Network Working Group A. Bittau
Internet-Draft Google
Intended status: Standards Track D. Boneh
Expires: May 4, 2017 D. Giffin
M. Hamburg
Stanford University
M. Handley
University College London
D. Mazieres
Q. Slack
Stanford University
E. Smith
Kestrel Institute
October 31, 2016
Cryptographic protection of TCP Streams (tcpcrypt)
draft-ietf-tcpinc-tcpcrypt-03
Abstract
This document specifies tcpcrypt, a TCP encryption protocol designed
for use in conjunction with 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 transmitted.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on May 4, 2017.
Copyright Notice
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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 . . . . . . . . . . . . . . . . . . . . . 17
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5. Key agreement schemes . . . . . . . . . . . . . . . . . . . . 17
6. AEAD algorithms . . . . . . . . . . . . . . . . . . . . . . . 18
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 18
8. Security considerations . . . . . . . . . . . . . . . . . . . 19
9. Design notes . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1. Asymmetric roles . . . . . . . . . . . . . . . . . . . . 20
9.2. Verified liveness . . . . . . . . . . . . . . . . . . . . 21
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Protocol constant values . . . . . . . . . . . . . . 22
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.
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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 to the right of 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.
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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 the use of a
particular TCP encryption protocol or _TEP_ by including protocol
identifiers in ENO suboptions. This document associates four TEP
identifiers with the tcpcrypt protocol, as listed in Table 1. Future
standards may associate additional identifiers with tcpcrypt.
An active opener that wishes to negotiate the use of tcpcrypt will
include an ENO option in its SYN segment. That option will include
suboptions with TEP identifiers indicating the key-agreement schemes
it is willing to enable. The active opener MAY additionally include
suboptions indicating support for encryption protocols other than
tcpcrypt, as well as other general options as specified by TCP-ENO.
If a passive opener receives an ENO option including tcpcrypt TEPs it
supports, it MAY then attach an ENO option to its SYN-ACK segment,
including _solely_ the TEP it wishes to enable.
To establish distinct roles for the two hosts in each connection,
tcpcrypt depends on the role-negotiation mechanism of TCP-ENO
[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; but in
the case of simultaneous open, an additional mechanism breaks the
symmetry and assigns different roles to the two hosts. 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 TEP_ is
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
in that of host A. If there is no such TEP, 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 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.
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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
segment containing an ENO option, encryption MUST be enabled and
plaintext application data MUST NOT ever be exchanged on the
connection. If the negotiated TEP is among those listed in Table 1,
a host MUST follow the protocol described in this document.
3.3. Key exchange
Following successful negotiation of a tcpcrypt TEP, all further
signaling is performed in the Data portion of TCP segments. Except
when resumption was negotiated (described below in Section 3.4), the
two hosts perform key exchange through two messages, "Init1" and
"Init2", at the start of the data streams of host A and host B,
respectively. These messages may span multiple TCP segments and need
not end at a segment boundary. However, the segment containing the
last byte of an "Init1" or "Init2" message 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.
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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,
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 TEP 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
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Given a session secret "ss", the two sides compute a series of master
keys as follows:
mk[0] = CPRF (ss, CONST_REKEY, K_LEN)
mk[i] = CPRF (mk[i-1], CONST_REKEY, K_LEN)
Finally, each master key "mk" is used to generate keys for
authenticated encryption for the "A" and "B" roles. Key "k_ab" is
used by host A to encrypt and host B to decrypt, while "k_ba" is used
by host B to encrypt and host A to decrypt.
k_ab = CPRF (mk, CONST_KEY_A, ae_keylen)
k_ba = CPRF (mk, CONST_KEY_B, ae_keylen)
The value "ae_keylen" depends on the authenticated-encryption
algorithm selected, and is given under "Key Length" in Table 2.
After host B sends "Init2" or host A receives it, that host may
immediately begin transmitting protected application data as
described in Section 3.5.
3.4. Session caching
When two hosts have already negotiated session secret "ss[i-1]", they
can establish a new connection without public-key operations using
"ss[i]". Willingness to employ this facility is signalled by sending
a SYN segment with a resumption suboption: an ENO suboption
containing the negotiated TEP 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)
+--------+--------+---...---+--------+
| TEP- | SID[i]{0..8} |
| byte | |
+--------+--------+---...---+--------+
Figure 2: ENO suboption used to initiate session resumption. The
TEP-byte contains a tcpcrypt TEP identifier 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]".
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In a particular SYN segment, a host SHOULD NOT send more than one
resumption suboption, and MUST NOT send more than one resumption
suboption with the same TEP identifier. But in addition to any
resumption suboptions, an active opener MAY include non-resumption
suboptions describing other 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
"ss[i]", it SHOULD (with exceptions specified below) respond with an
ENO option containing an _empty resumption suboption_ indicating the
same key-exchange scheme; that is, a suboption whose initial byte
gives the TEP identifier 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 attempt to negotiate fresh key
exchange by responding with a single, non-resumption suboption with
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 it 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 fresh key exchange. 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.
A host 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 retain "ss[i+1]" 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.
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However, a given host must either encrypt with "k_ab" for all
sessions derived from the same pre-session key "ss[0]", or with
"k_ba". Thus, which keys a host uses to send segments is not
affected by the role it plays in the current connection: it depends
only on whether the host played the "A" or "B" role in the initial
session.
Implementations that perform session caching MUST provide a means for
applications to control session caching, including flushing cached
session secrets associated with an ESTABLISHED connection or
disabling the use of caching for a particular connection.
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.
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(That is, the offset in the framing stream, not the plaintext
application stream.) Because it is strictly necessary for the
security of the AEAD algorithm, an implementation MUST NOT ever
transmit distinct frames with the same nonce value under the same
encryption key. In particular, a retransmitted TCP segment MUST
contain the same payload bytes for the same TCP sequence numbers, and
a host MUST NOT transmit more than 2^64 bytes in the underlying TCP
datastream (which would cause the "offset" field to wrap) before re-
keying.
With reference to the "AEAD Interface" described in Section 2 of
[RFC5116], tcpcrypt invokes the AEAD algorithm with the secret key
"K" set to k_ab or k_ba, according to the host's role as described in
Section 3.3. The plaintext value serves as "P", the associated data
as "A", and the frame nonce as "N". The output of the encryption
operation, "C", is transmitted in the frame's "ciphertext" field.
When a frame is received, tcpcrypt reconstructs the associated data
and frame nonce values (the former contains only data sent in the
clear, and the latter is implicit in the TCP stream), and provides
these and the ciphertext value to the the AEAD decryption operation.
The output of this operation is either "P", a plaintext value, or the
special symbol FAIL. In the latter case, the implementation MUST
either ignore the frame or 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
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.
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We refer to the two encryption keys (k_ab, k_ba) as a _key-set_. We
refer to the key-set generated by mk[i] as the key-set with
_generation number_ "i" within a session. Each host maintains a
_current generation number_ that it uses to encrypt outgoing frames.
Initially, the two hosts have current generation number 0.
When a host has just incremented its current generation number and
has used the new key-set for the first time to encrypt an outgoing
frame, it MUST set that frame's "rekey" field (see Section 4.2) to 1.
It MUST set this field to zero in all other cases.
A host MAY increment its current 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 responsive, 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
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increment its current generation number and MUST immediately transmit
a new frame (with zero-length application data, if necessary).
Implementations MAY use TCP Keep-Alives for purposes that do not
require endpoint authentication, as discussed in Section 9.2.
4. Encodings
This section provides byte-level encodings for values transmitted or
computed by the protocol.
4.1. Key exchange messages
The "Init1" message has the following encoding:
byte 0 1 2 3
+-------+-------+-------+-------+
| INIT1_MAGIC |
| |
+-------+-------+-------+-------+
4 5 6 7
+-------+-------+-------+-------+
| message_len |
| = M |
+-------+-------+-------+-------+
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 |
| |
+-------+---...---+-------+
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The constant "INIT1_MAGIC" is defined in Table 3. The four-byte
field "message_len" gives the length of the entire "Init1" message,
encoded as a big-endian integer. The "nciphers" field contains an
integer value that specifies the number of one-byte symmetric-cipher
identifiers that follow. The "sym-cipher" bytes identify
cryptographic algorithms in Table 2. The length "N_A_LEN" and the
length of "PK_A" are both determined by the negotiated key-agreement
scheme, as described in Section 5.
When sending "Init1", implementations of this protocol MUST omit the
field "ignored"; that is, they must construct the message such that
its end, as determined by "message_len", coincides with the end of
the field "PK_A". When receiving "Init1", however, implementations
MUST permit and ignore any bytes following "PK_A".
The "Init2" message has the following encoding:
byte 0 1 2 3
+-------+-------+-------+-------+
| INIT2_MAGIC |
| |
+-------+-------+-------+-------+
4 5 6 7 8
+-------+-------+-------+-------+-------+
| message_len |sym- |
| = M |cipher |
+-------+-------+-------+-------+-------+
9 9 + N_B_LEN
| |
v v
+-------+---...---+-------+-------+---...---+-------+
| N_B | PK_B |
| | |
+-------+---...---+-------+-------+---...---+-------+
M - 1
+-------+---...---+-------+
| ignored |
| |
+-------+---...---+-------+
The constant "INIT2_MAGIC" is defined in Table 3. The four-byte
field "message_len" gives the length of the entire "Init2" message,
encoded as a big-endian integer. The "sym-cipher" value is a
selection from the symmetric-cipher identifiers in the previously-
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received "Init1" message. The length "N_B_LEN" and the length of
"PK_B" are both determined by the negotiated key-agreement scheme, as
described in Section 5.
When sending "Init2", implementations of this protocol MUST omit the
field "ignored"; that is, they must construct the message such that
its end, as determined by "message_len", coincides with the end of
the "PK_B" field. When receiving "Init2", however, implementations
MUST permit and ignore any bytes following "PK_B".
4.2. Application frames
An _application frame_ comprises a control byte and a length-prefixed
ciphertext value:
byte 0 1 2 3 clen+2
+-------+-------+-------+-------+---...---+-------+
|control| clen | ciphertext |
+-------+-------+-------+-------+---...---+-------+
The field "clen" is an integer in big-endian format and gives the
length of the "ciphertext" field.
The byte "control" has this structure:
bit 7 1 0
+-------+---...---+-------+-------+
| cres | rekey |
+-------+---...---+-------+-------+
The seven-bit field "cres" is reserved; implementations MUST set
these bits to zero when sending, and MUST ignore them when receiving.
The use of the "rekey" field is described in Section 3.7.
4.2.1. Plaintext
The "ciphertext" field is the result of applying the negotiated
authenticated-encryption algorithm to a "plaintext" value, which has
one of these two formats:
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byte 0 1 plen-1
+-------+-------+---...---+-------+
| flags | data |
+-------+-------+---...---+-------+
byte 0 1 2 3 plen-1
+-------+-------+-------+-------+---...---+-------+
| flags | urgent | data |
+-------+-------+-------+-------+---...---+-------+
(Note that "clen" in the previous section will generally be greater
than "plen", as the ciphertext produced by the authenticated-
encryption scheme must both encrypt the application data and provide
a way to verify its integrity.)
The "flags" byte has this structure:
bit 7 6 5 4 3 2 1 0
+----+----+----+----+----+----+----+----+
| fres |URGp|FINp|
+----+----+----+----+----+----+----+----+
The six-bit value "fres" is reserved; implementations MUST set these
six bits to zero when sending, and MUST ignore them when receiving.
When the "URGp" bit is set, it indicates that the "urgent" field is
present, and thus that the plaintext value has the second structure
variant above; otherwise the first variant is used.
The meaning of "urgent" and of the flag bits is described in
Section 3.6.
4.2.2. Associated data
An application frame's "associated data" (which is supplied to the
AEAD algorithm when decrypting the ciphertext and verifying the
frame's integrity) has this format:
byte 0 1 2
+-------+-------+-------+
|control| clen |
+-------+-------+-------+
It contains the same values as the frame's "control" and "clen"
fields.
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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.
5. Key agreement schemes
The TEP 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 | |
+-------+-------+-------+---...---+-------+
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Implementations SHOULD encode these "pubkey" values in "compressed
format", and MUST accept values encoded in "compressed",
"uncompressed" or "hybrid" formats.
Key-agreement schemes ECDHE-Curve25519 and ECDHE-Curve448 use the
functions X25519 and X448, respectively, to perform the Diffie-Helman
protocol as described in [RFC7748]. When using these ciphers,
public-key values "PK_A" and "PK_B" are transmitted directly with no
length prefix: 32 bytes for Curve25519, and 56 bytes for Curve448.
A tcpcrypt implementation MUST support at least the schemes
ECDHE-P256 and ECDHE-P521, although system administrators need not
enable them.
6. AEAD algorithms
Specifiers and key-lengths for AEAD algorithms are given in Table 2.
The algorithms "AEAD_AES_128_GCM" and "AEAD_AES_256_GCM" are
specified in [RFC5116]. The algorithm "AEAD_CHACHA20_POLY1305" is
specified in [RFC7539].
7. IANA considerations
Tcpcrypt's TEP identifiers will need to be incorporated in IANA's
TCP-ENO encryption protocol identifier registry, as follows:
+------+---------------------------+
| cs | Spec name |
+------+---------------------------+
| 0x21 | TCPCRYPT_ECDHE_P256 |
| 0x22 | TCPCRYPT_ECDHE_P521 |
| 0x23 | TCPCRYPT_ECDHE_Curve25519 |
| 0x24 | TCPCRYPT_ECDHE_Curve448 |
+------+---------------------------+
Table 1: TEP identifiers 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.
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+------------------------+------------+------------+
| 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,
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
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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
to failed integrity checks, causing the application's connection to
hang. This is not a new threat because in plain TCP, an active
attacker could have modified sequence and acknowledgement numbers to
hang the connection anyway.
Tcpcrypt uses short-lived public keys to provide forward secrecy.
All currently specified key agreement schemes involve ECDHE-based key
agreement, meaning a new key can be efficiently computed for each
connection. If implementations reuse these parameters, they SHOULD
limit the lifetime of the private parameters, ideally to no more than
two minutes.
Attackers cannot force passive openers to move forward in their
session caching chain without guessing the content of the resumption
suboption, which will be difficult 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).
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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. Acknowledgments
We are grateful for contributions, help, discussions, and feedback
from the TCPINC working group, including Marcelo Bagnulo, David
Black, Bob Briscoe, Jana Iyengar, Tero Kivinen, Mirja Kuhlewind, Yoav
Nir, Christoph Paasch, Eric Rescorla, and Kyle Rose.
This work was funded by gifts from Intel (to Brad Karp) and from
Google; by NSF award CNS-0716806 (A Clean-Slate Infrastructure for
Information Flow Control); by DARPA CRASH under contract
#N66001-10-2-4088; and by the Stanford Secure Internet of Things
Project.
11. References
11.1. 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-06 (work in progress),
October 2016.
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[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>.
11.2. 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>.
[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
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+------------+-------------------+
| 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
Google
345 Spear Street
San Francisco, CA 94105
US
Email: bittau@google.com
Dan Boneh
Stanford University
353 Serra Mall, Room 475
Stanford, CA 94305
US
Email: dabo@cs.stanford.edu
Daniel B. Giffin
Stanford University
353 Serra Mall, Room 288
Stanford, CA 94305
US
Email: dbg@scs.stanford.edu
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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
Eric W. Smith
Kestrel Institute
3260 Hillview Avenue
Palo Alto, CA 94304
US
Email: eric.smith@kestrel.edu
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