Celo Tech Talks Part 10 - Keeping Phone Numbers Private


Judy Piper, an engineer at Sea Labs and host of the Celo Tech Talk series, welcomes the audience and highlights the purpose of the educational sessions provided by Celo Foundation. The focus is on spreading knowledge about Celo technology within the Celo community, with a particular emphasis on maintaining phone number privacy and security.

Introducing Rossi and C-Labs

Rossi, joining from Montreal, introduces himself and his role at C-Labs. He explains that C-Labs works on the application layer of the Celo platform, which consists of the blockchain layer, smart contract layer, and various applications. Rossi emphasizes Celo’s compatibility with Ethereum and its privacy-preserving capabilities.

Celo’s Mobile Client Focus

Celo has implemented features to cater to mobile clients. These features include Celo Dollar (cUSD), a stable value currency pegged to the US dollar, which addresses the issue of cryptocurrency volatility. Additionally, Celo has significantly faster syncing times compared to Ethereum, making it more suitable for mobile devices.

Phone Number Privacy on Celo

The system of identity on Celo is built on two key pillars: attestations and mappings. Attestations refer to the proof that individuals have control over the phone numbers they claim. Mappings involve associating phone numbers with addresses on the Celo blockchain.

Attestation Flow

To prove phone number ownership, users provide their phone numbers and pay a small fee for an attestation. The attestation smart contract assigns a randomly selected validator to validate the claim. The process is repeated three times with three different validators to ensure security. The attestation codes are then stored on the smart contract.

Privacy Challenges and OTIS

Publishing phone numbers in plain text on the blockchain poses a privacy issue. Celo addresses this concern with OTIS (Oblivious Identifier Service). OTIS generates and provides peppers (secret salts) in a decentralized manner, making the mapping of phone numbers more secure. OTIS partners, including C-Labs, run the nodes for this service.

Threshold Signatures and Quotas

Celo uses threshold signatures to combine partial signatures from different parties to create a valid full signature. The process involves blinding the phone number, partial sign requests, and unblinding the signature. OTIS has nine trusted partners, and the more transactions a user performs, the higher their quota for accessing the service.

Future Improvements and Challenges

Celo is continuously working on privacy-preserving improvements for the platform. These include addressing transaction privacy and enabling seamless onboarding and verification processes for users without funds in their accounts. The team is actively exploring medium to long-term solutions to enhance privacy and security.

Q&A Session

During the Q&A session, audience members inquire about observing balances and transactions, mobile security for users without smartphones, and the factors determining the quota. The speakers provide insights into these topics, addressing concerns and explaining the functionalities and limitations of the Celo platform.