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 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):

// 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:

data: [
  ...Peer.base64ToBytes(message.initialNonce),
  ...Peer.base64ToBytes(sessionNonce)
]

Why This is Critical: 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 - An attacker could potentially exploit this to bypass authentication checks

PoC

The cross-language test suite demonstrates this vulnerability:

1. Setup: Use identical nonces and cryptographic inputs across TypeScript, Python, and Go SDKs
2. Vulnerable behavior: TypeScript SDK produces different signature data than Go/Python reference implementations
3. Impact demonstration: Authentication attempts between TypeScript clients and Go/Python servers fail due to signature mismatch

Test Evidence:

// Buggy approach (produces ~32-34 bytes)
const concatenatedB64 = INITIAL_NONCE_B64 + SESSION_NONCE_B64;
const buggyResult = Array.from(Buffer.from(concatenatedB64, 'base64'));

// Correct approach (produces 64 bytes)
const correctResult = [...INITIAL_NONCE_BYTES, ...SESSION_NONCE_BYTES];

Base64 Padding Short Circuit Analysis:

The vulnerability occurs because base64 padding characters (=) act as early termination signals for base64 decoders. When concatenating base64 strings before decoding:

1. Individual nonces: Each 44-character base64 string decodes to 32 bytes
2. Concatenated string: 88-character string containing padding in the middle
3. Decoding result: Base64 decoder stops at the first = padding character, producing only 32 bytes instead of 64

Example with test data: - INITIAL_NONCE_B64: "QUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUE=" (44 chars → 32 bytes) - SESSION_NONCE_B64: "QkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkI=" (44 chars → 32 bytes) - Concatenated: "QUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUE=QkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkJCQkI=" - Buggy decode: Only 32 bytes (decoder stops at first =) - Correct decode: 64 bytes (32 + 32, decoded separately then concatenated)

Impact

Vulnerability Type: Cryptographic signature verification bypass

Severity: Critical (CVSS 9.1 - Critical)

Affected Systems: - TypeScript SDK clients attempting to authenticate with Go or Python SDK servers - Any BRC-104 implementation relying on cross-SDK compatibility - Mutual authentication protocols using the affected signature preparation

Who is Impacted: - Applications using the TypeScript SDK for BRC-104 authentication - Systems requiring cross-language/SDK authentication compatibility - Any peer-to-peer authentication scenarios where TypeScript clients communicate with non-TypeScript servers

Potential Attack Vectors: - Authentication bypass through signature verification failure - Man-in-the-middle attacks if authentication is silently ignored - Denial of service through failed authentication attempts

The fix ensures all SDKs now produce identical cryptographic signatures, restoring proper mutual authentication across implementations.