Category: Cryptography & Encryption
Transport Layer Security
Also known as: TLS, TLS protocol
Simply put
Transport Layer Security (TLS) is a protocol that protects information as it travels between two applications over a network such as the Internet, keeping it private and unaltered. It is commonly used to secure web traffic and other communications. TLS is the successor to the older Secure Sockets Layer (SSL) protocol and was introduced to address weaknesses in SSL.
Formal definition
Transport Layer Security (TLS) is a cryptographic protocol that provides privacy (confidentiality) and data integrity between two communicating applications over a computer network. It is designed to encapsulate other protocols, such as HTTP, and generally provides authentication of the communicating parties in addition to encrypting the session. TLS evolved from and replaced the earlier SSL protocol, addressing vulnerabilities present in SSL; note that specific version behavior, supported cipher suites, and approved configurations vary across TLS revisions and are subject to agency and baseline-specific requirements that a reader should confirm against current authoritative sources.
Why it matters
Transport Layer Security is one of the foundational protections for data in transit, addressing a core security objective that appears across virtually every compliance framework: preventing the interception, disclosure, or tampering of information as it moves between systems. Because TLS provides confidentiality and data integrity between two communicating applications, and generally supports authentication of the parties involved, it is a primary technical mechanism organizations rely on to satisfy transmission confidentiality and integrity control requirements. In defense and public sector contexts, protecting information such as Controlled Unclassified Information (CUI) as it traverses untrusted networks like the Internet is a recurring obligation, and TLS is frequently the control implemented to meet it.
Who it's relevant to
Information System Security Managers and Engineers
Those responsible for implementing transmission confidentiality and integrity protections often rely on TLS to secure data in transit. They must ensure that deployed versions and cipher suite configurations align with current approved settings under the applicable baseline, and that certificates and authentication are properly managed rather than assuming that enabling TLS alone satisfies the requirement.
Assessors and Auditors
Personnel evaluating whether an information system adequately protects data in transit frequently examine TLS implementations. Their review typically extends beyond confirming that TLS is present to verifying that deprecated protocol versions and weak cipher suites are not in use and that configurations meet the requirements applicable to the system, which vary by revision and by agency or baseline tailoring.
Government Contractors Handling CUI
Organizations transmitting sensitive information such as Controlled Unclassified Information over networks like the Internet commonly use TLS as a safeguard for data in transit. They should confirm the specific cryptographic and configuration requirements that apply to their contractual and regulatory obligations against current authoritative sources, since acceptable TLS configurations are subject to baseline-specific requirements.
Authorizing Officials and Compliance Officers
Those accountable for risk decisions and for demonstrating compliance benefit from understanding that TLS is a control supporting confidentiality and integrity of transmitted data, not a standalone assurance of security or compliance. They should recognize that the adequacy of a TLS implementation depends on version, configuration, certificate and authentication management, and continued alignment with evolving authoritative guidance.
Inside TLS
TLS Handshake
The negotiation phase in which a client and server agree on the protocol version, cipher suite, and session keys, and authenticate one another using certificates. The specific messages and key exchange mechanisms differ between TLS versions, so implementers should confirm behavior against the applicable protocol specification.
Cipher Suites
The named combinations of cryptographic algorithms (for key exchange, authentication, bulk encryption, and message integrity) negotiated during the handshake. Approved algorithms and permitted suites generally depend on the applicable cryptographic guidance, and readers should verify against current authoritative sources such as the relevant NIST publications and, where cryptographic module validation applies, FIPS 140 validation status.
X.509 Certificates and PKI
The digital certificates and supporting public key infrastructure used to authenticate endpoints and establish trust. Certificate validation, trust anchors, and revocation checking are integral to TLS assurance, though the specific requirements vary by environment and policy.
Protocol Versions
Successive versions of the protocol define different handshakes, supported algorithms, and security properties. Which versions are permitted or prohibited in a given system generally depends on applicable guidance and agency tailoring, and this should be confirmed against the current authoritative text rather than assumed.
Encryption in Transit
TLS provides confidentiality and integrity for data as it traverses a network, and is one of the mechanisms commonly cited to support protection of information in transit. It does not address data at rest or data in use, which are handled by separate controls.
Common questions
Answers to the questions practitioners most commonly ask about TLS.
Does using TLS mean my system is compliant with federal encryption requirements?
Not necessarily. TLS is a protocol that provides encryption in transit, but compliance depends on how it is configured and which requirements apply to your system. Enabling TLS alone does not guarantee compliance; federal and defense contexts generally require specific protocol versions, approved cipher suites, and validated cryptographic modules. Compliance is a separate determination that must be evaluated against the applicable control baseline and governing guidance, and it should not be assumed from the mere presence of TLS. Verify your configuration against current authoritative sources.
Is TLS the same thing as FIPS-validated encryption?
No. These are distinct concepts that are commonly conflated. TLS is a transport protocol, while FIPS validation refers to a formal process under which a cryptographic module is tested against a federal standard maintained by NIST. A TLS implementation may or may not rely on a validated cryptographic module. Using TLS does not by itself satisfy requirements that call for validated cryptography, and readers should confirm the validation status of the underlying module against the current authoritative program listings rather than assuming that TLS support implies validation.
Which TLS versions should I plan to support or disable?
This publication does not assign specific version numbers as universally required, because acceptable versions vary by applicable guidance and revision, and older versions are progressively deprecated over time. In most implementations, organizations are expected to move toward current, supported protocol versions and to disable versions that have been deprecated. Consult the current governing standards and any agency-specific tailoring that applies to your system to determine which versions are permitted, and re-verify as guidance is revised.
How do I choose cipher suites for a compliant TLS deployment?
Cipher suite selection generally should follow the approved algorithms and configurations identified in the applicable authoritative guidance for your environment, rather than vendor defaults. In many federal and defense contexts this means limiting the deployment to approved suites and disabling weaker or deprecated ones. Because approved lists change across revisions and may be tailored by an agency, confirm the current permitted cipher suites against the official source that governs your system before finalizing a configuration.
How should TLS configurations be validated during an assessment?
Assessment approaches typically involve verifying that TLS is enabled where required, confirming that supported protocol versions and cipher suites match the applicable guidance, and reviewing evidence of the configuration on relevant components. Keep in mind that a favorable assessment finding is distinct from authorization; assessment evaluates whether controls are implemented, while authorization is a separate decision. Refer to the assessment procedures associated with your governing framework, and verify the current expectations against authoritative sources.
Does configuring TLS once satisfy ongoing requirements?
No. TLS configurations should be maintained over time, because supported protocol versions, cipher suites, and certificates change and can expire or be deprecated. In most implementations this maintenance falls within continuous monitoring expectations, meaning configurations are periodically reviewed and updated rather than set once and left unchanged. Confirm the specific monitoring and maintenance obligations that apply to your system against the current governing guidance.
Common misconceptions
Deploying TLS means the connection and the data are fully secure.
TLS protects data in transit between negotiated endpoints, but compliance and security are not the same thing. Weak cipher suites, deprecated protocol versions, improper certificate validation, misconfiguration, or endpoint compromise can undermine the protection. TLS is one control among many and does not by itself satisfy a system's confidentiality, integrity, or authentication requirements.
Any TLS configuration will satisfy federal or defense cryptographic requirements.
Permitted protocol versions, cipher suites, and algorithms generally depend on the applicable guidance and tailoring for the environment, such as requirements referenced for CUI, DoD systems under the RMF, or civilian systems under FISMA, and may involve FIPS-validated cryptographic modules. Requirements differ by scope and change across revisions, so the specific configuration must be verified against the current authoritative text.
Enabling TLS is a one-time task that stays compliant over time.
Approved algorithms, protocol versions, and configuration expectations evolve as guidance is revised and as older mechanisms are deprecated. Maintaining an acceptable TLS posture is an ongoing activity that fits within continuous monitoring, and configurations should be periodically reassessed against current authoritative sources.
Best practices
Restrict deployments to protocol versions and cipher suites permitted by the guidance applicable to your system's scope (for example CUI, DoD, or civilian FISMA environments), and verify permitted algorithms against the current authoritative text rather than relying on defaults.
Where cryptographic module validation applies to your environment, confirm that the TLS implementation uses cryptography meeting the applicable FIPS 140 validation requirements.
Implement rigorous certificate management, including validation of trust anchors, checking certificate validity and revocation, and controlling the lifecycle of X.509 certificates and private keys.
Disable deprecated protocol versions, weak cipher suites, and outdated algorithms, and periodically reassess configurations as guidance is revised and mechanisms are deprecated.
Treat TLS configuration as part of continuous monitoring rather than a one-time setting, and document that TLS is one layered control that does not by itself satisfy broader confidentiality, integrity, or authentication requirements.
Confirm the specific TLS requirements, tailoring, and any agency-specific interpretations against current official sources for your environment before treating a configuration as compliant.