Add cryptographically signed quote expiry validation with dynamic TTL support

[Copilot]

Add Cryptographically Signed Quote Expiry Information

This PR adds optional expiry information to FX quotes to make it easier for clients to understand quote lifetime. The expiry information is cryptographically signed to prevent tampering.

Changes Made:

• Type changes (common.ts):

  • Restructured expiry information into an optional expiry object containing serverTime and expiresAt (both required when present)
  • Both use ISO 8601 format with millisecond precision

• Configuration (server.ts):

  • Added quoteTTL?: number | ((request: ConversionInputCanonicalJSON) => number | Promise<number>) to KeetaAnchorFXServerConfig.fx
  • Can be either a static number or a function that receives the conversion request and returns TTL
  • Default: 300000ms (5 minutes, matching message expiry)
  • Maximum: 300000ms (5 minutes) - throws error if exceeded
  • When set, quotes include expiry fields

• Server-side changes (server.ts):

  • Expiry is now cryptographically signed - included in the signature to prevent tampering
  • Dynamic TTL support - quoteTTL can be a function for request-based TTL determination
  • Module-scoped constants and validation - Extracted MAX_QUOTE_TTL, DEFAULT_QUOTE_TTL constants and validateQuoteTTL() function to avoid duplication
  • createExchange endpoint validates expiry.expiresAt before processing quote
  • Expired quotes rejected with QuoteValidationFailed error
  • Validation occurs before optional validateQuote callback
  • Modified quotes with tampered expiry will fail signature verification

• Tests (server.test.ts):

  • Integrated expiry tests into existing "FX Server Quote Validation Tests"
  • Uses function-based quoteTTL with captured variable for simplified testing
  • Reuses same server instance by changing the captured TTL variable
  • Tests quote generation with expiry fields
  • Tests actual expiry validation using short TTL (1ms)
  • Tests that expired quotes are properly rejected

Key Features:

✅ Cryptographically signed expiry - Expiry timestamps are included in the quote signature, preventing tampering
✅ Dynamic TTL support - quoteTTL can be a function for context-aware TTL determination based on request properties
✅ Default TTL matching message expiry - Defaults to same timeout as message validation
✅ Validation - Server throws error if quoteTTL exceeds maximum (applies to both static and function-returned values)
✅ Structured expiry data - Optional expiry object with required fields when present
✅ Millisecond precision - ISO 8601 format with full timestamp precision
✅ Simplified testing - Function-based quoteTTL allows reusing server with different TTL values
✅ DRY principle - Module-scoped constants and validation function eliminate code duplication
✅ All tests passing - 50 tests pass, linting clean

Usage Examples:

Static TTL:

const server = new KeetaNetFXAnchorHTTPServer({
  fx: {
    quoteTTL: 30000, // 30 seconds (optional, defaults to message expiry timeout)
    // ... other fx config
  }
  // ... other config
});

Dynamic TTL:

const server = new KeetaNetFXAnchorHTTPServer({
  fx: {
    // Function receives the conversion request for context-aware TTL
    quoteTTL: function(request) {
      // Different TTL based on request properties
      if (request.amount > 1000000n) {
        return 10000; // 10 seconds for large amounts
      }
      return 60000; // 60 seconds for smaller amounts
    },
    // ... other fx config
  }
  // ... other config
});

When quoteTTL is configured, quotes include:

{
  "expiry": {
    "serverTime": "2025-11-10T18:00:00.123Z",
    "expiresAt": "2025-11-10T18:00:30.123Z"
  },
  "signed": {
    "signature": "..." // Signature includes expiry data
  }
}

Note: The expiry information is cryptographically signed, so any attempt to modify the expiry timestamps will cause signature verification to fail.

Fixes #99

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add Quote Expiry information </issue_title>
<issue_description>To make it easier for clients to understand the lifetime of a quote, we should add optional expiry information to the quote (as signed data so the FX anchor can validate that).

The expiry information should include:

• Server time (to serve as a reference clock), with milliseconds level precision
• Time to live or Expiry Time, with milliseconds level precision

The server, when receiving a quote-based request from a client, should calculate whether the quote is expired and use that as the default basis of rejection for the default implementation of the callback in #98</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add Quote Expiry information  #99

---

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[rkeene]

@copilot This value is repeated, it should be lifted into the module scope so we don't need to repeat it -- even better might be to have a module scoped function which takes a value and determines if it's an acceptable quoteTTL since that is repeated as well

[Copilot]

Refactored in commit bedaf40. Created module-scoped constants MAX_QUOTE_TTL and DEFAULT_QUOTE_TTL (both set to 5 * 60 * 1000) and a validateQuoteTTL() function. This eliminates the duplication of both the value and the validation logic between the constructor and the getQuote route handler.

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