Internet DRAFT - draft-brockners-proof-of-transit
draft-brockners-proof-of-transit
Network Working Group F. Brockners
Internet-Draft S. Bhandari
Intended status: Experimental S. Dara
Expires: November 8, 2018 C. Pignataro
Cisco
J. Leddy
Comcast
S. Youell
JPMC
D. Mozes
T. Mizrahi
Marvell
May 7, 2018
Proof of Transit
draft-brockners-proof-of-transit-05
Abstract
Several technologies such as Traffic Engineering (TE), Service
Function Chaining (SFC), and policy based routing are used to steer
traffic through a specific, user-defined path. This document defines
mechanisms to securely prove that traffic transited said defined
path. These mechanisms allow to securely verify whether, within a
given path, all packets traversed all the nodes that they are
supposed to visit.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 8, 2018.
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Proof of Transit . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Basic Idea . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Solution Approach . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Setup . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. In Transit . . . . . . . . . . . . . . . . . . . . . 7
3.2.3. Verification . . . . . . . . . . . . . . . . . . . . 7
3.3. Illustrative Example . . . . . . . . . . . . . . . . . . 7
3.3.1. Basic Version . . . . . . . . . . . . . . . . . . . . 7
3.3.1.1. Secret Shares . . . . . . . . . . . . . . . . . . 8
3.3.1.2. Lagrange Polynomials . . . . . . . . . . . . . . 8
3.3.1.3. LPC Computation . . . . . . . . . . . . . . . . . 8
3.3.1.4. Reconstruction . . . . . . . . . . . . . . . . . 9
3.3.1.5. Verification . . . . . . . . . . . . . . . . . . 9
3.3.2. Enhanced Version . . . . . . . . . . . . . . . . . . 9
3.3.2.1. Random Polynomial . . . . . . . . . . . . . . . . 9
3.3.2.2. Reconstruction . . . . . . . . . . . . . . . . . 10
3.3.2.3. Verification . . . . . . . . . . . . . . . . . . 10
3.3.3. Final Version . . . . . . . . . . . . . . . . . . . . 11
3.4. Operational Aspects . . . . . . . . . . . . . . . . . . . 11
3.5. Alternative Approach . . . . . . . . . . . . . . . . . . 12
3.5.1. Basic Idea . . . . . . . . . . . . . . . . . . . . . 12
3.5.2. Pros . . . . . . . . . . . . . . . . . . . . . . . . 12
3.5.3. Cons . . . . . . . . . . . . . . . . . . . . . . . . 12
4. Sizing the Data for Proof of Transit . . . . . . . . . . . . 12
5. Node Configuration . . . . . . . . . . . . . . . . . . . . . 13
5.1. Procedure . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2. YANG Model . . . . . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7. Manageability Considerations . . . . . . . . . . . . . . . . 17
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8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8.1. Proof of Transit . . . . . . . . . . . . . . . . . . . . 18
8.2. Cryptanalysis . . . . . . . . . . . . . . . . . . . . . . 18
8.3. Anti-Replay . . . . . . . . . . . . . . . . . . . . . . . 19
8.4. Anti-Preplay . . . . . . . . . . . . . . . . . . . . . . 19
8.5. Anti-Tampering . . . . . . . . . . . . . . . . . . . . . 20
8.6. Recycling . . . . . . . . . . . . . . . . . . . . . . . . 20
8.7. Redundant Nodes and Failover . . . . . . . . . . . . . . 20
8.8. Controller Operation . . . . . . . . . . . . . . . . . . 20
8.9. Verification Scope . . . . . . . . . . . . . . . . . . . 21
8.9.1. Node Ordering . . . . . . . . . . . . . . . . . . . . 21
8.9.2. Stealth Nodes . . . . . . . . . . . . . . . . . . . . 21
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Several deployments use Traffic Engineering, policy routing, Segment
Routing (SR), and Service Function Chaining (SFC) [RFC7665] to steer
packets through a specific set of nodes. In certain cases,
regulatory obligations or a compliance policy require operators to
prove that all packets that are supposed to follow a specific path
are indeed being forwarded across and exact set of pre-determined
nodes.
If a packet flow is supposed to go through a series of service
functions or network nodes, it has to be proven that indeed all
packets of the flow followed the path or service chain or collection
of nodes specified by the policy. In case some packets of a flow
weren't appropriately processed, a verification device should
determine the policy violation and take corresponding actions
corresponding to the policy (e.g., drop or redirect the packet, send
an alert etc.) In today's deployments, the proof that a packet
traversed a particular path or service chain is typically delivered
in an indirect way: Service appliances and network forwarding are in
different trust domains. Physical hand-off-points are defined
between these trust domains (i.e. physical interfaces). Or in other
terms, in the "network forwarding domain" things are wired up in a
way that traffic is delivered to the ingress interface of a service
appliance and received back from an egress interface of a service
appliance. This "wiring" is verified and then trusted upon. The
evolution to Network Function Virtualization (NFV) and modern service
chaining concepts (using technologies such as Locator/ID Separation
Protocol (LISP), Network Service Header (NSH), Segment Routing (SR),
etc.) blurs the line between the different trust domains, because the
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hand-off-points are no longer clearly defined physical interfaces,
but are virtual interfaces. As a consequence, different trust layers
should not to be mixed in the same device. For an NFV scenario a
different type of proof is required. Offering a proof that a packet
indeed traversed a specific set of service functions or nodes allows
operators to evolve from the above described indirect methods of
proving that packets visit a predetermined set of nodes.
The solution approach presented in this document is based on a small
portion of operational data added to every packet. This "in-situ"
operational data is also referred to as "proof of transit data", or
POT data. The POT data is updated at every required node and is used
to verify whether a packet traversed all required nodes. A
particular set of nodes "to be verified" is either described by a set
of secret keys, or a set of shares of a single secret. Nodes on the
path retrieve their individual keys or shares of a key (using for
e.g., Shamir's Secret Sharing scheme) from a central controller. The
complete key set is only known to the controller and a verifier node,
which is typically the ultimate node on a path that performs
verification. Each node in the path uses its secret or share of the
secret to update the POT data of the packets as the packets pass
through the node. When the verifier receives a packet, it uses its
key(s) along with data found in the packet to validate whether the
packet traversed the path correctly.
2. Conventions
Abbreviations used in this document:
HMAC: Hash based Message Authentication Code. For example,
HMAC-SHA256 generates 256 bits of MAC
IOAM: In-situ Operations, Administration, and Maintenance
LISP: Locator/ID Separation Protocol
LPC: Lagrange Polynomial Constants
MTU: Maximum Transmit Unit
NFV: Network Function Virtualization
NSH: Network Service Header
POT: Proof of Transit
POT-profile: Proof of Transit Profile that has the necessary data
for nodes to participate in proof of transit
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RND: Random Bits generated per packet. Packet fields that
donot change during the traversal are given as input to
HMAC-256 algorithm. A minimum of 32 bits (left most) need
to be used from the output if RND is used to verify the
packet integrity. This is a standard recommendation by
NIST.
SEQ_NO: Sequence number initialized to a predefined constant.
This is used in concatenation with RND bits to mitigate
different attacks discussed later.
SFC: Service Function Chain
SR: Segment Routing
3. Proof of Transit
This section discusses methods and algorithms to provide for a "proof
of transit" for packets traversing a specific path. A path which is
to be verified consists of a set of nodes. Transit of the data
packets through those nodes is to be proven. Besides the nodes, the
setup also includes a Controller that creates secrets and secrets
shares and configures the nodes for POT operations.
The methods how traffic is identified and associated to a specific
path is outside the scope of this document. Identification could be
done using a filter (e.g., 5-tuple classifier), or an identifier
which is already present in the packet (e.g., path or service
identifier, NSH Service Path Identifier (SPI), flow-label, etc.)
The solution approach is detailed in two steps. Initially the
concept of the approach is explained. This concept is then further
refined to make it operationally feasible.
3.1. Basic Idea
The method relies on adding POT data to all packets that traverse a
path. The added POT data allows a verifying node (egress node) to
check whether a packet traversed the identified set of nodes on a
path correctly or not. Security mechanisms are natively built into
the generation of the POT data to protect against misuse (i.e.
configuration mistakes, malicious administrators playing tricks with
routing, capturing, spoofing and replaying packets). The mechanism
for POT leverages "Shamir's Secret Sharing" scheme [SSS].
Shamir's secret sharing base idea: A polynomial (represented by its
coefficients) is chosen as a secret by the controller. A polynomial
represents a curve. A set of well-defined points on the curve are
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needed to construct the polynomial. Each point of the polynomial is
called "share" of the secret. A single secret is associated with a
particular set of nodes, which typically represent the path, to be
verified. Shares of the single secret (i.e., points on the curve)
are securely distributed from a Controller to the network nodes.
Nodes use their respective share to update a cumulative value in the
POT data of each packet. Only a verifying node has access to the
complete secret. The verifying node validates the correctness of the
received POT data by reconstructing the curve.
The polynomial cannot be constructed if any of the points are missed
or tampered. Per Shamir's Secret Sharing Scheme, any lesser points
means one or more nodes are missed. Details of the precise
configuration needed for achieving security are discussed further
below.
While applicable in theory, a vanilla approach based on Shamir's
secret sharing could be easily attacked. If the same polynomial is
reused for every packet for a path a passive attacker could reuse the
value. As a consequence, one could consider creating a different
polynomial per packet. Such an approach would be operationally
complex. It would be complex to configure and recycle so many curves
and their respective points for each node. Rather than using a
single polynomial, two polynomials are used for the solution
approach: A secret polynomial which is kept constant, and a per-
packet polynomial which is public. Operations are performed on the
sum of those two polynomials - creating a third polynomial which is
secret and per packet.
3.2. Solution Approach
Solution approach: The overall algorithm uses two polynomials: POLY-1
and POLY-2. POLY-1 is secret and constant. Each node gets a point
on POLY-1 at setup-time and keeps it secret. POLY-2 is public,
random and per packet. Each node generates a point on POLY-2 each
time a packet crosses it. Each node then calculates (point on POLY-1
+ point on POLY-2) to get a (point on POLY-3) and passes it to
verifier by adding it to each packet. The verifier constructs POLY-3
from the points given by all the nodes and cross checks whether
POLY-3 = POLY-1 + POLY-2. Only the verifier knows POLY-1. The
solution leverages finite field arithmetic in a field of size "prime
number".
Detailed algorithms are discussed next. A simple example is
discussed in Section 3.3.
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3.2.1. Setup
A controller generates a first polynomial (POLY-1) of degree k and
k+1 points on the polynomial. The constant coefficient of POLY-1 is
considered the SECRET. The non-constant coefficients are used to
generate the Lagrange Polynomial Constants (LPC). Each of the k
nodes (including verifier) are assigned a point on the polynomial
i.e., shares of the SECRET. The verifier is configured with the
SECRET. The Controller also generates coefficients (except the
constant coefficient, called "RND", which is changed on a per packet
basis) of a second polynomial POLY-2 of the same degree. Each node
is configured with the LPC of POLY-2. Note that POLY-2 is public.
3.2.2. In Transit
For each packet, the ingress node generates a random number (RND).
It is considered as the constant coefficient for POLY-2. A
cumulative value (CML) is initialized to 0. Both RND, CML are
carried as within the packet POT data. As the packet visits each
node, the RND is retrieved from the packet and the respective share
of POLY-2 is calculated. Each node calculates (Share(POLY-1) +
Share(POLY-2)) and CML is updated with this sum. This step is
performed by each node until the packet completes the path. The
verifier also performs the step with its respective share.
3.2.3. Verification
The verifier cross checks whether CML = SECRET + RND. If this
matches then the packet traversed the specified set of nodes in the
path. This is due to the additive homomorphic property of Shamir's
Secret Sharing scheme.
3.3. Illustrative Example
This section shows a simple example to illustrate step by step the
approach described above.
3.3.1. Basic Version
Assumption: It is to be verified whether packets passed through 3
nodes. A polynomial of degree 2 is chosen for verification.
Choices: Prime = 53. POLY-1(x) = (3x^2 + 3x + 10) mod 53. The
secret to be re-constructed is the constant coefficient of POLY-1,
i.e., SECRET=10. It is important to note that all operations are
done over a finite field (i.e., modulo prime).
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3.3.1.1. Secret Shares
The shares of the secret are the points on POLY-1 chosen for the 3
nodes. For example, let x0=2, x1=4, x2=5.
POLY-1(2) = 28 => (x0, y0) = (2, 28)
POLY-1(4) = 17 => (x1, y1) = (4, 17)
POLY-1(5) = 47 => (x2, y2) = (5, 47)
The three points above are the points on the curve which are
considered the shares of the secret. They are assigned to three
nodes respectively and are kept secret.
3.3.1.2. Lagrange Polynomials
Lagrange basis polynomials (or Lagrange polynomials) are used for
polynomial interpolation. For a given set of points on the curve
Lagrange polynomials (as defined below) are used to reconstruct the
curve and thus reconstruct the complete secret.
l0(x) = (((x-x1) / (x0-x1)) * ((x-x2)/x0-x2))) mod 53 =
(((x-4) / (2-4)) * ((x-5)/2-5))) mod 53 =
(10/3 - 3x/2 + (1/6)x^2) mod 53
l1(x) = (((x-x0) / (x1-x0)) * ((x-x2)/x1-x2))) mod 53 =
(-5 + 7x/2 - (1/2)x^2) mod 53
l2(x) = (((x-x0) / (x2-x0)) * ((x-x1)/x2-x1))) mod 53 =
(8/3 - 2 + (1/3)x^2) mod 53
3.3.1.3. LPC Computation
Since x0=2, x1=4, x2=5 are chosen points. Given that computations
are done over a finite arithmetic field ("modulo a prime number"),
the Lagrange basis polynomial constants are computed modulo 53. The
Lagrange Polynomial Constant (LPC) would be 10/3 , -5 , 8/3.
LPC(x0) = (10/3) mod 53 = 21
LPC(x1) = (-5) mod 53 = 48
LPC(x2) = (8/3) mod 53 = 38
For a general way to compute the modular multiplicative inverse, see
e.g., the Euclidean algorithm.
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3.3.1.4. Reconstruction
Reconstruction of the polynomial is well-defined as
POLY1(x) = l0(x) * y0 + l1(x) * y1 + l2(x) * y2
Subsequently, the SECRET, which is the constant coefficient of
POLY1(x) can be computed as below
SECRET = (y0*LPC(l0)+y1*LPC(l1)+y2*LPC(l2)) mod 53
The secret can be easily reconstructed using the y-values and the
LPC:
SECRET = (y0*LPC(l0) + y1*LPC(l1) + y2*LPC(l2)) mod 53 = mod (28 * 21
+ 17 * 48 + 47 * 38) mod 53 = 3190 mod 53 = 10
One observes that the secret reconstruction can easily be performed
cumulatively hop by hop. CML represents the cumulative value. It is
the POT data in the packet that is updated at each hop with the
node's respective (yi*LPC(i)), where i is their respective value.
3.3.1.5. Verification
Upon completion of the path, the resulting CML is retrieved by the
verifier from the packet POT data. Recall that verifier is
preconfigured with the original SECRET. It is cross checked with the
CML by the verifier. Subsequent actions based on the verification
failing or succeeding could be taken as per the configured policies.
3.3.2. Enhanced Version
As observed previously, the vanilla algorithm that involves a single
secret polynomial is not secure. Therefore, the solution is further
enhanced with usage of a random second polynomial chosen per packet.
3.3.2.1. Random Polynomial
Let the second polynomial POLY-2 be (RND + 7x + 10 x^2). RND is a
random number and is generated for each packet. Note that POLY-2 is
public and need not be kept secret. The nodes can be pre-configured
with the non-constant coefficients (for example, 7 and 10 in this
case could be configured through the Controller on each node). So
precisely only RND value changes per packet and is public and the
rest of the non-constant coefficients of POLY-2 kept secret.
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3.3.2.2. Reconstruction
Recall that each node is preconfigured with their respective
Share(POLY-1). Each node calculates its respective Share(POLY-2)
using the RND value retrieved from the packet. The CML
reconstruction is enhanced as below. At every node, CML is updated
as
CML = CML+(((Share(POLY-1)+ Share(POLY-2)) * LPC) mod Prime
Let us observe the packet level transformations in detail. For the
example packet here, let the value RND be 45. Thus POLY-2 would be
(45 + 7x + 10x^2).
The shares that could be generated are (2, 46), (4, 21), (5, 12).
At ingress: The fields RND = 45. CML = 0.
At node-1 (x0): Respective share of POLY-2 is generated i.e., (2,
46) because share index of node-1 is 2.
CML = 0 + ((28 + 46)* 21) mod 53 = 17
At node-2 (x1): Respective share of POLY-2 is generated i.e., (4,
21) because share index of node-2 is 4.
CML = 17 + ((17 + 21)*48) mod 53 = 17 + 22 = 39
At node-3 (x2), which is also the verifier: The respective share
of POLY-2 is generated i.e., (5, 12) because the share index of
the verifier is 12.
CML = 39 + ((47 + 12)*38) mod 53 = 39 + 16 = 55 mod 53 = 2
The verification using CML is discussed in next section.
3.3.2.3. Verification
As shown in the above example, for final verification, the verifier
compares:
VERIFY = (SECRET + RND) mod Prime, with Prime = 53 here
VERIFY = (RND-1 + RND-2) mod Prime = ( 10 + 45 ) mod 53 = 2
Since VERIFY = CML the packet is proven to have gone through nodes 1,
2, and 3.
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3.3.3. Final Version
The enhanced version of the protocol is still prone to replay and
preplay attacks. An attacker could reuse the POT metadata for
bypassing the verification. So additional measures using packet
integrity checks (HMAC) and sequence numbers (SEQ_NO) are discussed
later "Security Considerations" section.
3.4. Operational Aspects
To operationalize this scheme, a central controller is used to
generate the necessary polynomials, the secret share per node, the
prime number, etc. and distributing the data to the nodes
participating in proof of transit. The identified node that performs
the verification is provided with the verification key. The
information provided from the Controller to each of the nodes
participating in proof of transit is referred to as a proof of
transit profile (POT-profile). Also note that the set of nodes for
which the transit has to be proven are typically associated to a
different trust domain than the verifier. Note that building the
trust relationship between the Controller and the nodes is outside
the scope of this document. Techniques such as those described in
[I-D.ietf-anima-autonomic-control-plane] might be applied.
To optimize the overall data amount of exchanged and the processing
at the nodes the following optimizations are performed:
1. The points (x, y) for each of the nodes on the public and private
polynomials are picked such that the x component of the points
match. This lends to the LPC values which are used to calculate
the cumulative value CML to be constant. Note that the LPC are
only depending on the x components. They can be computed at the
controller and communicated to the nodes. Otherwise, one would
need to distributed the x components to all the nodes.
2. A pre-evaluated portion of the public polynomial for each of the
nodes is calculated and added to the POT-profile. Without this
all the coefficients of the public polynomial had to be added to
the POT profile and each node had to evaluate them. As stated
before, the public portion is only the constant coefficient RND
value, the pre-evaluated portion for each node should be kept
secret as well.
3. To provide flexibility on the size of the cumulative and random
numbers carried in the POT data a field to indicate this is
shared and interpreted at the nodes.
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3.5. Alternative Approach
In certain scenarios preserving the order of the nodes traversed by
the packet may be needed. An alternative, "nested encryption" based
approach is described here for preserving the order
3.5.1. Basic Idea
1. The controller provisions all the nodes with their respective
secret keys.
2. The controller provisions the verifier with all the secret keys
of the nodes.
3. For each packet, the ingress node generates a random number RND
and encrypts it with its secret key to generate CML value
4. Each subsequent node on the path encrypts CML with their
respective secret key and passes it along
5. The verifier is also provisioned with the expected sequence of
nodes in order to verify the order
6. The verifier receives the CML, RND values, re-encrypts the RND
with keys in the same order as expected sequence to verify.
3.5.2. Pros
Nested encryption approach retains the order in which the nodes are
traversed.
3.5.3. Cons
1. Standard AES encryption would need 128 bits of RND, CML. This
results in a 256 bits of additional overhead is added per packet
2. In hardware platforms that do not support native encryption
capabilities like (AES-NI). This approach would have
considerable impact on the computational latency
4. Sizing the Data for Proof of Transit
Proof of transit requires transport of two data fields in every
packet that should be verified:
1. RND: Random number (the constant coefficient of public
polynomial)
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2. CML: Cumulative
The size of the data fields determines how often a new set of
polynomials would need to be created. At maximum, the largest RND
number that can be represented with a given number of bits determines
the number of unique polynomials POLY-2 that can be created. The
table below shows the maximum interval for how long a single set of
polynomials could last for a variety of bit rates and RND sizes: When
choosing 64 bits for RND and CML data fields, the time between a
renewal of secrets could be as long as 3,100 years, even when running
at 100 Gbps.
+-------------+--------------+------------------+-------------------+
| Transfer | Secret/RND | Max # of packets | Time RND lasts |
| rate | size | | |
+-------------+--------------+------------------+-------------------+
| 1 Gbps | 64 | 2^64 = approx. | approx. 310,000 |
| | | 2*10^19 | years |
| 10 Gbps | 64 | 2^64 = approx. | approx. 31,000 |
| | | 2*10^19 | years |
| 100 Gbps | 64 | 2^64 = approx. | approx. 3,100 |
| | | 2*10^19 | years |
| 1 Gbps | 32 | 2^32 = approx. | 2,200 seconds |
| | | 4*10^9 | |
| 10 Gbps | 32 | 2^32 = approx. | 220 seconds |
| | | 4*10^9 | |
| 100 Gbps | 32 | 2^32 = approx. | 22 seconds |
| | | 4*10^9 | |
+-------------+--------------+------------------+-------------------+
Table assumes 64 octet packets
Table 1: Proof of transit data sizing
5. Node Configuration
A POT system consists of a number of nodes that participate in POT
and a Controller, which serves as a control and configuration entity.
The Controller is to create the required parameters (polynomials,
prime number, etc.) and communicate those to the nodes. The sum of
all parameters for a specific node is referred to as "POT-profile".
This document does not define a specific protocol to be used between
Controller and nodes. It only defines the procedures and the
associated YANG data model.
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5.1. Procedure
The Controller creates new POT-profiles at a constant rate and
communicates the POT-profile to the nodes. The controller labels a
POT-profile "even" or "odd" and the Controller cycles between "even"
and "odd" labeled profiles. The rate at which the POT-profiles are
communicated to the nodes is configurable and is more frequent than
the speed at which a POT-profile is "used up" (see table above).
Once the POT-profile has been successfully communicated to all nodes
(e.g., all NETCONF transactions completed, in case NETCONF is used as
a protocol), the controller sends an "enable POT-profile" request to
the ingress node.
All nodes maintain two POT-profiles (an even and an odd POT-profile):
One POT-profile is currently active and in use; one profile is
standby and about to get used. A flag in the packet is indicating
whether the odd or even POT-profile is to be used by a node. This is
to ensure that during profile change the service is not disrupted.
If the "odd" profile is active, the Controller can communicate the
"even" profile to all nodes. Only if all the nodes have received the
POT-profile, the Controller will tell the ingress node to switch to
the "even" profile. Given that the indicator travels within the
packet, all nodes will switch to the "even" profile. The "even"
profile gets active on all nodes and nodes are ready to receive a new
"odd" profile.
Unless the ingress node receives a request to switch profiles, it'll
continue to use the active profile. If a profile is "used up" the
ingress node will recycle the active profile and start over (this
could give rise to replay attacks in theory - but with 2^32 or 2^64
packets this isn't really likely in reality).
5.2. YANG Model
This section defines that YANG data model for the information
exchange between the Controller and the nodes.
<CODE BEGINS> file "ietf-pot-profile@2016-06-15.yang"
module ietf-pot-profile {
yang-version 1;
namespace "urn:ietf:params:xml:ns:yang:ietf-pot-profile";
prefix ietf-pot-profile;
organization "IETF xxx Working Group";
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contact "";
description "This module contains a collection of YANG
definitions for proof of transit configuration
parameters. The model is meant for proof of
transit and is targeted for communicating the
POT-profile between a controller and nodes
participating in proof of transit.";
revision 2016-06-15 {
description
"Initial revision.";
reference
"";
}
typedef profile-index-range {
type int32 {
range "0 .. 1";
}
description
"Range used for the profile index. Currently restricted to
0 or 1 to identify the odd or even profiles.";
}
grouping pot-profile {
description "A grouping for proof of transit profiles.";
list pot-profile-list {
key "pot-profile-index";
ordered-by user;
description "A set of pot profiles.";
leaf pot-profile-index {
type profile-index-range;
mandatory true;
description
"Proof of transit profile index.";
}
leaf prime-number {
type uint64;
mandatory true;
description
"Prime number used for module math computation";
}
leaf secret-share {
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type uint64;
mandatory true;
description
"Share of the secret of polynomial 1 used in computation";
}
leaf public-polynomial {
type uint64;
mandatory true;
description
"Pre evaluated Public polynomial";
}
leaf lpc {
type uint64;
mandatory true;
description
"Lagrange Polynomial Coefficient";
}
leaf validator {
type boolean;
default "false";
description
"True if the node is a verifier node";
}
leaf validator-key {
type uint64;
description
"Secret key for validating the path, constant of poly 1";
}
leaf bitmask {
type uint64;
default 4294967295;
description
"Number of bits as mask used in controlling the size of the
random value generation. 32-bits of mask is default.";
}
}
}
container pot-profiles {
description "A group of proof of transit profiles.";
list pot-profile-set {
key "pot-profile-name";
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ordered-by user;
description
"Set of proof of transit profiles that group parameters
required to classify and compute proof of transit
metadata at a node";
leaf pot-profile-name {
type string;
mandatory true;
description
"Unique identifier for each proof of transit profile";
}
leaf active-profile-index {
type profile-index-range;
description
"Proof of transit profile index that is currently active.
Will be set in the first hop of the path or chain.
Other nodes will not use this field.";
}
uses pot-profile;
}
/*** Container: end ***/
}
/*** module: end ***/
}
<CODE ENDS>
6. IANA Considerations
IANA considerations will be added in a future version of this
document.
7. Manageability Considerations
Manageability considerations will be addressed in a later version of
this document.
8. Security Considerations
Different security requirements achieved by the solution approach are
discussed here.
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8.1. Proof of Transit
Proof of correctness and security of the solution approach is per
Shamir's Secret Sharing Scheme [SSS]. Cryptographically speaking it
achieves information-theoretic security i.e., it cannot be broken by
an attacker even with unlimited computing power. As long as the
below conditions are met it is impossible for an attacker to bypass
one or multiple nodes without getting caught.
o If there are k+1 nodes in the path, the polynomials (POLY-1, POLY-
2) should be of degree k. Also k+1 points of POLY-1 are chosen
and assigned to each node respectively. The verifier can re-
construct the k degree polynomial (POLY-3) only when all the
points are correctly retrieved.
o Precisely three values are kept secret by individual nodes. Share
of SECRET (i.e. points on POLY-1), Share of POLY-2, LPC, P. Note
that only constant coefficient, RND, of POLY-2 is public. x values
and non-constant coefficient of POLY-2 are secret
An attacker bypassing a few nodes will miss adding a respective point
on POLY-1 to corresponding point on POLY-2 , thus the verifier cannot
construct POLY-3 for cross verification.
Also it is highly recommended that different polynomials should be
used as POLY-1 across different paths, traffic profiles or service
chains.
8.2. Cryptanalysis
A passive attacker could try to harvest the POT data (i.e., CML, RND
values) in order to determine the configured secrets. Subsequently
two types of differential analysis for guessing the secrets could be
done.
o Inter-Node: A passive attacker observing CML values across nodes
(i.e., as the packets entering and leaving), cannot perform
differential analysis to construct the points on POLY-1. This is
because at each point there are four unknowns (i.e. Share(POLY-
1), Share(Poly-2) LPC and prime number P) and three known values
(i.e. RND, CML-before, CML-after).
o Inter-Packets: A passive attacker could observe CML values across
packets (i.e., values of PKT-1 and subsequent PKT-2), in order to
predict the secrets. Differential analysis across packets could
be mitigated using a good PRNG for generating RND. Note that if
constant coefficient is a sequence number than CML values become
quite predictable and the scheme would be broken.
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8.3. Anti-Replay
A passive attacker could reuse a set of older RND and the
intermediate CML values to bypass certain nodes in later packets.
Such attacks could be avoided by carefully choosing POLY-2 as a
(SEQ_NO + RND). For example, if 64 bits are being used for POLY-2
then first 16 bits could be a sequence number SEQ_NO and next 48 bits
could be a random number.
Subsequently, the verifier could use the SEQ_NO bits to run classic
anti-replay techniques like sliding window used in IPSEC. The
verifier could buffer up to 2^16 packets as a sliding window.
Packets arriving with a higher SEQ_NO than current buffer could be
flagged legitimate. Packets arriving with a lower SEQ_NO than
current buffer could be flagged as suspicious.
For all practical purposes in the rest of the document RND means
SEQ_NO + RND to keep it simple.
The solution discussed in this memo does not currently mitigate
replay attacks. An anti-replay mechanism may be included in future
versions of the solution.
8.4. Anti-Preplay
An active attacker could try to perform a man-in-the-middle (MITM)
attack by extracting the POT of PKT-1 and using it in PKT-2.
Subsequently attacker drops the PKT-1 in order to avoid duplicate POT
values reaching the verifier. If the PKT-1 reaches the verifier,
then this attack is same as Replay attacks discussed before.
Preplay attacks are possible since the POT metadata is not dependent
on the packet fields. Below steps are recommended for remediation:
o Ingress node and Verifier are configured with common pre shared
key
o Ingress node generates a Message Authentication Code (MAC) from
packet fields using standard HMAC algorithm.
o The left most bits of the output are truncated to desired length
to generate RND. It is recommended to use a minimum of 32 bits.
o The verifier regenerates the HMAC from the packet fields and
compares with RND. To ensure the POT data is in fact that of the
packet.
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If an HMAC is used, an active attacker lacks the knowledge of the
pre-shared key, and thus cannot launch preplay attacks.
The solution discussed in this memo does not currently mitigate
prereplay attacks. A mitigation mechanism may be included in future
versions of the solution.
8.5. Anti-Tampering
An active attacker could not insert any arbitrary value for CML.
This would subsequently fail the reconstruction of the POLY-3. Also
an attacker could not update the CML with a previously observed
value. This could subsequently be detected by using timestamps
within the RND value as discussed above.
8.6. Recycling
The solution approach is flexible for recycling long term secrets
like POLY-1. All the nodes could be periodically updated with shares
of new SECRET as best practice. The table above could be consulted
for refresh cycles (see Section 4).
8.7. Redundant Nodes and Failover
A "node" or "service" in terms of POT can be implemented by one or
multiple physical entities. In case of multiple physical entities
(e.g., for load-balancing, or business continuity situations -
consider for example a set of firewalls), all physical entities which
are implementing the same POT node are given that same share of the
secret. This makes multiple physical entities represent the same POT
node from an algorithm perspective.
8.8. Controller Operation
The Controller needs to be secured given that it creates and holds
the secrets, as need to be the nodes. The communication between
Controller and the nodes also needs to be secured. As secure
communication protocol such as for example NETCONF over SSH should be
chosen for Controller to node communication.
The Controller only interacts with the nodes during the initial
configuration and thereafter at regular intervals at which the
operator chooses to switch to a new set of secrets. In case 64 bits
are used for the data fields "CML" and "RND" which are carried within
the data packet, the regular intervals are expected to be quite long
(e.g., at 100 Gbps, a profile would only be used up after 3100 years)
- see Section 4 above, thus even a "headless" operation without a
Controller can be considered feasible. In such a case, the
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Controller would only be used for the initial configuration of the
POT-profiles.
8.9. Verification Scope
The POT solution defined in this document verifies that a data-packet
traversed or transited a specific set of nodes. From an algorithm
perspective, a "node" is an abstract entity. It could be represented
by one or multiple physical or virtual network devices, or is could
be a component within a networking device or system. The latter
would be the case if a forwarding path within a device would need to
be securely verified.
8.9.1. Node Ordering
POT using Shamir's secret sharing scheme as discussed in this
document provides for a means to verify that a set of nodes has been
visited by a data packet. It does not verify the order in which the
data packet visited the nodes. In case the order in which a data
packet traversed a particular set of nodes needs to be verified as
well, alternate schemes that e.g., rely on "nested encryption" could
to be considered.
8.9.2. Stealth Nodes
The POT approach discussed in this document is to prove that a data
packet traversed a specific set of "nodes". This set could be all
nodes within a path, but could also be a subset of nodes in a path.
Consequently, the POT approach isn't suited to detect whether
"stealth" nodes which do not participate in proof-of-transit have
been inserted into a path.
9. Acknowledgements
The authors would like to thank Eric Vyncke, Nalini Elkins, Srihari
Raghavan, Ranganathan T S, Karthik Babu Harichandra Babu, Akshaya
Nadahalli, Erik Nordmark, and Andrew Yourtchenko for the comments and
advice.
10. References
10.1. Normative References
[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015, <https://www.rfc-
editor.org/info/rfc7665>.
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[SSS] "Shamir's Secret Sharing", <https://en.wikipedia.org/wiki/
Shamir%27s_Secret_Sharing>.
10.2. Informative References
[I-D.ietf-anima-autonomic-control-plane]
Behringer, M., Eckert, T., and S. Bjarnason, "An Autonomic
Control Plane", draft-ietf-anima-autonomic-control-
plane-03 (work in progress), July 2016.
Authors' Addresses
Frank Brockners
Cisco Systems, Inc.
Hansaallee 249, 3rd Floor
DUESSELDORF, NORDRHEIN-WESTFALEN 40549
Germany
Email: fbrockne@cisco.com
Shwetha Bhandari
Cisco Systems, Inc.
Cessna Business Park, Sarjapura Marathalli Outer Ring Road
Bangalore, KARNATAKA 560 087
India
Email: shwethab@cisco.com
Sashank Dara
Cisco Systems, Inc.
Cessna Business Park, Sarjapura Marathalli Outer Ring Road
BANGALORE, Bangalore, KARNATAKA 560 087
INDIA
Email: sadara@cisco.com
Carlos Pignataro
Cisco Systems, Inc.
7200-11 Kit Creek Road
Research Triangle Park, NC 27709
United States
Email: cpignata@cisco.com
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John Leddy
Comcast
Email: John_Leddy@cable.comcast.com
Stephen Youell
JP Morgan Chase
25 Bank Street
London E14 5JP
United Kingdom
Email: stephen.youell@jpmorgan.com
David Mozes
Email: mosesster@gmail.com
Tal Mizrahi
Marvell
6 Hamada St.
Yokneam 20692
Israel
Email: talmi@marvell.com
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