ICNRG Working Group M. Mosko
Internet-Draft PARC, Inc.
Intended status: Experimental C. Wood
Expires: September 14, 2017 University of California Irvine
March 13, 2017
Encrypted Sessions In CCNx (ESIC)
draft-wood-icnrg-esic-00
Abstract
This document describes how to transport CCNx packets inside an
encrypted session between peers - a sender and receiver - that share
a traffic secret, such as that which is derived from [CCNxKE]. The
peers create an outer naming context to identify the encryption
session in one direction between the sender and the receiver. The
sender issues encrypted Interest messages to the receiver, who
responds with encrypted Content Objects. Inside the outer context,
the sender sends Interests with different names, for which the
receiver may reply to or send InterestReturns in response. There
does not need to be a naming relationship between the outer names and
the inner names. The inner content is still protected by normal CCNx
authentication mechanisms and possibly encrypted under other schemes.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 14, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 3
2. Stateless packet keys . . . . . . . . . . . . . . . . . . . . 4
3. Inner and Outer Contexts . . . . . . . . . . . . . . . . . . 4
3.1. Outer Context Names . . . . . . . . . . . . . . . . . . . 5
3.2. Outer Packet . . . . . . . . . . . . . . . . . . . . . . 5
3.2.1. Sender Outer Packet . . . . . . . . . . . . . . . . . 6
3.2.2. Receiver Outer Packet . . . . . . . . . . . . . . . . 6
3.3. Processing Chain . . . . . . . . . . . . . . . . . . . . 6
3.4. Transport State Machine . . . . . . . . . . . . . . . . . 7
4. Control Channel . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. ESIC Control Packets . . . . . . . . . . . . . . . . . . 9
4.2. ESIC Control Messages . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Normative References . . . . . . . . . . . . . . . . . . 11
6.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Sample API . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
CCNx packets [CCNxMessages] contain a fixed header, optional hop-by-
hop headers, a CCNx Message, and a validation section. Encrypted
Sessions in CCNx (ESIC) describes how to to transport encrypted CCNx
packets inside other CCNx packets. The outer packet (the wrapper)
uses a CCNx name that identifies the encrypted session while the
inner (encrypted) portion remains hidden and private to an outside
observer.
ESIC defines a new field Encapsulated (T_ENCAP) that may occur in
both an Interest (T_INTEREST) and Content Object (T_OBJECT). The
T_ENCAP field contains the encryption of the inner CCNx packet, be it
an Interest or Content Object.
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Because the use of an outer CCNxPacket, the total packet length of
the inner CCNxPacket may need to be limited to less than the maximum
of 64 KB. ESIC allows the use of a compressor before the encryptor,
so it is likely that a packet that would overflow the 64 KB limit
could be compressed by enough to allow for an outer CCNxPacket. This
consideration for the PacketLength is separate from concerns about
path MTU.
It is a requirement of ESIC that one inner packet fit in one outer
packet. This is because ESIC does not define a method to issue extra
outer interests to fetch extra outer content objects. It relies
entirely on Interests generated by the sender application.
ESIC defines a control channel within the outer context by using
special names with the inner packets. These names allow signaling
between the two encryption endpoints for features such as alerts and
rekeying requests.
ESIC defines how to use a traffic secret (TS), such as derived from
CCNxKE, to encrypt multiple packets in a sender-receiver session.
Each direction will use separate derived keys. If one wishes to have
a reverse traffic flow (interests from receiver fetching content
objects from the sender), then one must share a second TS and use it
with the roles reversed, but otherwise it works exactly as in the
first case.
The mechanism by which this symmetric key is obtained is outside the
scope of this document; These keys could be pre-shared or derived
from an online key-exchange protocol [CCNxKE].
1.1. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
The following terms are used:
o Inner Packet: A fully-formed CCNx packet (fixed header through
validation) that is carried encrypted inside a T_ENCAP TLV.
o Outer Packet: A fully-formed CCNx packet that carries the outer
context of an encrypted session.
o Outer Name: The name of the outer packet.
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o Inner Name: The name of the inner packet (not visible in
transport).
o Control channel: the use of Inner Packets to convey control
signaling between encryption endpoints using a special Inner Name.
2. Stateless packet keys
ESIC assumes that the sender and receiver share a Traffic Secret
(TS), usually derived as per CCNxKE. Regardless of how the TS is
derived, there are four secrets derived from it, as per [CCNxKE].
This specifies how to generate the Sender Write Key, Receiver Write
Key, Sender Write IV, and Receiver Write IV.
The AEAD nonce (IV) is derived as specified in [TLS13]. In
particular, the length of the IV for each AEAD operation is set to
max(8 bytes, N_MAX), where N_MIN must be at least 8 bytes [RFC5116].
With this length, the nonce is initialized by:
1. Padding the 64-bit per-packet AEAD sequence number to the left
with zeroes so that its length is equal to the IV length.
2. This padded sequence number is then XORed with the sender or
receiver IV, depending on the role.
3. Inner and Outer Contexts
The inner context is a CCNx packet with meaning to the sender and
receiver. They may be clear text or they make use additional
encryption as needed. The sender transmits an Interest packet with
an Inner Name (plus other optional fields as normal) and expects to
get back a Content Object or InterestReturn packet with corresponding
name and fields.
The outer context names the encryption session and sequences packets.
ESIC does not expect a one-to-one correspondence of outer name and
inner name. If a sender, for example, transmits 3 interests with
outer names NO1, NO2, NO3 and inner names NI1, NI2, and NI3, the
receiver can return those names in any order. It could put content
objects with name NI3 in NO1, NI1 in NO2, and NI2 in NO3. ESIC does
expect normal CCNx processing rules to be followed for the inner
packets, therefore we would expect at most one inner packet returned
for each inner Interest. That inner packet could be either a Content
Object or Interest Return.
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3.1. Outer Context Names
The outer context name is a routable prefix PREFIX followed by a
session ID (SID) followed by a ChunkNumber (Chunk). The chunk number
is a monotonically increasing number.
The outer name is clear text, visible to all observers.
The PREFIX and SID are derived outside of ESIC. In normal use with
CCNxKE, the PREFIX is that which is set after the handshake
completes, be it the original producer (receiver) prefix or the
MovePrefix. The SID is created by the receiver and given to the
sender inside CCNxKE.
OuterName := ccnx:/PREFIX/SID=sid/CHUNK=chunk
Chunk numbers are limited to 8 bytes and do not wrap around. When
the sender gets near the end of the sequence number space, it must
request a re-keying via the control channel. Because CCNx in a pull-
driven model, the sender is responsible for the chunk number and thus
responsible for requesting the re-keying. The receiver may also
request a re-keying for its own reasons.
3.2. Outer Packet
The outer packet will have a Fixed Header, per hop headers, a CCNx
Message with the Outer Name, and a Validation section (ValidationAlg
and ValidationPayload). The outer packet is visible to 3rd parties
in its entirety. Only the 'value' of T_ENCAP TLV field inside the
CCNx Message is encrypted. The T_ENCAP TLV Value is the AEAD
'plaintext' that will be converted to the 'ciphertext'. In the outer
packet, only the CCNx Message and the ValidationAlg are covered by
the authentication token
The Outer Packet has a Validation section. The ValidationAlg will
have a 0-length ValidationType of T_SESSION, which indicates that the
encryption context must be derived from the SID in the name.
The Associated Data (in AEAD) covered by the validation output is
from the beignning of the CCNx Message up to but not including the
T_ENCAP Value concatenated with the ValidationAlg TLV. That is, it
skips the T_ENCAP TLV Value.
The ValidationPayload contains the AEAD authentication token.
If the receiver cannot satisfy an Inner Packet Interest, it will
encapsulate an InterestReturn inside an OuterPacket of PacketType
ContentObject. That is, the InterestReturn is end-to-end signaling
about the inner context.
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If the receiver has an error with the Outer Context, it may return an
InterestReturn for the outer context as normal for Interest
processing.
3.2.1. Sender Outer Packet
The outer packet from the sender to the receiver will always be of
PacketType Interest. They may have any of the normal Interest per-
hop headers (e.g. InterestLifetime), which will be visible to 3rd
parties and not protected by the encryption or authentication.
The Outer Context has a T_INTEREST message type, which contains a
T_NAME of the Outer Name. It may have other additional metadata in
clear text. The T_INTEREST container is protected by the encryption
authenticator. Finally, the T_INTEREST has a T_ENCAP field that
contains the encryption of the Inner Packet. The encryption will use
the algorithm negotiated as part of the SID (i.e. AES-GCM).
3.2.2. Receiver Outer Packet
The receiver will only send PacketType ContentObject back to the
sender. The Inner packet may be either an InterestReturn or a
ContentObject corresponding to the Inner Packet interest.
The outer packet may have per-hop headers (e.g.
RecommendedCacheTime) that affect the encrypted packet. These are
independent from the inner Per Hop headers. The outer MessageType is
always T_OBJECT. It may have normal metadata for a content object,
such as ExpiryTime, which affect only the outer packet. Finally, it
has a T_ENCAP that contains the wrapped inner Packet.
3.3. Processing Chain
The processing chain from the Source to the Sink is shown below. The
compression/decompression stages are optional and are not strongly
tied to the encrypted session. If used, we assume the compression
protocol is session specific to avoid state snooping (e.g. such as in
CRIME attack).
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() indicates output of stage
+------------+ +-------------+ +-----------------+ +---------+
| Source | - | Compresser | - | Encypter/Framer | - | Channel |
|(CCNxPacket)| |(CCNxzPacket)| | (CCNxPacket) | | |
+------------+ +-------------+ +-----------------+ +---------+
+------------+ +--------------------+ +-------------+ +------+
| Channel | - | Deframer/Decrypter | - | Decompressor| - | Sink |
|(CCNxPacket)| | (CCNxzPacket) | | (CCNxPacket)| | |
+------------+ +--------------------+ +-------------+ +------+
o Source: The source of an Inner Packet.
o Compressor: Optional component to reduce the size before
encryption.
o Encrypter/Framer: Creates the ciphertext of the CCNx(z)packet to
produce the T_ENCAP, constructs the outer packet, computes the
authentication token and generates the ValidationPayload.
o Channel: Carries the wire format outer packet
o Deframer/Decrypter: Verifies the authenticator, decrypts the
T_ENCAP, and passes the Inner Packet to the Decompressor.
o Decompressor: Optional component to expand the inner packet
o Sink: The sink of an Inner Packet.
The Encrypter/Framer will generate outer names with sequential outer
name chunk numbers.
The Deframer/Decryptor will extract the SID and chunk number from the
outer name and use those to create the packet key (see below). Using
the packet key, it will verify the authentication token and if
successful decrypt the T_ENCAP. The output of the T_ENCAP will then
be passed to the Sink.
3.4. Transport State Machine
ESIC uses a state machine to manage the ephemeral session such that
the receiver knows when the sender is finished with the SID. It also
will try to re-request packets that fail authentication before
sending its own InterestReturn up the Sink.
The protocol begins with each side knowing the four keys (see
Stateless Packet Keys below), the Session ID (SID), and the routable
prefix PREFIX.
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The receiving process uses a replay buffer to prevent replay attacks.
The buffer tracks the last N out-of-order verified chunks plus the
cumulative verified chunk number.
((TODO: Sort this out how to avoid replay attacks without requiring
reliable in-order delivery.))
Protocol of Encrypter/Framer:
o Initialize: set NextChunkNumber = 0, State = Waiting
o Waiting: Wait for packet from Source (or compressor). On packet
receive, State = Send
o Send:
* Generate packet key for NextChunkNumber
* Create outer packet with name /PREFIX/SID=sid/
CHUNK=NextChunkNumber and the input packet as cleartext in the
T_ENCAP.
* Run the AEAD scheme authenticating and encrypting. Note the
prior description of the split Associated Data before and after
the plaintext.
* Increment NextChunkNumber
* Send the packet
* State = Waiting
Protocol of the Deframer/Decrypter:
o Initialize the replay buffer to empty, State = Waiting.
o Waiting: wait for packet, on input from channel State = Receive
o Receive:
* Extract the SID and ChunkNumber from name
* If replay, drop
* Authenticate the packet
+ If failed on sender, send InterestReturn to Source with "X
Error" (TBD)
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+ If failed on receiver, send failure message to Sink so it
can send end-to-end InterestReturn back over channel (if
desired) with "Y Error" (TBD)
* Add packet to replay buffer
* Decrypt packet
* Pass decrypted packet to Sink/Source (or decompressor)
4. Control Channel
The sender and receiver will need to exchange signaling about the
encryption context. Control and data traffic should be
indistinguishable to an external observer. Therefore, all control
signaling is done within the same outer names as data traffic.
Control signaling is done with a normal Inner Packet that pushes data
to the other side. We use an Interest with an Inner Name of the form
shown below, where '_direction_' is 'up' from the sender to receiver
or 'down' for the receiver to sender. This allows each side to
maintain its own sequence number space in the 'seqnum'. This is
similar to the use of the sequence number in the DTLS record layer.
Like DTLS, ESIC control messages are unreliable, though they are
uniquely named.
The payload of the control Interest uses a TLV equivalent of the TLS
record format for handshake and alert messages. Application data is
never communicated in these records, as they use an Inner Packet with
a different Inner Name. Inside the payload, a TLV type of Alert (21)
or Handshake (22) indicates the purpose of the TLV value. One may
concatenate multiple records into one payload.
ControlName := ccnx:/localhost/esic/_direction_/SID=sid/SEQNUM=seqnum
4.1. ESIC Control Packets
A control packet is a CCNx Interest Inner Packet. The name of the
control packet is as above in the /localhost/esic namesapce. The
Payload of the Interest is the actual data.
The ESIC control packet SHOULD be padded out to a length that is
indistinguishable from other traffic in the given _direction_.
The Payload of the Interest contains a set of TLV records using the
normal CCNx TLV encoding. The TLV types and values are defined in
the next section.
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In the 'up' direction from the sender to the receiver, a control
packet can be inserted into the Interest stream as normal. The
receiver may use this extra outer name to return its own control
message or send a "no-op" back to consume the extra name.
In the 'down' direction from the receiver to the sender, there is no
pre-allocated outer name available. The receiver can only send the
sender a control message if the sender has outstanding Interests up
to the receiver. If there is one or more outstanding interests in
the outer name space, the receiver normally would send a Content
Object or Interest Return corresponding to some inner name. In this
case, the receiver would instead inject a control packet Interest in
the downstream. This means the receiver is now short one outer
Interest in the upstream direction. Therefore, whenever the
Deframer/Decrypter sees a control message in the downstream
direction, it MUST insert an upstream "no-op" packet, padded out to
statistically undetectable length, to give the receiver back a
missing name slot.
We allow one ESIC control packet in one outer packet. However, we
allow multiple Alert messages to be encoded in the payload, so long
as it remains indistinguishable from other packets in the given
_direction_.
Example from a sender to a receiver, where "NO" means "name outer"
and "NI" means "name inner".
Sender Receiver
| >------- NO1 : NI1 (Interest) --------> |
| >------- NO2 : NI2 (Interest) --------> |
| <------- NO1 : NI1 (ContentObject) ---< |
| >------- NO3 : NI /local/esic/up/2/1 -> |
| <------- NO3 : no-op -----------------< | (no-op)
| <------- NO2 : NI2 (ContentObject) ---< |
Here is an example from a receiver to a sender. The receiver uses
the second available name NO2 to send a control message to the
sender. The sender must then send a no-op packet back up to the
receiver so it can return the final data packet NI2 inside NO3.
Sender Receiver
| >------- NO1 : NI1 (Interest) --------> |
| >------- NO2 : NI2 (Interest) --------> |
| <------- NO1 : NI1 (ContentObject) ---< |
| <------- NO2 : NI /local/esic/dn/2/1 -< |
| >------- NO3 : -----------------------> | (no-op)
| <------- NO3 : NI2 (ContentObject) ---< |
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((TODO: Add examples with loss))
4.2. ESIC Control Messages
ESIC adopts the TLS 1.3 Alert Protocol for its control messages. The
TLV type of the message inside the control packet payload is taken
from the enum AlertDescription. As per TLS 1.3, fatal Alert messages
are an immediate close of the ESIC session.
As per TLS 1.3, each party MUST send a close_notify message closing
the write side of the connection. In ESIC, this means that when a
sender is done requesting data, it should send a final close_notify.
The receiver should then use this outer name to send back its own
close_notify. If for some reason the receiver must close before the
sender, it should inject its own close_notify discarding all
remaining data and the receiver should send back upstream a
close_notify.
The KeyUpdate messages function as per TLS 1.3 Sec 6.3.5.3. Either
side may generate a KeyUpdate message and begin transmitting with the
new key. The other side must update their own key and issue its own
KeyUpdate message.
5. Security Considerations
It may be possible for an observer to identify which outer packets
contain a control (alert) message if the ACK response time shows
significant statistical timing different from the normal flow of
messages.
6. References
6.1. Normative References
[CCNxKE] "CCNx Key Exchange Protocol Version 1.0", n.d.,
<https://github.com/parc/ccnx-keyexchange-rfc>.
[CCNxMessages]
Mosko, M., Solis, I., and C. Wood, "CCNx Semantics", n.d.,
<https://tools.ietf.org/html/draft-irtf-icnrg-
ccnxmessages-04>.
[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>.
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[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>.
[TLS13] RTFM, Inc, ., "The Transport Layer Security (TLS) Protocol
Version 1.3", n.d., <https://tools.ietf.org/html/draft-
ietf-tls-tls13-19>.
6.2. Informative References
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
DOI 10.17487/RFC5288, August 2008,
<http://www.rfc-editor.org/info/rfc5288>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<http://www.rfc-editor.org/info/rfc5389>.
Appendix A. Sample API
In this section we describe the ESIC API. Before doing so, we
highlight some details that molded the API for both senders and
receivers.
o Encrypted sessions are bound to names instead of addresses.
Consequently, in addition to a set of trusted keys, sessions
between a sender and receiver require only a name to be created.
o Sessions are created by an active sender with a passive peer
(receiver). Thus, the API must reflect these roles.
o Senders transmit and receive whole CCNx messages over a session.
Thus, simple read and write functions must be exposed via the API.
o Sessions are not full duplex by default. A receiver must specify
in its ServerConfiguration construct that it wishes to send
interests to the sender. To maintain transparency, the modality
of the resulting session is not reflected in the API.
These observations are distilled in the following ESIC API.
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# @Sender: create a secure session with a receiver
CCNxSecureSession *ccnxSecureSession_Connect(CCNxPortal *portal,
PARCIdentity *identity, CCNxName *servicePrefix);
# @Receiver: create a passive listener
CCNxSecureSession *ccnxSecureSession_CreateServer(CCNxPortal *portal,
CCNxKeyExchangeConfig *config, CCNxName *servicePrefix);
# @Receiver: accept uni- and bi-directional sessions
CCNxSecureSession *ccnxSecureSession_AcceptConnection(CCNxSecureSession *session);
CCNxSecureSession *ccnxSecureSession_AcceptBidirectionalConnection(CCNxSecureSession *session);
# Send a CCNx message
# Override the outer name with the `response` parameter if needed
void ccnxSecureSession_SendMessage(CCNxSecureSession *session,
CCNxTlvDictionary *message, const CCNxStackTimeout *timeout, CCNxName *response);
# Receive and decapsulate a CCNx message
# Store the outer name in the `response` parameter.
CCNxMetaMessage *ccnxSecureSession_ReceiveMessage(CCNxSecureSession *session,
const CCNxStackTimeout *timeout, CCNxName **response);
Authors' Addresses
Marc Mosko
PARC, Inc.
Email: marc.mosko@parc.com
Christopher A. Wood
University of California Irvine
Email: woodc1@uci.edu
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