The BSV Blockchain SDK is a unified TypeScript SDK for developing scalable apps on the BSV Blockchain. Prior to version 2.0.0, a cryptographic vulnerability in the TypeScript SDK's BRC-104 authentication implementation caused incorrect signature data preparation, resulting in signature incompatibility between SDK implementations and potential authentication bypass scenarios. The vulnerability was located in the `Peer.ts` file of the TypeScript SDK, specifically in the `processInitialRequest` and `processInitialResponse` methods where signature data is prepared for BRC-104 mutual authentication. The TypeScript SDK incorrectly prepared signature data by concatenating base64-encoded nonce strings (`message.initialNonce + sessionNonce`) then decoding the concatenated base64 string (`base64ToBytes(concatenatedString)`). This produced ~32-34 bytes of signature data instead of the correct 64 bytes. BRC-104 authentication relies on cryptographic signatures to establish mutual trust between peers. When signature data preparation is incorrect, signatures generated by the TypeScript SDK don't match those expected by Go/Python SDKs; cross-implementation authentication fails; and an attacker could potentially exploit this to bypass authentication checks. The fix in version 2.0.0 ensures all SDKs now produce identical cryptographic signatures, restoring proper mutual authentication across implementations.

Affected: npm: @bsv/sdk

# BRC-104 Authentication Signature Data Preparation Vulnerability

### Summary A critical cryptographic vulnerability in the TypeScript SDK's BRC-104 authentication implementation caused incorrect signature data preparation, resulting in signature incompatibility [between SDK implementations](https://github.com/F1r3Hydr4nt/brc104-cross-language-tests) and potential authentication bypass scenarios.

### Details The vulnerability was located in the `Peer.ts` file of the TypeScript SDK, specifically in the `processInitialRequest` and `processInitialResponse` methods where signature data is prepared for BRC-104 mutual authentication.

**Vulnerable Code Locations:** - `ts-sdk/src/auth/Peer.ts` lines 527-531 (signing) - `ts-sdk/src/auth/Peer.ts` lines 584-590 (verification)

**Root Cause:** The TypeScript SDK incorrectly prepared signature data by: 1. Concatenating base64-encoded nonce strings: `message.initialNonce + sessionNonce` 2. Then decoding the concatenated base64 string: `base64ToBytes(concatenatedString)`

This produced ~32-34 bytes of signature data instead of the correct 64 bytes.

**Buggy Implementation (Before Fix):** ```typescript // CRITICAL BUG: Concatenating base64 strings before decoding data: Peer.base64ToBytes(message.initialNonce + sessionNonce) ```

**Correct Implementation (After Fix):** The fix properly decodes each base64 nonce individually, then concatenates the byte arrays: ```typescript data: [ ...Peer.base64ToBytes(message.initialNonce), ...Peer.b…