# JWT Algorithm Investigation Results

**Date**: 2025-10-31  
**Investigator**: Auto (AI Assistant)  
**Status**: ✅ **INVESTIGATION COMPLETE**

---

## 🔴 CRITICAL FINDING: DID-Based JWT Signing Required (ES256K)

### Conclusion

**The endorser-ch API expects DID-based JWTs signed with ES256K (or ES256K-R), NOT HMAC-SHA256 (HS256).**

The current implementation in `TestNativeFetcher.java` using HMAC-SHA256 is **INCORRECT** and will fail authentication.

---

## Investigation Details

### TimeSafari Repository Findings

**Location**: `~/projects/timesafari/crowd-funder-for-time-pwa/src/libs/crypto/vc/index.ts`

**Function**: `createEndorserJwtForKey()`

**Implementation**:
```typescript
export async function createEndorserJwtForKey(
  account: KeyMetaWithPrivate,
  payload: object,
  expiresIn?: number,
) {
  if (account?.identity) {
    const identity: IIdentifier = JSON.parse(account.identity!);
    const privateKeyHex = identity.keys[0].privateKeyHex;
    const signer = await SimpleSigner(privateKeyHex as string);
    const options = {
      // alg: "ES256K", // "K" is the default, "K-R" is used by the server in tests
      issuer: account.did,
      signer: signer,
      expiresIn: undefined as number | undefined,
    };
    if (expiresIn) {
      options.expiresIn = expiresIn;
    }
    return didJwt.createJWT(payload, options);
  }
  // ... passkey handling ...
}
```

**Key Points**:
- Uses `did-jwt.createJWT()` library
- Uses `SimpleSigner(privateKeyHex)` - signs with Ethereum private key
- Algorithm is **ES256K** (default)
- Signs with DID private key, NOT a shared secret

**Library Used**: `did-jwt` (DID-based JWT library)

---

### endorser-ch Repository Findings

**Location**: `~/projects/timesafari/endorser-ch/src/api/services/vc/index.js`

**Function**: `decodeAndVerifyJwt()`

**Implementation**:
```javascript
export async function decodeAndVerifyJwt(jwt) {
  const pieces = jwt.split('.')
  const header = JSON.parse(base64url.decode(pieces[0]))
  const payload = JSON.parse(base64url.decode(pieces[1]))
  const issuerDid = payload.iss

  if (issuerDid.startsWith(ETHR_DID_PREFIX)) {
    try {
      const verifiedResult = await didJwt.verifyJWT(jwt, {resolver})
      return verifiedResult
    } catch (e) {
      return Promise.reject({
        clientError: {
          message: `JWT failed verification: ` + e.toString(),
          code: JWT_VERIFY_FAILED_CODE
        }
      })
    }
  }
  // ... other DID methods ...
}
```

**Key Points**:
- Uses `did-jwt.verifyJWT(jwt, {resolver})` - DID-based verification
- Verifies signature using DID resolver (resolves DID to public key)
- **NO shared secret used** - uses DID public key from resolver
- Algorithm: **ES256K** (implicit from did-jwt library)

**Middleware Location**: `~/projects/timesafari/endorser-ch/src/common/server.js`

**Authentication Flow**:
```javascript
decodeAndVerifyJwt(authorizationJwt)
  .then((result) => {
    const { header, issuer, payload, verified } = result
    if (!verified) {
      res.status(400).json({
        error: {
          message: "Signature failed validation.",
          code: ERROR_CODES.JWT_VERIFY_FAILED
        }
      }).end()
    } else {
      res.locals.authTokenIssuer = issuer
      next()
    }
  })
```

---

## Impact on Current Implementation

### Current Implementation (WRONG)

**File**: `test-apps/daily-notification-test/android/app/src/main/java/com/timesafari/dailynotification/test/TestNativeFetcher.java`

**Current Approach**: HMAC-SHA256 with shared secret
```java
Mac hmac = Mac.getInstance("HmacSHA256");
SecretKeySpec secretKey = new SecretKeySpec(
    jwtSecret.getBytes(StandardCharsets.UTF_8), 
    "HmacSHA256"
);
hmac.init(secretKey);
byte[] signatureBytes = hmac.doFinal(unsignedToken.getBytes(StandardCharsets.UTF_8));
```

**Problem**: Server expects ES256K with DID private key, NOT HMAC-SHA256 with shared secret

---

## Required Changes

### Action Required

1. **Remove HMAC-SHA256 implementation** - This is completely wrong
2. **Implement DID-based signing (ES256K)** - Sign with Ethereum private key
3. **Access DID private key** - Need to retrieve private key from DID/account storage
4. **Use did-jwt-java or web3j** - Java library for DID-based JWT signing

### Implementation Options

#### Option 1: Use did-jwt-java Library (Recommended)

```java
// Add dependency to build.gradle
implementation 'io.uport:uport-did-jwt:3.1.0'

// Sign with DID private key
import io.uport.sdk.did.jwt.DIDJWT;
import io.uport.sdk.did.jwt.SimpleSigner;

String privateKeyHex = getPrivateKeyForDid(activeDid); // Need to implement
SimpleSigner signer = new SimpleSigner(privateKeyHex);
String jwt = DIDJWT.createJWT(payload, signer, issuer: activeDid);
```

#### Option 2: Use web3j for Ethereum Signing

```java
// Add dependency
implementation 'org.web3j:core:4.9.8'

import org.web3j.crypto.ECKeyPair;
import org.web3j.crypto.Sign;

ECKeyPair keyPair = getKeyPairForDid(activeDid); // Need to implement
Sign.SignatureData signature = Sign.signMessage(
    unsignedToken.getBytes(StandardCharsets.UTF_8),
    keyPair
);
// Then encode signature according to ES256K format
```

---

## Next Steps

1. **Remove `jwtSecret` parameter** - No longer needed (shared secret not used)
2. **Add DID private key retrieval** - Need mechanism to get private key for `activeDid`
3. **Implement ES256K signing** - Using did-jwt-java or web3j
4. **Update `configureNativeFetcher()`** - Remove `jwtSecret`, add private key retrieval mechanism
5. **Test with real API** - Verify JWTs are accepted by endorser-ch server

---

## References

- **TimeSafari Implementation**: `~/projects/timesafari/crowd-funder-for-time-pwa/src/libs/crypto/vc/index.ts`
- **endorser-ch Verification**: `~/projects/timesafari/endorser-ch/src/api/services/vc/index.js`
- **did-jwt Library**: https://github.com/decentralized-identity/did-jwt
- **did-jwt-java**: https://github.com/uport-project/uport-did-jwt (if available) or alternative Java DID libraries

---

## Evidence Summary

| Component | Finding | Evidence |
|-----------|---------|----------|
| **TimeSafari JWT Creation** | ✅ DID-based (ES256K) | Uses `didJwt.createJWT()` with `SimpleSigner(privateKeyHex)` |
| **endorser-ch JWT Verification** | ✅ DID-based (ES256K) | Uses `didJwt.verifyJWT(jwt, {resolver})` |
| **Current TestNativeFetcher** | ❌ HMAC-SHA256 | Uses `Mac.getInstance("HmacSHA256")` with shared secret |
| **Shared Secret Config** | ❌ Not Used | No `JWT_SECRET` found in endorser-ch, no shared secret in TimeSafari |

---

**Status**: Investigation complete. Implementation changes required.