A public key is shared openly and is used to encrypt data or verify a digital signature. A private key is kept secret and is used to decrypt data or create the signature. The two keys are mathematically related, which lets organisations communicate securely without exposing the secret key to everyone on the network.
What the public key does, and why it can be shared
The public key is the outward-facing half of a key pair. Its purpose is to let others verify, encrypt, or otherwise interact with the holder’s cryptographic identity without exposing the secret needed to complete the operation. That asymmetry is what makes PKI usable at scale: the public key can be distributed widely, while trust still depends on the private key staying protected.
In practical PKI workflows, the public key is usually embedded in a certificate, which lets other systems bind the key to an asserted identity. That binding is what turns a raw key into something operationally useful, because peers can rely on the certificate chain rather than guessing whether the key belongs to the right subject. The CA/Browser Forum sets baseline requirements for publicly trusted certificate issuance and revocation, which matters when the public key is meant to be accepted beyond a single internal environment.
What the private key does, and why secrecy is non-negotiable
The private key is the secret half of the pair. It is used to decrypt data that was encrypted with the matching public key, or to create a digital signature that others can verify with that public key. If the private key is exposed, the trust model collapses: anyone holding it can impersonate the owner, decrypt protected content, or sign as though they were the legitimate holder.
That is why the private key is treated as sensitive authentication material, not just another file. In well-run environments it should be protected by strong access controls, hardware-backed storage where possible, and a lifecycle that includes rotation, revocation, and disposal. NIST SP 800-57 Key Management is the clearest external reference for those lifecycle decisions, especially cryptoperiod and key protection expectations.
How PKI uses both keys together in trust and verification
PKI works because the two keys solve different halves of the same problem. The public key lets outsiders verify a signature or encrypt to the owner, while the private key lets the owner prove possession or recover the protected content. The mathematical relationship between them is one-way by design, so sharing the public key does not reveal the private key.
That split is also why PKI can support signatures, TLS, document signing, and client authentication without using a shared secret. The model becomes fragile when the private key is copied into too many systems, reused across environments, or embedded in application code. For authentication patterns such as signed client assertions, RFC 7523: JWT Profile for OAuth 2.0 Client Authentication and Authorization Grants shows how the private key can replace shared secrets while preserving verifiable trust.
Risk and Threat Considerations
The main risk in PKI is not the public key, it is uncontrolled private key exposure. Once a private key leaks, an attacker may be able to impersonate the subject, forge signatures, or decrypt data that was meant to remain confidential.
Failure mechanism: Private keys are often stolen through endpoint compromise, secret sprawl, weak file permissions, build-time leakage, or careless reuse across systems. Because the public key remains valid, the compromise can be hard to spot until signatures, sessions, or encrypted data begin failing trust checks.
Impact: The result can be authentication bypass, data disclosure, non-repudiation loss, and revocation work that is operationally expensive and sometimes disruptive. In certificate-heavy environments, one compromised key can affect many dependent systems at once.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP API Security Top 10 addresses the attack surface, NIST SP 800-57 and NIST SP 800-53 Rev 5 set the technical controls, and ISO/IEC 27001:2022 defines the regulatory obligations.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST SP 800-57 | Key Management | PKI hinges on private key lifecycle, protection, rotation, and disposal. |
| Recommendation — Apply key lifecycle controls to protect, rotate, and retire private keys safely. | ||
| NIST SP 800-53 Rev 5 | IA-5 — Authenticator Management | Private keys function as authenticators and require strict lifecycle control. |
| IA-9 — Service Identification and Authentication | PKI commonly authenticates systems and services with key pairs and certificates. | |
| Recommendation — Manage private keys as authenticators with controlled issuance, rotation, and revocation. Use key-based authentication for services and enforce certificate trust checks. | ||
| ISO/IEC 27001:2022 | A.5.17 — Authentication information | Private keys are authentication information that must be protected and controlled. |
| Recommendation — Protect authentication information with strict handling, storage, and access rules. | ||
| OWASP API Security Top 10 | API2 — Broken Authentication | Key handling failures can undermine authentication that depends on signed assertions. |
| Recommendation — Enforce strong authentication controls wherever keys are used to prove identity. | ||
Practitioner Guidance
What to verify: Treat every private key as a high-value secret with an owner, purpose, expiry, and revocation path. Confirm where the key lives, who can export it, whether it is reused anywhere else, and whether the certificate chain and revocation status are actually enforced by the relying party.
Decision rule: If the key is used for production authentication or signing, prioritise key protection, rotation, and revocation readiness before debating whether the underlying algorithm is strong enough. A strong algorithm does not compensate for weak storage or uncontrolled distribution.
Practitioner takeaway: The practical distinction is simple: the public key is meant to be distributed, while the private key is meant to remain under exclusive control. Most PKI failures come from breaking that boundary, not from misunderstanding the math.
Related resources from NHI Mgmt Group
- What is the difference between public PKI and private PKI for workload identity?
- What is the difference between private key encryption and public key encryption for practitioners?
- What is the difference between public PKI and private PKI in enterprise use cases?
- What is the difference between public TLS and private PKI for non-browser authentication use cases?
Deepen Your Knowledge
Reviewed and updated by the NHIMG editorial team on September 25, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org
