An encryption key is a cryptographic value used with an algorithm to protect information. What the term means depends on the method: symmetric encryption uses one shared secret key, while public-key encryption uses the public key of a public/private pair to encrypt. The corresponding private key is kept secret. That distinction matters because not every key in a cryptographic system has the same purpose or secrecy requirement.
What are encryption keys?
A cryptographic key is a value or parameter that controls a cryptographic operation. Depending on its role, a key can support encryption, decryption, signature generation or verification, authentication, or protection of other keys. “Encryption key” is narrower: it refers to the key used for encryption, and its specific meaning depends on whether the system is symmetric or public-key cryptography.
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NIST’s glossary definition of an encryption key distinguishes these cases: in symmetric encryption, the encryption key is the symmetric key; in public-key encryption, it is the public key in the pair.
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| Feature | Symmetric key | Public/private key pair |
|---|---|---|
| Structure | One shared secret key | A public key paired with a private key |
| Encryption and paired operation | The same key is used for encryption and decryption | For public-key encryption, the public key encrypts and the corresponding private key performs the paired decryption operation |
| Secrecy | Authorized parties may share the key; it must be protected from unauthorized disclosure | The public key can be shared; the private key must be kept private |
| Other possible roles | May be used for data encryption or other symmetric-key purposes | Depending on the system, keys may support encryption, key establishment, signing, or authentication |
NIST defines a symmetric key by its use for an operation and its inverse, such as encryption and decryption. The phrase “public/private key pair” alone does not tell you which operation each key supports; describe the specific role rather than calling both keys encryption keys.
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What are the main classifications of cryptographic keys?
Keys can be classified by structure—symmetric or asymmetric—and by purpose. The purpose determines what a key does and what protection it needs.
Data-encryption keys
These keys are used to encrypt data and support the corresponding decryption operation. A symmetric data-encryption key is secret; in public-key encryption, the public key encrypts and its paired private key decrypts.
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Signature-generation and signature-verification keys
These keys support different sides of a digital-signature process: generating a signature and verifying it. They are not simply interchangeable with encryption keys. In particular, signing is a distinct cryptographic purpose from keeping data confidential.
Authentication keys
Authentication keys support establishing or checking identity or authenticity. Their exact operation depends on the cryptographic system, so the label does not by itself specify whether a key is public or secret.
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Key-wrapping keys
A key-wrapping key protects other keys rather than directly encrypting ordinary application data. Because it protects keying material, its own access and handling require protection appropriate to that role.
These are examples, not an exhaustive list. NIST’s SP 800-57 Part 1 Revision 5 discusses key types, protection, and key-management functions.
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Why does key management matter?
Encryption only helps if the keys are handled appropriately. Key management covers selecting, protecting, using, and replacing keying material. The details depend on the key’s purpose and cryptographic method: a symmetric secret key and an asymmetric private key must be protected from unauthorized access, while a public key is intended to be shared for its designated role.
NIST’s SP 800-57 Part 1 Revision 5, authored by Elaine B. Barker and published May 4, 2020, provides general guidance and best practices for cryptographic key management. It is guidance on key types and management, not a claim that one cryptographic method is universally safer or better than another.
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How should you identify a key’s role?
- Identify the cryptographic method: symmetric or public-key.
- Name the specific operation: encryption, decryption, signing, verification, authentication, or key wrapping.
- Determine which key must remain secret and who is authorized to use or share it.
- Apply protection and lifecycle controls suited to that key’s role.
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