sip-authentication-security

# SIP Authentication and Security

Master SIP authentication mechanisms (HTTP Digest), TLS encryption, SIPS,
and security best practices for building secure VoIP applications.

## HTTP Digest Authentication

### Challenge-Response Flow

```
Client                                    Server
  |                                         |
  | REGISTER (no credentials)               |
  |---------------------------------------->|
  |                                         |
  |    401 Unauthorized                     |
  |    WWW-Authenticate: Digest             |
  |      realm="atlanta.com"                |
  |      nonce="dcd98b7102dd..."            |
  |      algorithm=MD5                      |
  |      qop="auth"                         |
  |<----------------------------------------|
  |                                         |
  | REGISTER (with Authorization)           |
  |    Authorization: Digest                |
  |      username="alice"                   |
  |      realm="atlanta.com"                |
  |      nonce="dcd98b7102dd..."            |
  |      uri="sip:atlanta.com"              |
  |      response="6629fae49393..."         |
  |      algorithm=MD5                      |
  |      qop=auth                           |
  |      nc=00000001                        |
  |      cnonce="0a4f113b"                  |
  |---------------------------------------->|
  |                                         |
  |    200 OK                               |
  |<----------------------------------------|
  |                                         |
```

### Digest Authentication Implementation

```typescript
import crypto from 'crypto';

interface DigestChallenge {
  realm: string;
  nonce: string;
  algorithm: 'MD5' | 'SHA-256';
  qop?: 'auth' | 'auth-int';
  opaque?: string;
  stale?: boolean;
}

interface DigestCredentials {
  username: string;
  realm: string;
  nonce: string;
  uri: string;
  response: string;
  algorithm: 'MD5' | 'SHA-256';
  cnonce?: strin