Add "dns-account-01" support from draft-ietf-acme-scoped-dns-challeng…

…es (#435)

This change implements the `dns-account-01` ACME challenge as specified
in
[draft-ietf-acme-scoped-dns-challenges](https://datatracker.ietf.org/doc/draft-ietf-acme-scoped-dns-challenges/).

The relevant [validation label
computation](https://github.com/aaomidi/draft-ietf-acme-scoped-dns-challenges/blob/0058e0800056698fb37f3b2cb31a727c826675fb/draft-ietf-acme-scoped-dns-challenges.mkd#dns-account-01-challenge)
is:
```plain
"_" || base32(SHA-256(<ACCOUNT_RESOURCE_URL>)[0:10]) || "._acme-" || <SCOPE> || "-challenge"
```
where SCOPE is one of { `host`, `wildcard` }. A SCOPE of { `domain` } is
unimplemented.

This implementation is interoperable with the
https://github.com/eggsampler/acme changes in
eggsampler/acme#21 and passes the
`TestWildcardDNSAccount` test.

Solves #425.

acme/common.go

@@ -15,9 +15,10 @@ const (
 	IdentifierDNS = "dns"
 	IdentifierIP  = "ip"
 
-	ChallengeHTTP01    = "http-01"
-	ChallengeTLSALPN01 = "tls-alpn-01"
-	ChallengeDNS01     = "dns-01"
+	ChallengeHTTP01       = "http-01"
+	ChallengeTLSALPN01    = "tls-alpn-01"
+	ChallengeDNS01        = "dns-01"
+	ChallengeDNSAccount01 = "dns-account-01"
 
 	HTTP01BaseURL = ".well-known/acme-challenge/"
 

va/va.go

@@ -7,6 +7,7 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/asn1"
+	"encoding/base32"
 	"encoding/base64"
 	"fmt"
 	"io"
@@ -92,6 +93,8 @@ type vaTask struct {
 	Identifier acme.Identifier
 	Challenge  *core.Challenge
 	Account    *core.Account
+	AccountURL string
+	Wildcard   bool
 }
 
 type VAImpl struct {
@@ -157,11 +160,13 @@ func New(
 	return va
 }
 
-func (va VAImpl) ValidateChallenge(ident acme.Identifier, chal *core.Challenge, acct *core.Account) {
+func (va VAImpl) ValidateChallenge(ident acme.Identifier, chal *core.Challenge, acct *core.Account, acctURL string, wildcard bool) {
 	task := &vaTask{
 		Identifier: ident,
 		Challenge:  chal,
 		Account:    acct,
+		AccountURL: acctURL,
+		Wildcard:   wildcard,
 	}
 	// Submit the task for validation
 	va.tasks <- task
@@ -299,6 +304,8 @@ func (va VAImpl) performValidation(task *vaTask, results chan<- *core.Validation
 		results <- va.validateTLSALPN01(task)
 	case acme.ChallengeDNS01:
 		results <- va.validateDNS01(task)
+	case acme.ChallengeDNSAccount01:
+		results <- va.validateDNSAccount01(task)
 	default:
 		va.log.Printf("Error: performValidation(): Invalid challenge type: %q", task.Challenge.Type)
 	}
@@ -342,6 +349,49 @@ func (va VAImpl) validateDNS01(task *vaTask) *core.ValidationRecord {
 	return result
 }
 
+func (va VAImpl) validateDNSAccount01(task *vaTask) *core.ValidationRecord {
+	acctHash := sha256.Sum256([]byte(task.AccountURL))
+	acctLabel := strings.ToLower(base32.StdEncoding.EncodeToString(acctHash[0:10]))
+	scope := "host"
+	if task.Wildcard {
+		scope = "wildcard"
+	}
+	challengeSubdomain := fmt.Sprintf("_%s._acme-%s-challenge.%s", acctLabel, scope, task.Identifier.Value)
+
+	result := &core.ValidationRecord{
+		URL:         challengeSubdomain,
+		ValidatedAt: time.Now(),
+	}
+
+	txts, err := va.getTXTEntry(challengeSubdomain)
+	if err != nil {
+		result.Error = acme.UnauthorizedProblem(fmt.Sprintf("Error retrieving TXT records for DNS-ACCOUNT-01 challenge (%q)", err))
+		return result
+	}
+
+	if len(txts) == 0 {
+		msg := "No TXT records found for DNS-ACCOUNT-01 challenge"
+		result.Error = acme.UnauthorizedProblem(msg)
+		return result
+	}
+
+	task.Challenge.RLock()
+	expectedKeyAuthorization := task.Challenge.ExpectedKeyAuthorization(task.Account.Key)
+	h := sha256.Sum256([]byte(expectedKeyAuthorization))
+	task.Challenge.RUnlock()
+	authorizedKeysDigest := base64.RawURLEncoding.EncodeToString(h[:])
+
+	for _, element := range txts {
+		if subtle.ConstantTimeCompare([]byte(element), []byte(authorizedKeysDigest)) == 1 {
+			return result
+		}
+	}
+
+	msg := "Correct value not found for DNS-ACCOUNT-01 challenge"
+	result.Error = acme.UnauthorizedProblem(msg)
+	return result
+}
+
 func (va VAImpl) validateTLSALPN01(task *vaTask) *core.ValidationRecord {
 	portString := strconv.Itoa(va.tlsPort)
 

wfe/wfe.go

@@ -1597,30 +1597,27 @@ func (wfe *WebFrontEndImpl) makeChallenge(
 func (wfe *WebFrontEndImpl) makeChallenges(authz *core.Authorization, request *http.Request) error {
 	var chals []*core.Challenge
 
-	// Authorizations for a wildcard identifier only get a DNS-01 challenges to
-	// match Boulder/Let's Encrypt wildcard issuance policy
+	// Determine which challenge types are enabled for this identifier
+	var enabledChallenges []string
 	if strings.HasPrefix(authz.Identifier.Value, "*.") {
-		chal, err := wfe.makeChallenge(acme.ChallengeDNS01, authz, request)
-		if err != nil {
-			return err
-		}
-		chals = []*core.Challenge{chal}
+		// Authorizations for a wildcard identifier get DNS-based challenges to
+		// match Boulder/Let's Encrypt wildcard issuance policy
+		enabledChallenges = []string{acme.ChallengeDNS01, acme.ChallengeDNSAccount01}
 	} else {
 		// IP addresses get HTTP-01 and TLS-ALPN challenges
-		var enabledChallenges []string
 		if authz.Identifier.Type == acme.IdentifierIP {
 			enabledChallenges = []string{acme.ChallengeHTTP01, acme.ChallengeTLSALPN01}
 		} else {
 			// Non-wildcard, non-IP identifier authorizations get all of the enabled challenge types
-			enabledChallenges = []string{acme.ChallengeHTTP01, acme.ChallengeTLSALPN01, acme.ChallengeDNS01}
+			enabledChallenges = []string{acme.ChallengeHTTP01, acme.ChallengeTLSALPN01, acme.ChallengeDNS01, acme.ChallengeDNSAccount01}
 		}
-		for _, chalType := range enabledChallenges {
-			chal, err := wfe.makeChallenge(chalType, authz, request)
-			if err != nil {
-				return err
-			}
-			chals = append(chals, chal)
+	}
+	for _, chalType := range enabledChallenges {
+		chal, err := wfe.makeChallenge(chalType, authz, request)
+		if err != nil {
+			return err
 		}
+		chals = append(chals, chal)
 	}
 
 	// Lock the authorization for writing to update the challenges
@@ -2377,8 +2374,12 @@ func (wfe *WebFrontEndImpl) updateChallenge(
 
 	// If the identifier value is for a wildcard domain then strip the wildcard
 	// prefix before dispatching the validation to ensure the base domain is
-	// validated.
-	ident.Value = strings.TrimPrefix(ident.Value, "*.")
+	// validated. Set a flag to indicate validation scope.
+	wildcard := false
+	if strings.HasPrefix(ident.Value, "*.") {
+		ident.Value = strings.TrimPrefix(ident.Value, "*.")
+		wildcard = true
+	}
 
 	// Confirm challenge status again and update it immediately before sending it to the VA
 	prob = nil
@@ -2395,8 +2396,11 @@ func (wfe *WebFrontEndImpl) updateChallenge(
 		return
 	}
 
+	// Reconstruct account URL for use in scoped validation methods
+	acctURL := wfe.relativeEndpoint(request, fmt.Sprintf("%s%s", acctPath, existingAcct.ID))
+
 	// Submit a validation job to the VA, this will be processed asynchronously
-	wfe.va.ValidateChallenge(ident, existingChal, existingAcct)
+	wfe.va.ValidateChallenge(ident, existingChal, existingAcct, acctURL, wildcard)
 
 	// Lock the challenge for reading in order to write the response
 	existingChal.RLock()