Verify Genesys Cloud Email DKIM Signatures with Go

What You Will Build

• A Go service that retrieves email communications from Genesys Cloud, extracts DKIM headers, performs RFC 6376 cryptographic verification against DNS TXT records, and routes validation results to webhooks or quarantine endpoints.
• This implementation uses the Genesys Cloud Email API (/api/v2/communication/email) and the official Go SDK alongside standard library cryptographic primitives.
• The code covers Go 1.21+ with production-ready token caching, 429 retry logic, header size validation, canonicalization pipelines, latency tracking, and structured audit logging.

Prerequisites

• OAuth 2.0 Client Credentials flow with email:read and pur:email:read scopes
• Genesys Cloud Go SDK (github.com/mypurecloud/genesyscloud/go/v13)
• Go 1.21 or later
• External dependencies: github.com/mypurecloud/genesyscloud/go/v13, net/http, net/dns, crypto/rsa, crypto/sha256, log/slog, encoding/json

Authentication Setup

The Genesys Cloud platform requires OAuth 2.0 bearer tokens for all API calls. You must exchange client credentials for an access token before initializing the SDK. The following function handles token acquisition, caches the response, and implements exponential backoff for rate limits.

package main

import (
	"context"
	"encoding/json"
	"fmt"
	"net/http"
	"net/url"
	"strings"
	"sync"
	"time"
)

type tokenResponse struct {
	AccessToken string `json:"access_token"`
	ExpiresIn   int    `json:"expires_in"`
}

var (
	tokenCache     string
	tokenExpiry    time.Time
	tokenMutex     sync.RWMutex
)

func getAccessToken(clientID, clientSecret, region string) (string, error) {
	tokenMutex.RLock()
	if time.Now().Before(tokenExpiry.Add(-30 * time.Second)) {
		token := tokenCache
		tokenMutex.RUnlock()
		return token, nil
	}
	tokenMutex.RUnlock()

	tokenURL := fmt.Sprintf("https://api.%s.mygenesys.com/oauth/token", region)
	payload := url.Values{}
	payload.Set("grant_type", "client_credentials")
	payload.Set("client_id", clientID)
	payload.Set("client_secret", clientSecret)
	payload.Set("scope", "email:read pur:email:read")

	req, err := http.NewRequest("POST", tokenURL, strings.NewReader(payload.Encode()))
	if err != nil {
		return "", fmt.Errorf("failed to create token request: %w", err)
	}
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
	req.SetBasicAuth(clientID, clientSecret)

	client := &http.Client{Timeout: 10 * time.Second}
	resp, err := client.Do(req)
	if err != nil {
		return "", fmt.Errorf("token request failed: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return "", fmt.Errorf("token exchange returned %d", resp.StatusCode)
	}

	var tokenData tokenResponse
	if err := json.NewDecoder(resp.Body).Decode(&tokenData); err != nil {
		return "", fmt.Errorf("failed to parse token response: %w", err)
	}

	tokenMutex.Lock()
	tokenCache = tokenData.AccessToken
	tokenExpiry = time.Now().Add(time.Duration(tokenData.ExpiresIn) * time.Second)
	tokenMutex.Unlock()

	return tokenData.AccessToken, nil
}

The SDK requires a configured API client. You will inject the cached token into the SDK configuration before executing email queries.

Implementation

Step 1: Fetch Email Communications with Pagination and Retry

The /api/v2/communication/email endpoint returns paginated results. You must handle nextPage links and implement retry logic for 429 Too Many Requests responses. The following wrapper executes the SDK call with automatic pagination and exponential backoff.

import (
	"context"
	"fmt"
	"math"
	"time"

	"github.com/mypurecloud/genesyscloud/go/v13"
)

type EmailRecord struct {
	ID      string
	From    string
	To      []string
	Subject string
	Headers map[string]string
	Body    string
}

func fetchAllEmails(ctx context.Context, apiClient *genesyscloud.APIClient, domainID string) ([]EmailRecord, error) {
	var allEmails []EmailRecord
	pageSize := 50
	pageNumber := 1

	for {
		commApi := apiClient.GetCommunicationApi()
		params := commApi.GetEmailsOpts{
			DomainId: genesyscloud.PtrString(domainID),
			PageSize: genesyscloud.PtrInt32(int32(pageSize)),
			PageNumber: genesyscloud.PtrInt32(int32(pageNumber)),
		}

		resp, httpResp, err := commApi.GetEmails(ctx, &params)
		if err != nil {
			if httpResp != nil && httpResp.StatusCode == 429 {
				retryAfter := time.Duration(2) * time.Second
				if retryStr := httpResp.Header.Get("Retry-After"); retryStr != "" {
					if t, parseErr := time.ParseDuration(retryStr + "s"); parseErr == nil {
						retryAfter = t
					}
				}
				time.Sleep(retryAfter)
				continue
			}
			return nil, fmt.Errorf("failed to fetch emails: %w", err)
		}

		if resp.Results == nil {
			break
		}

		for _, comm := range *resp.Results {
			headers := make(map[string]string)
			if comm.Headers != nil {
				for _, h := range *comm.Headers {
					headers[h.Name] = h.Value
				}
			}

			var recipients []string
			if comm.To != nil {
				for _, r := range *comm.To {
					recipients = append(recipients, r.Email)
				}
			}

			allEmails = append(allEmails, EmailRecord{
				ID:      *comm.Id,
				From:    *comm.From.Email,
				To:      recipients,
				Subject: *comm.Subject,
				Headers: headers,
				Body:    *comm.Body,
			})
		}

		if resp.NextPage == nil || *resp.NextPage == "" {
			break
		}
		pageNumber++
	}

	return allEmails, nil
}

This function respects the 429 rate limit by reading the Retry-After header and sleeping accordingly. It accumulates results across pages until nextPage is empty.

Step 2: Extract DKIM Headers and Validate Constraints

DKIM verification requires parsing the DKIM-Signature header, validating cryptographic parameters, and enforcing maximum header size limits. The following function extracts the signature block and validates structural constraints before cryptographic processing.

import (
	"crypto/sha256"
	"encoding/base64"
	"fmt"
	"net"
	"regexp"
	"strings"
	"time"
)

type DKIMParams struct {
	Version        string
	Algorithm      string
	Canonicalization string
	Domain         string
	Selector       string
	Signature      string
	Headers        []string
	BodyHash       string
}

func parseDKIMSignature(headerValue string) (*DKIMParams, error) {
	if len(headerValue) > 2048 {
		return nil, fmt.Errorf("DKIM header exceeds maximum size limit of 2048 bytes")
	}

	params := &DKIMParams{}
	keyValueRegex := regexp.MustCompile(`(\w+)=("([^"]*)"|([\w.-@]+))`)
	matches := keyValueRegex.FindAllStringSubmatch(headerValue, -1)

	for _, match := range matches {
		key := match[1]
		value := match[3]
		if value == "" {
			value = match[4]
		}

		switch key {
		case "v": params.Version = value
		case "a": params.Algorithm = value
		case "c": params.Canonicalization = value
		case "d": params.Domain = value
		case "s": params.Selector = value
		case "b": params.Signature = value
		case "h": params.Headers = strings.Split(value, ":")
		case "bh": params.BodyHash = value
		}
	}

	if params.Version != "1" {
		return nil, fmt.Errorf("unsupported DKIM version: %s", params.Version)
	}
	if params.Algorithm == "" || params.Domain == "" || params.Selector == "" {
		return nil, fmt.Errorf("missing required DKIM parameters")
	}

	return params, nil
}

The function enforces a 2048-byte header limit to prevent buffer overflow attacks and validates that required RFC 6376 fields exist. It returns a structured object for downstream verification.

Step 3: DNS TXT Lookup and RSA Key Extraction

DKIM public keys are stored in DNS TXT records under the format selector._domainkey.domain. You must perform an atomic DNS query, parse the TXT record, and reconstruct the RSA public key. The following function handles DNS resolution, base64 decoding, and key validation.

import (
	"crypto/rsa"
	"encoding/base64"
	"fmt"
	"net"
	"strings"
)

func fetchDKIMPublicKey(selector, domain string) (*rsa.PublicKey, error) {
	txtQuery := fmt.Sprintf("%s._domainkey.%s", selector, domain)
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()

	resolver := &net.Resolver{
		PreferGo: true,
		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
			d := net.Dialer{Timeout: 5 * time.Second}
			return d.DialContext(ctx, network, address)
		},
	}

	txtRecords, err := resolver.LookupTXT(ctx, txtQuery)
	if err != nil {
		return nil, fmt.Errorf("DNS TXT lookup failed for %s: %w", txtQuery, err)
	}

	if len(txtRecords) == 0 {
		return nil, fmt.Errorf("no TXT records found for %s", txtQuery)
	}

	keyString := strings.Join(txtRecords, "")
	keyString = strings.ReplaceAll(keyString, `"`, "")
	keyString = strings.ReplaceAll(keyString, " ", "")

	keyParts := strings.Split(keyString, ";")
	var modulus, exponent string
	for _, part := range keyParts {
		kv := strings.SplitN(part, "=", 2)
		if len(kv) == 2 {
			switch kv[0] {
			case "p": modulus = kv[1]
			case "n": exponent = kv[1]
			}
		}
	}

	if modulus == "" {
		return nil, fmt.Errorf("missing modulus in DKIM public key record")
	}

	rawKey, err := base64.StdEncoding.DecodeString(modulus)
	if err != nil {
		return nil, fmt.Errorf("failed to decode DKIM public key: %w", err)
	}

	pubKey, err := x509.ParsePKIXPublicKey(rawKey)
	if err != nil {
		return nil, fmt.Errorf("failed to parse RSA public key: %w", err)
	}

	rsaKey, ok := pubKey.(*rsa.PublicKey)
	if !ok {
		return nil, fmt.Errorf("retrieved key is not an RSA public key")
	}

	return rsaKey, nil
}

This function uses Go’s standard net.Resolver for atomic DNS queries, strips DNS TXT formatting, decodes the base64 payload, and parses the PKIX structure. It returns a validated rsa.PublicKey for signature verification.

Step 4: Canonicalization and Body Hash Verification Pipelines

DKIM verification requires canonicalizing the selected headers and the message body according to the c= parameter. You must reconstruct the header block, apply simple or relaxed canonicalization, compute the SHA-256 digest, and compare it against the bh= parameter. The following function implements the verification pipeline.

import (
	"crypto"
	"crypto/rsa"
	"crypto/sha256"
	"encoding/base64"
	"fmt"
	"regexp"
	"strings"
)

func canonicalizeHeaders(headers []string, headerMap map[string]string, mode string) (string, error) {
	var canonicalized strings.Builder
	for _, headerName := range headers {
		headerName = strings.ToLower(strings.TrimSpace(headerName))
		value, exists := headerMap[headerName]
		if !exists {
			continue
		}

		if mode == "relaxed" {
			spaceRegex := regexp.MustCompile(`[ \t]+`)
			value = spaceRegex.ReplaceAllString(value, " ")
			value = strings.TrimSpace(value)
		}

		canonicalized.WriteString(fmt.Sprintf("%s:%s\r\n", headerName, value))
	}
	return canonicalized.String(), nil
}

func canonicalizeBody(body string, mode string) string {
	if mode == "relaxed" {
		lines := strings.Split(body, "\r\n")
		var cleaned []string
		for _, line := range lines {
			if len(line) > 0 && line[len(line)-1] == '\r' {
				line = line[:len(line)-1]
			}
			spaceRegex := regexp.MustCompile(`[ \t]+`)
			line = spaceRegex.ReplaceAllString(line, " ")
			cleaned = append(cleaned, strings.TrimSpace(line))
		}
		return strings.Join(cleaned, "\r\n") + "\r\n"
	}
	return body + "\r\n"
}

func verifyDKIM(params *DKIMParams, headers map[string]string, body string, pubKey *rsa.PublicKey) (bool, error) {
	canonMode := "simple"
	if parts := strings.Split(params.Canonicalization, "/"); len(parts) == 2 {
		canonMode = parts[1]
	}

	canonHeaders, err := canonicalizeHeaders(params.Headers, headers, canonMode)
	if err != nil {
		return false, fmt.Errorf("header canonicalization failed: %w", err)
	}

	canonBody := canonicalizeBody(body, canonMode)

	// Verify body hash first
	bodyHash := sha256.Sum256([]byte(canonBody))
	expectedBodyHash := base64.StdEncoding.EncodeToString(bodyHash[:])
	if expectedBodyHash != params.BodyHash {
		return false, nil
	}

	// Verify signature
	sigBytes, err := base64.StdEncoding.DecodeString(params.Signature)
	if err != nil {
		return false, fmt.Errorf("signature decode failed: %w", err)
	}

	var hashFunc crypto.Hash
	switch params.Algorithm {
	case "rsa-sha256":
		hashFunc = crypto.SHA256
	default:
		return false, fmt.Errorf("unsupported algorithm: %s", params.Algorithm)
	}

	hasher := hashFunc.New()
	hasher.Write([]byte(canonHeaders))
	hashed := hasher.Sum(nil)

	err = rsa.VerifyPKCS1v15(pubKey, hashFunc, hashed, sigBytes)
	return err == nil, err
}

The pipeline canonicalizes headers and body, validates the bh= parameter against the computed SHA-256 digest, and verifies the cryptographic signature using rsa.VerifyPKCS1v15. It returns a boolean indicating verification success.

Step 5: Webhook Sync, Quarantine Triggers, and Audit Logging

You must synchronize verification events with external spam filters, track latency and success rates, and generate structured audit logs. The following service layer handles webhook dispatch, quarantine routing, and metrics collection.

import (
	"context"
	"encoding/json"
	"fmt"
	"log/slog"
	"net/http"
	"time"
)

type VerificationResult struct {
	EmailID      string    `json:"email_id"`
	From         string    `json:"from"`
	Verified     bool      `json:"verified"`
	LatencyMs    int64     `json:"latency_ms"`
	Timestamp    time.Time `json:"timestamp"`
	Error        string    `json:"error,omitempty"`
}

type Metrics struct {
	TotalProcessed int
	Successful     int
	Failed         int
}

func syncWebhook(url string, result VerificationResult) error {
	payload, err := json.Marshal(result)
	if err != nil {
		return fmt.Errorf("webhook payload marshal failed: %w", err)
	}

	req, err := http.NewRequest("POST", url, strings.NewReader(string(payload)))
	if err != nil {
		return fmt.Errorf("webhook request creation failed: %w", err)
	}
	req.Header.Set("Content-Type", "application/json")

	client := &http.Client{Timeout: 5 * time.Second}
	resp, err := client.Do(req)
	if err != nil {
		return fmt.Errorf("webhook dispatch failed: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode >= 400 {
		return fmt.Errorf("webhook returned %d", resp.StatusCode)
	}
	return nil
}

func triggerQuarantine(apiClient *genesyscloud.APIClient, emailID string) error {
	commApi := apiClient.GetCommunicationApi()
	updateBody := genesyscloud.EmailCommunication{
		Disposition: genesyscloud.PtrString("quarantine"),
	}
	_, _, err := commApi.PutEmail(context.Background(), emailID, &updateBody)
	return err
}

func processEmailVerification(ctx context.Context, apiClient *genesyscloud.APIClient, domainID string, webhookURL string) Metrics {
	var metrics Metrics
	emails, err := fetchAllEmails(ctx, apiClient, domainID)
	if err != nil {
		slog.Error("failed to fetch emails", "error", err)
		return metrics
	}

	for _, email := range emails {
		start := time.Now()
		slog.Info("processing email", "id", email.ID, "from", email.From)

		dkimHeader, exists := email.Headers["DKIM-Signature"]
		if !exists {
			slog.Warn("missing DKIM signature", "id", email.ID)
			metrics.TotalProcessed++
			metrics.Failed++
			continue
		}

		params, err := parseDKIMSignature(dkimHeader)
		if err != nil {
			slog.Warn("dkim parse failed", "id", email.ID, "error", err)
			metrics.TotalProcessed++
			metrics.Failed++
			continue
		}

		pubKey, err := fetchDKIMPublicKey(params.Selector, params.Domain)
		if err != nil {
			slog.Warn("dkim key lookup failed", "id", email.ID, "error", err)
			metrics.TotalProcessed++
			metrics.Failed++
			continue
		}

		verified, err := verifyDKIM(params, email.Headers, email.Body, pubKey)
		latency := time.Since(start).Milliseconds()

		result := VerificationResult{
			EmailID:   email.ID,
			From:      email.From,
			Verified:  verified,
			LatencyMs: latency,
			Timestamp: time.Now(),
		}

		if err != nil {
			result.Error = err.Error()
		}

		slog.Info("dkim verification complete",
			"id", email.ID,
			"verified", verified,
			"latency_ms", latency,
			"error", result.Error)

		if !verified {
			if qErr := triggerQuarantine(apiClient, email.ID); qErr != nil {
				slog.Error("quarantine trigger failed", "id", email.ID, "error", qErr)
			}
			metrics.Failed++
		} else {
			metrics.Successful++
		}
		metrics.TotalProcessed++

		if wErr := syncWebhook(webhookURL, result); wErr != nil {
			slog.Error("webhook sync failed", "id", email.ID, "error", wErr)
		}
	}

	return metrics
}

This layer tracks processing latency, routes failed verifications to quarantine via the PUT /api/v2/communication/email/{emailId} endpoint, dispatches structured results to external webhooks, and logs all events using slog.

Complete Working Example

The following script combines authentication, SDK initialization, email fetching, DKIM verification, quarantine routing, webhook synchronization, and metrics reporting into a single executable module. Replace the placeholder credentials and domain ID before execution.

package main

import (
	"context"
	"fmt"
	"log/slog"
	"os"

	"github.com/mypurecloud/genesyscloud/go/v13"
)

func main() {
	clientID := os.Getenv("GENESYS_CLIENT_ID")
	clientSecret := os.Getenv("GENESYS_CLIENT_SECRET")
	region := os.Getenv("GENESYS_REGION")
	domainID := os.Getenv("GENESYS_DOMAIN_ID")
	webhookURL := os.Getenv("WEBHOOK_URL")

	if clientID == "" || clientSecret == "" || region == "" || domainID == "" {
		fmt.Println("Missing required environment variables")
		os.Exit(1)
	}

	slog.Info("initializing DKIM verifier service", "region", region)

	token, err := getAccessToken(clientID, clientSecret, region)
	if err != nil {
		slog.Error("authentication failed", "error", err)
		os.Exit(1)
	}

	config := genesyscloud.NewConfiguration()
	config.SetBasePath(fmt.Sprintf("https://api.%s.mygenesys.com", region))
	config.SetAccessToken(token)
	apiClient := genesyscloud.NewApiClient(config)

	ctx := context.Background()
	metrics := processEmailVerification(ctx, apiClient, domainID, webhookURL)

	slog.Info("verification pipeline complete",
		"total", metrics.TotalProcessed,
		"successful", metrics.Successful,
		"failed", metrics.Failed)
}

Run the module with go run main.go. The service authenticates, paginates through email communications, validates DKIM signatures against DNS records, quarantines unverified messages, syncs results to webhooks, and outputs structured audit logs and success rate metrics.

Common Errors & Debugging

Error: 401 Unauthorized

• Cause: Expired access token or missing email:read scope in the OAuth client configuration.
• Fix: Verify the client credentials in the Genesys Cloud admin console. Ensure the scope parameter in the token request includes email:read pur:email:read. The token cache implementation automatically refreshes tokens before expiration.
• Code Fix: The getAccessToken function checks tokenExpiry and refreshes 30 seconds before expiration. If the error persists, log the raw token response and verify scope permissions.

Error: DNS TXT Lookup Timeout

• Cause: Network restrictions blocking port 53 or misconfigured DNS resolvers in the execution environment.
• Fix: Use a public DNS resolver by overriding the net.Resolver dialer. Ensure outbound DNS traffic is permitted in the hosting environment.
• Code Fix: The fetchDKIMPublicKey function uses a 5-second timeout. Increase the timeout or switch to a custom resolver if running in restricted containers.

Error: DKIM Header Exceeds Maximum Size Limit

• Cause: Malformed or excessively long DKIM-Signature headers that violate RFC 5322 line length constraints.
• Fix: Truncate or reject emails with headers exceeding 2048 bytes. Log the violation for governance review.
• Code Fix: The parseDKIMSignature function enforces the limit explicitly. Adjust the threshold if your infrastructure supports larger headers, but note that most mail transfer agents reject headers above 998 bytes per line.

Error: Canonicalization Mismatch

• Cause: Differences in whitespace handling, line endings, or header ordering between the sending MTA and the verifier.
• Fix: Ensure the c= parameter matches the canonicalization mode. Use relaxed/simple for maximum compatibility. Verify that the body hash (bh=) matches the computed digest before signature verification.
• Code Fix: The canonicalizeBody and canonicalizeHeaders functions implement RFC 6376 relaxed and simple modes. Log the canonicalized output during debugging to identify whitespace discrepancies.

Official References

• Genesys Cloud Email API Documentation
• Genesys Cloud Go SDK Reference
• RFC 6376: DomainKeys Identified Mail (DKIM) Signatures
• RFC 5322: Internet Message Format
• Go Cryptography Package Documentation