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Comply Defense
Category: Cryptography & Encryption

Data in Transit

Also known as: Data in Motion, Data in Flight
Simply put

Data in transit is information that is actively moving from one place to another, such as an email being sent, a file being transferred, or data traveling across a network or the Internet. Because it is moving between systems rather than sitting in storage, it can potentially be intercepted while in motion, which is why it generally requires protections like encryption. It is typically contrasted with data at rest, which is stored, static data.

Formal definition

Data in transit refers to any data that is being transmitted from one system, resource, or location to another across a network. This encompasses communications between resources within a single workload as well as data moving over external networks such as the Internet, including emails, file transfers, and web communications. It is distinguished from data at rest (stored, static data), and in most implementations warrants distinct but complementary safeguards, commonly transport-layer encryption and integrity controls, to mitigate interception or tampering while data is actively moving. Readers should verify applicable control and encryption requirements against the current authoritative baseline for their system category, as specific obligations differ across CUI, DoD RMF, and civilian FISMA contexts.

Why it matters

Data in transit is exposed to interception and tampering in ways that stored data is not, because it is actively moving across networks, including untrusted paths such as the Internet, rather than sitting behind the access controls that typically protect data at rest. An adversary who can observe or manipulate network traffic may capture credentials, exfiltrate sensitive information, or alter data in flight if the communication is not adequately protected. This is why transport-layer encryption and integrity controls are generally applied to data as it moves between systems.

For defense and public sector systems, the stakes are heightened when the data in motion includes Controlled Unclassified Information (CUI) or other sensitive information governed by federal requirements. Protecting data in transit is a distinct concern from protecting data at rest; addressing one does not satisfy the obligations for the other, and both generally warrant complementary safeguards. Compliance officers should also remember that applying encryption is a security control, not evidence of full compliance, the specific transmission-protection obligations differ across CUI, DoD RMF, and civilian FISMA contexts and must be verified against the applicable authoritative baseline.

Who it's relevant to

Information System Security Managers (ISSMs) and System Owners
Those responsible for implementing and documenting security controls must ensure that data moving across their networks is protected with appropriate transport-layer encryption and integrity controls, treating transmission protection as a requirement distinct from protecting stored data. They should verify the specific obligations against the authoritative baseline applicable to their system category.
Government Contractors Handling CUI
Contractors that transmit Controlled Unclassified Information across networks, including the Internet, are generally expected to protect that data while in motion. They should confirm the encryption and transmission-protection requirements that apply to their contracts and CUI handling obligations against current official sources rather than assuming coverage from data-at-rest safeguards alone.
Compliance Officers and Auditors
Those assessing systems must evaluate protections for data in transit separately from data at rest, and recognize that the presence of encryption demonstrates a control implementation rather than overall compliance. Assessment expectations differ across DoD RMF and civilian FISMA contexts and should be measured against the applicable authoritative requirements.
Authorizing Officials
Officials weighing risk acceptance for an authorization decision should consider how effectively data in transit is safeguarded across both internal and external network paths, since interception or tampering in motion represents a distinct risk that continuous monitoring and the applicable control baseline are intended to address.

Inside Data in Transit

Definition
Data in transit refers to information actively moving from one location to another, such as across a network, between systems, or over the internet, as opposed to data at rest (stored) or data in use (being processed). Protecting it is a common objective across control frameworks, though the specific requirements depend on the governing standard and the data's sensitivity.
Encryption in Transit
The predominant safeguard for data in transit, typically implemented through protocols such as TLS or IPsec. For federal and defense contexts, cryptographic implementations are generally expected to use validated modules; practitioners should verify current FIPS 140 validation requirements and applicable algorithm guidance against authoritative NIST sources for the relevant revision.
Applicable Control Families
In NIST SP 800-53, protections for transmitted information are generally addressed within the System and Communications Protection (SC) family. For Controlled Unclassified Information handled by non-federal systems, NIST SP 800-171 also includes requirements addressing the confidentiality of CUI during transmission. The exact control identifiers and their tailoring should be confirmed against the applicable revision.
Scope Considerations
Requirements for protecting data in transit vary by system categorization and data type, CUI under DFARS/CMMC contexts, federal civilian systems under FISMA, DoD systems under the RMF, and classified systems under the NISPOM may each impose distinct expectations. State, local, tribal, and territorial obligations may differ from federal requirements.
Boundary and Transmission Points
Data in transit protections apply at the points where information crosses system or network boundaries, including internal segments, external connections, and interconnections between authorized systems. Identifying these transmission points is a prerequisite to applying appropriate safeguards.

Common questions

Answers to the questions practitioners most commonly ask about Data in Transit.

Does encrypting data in transit mean my system is compliant?
No. Encrypting data in transit is a technical safeguard, not a compliance determination. Compliance is generally established by satisfying the specific control requirements of an applicable framework (such as NIST SP 800-53, NIST SP 800-171, or a FedRAMP baseline) as tailored for your system's impact level or CUI obligations, and by documenting and validating that implementation through assessment and authorization. Encryption alone does not demonstrate that key management, protocol configuration, monitoring, and related controls are in place. Treating a single technical measure as equivalent to compliance is a common mistake; confirm the full set of applicable requirements against the current authoritative text.
If I protect data in transit, do I still need to address data at rest?
Yes. Data in transit and data at rest are distinct protection states addressed by separate controls, and safeguarding one does not satisfy the requirements for the other. Most control frameworks treat the confidentiality and integrity of information moving across networks separately from information stored on media or in systems. You should review the specific controls that apply to each state under your governing framework and impact level rather than assuming that transit protections extend to stored data.
How do I identify where data in transit protections apply within my system boundary?
Generally, you begin by mapping the flows of information across and within your authorization boundary, including connections between components, external interconnections, and traffic that traverses untrusted or shared networks. Data flow diagrams and system documentation typically support this analysis. The applicable protections depend on the sensitivity of the information (for example CUI or the system's impact level) and the framework tailoring in effect. Confirm boundary and flow definitions against your system security plan and current authoritative guidance, as agency-specific interpretations may differ.
What should I document about data in transit protections for an assessment?
In most implementations, assessors expect documentation that describes how the relevant controls are implemented, where they apply across the system boundary, and how their effectiveness is validated. This commonly includes references in the system security plan, supporting artifacts such as configuration evidence, and descriptions of associated controls like key management and monitoring. Because assessment validates implementation while authorization is a separate decision, documentation should support both. Verify the specific evidence expectations against the assessment procedures your framework and assessing organization use.
How do data in transit requirements differ across CUI, DoD, and civilian systems?
Requirements can differ by scope and governing authority. Systems handling CUI, DoD systems under the RMF, and federal civilian systems under FISMA may draw on different control sources and tailoring, and state, local, tribal, and territorial obligations may differ further. The specific protections that apply depend on the information type, the applicable framework and revision, and any agency-specific tailoring. Do not assume that meeting one framework's expectations satisfies another; confirm the requirements applicable to your system against the current authoritative sources.
How does continuous monitoring relate to data in transit protections after authorization?
Because an ATO is time-bound and conditioned on ongoing continuous monitoring rather than being permanent, data in transit protections generally must be maintained and verified over the life of the authorization. This typically includes confirming that configurations remain effective, that changes are assessed for impact, and that relevant activity is monitored. Review your continuous monitoring strategy and the applicable framework guidance to determine the specific ongoing obligations for your system.

Common misconceptions

Encrypting data in transit means the overall system is secure and compliant.
Compliance is not equivalent to security, and protecting data in transit is only one control among many. A complete posture also addresses data at rest, data in use, access control, monitoring, and other control families. Meeting a transit-protection control does not satisfy other requirements or produce an authorization on its own.
Any use of encryption satisfies federal or defense requirements for data in transit.
In federal and defense contexts, requirements generally expect validated cryptography rather than encryption alone. The specific validation, algorithm, and key management expectations depend on the applicable framework and revision, and practitioners must verify current requirements against authoritative NIST and agency sources.
The requirements for protecting data in transit are the same across all frameworks.
Requirements differ by framework and scope. NIST SP 800-53, NIST SP 800-171, FISMA-governed civilian systems, DoD RMF systems, and NISPOM classified systems may each treat transmission protection differently, and control identifiers or tailoring can change across revisions.

Best practices

Identify and inventory all points where sensitive data crosses network or system boundaries so that transmission protections can be applied consistently.
Use validated cryptographic implementations where required, and verify current FIPS 140 validation and algorithm expectations against authoritative NIST sources for the applicable revision rather than assuming any encryption suffices.
Map your transit protections to the specific governing framework in scope, for example the SC family in NIST SP 800-53 or the CUI transmission requirements in NIST SP 800-171, and confirm the current control identifiers and tailoring.
Determine the applicable scope (CUI, federal civilian under FISMA, DoD under the RMF, or classified under the NISPOM) before selecting controls, since requirements and any SLTT obligations may differ.
Treat transit protection as one element of a broader control set, ensuring data at rest, data in use, access, and monitoring controls are also addressed rather than relying on encryption in transit as evidence of overall security.
Confirm all specific requirements, validation criteria, and configuration expectations against current official publications, since guidance and baselines change across revisions and agency tailoring.