keypolicy

Keypolicy, in the context of attribute-based encryption (ABE), refers to the access structure encoded into a private key that determines which ciphertexts a holder is permitted to decrypt. In a typical key-policy ABE (KP-ABE) system, ciphertexts are labeled with a set of attributes, while each user receives a private key that carries an associated policy. A decryption is possible only when the ciphertext’s attributes satisfy the policy embedded in the user’s key. This separation of policy (in the key) from data attributes (in the ciphertext) enables fine-grained access control over encrypted data.

There is a closely related paradigm called ciphertext-policy ABE (CP-ABE), where the access policy is attached to the ciphertext and a user’s private key contains a set of attributes. The terminology “keypolicy” is most directly tied to KP-ABE, but the broader concept of policies governing access can be expressed either in keys or in ciphertexts depending on the scheme. Both approaches rely on cryptographic constructions such as monotone access structures and, in many implementations, linear secret sharing schemes to realize complex policies.

Policies are typically represented as logical structures or trees that specify required attribute combinations for decryption. Examples include formulas like “department: cardiology AND role: physician” or more complex thresholds. These schemes support fine-grained access control in scenarios such as cloud storage, healthcare data sharing, and corporate data collaboration, while also presenting challenges in key management, revocation, and efficiency for large user sets.