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.
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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