icnrg M. Mosko
Internet-Draft E. Uzun
Intended status: Standards Track C. Wood
Expires: May 4, 2017 PARC
October 31, 2016
CCNx Key Exchange Protocol Version 1.0
draft-wood-icnrg-ccnxkeyexchange-01
Abstract
This document specifies Version 1.0 of the CCNx Key Exchange (CCNxKE)
protocol. The CCNxKE protocol allows two peers to establish a
shared, forward-secure key for secure and confidential communication.
The protocol is designed to prevent eavesdropping, tampering, and
message forgery between two peers. It is also designed to minimize
the number of rounds required to establish a shared key. In the
worst case, it requires two RTTs between a consumer and producer to
establish a shared key. In the best case, one RTT is required before
sending any application data. This document outlines how to derive
the keys used to encrypt traffic. An annex provides an example peer-
to-peer transport protocol for exchanging encrypted CCNx
communications.
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 May 4, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 5
2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Presentation Language . . . . . . . . . . . . . . . . . . . . 7
5. CCNxKE Overview . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Connection Establishment Latency . . . . . . . . . . . . 8
5.2. Connection Migration and Resumption . . . . . . . . . . . 8
5.3. Re-Transmissions, Timeouts, and Replay Prevention . . . . 8
5.4. Loss Sensitivity . . . . . . . . . . . . . . . . . . . . 9
6. The CCNxKE Protocol . . . . . . . . . . . . . . . . . . . . . 9
6.1. Round Overview . . . . . . . . . . . . . . . . . . . . . 10
6.2. Round 1 . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.3. Round 2 . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.4. Round 3 . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Alternative Exchanges . . . . . . . . . . . . . . . . . . . . 18
7.1. One-RTT Exchange . . . . . . . . . . . . . . . . . . . . 19
8. Resumption and PSK Mode . . . . . . . . . . . . . . . . . . . 20
9. Secret Derivation . . . . . . . . . . . . . . . . . . . . . . 21
9.1. SourceCookie Derivation . . . . . . . . . . . . . . . . . 21
9.2. Move Derivation . . . . . . . . . . . . . . . . . . . . . 21
9.3. SessionID and ResumptionCookie Properties, Derivation,
and Usage . . . . . . . . . . . . . . . . . . . . . . . . 22
9.4. Key Derivation . . . . . . . . . . . . . . . . . . . . . 23
9.5. Secret Generation and Lifecycle . . . . . . . . . . . . . 24
10. Re-Key Message . . . . . . . . . . . . . . . . . . . . . . . 25
11. Application Data Protocol . . . . . . . . . . . . . . . . . . 25
12. Security Considerations . . . . . . . . . . . . . . . . . . . 26
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
13.1. Normative References . . . . . . . . . . . . . . . . . . 26
13.2. Informative References . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
DISCLAIMER: This is a WIP draft of CCNxKE and has not yet seen
rigorous security analysis.
CCNx Key Exchange (CCNxKE) establishes ephemeral forward secure keys
between two peers, called the consumer (client) and producer
(server). The underlying cryptography of CCNxKE is similar to TLS
1.3, though there are some protocol changes due to the ICN nature of
CCNxKE. CCNxKE also supports the concept of a MoveToken, which
allows the authenticating producer to shift a session to one (or
more) co-operating replicas.
CCNxKE does not specify how the keys are used. It only specifies how
to derive the traffic secret that could be used to encrypt/decrypt
data. The draft [draft-wood-icnrg-tlvencap] specifies one way to use
the traffic secret to carry out communications in a session.
Annex A also sketches out an example CCNx protocol for exchanging
encrypted messages, though it is not part of this standard. Other
protocols may use CCNxKE.
For example, a producer and replica may use CCNxKE to establish a
shared key to use in Move Tokens. Two routers may use CCNxKE to
establish MACSEC keys. A consumer and publisher could establish a
symmetric key while on-line then publish content later for an off-
line consumer. In short, the use of CCNxKE is not limited to a TLS-
like transport protocol.
CCNxKE allows upper-layer data to be returned in Round 3, like TLS
1.3. In this sense, one can achieve 3 RTT (worst case) or 1 RTT
(best case) communcations. The data put in this response is up to
the protocol using CCNxKE and may or may not be used.
CCNxKE is not a substitue for data authenticity, such as Content
Object provenance via signatures, group encryption of cached objects,
or DRM protections. CCNxKE only creates a private, ephemeral tunnel
between a consumer and a producer. CCNxKE expects that the encrypted
communications protocol still carries normal CCNx packets with normal
CCNx attributes such as signatures.
Some types of ICN communications require emphemeral, forward secure
encryption. Typical examples are on-line banking, real-time voice,
or on-line shopping. Other applications may need different types of
encryption and thus not use CCNxKE. There is currently no standard
way for CCNx peers to exchange emphemeral, forward secure keys, thus
this RFC specifies the standard mechanism that should be used by all
CCNx peers for such keys. CCNxKE is built on the CCNx 1.0 protocol
and only relies upon standard Interest and Content Objects as a
vehicle for communication.
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In this document, the term 'CCNxKE session' refers to the key
exchange session. It does not refer to a transport protocol session
(like TLS) that uses the derived keys.
This protocol has the following four main properties:
- Each peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA [RSA], ECDSA [ECDSA], etc.).
Server authentication is mandatory whereas mutual authentication
is optional.
- The negotiation of a forward-secure shared secret is protected
from eavesdroppers and man-in-the-middle (MITM) attacks.
- The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties to
the communication.
- The state of a CCNxKE session can be securely migrated between an
endpoint performing authentication and that which provides content
using a "move token." This allows authentication and
authorization to be separated from encryption for a session,
enabling different systems to complete these steps.
Usage of CCNxKE is entirely independent of upper-layer application
protocols. CCNxKE may be used for any purpose that requires producer
authentication and shared emphemeral forward-secure keys.
CCNxKE also introduces a new type of cookie based on reverse hash
chains [HASHCHAIN] to help limit the amount of significant server
work done in response to a client or consumer Interest. TCP-based
protocols, such as TLS [TLS13], use the TCP 3-way handshake for such
proof. UDP-based protocols, such as QUIC [QUIC] and DTLS 1.2
[DTLS12], use an optional session address token or cookie that must
be presented by the client (consumer) to prove ownership of an
address during a key exchange procedure. Without source addresses,
our cookie technique ensures that the same entity which requested
server information, e.g., the public configuration data, is the same
entity that wishes to complete a key exchange.
The main contribution of this work is adapting key exchange
principles to the pull-based CCNx communication model. CCNxKE only
assumes that a consumer knows a first name prefix to initiate the key
exchange. The first Interest does not need to be a CCNxKE packet --
the producer can signal back to the consumer that it requires a
transport protocol using CCNxKE in the response.
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This specification does not subsume other ICN-compliant key exchange
protocols. Nor does its existence imply that all encryption in an
ICN must be based on sessions. It was designed specifically to solve
the problem of session-based encryption in ICN.
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:
Consumer/Client: The CCN consumer initiating the CCNxKE key exchange
via a first Interest.
Producer/Server: The CCN producer receiving or accepting the CCNxKE
key exchange request request Interest.
Sender: An endpoint that originates a message.
Receiver: An endpoint that is receiving messages.
Peer: An endpoint. When discussing a particular endpoint, "peer"
refers to the endpoint that is remote to the primary subject of
discussion.
Connection: A network path of n >= 1 hops between the consumer and
producer.
Endpoint: Either the consumer or producer of the connection.
Handshake: A series of message exchanges between two peers that is
used to perform a task (e.g., perform key exchange and derivation).
Session: An association between a consumer and a producer resulting
from a CCNxKE handshake.
DH: A Diffie Hellman key exchange procedure [RFC2631] [DH].
Key Share: One half of the shared-secret provided by one peer
performing a DH key exchange.
Forward-secure: The property that compromising any long-term secrets
(e.g., cryptographic keys) does not compromise any session keys
derived from those long-term secrets.
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CONFIG information: A data structure created by a producer which
contains long-term cryptographic material and associated information
needed by a client to initiate a key-exchange with the producer.
HELLO exchange: An exchange between a consumer and producer wherein
the consumer retrieves the CONFIG information from the producer.
Payload: The payload section of a CCNxMessage as defined in
[CCNxMessages].
KEPayload: A payload for information used in the CCNxKE protocol
which is a generic key-value store. The KEPayload is _not_ the
CCNxMessage payload.
CCNxName: A CCNxName as defined in [CCNxMessages].
Semi-static: Short-term.
Short-term Secret (SS): A secret which is derived from the server's
semi-static DH share and the client's fresh DH share.
Forward-secure Secret (FSK): A secret which is derived from fresh
(i.e., generated on demand at random) DH shares from both the
consumer and producer for the given connection.
HKDF: Hash-based key-derivation function [RFC5869].
2. Goals
The goals of the CCNxKE protocol, in order of priority, are as
follows:
1. Cryptographic security: CCNxKE should be used to securely
establish a session and all related shared secrets between two
peers. Cryptographic properties of interest include: (a)
forward-secure session key derivation and (b) (state and
computational) denial-of-service prevention at the producer (see
[RFC4987]) that is no worse than DTLS 1.2 [DTLS12]}. For property
(a), different keys (and relevant algorithm parameters such as
IVs) are established for each communication direction, i.e., from
consumer to producer and producer to consumer. For property (b),
we use a new type of stateless cookie inspired by that of DTLS
1.2.
2. Interoperability: Independent programmers should be able to
develop applications utilizing CCNxKE that can successfully
exchange cryptographic parameters without knowledge of one
another's code.
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3. Extensibility: CCNxKE seeks to provide a framework into which new
public key and symmetric key methods and algorithms can be
incorporated without breaking backwards compatibility or
requiring all clients to implement new functionality. Moreover,
the protocol should be able to support a variety of peer
authentication protocols, e.g., EAP-TLS, EAP-PWD, or a simple
challenge-response protocol.
4. Relative efficiency: CCNxKE tries to create sessions with minimal
computation, bandwidth, and message complexity. In particular,
it seeks to create sessions with as few end-to-end round trips as
possible, and also provide support for accelerated session
establishment and resumption when appropriate. At most 2 round-
trip-times (RTTs) should be used to establish a session key, with
the possibility of 1-RTT accelerated starts and resumption.
3. Scope
This document and the CCNxKE protocol are influenced by the TLS 1.3
[TLS13], QUIC [QUIC], and DTLS 1.2 [DTLS12] protocols. The reader,
however, does not need a detailed understanding of those protocols to
understand this document. Moreover, where appropriate, references to
related protocols are made for brevity and technical clarity. This
document is intended primarily for readers who will be implementing
the protocol and for those doing cryptographic analysis of it. The
specification has been written with this in mind and it is intended
to reflect the needs of those two groups.
Unlike TLS, this document does not specify the transport protocol.
It specifies the establishment of a session ID and shared keys.
Other documents specify the use of CCKxKE within a transport
protocol.
This document is not intended to supply any details of service
definition or of interface definition, although it does cover select
areas of policy as they are required for the maintenance of solid
security.
4. Presentation Language
This document uses a presentation language of remote calls (i.e.
packet messages) similar to the format used by TLS [TLS13].
5. CCNxKE Overview
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5.1. Connection Establishment Latency
CCNxKE operates in three rounds, where each round requires a single
RTT to complete. The full execution of the protocol therefore
requires 2 RTTs before a session is fully established. The full
version is used when consumers have no a priori information about the
producer. An accelerated one round version is used when the consumer
has valid configuration information and a source cookie from the
producer; this variant requires 1 RTT before a session is
established.
5.2. Connection Migration and Resumption
CCN end hosts lack the notion of addresses. Thus, the producer
endpoint for a given execution of the CCNxKE protocol is one which
can authoritatively serve as the owner of a particular namespace.
For example, a consumer may wish to establish a session with a
producer who owns the /company/foo namespace. The specific end host
which partakes in the protocol instance is not specified, by virtue
of the fact that all CCNxKE messages are based on well-defined names.
This enables the producer end-host which partakes in the protocol to
change based on the name of the CCNxKE messages. Consequently, to
maintain correctness, it is important that a single execution of the
protocol operates within the same trusted context; this does not mean
that the same producer end-host is required to participate in all
three steps of the protocol. Rather, it means that the end-host
responding to a CCNxKE message must be trusted by the consumer to
complete the exchange. CCNxKE is designed to enable this sort of
producer migration.
For example, a consumer may use an initial name like '/parc/
index.html' that works like an IP any cast address and could got to
one of several systems. CCNxKE allows the responding endpoint to
include a localized name to ensure that subsequent messages from the
consumer come back to the same producer. CCNxKE also allows the key
exchange peer to securely hand-off the session to a content producer
peer via another name and session token once the client is
authenticated and keying material is exchanged.
5.3. Re-Transmissions, Timeouts, and Replay Prevention
CCNxKE timeouts and retransmissions are handled using the approach in
[RFC6347]. One primary difference is that timer values may need to
be adjusted (elongated) due to prefix shifts and the need for a
producer to transfer security information between different machines.
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Replay attack prevention is also an optional feature, and if used,
MAY be done using one of the following two approaches at the receiver
(producer):
- IPSec AH [RFC4302] and ESP [RFC4303] style replay detection based
on sliding windows and monotonically increasing sequence numbers
for windows. Note that the sliding window inherently limits the
performance of the protocol to the window size, since only a
finite number of messages may be received within a given window
(based on the window size).
- The optimized anti-replay algorithm of [RFC6479].
5.4. Loss Sensitivity
CCNxKE messages are transferred using standard CCN Interest and
Content Objects and are therefore subject to loss as any datagram.
This means that traffic encrypted with keys derived from CCNxKE must
be stateless. They cannot depend on in-order arrival. This problem
is solved by two mechanisms: (1) by prohibiting stream ciphers of any
kind and (2) adding sequence numbers to each message that allow the
receiver to identify and use the correct cryptographic state to
decrypt the message. Moreover, sequence numbers permit anti-replay
mechanisms similar to those used in DTLS [DTLS12] as mentioned above.
6. The CCNxKE Protocol
This section describes the CCNxKE protocol in detail at the message
level. The specific encoding of those messages is given later.
CCNxKE could be adapted to different wire format encodings, such as
those used by the NDN protocol.
The following assumptions are made about peers participating in the
CCNxKE protocol:
- Consumers know the namespace prefix of the producer for which they
wish to execute the CCNxKE protocol.
- CCNxKE protocol information is carried in a distinguished field
outside of the payload of CCN messages. This is done to
distinguish key exchange material with application data in a
message. This is necessary for 1 RTT packets that carry both
keying material and application payload.
- CCNxKE does not require any special behavior of intermediate
systems to forward packets.
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- CCNxKE packets generally should not be cached for significant
periods of time, as use normal protocol methods to limit caching.
Part of this is achieved through the use of consumer-specific
nonces in names.
6.1. Round Overview
CCNxKE is composed of three rounds. The purpose of each round is
described below.
- Round 1: Perform a bare HELLO exchange to obtain the extensions
(parameters) for the key exchange provided by the producer and a
source cookie to prove ownership of the "source" of the request.
- Round 2: Perform the initial FULL-HELLO exchange to establish a
forward-secure key used for future communication, i.e., Interest
and Content Object exchanges in the context of the newly
established session.
- Round 3: Send the first bit of application data and (optionally)
transfer resumption cookie(s) from the producer to the consumer.
Conceptually, there are two secrets established during a single
execution of CCNxKE:
- Static Secret (SS): A secret which is derived in one of two ways:
(a) from the client and server ephemeral key shares and (b) from
the server's semi-static share and the client's ephemeral key
share. Keying material derived from SS in option (a) is not
forward secure.
- Ephemeral Secret (ES): A secret which is derived from both the
client and server ephemeral key shares.
Depending on the mode in which CCNxKE is used, these secrets can be
established in a variety of ways. Key derivation details are
outlined in Section Section 9.
All secrets are derived with the appropriate amount of randomness
[RFC4086]. An overview of the messages sent in each of the three
rounds to establish and use these secrets is shown in Figure Figure 1
below. This diagram omits some parts of each message for brevity.
Consumer Producer
HELLO:
+ SourceChallenge
I[/prefix/random-1]
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-------->
HELLO-REJECT:
+ Timestamp
+ SourceCookie
+ pinned-prefix*
+ ServerChallenge*
+ ServerConfiguration*
CO[/prefix/random-1]
<---------
FULL-HELLO:
+ ClientKeyShare
+ SourceCookie
+ SourceProof
+ Timestamp
I[/pinned-prefix/random-2]
-------->
HELLO-ACCEPT:
+ ServerKeyShare
+ SessionID
+ [CertificateRequest*]
+ [CertificateVerify*]
+ [MovePrefix*, MoveToken)*]
+ [Finished]
CO[/pinned-prefix/random-2]
<--------
**key exchange complete**
Payload:
+ MoveToken*
+ MoveProof*
+ [ConsumerData]
I[/prefix/SessionID/[...]]
-------->
+ NewSessionID*
+ NewSessionIDTag*
Payload:
[ProducerData]
CO[/prefix/SessionID/[...]]
<--------
Repeat with data <--------> Repeat with data
* Indicates optional or situation-dependent
messages that are not always sent.
{} Indicates messages protected using keys
derived from the short-term secret (SS).
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() Indicates messages protected using keys
derived from the ephemeral secret (ES).
[] Indicates messages protected using keys
derived from the traffic secret (TS).
Figure 1: High-level message flow for full CCNxKE protocol with a
maximum 2-RTT delay.
In the following sections, we will describe the format of each round
in this protocol in more detail.
We do not specify the encoding of CCNxKE data sent in Interest and
Content Object payloads. Any viable encoding will suffice, so long
as both parties agree upon the type. For example, the payload could
be structured and encoded as a JSON object, e.g.,
{ "ClientKeyShare" : 0xaa, "SourceCookie" : 0xbb, "SourceProof" :
0xbb, ... }
For now, we assume some valid encoding mechanism is used to give
structure to message payloads. Moreover, we assume that these
payloads are carried in a distinguished CCNxKE payload field
contained in the Interest and Content Objects.
6.2. Round 1
The purpose of Round 1 is to acquire a cookie to binding the exchange
to the initial consumer and the public configuration information
contained in the ServerConfiguration structure. This information is
used in the second round when performing the actual key exchange. To
that end, the format of the Round 1 message is trivial. First, the
client issues an Interest with the following name
/prefix/random-1
where random-1 is a randomly generated 64-bit nonce. This interest
carries a KEPayload with the following information:
+-----------------+-------------------------------------+-----------+
| HELLO Field | Description | Optional? |
+-----------------+-------------------------------------+-----------+
| SourceChallenge | A random value generated to prove | No |
| | ownership of the consumer's | |
| | "source" | |
+-----------------+-------------------------------------+-----------+
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Upon receipt of this interest, the producer responds with a HELLO-
REJECT Content Object whose KEPayload has the following fields:
+---------------------+---------------------------------+-----------+
| HELLO-REJECT Field | Description | Optional? |
+---------------------+---------------------------------+-----------+
| Timestamp | Current server timestamp | No |
| | | |
| SourceCookie | A cookie that binds the | No |
| | consumer's challenge to the | |
| | current timestamp | |
| | | |
| PinnedPrefix | A new prefix that pins the key | Yes |
| | exchange to a particular server | |
| | | |
| ServerConfiguration | The public server configuration | Yes |
| | information | |
| | | |
| ServerChallenge | A random value for the consumer | Yes |
| | to include in its | |
| | CertificateVerify if the server | |
| | requires client authentication | |
+---------------------+---------------------------------+-----------+
The Timestamp and SourceCookie are used in Round 2. Their derivation
is described later. If the server provides a PinnedPrefix then the
consumer must use this prefix in Round 2 in lieu of the Round 1 name
prefix. (This is because the PinnedPrefix identifies a particular
endpoint that is capable of completing the key exchange.)
The ServerConfiguration information is a semi-static catalog of
information that consumers may use to complete future key exchanges
with the producer. The fields of the ServerConfiguration information
are shown below.
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+---------------------+---------------------------------+-----------+
| ServerConfiguration | Description | Optional? |
| Field | | |
+---------------------+---------------------------------+-----------+
| KEXS | Supported elliptic-curve key- | No |
| | exchange algorithms | |
| | | |
| AEAD | Supported AEAD algorithms | No |
| | | |
| PUBS | List of public values (for key | No |
| | exchange algorithm) encoded | |
| | appropriately for the given | |
| | group | |
| | | |
| EXPRY | Expiration timestamp (i.e., | No |
| | longevity of the | |
| | ServerConfiguration structure) | |
| | | |
| VER | Version of the CONFIG structure | Yes |
| | | |
| CERT | Server certificate | No |
| | | |
| SIG | Signature produced by the | No |
| | server over the entire | |
| | ServerConfiguration message | |
+---------------------+---------------------------------+-----------+
The KEXS is a data structure that enumerates the elliptic curve key-
exchange algorithms that are supported by the producer (see [QUIC]
for more details). Currently, only the following curves are
supported:
- Curve25519
- P-256
Selection criteria for these curves is given at
http://safecurves.cr.yp.to/.
The AEAD structure enumerates the supported AEAD algorithms used for
symmetric-key authenticated encryption after the session has been
established. Currently, the only supported algorithms are:
- AES-GCM-(128,192,256) [GCM]: a 12-byte tag is used, where the
first four bytes are taken from the FSK key-derivation step and
the last eight are taken from the initial consumer nonce.
- Salsa20 [SALSA20] (stream cipher) with Poly1305 (MAC).
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The key sizes and related parameters are provided with the AEAD tag
in the CONFIG structure.
The PUBS structure contains the public values for the initial key
exchange. Both Curve25519 and P-256 provide their own set of
accepted parameters. Thus, the only values provided here are the
random curve elements used in the DH operation.
The EXPRY value is an absolute timestamp that indicates the longevity
of the ServerConfiguration.
The CERT and SIG values contain the server's certificate and a
signature generated over the entire ServerConfiguration field. This
signature is generated with the corresponding private key.
6.3. Round 2
The purpose of Round 2 is to perform the initial FULL-HELLO exchange
to establish a forward-secure key used for future communication. It
is assumed that the consumer already has the ServerConfiguration
information that is provided from the producer in Round 1. It is
also assumed that the consumer has a
Moreover, assume that nonce2 is a ephemeral nonce provided by the
producer in Round 1. Then, the consumer issues an Interest with the
following name:
/prefix/random-2
and a KEPayload with the following information:
+----------------------+--------------------------------+-----------+
| FULL-HELLO Field | Description | Optional? |
+----------------------+--------------------------------+-----------+
| ClientKeyShare | The client's key share for the | No |
| | key exchange | |
| | | |
| SourceCookie | SourceCookie provided by the | No |
| | server in Round 1 | |
| | | |
| SourceProof | The SourceCookie construction | No |
| | proof provided by the client | |
| | | |
| Timestamp | The timestamp provided by the | No |
| | server in Round 1 | |
| | | |
| ConsumerPrefix | The consumer's prefix that can | Yes |
| | be used for the producer to | |
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| | send interests to the consumer | |
| | | |
| PreSharedKey | A pre-shared key that can be | Yes |
| | configured between a consumer | |
| | and producer | |
| | | |
| ResumptionCookie | The ResumptionCookie derived | Yes |
| | from a past session | |
| | | |
| {MoveChallenge} | A move challenge generated | Yes |
| | identically to the | |
| | SourceChallenge | |
| | | |
| {AlgChoice} | Algorithm (KEXS and AEAD) | No |
| | options choice (a list of tags | |
| | echoed from the | |
| | ServerConfiguration) | |
| | | |
| {Proof} | Proof of demand (i.e., a | No |
| | sorted list of types of proof | |
| | the consumer will expect) | |
| | | |
| {CCS} | Compressed certificate set | No |
| | that the consumer possesses | |
| | | |
| {ConsumerData} | Application data encrypted | Yes |
| | under a key derived from SS | |
| | (in a 1-RTT exchange) | |
| | | |
| ServerNameIndication | A server name indication (as a | Yes |
| | CCNxName) defined in Section 3 | |
| | of [RFC6066] | |
| | | |
| Certificate | The client's certificate | Yes |
| | | |
| CertificateVerify | A signature generated over the | Yes |
| | entire FULL-HELLO message | |
+----------------------+--------------------------------+-----------+
((TODO: provide more details about each of these fields))
Upon receipt of this interest, the producer performs the DH
computation to compute ES and SS, decrypts all protected fields in
the consumer's KEPayload, and validates the algorithm choice
selection (AlgChoice). If any of these steps fail, the producer
replies with with a HELLO-REJECT Content Object whose KEPayload
contains a REJ flag and the reason of the error. The REJ flag and
value are encrypted by the SS (if possible).
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If the above steps complete without failure or error, then the
producer responds with a Content Object whose KEPayload has the
following fields:
+--------------------------+----------------------------+-----------+
| HELLO-ACCEPT Field | Description | Optional? |
+--------------------------+----------------------------+-----------+
| SessionID | Cleartext session | No |
| | identifier | |
| | | |
| ServerKeyShare | Server's key share for the | No |
| | ES derivation | |
| | | |
| {ServerExtensions} | Additional extensions | Yes |
| | provided by the server, | |
| | encrypted under ES | |
| | | |
| [ResumptionCookie] | Resumption cookie | Yes |
| | encrypted under a TS- | |
| | derived key | |
| | | |
| {(MovePrefix,MoveToken)} | Third CCNxName prefix and | Yes |
| | token to use when moving | |
| | to session establishment | |
| | | |
| CertificateRequest* | Server certificate that | Yes |
| | matches the type of proof | |
| | provided by the client | |
| | | |
| CertificateVerify* | Signature generated over | Yes |
| | the entire HELLO-ACCEPT | |
| | message | |
+--------------------------+----------------------------+-----------+
If a MovePrefix and MoveToken tuple is provided then in the HELLO-
ACCEPT message then a CertificateVerify (signature) MUST also be
provided in the response.
6.4. Round 3
In Round 3, the consumer sends interests whose name and optional
Payload are encrypted using one of the forward-secure keys derived
after Round 2. In normal operation, the producer will respond with
Content Objects whose Payloads are encrypted using a different
forward-secure key. That is, interests and Content Objects are
encrypted and authenticated using two separate keys. The producer
may also optionally provide a new resumption cookie (RC) with a
Content Object response. This is used to keep the consumer's
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resumption cookie fresh and to also support 0 RTT resumption. In
this case, the producer's Content Object response has the following
fields:
- Payload: the actual Content Object payload data encrypted with the
producer's forward-secure key.
- ResumptionCookie: A new resumption cookie to be used for resuming
this session in the future.
The producer is free to choose the frequency at which new resumption
cookies are issued to the consumer.
The producer may also reply with a new SessionID. This is done if
the client presented a MoveToken and MoveProof. A NewSessionID must
be accompanied with a NewSessionIDTag, which is equal to the HMAC of
NewSessionID computed with the traffic-secret key. A client MUST
then use NewSessionID instead of SessionID after verifying the
NewSessionIDTag.
7. Alternative Exchanges
CCNxKE also supports one-round key exchange and session resumption.
These variants are outlined below. The key material differences are
described later. In these variants, we use message
ExchangeSourceCookie to denote the following exchange:
Consumer Producer
HELLO:
+ SourceChallenge
I[/prefix/random-1]
-------->
HELLO-REJECT:
+ Timestamp
+ SourceCookie
ServerChallenge*
ServerConfiguration*
CO[/prefix/random-1]
<---------
Figure 2: SourceCookie exchange -- ExchangeSourceCookie.
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7.1. One-RTT Exchange
Consumer Producer
-------->
ExchangeSourceCookie
<---------
FULL-HELLO:
+ ClientKeyShare
+ SourceCookie
+ SourceProof
+ Timestamp
+ Certificate*
+ CertificateVerify*
+ {ConsumerData*}
I[/prefix/random-2]
-------->
HELLO-ACCEPT:
+ ServerKeyShare
+ SessionID
+ [ServerExtensions]
+ [ResumptionCookie]
+ [CertificateRequest*]
+ [CertificateVerify*]
+ [MovePrefix*, MoveToken*]
+ [Finished]
CO[/prefix/random-2]
<--------
**key exchange complete**
Send encrypted data <--------> Send encrypted data
* Indicates optional or situation-dependent
messages that are not always sent.
{} Indicates messages protected using keys
derived from the short-term secret (SS).
() Indicates messages protected using keys
derived from the ephemeral secret (ES).
[] Indicates messages protected using keys
derived from the traffic secret (TS).
Figure 3: Exchange with 1 RTT.
As with TLS, the initial application data is protected with the
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8. Resumption and PSK Mode
In this mode, the client uses its ResumptionCookie to re-create a
previous session. The client also provides a key share in case the
server opts to fall back and establish a fresh key. If the server
accepts the ResumptionCookie then it MUST issue a new SessionID and
ResumptionCookie for future use with the client.
Consumer Producer
-------->
ExchangeSourceCookie
<---------
FULL-HELLO:
+ ClientKeyShare
+ SourceCookie
+ SourceProof
+ Timestamp
+ PreSharedKey
+ ResumptionCookie
I[/prefix/random-2]
-------->
HELLO-ACCEPT:
+ ServerKeyShare
+ SessionID
+ [ServerExtensions]
+ [ResumptionCookie]
+ [MovePrefix*, MoveToken*]
+ [Finished]
CO[/prefix/random-2]
<--------
**key exchange complete**
Send encrypted data <--------> Send encrypted data
* Indicates optional or situation-dependent
messages that are not always sent.
{} Indicates messages protected using keys
derived from the short-term secret (SS).
() Indicates messages protected using keys
derived from the ephemeral secret (ES).
[] Indicates messages protected using keys
derived from the traffic secret (TS).
Figure 4: Exchange with 1 RTT.
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9. Secret Derivation
In this section we describe how secrets used in the protocol are
derived. We cover the SourceCookie, MoveToken, SessionID,
ResumptionCookie, and the actual traffic keys.
9.1. SourceCookie Derivation
The intention of the SourceCookie is to prove that a client is
sending interests from a legitimate location before any server
computation is done. Without this, a Denial of Service attack could
be carried out by sending interests to the server with the intention
of triggering wasted computation. TCP-based protocols prevent this
with the SYN-flood cookie mechanism. Protocols based on UDP use
cookies that bind to the client address [DTLS12]. Since CCN lacks
any notion of a source address, these cookie mechanisms do not apply.
Instead, we need a way for clients to prove that they initiated a key
exchange from the "same origin." We now describe the cookie
mechanism that gives us this guarantee.
Instead of a source address, a SourceCookie is computed using a
challenge provided by a consumer. To create this challenge, a
consumer first generates a a randomly generated 256-bit string X.
The consumer then computes SourceChallenge = SHA256(X). Upon receipt
of this challenge, the producer generates a SourceCookie as follows:
SourceCookie = HMAC(k, SourceChallenge || timestamp)
where timestamp is the current server timestamp and k is the server's
secret key. To prove ownership of the "source," the consumer then
provides the SourceCookie and a SourceProof in the round 2 Interest.
The SourceProof is set to the value X used to derive the
SourceChallenge. Upon receipt of the SourceProof, the server
verifies the following equality:
SourceCookie = HMAC(k, SHA256(SourceProof) || timestamp)
If this check passes, then the server continues with the
computationally expensive part of the key exchange protocol.
9.2. Move Derivation
The MoveChallenge and MoveProof are computed identically to the
SourceChallenge and SourceProof. The MoveToken, however, is left as
an opaque bit string. Extensions may be specified to describe how to
compute this value.
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9.3. SessionID and ResumptionCookie Properties, Derivation, and Usage
The purpose of the session identifier SessionID is to uniquely
identify a single session for the producer and consumer. A Producer
MAY use a random bit string or MAY use the method described in this
section or MAY use another proprietary method to distinguish clients.
We provide a more secure creation of the SessionID since it is used
with the ResumptionCookie derivation (defined later). Specifically,
the SessionID is derived as the encryption of the hash digest of a
server secret, TS, and an optional prefix (e.g., MovePrefix).
Encryption is done by the using a long-term secret key owned by the
server used for only this purpose, i.e., it is not used for consumer
traffic encryption. Mechanically, this derivation is:
SessionID = Enc(k1, H(TS || (Prefix3))),
where k1 is the long-term producer key.
For the resumption cookie, we require that it must be able to be used
to recover the TS for a given session. Without TS, correct session
communication is not possible. We derive it as the encryption of the
hash digest of the server secret, TS, and the optional (MovePrefix,
MoveToken) tuple (if created for the session). The producer must use
a long-term secret key for this encryption. Mechanically, this
derivation is:
ResumptionCookie = Enc(k2, TS || ( (Prefix3 || MoveToken) )),
where k2 is again a long-term producer key. Note that it may be the
case that k1 = k2 (see above), though this is not required.
With this SessionID and ResumptionCookie, the consumer then resumes a
session by providing both the SessionID and ResumptionCookie to the
producer. This is done to prove to the producer that the consumer
who knows the SessionID is also in possession of the correct
ResumptionCookie. The producer verifies this by computing
(TS || ( (Prefix3 || MoveToken) )) = Dec(k2, ResumptionCookie)
and checking the following equality
SessionID = Enc(k1, H(TS || (Prefix3)))
If equality holds, the producer uses the TS recovered from
ResumptionCookie to re-initialize the previous session with the
consumer.
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9.4. Key Derivation
CCNxKE adopts the key schedule and derivation techniques defined in
TLS 1.3 [TLS13]. Specifically, it uses the SS and ES to establish a
common master secret (MS) and, from that, the traffic secret (TS).
These dependencies are shown below.
+------+ +------+
| KE-1 | | KE-2 |
+------+ +----+-+
| |
| |
| |
+---v--+ +----v-+
| SS +---+ +--+ ES |
+------+ | | +------+
| |
| |
+-v----v-|
| MK |
+---+----+
|
|
|
+-v--+
| TS |
+----+
In this figure, KE-1 and KE-2 are two "sources" of keying material.
The following table shows what these two sources are in different key
exchange scenarios.
+-------------+------------------------------+----------------------+
| Key | KE-1 | KE-2 |
| Exchange | | |
+-------------+------------------------------+----------------------+
| Full | ClientKeyShare and | ClientKeyShare and |
| handshake | ServerKeyShare DH | ServerKeyShare DH |
| | | |
| Handshake | ClientKeyShare and | ClientKeyShare and |
| with 1-RTT | ServerConfiguration public | ServerKeyShare DH |
| | share DH | |
| | | |
| PSK | Pre-shared key | Pre-shared key |
+-------------+------------------------------+----------------------+
Given the values for SS and ES, the remaining derivation steps are
below as defined in [TLS13]. They are repeated here for posterity.
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1. xSS = HKDF-Extract(0, SS). Note that HKDF-Extract always
produces a value the same length as the underlying hash function.
2. xES = HKDF-Extract(0, ES)
3. mSS = HKDF-Expand-Label(xSS, "expanded static secret",
handshake_hash, L)
4. mES = HKDF-Expand-Label(xES, "expanded ephemeral secret",
handshake_hash, L)
5. master_secret = HKDF-Extract(mSS, mES)
6. traffic_secret_0 = HKDF-Expand-Label(master_secret, "traffic
secret", handshake_hash, L)
In all computations, the value "handshake_hash" is defined as the
SHA256 hash digest of all CCNxKE messages contained up to the point
of derivation. More details are given in Section 7.3.1 of [TLS13].
Updating the traffic secret using the re-key message (defined later)
increments traffic_secret_N to traffic_secret_(N+1). This update
procedure works as follows:
traffic_secret_N+1 = HKDF-Expand-Label(traffic_secret_N, "traffic
secret", "", L)
9.5. Secret Generation and Lifecycle
The secrets (keys and IVs) used to encrypt and authenticate traffic
are derived from the traffic secret. The explicit derivation
formula, as is defined in [TLS13], is as follows:
secret = HKDF-Expand-Label(Secret, phase + ", " + purpose,
handshake_context, key_length)
In this context, secret can be a key or IV. This formula is used
when deriving keys based on a non-forward-secure SS and the forward-
secure TS. The following table enumerates the values for "phase",
and "handshake_context" to be used when defining keys for different
purposes.
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+-------------+--------+------------------+-------------------------+
| Record Type | Secret | Phase | Handshake Context |
+-------------+--------+------------------+-------------------------+
| 1-RTT | xSS | "early handshake | HELLO + |
| Handshake | | key expansion" | ServerConfiguration + |
| | | | Server Certificate |
| | | | |
| 1-RTT Data | xSS | "early | HELLO + |
| | | application data | ServerConfiguration + |
| | | key expansion" | Server Certificate |
| | | | |
| Application | TS | "application | HELLO ... Finished |
| Data | | data key | |
| | | expansion" | |
+-------------+--------+------------------+-------------------------+
Moreover, the following table indicates the values of "purpose" used
in the generation of each secret.
+------------------+--------------------+
| Secret | Purpose |
+------------------+--------------------+
| Client Write Key | "client write key" |
| | |
| Server Write Key | "server write key" |
| | |
| Client Write IV | "client write IV" |
| | |
| Server Write IV | "server write IV" |
+------------------+--------------------+
(( TODO: should we add examples for each of the above variants? ))
10. Re-Key Message
Either the client and server can trigger a key update by sending an
Interest or Content Object with a KEPayload field containing the flag
KeyUpdate. The KEPayload will be encrypted by the traffic key. Upon
receipt, the recipient MUST update the traffic secret as defined
above and re-compute the traffic encryption and authentication keys.
The previous traffic key must be securely discarded.
11. Application Data Protocol
Once traffic keys and the associated IVs are derived from the CCNxKE
protocol, all subsequent Interest and Content Object messages are
encrypted. Packet encryption uses the TLV encapsulation mechanism
specified in [TLVENCAP]. For Interest encryption, the Salt in
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[TLVENCAP] is set to the packet sequence number. The same
substitution is done for Content Object encryption. Similarly, the
KeyId field is substituted with the SessionID derived by the CCNxKE
protocol. Packet sequence numbers are 64-bit numbers initialized to
0 when after the traffic secret is calculated. Each message
increments and uses the sequence number when sending a new datagram
(Interest). The sequence number for an Interest matches that of the
Content Object response.
12. Security Considerations
For CCNxKE to be able to provide a secure connection, both the
consumer and producer systems, keys, and applications must be secure.
In addition, the implementation must be free of security errors.
The system is only as strong as the weakest key exchange and
authentication algorithm supported, and only trustworthy
cryptographic functions should be used. Short public keys and
anonymous servers should be used with great caution. Implementations
and users must be careful when deciding which certificates and
certificate authorities are acceptable; a dishonest certificate
authority can do tremendous damage.
13. References
13.1. Normative References
[CCNxMessages]
Mosko, M. and I. Solis, "CCNx Messages in TLV Format",
January 2016, <https://tools.ietf.org/html/draft-irtf-
icnrg-ccnxmessages-01>.
[DH] Diffie, W. and M. Hellman, "New Directions in
Cryptography", IEEE Transactions on Information Theory,
V.IT-22 n.6 , June 1977.
[DTLS12] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", January 2012,
<https://tools.ietf.org/html/rfc6347>.
[ECDSA] American National Standards Institute, "Public Key
Cryptography for the Financial Services Industry: The
Elliptic Curve Digital Signature Algorithm (ECDSA)",
ANSI ANS X9.62-2005, November 2005.
[GCM] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC",
NIST Special Publication 800-38D, November 2007.
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[QUIC] Iyengar, J. and I. Swett, "QUIC: A UDP-Based Secure and
Reliable Transport for HTTP/2", December 2015.
[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>.
[RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method",
RFC 2631, DOI 10.17487/RFC2631, June 1999,
<http://www.rfc-editor.org/info/rfc2631>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<http://www.rfc-editor.org/info/rfc4302>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<http://www.rfc-editor.org/info/rfc4303>.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<http://www.rfc-editor.org/info/rfc4987>.
[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>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC6479] Zhang, X. and T. Tsou, "IPsec Anti-Replay Algorithm
without Bit Shifting", RFC 6479, DOI 10.17487/RFC6479,
January 2012, <http://www.rfc-editor.org/info/rfc6479>.
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[RSA] Rivest, R., Shamir, A., and L. Adleman, "A Method for
Obtaining Digital Signatures and Public-Key
Cryptosystems", Communications of the ACM v. 21, n. 2, pp.
120-126., February 1978.
[SALSA20] Bernstein, D., "Salsa20 specification",
www.http://cr.yp.to/snuffle/spec.pdf , April 2005.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", December 2015, <https://tools.ietf.org/html/
draft-ietf-tls-tls13-11>.
[TLVENCAP]
Mosko, M. and C. Wood, "CCNx Packet Encapsulation", n.d.,
<https://github.com/PARC/ccnx-tlvencap-rfc>.
13.2. Informative References
[HASHCHAIN]
L. Lamport, "Password Authentication with Insecure
Communication", ANSI Communications of the ACM 24.11, pp
770-772, November 1981.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <http://www.rfc-editor.org/info/rfc5077>.
Authors' Addresses
M. Mosko
PARC
EMail: marc.mosko@parc.com
Ersin Uzun
PARC
EMail: ersin.uzun@parc.com
Christopher A. Wood
PARC
EMail: christopher.wood@parc.com
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