Add DKIM control

Signed-off-by: Bryan Frimin <bryan@frimin.fr>

controls/communication-security/email-security/COM-EMAIL-001_dkim_implementation.md

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+---
+id: "COM-EMAIL-001"
+category: communication-security/email-security
+revision-version: 1
+revision-date: 2024-01-07
+estimate-time: "30m"
+frameworks:
+  - name: "soc2"
+	sections: ["CC6.1", "CC6.8"]
+---
+
+## Purpose
+DKIM (DomainKeys Identified Mail) helps prevent email spoofing by
+adding a digital signature to outgoing messages, allowing receiving
+mail systems to verify that emails genuinely came from your domain and
+weren't modified in transit.
+
+## Risk Assessment
+
+| Risk                            | Impact | Reason                                                               |
+|---------------------------------|--------|----------------------------------------------------------------------|
+| Phishing and Social Engineering | 8      | Reduces phishing by preventing email spoofing of company domains     |
+| Business Email Compromise (BEC) | 8      | Prevents attackers from impersonating trusted partners or executives |
+
+## Implementation
+
+### Google Workspace
+
+1. Go to [Google Admin console](admin.google.com).
+2. Navigate to Apps > Google Workspace > Gmail > Authenticate Email.
+3. Select your domain and click "Generate new record".
+4. Copy the DKIM TXT record provided by Google.
+5. Add this TXT record to your DNS.
+6. After DNS propagation, return to Admin console and click "Start authentication".
+
+## Evidence
+
+- Screenshot of published DKIM DNS record
+- Sample email headers showing DKIM pass