ABEnc - Attribute-Based Encryption¶

This module provides the base class for implementing Attribute-Based Encryption (ABE) schemes in the Charm cryptographic library.

Overview¶

Attribute-Based Encryption (ABE) is an advanced public-key encryption paradigm where ciphertexts and keys are associated with sets of attributes or access policies rather than specific identities. Decryption is only possible when a user’s attribute set satisfies the access structure embedded in either the ciphertext (Ciphertext-Policy ABE) or the key (Key-Policy ABE).

ABE enables fine-grained access control over encrypted data, allowing data owners to specify complex access policies without needing to know the specific identities of authorized users in advance.

Variants:

Ciphertext-Policy ABE (CP-ABE): The access policy is embedded in the ciphertext, and user keys are associated with attribute sets. A user can decrypt if their attributes satisfy the ciphertext’s policy.
Key-Policy ABE (KP-ABE): The access policy is embedded in the user’s key, and ciphertexts are associated with attribute sets. A user can decrypt if the ciphertext’s attributes satisfy their key’s policy.

Security Properties¶

ABE schemes in Charm support the following security definitions:

Security Definition	Description
`IND_AB_CPA`	Indistinguishability under Chosen-Plaintext Attack for attribute-based settings. An adversary cannot distinguish between encryptions of two messages of their choice.
`IND_AB_CCA`	Indistinguishability under Chosen-Ciphertext Attack. Provides security even when the adversary can obtain decryptions of ciphertexts other than the challenge.
`sIND_AB_CPA`	Selective security variant where the adversary commits to the challenge attributes/policy before seeing public parameters.
`sIND_AB_CCA`	Selective CCA security combining selective-ID with chosen-ciphertext attacks.

Additional Security Guarantees:

Collusion Resistance: Multiple users cannot combine their keys to decrypt ciphertexts they couldn’t individually access. This is a fundamental property that distinguishes ABE from simpler broadcast encryption schemes.
Attribute Privacy (in some schemes): The ciphertext does not reveal which attributes were used in the access policy.

Typical Use Cases¶

Fine-grained Access Control in Cloud Storage

Encrypt files so only users with certain role attributes can decrypt. For example, a policy like (Manager AND Engineering) OR (Director) allows access only to engineering managers or any director.
```
policy = '((MANAGER and ENGINEERING) or DIRECTOR)'
cipher_text = cpabe.encrypt(master_public_key, msg, policy)
```
Healthcare Record Sharing

Allow only authorized medical staff with specific credentials to access patient records. Policies can encode requirements like Doctor AND (Cardiology OR Emergency).
Broadcast Encryption

Encrypt content once and allow decryption by any user meeting attribute requirements, without needing to know the specific recipients in advance.

Example Schemes¶

The following concrete ABE implementations are available in Charm:

CP-ABE (Ciphertext-Policy ABE):

charm.schemes.abenc.abenc_bsw07 - CPabe_BSW07: The Bethencourt-Sahai-Waters scheme from IEEE S&P 2007. Supports arbitrary access policies as Boolean formulas.

from charm.toolbox.pairinggroup import PairingGroup, GT
from charm.schemes.abenc.abenc_bsw07 import CPabe_BSW07

group = PairingGroup('SS512')
cpabe = CPabe_BSW07(group)

# Setup
(master_public_key, master_key) = cpabe.setup()

# Key generation for user with attributes
attributes = ['ONE', 'TWO', 'THREE']
secret_key = cpabe.keygen(master_public_key, master_key, attributes)

# Encryption with access policy
msg = group.random(GT)
access_policy = '((four or three) and (three or one))'
cipher_text = cpabe.encrypt(master_public_key, msg, access_policy)

# Decryption (succeeds if attributes satisfy policy)
decrypted_msg = cpabe.decrypt(master_public_key, secret_key, cipher_text)
assert msg == decrypted_msg

charm.schemes.abenc.abenc_waters09 - CPabe09: Waters’ CP-ABE scheme from 2009.

KP-ABE (Key-Policy ABE):

charm.schemes.abenc.abenc_lsw08 - KPabe: The Lewko-Sahai-Waters KP-ABE scheme.
charm.schemes.abenc.abenc_yct14 - EKPabe: Extended KP-ABE with additional features.

API Reference¶

Base class for attribute-based encryption

Notes: This class implements an interface for a standard attribute-based encryption scheme.

A public key attribute-based encryption scheme consists of four algorithms: (setup, keygen, encrypt, decrypt).

class ABEnc.ABEnc[source]¶

Bases: SchemeBase

decrypt(pk, sk, ct)[source]¶

encrypt(pk, M, object)[source]¶

keygen(pk, mk, object)[source]¶

setup()[source]¶