Network Working Group A. Bittau
Internet-Draft D. Boneh
Intended status: Standards Track D. Giffin
Expires: January 9, 2017 M. Hamburg
Stanford University
M. Handley
University College London
D. Mazieres
Q. Slack
Stanford University
E. Smith
Kestrel Institute
July 8, 2016
Cryptographic protection of TCP Streams (tcpcrypt)
draft-ietf-tcpinc-tcpcrypt-02
Abstract
This document specifies tcpcrypt, a cryptographic protocol that
protects TCP payload data. Use of the protocol is negotiated by
means of the TCP Encryption Negotiation Option (TCP-ENO)
[I-D.ietf-tcpinc-tcpeno]. Tcpcrypt coexists with middleboxes by
tolerating resegmentation, NATs, and other manipulations of the TCP
header. The protocol is self-contained and specifically tailored to
TCP implementations, which often reside in kernels or other
environments in which large external software dependencies can be
undesirable. Because the size of TCP options is limited, the
protocol requires one additional one-way message latency to perform
key exchange before application data may be transmited. However,
this cost can be avoided between two hosts that have recently
established a previous tcpcrypt connection.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
Bittau, et al. Expires January 9, 2017 [Page 1]
Internet-Draft tcpcrypt July 2016 This Internet-Draft will expire on January 9, 2017. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Requirements language . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Encryption protocol . . . . . . . . . . . . . . . . . . . . . 3 3.1. Cryptographic algorithms . . . . . . . . . . . . . . . . 4 3.2. Protocol negotiation . . . . . . . . . . . . . . . . . . 5 3.3. Key exchange . . . . . . . . . . . . . . . . . . . . . . 6 3.4. Session caching . . . . . . . . . . . . . . . . . . . . . 8 3.5. Data encryption and authentication . . . . . . . . . . . 10 3.6. TCP header protection . . . . . . . . . . . . . . . . . . 11 3.7. Re-keying . . . . . . . . . . . . . . . . . . . . . . . . 11 3.8. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 12 4. Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1. Key exchange messages . . . . . . . . . . . . . . . . . . 13 4.2. Application frames . . . . . . . . . . . . . . . . . . . 15 4.2.1. Plaintext . . . . . . . . . . . . . . . . . . . . . . 15 4.2.2. Associated data . . . . . . . . . . . . . . . . . . . 16 4.2.3. Frame nonce . . . . . . . . . . . . . . . . . . . . . 16 Bittau, et al. Expires January 9, 2017 [Page 2]
Internet-Draft tcpcrypt July 2016 5. Key agreement schemes . . . . . . . . . . . . . . . . . . . . 17 6. AEAD algorithms . . . . . . . . . . . . . . . . . . . . . . . 17 7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 18 8. Security considerations . . . . . . . . . . . . . . . . . . . 18 9. Design notes . . . . . . . . . . . . . . . . . . . . . . . . 20 9.1. Asymmetric roles . . . . . . . . . . . . . . . . . . . . 20 9.2. Verified liveness . . . . . . . . . . . . . . . . . . . . 20 10. API extensions . . . . . . . . . . . . . . . . . . . . . . . 20 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 12.1. Normative References . . . . . . . . . . . . . . . . . . 22 12.2. Informative References . . . . . . . . . . . . . . . . . 22 Appendix A. Protocol constant values . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 1. Requirements language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. Introduction This document describes tcpcrypt, an extension to TCP for cryptographic protection of session data. Tcpcrypt was designed to meet the following goals: o Meet the requirements of the TCP Encryption Negotiation Option (TCP-ENO) [I-D.ietf-tcpinc-tcpeno] 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. 3. Encryption protocol This section describes the tcpcrypt protocol at an abstract level. The concrete format of all messages is specified in Section 4. Bittau, et al. Expires January 9, 2017 [Page 3]
Internet-Draft tcpcrypt July 2016 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 from some initial keying material, typically the output of 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. We use the notation CPRF(K, CONST, L) to designate the output of L bytes of the pseudo-random function identified by key K on CONST. The Extract and CPRF functions used by default are the Extract and Expand functions of HKDF [RFC5869]. These are defined as follows in terms of the PRF "HMAC-Hash(key, value)" for a negotiated "Hash" function: HKDF-Extract(salt, IKM) -> PRK PRK = HMAC-Hash(salt, IKM) 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) | ... Figure 1: The symbol | denotes concatenation, and the counter concatenated after CONST is a single octet. Lastly, once tcpcrypt has been successfully set up, an _authenticated encryption mode_ is used to protect the confidentiality and integrity of all transmitted application data. Bittau, et al. Expires January 9, 2017 [Page 4]
Internet-Draft tcpcrypt July 2016 3.2. Protocol negotiation Tcpcrypt depends on TCP-ENO [I-D.ietf-tcpinc-tcpeno] to negotiate whether encryption will be enabled for a connection, and also which key agreement scheme to use. TCP-ENO negotiates an _encryption spec_ by means of suboptions embedded in SYN segments. Each suboption is identified by a byte consisting of a seven-bit _encryption spec identifier_ value, "cs", and a one-bit additional-data indicator, "v". This document reserves and associates four "cs" values with tcpcrypt, as listed in Table 1; future standards may associate additional values with tcpcrypt. An active opener that wishes to negotiate the use of tcpcrypt will include an ENO option in its SYN segment, and that option will include tcpcrypt suboptions corresponding to the key-agreement schemes it is willing to enable. The active opener MAY additionally include suboptions indicating support for encryption protocols other than tcpcrypt, as well as other general options as specified by TCP- ENO. If a passive opener receives an ENO option including tcpcrypt suboptions it supports, it MAY then attach an ENO option to its SYN- ACK segment, including _solely_ the suboption it wishes to enable. To establish distinct roles for the two hosts in each connection, tcpcrypt depends on the role-negotiation mechanism of TCP-ENO [I-D.ietf-tcpinc-tcpeno]. As part of the negotiation process, TCP- ENO assigns hosts unique roles abstractly called "A" at one end of the connection and "B" at the other. Generally, an active opener plays the "A" role and a passive opener plays the "B" role, though an additional mechanism breaks the symmetry of simultaneous open. This document adopts the terms "host A" and "host B" to identify each end of a connection uniquely, following TCP-ENO's designation. Once two hosts have exchanged SYN segments, the _negotiated spec_ is the last spec identifier in the SYN segment of host B (that is, the passive opener in the absence of simultaneous open) that also occurs in that of host A. If there is no such spec, hosts MUST disable TCP- ENO and tcpcrypt. The _negotiated suboption_ is the ENO suboption from the SYN segment of host B that contains the negotiated spec, if it exists. As required by TCP-ENO, once a host has both sent and received an ACK segment containing an ENO option, encryption MUST be enabled and plaintext application data MUST NOT ever be exchanged on the connection. If the negotiated spec is a "cs" value associated with tcpcrypt, a host MUST follow the protocol described in this document. Bittau, et al. Expires January 9, 2017 [Page 5]
Internet-Draft tcpcrypt July 2016 In particular, if the negotiated suboption contains "v = 0", a fresh key agreement will be perfomed as described below in Section 3.3; if it contains "v = 1", the key-exchange messages are omitted in favor of determining keys via session-caching as described in Section 3.4, and protected application data may immediately be sent as detailed in Section 3.5. 3.3. Key exchange Following successful negotiation of a tcpcrypt spec, all further signaling is performed in the Data portion of TCP segments. Except when resumption was negotiated (described below in Section 3.4), the two hosts perform key exchange through two messages, "Init1" and "Init2", at the start of host A's and host B's data streams, respectively. These messages may span multiple TCP segments and need not end at a segment boundary. However, the segment containing the last byte of an "Init1" or "Init2" message SHOULD have TCP's PSH bit set. The key exchange protocol, in abstract, proceeds as follows: A -> B: Init1 = { INIT1_MAGIC, sym-cipher-list, N_A, PK_A } B -> A: Init2 = { INIT2_MAGIC, sym-cipher, N_B, PK_B } The concrete format of these messages is specified in further detail in Section 4.1. The parameters are defined as follows: o "INIT1_MAGIC", "INIT2_MAGIC": constants defined in Table 3. o "sym-cipher-list": a list of symmetric ciphers (AEAD algorithms) acceptable to host A. These are specified in Table 2. 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 "PK_A", "PK_B": ephemeral public keys for hosts A and B, respectively. These, as well as their corresponding private keys, are short-lived values that SHOULD be refreshed periodically. The private keys SHOULD NOT ever be written to persistent storage. The ephemeral secret ("ES") is defined to be the result of the key- agreement algorithm whose inputs are the local host's ephemeral private key and the remote host's ephemeral public key. For example, Bittau, et al. Expires January 9, 2017 [Page 6]
Internet-Draft tcpcrypt July 2016 host A would compute "ES" using its own private key (not transmitted) and host B's public key, "PK_B". The two sides then compute a pseudo-random key ("PRK"), from which all session keys are derived, as follows: PRK = Extract (N_A, eno-transcript | Init1 | Init2 | ES) Above, "|" denotes concatenation; "eno-transcript" is the protocol- negotiation transcript defined in TCP-ENO; and "Init1" and "Init2" are the transmitted encodings of the messages described in Section 4.1. A series of "session secrets" and corresponding session identifiers are then computed from "PRK" as follows: ss[0] = PRK ss[i] = CPRF (ss[i-1], CONST_NEXTK, K_LEN) SID[i] = CPRF (ss[i], CONST_SESSID, K_LEN) The value "ss[0]" is used to generate all key material for the current connection. "SID[0]" is the _bare session ID_ for the current connection, and will with overwhelming probability be unique for each individual TCP connection. The values of "ss[i]" for "i > 0" can be used to avoid public key cryptography when establishing subsequent connections between the same two hosts, as described in Section 3.4. The "CONST_*" values are constants defined in Table 3. The length "K_LEN" depends on the tcpcrypt spec in use, and is specified in Section 5. To yield the _session ID_ required by TCP-ENO [I-D.ietf-tcpinc-tcpeno], tcpcrypt concatenates the first byte of the negotiated suboption (that is, including the "v" bit as transmitted by host B) with the bare session ID for a particular connection: session ID = subopt-byte | SID Given a session secret "ss", the two sides compute a series of master keys as follows: mk[0] = CPRF (ss, CONST_REKEY, K_LEN) mk[i] = CPRF (mk[i-1], CONST_REKEY, K_LEN) Finally, each master key "mk" is used to generate keys for authenticated encryption for the "A" and "B" roles. Key "k_ab" is Bittau, et al. Expires January 9, 2017 [Page 7]
Internet-Draft tcpcrypt July 2016 used by host A to encrypt and host B to decrypt, while "k_ba" is used by host B to encrypt and host A to decrypt. k_ab = CPRF (mk, CONST_KEY_A, ae_keylen) k_ba = CPRF (mk, CONST_KEY_B, ae_keylen) The value "ae_keylen" depends on the authenticated-encryption algorithm selected, and is given under "Key Length" in Table 2. After host B sends "Init2" or host A receives it, that host may immediately begin transmitting protected application data as described in Section 3.5. 3.4. Session caching When two hosts have already negotiated session secret "ss[i-1]", they can establish a new connection without public-key operations using "ss[i]". Willingness to employ this facility is signalled by sending a SYN segment with a _resumption suboption_: an ENO suboption containing the negotiated spec identifier from the original session and the flag "v = 1" (indicating variable-length data). An active opener wishing to resume from a cached session may send a resumption suboption whose content is the nine-byte prefix of the associated bare session ID: byte 0 1 9 (10 bytes total) +--------+--------+---...---+--------+ | spec- | SID[i]{0..8} | | byte | | +--------+--------+---...---+--------+ Figure 2: ENO suboption used to initiate session resumption. The spec-byte contains a tcpcrypt cs value and v = 1. The active opener MUST use the lowest value of "i" that has not already been used to successfully negotiate resumption with the same host and for the same pre-session key "ss[0]". A host SHOULD also include ENO suboptions describing the key- agreement schemes it supports in addition to the resume suboption, so as to fall back to full key exchange in the event that resumption fails. Implementations MUST NOT send more than one resumption suboption for the same "cs" value in the same SYN segment. If the passive opener recognizes the prefix of "SID[i]" and knows "ss[i]", it SHOULD (with exceptions specified below) respond with an ENO option containing an _empty resumption suboption_ with matching Bittau, et al. Expires January 9, 2017 [Page 8]
Internet-Draft tcpcrypt July 2016 spec identifier; that is, a suboption whose initial byte gives the "cs" value from host A's resumption suboption and sets "v = 1", but whose contents are empty. (The only way to encode this is as the last ENO suboption.) Otherwise, the passive opener SHOULD inspect any other ENO suboptions in hopes of negotiating a fresh key exchange as described in Section 3.3. A host MUST ignore a resumption suboption if has successfully negotiated resumption in the past, in either role, with the same "SID[i]". In the event that two hosts simultaneously send SYN segments to each other with the same "SID[i]", but the two segments are not part of a simultaneous open, both connections will have to revert to public key cryptography. To avoid this limitation, implementations MAY choose to implement session caching such that a given pre-session key "ss[0]" is only used for either passive or active opens at the same host, not both. In the case of simultaneous open where TCP-ENO is able to establish asymmetric roles, two hosts that simultaneously send SYN segments with resumption suboptions containing the same "SID[i]" may resume the associated session. Hosts MUST NOT send, and upon receipt MUST ignore, an empty resumption suboption in a SYN-only segment. After using "ss[i]" to compute "mk[0]", implementations SHOULD compute and cache "ss[i+1]" for possible use by a later session, then erase "ss[i]" from memory. Hosts SHOULD keep "ss[i+1]" around for a period of time until it is used or the memory needs to be reclaimed. Hosts SHOULD NOT write a cached "ss[i+1]" value to non-volatile storage. When two hosts have previously negotiated a tcpcrypt session, either host may initiate session resumption regardless of which host was the active opener or played the "A" role in the previous session. However, a given host must either encrypt with "k_ab" for all sessions derived from the same pre-session key "ss[0]", or with "k_ba". Thus, which keys a host uses to send segments is not affected by the role it plays in the current connection: it depends only on whether the host played the "A" or "B" role in the initial session. Implementations that perform session caching MUST provide a means for applications to control session caching, including flushing cached session secrets associated with an ESTABLISHED connection or Bittau, et al. Expires January 9, 2017 [Page 9]
Internet-Draft tcpcrypt July 2016 disabling the use of caching for a particular connection. A recommended interface is described in Section 10. The session ID required by TCP-ENO and exposed to applications is constructed in the same way for resumed sessions as it is for fresh ones, as described above in Section 3.3. In particular, the first byte of the session ID is the first byte of the current connection's negotiated suboption, which means the byte will contain "v = 1"; and the remainder is "SID[i]", the bare session ID for the resumed session. 3.5. Data encryption and authentication Following key exchange (or its omission via session caching), all further communication in a tcpcrypt-enabled connection is carried out within delimited _application frames_ that are encrypted and authenticated using the agreed keys. This protection is provided via algorithms for Authenticated Encryption with Associated Data (AEAD). The particular algorithms that may be used are listed in Table 2. One algorithm is selected during the negotiation described in Section 3.3. The format of an application frame is specified in Section 4.2. A sending host breaks its stream of application data into a series of chunks. Each chunk is placed in the "data" portion of a "plaintext" value, which is then encrypted to yield a frame's "ciphertext" field. Chunks must be small enough that the ciphertext (whose length depends on the AEAD cipher used, and is generally slightly longer than the plaintext) has length less than 2^16 bytes. An "associated data" value (see Section 4.2.2) is constructed for the frame. It contains the frame's "control" field and the length of the ciphertext. A "frame nonce" value (see Section 4.2.3) is also constructed for the frame (but not explicitly transmitted), containing an "offset" field whose integer value is the zero-indexed byte offset of the beginning of the current application frame in the underlying TCP datastream. (That is, the offset in the framing stream, not the plaintext application stream.) Because it is strictly necessary for the security of the AEAD algorithm, an implementation MUST NOT ever transmit distinct frames with the same nonce value under the same encryption key. In particular, a retransmitted TCP segment MUST contain the same payload bytes for the same TCP sequence numbers, and a host MUST NOT transmit more than 2^64 bytes in the underlying TCP datastream (which would cause the "offset" field to wrap) before re- keying. Bittau, et al. Expires January 9, 2017 [Page 10]
Internet-Draft tcpcrypt July 2016 With reference to the "AEAD Interface" described in Section 2 of [RFC5116], tcpcrypt invokes the AEAD algorithm with the secret key "K" set to k_ab or k_ba, according to the host's role as described in Section 3.3. The plaintext value serves as "P", the associated data as "A", and the frame nonce as "N". The output of the encryption operation, "C", is transmitted in the frame's "ciphertext" field. When a frame is received, tcpcrypt reconstructs the associated data and frame nonce values (the former contains only data sent in the clear, and the latter is implicit in the TCP stream), and provides these and the ciphertext value to the the AEAD decryption operation. The output of this operation is either "P", a plaintext value, or the special symbol FAIL. In the latter case, the implementation MUST either ignore the frame or terminate the connection. 3.6. TCP header protection The "ciphertext" field of the application frame contains protected versions of certain TCP header values. When "URGp" is set, the "urgent" value indicates an offset from the current frame's beginning offset; the sum of these offsets gives the index of the last byte of urgent data in the application datastream. When "FINp" is set, it indicates that the sender will send no more application data after this frame. A receiver MUST ignore the TCP FIN flag and instead wait for "FINp" to signal to the local application that the stream is complete. 3.7. Re-keying Re-keying allows hosts to wipe from memory keys that could decrypt previously transmitted segments. It also allows the use of AEAD ciphers that can securely encrypt only a bounded number of messages under a given key. We refer to the two encryption keys (k_ab, k_ba) as a _key-set_. We refer to the key-set generated by mk[i] as the key-set with _generation number_ "i" within a session. Each host maintains a _current generation number_ that it uses to encrypt outgoing frames. Initially, the two hosts have current generation number 0. When a host has just incremented its current generation number and has used the new key-set for the first time to encrypt an outgoing frame, it MUST set that frame's "rekey" field (see Section 4.2) to 1. It MUST set this field to zero in all other cases. Bittau, et al. Expires January 9, 2017 [Page 11]
Internet-Draft tcpcrypt July 2016 A host MAY increment its generation number beyond the highest generation it knows the other side to be using. We call this action _initiating re-keying_. A host SHOULD NOT initiate more than one concurrent re-key operation if it has no data to send; that is, it should not initiate re-keying with an empty application frame more than once while its record of the remote host's current generation number is less than its own. On receipt, a host increments its record of the remote host's current generation number if and only if the "rekey" field is set to 1. If a received frame's generation number is greater than the receiver's current generation number, the receiver MUST immediately increment its current generation number to match. After incrementing its generation number, if the receiver does not have any application data to send, it MUST send an empty application frame with the "rekey" field set to 1. When retransmitting, implementations must always transmit the same bytes for the same TCP sequence numbers. Thus, a frame in a retransmitted segment MUST always be encrypted with the same key as when it was originally transmitted. Implementations SHOULD delete older-generation keys from memory once they have received all frames they will need to decrypt with the old keys and have encrypted all outgoing frames under the old keys. 3.8. Keep-alive Instead of using TCP Keep-Alives to verify that the remote endpoint is still alive, tcpcrypt implementations SHOULD employ the re-keying mechanism, as follows. When necessary, a host SHOULD probe the liveness of its peer by initiating re-keying as described in Section 3.7, and then transmitting a new frame (with zero-length application data if necessary). A host receiving a frame whose key generation number is greater than its current generation number MUST increment its current generation number and MUST immediately transmit a new frame (with zero-length application data, if necessary). Implementations MAY use TCP Keep-Alives for purposes that do not require endpoint authentication, as discussed in Section 9.2. 4. Encodings This section provides byte-level encodings for values transmitted or computed by the protocol. Bittau, et al. Expires January 9, 2017 [Page 12]
Internet-Draft tcpcrypt July 2016 4.1. Key exchange messages The "Init1" message has the following encoding: byte 0 1 2 3 +-------+-------+-------+-------+ | INIT1_MAGIC | | | +-------+-------+-------+-------+ 4 5 6 7 +-------+-------+-------+-------+ | message_len | | = M | +-------+-------+-------+-------+ 8 +--------+-------+-------+---...---+-------+ |nciphers|sym- |sym- | |sym- | | =K+1 |cipher0|cipher1| |cipherK| +--------+-------+-------+---...---+-------+ K + 10 K + 10 + N_A_LEN | | v v +-------+---...---+-------+-------+---...---+-------+ | N_A | PK_A | | | | +-------+---...---+-------+-------+---...---+-------+ M - 1 +-------+---...---+-------+ | ignored | | | +-------+---...---+-------+ The constant "INIT1_MAGIC" is defined in Table 3. The four-byte field "message_len" gives the length of the entire "Init1" message, encoded as a big-endian integer. The "nciphers" field contains an integer value that specifies the number of one-byte symmetric-cipher identifiers that follow. The "sym-cipher" bytes identify cryptographic algorithms in Table 2. The length "N_A_LEN" and the length of "PK_A" are both determined by the negotiated key-agreement scheme, as described in Section 5. When sending "Init1", implementations of this protocol MUST omit the field "ignored"; that is, they must construct the message such that its end, as determined by "message_len", coincides with the end of Bittau, et al. Expires January 9, 2017 [Page 13]
Internet-Draft tcpcrypt July 2016 the field "PK_A". When receiving "Init1", however, implementations MUST permit and ignore any bytes following "PK_A". The "Init2" message has the following encoding: byte 0 1 2 3 +-------+-------+-------+-------+ | INIT2_MAGIC | | | +-------+-------+-------+-------+ 4 5 6 7 8 +-------+-------+-------+-------+-------+ | message_len |sym- | | = M |cipher | +-------+-------+-------+-------+-------+ 9 9 + N_B_LEN | | v v +-------+---...---+-------+-------+---...---+-------+ | N_B | PK_B | | | | +-------+---...---+-------+-------+---...---+-------+ M - 1 +-------+---...---+-------+ | ignored | | | +-------+---...---+-------+ The constant "INIT2_MAGIC" is defined in Table 3. The four-byte field "message_len" gives the length of the entire "Init2" message, encoded as a big-endian integer. The "sym-cipher" value is a selection from the symmetric-cipher identifiers in the previously- received "Init1" message. The length "N_B_LEN" and the length of "PK_B" are both determined by the negotiated key-agreement scheme, as described in Section 5. When sending "Init2", implementations of this protocol MUST omit the field "ignored"; that is, they must construct the message such that its end, as determined by "message_len", coincides with the end of the "PK_B" field. When receiving "Init2", however, implementations MUST permit and ignore any bytes following "PK_B". Bittau, et al. Expires January 9, 2017 [Page 14]
Internet-Draft tcpcrypt July 2016 4.2. Application frames An _application frame_ comprises a control byte and a length-prefixed ciphertext value: byte 0 1 2 3 clen+2 +-------+-------+-------+-------+---...---+-------+ |control| clen | ciphertext | +-------+-------+-------+-------+---...---+-------+ The field "clen" is an integer in big-endian format and gives the length of the "ciphertext" field. The byte "control" has this structure: bit 7 1 0 +-------+---...---+-------+-------+ | cres | rekey | +-------+---...---+-------+-------+ The seven-bit field "cres" is reserved; implementations MUST set these bits to zero when sending, and MUST ignore them when receiving. The use of the "rekey" field is described in Section 3.7. 4.2.1. Plaintext The "ciphertext" field is the result of applying the negotiated authenticated-encryption algorithm to a "plaintext" value, which has one of these two formats: byte 0 1 plen-1 +-------+-------+---...---+-------+ | flags | data | +-------+-------+---...---+-------+ byte 0 1 2 3 plen-1 +-------+-------+-------+-------+---...---+-------+ | flags | urgent | data | +-------+-------+-------+-------+---...---+-------+ (Note that "clen" in the previous section will generally be greater than "plen", as the ciphertext produced by the authenticated- encryption scheme must both encrypt the application data and provide a way to verify its integrity.) The "flags" byte has this structure: Bittau, et al. Expires January 9, 2017 [Page 15]
Internet-Draft tcpcrypt July 2016 bit 7 6 5 4 3 2 1 0 +----+----+----+----+----+----+----+----+ | fres |URGp|FINp| +----+----+----+----+----+----+----+----+ The six-bit value "fres" is reserved; implementations MUST set these six bits to zero when sending, and MUST ignore them when receiving. When the "URGp" bit is set, it indicates that the "urgent" field is present, and thus that the plaintext value has the second structure variant above; otherwise the first variant is used. The meaning of "urgent" and of the flag bits is described in Section 3.6. 4.2.2. Associated data An application frame's "associated data" (which is supplied to the AEAD algorithm when decrypting the ciphertext and verifying the frame's integrity) has this format: byte 0 1 2 +-------+-------+-------+ |control| clen | +-------+-------+-------+ It contains the same values as the frame's "control" and "clen" fields. 4.2.3. Frame nonce Lastly, a "frame nonce" (provided as input to the AEAD algorithm) has this format: byte +------+------+------+------+ 0 | FRAME_NONCE_MAGIC | +------+------+------+------+ 4 | | + offset + 8 | | +------+------+------+------+ The 4-byte magic constant is defined in Table 3. The 8-byte "offset" field contains an integer in big-endian format. Its value is specified in Section 3.5. Bittau, et al. Expires January 9, 2017 [Page 16]
Internet-Draft tcpcrypt July 2016 5. Key agreement schemes The encryption spec negotiated via TCP-ENO may indicate the use of one of the key-agreement schemes named in Table 1. For example, "TCPCRYPT_ECDHE_P256" names the tcpcrypt protocol with key-agreement scheme ECDHE-P256. All schemes listed there use HKDF-Expand-SHA256 as the CPRF, and these lengths for nonces and session keys: N_A_LEN: 32 bytes N_B_LEN: 32 bytes K_LEN: 32 bytes Key-agreement schemes ECDHE-P256 and ECDHE-P521 employ the ECSVDP-DH secret value derivation primitive defined in [ieee1363]. The named curves are defined in [nist-dss]. When the public-key values "PK_A" and "PK_B" are transmitted as described in Section 4.1, they are encoded with the "Elliptic Curve Point to Octet String Conversion Primitive" described in Section E.2.3 of [ieee1363], and are prefixed by a two-byte length in big-endian format: byte 0 1 2 L - 1 +-------+-------+-------+---...---+-------+ | pubkey_len | pubkey | | = L | | +-------+-------+-------+---...---+-------+ Implementations SHOULD encode these "pubkey" values in "compressed format", and MUST accept values encoded in "compressed", "uncompressed" or "hybrid" formats. Key-agreement schemes ECDHE-Curve25519 and ECDHE-Curve448 use the functions X25519 and X448, respectively, to perform the Diffie-Helman protocol as described in [RFC7748]. When using these ciphers, public-key values "PK_A" and "PK_B" are transmitted directly with no length prefix: 32 bytes for Curve25519, and 56 bytes for Curve448. A tcpcrypt implementation MUST support at least the schemes ECDHE-P256 and ECDHE-P521, although system administrators need not enable them. 6. AEAD algorithms Specifiers and key-lengths for AEAD algorithms are given in Table 2. The algorithms "AEAD_AES_128_GCM" and "AEAD_AES_256_GCM" are specified in [RFC5116]. The algorithm "AEAD_CHACHA20_POLY1305" is specified in [RFC7539]. Bittau, et al. Expires January 9, 2017 [Page 17]
Internet-Draft tcpcrypt July 2016 7. IANA considerations Tcpcrypt's spec identifiers ("cs" values) will need to be added to IANA's ENO suboption registry, as follows: +------+---------------------------+ | cs | Spec name | +------+---------------------------+ | 0x21 | TCPCRYPT_ECDHE_P256 | | 0x22 | TCPCRYPT_ECDHE_P521 | | 0x23 | TCPCRYPT_ECDHE_Curve25519 | | 0x24 | TCPCRYPT_ECDHE_Curve448 | +------+---------------------------+ Table 1: cs values for use with tcpcrypt A "tcpcrypt AEAD parameter" registry needs to be maintained by IANA as in the following table. The use of encryption is described in Section 3.5. +------------------------+------------+------------+ | AEAD Algorithm | Key Length | sym-cipher | +------------------------+------------+------------+ | AEAD_AES_128_GCM | 16 bytes | 0x01 | | AEAD_AES_256_GCM | 32 bytes | 0x02 | | AEAD_CHACHA20_POLY1305 | 32 bytes | 0x10 | +------------------------+------------+------------+ Table 2: Authenticated-encryption algorithms corresponding to sym- cipher specifiers in Init1 and Init2 messages. 8. Security considerations Public-key generation, public-key encryption, and shared-secret generation all require randomness. Other tcpcrypt functions may also require randomness, depending on the algorithms and modes of operation selected. A weak pseudo-random generator at either host will compromise tcpcrypt's security. Many of tcpcrypt's cryptographic functions require random input, and thus any host implementing tcpcrypt MUST have access to a cryptographically-secure source of randomness or pseudo-randomness. Most implementations will rely on system-wide pseudo-random generators seeded from hardware events and a seed carried over from the previous boot. Once a pseudo-random generator has been properly seeded, it can generate effectively arbitrary amounts of pseudo- random data. However, until a pseudo-random generator has been seeded with sufficient entropy, not only will tcpcrypt be insecure, Bittau, et al. Expires January 9, 2017 [Page 18]
Internet-Draft tcpcrypt July 2016 it will reveal information that further weakens the security of the pseudo-random generator, potentially harming other applications. As required by TCP-ENO, implementations MUST NOT send ENO options unless they have access to an adequate source of randomness. The cipher-suites specified in this document all use HMAC-SHA256 to implement the collision-resistant pseudo-random function denoted by "CPRF". A collision-resistant function is one on which, for sufficiently large L, an attacker cannot find two distinct inputs "K_1", "CONST_1" and "K_2", "CONST_2" such that "CPRF(K_1, CONST_1, L) = CPRF(K_2, CONST_2, L)". Collision resistance is important to assure the uniqueness of session IDs, which are generated using the CPRF. All of the security considerations of TCP-ENO apply to tcpcrypt. In particular, tcpcrypt does not protect against active eavesdroppers unless applications authenticate the session ID. If it can be established that the session IDs computed at each end of the connection match, then tcpcrypt guarantees that no man-in-the-middle attacks occurred unless the attacker has broken the underlying cryptographic primitives (e.g., ECDH). A proof of this property for an earlier version of the protocol has been published [tcpcrypt]. To gain middlebox compatibility, tcpcrypt does not protect TCP headers. Hence, the protocol is vulnerable to denial-of-service from off-path attackers. Possible attacks include desynchronizing the underlying TCP stream, injecting RST packets, and forging or suppressing rekey bits. These attacks will cause a tcpcrypt connection to hang or fail with an error. Implementations MUST give higher-level software a way to distinguish such errors from a clean end-of-stream (indicated by an authenticated "FINp" bit) so that applications can avoid semantic truncation attacks. There is no "key confirmation" step in tcpcrypt. This is not required because tcpcrypt's threat model includes the possibility of a connection to an adversary. If key negotiation is compromised and yields two different keys, all subsequent frames will be ignored due failed integrity checks, causing the application's connection to hang. This is not a new threat because in plain TCP, an active attacker could have modified sequence and acknowledgement numbers to hang the connection anyway. Tcpcrypt uses short-lived public keys to provide forward secrecy. All currently specified key agreement schemes involve ECDHE-based key agreement, meaning a new key can be efficiently computed for each connection. If implementations reuse these parameters, they SHOULD limit the lifetime of the private parameters, ideally to no more than two minutes. Bittau, et al. Expires January 9, 2017 [Page 19]
Internet-Draft tcpcrypt July 2016 Attackers cannot force passive openers to move forward in their session caching chain without guessing the content of the resumption suboption, which will be hard without key knowledge. 9. Design notes 9.1. Asymmetric roles Tcpcrypt transforms a shared pseudo-random key (PRK) into cryptographic session keys for each direction. Doing so requires an asymmetry in the protocol, as the key derivation function must be perturbed differently to generate different keys in each direction. Tcpcrypt includes other asymmetries in the roles of the two hosts, such as the process of negotiating algorithms (e.g., proposing vs. selecting cipher suites). 9.2. Verified liveness Many hosts implement TCP Keep-Alives [RFC1122] as an option for applications to ensure that the other end of a TCP connection still exists even when there is no data to be sent. A TCP Keep-Alive segment carries a sequence number one prior to the beginning of the send window, and may carry one byte of "garbage" data. Such a segment causes the remote side to send an acknowledgment. Unfortunately, tcpcrypt cannot cryptographically verify Keep-Alive acknowledgments. Hence, an attacker could prolong the existence of a session at one host after the other end of the connection no longer exists. (Such an attack might prevent a process with sensitive data from exiting, giving an attacker more time to compromise a host and extract the sensitive data.) Thus, tcpcrypt specifies a way to stimulate the remote host to send verifiably fresh and authentic data, described in Section 3.8. The TCP keep-alive mechanism has also been used for its effects on intermediate nodes in the network, such as preventing flow state from expiring at NAT boxes or firewalls. As these purposes do not require the authentication of endpoints, implementations may safely accomplish them using either the existing TCP keep-alive mechanism or tcpcrypt's verified keep-alive mechanism. 10. API extensions Applications aware of tcpcrypt will need an API for interacting with the protocol. They can do so if implementations provide the recommended API for TCP-ENO. This section recommends several Bittau, et al. Expires January 9, 2017 [Page 20]
Internet-Draft tcpcrypt July 2016 additions to that API, described in the style of socket options. However, these recommendations are non-normative: The following option is read-only: TCP_CRYPT_CONF: Returns the one-byte authenticated encryption algorithm in use by the connection (as specified in Table 2). The following option is write-only: TCP_CRYPT_CACHE_FLUSH: Setting this option to non-zero wipes cached session keys as specified in Section 3.4. Useful if application-level authentication discovers a man in the middle attack, to prevent the next connection from using session caching. The following options should be readable and writable: TCP_CRYPT_ACONF: Set of allowed symmetric ciphers and message authentication codes this host advertises in "Init1" messages. TCP_CRYPT_BCONF: Order of preference of symmetric ciphers. Finally, system administrators must be able to set the following system-wide parameters: o Default TCP_CRYPT_ACONF value o Default TCP_CRYPT_BCONF value o Types, key lengths, and regeneration intervals of local host's short-lived public keys for implementations that do not use fresh ECDH parameters for each connection. 11. Acknowledgments We are grateful for contributions, help, discussions, and feedback from the TCPINC working group, including Marcelo Bagnulo, David Black, Bob Briscoe, Jana Iyengar, Tero Kivinen, Mirja Kuhlewind, Yoav Nir, Christoph Paasch, Eric Rescorla, and Kyle Rose. This work was funded by gifts from Intel (to Brad Karp) and from Google; by NSF award CNS-0716806 (A Clean-Slate Infrastructure for Information Flow Control); by DARPA CRASH under contract Bittau, et al. Expires January 9, 2017 [Page 21]
Internet-Draft tcpcrypt July 2016 #N66001-10-2-4088; and by the Stanford Secure Internet of Things Project. 12. References 12.1. Normative References [I-D.ietf-tcpinc-tcpeno] Bittau, A., Boneh, D., Giffin, D., Handley, M., Mazieres, D., and E. Smith, "TCP-ENO: Encryption Negotiation Option", draft-ietf-tcpinc-tcpeno-03 (work in progress), July 2016. [ieee1363] "IEEE Standard Specifications for Public-Key Cryptography (IEEE Std 1363-2000)", 2000. [nist-dss] "Digital Signature Standard, FIPS 186-2", 2000. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, <http://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, <http://www.rfc-editor.org/info/rfc5869>. [RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015, <http://www.rfc-editor.org/info/rfc7539>. [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016, <http://www.rfc-editor.org/info/rfc7748>. 12.2. Informative References [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989, <http://www.rfc-editor.org/info/rfc1122>. Bittau, et al. Expires January 9, 2017 [Page 22]
Internet-Draft tcpcrypt July 2016 [tcpcrypt] Bittau, A., Hamburg, M., Handley, M., Mazieres, D., and D. Boneh, "The case for ubiquitous transport-level encryption", USENIX Security , 2010. Appendix A. Protocol constant values +------------+-------------------+ | Value | Name | +------------+-------------------+ | 0x01 | CONST_NEXTK | | 0x02 | CONST_SESSID | | 0x03 | CONST_REKEY | | 0x04 | CONST_KEY_A | | 0x05 | CONST_KEY_B | | 0x15101a0e | INIT1_MAGIC | | 0x097105e0 | INIT2_MAGIC | | 0x44415441 | FRAME_NONCE_MAGIC | +------------+-------------------+ Table 3: Protocol constants Authors' Addresses Andrea Bittau Stanford University 353 Serra Mall, Room 288 Stanford, CA 94305 US Email: bittau@cs.stanford.edu Dan Boneh Stanford University 353 Serra Mall, Room 475 Stanford, CA 94305 US Email: dabo@cs.stanford.edu Bittau, et al. Expires January 9, 2017 [Page 23]
Internet-Draft tcpcrypt July 2016 Daniel B. Giffin Stanford University 353 Serra Mall, Room 288 Stanford, CA 94305 US Email: dbg@scs.stanford.edu Mike Hamburg Stanford University 353 Serra Mall, Room 475 Stanford, CA 94305 US Email: mike@shiftleft.org Mark Handley University College London Gower St. London WC1E 6BT UK Email: M.Handley@cs.ucl.ac.uk David Mazieres Stanford University 353 Serra Mall, Room 290 Stanford, CA 94305 US Email: dm@uun.org Quinn Slack Stanford University 353 Serra Mall, Room 288 Stanford, CA 94305 US Email: sqs@cs.stanford.edu Bittau, et al. Expires January 9, 2017 [Page 24]
Internet-Draft tcpcrypt July 2016
Eric W. Smith
Kestrel Institute
3260 Hillview Avenue
Palo Alto, CA 94304
US
Email: eric.smith@kestrel.edu
Bittau, et al. Expires January 9, 2017 [Page 25]
Datatracker
draft-ietf-tcpinc-tcpcrypt-02
This is an older version of an Internet-Draft that was ultimately published as RFC 8548.
| Document | Document type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8548.
|
|
|---|---|---|---|
| Select version | |||
| Compare versions | |||
| Authors | Andrea Bittau, Dan Boneh , Daniel B. Giffin , Mike Hamburg , Mark J. Handley , David Mazieres , Quinn Slack , Eric W. Smith | ||
| Replaces | draft-bittau-tcpinc-tcpcrypt | ||
| RFC stream |
|
||
| Other formats | |||
| Additional resources | Mailing list discussion |