Internet DRAFT - draft-sheffer-ietf-ciphertext-format
draft-sheffer-ietf-ciphertext-format
Network Working Group Y. Sheffer
Internet-Draft G. Keselman
Intended status: Standards Track Intuit
Expires: July 19, 2021 Y. Nir
Dell Technologies
January 15, 2021
A Generic Ciphertext Format
draft-sheffer-ietf-ciphertext-format-01
Abstract
This document defines a set of structured headers for encrypted data.
The main goal of this format is to enable detection of encrypted data
in large data stores, and associating it back to the system where it
was created and the key with which it was encrypted. This allows
organizations to extend the concept of data governance to encrypted
data, and to manage such data even when encrypted by multiple
different systems and cloud providers.
Status of This Memo
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Table of Contents
1. Introduction and Design Principles . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Design Goals . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Previous Work . . . . . . . . . . . . . . . . . . . . . . 4
3. The Ciphertext Format . . . . . . . . . . . . . . . . . . . . 4
3.1. Format Overview . . . . . . . . . . . . . . . . . . . . . 4
3.1.1. Fixed Header . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Variable Header . . . . . . . . . . . . . . . . . . . 5
3.1.3. Deriving a Specific Key . . . . . . . . . . . . . . . 6
3.2. Receiving Ciphertext . . . . . . . . . . . . . . . . . . 7
3.3. Fixed Header Rationale . . . . . . . . . . . . . . . . . 7
4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Fixed Header . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Variable Header: CBOR Diagnostic Notation . . . . . . . . 8
4.3. Variable Header: Binary . . . . . . . . . . . . . . . . . 8
4.4. Complete Header . . . . . . . . . . . . . . . . . . . . . 8
4.5. CDDL . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6.1. Integrity Protection . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 10
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Document History . . . . . . . . . . . . . . . . . . 11
A.1. draft-sheffer-ietf-ciphertext-format-01 . . . . . . . . . 11
A.2. draft-sheffer-ietf-ciphertext-format-00 . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction and Design Principles
Organizations that manage sensitive data often employ application-
level encryption to protect data at rest. When this solution is
used, it is common that very large numbers of encrypted data items
are stored, potentially for a long time. Security best practices,
complicated organizational structures, as well as the existence of
modern key management systems, lead to the proliferation of large
numbers of encryption keys. After a while it becomes difficult to
identify the encryption key that was used for a particular piece of
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data, with the situation becoming even more complicated when multiple
key management systems are used by the same organization.
Application-level encryption can be deployed at different scales: in
some cases a multi-megabyte file may be encrypted with a single key.
In other cases, we may want to deploy encryption for specific
database fields, which can easily manifest itself as millions of keys
for a single database table.
Tagging encrypted data with metadata supports a number of important
use cases: it allows the organization to better catalog the data
(a.k.a. "data governance"), to discover the owner of each piece of
encrypted data, to detect data encrypted with outdated keys.
1.1. 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.
2. Motivation
Our main goal in defining a common ciphertext format is to allow
organizations to manage large scale data, encrypted at rest using
multiple key management and encryption services. Additional
motivations for an enterprise to use a common format are:
- Cross-KMS-provider interoperability, to simplify automated
management of data sourced from multiple origins.
- Proprietary data encryption formats mean that the data remains
tied to a single vendor.
- Standardization around key management best practices.
2.1. Design Goals
Some of the goals behind this design include:
- The format should allow simple and efficient detection of
encrypted data, in support of automated data governance and key
lifecycle management.
- The format should be space-efficient, since it may be used for
very large numbers of small encrypted items. As a result,
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important information is associated with the (stored) key, rather
than the ciphertext.
- Specifically, following security best practices, a given key
material should be used with only a single cryptographic
algorithm. Therefore, the algorithm identifier should be stored
with the key (or the key version), rather than with the
ciphertext.
- The format defined here only covers the ciphertext header, and not
the ciphertext itself (referred to as "body" in this document).
The body is defined elsewhere, such as [NISTSP800-38D] for AES-
GCM.
- The header is not encrypted. Integrity-protection is optional.
See Section 6.1 for details.
- The format should support key versioning, i.e. automated, periodic
rotation of keys.
- The format should support granular key management by allowing for
key derivation and key wrapping.
- The format should allow for generic tools to perform partial
attribution of ciphertext, i.e. to associate it with a specific
key provider. More specific, possibly provider-specific tools are
required for full attribution.
2.2. Previous Work
A few notable formats are:
- The Amazon Web Services SDK message format, documented here [1].
This format is specific to the AWS library, and aimed at users of
the AWS Key Management System (KMS).
- The wire format [2] defined by Google's Tink library.
- The format defined by the KMIP 2.1 [3] specification, which is
targeted at data transmittal, rather than storage.
3. The Ciphertext Format
3.1. Format Overview
The ciphertext is prefixed by a header, which in turn, consists of a
short fixed header and variable header. The variable header is a
CBOR [RFC8949] map.
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Following the header is the body of the ciphertext. The format
(including length) of the body is out of scope for this document.
3.1.1. Fixed Header
The fixed header consists of:
- A single constant octet 0x08 (see Section 3.3).
- A single octet denoting the format version. The version is 0x01
for the format defined in this document.
3.1.2. Variable Header
The variable header is a CBOR map consisting of elements from the
following table.
+----------------+-----+----------+---------------------+-----------+
| Field Name | Map | Value | Meaning | Mandatory |
| | Key | Type | | |
+----------------+-----+----------+---------------------+-----------+
| Key Provider | 1 | Unsigned | The organization | Y |
| | | integer | responsible for the | |
| | | | key management | |
| | | | system. | |
| | | | | |
| Key ID | 2 | Byte | An encryption key | Y |
| | | string | identifier, where | |
| | | | the key is stored | |
| | | | in a key management | |
| | | | system. This must | |
| | | | denote a unique | |
| | | | key, even if the | |
| | | | Provider supports | |
| | | | multiple tenants. | |
| | | | Encoding of this | |
| | | | field is Provider- | |
| | | | specific. The field | |
| | | | must appear once. | |
| | | | | |
| Key Version | 3 | Unsigned | A version of a key, | N |
| | | integer | where the key is | |
| | | | rotated on a | |
| | | | periodic basis. | |
| | | | Encoding of this | |
| | | | field is Provider- | |
| | | | specific. The field | |
| | | | must appear at most | |
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| | | | once. | |
| | | | | |
| Auxiliary Data | 4 | Byte | Additional data | N |
| | | string | required to derive | |
| | | | a specific key from | |
| | | | the referenced key | |
| | | | (and key version, | |
| | | | if any), see also | |
| | | | Section 3.1.3. The | |
| | | | field must appear | |
| | | | at most once. | |
| | | | | |
| Nonce | 5 | Byte | A nonce or | N |
| | | string | initialization | |
| | | | vector (IV), if | |
| | | | required by the | |
| | | | cipher algorithm. | |
| | | | We note that an | |
| | | | implementation may | |
| | | | prefer to store the | |
| | | | nonce and | |
| | | | authentication tag | |
| | | | in-line with the | |
| | | | ciphertext. | |
| | | | | |
| Authentication | 6 | Byte | An authentication | N |
| Tag | | string | tag or integrity | |
| | | | check value (ICV), | |
| | | | if required by the | |
| | | | cipher algorithm. | |
| | | | | |
| Additional | 7 | Byte | Additional | N |
| Authenticated | | string | authenticated data | |
| Data | | | (AAD), which is | |
| | | | integrity-protected | |
| | | | but not encrypted | |
| | | | by the cipher. | |
+----------------+-----+----------+---------------------+-----------+
3.1.3. Deriving a Specific Key
The Auxiliary Data field is used to support derivation of a key,
specific to the ciphertext being managed. There are two common ways
to obtain this specific key:
- Using a key derivation function: SK = KDF(key, aux-data)
- Decryption of a wrapped key: SK = Decrypt(key, aux-data)
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The exact algorithm is implementation dependent, and should be
uniquely defined by the combination of Key Provider, Key ID and (if
given) Key Version.
3.2. Receiving Ciphertext
Correct interpretation of the format may have security implications,
making it important to define the exact semantics even when the
entity that receives a ciphertext may not understand parts of the
header.
- A recipient MUST reject a malformed header, e.g. if the total
length is larger than the physical length allocated to it based on
higher-level network protocols or storage formats.
- A recipient MUST reject a ciphertext if it does not recognize the
format version.
- A recipient MUST reject a ciphertext if the variable header is not
valid CBOR, as per [RFC8949] Sec. 5.3.1. In particular, it MUST
reject duplicate map keys.
- A recipient MUST accept a ciphertext even if it does not recognize
some of the map keys. It MUST ignore the unknown map keys and
MUST interpret all known ones. In other words, the only way to
introduce new mandatory map keys is by incrementing the format
version.
- If ciphertext integrity protection coverage includes the header, a
recipient MUST reject the header as well as the ciphertext if the
integrity protection fails to validate.
3.3. Fixed Header Rationale
We chose the initial byte 0x08, since strings are very unlikely to
start with it, as we explain below. Automated tools can detect
encrypted data in structured contexts (e.g., a SQL database column)
by sampling a number of data items and if all start with this byte,
determining that they are encrypted with a high probability.
The byte 0x08 encodes the ASCII control character "backspace". It
has the same meaning in UTF-8, and the 08 block of UTF-16 characters
is only populated by two very small languages and rarely-used
extended Arabic characters [4].
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4. Example
4.1. Fixed Header
"08 01"
4.2. Variable Header: CBOR Diagnostic Notation
" {1: 65535, 2: h'1122334455', 3: 6, } "
4.3. Variable Header: Binary
" a3 01 19 ff ff 02 45 11 22 33 44 55 03 06 "
4.4. Complete Header
" 08 01 a3 01 19 ff ff 02 45 11 22 33 44 55 03 06 "
4.5. CDDL
The following non-normative snippet defines the format of the
variable header using CDDL [RFC8610].
var_header = {
K_KEY_PROVIDER: uint,
K_KEY_ID: bstr,
? K_KEY_VERSION: uint,
? K_AUX_DATA: bstr,
? K_NONCE : bstr,
? K_AUTH_TAG : bstr,
? K_AAD : bstr,
*uint => any ; extensions
}
K_RESERVED = 0
K_KEY_PROVIDER = 1
K_KEY_ID = 2
K_KEY_VERSION = 3
K_AUX_DATA = 4
K_NONCE = 5
K_AUTH_TAG = 6
K_AAD = 7
; extend here
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5. IANA Considerations
TBD: establish a registry for Types, with 128-255 as private use.
TBD: establish a registry of Key Providers.
6. Security Considerations
6.1. Integrity Protection
The format defined here does not include integrity protection for the
header, and neither does it mandate that the encrypted item's
integrity protection should include the header.
Data encrypted at rest is typically vulnerable to denial of service
attacks, since (assuming the data is integrity protected) an attacker
that can change the ciphertext can trivially cause it to fail
validation.
There are cases where it is convenient to manipulate the ciphertext
header, even if the data itself remains encrypted and unmodified.
For example, when migrating between formats or when bulk-changing
metadata associated with the ciphertext. On the other hand, it is a
best practice to protect cryptographic metadata against malicious
modification. We are currently not aware of a specific threat vector
associated with malicious changes to the proposed format, at least
assuming the use of AEAD ciphers.
7. References
7.1. Normative References
[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>.
[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>.
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7.2. Informative References
[NISTSP800-38D]
Dworkin, M., "Recommendation for block cipher modes of
operation :: GaloisCounter Mode (GCM) and GMAC", National
Institute of Standards and Technology report,
DOI 10.6028/nist.sp.800-38d, 2007.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
7.3. URIs
[1] https://docs.aws.amazon.com/encryption-sdk/latest/developer-
guide/message-format.html
[2] https://github.com/google/tink/blob/master/docs/WIRE-FORMAT.md
[3] https://docs.oasis-open.org/kmip/kmip-profiles/v2.1/csprd01/kmip-
profiles-v2.1-csprd01.html
[4] https://en.wikipedia.org/wiki/Arabic_Extended-A
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Appendix A. Document History
A.1. draft-sheffer-ietf-ciphertext-format-01
- SAAG feedback: the variable header is now CBOR.
- Binary example.
- Non-normative CDDL.
- Additional types for non-inline AEAD.
A.2. draft-sheffer-ietf-ciphertext-format-00
- Initial version.
Authors' Addresses
Yaron Sheffer
Intuit
EMail: yaronf.ietf@gmail.com
Gleb Keselman
Intuit
EMail: gleb.keselman@gmail.com
Yoav Nir
Dell Technologies
EMail: ynir.ietf@gmail.com
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