DTN Bundle Protocol Security COSE Security Contexts
draft-bsipos-dtn-bpsec-cose-01

Abstract

This document defines an integrity security context and a confidentiality security context suitable for using CBOR Object Signing and Encryption (COSE) algorithms within Bundle Protocol Security (BPSec) blocks. A profile of COSE is also defined for BPSec interoperation.

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 December 28, 2020.

Copyright Notice

Copyright (c) 2020 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 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 2. Requirements Language
• 3. BPSec Security Contexts
• 3.1. COSE Integrity Context
• 3.2. COSE Confidentiality Context
• 4. COSE Profile for BPSec
• 4.1. Interoperability Algorithms
• 5. Implementation Status
• 6. Security Considerations
• 6.1. Threat: BPSec Block Replay
• 6.2. Threat: Unidentifiable Key
• 6.3. Threat: Algorithm Vulnerabilities
• 7. IANA Considerations
• 7.1. BPSec Security Contexts
• 8. Acknowledgments
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Examples
• A.1. Symmetric Key COSE_Mac0
• A.2. RSA Keypair COSE_Sign1
• A.3. Symmetric Key COSE_Encrypt0
• Author's Address

1. Introduction

The Bundle Protocol Security (BPSec) Specification [I-D.ietf-dtn-bpsec] defines structure and encoding for Block Integrity Block (BIB) and Block Confidentiality Block (BCB) types but does not specify any security contexts to be used by either of the security block types. The CBOR Object Signing and Encryption (COSE) specification [RFC8152] defines a structure, encoding, and algorithms to use for cryptographic signing and encryption.

This document describes how to use the algorithms and encodings of COSE within BPSec blocks to apply those algorithms to Bundle security in Section 3. A bare minimum of interoperability algorithms and algorithm parameters is specified by this document in Section 4.

This document does not address how those COSE algorithms are intended to be used within a larger security context.

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. BPSec Security Contexts

Rather than defining a single security context for both integrity and confidentiality blocks, this document specifies two separate security contexts which are analogous to the two BPSec block types. Each security context allows a specific set of BPSec Result IDs.

The existing COSE message-marking tags in Section 2 of [RFC8152] SHALL be used as BPSec Result ID values for all COSE security contexts (see Table 1 and Table 2). This avoids the need for value-mapping between code points of the two registries.

When embedding COSE messages, the CBOR-tagged form SHALL NOT be used. The Result ID values already provide the same information as the COSE tags.

3.1. COSE Integrity Context

The COSE Integrity Context has a Security Context ID of TBD-CI.

The integrity context SHALL allow only the Result IDs from Table 1. Each integrity context result value SHALL consist of the COSE message indicated by Table 1 in its decoded form.

Each integrity result SHALL use the "detached" payload form with nil payload value. The integrity result for COSE_Mac and COSE_Mac0 messages are computed by the procedure in Section 6.3 of [RFC8152]. The integrity result for COSE_Sign and COSE_Sign1 messages are computed by the procedure in Section 4.4 of [RFC8152].

[NOTE: This differs from base BPSec in that the entire block and the bundle primary is signed] The COSE "payload" used to generate a signature or MAC result SHALL be the canonically serialized target block, including the canonical block array structure. The COSE "protected attributes from the application" used to generate a signature or MAC result SHALL be either:

For a primary block target:

An empty byte string.

For a canonical block target:

The canonically serialized primary block of the bundle.

3.2. COSE Confidentiality Context

The COSE Confidentiality Context has a Security Context ID of TBD-CC.

The confidentiality context SHALL allow only the Result IDs from Table 2. Each confidentiality context result value SHALL consist of the COSE message indicated by Table 2 in its decoded form.

Only algorithms which support Authenticated Encryption with Authenticated Data (AEAD) SHALL be usable in the first (content) layer of a confidentiality result. Because COSE encryption with AEAD appends the authentication tag with the ciphertext, the size of the block-type-specific-data will grow after an encryption operation.

Each confidentiality result SHALL use the "detached" payload form with nil payload value. The COSE plaintext and ciphertext correspond exactly with the target block-type-specific-data. The confidentiality result for COSE_Encrypt and COSE_Encrypt0 messages are computed by the procedure in Section 5.3 of [RFC8152].

[NOTE: This differs from base BPSec in that AAD from the block and the bundle primary is used] The COSE "plaintext" used to generate an encrypt result SHALL be the block-type-specific-data of the target block, the decoded byte string itself (not including the encoded CBOR item header). The COSE "protected attributes from the application" used to generate an encrypt result SHALL be the concatenation of the following:

1. The canonically serialized primary block of the bundle.
2. The canonically serialized augmented target block, which has its block-type-specific-data substituted with an empty byte string.

4. COSE Profile for BPSec

This section contains requirements which apply to the use of COSE within BPSec across any security context use.

When used in a BPSec result, each COSE message SHALL contain an explicit algorithm identifier in the lower (content) layers. A BPSec security operation always occurs within the context of the immutable primary block and its parameters. When available and not implied by the bundle source, a COSE message SHOULD contain a key identifier in the highest layer. When a key identifier is not available, BPSec acceptors SHOULD use the Security Source (if available) and the Bundle Source to imply which keys can be used for security operations.

The algorithms required by this profile focuses on networks using shared symmetric-keys, with recommended algorithms for Elliptic Curve (EC) keypairs and RSA keypairs. The focus of this profile is to enable interoperation between security sources and acceptors on an open network, where more explicit COSE parameters make it easier for BPSec acceptors to avoid assumptions and avoid out-of-band parameters. The requirements of this profile still allow the use of potentially not-easily-interoperable algorithms and message/recipient configurations for use by private networks, where message size is more important than explicit COSE parameters.

4.1. Interoperability Algorithms

[NOTE: The required list is identical to the [I-D.ietf-dtn-bpsec-interop-sc] list.] The set of integrity algorithms needed for interoperability is listed here. The full set of COSE algorithms available is managed at [IANA-COSE].

Implementations conforming to this specification SHALL support the symmetric keyed algorithms of Table 3. Implementations capable of doing so SHOULD support the asymmetric keyed and key-encryption algorithms of Table 3.

The following are recommended key and recipient uses within COSE/BPSec:

Symmetric Key Integrity:

When generating a BIB result from a symmetric key, implementations SHOULD use either a COSE_Mac0 or a COSE_Mac using the private key directly. When a COSE_Mac is used with a direct key, the recipient layer SHOULD include a key identifier.

EC Keypair Integrity:

When generating a BIB result from an EC keypair, implementations SHOULD use either a COSE_Sign1 or a COSE_Sign using the private key directly or a COSE_Mac from a symmetric key with a layer-2 encryption of the symmetric key. When a COSE_Sign or COSE_Mac is used with EC keypair, the recipient layer SHOULD include a public key identifier.

RSA Keypair Integrity:

When generating a BIB result from an RSA keypair, implementations SHOULD use either a COSE_Sign1 or a COSE_Sign using the private key directly or a COSE_Mac from a symmetric key with a layer-2 key-wrap of the symmetric key. When a COSE_Sign or COSE_Mac is used with RSA keypair, the recipient layer SHOULD include a public key identifier. When a COSE_Sign or COSE_Sign1 is used with RSA keypair, the signature uses a maximum-length PSS salt in accordance with [RFC8230].

Symmetric Key Confidentiality:

When generating a BCB result from an symmetric key, implementations SHOULD use a COSE_Encrypt message with a recipient containing a key-wrapped CEK. When generating a BCB result from a symmetric key, implementations SHOULD NOT use COSE_Encrypt0 or COSE_Encrypt with direct content encryption key (CEK). Doing so risks key overuse and the vulnerabilities associated with large amount of ciphertext from the same key.

EC Keypair Confidentiality:

When generating a BCB result from an EC keypair, implementations SHOULD use a COSE_Encrypt message with a recipient containing a key-wrapped CEK.

RSA Keypair Confidentiality:

When generating a BCB result from an RSA keypair, implementations SHOULD use a COSE_Encrypt message with a recipient containing a key-wrapped CEK.

5. Implementation Status

[NOTE to the RFC Editor: please remove this section before publication, as well as the reference to [RFC7942] and [github-dtn-bpsec-cose].]

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations can exist.

An example implementation of COSE over Blocks has been created as a GitHub project [github-dtn-bpsec-cose] and is intended to use as a proof-of-concept and as a possible source of interoperability testing. This example implementation only handles CBOR encoding/decoding and cryptographic functions, it does not construct actual BIB or BCB and does not integrate with a BP Agent.

6. Security Considerations

This section separates security considerations into threat categories based on guidance of BCP 72 [RFC3552].

All of the security considerations of the underlying BPSec [I-D.ietf-dtn-bpsec] apply to these new security contexts.

6.1. Threat: BPSec Block Replay

The bundle's primary block contains fields which uniquely identify a bundle: the Source Node ID, Creation Timestamp, and fragment parameters (see Section 4.2.2 of [I-D.ietf-dtn-bpbis]). These same fields are used to correlate Administrative Records with the bundles for which the records were generated. Including the primary block in the AAD for BPSec integrity and confidentiality binds the verification of the secured block to its parent bundle and disallows replay of any block with its BIB or BCB.

This profile of COSE limits the encryption algorithms to only AEAD in order to include the context of the encrypted data as AAD. If an agent mistakenly allows the use of non-AEAD encryption when decrypting and verifying a BCB, the possibility of block replay attack is present.

6.2. Threat: Unidentifiable Key

The profile in Section 4.1 recommends key identifiers when possible. If the application using a COSE Integrity or COSE Confidentiality context leaves out key identification data (in a COSE recipient structure), the security acceptor for those BPSec blocks only has the primary block available to use when verifying or decrypting the target block. This leads to a situation, identified in BPSec Security Considerations, where a signature is verified to be valid but not from the expected Security Source.

6.3. Threat: Algorithm Vulnerabilities

Because this use of COSE leaves the specific algorithms chosen for BIB and BCB use up to the applications securing bundle data, it is important to use only COSE algorithms which are marked as recommended in the IANA registry [IANA-COSE].

7. IANA Considerations

Registration procedures referred to in this section are defined in [RFC8126].

7.1. BPSec Security Contexts

Within the "Bundle Protocol" registry [IANA-BUNDLE], the following entry has been added to the "BPSec Security Context Identifiers" sub-registry.

8. Acknowledgments

The interoperability minimum algorithms and parameters are based on the draft [I-D.ietf-dtn-bpsec-interop-sc].

9. References

9.1. Normative References

9.2. Informative References

Appendix A. Examples

A.1. Symmetric Key COSE_Mac0

This is an example of a MAC with implied recipient (and its key material). The two provided figures are CBOR diagnostic notation [RFC7049] of the target block being signed and the Abstract Security Block (which will itself be enveloped within a BIB).

The 256-bit key used is shown below.

/ signing key /
h'13bf9cead057c0aca2c9e52471ca4b19ddfaf4c0784e3f3e8e3999dbae4ce45c'
          

Symmetric Key

[
  7, / BP version /
  0, / flags /
  0, / CRC type /
  [1, '//dst/'], / destination /
  [1, '//src/'], / source /
  [1, '//src/'], / report-to /
  [0, 40], / timestamp /
  1000000 / lifetime /
]
          

Figure 1: Primary block CBOR diagnostic

[
  7, / type code - bundle age /
  2, / block num /
  0, / flags /
  0, / CRC type /
  h'19012c' / type-specific-data:
    300 \ age \
  /
]
          

Figure 2: Target block CBOR diagnostic

The external_aad is the encoded primary block. The payload is the encoded target block.

[
  'MAC0', / context /
  h'a10105', / protected /
  h'880700008201462f2f6473742f8201462f2f7372632f8201462f2f7372632f820018
    281a000f4240', / external_aad /
  h'85070200004319012c' / payload /
]
          

Figure 3: MAC_structure CBOR diagnostic

[
  [2], / targets /
  0, / security context TBD /
  0, / flags /
  [
    [ / target block #2 /
      [ / result /
        17, / COSE_Mac0 tag /
        [
          h'a10105' / protected {
             \ alg \ 1:5 \ HMAC 256//256 \
           } / ,
          { / unprotected /
            / kid / 4:'mykey'
          },
          null, / payload /
          h'd98308918d36dc4190a93f84c8d857015b75b78edea3360282555257c3be
            f847' / tag /
        ]
      ]
    ]
  ]
]
          

Figure 4: Abstract Security Block CBOR diagnostic

A.2. RSA Keypair COSE_Sign1

This is an example of a signature with implied recipient (and its key material). The two provided figures are CBOR diagnostic notation [RFC7049] of the target block being signed and the Abstract Security Block (which will itself be enveloped within a BIB).

The 512-bit private key used is below. It is not supposed to be a secure configuration, only intended to explain the procedure. This signature uses zero-length salt for deterministic output, which differs from the parameter specified by [RFC8230] and is not recommended for normal use.

-----BEGIN RSA PRIVATE KEY-----
MIIBOwIBAAJBAN21GdS0faAYgacepRmbr7TAT0wEuahjrBfAO0Dg1M5d37O9Tx9H
vZw2OEcLq2WTvf0Kja1JWpqdoJm17LghhPkCAwEAAQJBAMgkJo9n6EhQFyrgdTZq
3vES8gKz+U3TvJUsSdFFpZYsZhUaLKP9oxyIxl3IvK5iS0oAsW0nqI7aMcBoPmxZ
pQECIQDuyd5uzvS0wnrsDWoDhiTm6O+PJoMQix9yH99HBUhWKQIhAO2wDP7e/Nnr
A7rDSgM6+REGmt8I00NglFwShBUi4HJRAiAiJrLuTCEJXSsxaXW5DU1nzPa+FXb3
Pb6Alvha8vF2iQIgbC7WK2dJBNKv9uCOHlxIItSzxtIYfjFGNYYD8i7Wo5ECIQDp
5++fp04AMVAnE0uqNEnITkTWb91hAS8IjaYCqLGyEA==
-----END RSA PRIVATE KEY-----

[
  7, / BP version /
  0, / flags /
  0, / CRC type /
  [1, '//dst/'], / destination /
  [1, '//src/'], / source /
  [1, '//src/'], / report-to /
  [0, 40], / timestamp /
  1000000 / lifetime /
]
          

Figure 5: Primary block CBOR diagnostic

[
  7, / type code - bundle age /
  2, / block num /
  0, / flags /
  0, / CRC type /
  h'19012c' / type-specific-data:
    300 \ age \
  /
]
          

Figure 6: Target block CBOR diagnostic

The external_aad is the encoded primary block. The payload is the encoded target block.

[
  'Signature1', / context /
  h'a1013824', / protected /
  h'880700008201462f2f6473742f8201462f2f7372632f8201462f2f7372632f820018
    281a000f4240', / external_aad /
  h'85070200004319012c' / payload /
]
          

Figure 7: Sig_structure CBOR diagnostic

[
  [2], / targets /
  0, / security context TBD /
  0, / flags /
  [
    [ / target block #2 /
      [ / result /
        18, / COSE_Sign1 tag /
        [
          h'a1013824' / protected {
             \ alg \ 1:-37 \ PS256 \
           } / ,
          { / unprotected /
            / kid / 4:'mykey'
          },
          null, / payload /
          h'1a35746072396c74275fd7b443a0d7391a0f92012a53e0accc543daa51ae
            6faae551a4843a0bc7c3bf808e3638ddc381355b54cc60f4ca9dea15923b
            5986e758' / signature /
        ]
      ]
    ]
  ]
]
          

Figure 8: Abstract Security Block CBOR diagnostic

A.3. Symmetric Key COSE_Encrypt0

This is an example of an encryption with implied recipient (and its direct content encryption key). The provided figures are CBOR diagnostic notation [RFC7049] of the target block being encrypted, the Abstract Security Block (which will itself be enveloped within a BCB), and the resulting target block.

This example uses a single shared content encryption key, which is not recommended for normal use. The 256-bit key used is shown below. A random IV is generated for this operation and is indicated in a standard way in the unprotected header.

/ content encryption key /
h'13bf9cead057c0aca2c9e52471ca4b19ddfaf4c0784e3f3e8e3999dbae4ce45c'
          

Symmetric Keys

[
  7, / BP version /
  0, / flags /
  0, / CRC type /
  [1, '//dst/'], / destination /
  [1, '//src/'], / source /
  [1, '//src/'], / report-to /
  [0, 40], / timestamp /
  1000000 / lifetime /
]
          

Figure 9: Primary block CBOR diagnostic

[
  7, / type code - bundle age /
  2, / block num /
  0, / flags /
  0, / CRC type /
  h'19012c' / type-specific-data:
    300 \ age \
  /
]
          

Figure 10: Initial Target block CBOR diagnostic

The external_aad is a concatenation of the encoded primary block and the encoded augmented target block (its block data removed).

[
  'Encrypt0', / context /
  h'a10103', / protected /
  h'880700008201662f2f6473742f8201662f2f7372632f8201662f2f7372632f820018
    281a000f4240850702000040' / external_aad /
]
          

Figure 11: Enc_structure CBOR diagnostic

[
  [2], / targets /
  0, / security context TBD /
  0, / flags /
  [
    [ / target block #2 /
      [ / result /
        16, / COSE_Encrypt0 tag /
        [
          h'a10103', / protected {
             \ alg \ 1:3 \ A256GCM \
           } /
          { / unprotected /
            / kid / 4:'mykey',
            / iv / 5: h'6f3093eba5d85143c3dc484a'
          },
          null / payload /
        ]
      ]
    ]
  ]
]
          

Figure 12: Abstract Security Block CBOR diagnostic

[
  7, / type code - bundle age /
  2, / block num /
  0, / flags /
  0, / CRC type /
  h'63bb16685ef432a0e6f1d404da71959081a715' / ciphertext /
]
          

Figure 13: Encrypted Target block CBOR diagnostic

Author's Address