Exploring Registration-Based Encryption for Blockchain Key Distribution

Linking cryptographic keys to identities has been a long-standing challenge since the advent of public-key cryptography. The primary issue is providing and maintaining a publicly available and consistent mapping between identities and public keys. This challenge is particularly relevant in the context of web3, where transparency and anonymity are paramount.

According to a16z crypto, three main approaches exist for addressing this issue: public key directories, identity-based encryption (IBE), and the more recent registration-based encryption (RBE). Each method offers distinct trade-offs in terms of anonymity, interactivity, and efficiency.

The Three Approaches

The traditional approach involves a public key infrastructure (PKI) with a public key directory at its core. This method requires a trusted third party to maintain the directory, which can be costly and prone to errors. Additionally, the public key directory approach is not succinct, somewhat interactive, and lacks sender anonymity.

Identity-based encryption (IBE), proposed by Adi Shamir in 1984, eliminates the need for a public key directory by using identifiers like phone numbers or email addresses as public keys. However, IBE introduces a strong trust assumption, as it requires a trusted key generator to issue keys. This approach is more space-efficient and offers non-interactive encryption and decryption, but the risk associated with the master secret key is significant.

Registration-based encryption (RBE), proposed in 2018, replaces the trusted key generator with a transparent key curator. The blockchain setting, where a smart contract can serve as the key curator, makes RBE a natural fit. RBE offers the benefits of both PKI and IBE while mitigating their respective drawbacks. It uses less on-chain storage than a public key directory and avoids the strong trust assumption of IBE.

Evaluating the Trade-offs

RBE requires succinct parameters, meaning the size of parameters to be stored on-chain is sublinear in the number of users. This is smaller than the total storage required for a public key directory but still more than IBE. Encryption and decryption are somewhat interactive, requiring periodic updates to public parameters and auxiliary information. However, RBE provides sender anonymity and transparency, making it a compelling option for privacy-conscious users.

Performance Comparison

In terms of cost, RBE has a higher setup and registration cost compared to PKI and IBE. However, it offers stronger anonymity and reduced trust assumptions, making it a viable option for those who prioritize privacy and trustless setups. According to a performance evaluation by a16z crypto, RBE can be feasibly deployed on the Ethereum mainnet today, despite its higher costs.

Overall, while RBE is more expensive, it provides significant advantages in terms of privacy and trustlessness, making it an attractive option for blockchain key management.

Additional Considerations

Handling key updates and revocations is straightforward for a public key directory, but more complex for IBE and RBE. IBE requires periodic updates to keys, while RBE can be extended to support these functionalities through additional mechanisms. Moving data off-chain with data availability solutions can reduce on-chain storage for both public key directories and RBE, further enhancing their efficiency.

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