Guidelines for Writing Cryptography Specifications
draft-irtf-cfrg-cryptography-specification-01
| Document | Type | Active Internet-Draft (cfrg RG) | |
|---|---|---|---|
| Authors | Nick Sullivan , Christopher A. Wood | ||
| Last updated | 2024-04-10 | ||
| Replaces | draft-sullivan-cryptography-specification | ||
| RFC stream | Internet Research Task Force (IRTF) | ||
| Intended RFC status | Informational | ||
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draft-irtf-cfrg-cryptography-specification-01
Network Working Group N. Sullivan
Internet-Draft Cryptography Consulting LLC
Intended status: Informational C. A. Wood
Expires: 12 October 2024 Cloudflare, Inc.
10 April 2024
Guidelines for Writing Cryptography Specifications
draft-irtf-cfrg-cryptography-specification-01
Abstract
This document provides guidelines and best practices for writing
technical specifications for cryptography protocols and primitives,
targeting the needs of implementers, researchers, and protocol
designers. It highlights the importance of technical specifications
and discusses strategies for creating high-quality specifications
that cater to the needs of each community, including guidance on
representing mathematical operations, security definitions, and
threat models.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Crypto Forum Research
Group mailing list (cfrg@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/search/?email_list=cfrg.
Source for this draft and an issue tracker can be found at
https://github.com/cfrg/draft-irtf-cfrg-cryptography-specification.
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 12 October 2024.
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Copyright Notice
Copyright (c) 2024 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Goals and Requirements . . . . . . . . . . . . . . . . . . . 3
3. Guidelines for Cryptographic Specification Presentation . . . 5
3.1. Simplicity . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Precision . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Consistency . . . . . . . . . . . . . . . . . . . . . . . 7
3.3.1. Representing Mathematical Operations . . . . . . . . 9
4. Guidelines for Cryptography Specification Content . . . . . . 10
4.1. Reusability . . . . . . . . . . . . . . . . . . . . . . . 10
4.1.1. Build on Existing Specifications . . . . . . . . . . 11
4.1.2. Modular Design . . . . . . . . . . . . . . . . . . . 11
4.1.3. Clear Interfaces and Abstractions . . . . . . . . . . 11
4.1.4. Completeness . . . . . . . . . . . . . . . . . . . . 12
4.1.5. Documentation and Examples . . . . . . . . . . . . . 12
4.2. Defining Security Definitions and Threat Models . . . . . 12
4.2.1. Defining Security Goals . . . . . . . . . . . . . . . 13
4.2.2. Formalizing Security Definitions . . . . . . . . . . 13
4.2.3. Describing the Threat Model . . . . . . . . . . . . . 13
4.2.4. Addressing Known Vulnerabilities and Attacks . . . . 13
4.2.5. Providing Guidance on Secure Implementation and
Deployment . . . . . . . . . . . . . . . . . . . . . 14
5. Catering to Target Audiences . . . . . . . . . . . . . . . . 14
5.1. Catering to Implementers . . . . . . . . . . . . . . . . 14
5.1.1. Test vectors . . . . . . . . . . . . . . . . . . . . 15
5.2. Catering to Researchers . . . . . . . . . . . . . . . . . 16
5.3. Catering to Protocol Designers . . . . . . . . . . . . . 16
6. General Recommendations . . . . . . . . . . . . . . . . . . . 17
6.1. Encourage Open Communication and Feedback . . . . . . . . 18
6.2. Seek External Expertise . . . . . . . . . . . . . . . . . 18
6.3. Recognize and Address Conflicting Interests . . . . . . . 18
7. Examples of Well-Written Specifications . . . . . . . . . . . 18
7.1. ChaCha20 and Poly1305 for IETF Protocols (RFC 8439) . . . 18
7.1.1. Introduction and Overview . . . . . . . . . . . . . . 19
7.1.2. Algorithm Descriptions . . . . . . . . . . . . . . . 19
7.1.3. Test Vectors . . . . . . . . . . . . . . . . . . . . 19
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7.1.4. Security Considerations . . . . . . . . . . . . . . . 19
7.1.5. IANA Considerations and References . . . . . . . . . 19
7.1.6. Problematic Aspects . . . . . . . . . . . . . . . . . 20
8. Examples of Specifications That Could Be Improved . . . . . . 20
8.1. Test Vectors . . . . . . . . . . . . . . . . . . . . . . 20
8.2. Unnecessary Branching . . . . . . . . . . . . . . . . . . 20
8.3. Compatibility and Modularity . . . . . . . . . . . . . . 21
9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Normative References . . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
High-quality cryptography specifications are critical for the for
development and deployment of secure cryptographic protocols. This
document provides guidelines for specification writers to follow to
help ensure that their specifications are of high quality and are
useful for their intended audience. This document provides
guidelines for writing clear, precise, and consistent specifications
covering topics such as representing mathematical operations,
defining security definitions, and describing threat models.
Adhering to these guidelines helps ensure that specifications are
easier to understand, implement, and analyze, leading to high
assurance and interoperable systems.
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. Goals and Requirements
The primary goal of these guidelines is to help guide the authorship
of cryptographic specifications so that they are as useful as
possible when creating high-assurance cryptographic software.
Specifications that follow these guidelines should be able to be
easily understood, implemented, and analyzed by different audiences,
including the engineering community, research community, and
standardization community. By addressing the unique needs and
expectations of each group, these guidelines aim to:
* Minimize ambiguity and misinterpretations, leading to clearer
specifications and more accurate implementations.
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* Ensure consistent and correct implementations by providing a clear
description of both algorithms and their underlying mathematical
foundation.
* Facilitate review and analysis by the research community, allowing
for the verification of security properties and the identification
of potential vulnerabilities.
* Enable interoperability of implementations of these
specifications, promoting collaboration and compatibility between
various systems and protocols.
Each of these stakeholder groups contributes something different to
the overall process of deploying software:
1. Engineering community: This community identifies technical
problems to be solved and have the skills and resources necessary
to build solutions using computing tools. Engineers focus on why
certain problems should be addressed, producing requirements that
define the problem and solutions that meet those requirements.
Their ultimate goal is to implement and ship software or hardware
that effectively tackles these challenges.
2. Research community: Researchers are experts who explore the
design space of different subject areas and evaluate potential
solutions to problems with the goal of better understanding a
topic. This group develops methods for designing tools and
performing experiments to validate hypotheses. The research
community concentrates on how problems should be solved, creating
artifacts that help describe solutions. These may include
academic, peer-reviewed papers or software that studies or
supports the shipping of software.
3. Standardization community: This group is responsible for
developing technical specifications of protocols that others can
implement, analyze, and verify. The standardization community
produces technical specifications that capture the details of a
solution. These specifications serve as a foundation for
creating interoperable systems and ensuring the correct
implementation of cryptographic algorithms and protocols.
By following these guidelines and addressing the distinct needs of
each stakeholder group, specification authors can create well-
structured, informative specfications documents that facilitate the
development, analysis, and implementation of high assurance
cryptographic solutions.
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3. Guidelines for Cryptographic Specification Presentation
Technical specifications do not stand on their own. Their value is
derived from their usefulness to the various communities that rely on
them. A specification can have amazing content but without the
appropriate presentation, it may not be as useful as intended. The
guidelines in this section are a baseline set of recommendations for
authors to consider when writing a cryptographic specification and
are applicable beyond just cryptographic standards and are general
good practices for specification writers.
3.1. Simplicity
Complexity is one of the main causes of software bugs. The opposite
of complexity is simplicity, which is a key aspect of creating
effective cryptography specifications. By striving for simplicity in
problem statements, technical content, and presentation,
specification authors can make their documents more accessible to a
wider audience, including implementers, researchers, and protocol
designers. Simplicity reduces the cognitive load required to
understand the specification and minimizes the risk of
misinterpretation, which can lead to incorrect implementations and
security vulnerabilities.
To achieve simplicity, specification authors should focus on:
* Clearly defining the problem: Start by presenting a concise and
easily comprehensible description of the problem that the
specification aims to solve. Avoid unnecessary jargon and strive
to make the problem statement accessible to readers with varying
levels of expertise in the field. Ideally, each specification
addresses precisely one clearly defined problem.
* Reducing complex concepts to component parts: When explaing with
multi-step cryptographic algorithms or concepts, break them down
into smaller, more manageable components. This will make it
easier for readers to understand the individual parts and their
relationships to one another.
* Using straightforward language and terminology: Write the
specification using clear, concise language, and consistent and
broadly understood terminology.
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* Avoid overly technical jargon, and define any terms that may be
unfamiliar to some readers. Limiting scope: Keep the
specification focused on the primary problem or use case, i.e.,
avoid feature creep. Avoid introducing unrelated or peripheral
topics, as this can create confusion and detract from the primary
focus.
* Providing clear examples and diagrams: Include examples and visual
aids, such as diagrams or visualizations, to help illustrate
complex concepts or processes. These tools can greatly improve
the readability and understanding of the specification.
By focusing on simplicity in document structure and prose in the
specification writing process, authors can create documents that are
more accessible and easier to understand, ultimately resulting in
more reliable and secure implementations of cryptographic algorithms
and protocols. Focusing on simplicity in writing does not imply
imprecision or brevity. Even long documents can embody simplicity
with the right attention to detail and structuring of prose.
3.2. Precision
Precision is a crucial aspect of writing high-quality specifications,
particularly for cryptography, where small deviations or ambiguities
can lead to severe security vulnerabilities. A precise specification
ensures that resulting implementations are consistent and correct,
and that any analysis done matches the specification.
To achieve precision in your specifications, consider the following
recommendations:
1. Use clear and concise language: Write your specification using
straightforward and unambiguous language. Avoid jargon or
colloquialisms that may lead to misinterpretation. When
introducing technical terms or concepts, provide clear
definitions or explanations to ensure that all readers are on the
same page.
2. Provide explicit instructions and avoid undefined behavior: When
describing algorithms, protocols, or procedures, provide step-by-
step instructions that can be easily followed by implementers
with minimal or zero risk of misinterpretation. This will help
ensure that all implementations are consistent with the intended
design and minimize the risk of errors or vulnerabilities.
3. Provide test vectors. Test vectors help check for correctness of
all behavior in the specification, especially those near edge
cases. For example, if a specification involves a branch or
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condition, then test cases should ideally be written to exercise
both paths of the branch. Sometimes this is infeasible, e.g., if
probability of a particular branch happening is negligible,
though more often than not branches can be adequately covered.
4. Employ formal notation or pseudocode: Using formal notation or
pseudocode can help provide a precise description of algorithms,
data structures, and protocols. This will ensure that
implementers, researchers, and protocol designers can accurately
understand the intended behavior and interactions of the
components within the specification.
5. Specify data formats and encodings: Clearly define data formats,
encoding schemes, and serialization methods for all data types
used in the specification. This will help ensure that different
implementations can interoperate seamlessly and reduce the
likelihood of incompatibilities or communication errors.
6. Document assumptions and dependencies: Clearly state any
assumptions or dependencies on external components, including
other specifications or protocol descriptions. This includes
common dependencies like that of a random number generator. This
will help implementers and researchers understand the context in
which your specification operates and any potential limitations
or risks.
Precise specifications minimize ambiguity and reduce the likelihood
of implementation errors or inconsistencies.
3.3. Consistency
When a document is self-consistent and consistent with the
expectations of documents of its type, it helps reduce ambiguity and
is easier to consume. Ensuring consistency across concepts,
vocabulary, language, and presentation helps lower the cognitive load
necessary for readers to understand and work with the specification.
To achieve consistency in your specifications, consider the following
recommendations:
1. Establish a consistent terminology: Develop a clear and
consistent set of terms and definitions that will be used
throughout the document. Avoid using synonyms or multiple terms
for the same concept, as this can lead to confusion. When using
acronyms, always provide their full meaning upon first usage and
use the acronym consistently afterward.
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2. Maintain a uniform style and tone: Write the specification using
a consistent style and tone to ensure that readers can easily
follow the content. This includes consistent use of grammatical
structures, punctuation, and capitalization. If your
organization has a style guide, adhere to it when writing the
specification.
3. Use a logical structure: Organize your specification in a logical
manner, starting with an overview and then progressing through
the various components, algorithms, and protocols. Make use of
sections, subsections, and other structural elements to break up
the content and make it easier to navigate and comprehend. Use
forward or backward references to make navigation of the document
simpler.
4. Provide consistent formatting: Ensure that all elements within
the specification, such as tables, figures, pseudocode and
equations, are formatted consistently. This will help readers
quickly identify and understand these elements as they progress
through the document.
5. Be consistent with conventions and notations: When using
mathematical notation, programming languages, or other
conventions, apply them consistently throughout the document.
This will help prevent confusion and allow readers to focus on
the content rather than deciphering different notations.
6. Reference external documents consistently: When referring to
external documents or resources, such as other RFCs, standards,
or research papers, provide consistent and accurate citations.
This will enable readers to locate and review these resources as
needed.
7. Keep the broader context in mind: Try to adopt the same
terminology and conventions as other related documents the reader
may be familiar with, especially for specifications that are
developed based on peer-reviewed, published work. Consistency
across audiences is important to help lower the bar to successful
collaboration and effective communication. If the specification
is intended to be part of the RFC series, reuse conventions from
other documents in the series.
By focusing on consistency in your cryptography specification, you
will make it more accessible and easier to understand for
implementers, researchers, and protocol designers. This, in turn,
will facilitate the development of correct, secure, and interoperable
cryptographic systems based on your specification.
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Cryptography specifications are often unique in their use of
mathematical objects to define protocols. As such, presenting this
content requires special guidance.
3.3.1. Representing Mathematical Operations
Mathematical operations are fundamental to cryptographic algorithms
and protocols, and their clear and precise representation in
specifications is crucial for correct implementation and analysis.
This section provides guidelines for representing mathematical
operations in cryptography specifications in a way that is both
comprehensible and unambiguous to the target audiences, including
implementers, researchers, and protocol designers.
3.3.1.1. Notation Consistency
Consistency in the notation used to represent mathematical operations
is essential for avoiding confusion and ensuring that the
specification is easy to understand. Specification authors should
establish a clear notation system from the beginning and use it
consistently throughout the document. This notation should be
introduced with a comprehensive description or a reference to a well-
known notation system to ensure that readers can easily follow the
mathematical expressions. For example, exponentiation can be
represented by superscript or by a carat, but not by both.
3.3.1.2. Use of Standard Mathematical Symbols
Specification authors should use standard mathematical symbols to
represent mathematical operations whenever possible. This approach
promotes clarity and reduces the risk of misinterpretation. It is
important to remember that some symbols may have different meanings
in different contexts or disciplines. In such cases, specification
authors should clarify the intended meaning of a symbol within the
context of the specification. For example, when describing group
operations using multiplicative notation, the multiplication symbol *
should be used instead of the x symbol.
3.3.1.3. Explicitly Defining Custom Operations
When a specification requires custom mathematical operations or
notation, these should be explicitly defined and accompanied by clear
explanations and examples. Specification authors should take care to
minimize the use of custom operations to avoid creating unnecessary
complexity and potential confusion. When using custom operations, it
is essential to ensure their definitions are unambiguous and easily
understandable to the target audiences. A glossary of terms MAY be
useful when there are multiple custom or uncommon operations
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introduced in the specification.
3.3.1.4. Pseudocode and Algorithmic Descriptions
Providing algorithmic descriptions or pseudocode alongside
mathematical expressions can greatly improve the clarity of a
specification, particularly for implementers. Pseudocode should be
clear, concise, and closely resemble the structure of actual
programming languages. Including comments and explanations within
the pseudocode can further enhance its readability and ease of
understanding. Consistent notation for describing loops or if/then
statements should be used throughout the document.
3.3.1.5. Visual Representations
Visual representations, such as diagrams, tables or visualizations,
can be a valuable tool for conveying complex mathematical concepts or
relationships in a more digestible form. When including visual
representations, specification authors should ensure they are clear,
accurately labeled, and consistent with the overall notation system.
4. Guidelines for Cryptography Specification Content
In addition to cryptographic specification clarity and accessibility
through presentation format, the content of a specification also
influences the overall value of the specification. The syntax of
cryptographic objects introduced and their interfaces, as well as the
way in which the object is structured for use in applications, is
important for reliable and secure implementations of cryptographic
algorithms and protocols. In this section, we discuss factors that
relate to the content of the specifications and their impact on
overall quality.
4.1. Reusability
Cryptography specifications that rely on bespoke sub-algorithms or
lower-level components tend to be brittle and invite implementation
issues. To create efficient, interoperable, and widely adopted
cryptographic systems, it is preferable to reuse existing components
or primitives. Reusability allows developers to build on existing
work, reducing the time and effort required to create new
implementations while leveraging established security properties and
analyses. This section discusses the importance of reusability in
cryptography specifications and offers guidance for incorporating
reusability principles into the specification development process.
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4.1.1. Build on Existing Specifications
When developing a cryptography specification, it is advantageous to
build upon existing, well-established specifications, protocols, and
primitives where possible. By doing so, specification authors can
capitalize on the collective expertise of the community, as well as
existing security analyses, implementation experiences, and best
practices. This approach reduces the potential for introducing new
vulnerabilities and inconsistencies while promoting interoperability
between different systems.
4.1.2. Modular Design
Emphasizing modularity in the design of cryptography specifications
allows for greater flexibility and reusability. By breaking down
complex algorithms into smaller, self-contained components or
modules, specification authors facilitate the reuse of these
components in different contexts or applications. A modular design
also simplifies the process of updating or replacing specific
components without affecting the overall system, making it easier to
incorporate new research findings or technological advancements. An
example of a modular design is the prime-order group abstraction.
Algorithms that use this abstraction admit a modular design where the
group implementation is described in a separate document dedicated to
the details of the implementation of the group.
4.1.3. Clear Interfaces and Abstractions
To promote misuse resistance and elegant higher-level designs,
cryptography specifications should provide clear interfaces and
abstractions for the components and primitives they describe. Well-
defined interfaces enable developers to understand and interact with
a component without needing to know the details of its internal
implementation. This approach allows for the replacement or
modification of components with minimal impact on the overall system
and encourages the development of interchangeable components that can
be reused across different applications and within protocols.
Cryptographic objects typically have a set of functions associated
with them that make up the interface. Structuring the functions to
fit well-understood and existing abstractions help make the job of
using the object in higher-level algorithms easier and less prone to
code duplication.
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4.1.4. Completeness
The operations defined in a cryptography specification should be
complete, with defined behavior on all inputs. This includes error
handing, and edge cases which would otherwise not impact the
algorithm's cryptographic properties. In particular, when
deserializing a byte string, the behavior on all byte strings should
be defined, including cases which would not be valid outputs of the
corresponding serialization function. A complete specification help
avoids implementation variations. These variations can lead to
interoperability failures, gaps between formal analysis and real-
world practice, or security vulnerabilities.
Avoid defining multiple implementation behaviors as valid. Leaving
multiple options to implementators leads to compounding complexity:
downstream specifications may need to profile the algorithm to pick
the preferred option, and validation tools must be configurable to
assert either case.
4.1.5. Documentation and Examples
Thorough documentation and illustrative examples play a crucial role
in promoting reusability. By providing comprehensive explanations of
the specification's components, interfaces, and intended use cases,
specification authors make it easier for developers to understand and
implement the specification correctly. Including examples of how
components can be combined or applied in various scenarios further
clarifies their usage and encourages their reuse in different
contexts.
By focusing on reusability in cryptography specifications, authors
can help create secure, efficient, and adaptable cryptographic
systems that can be more easily integrated, maintained, and updated,
resulting in more robust and widely adopted solutions.
4.2. Defining Security Definitions and Threat Models
Cryptographic protocols are always used within a context of a broader
system whose security relies on an understanding capabilities of
potential attackers. An incorrect definition or assumption about the
threat models to a protocol can make a protocol that is safe in one
context unsafe in a different context. Precise definitions help
researchers assess the security of the proposed algorithms and
protocols, while comprehensible threat models enable implementers and
protocol designers to understand the potential risks and limitations
of the specification. This section provides guidelines for defining
security definitions and threat models in a way that caters to the
needs of all target audiences.
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4.2.1. Defining Security Goals
Specification authors should explicitly state the security goals that
the proposed algorithms or protocols aim to achieve. These goals
should be comprehensive, covering all relevant aspects, such as
confidentiality, integrity, authentication, non-repudiation, and
availability as well as resistance to implementation flaws such as
side-channels. Furthermore, authors should clarify any trade-offs or
limitations associated with the security goals, ensuring that the
target audiences understand the intended balance between security and
other factors, such as performance or ease of implementation.
4.2.2. Formalizing Security Definitions
Formalizing security definitions is essential for researchers to
rigorously analyze the algorithms and protocols described in the
specification. Specification authors should strive to express
security definitions in a formal language, using consistent notation
and terminology. The formal definitions should be accompanied by
clear explanations and examples to make them more accessible to
implementers and protocol designers who may not be familiar with
formal methods.
4.2.3. Describing the Threat Model
A well-defined threat model provides an overview of the potential
adversaries and the risks they pose to the security of the algorithms
or protocols. Specification authors should describe the threat model
in detail, including the capabilities, resources, and motivations of
adversaries. Additionally, authors should identify any assumptions
made about the adversarial model and explicitly state them to help
the target audiences understand the intended scope and limitations of
the specification's security guarantees.
4.2.4. Addressing Known Vulnerabilities and Attacks
Specification authors should discuss known vulnerabilities and
attacks relevant to the proposed algorithms or protocols. This
discussion should include an explanation of how the specification
addresses or mitigates these issues, as well as any residual risks
that remain. This information is valuable for implementers and
protocol designers to understand the potential threats and for
researchers to assess the robustness of the specification's security
claims.
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4.2.5. Providing Guidance on Secure Implementation and Deployment
To help ensure that the security definitions and threat models are
effectively realized in practice, specification authors should
provide guidance on secure implementation and deployment of the
proposed algorithms and protocols. This guidance may include best
practices for avoiding common pitfalls, recommendations for
cryptographic parameter selection, or considerations for securely
integrating the specification into existing systems.
By clearly defining security definitions and threat models in
cryptography specifications, authors can facilitate a better
understanding of the security properties and limitations of the
proposed algorithms and protocols among implementers, researchers,
and protocol designers. Following these guidelines and the
recommendations from [RFC3552] help make for a robust security
considerations section for the specification. Having a complete
discussion of the threat model and security definitions helps prevent
the use of cryptographic algorithms in insecure contexts.
5. Catering to Target Audiences
When writing a specification, it is important to consider the needs
of the three primary audiences: implementers, researchers, and
protocol designers. Each group has unique requirements and goals,
and the specification should be written in a way that addresses their
specific concerns.
5.1. Catering to Implementers
Implementers require a clear, concise, and unambiguous specification
to develop production-grade software. To cater to implementers:
* Provide step-by-step instructions for implementing algorithms or
processes, ensuring that all required inputs, outputs, and
intermediate steps are defined. Where exceptional cases occur,
those should be noted and recommended error-handling steps should
be given. Include test vectors to help implementers verify the
correctness of their implementations.
* Describe best practices for representing components of the
specification in code, addressing exceptional cases and
recommended error handling procedures, as well as aspects of the
specification that are difficult to implement correctly (e.g.,
where side-channel attacks might be possible).
* Clearly indicate any optional features, variations, or extensions,
specifying their impact on interoperability and security.
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5.1.1. Test vectors
Test vectors ideally cover all branches of the specification, with
reasonable exceptions, such as branches that occur with negligible
probability and as such are computationally infeasible to reproduce.
To facilitate writing tests, where possible, all functions should be
written with determinism in mind. In particular, this means that
functions that produce random outputs, such as a function that
produces random elements in a prime-order group, should accept
randomness as input and test vectors should specify this randomness
as an input to the function. Specifications should minimize internal
calls to PRNGs or similar and emphasize determinism.
Finally, specifications should make the connection between
specification and test vectors clear by including explicit
reproducibility steps that describe how test vectors were derived for
parts of the specification. This might mean pointing to a reference
implementation with instructions for how to run it, where the
reference implementation is written in a way that is clearly
consistent with the specification.
It's possible to include too many test vectors in a specification,
which increases document length and decreases readability. Authors
should provide test vectors that cover:
* Typical test cases that exercise all logical pathways within an
algorithm
* All valid but degenerate cases that result in error or early exit
of an algorithm
* Exceptions that can be reached by attacker-controlled inputs
It is NOT necessary to include test vectors for cases that are
statistically improbable to be triggered, even by attacker-controlled
input, based on the underlying cryptographic assumptions. For
example, if an error case is only reachable when an intermediate data
point matches the pre-image of a hash value that was randomly
generated, finding a test vector to trigger that case would require
the ability to compute a hash pre-image, which is deemed unfeasible
for sufficiently strong hash functions. Exceptional cases that don't
have test vectors should be explicitly noted in the algorithm
description.
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Lastly, specifications should provide references to machine-readable
test vectors (e.g., in JSON format) that persist alongside the
specification. This helps avoid possibly error-prone parsing in
translating test vectors from a textual specification to test code
inputs.
5.2. Catering to Researchers
Researchers need to understand the syntax and functionality of the
cryptographic protocol or primitive to ensure its correctness and
analyze its security properties. To cater to researchers:
* Clearly define the underlying mathematical concepts and notations
used in the specification, ensuring that all symbols, functions,
and variables are consistently and accurately represented as
explained in the section Representing Mathematical Operations.
* Provide detailed security definitions, goals, and threat models,
including the capabilities and limitations of adversaries and
their impact on parameter selection. In general, authors should
make input requirements that are important for the security of the
protocol or construction maximally clear. See: Defining Security
Definitions and Threat Models.
* Describe any assumptions made about the underlying primitives or
protocols and the justifications for these assumptions. Such
assumptions should include references to external documents that
describe these underlying primitives or protocols where
appropriate, unless there are gaps between how the underlying
primitive or protocol is used and how it is described externally.
* Clearly present any security proofs, analysis, or references to
existing literature that support the security claims of the
specification. If there are gaps between the specification and
formal security analysis, these gaps should be noted, along with
rationale that justifies the gaps.
5.3. Catering to Protocol Designers
Protocol designers in the standards community use specifications to
understand how to safely use the cryptographic protocol or primitive
when designing a higher-level protocol that depends on it. To cater
to protocol designers:
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* Clearly define the interfaces, APIs, or functions exposed by the
protocol or primitive, indicating how they should be used and any
potential risks associated with their misuse. For example, for
each input to the protocol, it should be made clear whether or not
these are attacker controlled and, if so, describe what steps must
be taken to validate that input.
* Describe any corner cases or situations that may impact security,
providing guidance on how to avoid or mitigate potential risks.
This includes explicitly stating the probability of an algorithm
failing due to invalid operations occurring (such as divide-by-
zero) both in the typical case and under attacker-controlled
inputs.
* Explain any dependencies or interactions with other protocols,
primitives, or system components, highlighting potential
compatibility or interoperability issues.
* Provide guidance on configuration, parameter selection, or
deployment considerations that may affect the security or
performance of the protocol in real-world scenarios. This
includes the impact of new discoveries that weaken the security
assumptions of a primitive.
By addressing the specific needs of implementers, researchers, and
protocol designers, a specification can be more effectively
understood, implemented, and analyzed, leading to more secure and
interoperable systems.
6. General Recommendations
Developing effective cryptography specifications often requires
collaboration between multiple stakeholders in the target audience,
including engineers, researchers, and standardization organizations.
Engaging in a collaborative process helps ensure that diverse
perspectives and expertise are considered, resulting in more robust
and widely applicable specifications. This section discusses the
importance of collaboration and compromise in specification
development and offers recommendations for fostering a collaborative
environment.
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6.1. Encourage Open Communication and Feedback
Effective collaboration relies on open communication and an ongoing
exchange of ideas and feedback. By creating channels for
communication, such as mailing lists, pull request threads (as
described in [RFC8874]), or regular meetings, specification authors
can facilitate discussions, address concerns, and gather valuable
input from various stakeholders. Encouraging an environment where
feedback is welcomed and valued helps ensure that the specification
benefits from diverse expertise and experiences.
6.2. Seek External Expertise
Involving external experts, such as researchers or engineers from
different organizations, can help identify potential issues, uncover
new insights, and provide a broader perspective on the specification.
Engaging with experts such as those in the IRTF Crypto Review Panel
who have different backgrounds or areas of expertise can also help
identify potential gaps in the specification or highlight areas where
further research or clarification is needed.
6.3. Recognize and Address Conflicting Interests
Collaboration often involves addressing conflicting interests or
opinions among stakeholders. It is essential to acknowledge these
differences and work towards finding mutually agreeable solutions.
This may require making compromises or revisiting previous decisions
to ensure that the specification meets the needs of all involved
parties. By maintaining a flexible and open-minded approach,
specification authors can build consensus and develop a more robust
specification.
7. Examples of Well-Written Specifications
To provide a better understanding of how to write high-quality
cryptography specifications, we will analyze specific sections from a
well-written example: ChaCha20 and Poly1305 for IETF Protocols
([RFC8439]).
7.1. ChaCha20 and Poly1305 for IETF Protocols (RFC 8439)
[RFC8439] is a specification that describes the use of the ChaCha20
stream cipher and the Poly1305 message authentication code for IETF
protocols. It demonstrates how to write a clear, comprehensive, and
precise specification while catering to different audiences.
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7.1.1. Introduction and Overview
The introduction in [RFC8439] clearly defines the purpose and
motivation for the specification. It provides context on the origins
of ChaCha20 and Poly1305, and how they are used together to provide
confidentiality and data integrity. By presenting a concise and
informative introduction, the specification sets the stage for the
detailed technical descriptions that follow.
7.1.2. Algorithm Descriptions
The specification provides detailed and precise descriptions of the
ChaCha20 and Poly1305 algorithms, including pseudocode, constants,
and mathematical operations. This section caters to implementers,
ensuring that they have all the necessary information to create
consistent and correct implementations. The mathematical operations
are expressed in a clear and unambiguous manner, which helps both
implementers and researchers understand the algorithms better.
7.1.3. Test Vectors
[RFC8439] includes test vectors for both ChaCha20 and Poly1305,
providing concrete examples of inputs and expected outputs for the
algorithms. This section is invaluable for implementers, allowing
them to verify that their implementations are correct and compatible
with others.
7.1.4. Security Considerations
The specification dedicates an entire section to security
considerations, catering to researchers and protocol designers. It
discusses potential attacks and their mitigations, recommendations
for nonce usage, and the security properties of the algorithms. This
section also provides references to academic papers and other
resources for further reading, enabling researchers to delve deeper
into the security aspects of the specified algorithms.
7.1.5. IANA Considerations and References
[RFC8439] concludes with IANA considerations and a list of
references, ensuring that the specification is well-integrated with
existing IETF processes and standards. The IANA considerations
section is essential for protocol designers who need to register new
values or coordinate with existing registries.
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7.1.6. Problematic Aspects
A criticism of this document is that it does not cater enough to
protocol designers in that it does not explicitly define a decryption
algorithm. Researchers familiar with the concept of a stream cipher
understand that decryption and encryption are identical in stream
cipher constructions, but this may not be clear to implementers.
In summary, [RFC8439] serves as an excellent example of a well-
written cryptography specification, providing clear, precise, and
comprehensive information for implementers, researchers, and protocol
designers alike. By studying and emulating the structure and content
of specifications like [RFC8439], authors can create high-quality
specifications that cater to the diverse needs of their target
audiences.
8. Examples of Specifications That Could Be Improved
[RFC8032] is a specification that describes the Edwards-curve Digital
Signature Algorithm (EdDSA). This specification had several errata
filed against it for corrections and has had documented criticisms
published online.
8.1. Test Vectors
The test vectors included in this document were not comprehensive and
did not cover all the cases described in the algorithm, resulting in
multiple incompatible implementations. There were also issues with a
"greater than" comparison which should have been a "greater than or
equal to" which were not explicitly covered by the test vectors.
8.2. Unnecessary Branching
Some components of the cryptographic algorithms in EdDSA had branches
that sometimes led to different implementation behavior. In
particular, in the verification step for Ed25519, the following text
exists: "Check the group equation [8][S]B = [8]R + [8][k]A'. It's
sufficient, but not required, to instead check [S]B = R + [k]A'."
This alternative branch has led to disagreement between what
signatures are valid or not, which has a profound effect on
applications. Minimizing and removing similar branches - especially
those that exist in the name of performance - should be a goal of all
cryptographic specifications.
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8.3. Compatibility and Modularity
EdDSA is a variant of the Schnorr signature scheme, but with some
small variations that make it incompatible with other related Schnorr
signature schemes. This includes a "clamping" operation that makes
EdDSA keys and operations incompatible with x25519 ([RFC7748]). Many
of the issues in the specification derive from the fact that the
specification was written to match an existing implementation rather
than define an algorithm. This limited the authors from focusing on
compatibility with other related standards and primitives, resulting
in numerous issues.
9. Conclusion
This document provides guidelines for writing effective cryptography
specifications, emphasizing the importance of catering to different
audiences, such as implementers, researchers, and protocol designers,
with the end goal of enabling high-assurance cryptographic software.
By focusing on simplicity, precision, consistency, reusability,
collaboration, and compromise, specification authors can create
documents that are easier to understand, implement, and analyze.
We have also discussed the representation of mathematical operations
and the importance of clearly defining security definitions and
threat models. These elements are critical in ensuring that
specifications are not only technically accurate but also convey the
necessary information to properly assess the security properties of
cryptographic algorithms and protocols.
Finally, we have examined a well-written example, [RFC8439], to
demonstrate how these guidelines can be applied in practice. By
highlighting specific sections of this specification, we have shown
how authors can create high-quality specifications that cater to the
diverse needs of their target audiences.
In conclusion, the process of writing cryptography specifications is
both an art and a science. The guidelines presented in this document
should serve as a foundation for authors, but it is essential to
remain open to feedback and collaboration with the broader community.
By doing so, we can continue to develop and refine the specifications
that underpin the secure and reliable communication systems of today
and the future.
10. Security Considerations
This document discusses best practices for writing and editing
cryptography specifications. It does not provide any guidance for
the semantic contents of those specifications.
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11. IANA Considerations
This document has no IANA actions.
12. References
12.1. 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/rfc/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/rfc/rfc8174>.
12.2. Informative References
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/rfc/rfc3552>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/rfc/rfc7748>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/rfc/rfc8032>.
[RFC8439] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,
<https://www.rfc-editor.org/rfc/rfc8439>.
[RFC8874] Thomson, M. and B. Stark, "Working Group GitHub Usage
Guidance", RFC 8874, DOI 10.17487/RFC8874, August 2020,
<https://www.rfc-editor.org/rfc/rfc8874>.
Authors' Addresses
Nick Sullivan
Cryptography Consulting LLC
San Francisco,
United States of America
Email: nicholas.sullivan+ietf@gmail.com
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Christopher A. Wood
Cloudflare, Inc.
101 Townsend St
San Francisco,
United States of America
Email: caw@heapingbits.net
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