DTN Research Group S. Symington
Internet-Draft The MITRE Corporation
Expires: October 26, 2007 S. Farrell
Trinity College Dublin
H. Weiss
P. Lovell
SPARTA, Inc.
April 24, 2007
Bundle Security Protocol Specification
draft-irtf-dtnrg-bundle-security-03
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Abstract
This document defines the bundle security protocol, which provides
data integrity and confidentiality services. We also describe
various bundle security considerations including policy options.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Related Documents . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Security Blocks . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Abstract Security Block . . . . . . . . . . . . . . . . . 7
2.2. Bundle Authentication Block . . . . . . . . . . . . . . . 10
2.3. Payload Security Block . . . . . . . . . . . . . . . . . . 11
2.4. Confidentiality Block . . . . . . . . . . . . . . . . . . 12
2.5. PSB and CB combinations . . . . . . . . . . . . . . . . . 14
3. Security Processing . . . . . . . . . . . . . . . . . . . . . 16
3.1. Nodes as policy enforcement points . . . . . . . . . . . . 16
3.2. Canonicalisation of bundles . . . . . . . . . . . . . . . 16
3.3. Endpoint ID confidentiality . . . . . . . . . . . . . . . 22
3.4. Bundles received from other nodes . . . . . . . . . . . . 22
3.5. The At-Most-Once-Delivery Option . . . . . . . . . . . . . 24
3.6. Bundle Fragmentation and Reassembly . . . . . . . . . . . 24
3.7. Reactive fragmentation . . . . . . . . . . . . . . . . . . 25
4. Mandatory Ciphersuites . . . . . . . . . . . . . . . . . . . . 27
4.1. BAB-HMAC . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.2. PSB-RSA-SHA256 . . . . . . . . . . . . . . . . . . . . . . 28
4.3. CB-RSA-AES128-PAYLOAD-PSB . . . . . . . . . . . . . . . . 28
5. Key Management . . . . . . . . . . . . . . . . . . . . . . . . 30
6. Default Security Policy . . . . . . . . . . . . . . . . . . . 31
7. Security Considerations . . . . . . . . . . . . . . . . . . . 33
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.1. Normative References . . . . . . . . . . . . . . . . . . . 35
9.2. Informative References . . . . . . . . . . . . . . . . . . 35
Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . .
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
Intellectual Property and Copyright Statements . . . . . . . . . . 39
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1. Introduction
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1].
This document defines security features for the bundle protocol [2]
intended for use in delay tolerant networks, in order to provide the
DTN security services as described in the DTN Security Overview and
Motivations document [8].
The bundle protocol is used in DTNs which overlay multiple networks,
some of which may be challenged by limitations such as intermittent
and possibly unpredictable loss of connectivity, long or variable
delay, asymmetric data rates, and high error rates. The purpose of
the bundle protocol is to support interoperability across such
stressed networks. The bundle protocol is layered on top of
underlay-network-specific convergence layers, on top of network-
specific lower layers, to enable an application in one network to
communicate with an application in another network, both of which are
spanned by the DTN.
Security will be important for the bundle protocol. The stressed
environment of the underlying networks over which the bundle protocol
will operate makes it important that the DTN be protected from
unauthorized use, and this stressed environment poses unique
challenges on the mechanisms needed to secure the bundle protocol.
Furthermore, DTNs may very likely be deployed in environments where a
portion of the network might become compromised, posing the usual
security challenges related to confidentiality, integrity and
availability.
1.1. Related Documents
This document is best read and understood within the context of the
following other DTN documents:
The Delay-Tolerant Network Architecture [9] defines the
architecture for delay-tolerant networks, but does not discuss
security at any length.
The DTN Bundle Protocol [2] defines the format and processing of
the blocks used to implement the bundle protocol, excluding the
security-specific blocks defined here.
The Delay-Tolerant Networking Security Overview [8] provides an
informative overview and high-level description of DTN security.
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1.2. Terminology
We introduce the following terminology for purposes of clarity:
source - the bundle node from which a bundle originates
destination - the bundle node to which a bundle is ultimately
destined
forwarder - the bundle node that forwarded the bundle on its most
recent hop
intermediate receiver or "next hop" - the neighboring bundle node
to which a forwarder forwards a bundle.
In the figure below, which is adapted from figure 1 in the Bundle
Protocol Specification, four bundle nodes (denoted BN1, BN2, BN3, and
BN4) reside above some transport layer(s). Three distinct transport
and network protocols (denoted T1/N1, T2/N2, and T3/N3) are also
shown.
+---------v-| +->>>>>>>>>>v-+ +->>>>>>>>>>v-+ +-^---------+
|BN1 v | | ^ BN2 v | | ^ BN3 v | | ^ BN4 |
+---------v-+ +-^---------v-+ +-^---------v-+ +-^---------+
|Trans1 v | + ^ T1/T2 v | + ^ T2/T3 v | | ^ Trans3 |
+---------v-+ +-^---------v-+ +-^---------v + +-^---------+
|Net1 v | | ^ N1/N2 v | | ^ N2/N3 v | | ^ Net3 |
+---------v-+ +-^---------v + +-^---------v-+ +-^---------+
| >>>>>>>>^ >>>>>>>>>>^ >>>>>>>>^ |
+-----------+ +------------+ +-------------+ +-----------+
| | | |
|<-- An Internet --->| |<--- An Internet --->|
| | | |
BN = "Bundle Node" (as defined in the Bundle Protocol Specification
Bundle Nodes Sit at the Application layer of the Internet Model.
Figure 1
Bundle node BN1 originates a bundle that it forwards to BN2. BN2
forwards the bundle to BN3, and BN3 forwards the bundle to BN4. BN1
is the source of the bundle and BN4 is the destination of the bundle.
BN1 is the first forwarder, and BN2 is the first intermediate
receiver; BN2 then becomes the forwarder, and BN3 the intermediate
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receiver; BN3 then becomes the last forwarder, and BN4 the last
intermediate receiver, as well as the destination.
If node BN2 originates a bundle (for example, a bundle status report
or a custodial signal), which is then forwarded on to BN3, and then
to BN4, then BN2 is the source of the bundle (as well as being the
first forwarder of the bundle) and BN4 is the destination of the
bundle (as well as being the final intermediate receiver).
We introduce the following security-specific DTN terminology:
security-source - a bundle node that adds a security block to a
bundle
security-destination - a bundle node that processes a security
block of a bundle
Referring to figure 1 again:
If the bundle that originates at BN1 as source is given a security
block by BN1, then BN1 is the security-source of this bundle with
respect to that security block, as well as being the source of the
bundle.
If the bundle that originates at BN1 as source is given a security
block by BN2, then BN2 is the security-source of this bundle with
respect to that security block, even though BN1 is the source.
If the bundle that originates at BN1 as source is given a security
block by BN1 that is intended to be processed by BN3, then BN1 is the
security-source and BN3 is the security destination with respect to
this security block.
A bundle may have multiple security blocks. The security-source of a
bundle with respect to a given security block in the bundle may be
the same as or different from the security-source of the bundle with
respect to a different security block in the bundle. Similarly, the
security-destination of a bundle with respect to each of that
bundle's security blocks may be the same or different.
Forwarding nodes MUST transmit blocks in the same order as they were
received. This requirement applies to all dtn nodes, not just ones
which implement security processing. Blocks in a bundle may be added
or deleted according to the applicable specification, but blocks
which are received and then transmitted MUST remain in the same
relative order.
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2. Security Blocks
There are three types of security blocks that MAY be included in a
bundle. These are the Bundle Authentication Block (BAB), the Payload
Security Block (PSB), and the Confidentiality Block (CB).
The BAB is used to assure the authenticity of the bundle along a
single hop from forwarder to intermediate receiver.
The PSB is used to assure the authenticity of the bundle from the
PSB security-source, which creates the PSB, to the PSB security-
destination, which verifies the PSB authenticator. The
authentication information in the PSB may (if the ciphersuite
allows) be verified by any node in between the PSB security-source
and the PSB security-destination that has access to the
cryptographic keys and revocation status information required to
do so.
Since a BAB protects on a "hop-by-hop" basis and a PSB protects on
a (sort of) "end-to-end" basis, whenever both are present the BAB
MUST form the "outer" layer of protection - that is, the BAB MUST
always be calculated and added to the bundle after the PSB has
been calculated and added to the bundle.
The CB indicates that some parts of the bundle have been encrypted
while en route between the CB security-source and the CB security-
destination.
Each of the security blocks uses the Canonical Bundle Block Format as
defined in the Bundle Protocol Specification. That is, each security
block is comprised of the following elements:
- Block type code
- Block processing control flags
- Block EID reference list (optional)
- Block data length
- Block-type-specific data fields
Since the three security blocks have most fields in common, we can
shorten the description of the Block-type-specific data fields of
each security block if we first define an abstract security block
(ASB) and then specify each of the real blocks in terms of the fields
which are present/absent in an ASB. Note that no bundle ever
contains an ASB, which is simply a specification artifact.
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2.1. Abstract Security Block
An ASB consists of the following mandatory and optional fields:
- Block-type code (one byte) - as in all bundle protocol blocks
except the primary bundle block. The block types codes for the
security blocks are:
BAB: 0x02
PSB: 0x03
CB: 0x04
- Block processing control flags (SDNV) - defined as in all bundle
protocol blocks except the primary bundle block (as described in
the Bundle Protocol [2]). SDNV encoding is described in the
bundle protocol. There are no constraints on the use of the block
processing flags.[Comment.1]
- EID references - composite field defined in [2] containing
references to one or two EIDs. Presence of EIDs is indicated by
by the setting of bit 6 ("block contains an EID-reference field")
of the block processing control flags. If one or more is present,
flags in the ciphersuite ID field, described below, specify which.
The possible EIDs are, in order:-
- (optional) Security-source - specifies the security source for
the service. If this is omitted, then the source of the bundle is
assumed to be the security-source.
- (optional) Security-destination - specifies the security
destination for the service. If this is omitted, then the
destination of the bundle is assumed to be the security-
destination.
Both EID fields may be omitted, in which case the composite field
itself is empty, as defined in [2]. In this case neither count
nor references appear, and bit 6 is not set.
- Block data length (SDNV) - as in all bundle protocol blocks
except the primary bundle block. SDNV encoding is described in
the bundle protocol.
- Block-type-specific data fields as follows:
- Ciphersuite ID - identifies the ciphersuite in use. This is
two bytes long, though the top five bits are used to indicate
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the presence or absence of the optional fields below.
- (optional) Correlator - when more than one related block is
inserted then this field must have the same value in each
related block instance. This is encoded as an SDNV. See note
in Section 3.6 with regard to correlator values in bundle
fragments.
- (optional) Ciphersuite parameters - compound field of next
two items
- Ciphersuite parameters length - specifies the length of
the following Ciphersuite parameters data field and is
encoded as an SDNV.
- Ciphersuite parameters data - parameters to be used with
the ciphersuite in use, e.g. a key identifier or
initialization vector (IV). The encoding rules for this
field are defined as part of the ciphersuite specification.
- (optional) Security result - compound field of next two items
- Security result length - contains the length of the next
field and is encoded as an SDNV.
- Security result data - contains the results of the
appropriate ciphersuite-specific calculation (e.g. a
signature, MAC or ciphertext block key).
+----------------+----------------+----------------+----------------+
| type | flags (SDNV) | EID ref list(comp) |
+----------------+----------------+----------------+----------------+
| length (SDNV) |
+----------------+----------------+----------------+----------------+
| ciphersuite | correlator (SDNV) |
+----------------+----------------+----------------+----------------+
|params len(SDNV)| ciphersuite params data |
+----------------+----------------+----------------+----------------+
|res-len (SDNV) | security result data |
+----------------+----------------+----------------+----------------+
The structure of an abstract security block
Figure 2
The ciphersuite ID is a 16-bit value with the top five bits
indicating which of the optional fields are present (value = "1") or
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absent (value="0"). The remaining 11 bits indicate the ciphersuite.
Some ciphersuites are specified in Section 4, which also specifies
the rules which MUST be satisfied by ciphersuite specifications.
Additional ciphersuites MAY be defined in separate specifications.
The structure of the ciphersuite ID bytes is shown in Figure 3. In
each case the presence of an optional field is indicated by setting
the value of the corresponding flag to one. A value of zero
indicates the corresponding optional field is missing.
src - the most significant bit (bit 0) indicates whether the ASB
contains the optional security-source-length and security-source
fields.
dest - bit 1 indicates whether the security-destination-length and
security-destination fields are present or not.
parm - bit 2 indicates whether the ciphersuite-parameters-length
and ciphersuite parameters data fields are present or not.
corr - bit 3 indicates whether or not the ASB contains an optional
correlator.
res - bit 4 indicates whether or not the ASB contains the security
result length and security result data fields.
bits 5-15 represent the ciphersuite number, giving a maximum of
2048 different ciphersuites.
Ciphersuite ID
Bit Bit Bit Bit Bit Bit Bit Bit
0 1 2 3 4 5 ... 15
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
|src |dest |parm |corr |res | ciphersuite ID |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
Figure 3
A little bit more terminology: when the block is a PSB then we refer
to the PSB-source when we mean the security source field in the PSB.
Similarly we may refer to the CB-dest, meaning the security-
destination field of the CB. For example, referring to Figure 1
again, if the bundle that originates at BN1 as source is given a
Confidentiality Block (CB) by BN1 that is protected using a key held
by BN3 and it is given a Payload Security Block (PSB) by BN1, then
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BN1 is both the CB-source and the PSB-source of the bundle, and BN3
is the CB-dest of the bundle.
The correlator field is used to associate several related instances
of a security block. This can be used to place a BAB that contains
the ciphersuite information at the "front" of a (probably large)
bundle , and another correlated BAB that contains the security result
at the "end" of the bundle. This allows even very memory-constrained
nodes to be able to process the bundle and verify the BAB. There are
similar use cases for multiple related instances of PSB and CB as
will be seen below.
The ciphersuite specification MUST make it clear whether or not
multiple block instances are allowed, and if so, under what
conditions. Some ciphersuites can of course leave flexibility to the
implementation, whereas others might mandate a fixed number of
instances.
2.2. Bundle Authentication Block
In this section we describe typical BAB field values for two
scenarios - where a single instance of the BAB contains all the
information and where two related instances are used, one "up front"
which contains the ciphersuite and another following the payload
which contains the security result (e.g. a MAC).
For the case where a single BAB is used:
The block-type code field value MUST be 0x02.
The block processing control flags value can be set to whatever
values are required by local policy.
The ciphersuite ID MUST be documented as a hop-by-hop
authentication-ciphersuite which requires one instance of the BAB.
The correlator field MUST NOT be present.
The ciphersuite parameters field MAY be present, if so specified
in the ciphersuite specification.
The security-source field SHOULD be present and, if it is present,
it MUST identify the forwarder of the bundle. (If the forwarding
node is identified in another block of the bundle that the next
hop supports, e.g., the Previous Hop Insertion Block, the
forwarding node need not be identified in the BAB.)
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The security-destination field SHOULD NOT be present unless the
ciphersuite requires this information (since the first node
receiving the bundle ought be the one to validate the BAB).
The security result MUST be present as it is effectively the
"output" from the ciphersuite calculation (e.g. the MAC or
signature) applied to the (relevant parts of) the bundle (as
specified in the ciphersuite definition).
For the case using two related BAB instances, the first instance is
as defined above, except the ciphersuite ID MUST be documented as a
hop-by-hop authentication ciphersuite that requires two instances of
the BAB. In addition, the correlator MUST be present and the
security result length and security result fields MUST be absent.
The second instance of the BAB MUST have the same correlator value
present and MUST contain security result length and a security result
fields. The other optional fields MUST NOT be present. Typically,
this second instance of a BAB will be the last block of the bundle.
2.3. Payload Security Block
A PSB is an ASB with the following additional restrictions:
The block type code value MUST be 0x03.
The block processing control flags value can be set to whatever
values are required by local policy.
The ciphersuite ID MUST be documented as an end-to-end
authentication-ciphersuite or as an end-to-end error-detection-
ciphersuite.
The correlator MUST be present if the ciphersuite requires more
than one related instance of a PSB be present in the bundle. The
correlator MUST NOT be present if the ciphersuite only requires
one instance of the PSB in the bundle.
The ciphersuite parameters field MAY be present.
The security-source field MAY be present.
The security-destination field MAY be present.
The security result is effectively the "output" from the
ciphersuite calculation (e.g. the MAC or signature) applied to the
(relevant parts of) the bundle. As in the case of the BAB, this
field MUST be present if the correlator is absent. If more than
one related instance of the PSB is required then this is handled
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in the same way as described for the BAB above.
For some ciphersuites, (e.g. those using asymmetric keying to produce
signatures or those using symmetric keying with a group key), the
security information can be checked at any hop on the way to the
destination that has access to the required keying information.
Most asymmetric PSB-ciphersuites will use the PSB-source to indicate
the signer and will not require the PSB-dest field because the key
needed to verify the PSB authenticator will be a public key
associated with the PSB-source.
2.4. Confidentiality Block
A typical confidentiality ciphersuite will encrypt the payload using
a randomly generated bundle encrypting key (BEK) and will use a CB
security result to carry the BEK encrypted with some long term key
encryption key (KEK). or well-known public key. If neither the
destination or security-destination resolves the key to use for
decryption, the ciphersuite parameters field can be used to indicate
the decryption key with which the BEK can be recovered. Subsequent
CB security results will contain blocks encrypted using the BEK if
non-payload blocks are to be encrypted.
The payload is encrypted "in-place", that is, following encryption,
the payload block payload field contains ciphertext, not plaintext.
The payload block processing flags are unmodified.[Comment.2]
Payload super-encryption is allowed. If a CB ciphersuite supports
such super-encryption, then the ciphersuite MUST provide an
unambiguous way to do the decryption operations in the correct order
(e.g. by encoding the "layer" information as a ciphersuite
parameter). This "in-place" encryption of payload bytes is so as to
allow bundle payload fragmentation and re-assembly to operate without
knowledge of whether encryption has occurred. A side-effect of this
"in-place" encryption is that the payload will typically be expanded
by up-to a ciphertext blocksize if the bulk cipher is a block cipher.
Another is that the 2nd application of confidentiality does not
generally protect the parameters of the first which represent a
vulnerability in some circumstances.
A CB is an ASB with the following additional restrictions:
The block type code value MUST be 0x04.
The block processing control flags value can be set to whatever
values are required by local policy.
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The ciphersuite ID MUST be documented as a confidentiality-
ciphersuite.
The correlator MUST be present if more than one related CB
instance is required. More than one CB instance might be required
if the payload is to be encrypted for more than one security-
destination so as to be robust in the face of routing
uncertainties. These multiple CB instances, however, would not be
related and would therefore not require correlators. On the other
hand, multiple related CB instances would be required if both the
payload and the PSB blocks in the bundle were to be encrypted.
These multiple CB instances would require correlators to associate
them with each other. The correlator MUST NOT be present if there
are no related CB instances.
The ciphersuite parameters field MAY be present
The security-source field MAY be present.
The security-destination field MAY be present (and typically will
be).
The security result MAY be present and normally represents an
encrypted bundle encryption key or encrypted versions of bundle
blocks other than the payload block.
As was the case for the BAB and PSB, if the ciphersuite requires more
than one instance of the CB, then the first occurrence MUST contain
any optional fields (e.g. security destination etc.) that apply to
all instances with this correlator. These MUST be contained in the
first instance and MUST NOT be repeated in other correlated blocks.
Fields that are specific to a particular instance of the CB MAY
appear in that CB. For example, the security result field MAY (and
probably will) be included in multiple related CB instances.
Similarly, subsequent CBs might each contain a ciphersuite parameters
field with an IV specific to that CB instance.
Put another way: when a node is encrypting some (non-payload) blocks,
it MUST first create a CB with the required ciphersuite ID,
parameters etc. as specified above. Typically, this CB will appear
"early" in the bundle. If this "first" CB doesn't contain all of the
ciphertext, then it may be followed by other (correlated) CB, which
MUST NOT repeat the ciphersuite parameters, security-source, or
security-destination fields from the first CB.
A CB ciphersuite may, or may not, specify which blocks are to be
encrypted. If the ciphersuite doesn't specify this, then the node is
free to encrypt whichever blocks it wishes. If a CB ciphersuite does
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specify which blocks are to be encrypted, then doing otherwise is an
error.
Since a single CB security result can contain the ciphertext for
multiple (non-payload) plaintext blocks, the node simply catenates
these plaintext blocks prior to encryption. After decryption the
recovered plaintext should then replace the CB in the bundle for
further processing (e.g. PSB verification). This recovered
plaintext MUST contain all the appropriate block type, processing
flags and length information. In other words delete the CB in
question and place the recovered plaintext, which consists of
additional (non-payload) blocks, in the bundle at the location from
which the CB was deleted.
Even if a to-be-encrypted block has the "discard" flag set, whether
or not the CB's "discard" flag is set is an implementation/policy
decision for the encrypting node. (The "discard" flag is more
properly called the "discard if block cannot be processed" flag.)
2.5. PSB and CB combinations
Given the above definitions, nodes are free to combine applications
of PSB and CB in any way they wish - the correlator value allows for
multiple applications of security services to be handled separately.
However, there are some clear security problems that could arise when
applying multiple services, for example, if we encrypted a payload
but left a PSB security result containing a signature in clear, this
would allow payload guesses to be confirmed.
We cannot, in general, prevent all such problems since we cannot
assume that every ciphersuite definition takes account of every other
ciphersuite definition. However, we can limit the potential for such
problems by requiring that any ciphersuite which applies to one
instance of a PSB or CB, must be applied to all instances with the
same correlator.
We now list the PSB and CB combinations which we envisage as being
useful to support:
Encrypted tunnels - a single bundle may be encrypted many times
en-route to its destination. Clearly it must be decrypted an
equal number of times, but we can imagine each encryption as
representing the entry into yet another layer of tunnel. This is
supported by using multiple instances of CB, but with the payload
encrypted multiple times, "in-place".
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Multiple parallel authenticators - a single security source might
wish to integrity protect a bundle in multiple ways, in the hope
that one of them will be useful. This could be required if the
path the bundle will take is unpredictable, and if various nodes
might be involved as security destinations. Similarly, if the
security source cannot determine in advance which algorithms to
use, then using all might be reasonable. This would result in
uses of PSB which presumably all protect the payload, and which
cannot in general protect one another. Note that this logic can
also apply to a BAB, if the unpredictable routing happens in the
convergence layer, so we also envisage support for multiple
parallel uses of BAB.
Multiple sequential authenticators - if some security destination
requires assurance about the route that bundles have taken, then
it might insist on each forwarding node adding its own PSB. More
likely however would be that outbound "bastion" nodes would be
configured to sign bundles as a way of allowing the sending
"domain" to take accountability for the bundle. In this case, the
various PSBs will likely be layered, so that each protects the
earlier applications of PSB.
Authenticated and encrypted bundles - a single bundle may require
both authenticity and confidentiality. In this case, most
specifications first apply the authenticator and follow this by
encrypting the payload and authenticator. As noted previously in
the case where the authenticator is a signature, there are
security reasons for this ordering. (See the CB-RSA-AES128-
PAYLOAD-PSB ciphersuite defined later in Section 4.3.)
There are no doubt other valid ways to combine PSB and CB instances,
but these are the "core" set we wish to support. Having said that,
as will be seen, the mandatory ciphersuites defined here are quite
specific and restrictive in terms of limiting the flexibility offered
by the correlator mechanism. This is primarily in order to keep this
specification as simple as possible, while at the same time
supporting the above scenarios.
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3. Security Processing
This section describes the security aspects of bundle processing.
3.1. Nodes as policy enforcement points
All nodes are REQUIRED to have and enforce their own configurable
security policies, whether these policies be explicit or default, as
defined in Section 6.
All nodes serve as Policy Enforcement Points (PEP) insofar as they
enforce polices that may restrict the permissions of bundle nodes to
inject traffic into the network. If a particular transmission
request satisfies the node's policy and is therefore accepted, then
an outbound bundle can be created and dispatched. If not, then in
its role as a PEP, the node will not create or forward a bundle.
Error handling for such cases is currently considered out of scope of
this document.[Comment.3]
Policy enforcing code MAY override all other processing steps
described here and elsewhere in this document. For example, it is
valid to implement a node which always attempts to attach a PSB.
Similarly it is also valid to implement a node which always rejects
all requests which imply the use of a PSB.
Nodes MUST consult their security policy to determine the criteria
that a received bundle ought to meet before it will be forwarded.
These criteria MUST include a determination of whether or not the
received bundle must include valid BAB, PSB or CB. If the bundle
does not meet the node's policy criteria, then the bundle MUST be
discarded and processed no further; in this case, a bundle status
report indicating the failure MAY be generated, destined for the
forwarding node's own endpoint.[Comment.4]
The node's policy MAY call for the node to add or subtract some
security blocks, for example, requiring the node attempt to encrypt
(parts of) the bundle for some security-destination, or requiring
that the node add a PSB. If the node's policy requires a BAB to be
added to the bundle, it MUST be added last so that the calculation of
its security result may take into consideration the values of all
other blocks in the bundle.
3.2. Canonicalisation of bundles
In order to verify a signature or MAC on a bundle the exact same
bits, in the exact same order, must be input to the calculation upon
verification as were input upon initial computation of the original
signature or MAC value. Consequently, a node SHOULD NOT change the
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encoding of any URI in the dictionary field, e.g., changing the DNS
part of some HTTP URL from lower case to upper case. Because bundles
may be modified while in transit (either correctly or due to
implementation errors), a canonical form of any given bundle (that
contains a BAB or PSB) must be defined.
This section defines two bundle canonicalisation algorithms which can
be used by various ciphersuites.
3.2.1. Strict canonicalisation
The first algorithm which can be used basically permits no changes at
all to the bundle between when it is forwarded at the security-source
and when it is received at the security-destination and is mainly
intended for use in BAB ciphersuites. This algorithm simply involves
catenating all blocks in the order presented, but omits all security
result fields which are present in blocks of the ciphersuite type in
question - that is, when a BAB ciphersuite specifies this algorithm
then we omit all BAB security results, when a PSB ciphersuite
specifies this algorithm then we omit all PSB security results. (All
security result length fields are included, even though their
corresponding security result length fields may be omitted.)
Notes:
- In the above we call for security results to be omitted. This
means that no bytes at all of the security result are input. We
do not set the security result length to zero. Rather, we are
assuming that the security result length will be known to the
module that implements the ciphersuite before the security result
is calculated, and that this value will be in the security result
length field even though the security result itself will be
omitted.
- The 'res' bit of the ciphersuite ID, which indicates whether or
not the security result length and security result data field are
present, is part of the canonical form.
-The value of the block data length field, which indicates the
length of the block, is also part of the canonical form. Its
value indicates the length of the entire bundle when the bundle
includes the security result field.
-BABs are always added to bundles after PSBs, so when a PSB
ciphersuite specifies this strict canonicalisation algorithm and
the PSB is received with a bundle that also includes one or more
BABs, application of strict canonicalisation as part of the PSB
security result verification process requires that all BABs in the
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bundle be ignored entirely.
3.2.2. Mutable canonicalisation
This algorithm is mainly intended to protect parts of the bundle
which should not be changed in-transit, and hence it omits the
mutable parts of the bundle.
The basic approach is to define a canonical form for the primary
block, and catenate that with the security and payload blocks in the
order that they will be transmitted. This algorithm ignores all
other blocks on the basis that we cannot tell whether or not they are
liable to change as the bundle transits the network.
Endpoint ID references in security blocks are canonicalized using the
de-referenced text form in place of the reference pair. The
reference count is not included.
The canoncial form of the primary block is shown below. Essentially,
it de-references the dictionary block, adjusts lengths where
necessary and ignores flags that may change in transit.
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+----------------+----------------+----------------+----------------+
| Version | Proc. Flags | COS Flags | SRR Flags |
+----------------+----------------+---------------------------------+
| Canonical primary block length |
+----------------+----------------+---------------------------------+
| Destination endpoint ID length |
+----------------+----------------+---------------------------------+
| |
| Destination endpoint ID |
| |
+----------------+----------------+---------------------------------+
| Source endpoint ID length |
+----------------+----------------+----------------+----------------+
| |
| Source endpoint ID |
| |
+----------------+----------------+---------------------------------+
| Report-to endpoint ID length |
+----------------+----------------+----------------+----------------+
| |
| Report-to endpoint ID |
| |
+----------------+----------------+----------------+----------------+
| |
+ Creation Timestamp (2 x SDNV) +
| |
+---------------------------------+---------------------------------+
| Lifetime |
+----------------+----------------+----------------+----------------+
| Fragment offset (optional) |
+----------------+----------------+---------------------------------+
| Total application data unit length (optional) |
+----------------+----------------+---------------------------------+
The canonical form of the primary bundle block.
Figure 4
The fields shown are:
Version, Processing Flags, COS, SRR - are all copied from the
first four bytes of the primary block and will contain the version
and the immutable flag values from the primary block. Formed by
copying the processing, COS and SRR flag fields from the primary
block and then subsequently setting all of the mutable bits to
zero. This requires ANDing with the (four byte) value 0xFF3E031F
so that the mutable and reserved bits are set to zero. The only
currently mutable bit masked out here is the "bundle is a
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fragment" bit - all others are reserved bits. Note also that,
since the flags fields are packed into an SDNV, adding a bit might
change its length and thereby invalidate the signature even though
the extra bit is not included in the canonicalization. To prevent
this error, canonicalization uses the non-SDNV form of the flags
field, along with the appropriate mask. As described in [2], the
maximum size supported for SDNV encoding is 64 bits.
Canonicalization therefore uses the 64-bit decoded value, masked
as described above.
There is an issue here which PSB ciphersuites MUST tackle. If a
bundle is fragmented before the PSB is applied then the PSB
applies to a fragment and not the entire bundle. However, the
protected fragment could be subsequently further fragmented, which
would leave the verifier unable to know which bytes were protected
by the PSB. For this reason, PSB ciphersuites which support
applying a PSB to fragments MUST specify which bytes of the bundle
payload are protected as part of the ciphersuite parameters. When
verifying such a fragment only the bytes from the fragment are
input to the PSB verification. Of course, if is also valid for a
ciphersuite to be specified so as to only apply to entire bundles
and not to fragments.
Length - a four-byte value containing the length (in bytes) of
this structure.[Comment.5]
Destination endpoint ID length and value - are the length (as a
four byte value) and value of the destination endpoint ID from the
primary bundle block. The URI is simply copied from the relevant
part(s) of the dictionary block and is not itself canonicalised.
Source endpoint ID length and value are handled similarly to the
destination.
Report-to endpoint ID length and value are handled similarly to
the destination.
Creation time and Lifetime are simply copied from the primary
block.
Fragment offset and Total application data unit length are copied
from the primary block if they are present there (which is
controlled by one of the flags).
Payload blocks are generally copied as-is, with the exception that
the processing flags value in the canonical version MUST be ANDed
with 0x37 to ensure that currently "reserved" flags are clear and
that the "last block" flag is ignored. The reason to ignore the
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"last block" flag is that if a bundle is created with both PSB and
BAB, then the next hop will remove the BAB instances, and if one of
those was the last block, it may set the "last block" flag of, e.g.,
a PSB instance. The "last block" flag is therefore a mutable bit and
should be omitted from the canonical form. Note that the "Block was
forwarded without being processed" flag is included in the canonical
form of the Payload Block because this flag is never expected to be
set by any node; all nodes are required to be able to process the
Payload Block. The SDNV considerations described above for the
primary block flags field apply also to the flags field of the
payload and other non-primary blocks.
Another exception occurs in cases where only a fragment of the
payload was protected, when only those bytes of the payload block
payload field are considered part of the canonical form.
Security blocks are handled likewise, with two exceptions:
- the "Block was forwarded without being processed" flag MUST be
omitted from the canonical form of the PSB and the CB because this
flag may be set by a node located between the security-source and
the security-destination. Therefore, the processing flags value
of the PSB and the CB in the canonical version MUST be ANDed with
0x17 to ensure that the mutable "Block was forwarded without being
processed" flag is ignored during canonicalization of these
blocks.
- the ciphersuite will likely specify that the "current" security
block security result field not be considered part of the
canonical form. This differs from the case in strict
canonicalisation since we might use the mutable canonicalisation
algorithm to handle sequential signatures, where later signatures
should cover earlier ones.
Notes:
- The canonical form of the bundle is not what is transmitted. It
is simply an artifact that is used as input to digesting.
- We omit the reserved flags on the basis that we cannot determine
whether or not they will change in transit. This means that this
algorithm may have to be revised if those flags are given a
definition and if we want to protect them.
- Our URI encoding does not preserve the "null-termination"
convention from the dictionary field, nor do we separate the
scheme and ssp as is done there.
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- Note that the URI encoding above will be a cause for errors if
any node rewrites the dictionary for example changing the DNS part
of some HTTP URL from lower-case to upper case. This could happen
transparently, for example, when a bundle is synched to disk using
one set of software and then read from disk and forwarded by a
second set of software. Because there are no exact general rules
for canonicalising URIs (or even worse IRIs), this problem may be
an unavoidable source of integrity failures.
- All length fields here are four byte values in network byte
order. We do not need to optimize the encoding since the values
are never sent over the network.
3.3. Endpoint ID confidentiality
Since every bundle MUST contain a primary block that cannot be
encrypted, and which contains the source endpoint ID (and others), if
we want to provide endpoint ID confidentiality, then we have to
invent a fake primary block with false values for these fields and
then a new block type to contain the actual values.
Similarly, there may be confidentiality requirements applying to
other parts of the primary block (e.g. the current-custodian) and we
support these in the same way.
Since we don't know if we'll do this...details are TBD:-)[Comment.6]
3.4. Bundles received from other nodes
Nodes implementing this specification SHALL consult their security
policy to determine whether or not a received bundle is required by
policy to include a BAB. If the bundle is not required to have a
BAB, then BAB processing on the received bundle is complete and the
bundle is ready to be further processed for CB/PSB handling or
delivery or forwarding.
If the bundle is required to have a BAB but it does not, then the
bundle MUST be discarded and processed no further. If the bundle is
required to have a BAB but all of its BABs identify a different node
other than the receiving node as the BAB security destination, then
the bundle MUST be discarded and processed no further.
Otherwise, if the bundle does have a BAB that either does not have a
security destination field or that identifies the receiving node as
the BAB security destination, then the value in the security result
field of the BAB MUST be verified according to the ciphersuite
specification. If for all such BABs in the bundle either the BAB
security source cannot be determined or the security result value
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check fails, the bundle has failed to authenticate and the bundle
SHALL be discarded and processed no further. Otherwise, if any of
the BABs present verify, the bundle is ready to have its CB processed
(if it includes one).
When forwarding a bundle that included some BABs when it was
received, these BABs MUST be stripped from the bundle. New BABs MAY
be added as required by policy. This might require correcting the
"last block" field of the to-be-forwarded bundle.
If the bundle has a CB and the receiving node is the CB destination
for the bundle (either because the node is listed in the bundle's CB-
dest field or because the node is listed as the bundle's destination
and there is no CB-dest field), the node MUST decrypt the relevant
parts of the bundle according to the ciphersuite specification and
delete the CB in question. If the relevant parts of the bundle
cannot be decrypted (i.e., the decryption key cannot be deduced or
decryption fails), then the bundle MUST be discarded and processed no
further; in this case a bundle deletion status report (see the Bundle
Protocol [2]) indicating the decryption failure MAY be generated,
destined for the receiving node's own endpoint. If the CB security
result included the ciphertext of anything other than an encrypted
BEK that was used to encrypt the bundle payload, the recovered
plaintext blocks MUST be placed in the bundle at the location from
which the CB was deleted.
If the bundle has a PSB and the receiving node is the PSB destination
for the bundle (either because the node is listed in the bundle's
PSB-dest field or because the node is listed as the bundle's
destination and there is no PSB-dest field), the node MUST verify the
value in the security result field of the PSB according to the
ciphersuite specification. If the check fails, the bundle has failed
to authenticate and the bundle SHALL be discarded and processed no
further; a bundle status report indicating the failure MAY be
generated, destined for the receiving node's own endpoint.
Otherwise, if the PSB verifies, the bundle is ready to be processed
for either delivery or forwarding. Before forwarding the bundle, the
node SHOULD remove the PSB from the bundle, unless there is the
likelihood that some downstream node will also be able to verify the
PSB.
If the bundle has a PSB and the receiving node is not the PSB-dest
for the bundle but the ciphersuite allows, the receiving node MAY, if
it is able, verify the value in the security result field. If the
check fails, the node SHALL discard the bundle and it MAY send a
bundle status report indicating the failure to the receiving node's
own endpoint.
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3.5. The At-Most-Once-Delivery Option
An application may request (in some implementation specific manner)
that a node be registered as a member of an endpoint and that
received bundles destined for that endpoint be delivered to that
application.
We define a new option for use in such cases, known as "at-most-once-
delivery". If this option is chosen, then the application is
indicating that it wants the node to check for duplicate bundles,
discard duplicates, and deliver at most one copy of each received
bundle to the application. If this option is not chosen, the
application is indicating that it wants the node to deliver all
received bundle copies to the application. If this option is chosen,
the node SHALL deliver at most one copy of each received bundle to
the application. If the option is not chosen, the node SHOULD
(subject to policy) deliver all bundles.
To enforce this the node MUST look at the (source, timestamp) pair
value of each complete (reassembled, if necessary) bundle received
and determine if this pair, which should uniquely identify a bundle,
has been received before. If it has, then the bundle is a duplicate.
If it has not, then the bundle is not a duplicate. The (source,
timestamp) pair SHALL be added to the list of pair values already
received by that node.
The duration for which a node maintains entries on such a list is an
implementation matter.
If any application has indicated that it wants a node to use the "at
most once" delivery option for a particular destination endpoint ID
that is in a bundle, then the node MUST compare the (source,
timestamp, fragment offset, fragment length) values of the bundle
with the local list of such values of already-received bundles.
Additional discussion relevant to at-most-delivery is in the DTN
Retransmission Block specification [10].
3.6. Bundle Fragmentation and Reassembly
If it is necessary for a node to fragment a bundle and security is
being used on that bundle, the following security-specific processing
is REQUIRED:
Firstly, a BAB, PSB or CB MUST NOT be fragmented. At this time, only
the payload field of the payload block MAY be fragmented.
If the bundle is required by the security policy to have a BAB before
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being forwarded, all fragments resulting from that bundle MUST
contain individual BAB values.
If the original bundle had a PSB, then each of the PSB instances MUST
be included in some fragment. A single PSB instance MUST NOT be sent
more than once.
If the original bundle had a CB, then the each of the CB instances
MUST be included in some fragment. A single CB instance MUST NOT be
sent more than once.
Note: various fragments may have additional security blocks added at
this or later stages and it is possible that correlators may collide.
In order to facilitate uniqueness, ciphersuites SHOULD include the
fragment-offset of the fragment as a high-order component of the
correlator.
3.7. Reactive fragmentation
When original bundle transmission is terminated before the entire
bundle has been transmitted, the receiving node SHALL consult its
security policy to determine whether it is permitted to transform the
received portion of the bundle into a bundle fragment for further
forwarding. Whether or not such reactive fragmentation is permitted
SHALL be dependent on the security policy in combination with the
ciphersuite used to calculate the BAB authentication information if
required. (Some BAB ciphersuites, i.e., the mandatory BAB-HMAC
ciphersuite defined in Section 4.1, do not accommodate reactive
fragmentation because the security result in the BAB requires that
the entire bundle be signed. It is conceivable, however, that a BAB
ciphersuite could be defined such that multiple security results are
calculated, each on a different segment of a bundle, and that these
security results could be interspersed between bundle payload
segments such that reactive fragmentation could be accommodated.)
If the original bundle is fragmented by the intermediate receiver
(reactively), and the BAB-ciphersuite is of an appropriate type (e.g.
with multiple security results embedded in the payload), the bundle
MUST be fragmented immediately after the last security result value
in the partial payload that is received. Any data received after the
last security result value MUST be dropped.
If an original bundle transmission is terminated before the entire
bundle has been transmitted, if the truncated bundle arriving at the
intermediate receiver is reactively fragmented and forwarded, only
the part of the bundle that was not received MUST be retransmitted,
though more of the bundle MAY be retransmitted. Before
retransmitting a portion of the bundle, it SHALL be changed into a
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fragment and, if the original bundle included a BAB, the fragmented
bundle MUST also, and its BAB SHALL be recalculated.
This specification does not currently define any ciphersuite which
can handle this reactive fragmentation case well.
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4. Mandatory Ciphersuites
This section defines the mandatory ciphersuites for this
specification. There is currently one mandatory ciphersuite for each
of BAB, PSB and CB. The BAB ciphersuite is based on shared secrets
using HMAC. The PSB ciphersuite is based on digital signatures using
RSA with SHA256. The CB ciphersuite is based on using RSA for key
transport and AES for bulk encryption.
4.1. BAB-HMAC
The BAB-HMAC ciphersuite has ciphersuite ID value 0x001.
Security parameters are optional with this scheme, but if used then
the value of the ciphersuite parameter MUST be used as a key
identifier. The exact type of key identifier to be used is an
implementation issue. In the absence of a key identifier the
intermediate receiver is expected to be able to find the correct key
based on the sending identity (from the security-source and/or
convergence layer).
BAB-HMAC uses the strict canonicalisation algorithm in Section 3.2.1.
The variant of HMAC to be used is HMAC-SHA1 as defined in RFC 2104
[3].[Comment.7]
This ciphersuite requires the use of two related instances of the
BAB. It involves placing the first BAB instance (as defined in
Section 2.2) just after the primary block. The second (correlated)
instance of the BAB MUST be placed after all other blocks (except
possibly other BAB blocks) in the bundle.
This means that normally, the BAB will be the second and last blocks
of the bundle. If a forwarder wishes to apply more than one
correlated BAB pair, then this can be done. There is no requirement
that each application "wrap" the others, but the forwarder MUST
insert all the "up front" BABs, and their "at back" "partners"
(without any security result), before canonicalising.
Inserting more than one correlated BAB pair would be useful if the
bundle could be routed to more than one potential "next-hop" or if
both an old or a new key were valid at sending time, with no
certainty about the situation that will obtain at reception time.
The security result is the output of the HMAC-SHA1 calculation with
input being the result of running the entire bundle through the
strict canonicalisation algorithm. Both required BAB instances MUST
be included in the bundle before canonicalisation.
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4.2. PSB-RSA-SHA256
The PSB-RSA-SHA256 ciphersuite has ciphersuite ID value 0x002.
If the bundle being signed has been fragmented before signing, then
we have to specify which bytes were signed, in case the signed bundle
is subsequently fragmented for a second time. So, if the bundle is a
fragment, then the ciphersuite parameters MUST include two SDNV
encoded numbers, representing the offset and length of the signed
fragment. If the entire bundle is signed then these numbers MUST be
omitted.
The ciphersuite parameters field MAY also contain a key identifier.
The exact type of key identifier to be used is an implementation
issue. In the absence of a key identifier the verifier of the PSB is
expected to be able to use the security source (if supplied) or else
the bundle source (if no security source is present) in order to
determine the correct public key to use for PSB verification.
PSB-RSA-SHA256 uses the mutable canonicalisation algorithm
Section 3.2.2. The resulting canonical form of the bundle is the
input to the signing process. This ciphersuite requires the use of a
single instance of the PSB.
RSA is used with SHA256 as specified for the sha256WithRSAEncryption
PKCSv1.5 signature scheme in RFC 4055 [4]. The output of the signing
process is the security result for the PSB.
"Commensurate strength" cryptography is generally held to be a good
idea. A combination of RSA with SHA256 is reckoned to require a 3076
bit RSA key according to this logic. Few implementations will choose
this length by default (and probably some just won't support such
long keys). Since this is an experimental protocol, we expect that
1024 or 2048 bit RSA keys will be used in many cases, and that that
will be fine since we also expect that the hash function "issues"
will be resolved before any standard would be derived from this
protocol.[Comment.8]
4.3. CB-RSA-AES128-PAYLOAD-PSB
[Comment.9]
The CB-RSA-AES128-PAYLOAD-PSB ciphersuite has ciphersuite ID value
0x003.
This scheme only allows for payload and PSB encryption and involves
encrypting every instance of a PSB as well as the payload.
This ciphersuite requires the use of a single CB instance if the
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bundle does not contain a PSB, and multiple CB instances if the
bundle includes one or more PSBs. A first CB is created which
contains the encrypted bundle encryption key (BEK). The key size for
this ciphersuite is 128 bits.
For the first CB, there MUST be a ciphersuite parameter which
contains a 16 byte IV, optionally followed by a key identifier (whose
format is again out of scope here). (If the ciphersuite parameters
length field has a value equal to 16, then the parameters data field
consists of only a 16-bye IV. If the ciphersuite parameters length
field has a value greater than 16, then the ciphersuite parameters
data field consists of a 16-byte IV followed by a key identifier, and
the length of that key identifier is the value in the ciphersuite
parameters length field minus 16.) The security result contains the
BEK encrypted using PKCSv1.5 rsaEncryption as specified in RFC 3370
[5].
For each subsequent PSB, the entire block is replaced by a CB that is
correlated with the first CB and whose security result is the
ciphertext form of the PSB, including the block type, etc. The
parameters field contains a 16-byte IV specific to this block.
For the payload, only the bytes of the bundle payload field are
affected, being replaced by ciphertext.
We separately encrypt the payload and each of the PSB blocks, using
the BEK and a different IV. The IV for the payload is contained in
the first CB, the IV for each of the PSBs is in the parameter field
of the replacement C block.
The BEK uses the AES algorithm in CBC mode as specified by the id-
aes-cbc object identifier in RFC 3565 [6]
[Comment.10][Comment.11][Comment.12]
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5. Key Management
Since key management in delay tolerant networks is still a research
topic we cannot provide much in the way of useful key management
here. However, solely in order to support implementation and
testing, implementations SHOULD support:
- Long-term pre-shared-symmetric keys for the BAB-HMAC
ciphersuite.
- The use of well-known RSA public keys for PSB-RSA-SHA256 and CB-
RSA-AES128-PAYLOAD-PSB ciphersuites.
Since endpoint IDs are URIs and URIs can be placed in X.509 [7]
public key certificates (in the subjectAltName extension)
implementations SHOULD support this way of distributing public keys.
Implementations SHOULD NOT be very strict in how they process X.509
though, for example, it would probably not be correct to insist on
Certificate Revocation List (CRL) checking in many DTN contexts.
Other than that, key management is for future study.
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6. Default Security Policy
Every node serves as a Policy Enforcement Point (PEP) insofar as it
enforces some policy that controls the forwarding and delivery of
bundles via one or more convergence layer protocol implementation.
Consequently, every node SHALL have and operate according to its own
configurable security policy, whether the policy be explicit or
default. The policy SHALL specify:
Under what conditions received bundles SHALL be forwarded.
Under what conditions received bundles SHALL be required to
include valid BABs.
Under what conditions the authentication information provided in a
bundle's BAB SHALL be deemed adequate to authenticate the bundle.
Under what conditions received bundles SHALL be required to have
valid PSBs and/or CBs.
Under what conditions the authentication information provided in a
bundle's PSB SHALL be deemed adequate to authenticate the bundle.
Under what conditions a BAB SHALL be added to a received bundle
before that bundle is forwarded.
Under what conditions a PSB SHALL be added to a received bundle
before that bundle is forwarded.
Under what conditions a CB SHALL be added to a received bundle
before that bundle is forwarded.
The actions that SHALL be taken in the event that a received
bundle does not meet the receiving node's security policy
criteria.
This specification does not address how security policies get
distributed to nodes. It only REQUIRES that nodes have and enforce
security policies. [Comment.13]
If no security policy is specified at a given node, or if a security
policy is only partially specified, that node's default policy
regarding unspecified criteria SHALL consist of the following:
Bundles that are not well-formed do not meet the security policy
criteria.
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The mandatory ciphersuites MUST be used.
All bundles received MUST have a BAB which MUST be verified to
contain a valid security result. If the bundle does not have a
BAB, then the bundle MUST be discarded and processed no further; a
bundle status report indicating the authentication failure MAY be
generated, destined for the receiving node's own endpoint.
No received bundles SHALL be required to have a PSB; if a received
bundle does have a PSB, however, the PSB can be ignored unless the
receiving node is the PSB-dest, in which case the PSB MUST be
verified.
No received bundles SHALL be required to have a CB; if a received
bundle does have a CB, however, the CB can be ignored unless the
receiving node is the CB-dest, in which case the CB MUST be
processed. If processing of a CB yields a PSB, that PSB SHALL be
processed by the node according to the node's security policy.
A PSB SHALL NOT be added to a bundle before sourcing or forwarding
it.
A CB SHALL NOT be added to a bundle before sourcing or forwarding
it.
A BAB MUST always be added to a bundle before that bundle is
forwarded.
If a destination node receives a bundle that has a PSB-destination
field but the value in that PSB-destination field is not the EID
of the destination node, the bundle SHALL be delivered at that
destination node.
If a received bundle does not satisfy the node's security policy
for any reason, then the bundle MUST be discarded and processed no
further; in this case, a bundle deletion status report (see the
Bundle Protocol [2]) indicating the failure SHOULD be generated,
destined for the receiving node's own endpoint.
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7. Security Considerations
[Comment.14]
If a BAB ciphersuite uses digital signatures but doesn't include the
security destination (which for a BAB is the next host), then this
allows the bundle to be sent to some node other than the intended
adjacent node. Because the BAB will still authenticate, the
receiving node may erroneously accept and forward the bundle. When
asymmetric BAB ciphersuites are used, the security destination field
SHOULD therefore be included in the BAB.
If a bundle's PSB-dest is not the same as its destination, then some
node other than the destination (the node identified as the PSB-dest)
is expected to validate the PSB security result while the bundle is
en route. However, if for some reason the PSB is not validated,
there is no way for the destination to become aware of this.
Typically, a PSB-dest will remove the PSB from the bundle after
verifying the PSB and before forwarding it. However, if there is a
possibility that the PSB will also be verified at a downstream node,
the PSB-dest will leave the PSB in the bundle. Therefore, if a
destination receives a bundle with a PSB that has a PSB-dest (which
isn't the destination), this may, but does not necessarily, indicate
a possible problem.
If a bundle is fragmented after being forwarded by its PSB-source but
before being received by its PSB-dest, the payload in the bundle MUST
be reassembled before validating the PSB security result in order for
the security result to validate correctly. Therefore, if the PSB-
dest is not capable of performing payload reassembly, its utility as
a PSB-dest will be limited to validating only those bundles that have
not been fragmented since being forwarded from the PSB-source.
Similarly, if a bundle is fragmented after being forwarded by its
PSB-source but before being received by its PSB-dest, all fragments
MUST be received at that PSB-dest in order for the bundle payload to
be able to be reassembled. If not all fragments are received at the
PSB-dest node, the bundle will not be able to be authenticated, and
will therefore never be forwarded by this PSB-dest node.
Specification of a security-destination other than the bundle
destination creates a routing requirement that the bundle somehow be
directed to the security-destination node on its way to the final
destination. This requirement is presently private to the
ciphersuite, since routing nodes are not required to implement
security processing.
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8. IANA Considerations
None at this time. If the bundle protocol becomes a standards track
protocol, then we may want to consider having IANA establish a
register of block types, and in particular for this specification a
separate register of ciphersuite specifications.
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9. References
9.1. Normative References
[1] Bradner, S. and J. Reynolds, "Key words for use in RFCs to
Indicate Requirement Levels", RFC 2119, October 1997.
[2] Scott, K. and S. Burleigh, "Bundle Protocol Specification",
draft-irtf-dtnrg-bundle-spec-09.txt, work-in-progress,
April 2007.
[3] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[4] Schaad, J., Kaliski, B., and R. Housley, "Additional Algorithms
and Identifiers for RSA Cryptography for use in the Internet
X.509 Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 4055, June 2005.
[5] Housley, R., "Cryptographic Message Syntax (CMS) Algorithms",
RFC 3370, August 2002.
[6] Schaad, J., "Use of the Advanced Encryption Standard (AES)
Encryption Algorithm in Cryptographic Message Syntax (CMS)",
RFC 3565, July 2003.
[7] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002.
9.2. Informative References
[8] Farrell, S., Symington, S., and H. Weiss, "Delay-Tolerant
Network Security Overview",
draft-irtf-dtnrg-sec-overview-03.txt, work-in-progress,
April 2007.
[9] Cerf, V., Burleigh, S., Durst, R., Fall, K., Hooke, A., Scott,
K., Torgerson, L., and H. Weiss, "Delay-Tolerant Network
Architecture", RFC 4838, April 2007.
[10] Symington, S., "Delay-Tolerant Network Retransmission Block",
draft-irtf-dtnrg-bundle-retrans-00.txt, work-in-progress,
April 2007.
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Editorial Comments
[] Stephen: I guess there could be some weird corner case
where a CB ciphersuite using counter-mode would allow
fragments to be individually decrypted, and in that
case, we might want to set replication for each
fragment. So we can't fully rule out setting that flag
for all PSB/CB.
[] Stephen: This to be revisited!
[] Stephen: Do we need to specify error handling for the
case where a node drops a bundle for policy reasons?
Does/can it signal back to the source that its done so?
[] Howie: The security policy database will need to be
discussed somewhere. Does it belong in this document,
the bundle protocol spec., both, some other document?
[] Editors: Check that mask value at the very last moment
(incl. during auth-48) to be sure its (still) correct.
[] Stephen: Should we support source confidentiality?
Might complicate PSB which is the downside IMO.
[] Editors: At the moment there appears to be no security
reason to move away from HMAC-SHA1 since the HMAC
construct is not as far as we know affected by
collisions in the underlying digest algorithm (which
are nearly practically computable for SHA-1).
Nevertheless, since we use SHA-256 in the signature
ciphersuite (since collisions do matter there), it may
be desirable to move to HMAC-SHA-256 as specified in
RFC 4321. So if you're writing code based on this...be
warned!
[] Editors: There are currently unresolved "issues" with
digest algorithms which might cause a change here prior
to, but more likely, after, an RFC has issued. So
expect change!
[] Editors: This entire section is to be treated as a
strawman for the present.
[] Speculation: There would be an interesting possibility
opened up were we to use a stream cipher with the REK.
That is that we could then encrypt and decrypt
independently - Alice could encrypt for Bob, then
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Charlie could encrypt for Dessie, then Bob could
decrypt, then Dessie. Better in terms of not adding
padding but worse in that it trivially allows known
plaintext manipulation if there's no PSB.
[] Editors: Another option might also be to switch to
using counter mode rather than CBC which would have the
benefit of allowing some fragments to be decrypted even
if not all fragments arrive. While that seems nice
enough to do, it would of course require us to think
more about fragments and so is for the next version if
at all.
[] Peter: there does not seem to be a suitable CTR mode
implementation for AES but perhaps CFB (also stream
mode) would be suitable. It also avoids padding.
[] Howie: Eventually we will need to state where the
security policy information/DB does get discussed/
specified.
[] Editors: Much more text is needed here no doubt.
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Authors' Addresses
Susan Flynn Symington
The MITRE Corporation
7515 Colshire Drive
McLean, VA 22102
US
Phone: +1 (703) 983-7209
Email: susan@mitre.org
URI: http://mitre.org/
Stephen Farrell
Trinity College Dublin
Distributed Systems Group
Department of Computer Science
Trinity College
Dublin 2
Ireland
Phone: +353-1-608-1539
Email: stephen.farrell@cs.tcd.ie
Howard Weiss
SPARTA, Inc.
7110 Samuel Morse Drive
Columbia, MD 21046
US
Phone: +1-443-430-8089
Email: hsw@sparta.com
Peter Lovell
SPARTA, Inc.
7110 Samuel Morse Drive
Columbia, MD 21046
US
Phone: +1-443-430-8052
Email: peter.lovell@sparta.com
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