Internet DRAFT - draft-bittau-tcpinc-tcpcrypt
draft-bittau-tcpinc-tcpcrypt
Network Working Group A. Bittau
Internet-Draft D. Boneh
Intended status: Standards Track D. Giffin
Expires: April 19, 2016 M. Hamburg
Stanford University
M. Handley
University College London
D. Mazieres
Q. Slack
Stanford University
E. Smith
Kestrel Institute
October 17, 2015
Cryptographic protection of TCP Streams (tcpcrypt)
draft-bittau-tcpinc-tcpcrypt-04
Abstract
This document specifies tcpcrypt, a cryptographic protocol that
protects TCP payload data and is negotiated by means of the TCP
Encryption Negotiation Option (TCP-ENO) [I-D.ietf-tcpinc-tcpeno].
Tcpcrypt coexists with middleboxes by tolerating resegmentation,
NATs, and other manipulations of the TCP header. The protocol is
self-contained and specifically tailored to TCP implementations,
which often reside in kernels or other environments in which large
external software dependencies can be undesirable. Because of option
size restrictions, the protocol requires one additional one-way
message latency to perform key exchange. However, this cost is
avoided between two hosts that have recently established a previous
tcpcrypt connection.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 19, 2016.
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. Roles . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Protocol negotiation . . . . . . . . . . . . . . . . . . 5
3.4. Key exchange . . . . . . . . . . . . . . . . . . . . . . 6
3.5. Session caching . . . . . . . . . . . . . . . . . . . . . 8
3.6. Data encryption and authentication . . . . . . . . . . . 10
3.7. TCP header protection . . . . . . . . . . . . . . . . . . 11
3.8. Re-keying . . . . . . . . . . . . . . . . . . . . . . . . 11
3.9. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 12
4. Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Key exchange messages . . . . . . . . . . . . . . . . . . 13
4.2. Application frames . . . . . . . . . . . . . . . . . . . 15
4.2.1. Plaintext . . . . . . . . . . . . . . . . . . . . . . 16
4.2.2. Associated data . . . . . . . . . . . . . . . . . . . 17
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4.2.3. Frame nonce . . . . . . . . . . . . . . . . . . . . . 17
5. API extensions . . . . . . . . . . . . . . . . . . . . . . . 17
6. Key agreement schemes . . . . . . . . . . . . . . . . . . . . 18
7. AEAD algorithms . . . . . . . . . . . . . . . . . . . . . . . 20
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Security considerations . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Protocol constant values . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Requirements language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Introduction
This document describes tcpcrypt, an extension to TCP for
cryptographic protection of session data. Tcpcrypt was designed to
meet the following goals:
o Meet the requirements of the TCP Encryption Negotiation Option
(TCP-ENO) [I-D.ietf-tcpinc-tcpeno] for protecting connection data.
o Be amenable to small, self-contained implementations inside TCP
stacks.
o Avoid unnecessary round trips.
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 TCP connections even when either end changes
IP address.
3. Encryption protocol
This section describes the tcpcrypt protocol at an abstract level, so
as to provide an overview and facilitate analysis. The next section
specifies the byte formats of all messages.
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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. 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.
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 with CONST is a single octet.
Once tcpcrypt has been successfully set up, we say the connection
moves to an ENCRYPTING phase, where it employs an _authenticated
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encryption mode_ to encrypt and integrity-protect all application
data.
Note that 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. Thus, any host implementing
tcpcrypt MUST have a cryptographically-secure source of randomness or
pseudo-randomness.
3.2. Roles
Tcpcrypt transforms a single 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).
To establish roles for the hosts, tcpcrypt depends on TCP-ENO
[I-D.ietf-tcpinc-tcpeno]. As part of the negotiation process, TCP-
ENO assigns hosts unique roles abstractly called "A" at one end of
the connection and "B" at the other. Generally, an active opener
plays the "A" role and a passive opener plays the "B" role, though an
additional mechanism breaks the symmetry of simultaneous open. This
document adopts the terms "A" and "B" to identify each end of a
connection uniquely, following TCP-ENO's designation.
3.3. Protocol negotiation
Tcpcrypt also depends on TCP-ENO [I-D.ietf-tcpinc-tcpeno] to
negotiate the use of tcpcrypt and a particular key agreement scheme.
TCP-ENO negotiates an _encryption spec_ by means of suboptions
embedded in SYN segments. Each suboption is identified by a byte
consisting of a seven-bit _encryption spec identifier_ value, "cs",
and a one-bit additional data indicator, "v". This document reserves
and associates four "cs" values with tcpcrypt, as listed in Table 1;
future standards can associate additional values with tcpcrypt.
A TCP connection MUST employ tcpcrypt and transition to the
ENCRYPTING phase when and only when:
1. The TCP-ENO negotiated spec contains a "cs" value associated with
tcpcrypt, and
2. The presence of variable-length data matches the suboption usage.
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Specifically, when the "cs" value is "TCPCRYPT_RESUME", whose use is
described in Section 3.5, there MUST be associated data (i.e., "v"
MUST be 1). For all other "cs" values specified in this document,
there MUST NOT be additional suboption data (i.e., "v" MUST be 0).
Future "cs" values associated with tcpcrypt might or might not
specify the use of associated data. Tcpcrypt implementations MUST
ignore suboptions whose "cs" and "v" values do not agree as specified
in this paragraph.
In normal usage, an active opener that wishes to negotiate the use of
tcpcrypt will include an ENO option in its SYN segment; that option
will include the tcpcrypt suboptions corresponding to the key-
agreement schemes it is willing to enable, and possibly also a
resumption suboption. The active opener MAY additionally include
suboptions indicating support for encryption protocols other than
tcpcrypt, as well as other general options as specified by TCP-ENO.
If a passive opener receives an ENO option including tcpcrypt
suboptions it supports, it MAY then attach an ENO option to its SYN-
ACK segment, including _solely_ the suboption it wishes to enable.
Once two hosts have exchanged SYN segments, the _negotiated spec_ is
the last spec identifier in the SYN segment of host B (that is, the
passive opener in the absence of simultaneous open) that also occurs
in that of host A. If there is no such spec, hosts MUST disable TCP-
ENO and tcpcrypt.
3.4. Key exchange
Following successful negotiation of a tcpcrypt spec, all further
signaling is performed in the Data portion of TCP segments. If the
negotiated spec is not TCPCRYPT_RESUME, the two hosts perform key
exchange through two messages, INIT1 and INIT2, at the start of host
A's and host B's data streams, respectively. INIT1 or INIT2 can 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 format of these messages is specified in detail in Section 4.1.
The parameters are defined as follows:
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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 B from the sym-
cipher-list sent by A.
o N_A, N_B: nonces chosen at random by A and B, respectively.
o PK_A, PK_B: ephemeral public keys for A and B, respectively.
These, as well as their corresponding private keys, are short-
lived values that SHOULD be refreshed periodically and SHOULD NOT
ever be written to persistent storage.
The pre-master secret (PMS) 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 PMS 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:
param := { eno-transcript, init1, init2 }
PRK := Extract (N_A, { param, PMS })
Above, "eno-transcript" is the protocol-negotiation transcript
defined in TCP-ENO; "init1" and "init2" are the transmitted encodings
of the INIT1 and INIT2 messages described in Section 4.1.
A series of "session secrets" and corresponding Session IDs are then
computed 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 Session ID for the current connection, and
will with overwhelming probability be unique for each individual TCP
connection. The most computationally expensive part of the key
exchange protocol is the public key cipher. 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 3.5. The CONST values are constants defined in Table 3. The
K_LEN values depend on the tcpcrypt spec in use, and are specified in
Figure 3.
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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 ae_keylen value depends on the authenticated-encryption algorithm
selected, and is given under "Key Length" in Table 2.
HKDF is not used directly for key derivation because tcpcrypt
requires multiple expand steps with different keys. This is needed
for forward secrecy, so that ss[n] can be forgotten once a session is
established, and mk[n] can be forgotten once a session is rekeyed.
There is no "key confirmation" step in tcpcrypt. This is not
required because tcpcrypt's threat model includes the possibility of
a connection to an adversary. If key negotiation is compromised and
yields two different keys, all subsequent frames will be ignored due
failed integrity checks, causing the application's connection to
hang. This is not a new threat because in plain TCP, an active
attacker could have modified sequence and acknowledgement numbers to
hang the connection anyway.
3.5. 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]. A host wishing to request this facility will include in its
SYN segment an ENO option whose last suboption contains the spec
identifier TCPCRYPT_RESUME:
byte 0 1 8
+--------+--------+---...---+--------+
| Opt = | SID[i]{0..7} |
| resume | |
+--------+--------+---...---+--------+
Figure 2: ENO suboption used to initiate session resumption
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Above, the "resume" value is the byte whose lower 7 bits are
TCPCRYPT_RESUME and whose top bit "v" is 1 (indicating variable-
length data follows). The remainder of the suboption is filled with
the first eight bytes of the Session ID SID[i].
A host SHOULD also include ENO suboptions describing the key-
agreement schemes it supports in addition to a resume suboption, so
as to fall back to full key exchange in the event that session
resumption fails.
Which symmetric keys a host uses for transmitted segments is
determined by its role in the original session ss[0]. It does not
depend on the role it plays in the current session. For example, if
a host had the "A" role in the first session, then it uses k_ab for
sending segments and k_ba for receiving.
After using ss[i] to compute mk[0], implementations SHOULD compute
and cache ss[i+1] for possible use by a later session, then erase
ss[i] from memory. Hosts SHOULD keep ss[i+1] around for a period of
time until it is used or the memory needs to be reclaimed. Hosts
SHOULD NOT write a cached ss[i+1] value to non-volatile storage.
It is an implementation-specific issue as to how long ss[i+1] should
be retained if it is unused. If the passive opener evicts it from
cache before the active opener does, the only cost is the additional
ten bytes to send the resumption suboption in the next connection.
The behavior then falls back to a normal public-key handshake.
The active opener MUST use the lowest value of "i" that has not
already appeared in a resumption suboption exchanged with the same
host and for the same pre-session seed.
If the passive opener recognizes SID[i] and knows ss[i], it SHOULD
respond with an ENO option containing a dataless resumption
suboption; that is, the suboption whose "cs" value is TCPCRYPT_RESUME
and whose "v" bit is zero.
If the passive opener does not recognize SID[i], or SID[i] is not
valid or has already been used, the passive opener SHOULD inspect any
other ENO suboptions in hopes of negotiating a fresh key exchange as
described in Section 3.4.
When two hosts have previously negotiated a tcpcrypt session, either
host may initiate session resumption regardless of which host was the
active opener or played the "A" role in the previous session.
However, a given host must either encrypt with k_ab for all sessions
derived from the same pre-session seed, or k_ba. Thus, which keys a
host uses to send segments depends only whether the host played the
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"A" or "B" role in the initial session that used ss[0]; it is not
affected by which host was the active opener transmitting the SYN
segment containing a resumption suboption.
A host MUST ignore a resumption suboption if it has previously sent
or received one with the same SID[i]. In the event that two hosts
simultaneously send SYN segments to each other with the same SID[i],
but the two segments are not part of a simultaneous open, both
connections will have to revert to public key cryptography. To avoid
this limitation, implementations MAY choose to implement session
caching such that a given pre-session key is only good 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.
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.
3.6. Data encryption and authentication
Following key exchange, 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 3.4.
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 frame's
"plaintext" value, which is then encrypted to yield the frame's
"ciphertext" field. Chunks must be small enough that the ciphertext
(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 byte-offset of the beginning of the
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current application frame in the underlying TCP datastream. (That
is, the offset in the framing stream, not the plaintext application
stream.) As the security of the AEAD algorithm depends on this nonce
being used to encrypt at most one distinct plaintext value, an
implementation MUST NOT ever transmit distinct frames at the same
location in the underlying TCP datastream.
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 3.4. 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 MAY
either ignore the frame or terminate the connection.
3.7. 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.
3.8. Re-keying
Re-keying allows hosts to wipe from memory keys that could decrypt
previously transmitted segments. It also allows the use of AEAD
ciphers that can securely encrypt only a bounded number of messages
under a given key.
We refer to the two encryption keys (k_ab, k_ba) as a _key-set_. We
refer to the key-set generated by mk[i] as the key-set with
_generation number_ "i" within a session. Each host maintains a
_current generation number_ that it uses to encrypt outgoing frames.
Initially, the two hosts have current generation number 0.
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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 the 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 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.
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.
3.9. Keep-alive
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.)
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Instead of TCP Keep-Alives, tcpcrypt implementations SHOULD employ
the re-keying mechanism to stimulate the remote host to send
verifiably fresh and authentic data. When required, a host SHOULD
probe the liveness of its peer by initiating re-keying as described
in Section 3.8, and then transmitting a new frame (with zero-length
application data if necessary). A host receiving a frame whose key
generation number is greater than its current generation number MUST
increment its current generation number and MUST immediately transmit
a new frame (with zero-length application data, if necessary).
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:
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byte 0 1 2 3
+-------+-------+-------+-------+
| INIT1_MAGIC |
| |
+-------+-------+-------+-------+
4 5 6 7
+-------+-------+-------+-------+
| message_len |
| = M |
+-------+-------+-------+-------+
8
+--------+-------+-------+---...---+-------+
|nciphers|sym- |sym- | |sym- |
| =K+1 |cipher0|cipher1| |cipherK|
+--------+-------+-------+---...---+-------+
K + 10 K + 10 + N_A_LEN
| |
v v
+-------+---...---+-------+-------+---...---+-------+
| N_A | PK_A |
| | |
+-------+---...---+-------+-------+---...---+-------+
M - 1
+-------+---...---+-------+
| ignored |
| |
+-------+---...---+-------+
The constant INIT1_MAGIC is defined in Table 3. The four-byte field
"message_len" gives the length of the entire INIT1 message, encoded
as a big-endian integer. The "nciphers" field contains an integer
value that specifies the number of one-byte symmetric-cipher
identifiers that follow. The "sym-cipher" bytes identify
cryptographic algorithms in Table 2. The length N_A_LEN and the
length of PK_A are both determined by the negotiated key-agreement
scheme, as shown in Figure 3.
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 PK_A field. When receiving INIT1, however, implementations MUST
permit and ignore any bytes following PK_A.
The INIT2 message has the following encoding:
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byte 0 1 2 3
+-------+-------+-------+-------+
| INIT2_MAGIC |
| |
+-------+-------+-------+-------+
4 5 6 7 8
+-------+-------+-------+-------+-------+
| message_len |sym- |
| = M |cipher |
+-------+-------+-------+-------+-------+
9 9 + N_B_LEN
| |
v v
+-------+---...---+-------+-------+---...---+-------+
| N_B | PK_B |
| | |
+-------+---...---+-------+-------+---...---+-------+
M - 1
+-------+---...---+-------+
| ignored |
| |
+-------+---...---+-------+
The constant INIT2_MAGIC is defined in Table 3. The four-byte field
"message_len" gives the length of the entire INIT2 message, encoded
as a big-endian integer. The "sym-cipher" value is a selection from
the symmetric-cipher identifiers in the previously-received INIT1
message. The length N_B_LEN and the length of PK_B are both
determined by the negotiated key-agreement scheme, as shown in
Figure 3.
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:
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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 3.8.
4.2.1. Plaintext
The "ciphertext" field is the result of applying the negotiated
authenticated-encryption algorithm to a "plaintext" value, which has
one of these two formats:
byte 0 1 plen-1
+-------+-------+---...---+-------+
| flags | data |
+-------+-------+---...---+-------+
byte 0 1 2 3 plen-1
+-------+-------+-------+-------+---...---+-------+
| flags | urgent | data |
+-------+-------+-------+-------+---...---+-------+
(Note that "clen" will generally be greater than "plen", as the
authenticated-encryption scheme attaches an integrity "tag" to the
encrypted input.)
The "flags" byte has this structure:
bit 7 6 5 4 3 2 1 0
+----+----+----+----+----+----+----+----+
| fres |URGp|FINp|
+----+----+----+----+----+----+----+----+
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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 3.7.
4.2.2. Associated data
An application frame's "associated data" (which is supplied to the
AEAD algorithm when decrypting the ciphertext and verifying the
frame's integrity) has this format:
byte 0 1 2
+-------+-------+-------+
|control| clen |
+-------+-------+-------+
It contains the same values as the frame's "control" and "clen"
fields.
4.2.3. Frame nonce
Lastly, a "frame nonce" (provided as input to the AEAD algorithm) has
this format:
byte
+------+------+------+------+
0 | 0x44 | 0x41 | 0x54 | 0x41 |
+------+------+------+------+
4 | |
+ offset +
8 | |
+------+------+------+------+
The 8-byte "offset" field contains an integer in big-endian format.
Its value is specified in Section 3.6.
5. API extensions
Applications aware of tcpcrypt will need an API for interacting with
the protocol. They can do so if implementations provide the
recommended API for TCP-ENO. This section recommends several
additions to that API, described in the style of socket options.
However, these recommendations are non-normative:
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The following options 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 3.5. 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.
6. Key agreement schemes
The encryption spec negotiated via TCP-ENO may indicate the use of
one of these key-agreement schemes:
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+---------------------------+----------------------------------+
| Encryption spec (cs) | Key-agreement scheme |
+---------------------------+----------------------------------+
| TCPCRYPT_ECDHE_P256 | Cipher: ECDHE-P256 |
| | Extract: HKDF-Extract-SHA256 |
| | CPRF: HKDF-Expand-SHA256 |
| | N_A_LEN: 32 bytes |
| | N_B_LEN: 32 bytes |
| | K_LEN: 32 bytes |
+---------------------------+----------------------------------+
| TCPCRYPT_ECDHE_P521 | Cipher: ECDHE-P521 |
| | Extract: HKDF-Extract-SHA256 |
| | CPRF: HKDF-Expand-SHA256 |
| | N_A_LEN: 32 bytes |
| | N_B_LEN: 32 bytes |
| | K_LEN: 32 bytes |
+---------------------------+----------------------------------+
| TCPCRYPT_ECDHE_Curve25519 | Cipher: ECDHE-Curve25519 |
| | Extract: HKDF-Extract-SHA256 |
| | CPRF: HKDF-Expand-SHA256 |
| | N_A_LEN: 32 bytes |
| | N_B_LEN: 32 bytes |
| | K_LEN: 32 bytes |
+---------------------------+----------------------------------+
Figure 3: Key agreement schemes
Ciphers ECDHE-P256 and ECDHE-P521 employ the ECSVDP-DH secret value
derivation primitive defined in [ieee1363]. The named curves are
defined in [nist-dss]. When the public-key values PK_A and PK_B are
transmitted as described in Section 4.1, they are encoded with the
"Elliptic Curve Point to Octet String Conversion Primitive" described
in Section E.2.3 of [ieee1363], and are prefixed by a two-byte length
in big-endian format:
byte 0 1 2 L - 1
+-------+-------+-------+---...---+-------+
| pubkey_len | pubkey |
| = L | |
+-------+-------+-------+---...---+-------+
Implementations SHOULD encode these "pubkey" values in "compressed
format", and MUST accept values encoded in "compressed",
"uncompressed" or "hybrid" formats.
The ECDHE-Curve25519 cipher uses the X25519 function described in
[I-D.irtf-cfrg-curves]. When using this cipher, public-key values
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PK_A and PK_B are transmitted directly as 32-byte values (with no
length prefix).
A tcpcrypt implementation MUST support at least the schemes
TCPCRYPT_ECDHE_P256 and TCPCRYPT_ECDHE_P521, although system
administrators need not enable them.
7. 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].
8. Acknowledgments
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), and by DARPA CRASH under contract
#N66001-10-2-4088.
9. IANA Considerations
Tcpcrypt's spec identifiers ("cs" values) will need to be added to
IANA's ENO suboption registry, as follows:
+------+---------------------------+--------------------------------+
| cs | Spec name | Meaning |
+------+---------------------------+--------------------------------+
| 0x20 | TCPCRYPT_RESUME | tcpcrypt session resumption |
| 0x21 | TCPCRYPT_ECDHE_P256 | tcpcrypt with ECDHE-P256 |
| 0x22 | TCPCRYPT_ECDHE_P521 | tcpcrypt with ECDHE-P521 |
| 0x23 | TCPCRYPT_ECDHE_Curve25519 | tcpcrypt with ECDHE-Curve25519 |
+------+---------------------------+--------------------------------+
Table 1: cs values for use with tcpcrypt
A "tcpcrypt AEAD parameter" registry needs to be maintained by IANA
as per the following table. The use of encryption is described in
Section 3.6.
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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.
10. Security considerations
It is worth reiterating just how crucial both the quality and
quantity of randomness are to tcpcrypt's security. 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. In the
absence of secure hardware random generators, implementations MUST
disable tcpcrypt after rebooting until the pseudo-random generator
has been reseeded (usually by a bootup script) or sufficient entropy
has been gathered.
Tcpcrypt guarantees that no man-in-the-middle attacks occurred if
Session IDs match on both ends of a connection, unless the attacker
has broken the underlying cryptographic primitives (e.g., ECDH). A
proof has been published [tcpcrypt].
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.
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.
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Similarly, tcpcrypt does not have a key confirmation step. Hence, an
active attacker can cause a connection to hang, though this is
possible even without tcpcrypt by altering sequence and ack numbers.
Tcpcrypt uses short-lived public key parameters to provide forward
secrecy. All currently specified key agreement schemes involve
ECDHE-based key agreement, meaning a new key can be chosen 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 hard without key knowledge.
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-00 (work in progress),
September 2015.
[I-D.irtf-cfrg-curves]
Langley, A. and M. Hamburg, "Elliptic Curves for
Security", draft-irtf-cfrg-curves-10 (work in progress),
October 2015.
[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>.
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[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>.
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
+------------+--------------+
| 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 |
+------------+--------------+
Table 3: Protocol constants
Authors' Addresses
Andrea Bittau
Stanford University
353 Serra Mall, Room 288
Stanford, CA 94305
US
Email: bittau@cs.stanford.edu
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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
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
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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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