The Software Bill of Function: A New Paradigm for Software Transparency

Modern software systems operate in environments that demand more than surface-level transparency. While current supply chain frameworks focus primarily on component origin and licensing through Software Bills of Materials (SBOMs), they often fail to capture the essential characteristics of how software behaves in real-world conditions. The Software Bill of Function (SBOF) addresses this gap by introducing a structured, verifiable account of what a piece of software does, rather than just what it is made of.

The SBOF offers a behavioral inventory that details executed functions, data interactions, system calls, and operational boundaries. This information is captured in a form that can be reviewed, signed, and shared as part of a broader trust framework. The SBOF is intended to serve both builders and operators. Builders can use it to document intended functionality with precision, and operators can verify the execution of that functionality with confidence. This approach strengthens situational awareness, operational assurance, and forensic readiness across the entire software lifecycle.

As software continues to integrate into national infrastructure, defense platforms, and regulated systems, the ability to account for functional behavior becomes a foundational requirement. Compliance is no longer limited to source code review or dependency traceability. Mission-aligned software must be able to demonstrate that it performs exactly as designed and that it remains within its declared operational envelope.

The Software Bill of Function offers a path forward. It introduces a framework where function-level trust can be established, measured, and inherited. This elevates software assurance from an administrative exercise to an active verification process grounded in observable behavior. In doing so, the SBOF supports a future in which critical systems are not only secure by design but also trustworthy by demonstration.

Background and Motivation

Software assurance has traditionally relied on documentation of origin, a review of included components, and adherence to compliance checklists. This approach made sense when software was largely monolithic and when the pace of change could be managed through periodic evaluations and manual approvals. Today, the situation is markedly different. Software is developed, deployed, and modified continuously. It operates within layered architectures that integrate third-party libraries, cloud services, microservices, AI agents, and dynamically loaded features. The environments in which these systems run are no longer stable or predictable. They are shaped by shifting networks, unpredictable inputs, adversarial threats, and mission-specific demands.

In this evolving context, a record of component provenance is no longer sufficient to determine trust. Knowing what a system is made of does not tell us what it is doing, how it is behaving, or whether it remains within its expected operational scope. The limitations of existing Software Bill of Materials approaches become clear when software is deployed to contested networks, zero trust environments, or compliance-sensitive sectors such as defense, healthcare, or critical infrastructure. When software functions autonomously, reacts to inputs in real time, and interacts with systems across trust boundaries, assurance must be rooted in behavior, not assumptions.

There is a growing recognition within both government and industry that operational confidence must be tied to verifiable evidence. This evidence must reflect real functionality, not theoretical design. Developers, security teams, and mission operators all require a shared understanding of what the software does in a given context. Without this shared foundation, efforts to mitigate risk or enforce accountability remain incomplete.

The Software Bill of Function emerges from this need. It provides a practical mechanism to document, verify, and communicate the actual behaviors of software in a structured and repeatable way. This concept builds upon decades of lessons in software instrumentation, behavior modeling, and functional auditing. It reorients the discussion around what matters most in high-risk environments: what code is doing, not just where it came from or who signed off on it.

This shift is not only strategic but also necessary. As systems become more distributed, as cyber threats grow more adaptive, and as operational environments become more chaotic, the ability to anchor trust in observable behavior becomes a mission-critical requirement. The Software Bill of Function is designed to meet that requirement. It establishes the groundwork for a more resilient, transparent, and functionally accountable software ecosystem.

Defining the Software Bill of Function

The Software Bill of Function is a formal record that captures what software actually does. It serves as a functional ledger that provides a clear, structured representation of a system's behavioral characteristics, independent of its underlying components or source code. While traditional documentation outlines what software is intended to do, the SBOF captures what it demonstrably performs within an operational context. It focuses on executed capabilities, interaction patterns, and data flows, offering a direct reflection of the system's real behavior across time and deployment environments.

At its core, an SBOF is composed of functional claims that describe the software's observable behaviors in a consistent and machine-readable format. These claims include details such as triggered functions, data transformations, access patterns, external system calls, conditional executions, and user-facing outcomes. The goal is not to speculate on theoretical functionality but to create a trustworthy reference that reflects operational execution. These behavioral claims are tied to metadata that allows stakeholders to verify provenance, measure consistency, and detect unexpected deviation.

An SBOF may be generated during build-time, test-time, or run-time. Depending on the operational requirements and the risk posture of the deployment environment, different levels of granularity can be employed. Some systems may only require high-level summaries of exposed capabilities, while others may benefit from detailed records of each function call, each system interaction, and each externally facing behavior. The model is flexible by design and can accommodate the unique needs of software used in constrained, adversarial, or safety-critical domains.

Importantly, the SBOF is not limited to traditional applications. It applies equally to embedded systems, containerized workloads, AI inference pipelines, and distributed edge services. Wherever software performs functions that affect outcomes, the SBOF provides a standardized way to represent and account for those functions. It creates a bridge between what the software claims to do and what it actually performs under real conditions, enabling a level of behavioral transparency that is often absent in today's assurance frameworks.

This approach allows for trust to be formed based on real evidence. Stakeholders are no longer dependent solely on declarations, static analysis, or external certification. They can observe, record, and verify the software's behavior with clarity. In operational environments where decisions must be made quickly and with high confidence, this type of functional accountability becomes foundational. The SBOF provides a consistent and auditable structure for building that accountability at every layer of the software lifecycle.

SBOF vs SBOM: Complementary, Not Redundant

The Software Bill of Function does not seek to replace the Software Bill of Materials. Rather, it builds upon the foundation that the SBOM has laid, offering a necessary extension into areas that remain unaddressed by component inventories alone. The SBOM has established itself as a valuable resource for identifying the ingredients of software—packages, libraries, dependencies, and their respective origins. This visibility has helped teams improve their vulnerability management practices, license compliance, and supply chain traceability. However, knowing what is included in the software only addresses part of the larger trust equation.

The SBOF complements the SBOM by focusing not on composition, but on behavior. While the SBOM tells us what was assembled, the SBOF tells us what actions the assembled parts perform once the software is operational. These two forms of documentation together create a more complete and meaningful representation of software intent, integrity, and function. Just as a bill of materials might describe the components used to build a machine, the bill of function describes what the machine does when it is turned on and put to use.

This distinction is not academic. In real-world scenarios, risks often arise from how components are configured and executed, not simply from their inclusion. A library may be present in two different applications, yet only one may invoke a sensitive function that introduces operational risk. Similarly, a component may be updated with no change in its interface or signature, but its functional behavior may shift in ways that affect downstream systems. The SBOF allows this kind of nuance to be captured and reviewed with clarity.

Together, the SBOM and SBOF establish a dual lens for understanding software. The SBOM enables static reasoning about potential exposure and inherited risk. The SBOF enables dynamic reasoning about actual impact and observable activity. Organizations that manage high-consequence systems will benefit from having both. The SBOM provides a starting point for supply chain scrutiny, while the SBOF enables ongoing trust through operational insight.

By embracing this layered approach, stakeholders can begin to make better-informed decisions about software usage, deployment, and remediation. The presence of a verified SBOF alongside a trusted SBOM introduces a new standard for completeness in software documentation. It ensures that transparency is not limited to the past or theoretical, but extended into the realm of execution and real-world behavior. In doing so, it brings software assurance closer to the level of accountability that modern systems demand.

Use Cases and Applications

The practical value of a Software Bill of Function becomes evident when applied to real-world scenarios where software behavior must be trusted, verified, and acted upon with precision. Modern software systems are deeply embedded in critical infrastructure, national defense platforms, financial systems, and regulated environments where misbehavior—intentional or accidental—can result in significant operational or security consequences. In these domains, insight into software behavior is not a luxury but a necessity.

One of the most immediate applications of the SBOF lies in the domain of zero trust architectures. Within such environments, trust is not granted based on identity or static attributes alone. Each action must be continuously verified, and access must be governed by policy-enforced behavior. The SBOF provides a concrete mechanism for enabling this model. By cataloging the functional behaviors of a given software instance, policy engines can make decisions based on what the software is permitted to do, and any deviation from the declared SBOF can trigger enforcement or isolation actions in real time.

Another critical use case emerges in secure software supply chain management. In complex supply chains, organizations often inherit risk from third-party code and external libraries. While SBOMs reveal which components are present, the SBOF provides insight into how those components behave within the assembled system. This is particularly useful when assessing whether a known-vulnerable function is ever actually invoked, or whether a mitigation is functionally effective. The SBOF makes it possible to validate that software behavior aligns with declared expectations, and that mitigations are not just present in theory but active in practice.

The SBOF also enhances the effectiveness of continuous authorization frameworks, such as those found in mission-critical or government environments that rely on Continuous Authority to Operate (cATO). In these settings, authorization cannot rely on point-in-time evaluations alone. It must be supported by an ongoing understanding of what software is doing throughout its operational lifecycle. The SBOF provides a mechanism for capturing this functional behavior, which can then be used to support risk assessments, runtime validation, and compliance reporting with far greater fidelity and responsiveness.

Emerging technologies such as AI and machine learning systems introduce further complexity. These systems often behave non-deterministically, adapting to inputs and producing outcomes that are difficult to predict through static analysis. The SBOF creates a pathway for documenting the functional boundaries of such systems, including model invocation patterns, inference triggers, and decision pipelines. This supports greater transparency in environments where AI is used in regulated workflows, mission-critical decisions, or operational automation.

In the domain of incident response and forensics, the SBOF provides a retrospective lens into what software actually performed. Following a security event, investigators need to reconstruct execution trails, understand the scope of impact, and determine whether the software behaved as expected. SBOFs can serve as behavioral baselines, allowing teams to identify unauthorized actions, detect lateral movement, or trace data exfiltration pathways with greater accuracy.

Finally, the SBOF serves as a foundational element in interoperability and certification efforts. In highly modular or federated systems, components are often certified independently and then integrated. Behavioral transparency enables stakeholders to assess whether the certified functions remain intact post-integration. This is especially relevant in defense acquisition, aerospace, and industrial systems where modular certification paths are common, and where the assurance of post-integration behavior is essential.

In each of these use cases, the common thread is the need for functional accountability. The SBOF delivers on this need by introducing a practical, extensible, and verifiable way to represent software behavior across environments and over time. It provides a critical layer of visibility for those responsible for securing, operating, auditing, or integrating complex systems. As these systems grow in complexity and importance, the SBOF offers the means to manage that complexity with confidence and clarity.

Trust, Attestation, and Verification

The utility of a Software Bill of Function rests on its ability to be trusted. Trust, in this context, is not an abstract concept. It is the product of verifiable evidence, cryptographic assurance, and transparent provenance. The SBOF is not merely a descriptive record. It is an artifact of accountability, one that must be grounded in methods that allow others to assess its authenticity and confirm that it accurately represents the behavior of the software to which it is attached.

Attestation is the process through which this trust is established. Each SBOF must be bound to the identity of the entity that generated or observed it. This may be a developer, a build system, a runtime monitor, or an independent auditor. The SBOF must then be signed using cryptographic methods that allow others to verify both its origin and its integrity. When this is done properly, the SBOF becomes more than a behavioral log. It becomes a trusted claim that can be validated across systems, reviewed by multiple stakeholders, and enforced within automated environments.

Verification mechanisms enable consumers of software to evaluate whether an observed SBOF matches expectations. This comparison can occur at multiple levels. During development, verification may involve matching the declared SBOF against unit tests or simulation data. In staging environments, teams may confirm that newly introduced code does not exceed the functional boundaries defined in prior versions. In production, operators may use SBOF verification to detect functional drift, unauthorized changes, or runtime anomalies that suggest tampering or misuse.

These verification activities do not rely on speculation. They are rooted in deterministic comparisons between declared behavior and recorded behavior. This allows policies to be built around specific functional claims. A system may, for example, allow only those binaries whose SBOF indicates that no network calls are made, or it may restrict deployment of containers that expose cryptographic functions unless explicitly authorized. These policies can be implemented through a variety of platforms, including workload orchestrators, secure enclaves, access control systems, and zero trust infrastructure.

Trust is reinforced when SBOFs are independently generated and corroborated. A function observed by one party can be compared with the same function observed by another, providing a cross-check that increases confidence. In high-security environments, this can form the basis for quorum-based verification, where multiple attestations must agree before trust is granted. In large enterprises, federated teams may use SBOFs to ensure that internally developed code adheres to centrally established behavioral standards before it is released into shared infrastructure.

The ability to verify behavior at the function level also strengthens the process of risk acceptance and mission alignment. Rather than relying on high-level documentation or historical certification, decision-makers can examine the specific behaviors of the software and determine whether those behaviors are appropriate for the operational context. This allows risk to be assessed with greater precision and enables authorizing officials to move from posture-based confidence to evidence-based assurance.

In the evolving software landscape, trust must be earned continuously. Attestation and verification of behavioral claims provide a structured method for doing so. The Software Bill of Function is a vehicle for this kind of trust formation. It empowers developers to make strong, verifiable claims about their software. It enables operators to confirm those claims with confidence. And it provides policy enforcers with the tools to automate judgment based on evidence rather than assumption. This convergence of accountability, assurance, and action creates the foundation for a more resilient and trustworthy software ecosystem.

Conclusion

The Software Bill of Function represents a meaningful shift in how software systems are understood, trusted, and governed. It offers a path toward a deeper kind of transparency—one grounded in what software actually performs, rather than what it was intended to do or how it was composed. As systems grow more interconnected and responsibilities for their assurance expand across teams, organizations, and governments, the ability to make claims about behavior that can be independently verified will become central to both mission success and operational resilience.

By formalizing the observation and representation of software behavior, the SBOF creates a shared language for functional accountability. It enables builders to describe the intent of their systems with precision. It empowers operators to verify that intent in production. It provides policymakers with an evidence-based mechanism to enforce boundaries, assess compliance, and detect deviation. This common foundation has the potential to bring alignment between stakeholders who often operate with different assumptions, timeframes, and levels of access.

The introduction of SBOFs does not require the reinvention of software engineering practices. It invites a thoughtful extension of them. It encourages teams to treat function-level behavior as a durable asset, one that can be versioned, signed, shared, and enforced. It brings rigor to the verification process, not by imposing burden, but by making verification observable, repeatable, and grounded in evidence. It strengthens resilience not by adding complexity, but by ensuring that complexity is accountable.

Organizations that invest in functional transparency today position themselves for a future in which trust must be earned continuously and systems must defend their behavior through verifiable action. The SBOF equips these organizations with the structure and clarity needed to meet that future with confidence. It supports software assurance that is not only responsive to today's challenges but adaptable to tomorrow's architectures.

As the technology landscape evolves and demands on digital systems continue to increase, the need for trustworthy behavior will remain constant. The Software Bill of Function addresses this need with precision and purpose. It offers a foundation upon which stronger systems, safer operations, and more confident decisions can be built. With disciplined implementation and sustained commitment, it will shape the standard for software assurance in the decades to come.