . . . . . . . . . . 4 3.2. Protocol Negotiation . . . . . . . . . . . . . . . . . . 6 3.3. Key Exchange . . . . . . . . . . . . . . . . . . . . . . 7 3.4. Session ID . . . . . . . . . . . . . . . . . . . . . . . 10 3.5. Session Resumption . . . . . . . . . . . . . . . . . . . 10 3.6. Data Encryption and Authentication . . . . . . . . . . . 14 3.7. TCP Header Protection . . . . . . . . . . . . . . . . . . 16 3.8. Rekeying . . . . . . . . . . . . . . . . . . . . . . . . 16 3.9. Keep-Alive . . . . . . . . . . . . . . . . . . . . . . . 17 4. Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.1. Key-Exchange Messages . . . . . . . . . . . . . . . . . . 18 4.2. Encryption Frames . . . . . . . . . . . . . . . . . . . . 20 4.2.1. Plaintext . . . . . . . . . . . . . . . . . . . . . . 20 4.2.2. Associated Data . . . . . . . . . . . . . . . . . . . 21 4.2.3. Frame ID . . . . . . . . . . . . . . . . . . . . . . 21 4.3. Constant Values . . . . . . . . . . . . . . . . . . . . . 22 5. Key-Agreement Schemes . . . . . . . . . . . . . . . . . . . . 22 6. AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . . . 24 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 8. Security Considerations . . . . . . . . . . . . . . . . . . . 25 8.1. Asymmetric Roles . . . . . . . . . . . . . . . . . . . . 27 8.2. Verified Liveness . . . . . . . . . . . . . . . . . . . . 27 8.3. Mandatory Key-Agreement Schemes . . . . . . . . . . . . . 27 9. Experiments . . . . . . . . . . . . . . . . . . . . . . . . . 28 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 10.1. Normative References . . . . . . . . . . . . . . . . . . 29 10.2. Informative References . . . . . . . . . . . . . . . . . 30 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 31 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
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 agreement scheme is used with a short-lived public key to agree upon a shared secret.
o An 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 pseudo-random function (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) for
a negotiated 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)
HKDF-Expand(PRK, CONST, L) -> OKM
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) | ...
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" 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 = 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 = 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 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 = 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 = 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" 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 and 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 or 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 defined in Section 4.3. o 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 symmetric cipher selected by host B from the sym_cipher_list sent by host A. o N_A, N_B: Nonces chosen at random by hosts A and B, respectively. o 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 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 using its own private key (not transmitted) and host B's public key, Pub_B. The two sides then compute a pseudo-random key, PRK, from which all session secrets 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 Section 4.8 of [RFC8547], and Init1 and Init2 are the transmitted encodings of the messages described in Section 4.1. A series of session secrets are 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. The values ss[i] for i > 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_* values are constants defined in Section 4.3. The length K_LEN depends on the tcpcrypt TEP in use, and is specified in Section 5. Given a session secret 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] are session nonces. For the initial session with i = 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] 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] are used by host A to encrypt and host B to decrypt, while keys 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; for example, if a host played role "A" in the first session, it will use keys k_ab[j] to encrypt in each derived session. The values ae_key_len and ae_nonce_len depend on the authenticated- encryption algorithm selected and are given in Table 3 of Section 6. The algorithm uses the first ae_key_len bytes of each traffic key as an authenticated-encryption key, and it uses the following 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 (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 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 with a sym_cipher value that was not present in the sym_cipher_list it previously transmitted in 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 [RFC793]. 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 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" 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 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], they can establish a new connection without public-key operations using ss[i], the next session secret in the sequence derived from the original PRK. A host signals 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]
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].
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".
The TEP-byte contains a tcpcrypt TEP identifier and v = 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".
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 = 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 = 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] as follows:
sn[i] = nonce_a | nonce_b
Master keys are then computed from s[i] and 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 = 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] to compute mk[0], implementations SHOULD compute and cache ss[i+1] for possible use by a later session and then erase ss[i] from memory. Hosts MAY retain 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" 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]. A given session secret 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] 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] for all sessions derived from the same original session secret ss[0], or with keys 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 that are encrypted and authenticated using the agreed-upon keys. This protection is provided via algorithms for Authenticated Encryption with Associated Data (AEAD). The permitted algorithms are listed in Table 5 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 and ae_nonce_len associated with each algorithm are found in Table 3 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 field 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 ID" value (see Section 4.2.3) is also constructed for the frame, but not explicitly transmitted. It contains a 64-bit 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. 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 field to wrap) before rekeying as described in Section 3.8.
Keys for AEAD encryption are taken from the traffic key k_ab[j] or k_ba[j] for some "j", according to the host's role as described in Section 3.3. First, the appropriate traffic key is divided into two parts: ae_key_len + ae_nonce_len - 1 | byte 0 ae_key_len | | | | v v v +----+----+--...--+----+----+----+--...--+----+ | K | NR | +----+----+--...--+----+----+----+--...--+----+ Figure 4: Format of Traffic Key With reference to the "AEAD Interface" described in Section 2 of [RFC5116], the first ae_key_len bytes of the traffic key provide the AEAD key K. The remaining ae_nonce_len bytes provide a nonce randomizer value NR, which is combined via bitwise exclusive-or with the frame ID to yield N, the AEAD nonce for the frame: N = frame_ID XOR NR The remaining AEAD inputs, P and A, are provided by the frame's plaintext value and associated data, respectively. The output of the AEAD operation, C, is transmitted in the frame's 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 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 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 field of the encryption frame contains protected versions of certain TCP header values. When the URGp bit is set, the urgent field 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 bit on the last frame it sends in the connection (unless it aborts the connection) and MUST NOT set 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 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 set, the host SHOULD abort the connection and raise an error condition distinct from the 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] is used to generate two encryption keys k_ab[j] and k_ba[j]. We refer to these as a key set with generation number "j". Each host maintains both a local generation number that determines which key set it uses to encrypt outgoing frames and a remote generation 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".
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 = 1 for that frame. It MUST set rekey = 0 in all other cases. When a host receives a frame with rekey = 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 = 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.
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