BPSec Default Security Contexts
draft-ietf-dtn-bpsec-default-sc-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9173.
|
|
|---|---|---|---|
| Author | Edward J. Birrane | ||
| Last updated | 2021-03-28 | ||
| Replaces | draft-ietf-dtn-bpsec-interop-sc | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-07)
by Thomas Fossati
Ready w/nits
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Scott Burleigh | ||
| Shepherd write-up | Show Last changed 2021-02-25 | ||
| IESG | IESG state | Became RFC 9173 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Zaheduzzaman Sarker | ||
| Send notices to | Scott.C.Burleigh@jpl.nasa.gov |
draft-ietf-dtn-bpsec-default-sc-03
Delay-Tolerant Networking E. Birrane
Internet-Draft JHU/APL
Intended status: Standards Track March 28, 2021
Expires: September 29, 2021
BPSec Default Security Contexts
draft-ietf-dtn-bpsec-default-sc-03
Abstract
This document defines default integrity and confidentiality security
contexts that may be used with the Bundle Protocol Security Protocol
(BPSec) implementations. These security contexts are intended to be
used for both testing the interoperability of BPSec implementations
and for providing basic security operations when no other security
contexts are defined or otherwise required for a network.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 29, 2021.
Copyright Notice
Copyright (c) 2021 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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Integrity Security Context BIB-HMAC-SHA2 . . . . . . . . . . 3
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Parameters . . . . . . . . . . . . . . . . . . . . . . . 5
3.3.1. SHA Variant . . . . . . . . . . . . . . . . . . . . . 6
3.3.2. Encapsulated Key . . . . . . . . . . . . . . . . . . 6
3.3.3. Integrity Scope Flags . . . . . . . . . . . . . . . . 6
3.3.4. Enumerations . . . . . . . . . . . . . . . . . . . . 7
3.4. Results . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Key Considerations . . . . . . . . . . . . . . . . . . . 8
3.6. Canonicalization Algorithms . . . . . . . . . . . . . . . 8
3.7. Processing . . . . . . . . . . . . . . . . . . . . . . . 9
3.7.1. Keyed Hash Generation . . . . . . . . . . . . . . . . 9
3.7.2. Keyed Hash Verification . . . . . . . . . . . . . . . 10
4. Security Context BCB-AES-GCM . . . . . . . . . . . . . . . . 11
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3. Parameters . . . . . . . . . . . . . . . . . . . . . . . 14
4.3.1. Initialization Vector (IV) . . . . . . . . . . . . . 14
4.3.2. AES Variant . . . . . . . . . . . . . . . . . . . . . 14
4.3.3. Encapsulated Key . . . . . . . . . . . . . . . . . . 15
4.3.4. AAD Scope Flags . . . . . . . . . . . . . . . . . . . 15
4.3.5. Enumerations . . . . . . . . . . . . . . . . . . . . 15
4.4. Results . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.4.1. Authentication Tag . . . . . . . . . . . . . . . . . 16
4.4.2. Enumerations . . . . . . . . . . . . . . . . . . . . 17
4.5. Key Considerations . . . . . . . . . . . . . . . . . . . 17
4.6. GCM Considerations . . . . . . . . . . . . . . . . . . . 18
4.7. Canonicalization Algorithms . . . . . . . . . . . . . . . 18
4.7.1. Cipher text related calculations . . . . . . . . . . 19
4.7.2. Additional Authenticated Data . . . . . . . . . . . . 19
4.8. Processing . . . . . . . . . . . . . . . . . . . . . . . 20
4.8.1. Encryption . . . . . . . . . . . . . . . . . . . . . 20
4.8.2. Decryption . . . . . . . . . . . . . . . . . . . . . 22
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
5.1. Security Context Identifiers . . . . . . . . . . . . . . 23
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
6.1. Key Handling . . . . . . . . . . . . . . . . . . . . . . 24
6.2. AES GCM . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.3. Bundle Fragmentation . . . . . . . . . . . . . . . . . . 24
7. Normative References . . . . . . . . . . . . . . . . . . . . 25
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Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
The Bundle Protocol Security Protocol (BPSec) [I-D.ietf-dtn-bpsec]
specification provides inter-bundle integrity and confidentiality
operations for networks deploying the Bundle Protocol (BP)
[I-D.ietf-dtn-bpbis]. BPSec defines BP extension blocks to carry
security information produced under the auspices of some security
context.
This document defines two security contexts (one for an integrity
service and one for a confidentiality service) for populating BPSec
Block Integrity Blocks (BIBs) and Block Confidentiality Blocks
(BCBs).
These contexts generate information that MUST be encoded using the
CBOR specification documented in [RFC8949].
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Integrity Security Context BIB-HMAC-SHA2
3.1. Overview
The BIB-HMAC-SHA2 security context provides a keyed hash over a set
of plain text information. This context uses the Secure Hash
Algorithm 2 (SHA-2) discussed in [SHS] combined with the HMAC keyed
hash discussed in [HMAC]. The combination of HMAC and SHA-2 as the
integrity mechanism for this security context was selected for two
reasons:
1. The use of symmetric keys allows this security context to be used
in places where an asymmetric-key infrastructure (such as a
public key infrastructure) may be impractical.
2. The combination HMAC-SHA2 represents a well-supported and well-
understood integrity mechanism with multiple implementations
available.
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BIB-HMAC-SHA2 supports three variants of HMAC-SHA, based on the
supported length of the SHA-2 hash value. These variants correspond
to "HMAC 256/256", "HMAC 384/384", and "HMAC 512/512" as defined in
[RFC8152] Table 7: HMAC Algorithm Values. The selection of which
variant is used by this context is provided as a security context
parameter.
The output of the HMAC MUST be equal to the size of the SHA2 hashing
function: 256 bits for SHA-256, 384 bits for SHA-384, and 512 bits
for SHA-512.
The BIB-HMAC-SHA2 security context MUST have the security context
identifier specified in Section 5.1.
3.2. Scope
The scope of BIB-HMAC-SHA2 is the set of information used to produce
the plain text over which a keyed hash is calculated. This plain
text is termed the "Integrity Protected Plain Text" (IPPT). The
content of the IPPT is constructed as the concatenation of
information whose integrity is being preserved from the BIB-HMAC-SHA2
security source to its security acceptor. There are four types of
information that can be used in the generation of the IPPT, based on
how broadly the concept of integrity is being applied. These four
types of information, whether they are required, and why they are
important for integrity, are discussed as follows.
Security target contents
The contents of the block-type-specific data field of the
security target MUST be included in the IPPT. Including this
information protects the security target data and is considered
the minimal, required set of information for an integrity service
on the security target.
Primary block
The primary block identifies a bundle and, once created, the
contents of this block are immutable. Changes to the primary
block associated with the security target indicate that the
security target (and BIB) may no longer be in the correct bundle.
For example, if a security target and associated BIB are copied
from one bundle to another bundle, the BIB may still contain a
verifiable signature for the security target unless information
associated with the bundle primary block is included in the keyed
hash carried by the BIB.
Including this information in the IPPT protects the integrity of
the association of the security target with a specific bundle.
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Security target other fields
The other fields of the security target include block
identification and processing information. Changing this
information changes how the security target is treated by nodes
in the network even when the "user data" of the security target
are otherwise unchanged.
For example, if the block processing control flags of a security
target are different at a security verifier than they were
originally set at the security source then the policy for
handling the security target has been modified.
Including this information in the IPPT protects the integrity of
the policy and identification of the security target data.
BIB other fields
The other fields of the BIB include block identification and
processing information. Changing this information changes how
the BIB is treated by nodes in the network, even when other
aspects of the BIB are unchanged.
For example, if the block processing control flags of the BIB are
different at a security verifier than they were originally set at
the security source, then the policy for handling the BIB has
been modified.
Including this information in the IPPT protects the integrity of
the policy and identification of the security service in the
bundle.
NOTE: The security context identifier and security context
parameters of the security block are not included in the IPPT
because these parameters, by definition, are required to verify
or accept the security service. Successful verification at
security verifiers and security acceptors implies that these
parameters were unchanged since being specified at the security
source.
The scope of the BIB-HMAC-SHA2 security context is configured using
an optional security context parameter.
3.3. Parameters
BIB-HMAC-SHA2 can be parameterized to select SHA-2 variants,
communicate key information, and define the scope of the IPPT.
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3.3.1. SHA Variant
This optional parameter identifies which variant of the SHA-2
algorithm is to be used in the generation of the authentication code.
This value MUST be encoded as a CBOR unsigned integer.
Valid values for this parameter are as follows.
SHA Variant Parameter Values
+-------+-----------------------------------------------------------+
| Value | Description |
+-------+-----------------------------------------------------------+
| 5 | HMAC 256/256 as defined in [RFC8152] Table 7: HMAC |
| | Algorithm Values |
| 6 | HMAC 384/384 as defined in [RFC8152] Table 7: HMAC |
| | Algorithm Values |
| 7 | HMAC 512/512 as defined in [RFC8152] Table 7: HMAC |
| | Algorithm Values |
+-------+-----------------------------------------------------------+
Table 1
When not provided, implementations SHOULD assume a value of 6
(indicating use of HMAC 384/384), unless an alternate default is
established by security policy at the security source, verifiers, or
acceptor of this integrity service.
3.3.2. Encapsulated Key
This optional parameter contains the output of a Key Encapsulation
Mechanism (KEM) run at the security source of this security context.
This value MUST be encoded as a CBOR byte string.
If provided, this information is used to retrieve the symmetric HMAC
key used in the generation of security results for this security
context. If not provided, security verifiers and acceptors MUST
determine the proper key as a function of their local BPSec policy
and configuration, as discussed in Section 3.5.
3.3.3. Integrity Scope Flags
This optional parameter contains a series of flags that describe what
information is to be included with the block-type-specific data when
constructing the IPPT value.
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This value MUST be represented as a CBOR unsigned integer, the value
of which MUST be processed as a bit field containing no more than 8
bits.
Bits in this field represent additional information to be included
when generating an integrity signature over the security target.
These bits are defined as follows.
- Bit 0 (the low-order bit, 0x1): Primary Block Flag.
- Bit 1 (0x02): Target Header Flag.
- Bit 2 (0x03): Security Header Flag.
- Bits 3-7 are reserved.
3.3.4. Enumerations
BIB-HMAC-SHA2 defines the following security context parameters.
BIB-HMAC-SHA2 Security Parameters
+----+-----------------------+--------------------+---------------+
| Id | Name | CBOR Encoding Type | Default Value |
+----+-----------------------+--------------------+---------------+
| 1 | SHA Variant | UINT | 6 |
| 2 | Encapsulated Key | Byte String | NONE |
| 3 | Integrity Scope Flags | UINT | 0x7 |
+----+-----------------------+--------------------+---------------+
Table 2
3.4. Results
BIB-HMAC-SHA2 defines the following security results.
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BIB-HMAC-SHA2 Security Results
+--------+----------+-------------+---------------------------------+
| Result | Result | CBOR | Description |
| Id | Name | Encoding | |
| | | Type | |
+--------+----------+-------------+---------------------------------+
| 1 | Expected | byte string | The output of the HMAC |
| | HMAC | | calculation at the security |
| | | | source. |
+--------+----------+-------------+---------------------------------+
Table 3
3.5. Key Considerations
BIB-HMAC-SHA2 does not define or otherwise mandate any method for key
exchange, encryption, or encapsulation. The derivation of an
appropriate key for use in the integrity service is considered
separate from the application of the integrity service for this
context.
HMAC keys used with this context MUST be symmetric and MUST have a
key length equal to the output of the HMAC.
It is assumed that any security verifier or security acceptor
performing an integrity verification can determine the proper HMAC
key to be used. Potential sources of the HMAC key include (but are
not limited to) the following:
Pre-placed keys selected based on local policy.
Keys extracted from encapsulated key material carried in the BIB.
Session keys negotiated via a mechanism external to the BIB.
As discussed in Section 6 and emphasized here, it is strongly
recommended that keys be protected once generated, both when they are
stored and when they are transmitted.
3.6. Canonicalization Algorithms
This section defines the canonicalization algorithm used to prepare
the IPPT input to the BIB-HMAC-SHA2 integrity mechanism. The
construction of the IPPT depends on the settings of the integrity
scope flags that may be provided as part of customizing the behavior
of this security context.
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In all cases, the canonical form of any portion of an extension block
MUST be performed as described in [I-D.ietf-dtn-bpsec]. The
canonicalization algorithms defined in [I-D.ietf-dtn-bpsec] adhere to
the canonical forms for extension blocks defined in
[I-D.ietf-dtn-bpbis] but resolve ambiguities related to how values
are represented in CBOR.
The IPPT is constructed using the following process.
1. The canonical form of the IPPT starts as the empty set with
length 0.
2. If the integrity scope parameter is present and the primary block
flag is set to 1, then a canonical form of the bundle's primary
block MUST be calculated and the result appended to the IPPT.
3. If the integrity scope parameter is present and the target header
flag is set to 1, then the canonical form of the block type code,
block number, and block processing control flags associated with
the security target MUST be calculated and, in that order,
appended to the IPPT.
4. If the integrity scope parameter is present and the security
header flag is set to 1, then the canonical form of the block
type code, block number, and block processing control flags
associated with the BIB MUST be calculated and, in that order,
appended to the IPPT.
5. The canonical form of the security target block-type-specific
data MUST be calculated and appended to the IPPT.
3.7. Processing
3.7.1. Keyed Hash Generation
During keyed hash generation, two inputs are prepared for the the
appropriate HMAC/SHA2 algorithm: the HMAC key and the IPPT. These
data items MUST be generated as follows.
The HMAC key MUST have the appropriate length as required by local
security policy. The key can be generated specifically for this
integrity service, given as part of local security policy, or
through some other key management mechanism as discussed in
Section 3.5.
Prior to the generation of the IPPT, if a CRC value is present for
the target block of the BIB, then that CRC value MUST be removed
from the target block. This involves both removing the CRC value
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from the target block and setting the CRC Type field of the target
block to "no CRC is present."
Once CRC information is removed, the IPPT MUST be generated as
discussed in Section 3.6.
Upon successful hash generation the following actions MUST occur.
The keyed hash produced by the HMAC/SHA2 variant MUST be added as
a security result for the BIB representing the security operation
on this security target, as discussed in Section 3.4).
Finally, the BIB containing information about this security operation
MUST be updated as follows. These operations may occur in any order.
The security context identifier for the BIB MUST be set to the
context identifier for BIB-HMAC-SHA2.
Any local flags used to generate the IPPT SHOULD be placed in the
integrity scope flags security parameter for the BIB unless these
flags are expected to be correctly configured at security
verifiers and acceptors in the network.
The HMAC key MAY be encapsulated using some key encapsulation
mechanism (to include encrypting with a key encryption key) and
the results of the encapsulation added as the encapsulated key
security parameter for the BIB.
The SHA variant used by this security context SHOULD be added as
the SHA variant security parameter for the BIB if it differs from
the default key length. Otherwise, this parameter MAY be omitted
if doing so provides a useful reduction in message sizes.
Problems encountered in the keyed hash generation MUST be processed
in accordance with local BPSec security policy.
3.7.2. Keyed Hash Verification
During keyed hash verification, the input of the security target and
a HMAC key are provided to the appropriate HMAC/SHA2 algorithm.
During keyed hash verification, two inputs are prepared for the
appropriate HMAC/SHA2 algorithm: the HMAC key and the IPPT. These
data items MUST be generated as follows.
The HMAC key MUST be derived using the encapsulated key security
parameter if such a parameter is included in the security context
parameters of the BIB. Otherwise, this key MUST be derived in
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accordance with security policy at the verifying node as discussed
in Section 3.5.
The IPPT MUST be generated as discussed in Section 3.6 with the
value of integrity scope flags being taken from the integrity
scope flags security context parameter. If the integrity scope
flags parameter is not included in the security context parameters
then these flags MAY be derived from local security policy.
The calculated HMAC output MUST be compared to the expected HMAC
output encoded in the security results of the BIB for the security
target. If the calculated HMAC and expected HMAC are identical, the
verification MUST be considered a success. Otherwise, the
verification MUST be considered a failure.
If the verification fails or otherwise experiences an error, or if
any needed parameters are missing, then the verification MUST be
treated as failed and processed in accordance with local security
policy.
This security service is removed from the bundle at the security
acceptor as required by the BPSec specification. If the security
acceptor is not the bundle destination and if no other integrity
service is being applied to the target block, then a CRC MUST be
included for the target block. The CRC type, as determined by
policy, is set in the target block's CRC type field and the
corresponding CRC value is added as the CRC field for that block.
4. Security Context BCB-AES-GCM
4.1. Overview
The BCB-AES-GCM security context replaces the block-type-specific
data field of its security target with cipher text generated using
the Advanced Encryption Standard (AES) cipher operating in Galois/
Counter Mode (GCM) [AES-GCM]. The use of AES-GCM was selected as the
cipher suite for this confidentiality mechanism for several reasons:
1. The selection of a symmetric-key cipher suite allows for
relatively smaller keys than asymmetric-key cipher suites.
2. The selection of a symmetric-key cipher suite allows this
security context to be used in places where an asymmetric-key
infrastructure (such as a public key infrastructure) may be
impractical.
3. The use of the Galois/Counter Mode produces cipher-text with the
same size as the plain text making the replacement of target
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block information easier as length fields do not need to be
changed.
4. The AES-GCM cipher suite provides authenticated encryption, as
required by the BPSec protocol.
Additionally, the BCB-AES-GCM security context generates an
authentication tag based on the plain text value of the block-type-
specific data and other additional authenticated data that may be
specified via parameters to this security context.
This security context supports two variants of AES-GCM, based on the
supported length of the symmetric key. These variants correspond to
A128GCM and A256GCM as defined in [RFC8152] Table 9: Algorithm Value
for AES-GCM.
The BCB-AES-GCM security context shall have the security context
identifier specified in Section 5.1.
4.2. Scope
There are two scopes associated with BCB-AES-GCM: the scope of the
confidentiality service and the scope of the authentication service.
The first defines the set of information provided to the AES-GCM
cipher for the purpose of producing cipher text. The second defines
the set of information used to generate an authentication tag.
The scope of the confidentiality service defines the set of
information provided to the AES-GCM cipher for the purpose of
producing cipher text. This MUST be the full set of plain text
contained in the block-type-specific data field of the security
target.
The scope of the authentication service defines the set of
information used to generate an authentication tag carried with the
security block. This information includes the data included in the
confidentiality service and MAY include other information (additional
authenticated data), as follows.
Primary block
The primary block identifies a bundle and, once created, the
contents of this block are immutable. Changes to the primary
block associated with the security target indicate that the
security target (and BCB) may no longer be in the correct bundle.
For example, if a security target and associated BCB are copied
from one bundle to another bundle, the BCB may still be able to
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decrypt the security target even though these blocks were never
intended to exist in the copied-to bundle.
Including this information as part of additional authenticated
data ensures that security target (and security block) appear in
the same bundle at the time of decryption as at the time of
encryption.
Security target other fields
The other fields of the security target include block
identification and processing information. Changing this
information changes how the security target is treated by nodes
in the network even when the "user data" of the security target
are otherwise unchanged.
For example, if the block processing control flags of a security
target are different at a security verifier than they were
originally set at the security source then the policy for
handling the security target has been modified.
Including this information as part of additional authenticated
data ensures that the cipher text in the security target will not
be used with a different set of block policy than originally set
at the time of encryption.
BCB other fields
The other fields of the BCB include block identification and
processing information. Changing this information changes how
the BCB is treated by nodes in the network, even when other
aspects of the BCB are unchanged.
For example, if the block processing control flags of the BCB are
different at a security acceptor than they were originally set at
the security source then the policy for handling the BCB has been
modified.
Including this information as part of additional authenticated
data ensures that the policy and identification of the security
service in the bundle has not changed.
NOTE: The security context identifier and security context
parameters of the security block are not included as additional
authenticated data because these parameters, by definition, are
those needed to verify or accept the security service.
Therefore, it is expected that changes to these values would
result in failures at security verifiers and security acceptors.
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The scope of the BCB-AES-GCM security context is configured using an
optional security context parameter.
4.3. Parameters
BCB-AES-GCM can be parameterized to specify the AES variant,
initialization vector, key information, and identify additional
authenticated data.
4.3.1. Initialization Vector (IV)
This optional parameter identifies the initialization vector (IV)
used to initialize the AES-GCM cipher.
The length of the initialization vector, prior to any CBOR encoding,
MUST be between 8-16 bytes. A value of 12 bytes SHOULD be used
unless local security policy requires a different length.
This value MUST be encoded as a CBOR byte string.
The initialization vector may have any value with the caveat that a
value MUST NOT be re-used for multiple encryptions using the same
encryption key. This value MAY be re-used when encrypting with
different keys. For example, if each encryption operation using BCB-
AES-GCM uses a newly generated key, then the same IV may be reused.
4.3.2. AES Variant
This optional parameter identifies the AES variant being used for the
AES-GCM encryption, where the variant is identified by the length of
key used.
This value MUST be encoded as a CBOR unsigned integer.
Valid values for this parameter are as follows.
AES Variant Parameter Values
+-------+-----------------------------------------------------------+
| Value | Description |
+-------+-----------------------------------------------------------+
| 1 | A128GCM as defined in [RFC8152] Table 9: Algorithm Values |
| | for AES-GCM |
| 3 | A256GCM as defined in [RFC8152] Table 9: Algorithm Values |
| | for AES-GCM |
+-------+-----------------------------------------------------------+
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When not provided, implementations SHOULD assume a value of 3
(indicating use of A256GCM), unless an alternate default is
established by security policy at the security source, verifier, or
acceptor of this integrity service.
Regardless of the variant, the generated authentication tag MUST
always be 128 bits.
4.3.3. Encapsulated Key
This optional parameter contains the output of a Key Encapsulation
Mechanism (KEM) run at the security source of this security context.
This value MUST be encoded as a CBOR byte string.
If provided, this information is used to retrieve the symmetric AES
key used in the generation of security results for this security
context. If not provided, security verifiers and acceptors MUST
determine the proper key as a function of their local BPSec policy
and configuration, as discussed in Section 4.5.
4.3.4. AAD Scope Flags
This optional parameter contains a series of flags that describe what
information is to be included with the block-type-specific data of
the security target as part of additional authenticated data (AAD).
This value MUST be represented as a CBOR unsigned integer, the value
of which MUST be processed as a bit field containing no more than 8
bits.
Bits in this field represent additional information to be included
when generating an integrity signature over the security target.
These bits are defined as follows.
- Bit 0 (the low-order bit, 0x1): Primary Block Flag.
- Bit 1 (0x02): Target Header Flag.
- Bit 2 (0x03): Security Header Flag.
- Bits 3-7 are reserved.
4.3.5. Enumerations
BCB-AES-GCM defines the following security context parameters.
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BCB-AES-GCM Security Parameters
+----+-----------------------+--------------------+---------------+
| Id | Name | CBOR Encoding Type | Default Value |
+----+-----------------------+--------------------+---------------+
| 1 | Initialization Vector | Byte String | NONE |
| 2 | AES Variant | UINT | 3 |
| 3 | Encapsulation Key | Byte String | NONE |
| 4 | AAD Scope Flags | UINT | 0x7 |
+----+-----------------------+--------------------+---------------+
Table 4
4.4. Results
The BCB-AES-GCM security context produces a single security result
carried in the security block: the authentication tag.
NOTES:
The cipher text generated by the cipher suite is not considered a
security result as it is stored in the block-type-specific data
field of the security target block. When operating in GCM mode,
AES produces cipher text of the same size as its plain text and,
therefore, no additional logic is required to handle padding or
overflow caused by the encryption in most cases (see below).
If the generated cipher text contains the authentication tag and
the tag can be separated from the cipher text then the tag MUST be
separated and stored in the authentication tag security result
field.
If the generated cipher text contains the authentication tag and
the tag cannot be separated from the cipher text then the tag MUST
NOT be included in the authentication tag security result field.
Instead the security target block MUST be resized to accommodate
the additional 128 bits of authentication tag included in the
generated cipher text.
4.4.1. Authentication Tag
The authentication tag is generated by the cipher suite over the
security target plain text input to the cipher suite as combined with
any optional additional authenticated data. This tag is used to
ensure that the plain text (and important information associated with
the plain text) is authenticated prior to decryption.
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If the authentication tag is included in the cipher text placed in
the security target block-type-specific data field, then this
security result MUST NOT be included in the BCB for that security
target.
The length of the authentication tag, prior to any CBOR encoding,
MUST be 128 bits.
This value MUST be encoded as a CBOR byte string.
4.4.2. Enumerations
BCB-AES-GCM defines the following security context parameters.
BCB-AES-GCM Security Results
+-----------+--------------------+--------------------+
| Result Id | Result Name | CBOR Encoding Type |
+-----------+--------------------+--------------------+
| 1 | Authentication Tag | Byte String |
+-----------+--------------------+--------------------+
Table 5
4.5. Key Considerations
BCB-AES-GCM does not define or otherwise mandate any method for key
exchange, encryption, or encapsulation. The derivation of an
appropriate key is considered separate from the application of the
authenticated confidentiality service provided by this context.
Keys used with this context MUST be symmetric and MUST have a key
length equal to the key length defined in the security context
parameters or as defined by local security policy at security
verifiers and acceptors.
It is assumed that any security verifier or security acceptor can
determine the proper key to be used. Potential sources of the key
include (but are not limited to) the following.
Pre-placed keys selected based on local policy.
Keys extracted from encapsulated key material carried in the BCB.
Session keys negotiated via a mechanism external to the BCB.
The security provided by block ciphers is reduced as more data is
processed with the same key. The total number of bytes processed
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with a single key for AES-GCM is recommended to be less than 2^64, as
described in Appendix B of [AES-GCM].
As discussed in Section 6 and emphasized here, it is strongly
recommended that keys be protected once generated, both when they are
stored and when they are transmitted.
4.6. GCM Considerations
The GCM cryptographic mode of AES has specific requirements that MUST
be followed by implementers for the secure function of the BCB-AES-
GCM security context. While these requirements are well documented
in [AES-GCM], some of them are repeated here for emphasis.
The pairing of an IV and a security key MUST be unique. An IV
MUST NOT be used with a security key more than one time. If an IV
and key pair are repeated then the GCM implementation may be
vulnerable to forgery attacks. More information regarding the
importance of the uniqueness of the IV value can be found in
Appendix A of [AES-GCM].
While any tag-based authentication mechanism has some likelihood
of being forged, this probability is increased when using AES-GCM.
In particular, short tag lengths combined with very long messages
SHOULD be avoided when using this mode. The BCB-AES-GCM security
context requires the use of 128-bit authentication tags at all
times. Concerns relating to the size of authentication tags is
discussed in Appendices B and C of [AES-GCM].
As discussed in Appendix B of [AES-GCM], implementations SHOULD
limit the number of unsuccessful verification attempts for each
key to reduce the likelihood of guessing tag values.
As discussed in the Security Considerations section of
[I-D.ietf-dtn-bpsec], delay-tolerant networks may have a higher
occurrence of replay attacks due to the store-and-forward nature
of the network. Because GCM has no inherent replay attack
protection, implementors SHOULD attempt to detect replay attacks
by using mechanisms such as those described in Appendix D of
[AES-GCM].
4.7. Canonicalization Algorithms
This section defines the canonicalization algorithms used to prepare
the inputs used to generate both the cipher text and the
authentication tag.
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In all cases, the canonical form of any portion of an extension block
MUST be performed as described in [I-D.ietf-dtn-bpsec]. The
canonicalization algorithms defined in [I-D.ietf-dtn-bpsec] adhere to
the canonical forms for extension blocks defined in
[I-D.ietf-dtn-bpbis] but resolve ambiguities related to how values
are represented in CBOR.
4.7.1. Cipher text related calculations
The plain text used during encryption MUST be calculated as the
single, definite-length CBOR byte string representing the block-type-
specific data field of the security target excluding the CBOR byte
string identifying byte and optional CBOR byte string length field.
For example, consider the following two CBOR byte strings and the
plain text that would be extracted from them.
CBOR Byte String Examples
+------------------------------+---------+--------------------------+
| CBOR Byte String (Hex) | CBOR | Plain Text Part (Hex) |
| | Part | |
| | (Hex) | |
+------------------------------+---------+--------------------------+
| 18ED | 18 | ED |
+------------------------------+---------+--------------------------+
| C24CDEADBEEFDEADBEEFDEADBEEF | C24C | DEADBEEFDEADBEEFDEADBEEF |
+------------------------------+---------+--------------------------+
Table 6
Similarly, the cipher text used during decryption MUST be calculated
as the single, definite-length CBOR byte string representing the
block-type-specific data field excluding the CBOR byte string
identifying byte and optional CBOR byte string length field.
All other fields of the security target (such as the block type code,
block number, block processing control flags, or any CRC information)
MUST NOT be considered as part of encryption or decryption.
4.7.2. Additional Authenticated Data
The construction of additional authenticated data depends on the AAD
scope flags that may be provided as part of customizing the behavior
of this security context.
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The canonical form of the AAD input to the BCB-AES-GCM mechanism is
constructed using the following process. This process MUST be
followed when generating AAD for either encryption or decryption.
1. The canonical form of the AAD starts as the empty set with length
0.
2. If the AAD scope parameter is present and the primary block flag
is set to 1, then a canonical form of the bundle's primary block
MUST be calculated and the result appended to the AAD.
3. If the AAD scope parameter is present and the target header flag
is set to 1, then the canonical form of the block type code,
block number, and block processing control flags associated with
the security target MUST be calculated and, in that order,
appended to the AAD.
4. If the AAD scope parameter is present and the security header
flag is set to 1, then the canonical form of the block type code,
block number, and block processing control flags associated with
the BIB MUST be calculated and, in that order, appended to the
AAD.
If, after this process, the AAD remains at length 0, then no AAD
exists to be input to the cipher suite.
4.8. Processing
4.8.1. Encryption
During encryption, four inputs are prepared for input to the AES/GCM
cipher: the encryption key, the IV, the security target plain text to
be encrypted, and any additional authenticated data. These data
items MUST be generated as follows.
Prior to encryption, if a CRC value is present for the target block,
then that CRC value MUST be removed. This requires removing the CRC
field from the target block and setting the CRC type field of the
target block to "no CRC is present."
The encryption key MUST have the appropriate length as required by
local security policy. The key may be generated specifically for
this encryption, given as part of local security policy, or
through some other key management mechanism as discussed in
Section 4.5.
The IV selected MUST be of the appropriate length. Because
replaying an IV in counter mode voids the confidentiality of all
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messages encrypted with said IV, this context also requires a
unique IV for every encryption performed with the same key. This
means the same key and IV combination MUST NOT be used more than
once.
The security target plain text for encryption MUST be generated as
discussed in Section 4.7.1.
Additional authenticated data, if present, MUST be generated as
discussed in Section 4.7.2 with the value of AAD scope flags being
taken from local security policy.
Upon successful encryption the following actions MUST occur.
The cipher text produced by AES/GCM MUST replace the bytes used to
define the plain text in the security target block's block-type-
specific data field. The block length of the security target MUST
be updated if the generated cipher text is larger than the plain
text (which can occur when the authentication tag is included in
the cipher text calculation, as discussed in Section 4.4).
The authentication tag calculated by the AES/GCM cipher MUST be
added as a security result for the security target in the BCB
holding results for this security operation.
Cases where the authentication tag is generated as part of the
cipher text MUST be processed as described in Section 4.4.
Finally, the BCB containing information about this security operation
MUST be updated as follows. These operations may occur in any order.
The security context identifier for the BCB MUST be set to the
context identifier for BCB-AES-GCM.
The IV input to the cipher MUST be added as the IV security
parameter for the BCB.
Any local flags used to generated AAD for this cipher MUST be
added as the AAD scope flags security parameter for the BCB.
The encryption key MAY be encapsulated using some key
encapsulation mechanism (to include encrypting with a key
encryption key) and the results of the encapsulation added as the
encapsulated key security parameter for the BCB.
The key length used by this security context MUST be considered
when setting the AES variant security parameter for the BCB if it
differs from the default AES variant. Otherwise, the AES variant
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MAY be omitted if doing so provides a useful reduction in message
sizes.
Problems encountered in the encryption MUST be processed in
accordance with local security policy. This MAY include restoring a
CRC value removed from the target block prior to encryption, if the
target block is allowed to be transmitted after an encryption error.
4.8.2. Decryption
During encryption, five inputs are prepared for input to the AES/GCM
cipher: the decryption key, the IV, the security target cipher text
to be decrypted, any additional authenticated data, and the
authentication tag generated from the original encryption. These
data items MUST be generated as follows.
The decryption key MUST be derived using the encapsulated key
security parameter if such a parameter is included in the security
context parameters of the BCB. Otherwise this key MUST be derived
in accordance with security policy at the decrypting node as
discussed in Section 4.5.
The IV MUST be set to the value of the IV security parameter
included in the BCB. If the IV parameter is not included as a
security parameter, an IV MAY be derived as a function of local
security policy and other BCB contents or a lack of an IV security
parameter in the BCB MAY be treated as an error by the decrypting
node.
The security target cipher text for decryption MUST be generated
as discussed in Section 4.7.1.
Additional authenticated data, if present, MUST be generated as
discussed in Section 4.7.2 with the value of AAD scope flags being
taken from the AAD scope flags security context parameter. If the
AAD scope flags parameter is not included in the security context
parameters then these flags MAY be derived from local security
policy in cases where the set of such flags is determinable in the
network.
The authentication tag MUST be present in the BCB security context
parameters field if additional authenticated data are defined for
the BCB (either in the AAD scope flags parameter or as specified
by local policy). This tag MUST be 128 bits in length.
Upon successful decryption the following actions MUST occur.
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The plain text produced by AES/GCM MUST replace the bytes used to
define the cipher text in the security target block's block-type-
specific data field. Any changes to the security target block
length field MUST be corrected in cases where the plain text has a
different length than the replaced cipher text.
If the security acceptor is not the bundle destination and if no
other integrity or confidentiality service is being applied to the
target block, then a CRC MUST be included for the target block. The
CRC type, as determined by policy, is set in the target block's CRC
type field and the corresponding CRC value is added as the CRC field
for that block.
If the cipher text fails to authenticate, if any needed parameters
are missing, or if there are other problems in the decryption then
the decryption MUST be treated as failed and processed in accordance
with local security policy.
5. IANA Considerations
5.1. Security Context Identifiers
This specification allocates two security context identifiers from
the "BPSec Security Context Identifier" registry defined in
[I-D.ietf-dtn-bpsec].
Additional Entries for the BPSec Security Context Identifiers
Registry:
+-------+---------------+---------------+
| Value | Description | Reference |
+-------+---------------+---------------+
| TBA | BIB-HMAC-SHA2 | This document |
| TBA | BCB-AES-GCM | This document |
+-------+---------------+---------------+
Table 7
6. Security Considerations
Security considerations specific to a single security context are
provided in the description of that context. This section discusses
security considerations that should be evaluated by implementers of
any security context described in this document. Considerations may
also be found in documents listed as normative references and they
should also be reviewed by security context implementors.
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6.1. Key Handling
In addition to the key considerations listed in each security
context, the following also apply to the generation, transmission,
and use of keys associated with all of the security contexts defined
in this document.
It is strongly RECOMMENDED that implementations protect keys both
when they are stored and when they are transmitted.
In the event that a key is compromised, any security operations
using a security context associated with that key SHOULD also be
considered compromised. This means that the BIB-HMAC-SHA2
security context SHOULD NOT provide integrity when used with a
compromised key and BCB-AES-GCM SHOULD NOT provide confidentiality
when used with a compromised key.
When keys are extracted from key material carried in a security
block, the encapsulated key SHOULD be protected by an approved
algorithm such as NIST SP-800-38F. The determination to use
approved algorithms increases interoperability and the specific
algorithm used is expected to be specified as part of local
security policy.
The same key SHOULD NOT be used for different algorithms as doing
so may leak information about the key.
6.2. AES GCM
There are a significant number of considerations related to the use
of the GCM mode of AES to provide a confidentiality service. These
considerations are provided in Section 4.6 as part of the
documentation of the BCB-AES-GCM security context.
6.3. Bundle Fragmentation
Bundle fragmentation may prevent security services in a bundle from
being verified after a bundle is fragmented and before the bundle is
re-assembled. Examples of potential issues include the following.
If a security block and its security target do not exist in the
same fragment, then the security block cannot be processed until
the bundle is re-assembled. If a fragment includes an encrypted
target block, but not its BCB, then a receiving bundle processing
agent (BPA) will not know that the target block has been
encrypted.
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If a security block is cryptographically bound to a bundle, it
cannot be processed even if the security block and target both
coexist in the fragment. This is because fragments have different
primary blocks than the original bundle.
If security blocks and their target blocks are repeated in
multiple fragments, policy must determine how to deal with issues
where a security operation verifies in one fragment but fails in
another fragment. This may happen, for example, if a BIB block
becomes corrupted in one fragment but not in another fragment.
Implementors should consider how security blocks are processed when a
BPA fragments a received bundle. For example, security blocks and
their targets could be placed in the same fragment if the security
block is not otherwise cryptographically bound to the bundle being
fragmented. Alternatively, if security blocks are cryptographically
bound to a bundle, then a fragmenting BPA should consider
encapsulating the bundle first and then fragmenting the encapsulating
bundle.
7. Normative References
[AES-GCM] Dworkin, M., "NIST Special Publication 800-38D:
Recommendation for Block Cipher Modes of Operation:
Galois/Counter Mode (GCM) and GMAC.", November 2007.
[HMAC] US NIST, "The Keyed-Hash Message Authentication Code
(HMAC).", FIPS-198-1, Gaithersburg, MD, USA, July 2008.
https://csrc.nist.gov/publications/detail/fips/198/1/final
[I-D.ietf-dtn-bpbis]
Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol
Version 7", draft-ietf-dtn-bpbis-31 (work in progress),
January 2021.
[I-D.ietf-dtn-bpsec]
Birrane, E. and K. McKeever, "Bundle Protocol Security
Specification", draft-ietf-dtn-bpsec-27 (work in
progress), February 2021.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[SHS] US NIST, "Secure Hash Standard (SHS).", FIPS-
180-4, Gaithersburg, MD, USA, August 2015.
https://csrc.nist.gov/publications/detail/fips/180/4/final
Appendix A. Acknowledgements
The following participants contributed useful review and analysis of
these security contexts: Amy Alford and Sarah Heiner of the Johns
Hopkins University Applied Physics Laboratory.
Author's Address
Edward J. Birrane, III
The Johns Hopkins University Applied
Physics Laboratory
11100 Johns Hopkins Rd.
Laurel, MD 20723
US
Phone: +1 443 778 7423
Email: Edward.Birrane@jhuapl.edu
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