IBSig - Identity-Based Signatures

This module provides the base class for implementing Identity-Based Signature (IBS) schemes in the Charm cryptographic library.

Overview

Identity-Based Signatures are the signing counterpart to Identity-Based Encryption. A signer’s public verification key is derived directly from their identity string (such as an email address), eliminating the need for certificates linking public keys to identities.

Anyone can verify a signature using just the signer’s identity string and the system’s global public parameters, without needing to look up or verify certificates.

How IBS Works:

  1. Setup: A trusted authority generates master public parameters and master secret key.

  2. KeyGen: The authority generates a signing key for a user based on their identity.

  3. Sign: The user signs messages using their identity-based signing key.

  4. Verify: Anyone can verify using the signer’s identity and public parameters.

Advantages over Traditional Signatures:

  • No certificate management or PKI required

  • Verification uses only the signer’s identity string

  • Simplified key distribution and revocation

Security Properties

IBS schemes in Charm support the following security definitions:

Security Definition

Description

EU_CMA

Existential Unforgeability under Chosen Message Attack. The adversary cannot forge a valid signature even after obtaining signatures on messages of their choice. This is the standard security notion for signatures.

wEU_CMA

Weak existential unforgeability. A relaxed notion that may allow certain types of signature malleability.

sEU_CMA

Strong existential unforgeability. Even producing a new valid signature on a previously signed message counts as a forgery.

Underlying Assumptions:

Security typically relies on assumptions in bilinear groups such as:

  • CDH (Computational Diffie-Hellman)

  • co-CDH (Computational co-Diffie-Hellman in asymmetric pairings)

Typical Use Cases

  1. Email Authentication

    Sign emails using identity-based keys derived from email addresses. Recipients can verify signatures without certificate lookup, using only the sender’s email address.

    # Signer gets key from authority
signer_id = 'alice@company.com'
signing_key = ibs.keygen(master_secret_key, signer_id)

# Sign a message
message = b"Quarterly report attached"
signature = ibs.sign(public_key, signing_key, message)

# Anyone can verify using just the identity
is_valid = ibs.verify(public_key, message, signature)

  2. IoT Device Authentication

    Lightweight signature scheme where device identity (e.g., serial number or MAC address) serves as the public key. Verifiers don’t need to store device certificates.

  3. Audit Logs

    Sign audit records where verifiers only need the signer’s identity string to verify. Useful for distributed systems where certificate distribution is impractical.

Example Schemes

The following IBS implementations are available in Charm:

BLS-based Signatures:

  • charm.schemes.pksig.pksig_bls04 - BLS01: The Boneh-Lynn-Shacham short signature scheme, which can be viewed as an identity-based signature.

from charm.toolbox.pairinggroup import PairingGroup
from charm.schemes.pksig.pksig_bls04 import BLS01

group = PairingGroup('MNT224')
ib = BLS01(group)

# Key generation
(public_key, secret_key) = ib.keygen()

# Sign messages
messages = {'a': "hello world!!!", 'b': "test message"}
signature = ib.sign(secret_key['x'], messages)

# Verify
is_valid = ib.verify(public_key, signature, messages)
assert is_valid == True

Other IBS Schemes:

  • charm.schemes.pksig.pksig_hess - Hess identity-based signature

  • charm.schemes.pksig.pksig_waters05 - Waters IBS (derived from IBE)

  • charm.schemes.pksig.pksig_waters09 - Waters 2009 IBS

API Reference

Base class for identity-based signatures

Notes: This class implements an interface for a standard identity-based signatures scheme.

Identity-based signatures consists of four algorithms: (setup, keygen, sign and verify).

class IBSig.IBSig[source]

Bases: SchemeBase

checkProperty(schemeObj, _reqProps)[source]
getProperty()[source]
keygen(msk, ID)[source]
printProperties()[source]
setProperty(secDef=None, assumption=None, messageSpace=None, secModel=None, **kwargs)[source]
setup()[source]
sign(pk, sk, M)[source]
updateProperty(scheme, secDef=None, assumption=None, messageSpace=None, secModel=None, **kwargs)[source]
verify(pk, M, sig)[source]

See Also