Update

vendor/tailscale.com/tempfork/acme/acme.go

@@ -270,10 +270,7 @@ func (c *Client) FetchRenewalInfo(ctx context.Context, leaf []byte) (*RenewalInf
 		return nil, fmt.Errorf("parsing leaf certificate: %w", err)
 	}
 
-	renewalURL, err := c.getRenewalURL(parsedLeaf)
-	if err != nil {
-		return nil, fmt.Errorf("generating renewal info URL: %w", err)
-	}
+	renewalURL := c.getRenewalURL(parsedLeaf)
 
 	res, err := c.get(ctx, renewalURL, wantStatus(http.StatusOK))
 	if err != nil {
@@ -288,16 +285,20 @@ func (c *Client) FetchRenewalInfo(ctx context.Context, leaf []byte) (*RenewalInf
 	return &info, nil
 }
 
-func (c *Client) getRenewalURL(cert *x509.Certificate) (string, error) {
+func (c *Client) getRenewalURL(cert *x509.Certificate) string {
 	// See https://www.ietf.org/archive/id/draft-ietf-acme-ari-04.html#name-the-renewalinfo-resource
 	// for how the request URL is built.
 	url := c.dir.RenewalInfoURL
 	if !strings.HasSuffix(url, "/") {
 		url += "/"
 	}
+	return url + certRenewalIdentifier(cert)
+}
+
+func certRenewalIdentifier(cert *x509.Certificate) string {
 	aki := base64.RawURLEncoding.EncodeToString(cert.AuthorityKeyId)
 	serial := base64.RawURLEncoding.EncodeToString(cert.SerialNumber.Bytes())
-	return fmt.Sprintf("%s%s.%s", url, aki, serial), nil
+	return aki + "." + serial
 }
 
 // AcceptTOS always returns true to indicate the acceptance of a CA's Terms of Service

vendor/tailscale.com/tempfork/acme/rfc8555.go

@@ -7,6 +7,7 @@ package acme
 import (
 	"context"
 	"crypto"
+	"crypto/x509"
 	"encoding/base64"
 	"encoding/json"
 	"encoding/pem"
@@ -205,6 +206,7 @@ func (c *Client) AuthorizeOrder(ctx context.Context, id []AuthzID, opt ...OrderO
 		Identifiers []wireAuthzID `json:"identifiers"`
 		NotBefore   string        `json:"notBefore,omitempty"`
 		NotAfter    string        `json:"notAfter,omitempty"`
+		Replaces    string        `json:"replaces,omitempty"`
 	}{}
 	for _, v := range id {
 		req.Identifiers = append(req.Identifiers, wireAuthzID{
@@ -218,6 +220,14 @@ func (c *Client) AuthorizeOrder(ctx context.Context, id []AuthzID, opt ...OrderO
 			req.NotBefore = time.Time(o).Format(time.RFC3339)
 		case orderNotAfterOpt:
 			req.NotAfter = time.Time(o).Format(time.RFC3339)
+		case orderReplacesCert:
+			req.Replaces = certRenewalIdentifier(o.cert)
+		case orderReplacesCertDER:
+			cert, err := x509.ParseCertificate(o)
+			if err != nil {
+				return nil, fmt.Errorf("failed to parse certificate being replaced: %w", err)
+			}
+			req.Replaces = certRenewalIdentifier(cert)
 		default:
 			// Package's fault if we let this happen.
 			panic(fmt.Sprintf("unsupported order option type %T", o))

vendor/tailscale.com/tempfork/acme/types.go

@@ -391,6 +391,30 @@ type orderNotAfterOpt time.Time
 
 func (orderNotAfterOpt) privateOrderOpt() {}
 
+// WithOrderReplacesCert indicates that this Order is for a replacement of an
+// existing certificate.
+// See https://datatracker.ietf.org/doc/html/draft-ietf-acme-ari-03#section-5
+func WithOrderReplacesCert(cert *x509.Certificate) OrderOption {
+	return orderReplacesCert{cert}
+}
+
+type orderReplacesCert struct {
+	cert *x509.Certificate
+}
+
+func (orderReplacesCert) privateOrderOpt() {}
+
+// WithOrderReplacesCertDER indicates that this Order is for a replacement of
+// an existing DER-encoded certificate.
+// See https://datatracker.ietf.org/doc/html/draft-ietf-acme-ari-03#section-5
+func WithOrderReplacesCertDER(der []byte) OrderOption {
+	return orderReplacesCertDER(der)
+}
+
+type orderReplacesCertDER []byte
+
+func (orderReplacesCertDER) privateOrderOpt() {}
+
 // Authorization encodes an authorization response.
 type Authorization struct {
 	// URI uniquely identifies a authorization.