Core Focus: The cryptographic technologies available to balance privacy and regulatory compliance in CBDC systems, including homomorphic encryption, zero-knowledge proofs, and secure multi-party computation.

In-Depth Notes:
The tension between user privacy and regulatory compliance has driven the development of sophisticated cryptographic technologies that allow for both. These technologies are not theoretical abstractions but practical tools being tested in CBDC pilots and implementations.

Homomorphic Encryption:
Homomorphic encryption allows computations to be performed on encrypted data without decrypting it. In a CBDC context, this means that transaction amounts can be processed in ciphertext, preserving privacy while enabling balance updates and regulatory checks . A privacy-preserving scheme based on Paillier partial homomorphic encryption has been proposed: the central bank generates key pairs and binds them to users’ digital identities for traceability; transaction amounts are encrypted using the public key, and balances are updated in ciphertext via homomorphic addition . This approach achieves 1.8-4.5 bits of average user behaviour entropy and 42-110ms regulatory response latency across transaction scales, with rule matching coverage reaching 92% ± 1.8% in typical money laundering scenarios .

Zero-Knowledge Proofs (ZKPs):
Zero-knowledge proofs allow a user to prove that a statement is true without revealing the information underlying that statement. In CBDC design, ZKPs can verify that a sender has sufficient balance to complete a transaction without revealing the actual balance. This prevents double-spending while preserving privacy . The PATROL protocol uses Groth16 zk-SNARKs over the BN254 curve, showing only 21.77 ms overhead per transaction (a 23.9% increase over existing approaches) while enabling sub-2 ms tracing in both directions .

Secure Multi-Party Computation (SMPC):
Secure multi-party computation allows multiple parties to jointly compute a function over their inputs while keeping those inputs private. In an AML/CFT context, SMPC can enable financial institutions to share data for fraud detection and suspicious activity identification without revealing customer identities. The Bank of England’s CBDC Technology Forum has explored privacy-preserving AML using secure multi-party computation .

The PATROL Framework:
A significant innovation in CBDC privacy technology is PATROL, a protocol that achieves bidirectional selective tracing while preserving transaction unlinkability under normal operations . PATROL encrypts both predecessor and successor serial numbers under the regulatory authority’s public key via ElGamal encryption, allowing authorised regulators to traverse transaction graphs in both directions under cryptographically-enforced conditions . Formal security proofs demonstrate that PATROL achieves transaction unforgeability, balance preservation, and user privacy under standard cryptographic assumptions .