Internet DRAFT - draft-ietf-cose-aes-ctr-and-cbc
draft-ietf-cose-aes-ctr-and-cbc
COSE R. Housley
Internet-Draft Vigil Security
Intended status: Standards Track H. Tschofenig
Expires: 26 November 2023 Arm Limited
25 May 2023
CBOR Object Signing and Encryption (COSE): AES-CTR and AES-CBC
draft-ietf-cose-aes-ctr-and-cbc-06
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 26 November 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. AES Modes of Operation . . . . . . . . . . . . . . . . . . . 3
4. AES Counter Mode . . . . . . . . . . . . . . . . . . . . . . 3
4.1. AES-CTR COSE Key . . . . . . . . . . . . . . . . . . . . 4
4.2. AES-CTR COSE Algorithm Identifiers . . . . . . . . . . . 5
5. AES Cipher Block Chaining Mode . . . . . . . . . . . . . . . 5
5.1. AES-CBC COSE Key . . . . . . . . . . . . . . . . . . . . 6
5.2. AES-CBC COSE Algoritm Identifiers . . . . . . . . . . . . 6
6. Implementation Considerations . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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. Encryption with COSE today uses
Authenticated Encryption with Associated Data (AEAD) [RFC5116]
algorithms, 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 [I-D.ietf-suit-manifest] is one example
where a digital signature is always present.
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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 Advanced Encryption
Standard (AES) [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 mode (AES-CTR) 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 (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.
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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. 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, and AES-CTR can be pipelined and parallelized.
AES-CTR also 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.
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.
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* 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.
In addition, the 'protected' header parameters encoded value MUST be
a zero-length byte string.
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 | TBD1 | 128 | AES-CTR w/ | Deprecated |
| | | | 128-bit key | |
+---------+-------+----------+------------------------+-------------+
| A192CTR | TBD2 | 192 | AES-CTR w/ | Deprecated |
| | | | 192-bit key | |
+---------+-------+----------+------------------------+-------------+
| A256CTR | TBD3 | 256 | AES-CTR w/ | Deprecated |
| | | | 256-bit key | |
+---------+-------+----------+------------------------+-------------+
Table 1
5. AES Cipher Block Chaining Mode
AES-CBC mode requires an 16 octet Initialization Vector (IV). Use of
a randomly or pseudo-randomly 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 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.
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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.
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.
In addition, the 'protected' header parameters encoded value MUST be
a zero-length byte string.
5.2. AES-CBC COSE Algoritm 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.
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+=========+=======+==========+========================+=============+
| Name | Value | Key Size | Description | Recommended |
+=========+=======+==========+========================+=============+
| A128CBC | TBD4 | 128 | AES-CBC w/ | Deprecated |
| | | | 128-bit key | |
+---------+-------+----------+------------------------+-------------+
| A192CBC | TBD5 | 192 | AES-CBC w/ | Deprecated |
| | | | 192-bit key | |
+---------+-------+----------+------------------------+-------------+
| A256CBC | TBD6 | 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 algorithm 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 is requested to register six COSE algorithm identifiers for AES-
CTR and AES-CBC in the COSE Algorithms Registry [IANA].
The information for the six COSE algorithm identifiers is provided in
Section 4.2 and Section 5.2. Also, for all six entries, the
"Capabilities" column should contain "[kty]", the "Change Controller"
column should contain "IESG", and the "Reference" column should
contain a reference to this document.
Ideally, the six values will be assigned in the -65534 to -261 range.
8. Security Considerations
This document specifies AES-CTR and AES-CBC for COSE, which are not
authenticated encryption with additional data (AEAD) ciphers. The
use of the ciphers is limited to special use cases where integrity
and authentication is provided by another mechanism, such as firmware
encryption.
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.
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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 are 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 his own choosing. The ciphertext is:
(P1 XOR K1), (P2 XOR K2), (P3 XOR K3)
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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) [RFC5116] algorithm. 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, guess of 16 bytes of plaintext at a time, and checking each
guess with padding oracle as discussed above.
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9. Acknowledgements
Many thanks to David Brown for raising the need for non-AEAD
algorithms to support encryption within the SUIT manifest. Many
thanks to David Brown, Ilari Liusvaara, Scott Arciszewski, John Preuß
Mattsson, Laurence Lundblade, Paul Wouters, Roman Danyliw, and John
Scudder for the review and thoughtful comments.
10. References
10.1. Normative References
[AES] National Institute of Standards and Technology (NIST),
"Advanced Encryption Standard (AES)", FIPS
Publication 197, November 2001.
[MODES] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Methods and Techniques", NIST Special
Publication 800-38A, December 2001.
[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>.
10.2. Informative References
[GCMMODE] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC", NIST
Special Publication 800-38D, November 2007.
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[I-D.ietf-suit-manifest]
Moran, B., Tschofenig, H., Birkholz, H., Zandberg, K., and
O. 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>.
[IANA] "IANA Registry for CBOR Object Signing and Encryption
(COSE)", n.d.,
<https://www.iana.org/assignments/cose/cose.xhtml>.
[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>.
[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>.
Authors' Addresses
Russ Housley
Vigil Security, LLC
Email: housley@vigilsec.com
Hannes Tschofenig
Arm Limited
Email: hannes.tschofenig@arm.com
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