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Tracks follow-up #2 from PR #17 security review. Blocked on mcp-server-v1 M3 (~Jul 28, 2026).

What

MCPPairingPanel compares the user-entered token to the expected token using === (constant-time-unsafe). Use a timing-safe comparison.

Why

=== short-circuits on the first byte difference, leaking the prefix-length-match through timing. For a 32-byte token, an attacker controlling the input can extract the correct token byte-by-byte via timing oracle in O(256 × 32 × N samples) requests. This is the textbook side-channel attack pattern.

The Web Crypto API has crypto.subtle.timingSafeEqual (proposed) — until that's universal, the manual fallback is a XOR-accumulator loop.

Acceptance criteria

• MCPPairingPanel.tsx compares user input to expected token via a timing-safe helper (no === on the raw token strings)
• Helper exported from src/utils/crypto.ts (or similar) with JSDoc + a comment citing the side-channel rationale
• Unit test verifies the helper returns false for mismatched-but-same-length inputs AND for different-length inputs
• Unit test verifies all execution paths take constant time within ±5% (use performance.now() x 1000 iterations + std-dev assertion)

Implementation hint

// src/utils/crypto.ts
/**
 * Compare two strings in constant time relative to the longer string's length.
 * Use this for token / secret comparisons to avoid timing side-channel attacks.
 *
 * NOTE: Once Web Crypto's timingSafeEqual ships universally
 * (https://github.com/whatwg/webcrypto/issues/270), prefer that. For now:
 */
export function timingSafeEqual(a: string, b: string): boolean {
  const maxLen = Math.max(a.length, b.length);
  let mismatch = a.length !== b.length ? 1 : 0;
  for (let i = 0; i < maxLen; i++) {
    const ca = i < a.length ? a.charCodeAt(i) : 0;
    const cb = i < b.length ? b.charCodeAt(i) : 0;
    mismatch |= (ca ^ cb);
  }
  return mismatch === 0;
}

Context

• Related PR: feat: improve MCPPairingPanel error messages for invalid token/port (#7) #17 (MCPPairingPanel)
• Blocked by: mcp-server-v1 M3 milestone
• Severity: High (security category C-2 in PR feat: improve MCPPairingPanel error messages for invalid token/port (#7) #17 review)
• References: OWASP — Authentication Cheat Sheet (Timing Attacks)