CBOR Object Signing and Encryption (COSE): AES-CTR and AES-CBC
RFC 9459



Internet Engineering Task Force (IETF)                        R. Housley
Request for Comments: 9459                                Vigil Security
Category: Standards Track                                  H. Tschofenig
ISSN: 2070-1721                                           September 2023

     CBOR Object Signing and Encryption (COSE): AES-CTR and AES-CBC

Abstract

   The Concise Binary Object Representation (CBOR) data format is
   designed for small code size and small message size.  CBOR Object
   Signing and Encryption (COSE) is specified in RFC 9052 to provide
   basic security services using the CBOR data format.  This document
   specifies the conventions for using AES-CTR and AES-CBC as content
   encryption algorithms with COSE.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9459.

Copyright Notice

   Copyright (c) 2023 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions and Terminology
   3.  AES Modes of Operation
   4.  AES Counter Mode
     4.1.  AES-CTR COSE Key
     4.2.  AES-CTR COSE Algorithm Identifiers
   5.  AES Cipher Block Chaining Mode
     5.1.  AES-CBC COSE Key
     5.2.  AES-CBC COSE Algorithm Identifiers
   6.  Implementation Considerations
   7.  IANA Considerations
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   This document specifies the conventions for using AES-CTR and AES-CBC
   as content encryption algorithms with the CBOR Object Signing and
   Encryption (COSE) [RFC9052] syntax.  Today, encryption with COSE uses
   Authenticated Encryption with Associated Data (AEAD) algorithms
   [RFC5116], which provide both confidentiality and integrity
   protection.  However, there are situations where another mechanism,
   such as a digital signature, is used to provide integrity.  In these
   cases, an AEAD algorithm is not needed.  The software manifest being
   defined by the IETF SUIT WG [SUIT-MANIFEST] is one example where a
   digital signature is always present.

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  AES Modes of Operation

   NIST has defined several modes of operation for the Advanced
   Encryption Standard [AES] [MODES].  AES supports three key sizes: 128
   bits, 192 bits, and 256 bits.  AES has a block size of 128 bits (16
   octets).  Each of these modes has different characteristics.  The
   modes include: CBC (Cipher Block Chaining), CFB (Cipher FeedBack),
   OFB (Output FeedBack), and CTR (Counter).

   Only AES Counter (AES-CTR) mode and AES Cipher Block Chaining (AES-
   CBC) are discussed in this document.

4.  AES Counter Mode

   When AES-CTR is used as a COSE content encryption algorithm, the
   encryptor generates a unique value that is communicated to the
   decryptor.  This value is called an "Initialization Vector" (or "IV")
   in this document.  The same IV and AES key combination MUST NOT be
   used more than once.  The encryptor can generate the IV in any manner
   that ensures the same IV value is not used more than once with the
   same AES key.

   When using AES-CTR, each AES encrypt operation generates 128 bits of
   key stream.  AES-CTR encryption is the XOR of the key stream with the
   plaintext.  AES-CTR decryption is the XOR of the key stream with the
   ciphertext.  If the generated key stream is longer than the plaintext
   or ciphertext, the extra key stream bits are simply discarded.  For
   this reason, AES-CTR does not require the plaintext to be padded to a
   multiple of the block size.

   AES-CTR has many properties that make it an attractive COSE content
   encryption algorithm.  AES-CTR uses the AES block cipher to create a
   stream cipher.  Data is encrypted and decrypted by XORing with the
   key stream produced by AES encrypting sequential IV block values,
   called "counter blocks", where:

   *  The first block of the key stream is the AES encryption of the IV.

   *  The second block of the key stream is the AES encryption of (IV +
      1) mod 2^128.

   *  The third block of the key stream is the AES encryption of (IV +
      2) mod 2^128, and so on.

   AES-CTR is easy to implement, can be pipelined and parallelized, and
   supports key stream precomputation.  Sending of the IV is the only
   source of expansion because the plaintext and ciphertext are the same
   size.

   When used correctly, AES-CTR provides a high level of
   confidentiality.  Unfortunately, AES-CTR is easy to use incorrectly.
   Being a stream cipher, reuse of the IV with the same key is
   catastrophic.  An IV collision immediately leaks information about
   the plaintext.  For this reason, it is inappropriate to use AES-CTR
   with static keys.  Extraordinary measures would be needed to prevent
   reuse of an IV value with the static key across power cycles.  To be
   safe, implementations MUST use fresh keys with AES-CTR.

   AES-CTR keys may be obtained either from a key structure or from a
   recipient structure.  Implementations encrypting and decrypting MUST
   validate that the key type, key length, and algorithm are correct and
   appropriate for the entities involved.

   With AES-CTR, it is trivial to use a valid ciphertext to forge other
   (valid to the decryptor) ciphertexts.  Thus, it is equally
   catastrophic to use AES-CTR without a companion authentication and
   integrity mechanism.  Implementations MUST use AES-CTR in conjunction
   with an authentication and integrity mechanism, such as a digital
   signature.

   The instructions in Section 5.4 of [RFC9052] are followed for AES-
   CTR.  Since AES-CTR cannot provide integrity protection for external
   additional authenticated data, the decryptor MUST ensure that no
   external additional authenticated data was supplied.  See Section 6.

   The 'protected' header MUST be a zero-length byte string.

4.1.  AES-CTR COSE Key

   When using a COSE key for the AES-CTR algorithm, the following checks
   are made:

   *  The 'kty' field MUST be present, and it MUST be 'Symmetric'.

   *  If the 'alg' field is present, it MUST match the AES-CTR algorithm
      being used.

   *  If the 'key_ops' field is present, it MUST include 'encrypt' when
      encrypting.

   *  If the 'key_ops' field is present, it MUST include 'decrypt' when
      decrypting.

4.2.  AES-CTR COSE Algorithm Identifiers

   The following table defines the COSE AES-CTR algorithm values.  Note
   that these algorithms are being registered as "Deprecated" to avoid
   accidental use without a companion integrity protection mechanism.

        +=========+========+==========+=============+=============+
        | Name    | Value  | Key Size | Description | Recommended |
        +=========+========+==========+=============+=============+
        | A128CTR | -65534 |   128    |  AES-CTR w/ |  Deprecated |
        |         |        |          | 128-bit key |             |
        +---------+--------+----------+-------------+-------------+
        | A192CTR | -65533 |   192    |  AES-CTR w/ |  Deprecated |
        |         |        |          | 192-bit key |             |
        +---------+--------+----------+-------------+-------------+
        | A256CTR | -65532 |   256    |  AES-CTR w/ |  Deprecated |
        |         |        |          | 256-bit key |             |
        +---------+--------+----------+-------------+-------------+

                                  Table 1

5.  AES Cipher Block Chaining Mode

   AES-CBC mode requires a 16-octet IV.  Use of a randomly or
   pseudorandomly generated IV ensures that the encryption of the same
   plaintext will yield different ciphertext.

   AES-CBC performs an XOR of the IV with the first plaintext block
   before it is encrypted.  For successive blocks, AES-CBC performs an
   XOR of the previous ciphertext block with the current plaintext
   before it is encrypted.

   AES-CBC requires padding of the plaintext; the padding algorithm
   specified in Section 6.3 of [RFC5652] MUST be used prior to
   encrypting the plaintext.  This padding algorithm allows the
   decryptor to unambiguously remove the padding.

   The simplicity of AES-CBC makes it an attractive COSE content
   encryption algorithm.  The need to carry an IV and the need for
   padding lead to an increase in the overhead (when compared to AES-
   CTR).  AES-CBC is much safer for use with static keys than AES-CTR.
   That said, as described in [RFC4107], the use of automated key
   management to generate fresh keys is greatly preferred.

   AES-CBC does not provide integrity protection.  Thus, an attacker can
   introduce undetectable errors if AES-CBC is used without a companion
   authentication and integrity mechanism.  Implementations MUST use
   AES-CBC in conjunction with an authentication and integrity
   mechanism, such as a digital signature.

   The instructions in Section 5.4 of [RFC9052] are followed for AES-
   CBC.  Since AES-CBC cannot provide integrity protection for external
   additional authenticated data, the decryptor MUST ensure that no
   external additional authenticated data was supplied.  See Section 6.

   The 'protected' header MUST be a zero-length byte string.

5.1.  AES-CBC COSE Key

   When using a COSE key for the AES-CBC algorithm, the following checks
   are made:

   *  The 'kty' field MUST be present, and it MUST be 'Symmetric'.

   *  If the 'alg' field is present, it MUST match the AES-CBC algorithm
      being used.

   *  If the 'key_ops' field is present, it MUST include 'encrypt' when
      encrypting.

   *  If the 'key_ops' field is present, it MUST include 'decrypt' when
      decrypting.

5.2.  AES-CBC COSE Algorithm Identifiers

   The following table defines the COSE AES-CBC algorithm values.  Note
   that these algorithms are being registered as "Deprecated" to avoid
   accidental use without a companion integrity protection mechanism.

        +=========+========+==========+=============+=============+
        | Name    | Value  | Key Size | Description | Recommended |
        +=========+========+==========+=============+=============+
        | A128CBC | -65531 |   128    |  AES-CBC w/ |  Deprecated |
        |         |        |          | 128-bit key |             |
        +---------+--------+----------+-------------+-------------+
        | A192CBC | -65530 |   192    |  AES-CBC w/ |  Deprecated |
        |         |        |          | 192-bit key |             |
        +---------+--------+----------+-------------+-------------+
        | A256CBC | -65529 |   256    |  AES-CBC w/ |  Deprecated |
        |         |        |          | 256-bit key |             |
        +---------+--------+----------+-------------+-------------+

                                  Table 2

6.  Implementation Considerations

   COSE libraries that support either AES-CTR or AES-CBC and accept
   Additional Authenticated Data (AAD) as input MUST return an error if
   one of these non-AEAD content encryption algorithms is selected.
   This ensures that a caller does not expect the AAD to be protected
   when the cryptographic algorithm is unable to do so.

7.  IANA Considerations

   IANA has registered six COSE algorithm identifiers for AES-CTR and
   AES-CBC in the "COSE Algorithms" registry [IANA-COSE].

   The information for the six COSE algorithm identifiers is provided in
   Sections 4.2 and 5.2.  Also, for all six entries, the "Capabilities"
   column contains "[kty]", the "Change Controller" column contains
   "IETF", and the "Reference" column contains a reference to this
   document.

8.  Security Considerations

   This document specifies AES-CTR and AES-CBC for COSE, which are not
   AEAD ciphers.  The use of the ciphers is limited to special use
   cases, such as firmware encryption, where integrity and
   authentication is provided by another mechanism.

   Since AES has a 128-bit block size, regardless of the mode employed,
   the ciphertext generated by AES encryption becomes distinguishable
   from random values after 2^64 blocks are encrypted with a single key.
   Implementations should change the key before reaching this limit.

   To avoid cross-protocol concerns, implementations MUST NOT use the
   same keying material with more than one mode.  For example, the same
   keying material must not be used with AES-CTR and AES-CBC.

   There are fairly generic precomputation attacks against all block
   cipher modes that allow a meet-in-the-middle attack against the key.
   These attacks require the creation and searching of huge tables of
   ciphertext associated with known plaintext and known keys.  Assuming
   that the memory and processor resources are available for a
   precomputation attack, then the theoretical strength of AES-CTR and
   AES-CBC is limited to 2^(n/2) bits, where n is the number of bits in
   the key.  The use of long keys is the best countermeasure to
   precomputation attacks.

   When used properly, AES-CTR mode provides strong confidentiality.
   Unfortunately, it is very easy to misuse this counter mode.  If
   counter block values are ever used for more than one plaintext with
   the same key, then the same key stream will be used to encrypt both
   plaintexts, and the confidentiality guarantees are voided.

   What happens if the encryptor XORs the same key stream with two
   different plaintexts?  Suppose two plaintext octet sequences P1, P2,
   P3 and Q1, Q2, Q3 are both encrypted with key stream K1, K2, K3.  The
   two corresponding ciphertexts are:

      (P1 XOR K1), (P2 XOR K2), (P3 XOR K3)

      (Q1 XOR K1), (Q2 XOR K2), (Q3 XOR K3)

   If both of these two ciphertext streams are exposed to an attacker,
   then a catastrophic failure of confidentiality results, since:

      (P1 XOR K1) XOR (Q1 XOR K1) = P1 XOR Q1
      (P2 XOR K2) XOR (Q2 XOR K2) = P2 XOR Q2
      (P3 XOR K3) XOR (Q3 XOR K3) = P3 XOR Q3

   Once the attacker obtains the two plaintexts XORed together, it is
   relatively straightforward to separate them.  Thus, using any stream
   cipher, including AES-CTR, to encrypt two plaintexts under the same
   key stream leaks the plaintext.

   Data forgery is trivial with AES-CTR mode.  The demonstration of this
   attack is similar to the key stream reuse discussion above.  If a
   known plaintext octet sequence P1, P2, P3 is encrypted with key
   stream K1, K2, K3, then the attacker can replace the plaintext with
   one of its own choosing.  The ciphertext is:

      (P1 XOR K1), (P2 XOR K2), (P3 XOR K3)

   The attacker simply XORs a selected sequence Q1, Q2, Q3 with the
   ciphertext to obtain:

      (Q1 XOR (P1 XOR K1)), (Q2 XOR (P2 XOR K2)), (Q3 XOR (P3 XOR K3))

   Which is the same as:

      ((Q1 XOR P1) XOR K1), ((Q2 XOR P2) XOR K2), ((Q3 XOR P3) XOR K3)

   Decryption of the attacker-generated ciphertext will yield exactly
   what the attacker intended:

      (Q1 XOR P1), (Q2 XOR P2), (Q3 XOR P3)

   AES-CBC does not provide integrity protection.  Thus, an attacker can
   introduce undetectable errors if AES-CBC is used without a companion
   authentication mechanism.

   If an attacker is able to strip the authentication and integrity
   mechanism, then the attacker can replace it with one of their own
   creation, even without knowing the plaintext.  The usual defense
   against such an attack is an Authenticated Encryption with Associated
   Data (AEAD) algorithm [RFC5116].  Of course, neither AES-CTR nor AES-
   CBC is an AEAD.  Thus, an implementation should provide integrity
   protection for the 'kid' field to prevent undetected stripping of the
   authentication and integrity mechanism; this prevents an attacker
   from altering the 'kid' to trick the recipient into using a different
   key.

   With AES-CBC mode, implementers should perform integrity checks prior
   to decryption to avoid padding oracle vulnerabilities [Vaudenay].

   With the assignment of COSE algorithm identifiers for AES-CTR and
   AES-CBC in the COSE Algorithms Registry, an attacker can replace the
   COSE algorithm identifiers with one of these identifiers.  Then, the
   attacker might be able to manipulate the ciphertext to learn some of
   the plaintext or extract the keying material used for authentication
   and integrity.

   Since AES-CCM [RFC3610] and AES-GCM [GCMMODE] use AES-CTR for
   encryption, an attacker can switch the algorithm identifier to AES-
   CTR and then strip the authentication tag to bypass the
   authentication and integrity, allowing the attacker to manipulate the
   ciphertext.

   An attacker can switch the algorithm identifier from AES-GCM to AES-
   CBC, guessing 16 bytes of plaintext at a time, and see if the
   recipient accepts the padding.  Padding oracle vulnerabilities are
   discussed further in [Vaudenay].

9.  References

9.1.  Normative References

   [AES]      National Institute of Standards and Technology (NIST),
              "Advanced Encryption Standard (AES)", NIST FIPS 197,
              DOI 10.6028/NIST.FIPS.197-upd1, May 2023,
              <https://doi.org/10.6028/NIST.FIPS.197-upd1>.

   [MODES]    Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Methods and Techniques", NIST Special
              Publication 800-38A, DOI 10.6028/NIST.SP.800-38A, December
              2001, <https://doi.org/10.6028/NIST.SP.800-38A>.

   [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>.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
              June 2005, <https://www.rfc-editor.org/info/rfc4107>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [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>.

   [RFC9052]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/info/rfc9052>.

9.2.  Informative References

   [GCMMODE]  Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Galois/Counter Mode (GCM) and GMAC", NIST
              Special Publication 800-38D, DOI 10.6028/NIST.SP.800-38D,
              November 2007, <https://doi.org/10.6028/NIST.SP.800-38D>.

   [IANA-COSE]
              IANA, "CBOR Object Signing and Encryption (COSE)",
              <https://www.iana.org/assignments/cose>.

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
              2003, <https://www.rfc-editor.org/info/rfc3610>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [SUIT-MANIFEST]
              Moran, B., Tschofenig, H., Birkholz, H., Zandberg, K., and
              Ø. Rønningstad, "A Concise Binary Object Representation
              (CBOR)-based Serialization Format for the Software Updates
              for Internet of Things (SUIT) Manifest", Work in Progress,
              Internet-Draft, draft-ietf-suit-manifest-22, 27 February
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              suit-manifest-22>.

   [Vaudenay] Vaudenay, S., "Security Flaws Induced by CBC Padding --
              Applications to SSL, IPSEC, WTLS...", EUROCRYPT 2002,
              2002, <https://www.iacr.org/cryptodb/archive/2002/
              EUROCRYPT/2850/2850.pdf>.

Acknowledgements

   Many thanks to David Brown for raising the need for non-AEAD
   algorithms to support encryption within the SUIT manifest.  Many
   thanks to Ilari Liusvaara, Scott Arciszewski, John Preuß Mattsson,
   Laurence Lundblade, Paul Wouters, Roman Danyliw, Sophie Schmieg,
   Stephen Farrell, Carsten Bormann, Scott Fluhrer, Brendan Moran, and
   John Scudder for the review and thoughtful comments.

Authors' Addresses

   Russ Housley
   Vigil Security, LLC
   Email: housley@vigilsec.com

   Hannes Tschofenig
   Email: hannes.tschofenig@gmx.net