sh-mcp/packages/shared/src/redact.ts

/**
 * MCP-layer PII redaction.
 *
 * Masks sensitive financial identifiers in tool response strings before they
 * leave the server and reach the LLM agent or the user.
 *
 * WHAT IS MASKED:
 *   - US bank routing numbers (9 digits, ABA)
 *   - US bank account numbers (4–17 digits following routing or account keywords)
 *   - Payment card numbers (13–19 digits, Luhn-matching encouraged; basic pattern here)
 *   - US Social Security Numbers (NNN-NN-NNNN / NNN NN NNNN / NNNNNNNNN)
 *
 * WHAT IS LEFT INTACT:
 *   - Vendor/company names
 *   - Contact information (phone numbers, email addresses, street addresses)
 *   - Dollar amounts and invoice/PO numbers
 *   - Any other non-financial-identity data
 *
 * See docs/design.md §2.5 for the requirement context.
 * See src/redact.test.ts for the full contract.
 */

// ---------------------------------------------------------------------------
// Mask helper
// ---------------------------------------------------------------------------

/** The string used to replace masked values. */
export const REDACTED = '[REDACTED]';

// ---------------------------------------------------------------------------
// Individual pattern redactors
// ---------------------------------------------------------------------------

/**
 * Mask US Social Security Numbers.
 *
 * Patterns matched:
 *   - NNN-NN-NNNN  (canonical)
 *   - NNN NN NNNN  (spaced)
 *   - NNNNNNNNN    (bare 9 digits) — only when preceded by an SSN keyword
 *     to avoid colliding with routing/account numbers handled below.
 */
function redactSSN(value: string): string {
  // Canonical and spaced formats (unambiguous)
  let result = value.replace(
    /\b(\d{3})[- ](\d{2})[- ](\d{4})\b/g,
    REDACTED,
  );
  // Bare 9-digit SSN preceded by an SSN keyword
  result = result.replace(
    /\b(ssn|social\s+security(?:\s+number)?|tax\s+id)\s*[:#]?\s*(\d{9})\b/gi,
    (_match, keyword) => `${keyword} ${REDACTED}`,
  );
  return result;
}

/**
 * Mask US ABA routing numbers.
 *
 * ABA routing numbers are exactly 9 digits. We match them when:
 *   a) preceded by a routing-number keyword, OR
 *   b) followed by a routing-number keyword
 *
 * We do NOT mask bare 9-digit strings without a keyword to avoid clobbering
 * zip+4 combos, phone fragments, etc.
 */
function redactRouting(value: string): string {
  // Keyword BEFORE the number: "routing number: 021000021"
  let result = value.replace(
    /\b(routing\s*(?:number|#|no\.?)?|aba\s*(?:number|#|no\.?)?)\s*[:#]?\s*(\d{9})\b/gi,
    (_match, keyword) => `${keyword.trim()} ${REDACTED}`,
  );
  // Keyword AFTER the number: "021000021 (routing)"
  result = result.replace(
    /\b(\d{9})\s*\((routing|aba)\)/gi,
    `${REDACTED} ($2)`,
  );
  return result;
}

/**
 * Mask US bank account numbers.
 *
 * Bank account numbers are 4–17 digits. We mask them only when a keyword
 * context makes them unambiguous, to avoid clobbering invoice/PO numbers,
 * phone numbers, etc.
 */
function redactAccountNumber(value: string): string {
  return value.replace(
    /\b(account\s*(?:number|#|no\.?)?|acct\.?\s*(?:#|no\.?)?)\s*[:#]?\s*(\d{4,17})\b/gi,
    (_match, keyword) => `${keyword.trim()} ${REDACTED}`,
  );
}

/**
 * Mask payment card numbers (credit / debit).
 *
 * Matches 13–19 consecutive digits (with optional spaces or hyphens between
 * groups of 4) that look like a PAN. We apply a basic Luhn check so common
 * non-card numeric strings (invoice IDs, phone numbers) do not get masked.
 */
function passesLuhn(digits: string): boolean {
  let sum = 0;
  let alternate = false;
  for (let i = digits.length - 1; i >= 0; i--) {
    let n = parseInt(digits[i]!, 10);
    if (alternate) {
      n *= 2;
      if (n > 9) n -= 9;
    }
    sum += n;
    alternate = !alternate;
  }
  return sum % 10 === 0;
}

function redactCard(value: string): string {
  // Match 13–19 digits, optionally separated by spaces or hyphens in groups of 4.
  return value.replace(
    /\b(\d{4}[-\s]?\d{4}[-\s]?\d{4}[-\s]?\d{1,7}|\d{13,19})\b/g,
    (match) => {
      const digits = match.replace(/[\s-]/g, '');
      if (digits.length < 13 || digits.length > 19) return match;
      if (!passesLuhn(digits)) return match;
      return REDACTED;
    },
  );
}

// ---------------------------------------------------------------------------
// Public API
// ---------------------------------------------------------------------------

/**
 * Redact PII from a string that may appear in a tool response.
 *
 * Applies all pattern redactors in a safe order (SSN first to avoid the bare
 * 9-digit pattern conflicting with the routing-number check).
 *
 * The function is PURE and has no side effects. It never makes network calls.
 *
 * @param value  The raw string (may be JSON, plain text, CSV, etc.)
 * @returns      A copy of `value` with sensitive fields replaced by `[REDACTED]`.
 */
export function redact(value: string): string {
  let result = value;
  result = redactSSN(result);
  result = redactRouting(result);
  result = redactAccountNumber(result);
  result = redactCard(result);
  return result;
}

/**
 * Mask a sensitive field value at the field level.
 *
 * Use this when you have an already-isolated sensitive field value (e.g. a
 * bank account number stored in its own database column) and need to produce
 * a display-safe string. Unlike `redact()`, which is designed for inline
 * pattern-matching inside free-form text, `maskValue` blindly replaces the
 * entire value with masking characters.
 *
 * Finance-tier tools MUST still call `redact()` on the field as well —
 * `maskValue` is a complementary, not a replacement, operation.
 *
 * @param value   The raw sensitive string (e.g. "123456789", "4111-1111-1111-1111").
 * @returns       A string of `●` characters the same length as `value` (max 16),
 *                safe to include in tool output or logs.
 */
export function maskValue(value: string): string {
  // Show at most 16 mask characters so excessively long values don't bloat output.
  return '●'.repeat(Math.min(value.length, 16));
}