draft-ietf-tcpinc-tcpcrypt-02.txt   draft-ietf-tcpinc-tcpcrypt-03.txt 
Network Working Group A. Bittau Network Working Group A. Bittau
Internet-Draft D. Boneh Internet-Draft Google
Intended status: Standards Track D. Giffin Intended status: Standards Track D. Boneh
Expires: January 9, 2017 M. Hamburg Expires: May 4, 2017 D. Giffin
M. Hamburg
Stanford University Stanford University
M. Handley M. Handley
University College London University College London
D. Mazieres D. Mazieres
Q. Slack Q. Slack
Stanford University Stanford University
E. Smith E. Smith
Kestrel Institute Kestrel Institute
July 8, 2016 October 31, 2016
Cryptographic protection of TCP Streams (tcpcrypt) Cryptographic protection of TCP Streams (tcpcrypt)
draft-ietf-tcpinc-tcpcrypt-02 draft-ietf-tcpinc-tcpcrypt-03
Abstract Abstract
This document specifies tcpcrypt, a cryptographic protocol that This document specifies tcpcrypt, a TCP encryption protocol designed
protects TCP payload data. Use of the protocol is negotiated by for use in conjunction with the TCP Encryption Negotiation Option
means of the TCP Encryption Negotiation Option (TCP-ENO) (TCP-ENO) [I-D.ietf-tcpinc-tcpeno]. Tcpcrypt coexists with
[I-D.ietf-tcpinc-tcpeno]. Tcpcrypt coexists with middleboxes by middleboxes by tolerating resegmentation, NATs, and other
tolerating resegmentation, NATs, and other manipulations of the TCP manipulations of the TCP header. The protocol is self-contained and
header. The protocol is self-contained and specifically tailored to specifically tailored to TCP implementations, which often reside in
TCP implementations, which often reside in kernels or other kernels or other environments in which large external software
environments in which large external software dependencies can be dependencies can be undesirable. Because the size of TCP options is
undesirable. Because the size of TCP options is limited, the limited, the protocol requires one additional one-way message latency
protocol requires one additional one-way message latency to perform to perform key exchange before application data may be transmitted.
key exchange before application data may be transmited. However, However, this cost can be avoided between two hosts that have
this cost can be avoided between two hosts that have recently recently established a previous tcpcrypt connection.
established a previous tcpcrypt connection.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 9, 2017. This Internet-Draft will expire on May 4, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Encryption protocol . . . . . . . . . . . . . . . . . . . . . 3 3. Encryption protocol . . . . . . . . . . . . . . . . . . . . . 3
3.1. Cryptographic algorithms . . . . . . . . . . . . . . . . 4 3.1. Cryptographic algorithms . . . . . . . . . . . . . . . . 4
3.2. Protocol negotiation . . . . . . . . . . . . . . . . . . 5 3.2. Protocol negotiation . . . . . . . . . . . . . . . . . . 5
3.3. Key exchange . . . . . . . . . . . . . . . . . . . . . . 6 3.3. Key exchange . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Session caching . . . . . . . . . . . . . . . . . . . . . 8 3.4. Session caching . . . . . . . . . . . . . . . . . . . . . 8
3.5. Data encryption and authentication . . . . . . . . . . . 10 3.5. Data encryption and authentication . . . . . . . . . . . 10
3.6. TCP header protection . . . . . . . . . . . . . . . . . . 11 3.6. TCP header protection . . . . . . . . . . . . . . . . . . 11
3.7. Re-keying . . . . . . . . . . . . . . . . . . . . . . . . 11 3.7. Re-keying . . . . . . . . . . . . . . . . . . . . . . . . 11
3.8. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 12 3.8. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 12
4. Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4. Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Key exchange messages . . . . . . . . . . . . . . . . . . 13 4.1. Key exchange messages . . . . . . . . . . . . . . . . . . 13
4.2. Application frames . . . . . . . . . . . . . . . . . . . 15 4.2. Application frames . . . . . . . . . . . . . . . . . . . 15
4.2.1. Plaintext . . . . . . . . . . . . . . . . . . . . . . 15 4.2.1. Plaintext . . . . . . . . . . . . . . . . . . . . . . 15
4.2.2. Associated data . . . . . . . . . . . . . . . . . . . 16 4.2.2. Associated data . . . . . . . . . . . . . . . . . . . 16
4.2.3. Frame nonce . . . . . . . . . . . . . . . . . . . . . 16 4.2.3. Frame nonce . . . . . . . . . . . . . . . . . . . . . 17
5. Key agreement schemes . . . . . . . . . . . . . . . . . . . . 17 5. Key agreement schemes . . . . . . . . . . . . . . . . . . . . 17
6. AEAD algorithms . . . . . . . . . . . . . . . . . . . . . . . 17 6. AEAD algorithms . . . . . . . . . . . . . . . . . . . . . . . 18
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 18 7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 18
8. Security considerations . . . . . . . . . . . . . . . . . . . 18 8. Security considerations . . . . . . . . . . . . . . . . . . . 19
9. Design notes . . . . . . . . . . . . . . . . . . . . . . . . 20 9. Design notes . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1. Asymmetric roles . . . . . . . . . . . . . . . . . . . . 20 9.1. Asymmetric roles . . . . . . . . . . . . . . . . . . . . 20
9.2. Verified liveness . . . . . . . . . . . . . . . . . . . . 20 9.2. Verified liveness . . . . . . . . . . . . . . . . . . . . 21
10. API extensions . . . . . . . . . . . . . . . . . . . . . . . 20 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 11.1. Normative References . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . 22 11.2. Informative References . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . 22 Appendix A. Protocol constant values . . . . . . . . . . . . . . 22
Appendix A. Protocol constant values . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Requirements language 1. Requirements language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. Introduction 2. Introduction
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HKDF-Expand(PRK, CONST, L) -> OKM HKDF-Expand(PRK, CONST, L) -> OKM
T(0) = empty string (zero length) T(0) = empty string (zero length)
T(1) = HMAC-Hash(PRK, T(0) | CONST | 0x01) T(1) = HMAC-Hash(PRK, T(0) | CONST | 0x01)
T(2) = HMAC-Hash(PRK, T(1) | CONST | 0x02) T(2) = HMAC-Hash(PRK, T(1) | CONST | 0x02)
T(3) = HMAC-Hash(PRK, T(2) | CONST | 0x03) T(3) = HMAC-Hash(PRK, T(2) | CONST | 0x03)
... ...
OKM = first L octets of T(1) | T(2) | T(3) | ... OKM = first L octets of T(1) | T(2) | T(3) | ...
Figure 1: The symbol | denotes concatenation, and the counter Figure 1: The symbol | denotes concatenation, and the counter
concatenated after CONST is a single octet. concatenated to the right of CONST is a single octet.
Lastly, once tcpcrypt has been successfully set up, an _authenticated Lastly, once tcpcrypt has been successfully set up, an _authenticated
encryption mode_ is used to protect the confidentiality and integrity encryption mode_ is used to protect the confidentiality and integrity
of all transmitted application data. of all transmitted application data.
3.2. Protocol negotiation 3.2. Protocol negotiation
Tcpcrypt depends on TCP-ENO [I-D.ietf-tcpinc-tcpeno] to negotiate Tcpcrypt depends on TCP-ENO [I-D.ietf-tcpinc-tcpeno] to negotiate
whether encryption will be enabled for a connection, and also which whether encryption will be enabled for a connection, and also which
key agreement scheme to use. TCP-ENO negotiates an _encryption spec_ key agreement scheme to use. TCP-ENO negotiates the use of a
by means of suboptions embedded in SYN segments. Each suboption is particular TCP encryption protocol or _TEP_ by including protocol
identified by a byte consisting of a seven-bit _encryption spec identifiers in ENO suboptions. This document associates four TEP
identifier_ value, "cs", and a one-bit additional-data indicator, identifiers with the tcpcrypt protocol, as listed in Table 1. Future
"v". This document reserves and associates four "cs" values with standards may associate additional identifiers with tcpcrypt.
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 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 an ENO option in its SYN segment. That option will include
include tcpcrypt suboptions corresponding to the key-agreement suboptions with TEP identifiers indicating the key-agreement schemes
schemes it is willing to enable. The active opener MAY additionally it is willing to enable. The active opener MAY additionally include
include suboptions indicating support for encryption protocols other suboptions indicating support for encryption protocols other than
than tcpcrypt, as well as other general options as specified by TCP- tcpcrypt, as well as other general options as specified by TCP-ENO.
ENO.
If a passive opener receives an ENO option including tcpcrypt If a passive opener receives an ENO option including tcpcrypt TEPs it
suboptions it supports, it MAY then attach an ENO option to its SYN- supports, it MAY then attach an ENO option to its SYN-ACK segment,
ACK segment, including _solely_ the suboption it wishes to enable. including _solely_ the TEP it wishes to enable.
To establish distinct roles for the two hosts in each connection, To establish distinct roles for the two hosts in each connection,
tcpcrypt depends on the role-negotiation mechanism of TCP-ENO tcpcrypt depends on the role-negotiation mechanism of TCP-ENO
[I-D.ietf-tcpinc-tcpeno]. As part of the negotiation process, TCP- [I-D.ietf-tcpinc-tcpeno]. As part of the negotiation process, TCP-
ENO assigns hosts unique roles abstractly called "A" at one end of ENO assigns hosts unique roles abstractly called "A" at one end of
the connection and "B" at the other. Generally, an active opener 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 plays the "A" role and a passive opener plays the "B" role; but in
additional mechanism breaks the symmetry of simultaneous open. This the case of simultaneous open, an additional mechanism breaks the
document adopts the terms "host A" and "host B" to identify each end symmetry and assigns different roles to the two hosts. This document
of a connection uniquely, following TCP-ENO's designation. 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 Once two hosts have exchanged SYN segments, the _negotiated TEP_ is
the last spec identifier in the SYN segment of host B (that is, the the last TEP identifier in the SYN segment of host B (that is, the
passive opener in the absence of simultaneous open) that also occurs 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- in that of host A. If there is no such TEP, hosts MUST disable TCP-
ENO and tcpcrypt. ENO and tcpcrypt.
The _negotiated suboption_ is the ENO suboption from the SYN segment The _negotiated suboption_ is the ENO suboption from the SYN segment
of host B that contains the negotiated spec, if it exists. of host B that contains the negotiated TEP, if it exists. This
suboption includes a one-bit flag "v" which indicates the presence of
additional data. For tcpcrypt TEPs, 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", it is a _resumption
suboption_: this indicates that the key-exchange messages will be
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.
Note that the negotiated TEP is determined without reference to the
"v" bits in ENO suboptions, so if host A offers a resumption
suboption with a particular TEP and host B replies with a non-
resumption suboption with the same TEP, that may become the
negotiated suboption and fresh key agreement will be performed. That
is, sending a resumption suboption also implies willingness to
perform fresh key-exchange with the indicated TEP.
As required by TCP-ENO, once a host has both sent and received an ACK 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 segment containing an ENO option, encryption MUST be enabled and
plaintext application data MUST NOT ever be exchanged on the plaintext application data MUST NOT ever be exchanged on the
connection. If the negotiated spec is a "cs" value associated with connection. If the negotiated TEP is among those listed in Table 1,
tcpcrypt, a host MUST follow the protocol described in this document. a host MUST follow the protocol described in this document.
In particular, if the negotiated suboption contains "v = 0", a fresh
key agreement will be perfomed as described below in Section 3.3; if
it contains "v = 1", the key-exchange messages are omitted in favor
of determining keys via session-caching as described in Section 3.4,
and protected application data may immediately be sent as detailed in
Section 3.5.
3.3. Key exchange 3.3. Key exchange
Following successful negotiation of a tcpcrypt spec, all further Following successful negotiation of a tcpcrypt TEP, all further
signaling is performed in the Data portion of TCP segments. Except signaling is performed in the Data portion of TCP segments. Except
when resumption was negotiated (described below in Section 3.4), the when resumption was negotiated (described below in Section 3.4), the
two hosts perform key exchange through two messages, "Init1" and two hosts perform key exchange through two messages, "Init1" and
"Init2", at the start of host A's and host B's data streams, "Init2", at the start of the data streams of host A and host B,
respectively. These messages may span multiple TCP segments and need respectively. These messages may span multiple TCP segments and need
not end at a segment boundary. However, the segment containing the 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 last byte of an "Init1" or "Init2" message SHOULD have TCP's PSH bit
set. set.
The key exchange protocol, in abstract, proceeds as follows: The key exchange protocol, in abstract, proceeds as follows:
A -> B: Init1 = { INIT1_MAGIC, sym-cipher-list, N_A, PK_A } A -> B: Init1 = { INIT1_MAGIC, sym-cipher-list, N_A, PK_A }
B -> A: Init2 = { INIT2_MAGIC, sym-cipher, N_B, PK_B } B -> A: Init2 = { INIT2_MAGIC, sym-cipher, N_B, PK_B }
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The value "ss[0]" is used to generate all key material for the 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. "SID[0]" is the _bare session ID_ for the
current connection, and will with overwhelming probability be unique current connection, and will with overwhelming probability be unique
for each individual TCP connection. for each individual TCP connection.
The values of "ss[i]" for "i > 0" can be used to avoid public key The values of "ss[i]" for "i > 0" can be used to avoid public key
cryptography when establishing subsequent connections between the cryptography when establishing subsequent connections between the
same two hosts, as described in Section 3.4. The "CONST_*" values 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 are constants defined in Table 3. The length "K_LEN" depends on the
tcpcrypt spec in use, and is specified in Section 5. tcpcrypt TEP in use, and is specified in Section 5.
To yield the _session ID_ required by TCP-ENO To yield the _session ID_ required by TCP-ENO
[I-D.ietf-tcpinc-tcpeno], tcpcrypt concatenates the first byte of the [I-D.ietf-tcpinc-tcpeno], tcpcrypt concatenates the first byte of the
negotiated suboption (that is, including the "v" bit as transmitted negotiated suboption (that is, including the "v" bit as transmitted
by host B) with the bare session ID for a particular connection: by host B) with the bare session ID for a particular connection:
session ID = subopt-byte | SID session ID = subopt-byte | SID
Given a session secret "ss", the two sides compute a series of master Given a session secret "ss", the two sides compute a series of master
keys as follows: keys as follows:
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After host B sends "Init2" or host A receives it, that host may After host B sends "Init2" or host A receives it, that host may
immediately begin transmitting protected application data as immediately begin transmitting protected application data as
described in Section 3.5. described in Section 3.5.
3.4. Session caching 3.4. Session caching
When two hosts have already negotiated session secret "ss[i-1]", they When two hosts have already negotiated session secret "ss[i-1]", they
can establish a new connection without public-key operations using can establish a new connection without public-key operations using
"ss[i]". Willingness to employ this facility is signalled by sending "ss[i]". Willingness to employ this facility is signalled by sending
a SYN segment with a _resumption suboption_: an ENO suboption a SYN segment with a resumption suboption: an ENO suboption
containing the negotiated spec identifier from the original session containing the negotiated TEP identifier from the original session
and the flag "v = 1" (indicating variable-length data). and the flag "v = 1" (indicating variable-length data).
An active opener wishing to resume from a cached session may send a An active opener wishing to resume from a cached session may send a
resumption suboption whose content is the nine-byte prefix of the resumption suboption whose content is the nine-byte prefix of the
associated bare session ID: associated bare session ID:
byte 0 1 9 (10 bytes total) byte 0 1 9 (10 bytes total)
+--------+--------+---...---+--------+ +--------+--------+---...---+--------+
| spec- | SID[i]{0..8} | | TEP- | SID[i]{0..8} |
| byte | | | byte | |
+--------+--------+---...---+--------+ +--------+--------+---...---+--------+
Figure 2: ENO suboption used to initiate session resumption. The Figure 2: ENO suboption used to initiate session resumption. The
spec-byte contains a tcpcrypt cs value and v = 1. TEP-byte contains a tcpcrypt TEP identifier and v = 1.
The active opener MUST use the lowest value of "i" that has not The active opener MUST use the lowest value of "i" that has not
already been used to successfully negotiate resumption with the same already been used to successfully negotiate resumption with the same
host and for the same pre-session key "ss[0]". host and for the same pre-session key "ss[0]".
A host SHOULD also include ENO suboptions describing the key- In a particular SYN segment, a host SHOULD NOT send more than one
agreement schemes it supports in addition to the resume suboption, so resumption suboption, and MUST NOT send more than one resumption
as to fall back to full key exchange in the event that resumption suboption with the same TEP identifier. But in addition to any
fails. Implementations MUST NOT send more than one resumption resumption suboptions, an active opener MAY include non-resumption
suboption for the same "cs" value in the same SYN segment. suboptions describing other key-agreement schemes it supports (in
addition to that indicated by the TEP in the resumption suboption).
If the passive opener recognizes the prefix of "SID[i]" and knows If the passive opener recognizes the prefix of "SID[i]" and knows
"ss[i]", it SHOULD (with exceptions specified below) respond with an "ss[i]", it SHOULD (with exceptions specified below) respond with an
ENO option containing an _empty resumption suboption_ with matching ENO option containing an _empty resumption suboption_ indicating the
spec identifier; that is, a suboption whose initial byte gives the same key-exchange scheme; that is, a suboption whose initial byte
"cs" value from host A's resumption suboption and sets "v = 1", but gives the TEP identifier from host A's resumption suboption and sets
whose contents are empty. (The only way to encode this is as the "v = 1", but whose contents are empty. (The only way to encode this
last ENO suboption.) is as the last ENO suboption.)
Otherwise, the passive opener SHOULD inspect any other ENO suboptions Otherwise, the passive opener SHOULD attempt to negotiate fresh key
in hopes of negotiating a fresh key exchange as described in exchange by responding with a single, non-resumption suboption with
Section 3.3. the same TEP as in the received resumption suboption, or with a TEP
from another received suboption.
A host MUST ignore a resumption suboption if has successfully A host MUST ignore a resumption suboption if it has successfully
negotiated resumption in the past, in either role, with the same negotiated resumption in the past, in either role, with the same
"SID[i]". In the event that two hosts simultaneously send SYN "SID[i]". In the event that two hosts simultaneously send SYN
segments to each other with the same "SID[i]", but the two segments 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 are not part of a simultaneous open, both connections will have to
revert to public key cryptography. To avoid this limitation, revert to fresh key exchange. To avoid this limitation,
implementations MAY choose to implement session caching such that a implementations MAY choose to implement session caching such that a
given pre-session key "ss[0]" is only used for either passive or given pre-session key "ss[0]" is only used for either passive or
active opens at the same host, not both. active opens at the same host, not both.
In the case of simultaneous open where TCP-ENO is able to establish In the case of simultaneous open where TCP-ENO is able to establish
asymmetric roles, two hosts that simultaneously send SYN segments asymmetric roles, two hosts that simultaneously send SYN segments
with resumption suboptions containing the same "SID[i]" may resume with resumption suboptions containing the same "SID[i]" may resume
the associated session. the associated session.
Hosts MUST NOT send, and upon receipt MUST ignore, an empty A host MUST NOT send, and upon receipt MUST ignore, an empty
resumption suboption in a SYN-only segment. resumption suboption in a SYN-only segment.
After using "ss[i]" to compute "mk[0]", implementations SHOULD After using "ss[i]" to compute "mk[0]", implementations SHOULD
compute and cache "ss[i+1]" for possible use by a later session, then 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 erase "ss[i]" from memory. Hosts SHOULD retain "ss[i+1]" until it is
period of time until it is used or the memory needs to be reclaimed. used or the memory needs to be reclaimed. Hosts SHOULD NOT write a
Hosts SHOULD NOT write a cached "ss[i+1]" value to non-volatile cached "ss[i+1]" value to non-volatile storage.
storage.
When two hosts have previously negotiated a tcpcrypt session, either When two hosts have previously negotiated a tcpcrypt session, either
host may initiate session resumption regardless of which host was the host may initiate session resumption regardless of which host was the
active opener or played the "A" role in the previous session. active opener or played the "A" role in the previous session.
However, a given host must either encrypt with "k_ab" for all However, a given host must either encrypt with "k_ab" for all
sessions derived from the same pre-session key "ss[0]", or with 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 "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 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 only on whether the host played the "A" or "B" role in the initial
session. session.
Implementations that perform session caching MUST provide a means for Implementations that perform session caching MUST provide a means for
applications to control session caching, including flushing cached applications to control session caching, including flushing cached
session secrets associated with an ESTABLISHED connection or session secrets associated with an ESTABLISHED connection or
disabling the use of caching for a particular connection. A disabling the use of caching for a particular connection.
recommended interface is described in Section 10.
The session ID required by TCP-ENO and exposed to applications is 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 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 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 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 negotiated suboption, which means the byte will contain "v = 1"; and
the remainder is "SID[i]", the bare session ID for the resumed the remainder is "SID[i]", the bare session ID for the resumed
session. session.
3.5. Data encryption and authentication 3.5. Data encryption and authentication
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Section 3.3. The plaintext value serves as "P", the associated data 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 as "A", and the frame nonce as "N". The output of the encryption
operation, "C", is transmitted in the frame's "ciphertext" field. operation, "C", is transmitted in the frame's "ciphertext" field.
When a frame is received, tcpcrypt reconstructs the associated data When a frame is received, tcpcrypt reconstructs the associated data
and frame nonce values (the former contains only data sent in the and frame nonce values (the former contains only data sent in the
clear, and the latter is implicit in the TCP stream), and provides clear, and the latter is implicit in the TCP stream), and provides
these and the ciphertext value to the the AEAD decryption operation. these and the ciphertext value to the the AEAD decryption operation.
The output of this operation is either "P", a plaintext value, or the The output of this operation is either "P", a plaintext value, or the
special symbol FAIL. In the latter case, the implementation MUST special symbol FAIL. In the latter case, the implementation MUST
either ignore the frame or terminate the connection. either ignore the frame or abort the connection; but if it aborts,
the implementation MUST raise an error condition distinct from the
end-of-file condition.
3.6. TCP header protection 3.6. TCP header protection
The "ciphertext" field of the application frame contains protected The "ciphertext" field of the application frame contains protected
versions of certain TCP header values. versions of certain TCP header values.
When "URGp" is set, the "urgent" value indicates an offset from the When "URGp" is set, the "urgent" value indicates an offset from the
current frame's beginning offset; the sum of these offsets gives 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. index of the last byte of urgent data in the application datastream.
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refer to the key-set generated by mk[i] as the key-set with refer to the key-set generated by mk[i] as the key-set with
_generation number_ "i" within a session. Each host maintains a _generation number_ "i" within a session. Each host maintains a
_current generation number_ that it uses to encrypt outgoing frames. _current generation number_ that it uses to encrypt outgoing frames.
Initially, the two hosts have current generation number 0. Initially, the two hosts have current generation number 0.
When a host has just incremented its current generation number and 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 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. 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. It MUST set this field to zero in all other cases.
A host MAY increment its generation number beyond the highest A host MAY increment its current generation number beyond the highest
generation it knows the other side to be using. We call this action generation it knows the other side to be using. We call this action
_initiating re-keying_. _initiating re-keying_.
A host SHOULD NOT initiate more than one concurrent re-key operation 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 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 with an empty application frame more than once while its record of
the remote host's current generation number is less than its own. 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 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. generation number if and only if the "rekey" field is set to 1.
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retransmitted segment MUST always be encrypted with the same key as retransmitted segment MUST always be encrypted with the same key as
when it was originally transmitted. when it was originally transmitted.
Implementations SHOULD delete older-generation keys from memory once Implementations SHOULD delete older-generation keys from memory once
they have received all frames they will need to decrypt with the old they have received all frames they will need to decrypt with the old
keys and have encrypted all outgoing frames under the old keys. keys and have encrypted all outgoing frames under the old keys.
3.8. Keep-alive 3.8. Keep-alive
Instead of using TCP Keep-Alives to verify that the remote endpoint Instead of using TCP Keep-Alives to verify that the remote endpoint
is still alive, tcpcrypt implementations SHOULD employ the re-keying is still responsive, tcpcrypt implementations SHOULD employ the re-
mechanism, as follows. When necessary, a host SHOULD probe the keying mechanism, as follows. When necessary, a host SHOULD probe
liveness of its peer by initiating re-keying as described in the liveness of its peer by initiating re-keying as described in
Section 3.7, and then transmitting a new frame (with zero-length Section 3.7, and then transmitting a new frame (with zero-length
application data if necessary). A host receiving a frame whose key application data if necessary). A host receiving a frame whose key
generation number is greater than its current generation number MUST generation number is greater than its current generation number MUST
increment its current generation number and MUST immediately transmit increment its current generation number and MUST immediately transmit
a new frame (with zero-length application data, if necessary). a new frame (with zero-length application data, if necessary).
Implementations MAY use TCP Keep-Alives for purposes that do not Implementations MAY use TCP Keep-Alives for purposes that do not
require endpoint authentication, as discussed in Section 9.2. require endpoint authentication, as discussed in Section 9.2.
4. Encodings 4. Encodings
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+ offset + + offset +
8 | | 8 | |
+------+------+------+------+ +------+------+------+------+
The 4-byte magic constant is defined in Table 3. The 8-byte "offset" 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 field contains an integer in big-endian format. Its value is
specified in Section 3.5. specified in Section 3.5.
5. Key agreement schemes 5. Key agreement schemes
The encryption spec negotiated via TCP-ENO may indicate the use of The TEP negotiated via TCP-ENO may indicate the use of one of the
one of the key-agreement schemes named in Table 1. For example, key-agreement schemes named in Table 1. For example,
"TCPCRYPT_ECDHE_P256" names the tcpcrypt protocol with key-agreement "TCPCRYPT_ECDHE_P256" names the tcpcrypt protocol with key-agreement
scheme ECDHE-P256. scheme ECDHE-P256.
All schemes listed there use HKDF-Expand-SHA256 as the CPRF, and All schemes listed there use HKDF-Expand-SHA256 as the CPRF, and
these lengths for nonces and session keys: these lengths for nonces and session keys:
N_A_LEN: 32 bytes N_A_LEN: 32 bytes
N_B_LEN: 32 bytes N_B_LEN: 32 bytes
K_LEN: 32 bytes K_LEN: 32 bytes
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6. AEAD algorithms 6. AEAD algorithms
Specifiers and key-lengths for AEAD algorithms are given in Table 2. Specifiers and key-lengths for AEAD algorithms are given in Table 2.
The algorithms "AEAD_AES_128_GCM" and "AEAD_AES_256_GCM" are The algorithms "AEAD_AES_128_GCM" and "AEAD_AES_256_GCM" are
specified in [RFC5116]. The algorithm "AEAD_CHACHA20_POLY1305" is specified in [RFC5116]. The algorithm "AEAD_CHACHA20_POLY1305" is
specified in [RFC7539]. specified in [RFC7539].
7. IANA considerations 7. IANA considerations
Tcpcrypt's spec identifiers ("cs" values) will need to be added to Tcpcrypt's TEP identifiers will need to be incorporated in IANA's
IANA's ENO suboption registry, as follows: TCP-ENO encryption protocol identifier registry, as follows:
+------+---------------------------+ +------+---------------------------+
| cs | Spec name | | cs | Spec name |
+------+---------------------------+ +------+---------------------------+
| 0x21 | TCPCRYPT_ECDHE_P256 | | 0x21 | TCPCRYPT_ECDHE_P256 |
| 0x22 | TCPCRYPT_ECDHE_P521 | | 0x22 | TCPCRYPT_ECDHE_P521 |
| 0x23 | TCPCRYPT_ECDHE_Curve25519 | | 0x23 | TCPCRYPT_ECDHE_Curve25519 |
| 0x24 | TCPCRYPT_ECDHE_Curve448 | | 0x24 | TCPCRYPT_ECDHE_Curve448 |
+------+---------------------------+ +------+---------------------------+
Table 1: cs values for use with tcpcrypt Table 1: TEP identifiers for use with tcpcrypt
A "tcpcrypt AEAD parameter" registry needs to be maintained by IANA A "tcpcrypt AEAD parameter" registry needs to be maintained by IANA
as in the following table. The use of encryption is described in as in the following table. The use of encryption is described in
Section 3.5. Section 3.5.
+------------------------+------------+------------+ +------------------------+------------+------------+
| AEAD Algorithm | Key Length | sym-cipher | | AEAD Algorithm | Key Length | sym-cipher |
+------------------------+------------+------------+ +------------------------+------------+------------+
| AEAD_AES_128_GCM | 16 bytes | 0x01 | | AEAD_AES_128_GCM | 16 bytes | 0x01 |
| AEAD_AES_256_GCM | 32 bytes | 0x02 | | AEAD_AES_256_GCM | 32 bytes | 0x02 |
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suppressing rekey bits. These attacks will cause a tcpcrypt suppressing rekey bits. These attacks will cause a tcpcrypt
connection to hang or fail with an error. Implementations MUST give connection to hang or fail with an error. Implementations MUST give
higher-level software a way to distinguish such errors from a clean higher-level software a way to distinguish such errors from a clean
end-of-stream (indicated by an authenticated "FINp" bit) so that end-of-stream (indicated by an authenticated "FINp" bit) so that
applications can avoid semantic truncation attacks. applications can avoid semantic truncation attacks.
There is no "key confirmation" step in tcpcrypt. This is not There is no "key confirmation" step in tcpcrypt. This is not
required because tcpcrypt's threat model includes the possibility of required because tcpcrypt's threat model includes the possibility of
a connection to an adversary. If key negotiation is compromised and a connection to an adversary. If key negotiation is compromised and
yields two different keys, all subsequent frames will be ignored due yields two different keys, all subsequent frames will be ignored due
failed integrity checks, causing the application's connection to to failed integrity checks, causing the application's connection to
hang. This is not a new threat because in plain TCP, an active hang. This is not a new threat because in plain TCP, an active
attacker could have modified sequence and acknowledgement numbers to attacker could have modified sequence and acknowledgement numbers to
hang the connection anyway. hang the connection anyway.
Tcpcrypt uses short-lived public keys to provide forward secrecy. Tcpcrypt uses short-lived public keys to provide forward secrecy.
All currently specified key agreement schemes involve ECDHE-based key All currently specified key agreement schemes involve ECDHE-based key
agreement, meaning a new key can be efficiently computed for each agreement, meaning a new key can be efficiently computed for each
connection. If implementations reuse these parameters, they SHOULD connection. If implementations reuse these parameters, they SHOULD
limit the lifetime of the private parameters, ideally to no more than limit the lifetime of the private parameters, ideally to no more than
two minutes. two minutes.
Attackers cannot force passive openers to move forward in their Attackers cannot force passive openers to move forward in their
session caching chain without guessing the content of the resumption session caching chain without guessing the content of the resumption
suboption, which will be hard without key knowledge. suboption, which will be difficult without key knowledge.
9. Design notes 9. Design notes
9.1. Asymmetric roles 9.1. Asymmetric roles
Tcpcrypt transforms a shared pseudo-random key (PRK) into Tcpcrypt transforms a shared pseudo-random key (PRK) into
cryptographic session keys for each direction. Doing so requires an cryptographic session keys for each direction. Doing so requires an
asymmetry in the protocol, as the key derivation function must be asymmetry in the protocol, as the key derivation function must be
perturbed differently to generate different keys in each direction. perturbed differently to generate different keys in each direction.
Tcpcrypt includes other asymmetries in the roles of the two hosts, Tcpcrypt includes other asymmetries in the roles of the two hosts,
skipping to change at page 20, line 47 skipping to change at page 21, line 31
Thus, tcpcrypt specifies a way to stimulate the remote host to send Thus, tcpcrypt specifies a way to stimulate the remote host to send
verifiably fresh and authentic data, described in Section 3.8. verifiably fresh and authentic data, described in Section 3.8.
The TCP keep-alive mechanism has also been used for its effects on The TCP keep-alive mechanism has also been used for its effects on
intermediate nodes in the network, such as preventing flow state from intermediate nodes in the network, such as preventing flow state from
expiring at NAT boxes or firewalls. As these purposes do not require expiring at NAT boxes or firewalls. As these purposes do not require
the authentication of endpoints, implementations may safely the authentication of endpoints, implementations may safely
accomplish them using either the existing TCP keep-alive mechanism or accomplish them using either the existing TCP keep-alive mechanism or
tcpcrypt's verified keep-alive mechanism. tcpcrypt's verified keep-alive mechanism.
10. API extensions 10. Acknowledgments
Applications aware of tcpcrypt will need an API for interacting with
the protocol. They can do so if implementations provide the
recommended API for TCP-ENO. This section recommends several
additions to that API, described in the style of socket options.
However, these recommendations are non-normative:
The following option is read-only:
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 We are grateful for contributions, help, discussions, and feedback
from the TCPINC working group, including Marcelo Bagnulo, David from the TCPINC working group, including Marcelo Bagnulo, David
Black, Bob Briscoe, Jana Iyengar, Tero Kivinen, Mirja Kuhlewind, Yoav Black, Bob Briscoe, Jana Iyengar, Tero Kivinen, Mirja Kuhlewind, Yoav
Nir, Christoph Paasch, Eric Rescorla, and Kyle Rose. Nir, Christoph Paasch, Eric Rescorla, and Kyle Rose.
This work was funded by gifts from Intel (to Brad Karp) and from This work was funded by gifts from Intel (to Brad Karp) and from
Google; by NSF award CNS-0716806 (A Clean-Slate Infrastructure for Google; by NSF award CNS-0716806 (A Clean-Slate Infrastructure for
Information Flow Control); by DARPA CRASH under contract Information Flow Control); by DARPA CRASH under contract
#N66001-10-2-4088; and by the Stanford Secure Internet of Things #N66001-10-2-4088; and by the Stanford Secure Internet of Things
Project. Project.
12. References 11. References
12.1. Normative References 11.1. Normative References
[I-D.ietf-tcpinc-tcpeno] [I-D.ietf-tcpinc-tcpeno]
Bittau, A., Boneh, D., Giffin, D., Handley, M., Mazieres, Bittau, A., Boneh, D., Giffin, D., Handley, M., Mazieres,
D., and E. Smith, "TCP-ENO: Encryption Negotiation D., and E. Smith, "TCP-ENO: Encryption Negotiation
Option", draft-ietf-tcpinc-tcpeno-03 (work in progress), Option", draft-ietf-tcpinc-tcpeno-06 (work in progress),
July 2016. October 2016.
[ieee1363] [ieee1363]
"IEEE Standard Specifications for Public-Key Cryptography "IEEE Standard Specifications for Public-Key Cryptography
(IEEE Std 1363-2000)", 2000. (IEEE Std 1363-2000)", 2000.
[nist-dss] [nist-dss]
"Digital Signature Standard, FIPS 186-2", 2000. "Digital Signature Standard, FIPS 186-2", 2000.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
skipping to change at page 22, line 46 skipping to change at page 22, line 34
<http://www.rfc-editor.org/info/rfc5869>. <http://www.rfc-editor.org/info/rfc5869>.
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF [RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015, Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
<http://www.rfc-editor.org/info/rfc7539>. <http://www.rfc-editor.org/info/rfc7539>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <http://www.rfc-editor.org/info/rfc7748>. 2016, <http://www.rfc-editor.org/info/rfc7748>.
12.2. Informative References 11.2. Informative References
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989, DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>. <http://www.rfc-editor.org/info/rfc1122>.
[tcpcrypt] [tcpcrypt]
Bittau, A., Hamburg, M., Handley, M., Mazieres, D., and D. Bittau, A., Hamburg, M., Handley, M., Mazieres, D., and D.
Boneh, "The case for ubiquitous transport-level Boneh, "The case for ubiquitous transport-level
encryption", USENIX Security , 2010. encryption", USENIX Security , 2010.
skipping to change at page 23, line 30 skipping to change at page 23, line 22
| 0x15101a0e | INIT1_MAGIC | | 0x15101a0e | INIT1_MAGIC |
| 0x097105e0 | INIT2_MAGIC | | 0x097105e0 | INIT2_MAGIC |
| 0x44415441 | FRAME_NONCE_MAGIC | | 0x44415441 | FRAME_NONCE_MAGIC |
+------------+-------------------+ +------------+-------------------+
Table 3: Protocol constants Table 3: Protocol constants
Authors' Addresses Authors' Addresses
Andrea Bittau Andrea Bittau
Stanford University Google
353 Serra Mall, Room 288 345 Spear Street
Stanford, CA 94305 San Francisco, CA 94105
US US
Email: bittau@cs.stanford.edu Email: bittau@google.com
Dan Boneh Dan Boneh
Stanford University Stanford University
353 Serra Mall, Room 475 353 Serra Mall, Room 475
Stanford, CA 94305 Stanford, CA 94305
US US
Email: dabo@cs.stanford.edu Email: dabo@cs.stanford.edu
Daniel B. Giffin Daniel B. Giffin
Stanford University Stanford University
353 Serra Mall, Room 288 353 Serra Mall, Room 288
Stanford, CA 94305 Stanford, CA 94305
US US
Email: dbg@scs.stanford.edu Email: dbg@scs.stanford.edu
Mike Hamburg Mike Hamburg
Stanford University Stanford University
353 Serra Mall, Room 475 353 Serra Mall, Room 475
Stanford, CA 94305 Stanford, CA 94305
US US
Email: mike@shiftleft.org Email: mike@shiftleft.org
Mark Handley Mark Handley
University College London University College London
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