Proving Ownership of BTC Multisig Addresses

Proving ownership of a Bitcoin (BTC) address is a crucial aspect in various scenarios. The post Proof of Ownership for BTC Addresses introduced the proof for non-multisig addresses. This guide explores how to prove ownership for different multisig BTC address types, including algorithm specifications and step-by-step methods.

What is a Multisig Address?

A Bitcoin multisig (multi-signature) address requires multiple private keys to authorize a transaction. It’s commonly used for added security and shared ownership.

Multisig Address Types

1. P2SH (Pay-to-Script-Hash): Often used for multisig.
2. P2WSH (Pay-to-Witness-Script-Hash): SegWit version of multisig.

Proving Ownership of a Multisig Address

Key Concepts

• M-of-N Scheme: Requires M out of N keys to sign a transaction.
• Redeem Script: Defines the conditions, including public keys and the required number of signatures.

Steps to Prove Ownership

1. Create a Redeem Script: Includes multiple public keys and the required number of signatures.

2. Generate a Message: Unique and non-reusable, similar to single-signature addresses.

3. Sign the Message: Each participant uses their private key to sign the message.

4. Verify the Signatures: Use the redeem script to verify that the correct number of valid signatures are present.

Detailed Algorithm

P2SH Multisig

Address Format: Begins with 3

Algorithm

1. Create a Redeem Script:

   • Define the M-of-N condition.
   • Include all involved public keys.

2. Signatures:

   • Each participant signs the message with their private key using ECDSA.
   • Collect M valid signatures.
3. Verification:
   • Validate the redeem script hash against the P2SH address.
   • Verify each signature against the corresponding public key in the script.

Steps

1. Generate a message: "I own this address. [timestamp]"
2. Each participant signs the message with their private key.
3. Verify signatures using the redeem script and public keys.

P2WSH Multisig

Address Format: Begins with bc1

Algorithm

1. Create a Witness Script:

   • Similar to the redeem script but for SegWit.
   • Include all public keys and the M-of-N condition.

2. Signatures:

   • Each participant signs the message using their private keys.
   • Collect M valid signatures.
3. Verification:
   • Validate the witness script hash against the P2WSH address.
   • Verify signatures using public keys defined in the witness script.

Steps

1. Generate a message: "I own this address. [timestamp]"
2. Each participant signs the message with their private key.
3. Verify signatures using the witness script and public keys.

Security Considerations

• Secure Key Management: Distribute private keys securely among participants.
• Unique Messages: Always use new messages to prevent replay attacks.
• Verification Process: Ensure all M required signatures are valid and correspond to the public keys in the script.

Conclusion

Proving ownership of a Bitcoin multisig address involves a collaborative cryptographic process. By following these steps and algorithms, participants can securely demonstrate ownership while leveraging the enhanced security that multisig provides.