diff --git a/acme/acme.go b/acme/acme.go
index 6be00d1d0c..7a51284f91 100644
--- a/acme/acme.go
+++ b/acme/acme.go
@@ -35,6 +35,7 @@ import (
 	"errors"
 	"fmt"
 	"math/big"
+	"net"
 	"net/http"
 	"strings"
 	"sync"
@@ -589,8 +590,9 @@ func (c *Client) TLSSNI02ChallengeCert(token string, opt ...CertOption) (tls.Cer
 
 // TLSALPN01ChallengeCert creates a certificate for TLS-ALPN-01 challenge response.
 // Servers can present the certificate to validate the challenge and prove control
-// over a domain name. For more details on TLS-ALPN-01 see
-// https://tools.ietf.org/html/draft-shoemaker-acme-tls-alpn-00#section-3
+// over an identifier (either a DNS name or the textual form of an IPv4 or IPv6
+// address). For more details on TLS-ALPN-01 see
+// https://www.rfc-editor.org/rfc/rfc8737 and https://www.rfc-editor.org/rfc/rfc8738
 //
 // The token argument is a Challenge.Token value.
 // If a WithKey option is provided, its private part signs the returned cert,
@@ -598,9 +600,13 @@ func (c *Client) TLSSNI02ChallengeCert(token string, opt ...CertOption) (tls.Cer
 // If no WithKey option is provided, a new ECDSA key is generated using P-256 curve.
 //
 // The returned certificate is valid for the next 24 hours and must be presented only when
-// the server name in the TLS ClientHello matches the domain, and the special acme-tls/1 ALPN protocol
+// the server name in the TLS ClientHello matches the identifier, and the special acme-tls/1 ALPN protocol
 // has been specified.
-func (c *Client) TLSALPN01ChallengeCert(token, domain string, opt ...CertOption) (cert tls.Certificate, err error) {
+//
+// Validation requests for IP address identifiers will use the reverse DNS form in the server name
+// in the TLS ClientHello since the SNI extension is not supported for IP addresses.
+// See RFC 8738 Section 6 for more information.
+func (c *Client) TLSALPN01ChallengeCert(token, identifier string, opt ...CertOption) (cert tls.Certificate, err error) {
 	ka, err := keyAuth(c.Key.Public(), token)
 	if err != nil {
 		return tls.Certificate{}, err
@@ -630,7 +636,7 @@ func (c *Client) TLSALPN01ChallengeCert(token, domain string, opt ...CertOption)
 	}
 	tmpl.ExtraExtensions = append(tmpl.ExtraExtensions, acmeExtension)
 	newOpt = append(newOpt, WithTemplate(tmpl))
-	return tlsChallengeCert([]string{domain}, newOpt)
+	return tlsChallengeCert(identifier, newOpt)
 }
 
 // popNonce returns a nonce value previously stored with c.addNonce
@@ -749,10 +755,14 @@ func defaultTLSChallengeCertTemplate() *x509.Certificate {
 }
 
 // tlsChallengeCert creates a temporary certificate for TLS-ALPN challenges
-// with the given SANs and auto-generated public/private key pair.
-// The Subject Common Name is set to the first SAN to aid debugging.
+// for the given identifier, using an auto-generated public/private key pair.
+//
+// If the provided identifier is a domain name, it will be used as a DNS type SAN and for the
+// subject common name. If the provided identifier is an IP address it will be used as an IP type
+// SAN.
+//
 // To create a cert with a custom key pair, specify WithKey option.
-func tlsChallengeCert(san []string, opt []CertOption) (tls.Certificate, error) {
+func tlsChallengeCert(identifier string, opt []CertOption) (tls.Certificate, error) {
 	var key crypto.Signer
 	tmpl := defaultTLSChallengeCertTemplate()
 	for _, o := range opt {
@@ -776,9 +786,12 @@ func tlsChallengeCert(san []string, opt []CertOption) (tls.Certificate, error) {
 			return tls.Certificate{}, err
 		}
 	}
-	tmpl.DNSNames = san
-	if len(san) > 0 {
-		tmpl.Subject.CommonName = san[0]
+
+	if ip := net.ParseIP(identifier); ip != nil {
+		tmpl.IPAddresses = []net.IP{ip}
+	} else {
+		tmpl.DNSNames = []string{identifier}
+		tmpl.Subject.CommonName = identifier
 	}
 
 	der, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, key.Public(), key)
diff --git a/acme/pebble_test.go b/acme/pebble_test.go
new file mode 100644
index 0000000000..63e6cc5836
--- /dev/null
+++ b/acme/pebble_test.go
@@ -0,0 +1,789 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package acme_test
+
+import (
+	"bytes"
+	"context"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/tls"
+	"crypto/x509"
+	"encoding/json"
+	"encoding/pem"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"golang.org/x/crypto/acme"
+)
+
+const (
+	// pebbleModVersion is the module version used for Pebble and Pebble's
+	// challenge test server. It is ignored if `-pebble-local-dir` is provided.
+	pebbleModVersion = "v2.7.0"
+	// startingPort is the first port number used for binding interface
+	// addresses. Each call to takeNextPort() will increment a port number
+	// starting at this value.
+	startingPort = 5555
+)
+
+var (
+	pebbleLocalDir = flag.String(
+		"pebble-local-dir",
+		"",
+		"Local Pebble to use, instead of fetching from source",
+	)
+	nextPort atomic.Uint32
+)
+
+func init() {
+	nextPort.Store(startingPort)
+}
+
+func TestWithPebble(t *testing.T) {
+	// We want to use process groups w/ syscall.Kill, and the acme package
+	// is very platform-agnostic, so skip on non-Linux.
+	if runtime.GOOS != "linux" {
+		t.Skip("skipping pebble tests on non-linux OS")
+	}
+
+	if testing.Short() {
+		t.Skip("skipping pebble tests in short mode")
+	}
+
+	tests := []struct {
+		name     string
+		challSrv func(*environment) (challengeServer, string)
+	}{
+		{
+			name: "TLSALPN01-Issuance",
+			challSrv: func(env *environment) (challengeServer, string) {
+				bindAddr := fmt.Sprintf(":%d", env.config.TLSPort)
+				return newChallTLSServer(bindAddr), bindAddr
+			},
+		},
+
+		{
+			name: "HTTP01-Issuance",
+			challSrv: func(env *environment) (challengeServer, string) {
+				bindAddr := fmt.Sprintf(":%d", env.config.HTTPPort)
+				return newChallHTTPServer(bindAddr), bindAddr
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			env := startPebbleEnvironment(t, nil)
+			challSrv, challSrvAddr := tt.challSrv(&env)
+			challSrv.Run()
+
+			t.Cleanup(func() {
+				challSrv.Shutdown()
+			})
+
+			waitForServer(t, challSrvAddr)
+			testIssuance(t, &env, challSrv)
+		})
+	}
+}
+
+// challengeServer abstracts over the details of running a challenge response
+// server for some supported acme.Challenge type. Responses are provisioned
+// during the test issuance process to be presented to the ACME server's
+// validation authority.
+type challengeServer interface {
+	Run()
+	Shutdown() error
+	Supported(chal *acme.Challenge) bool
+	Provision(client *acme.Client, ident acme.AuthzID, chal *acme.Challenge) error
+}
+
+// challTLSServer is a simple challenge response server that listens for TLS
+// connections on a specific port and if they are TLS-ALPN-01 challenge
+// requests, completes the handshake using the configured challenge response
+// certificate for the SNI value provided.
+type challTLSServer struct {
+	*http.Server
+	// mu protects challCerts.
+	mu sync.RWMutex
+	// challCerts is a map from SNI domain name to challenge response certificate.
+	challCerts map[string]*tls.Certificate
+}
+
+// https://datatracker.ietf.org/doc/html/rfc8737#section-4
+const acmeTLSAlpnProtocol = "acme-tls/1"
+
+func newChallTLSServer(address string) *challTLSServer {
+	challServer := &challTLSServer{Server: &http.Server{
+		Addr:         address,
+		ReadTimeout:  5 * time.Second,
+		WriteTimeout: 5 * time.Second,
+	}, challCerts: make(map[string]*tls.Certificate)}
+
+	// Configure the server to support the TLS-ALPN-01 challenge protocol
+	// and to use a callback for selecting the handshake certificate.
+	challServer.Server.TLSConfig = &tls.Config{
+		NextProtos:     []string{acmeTLSAlpnProtocol},
+		GetCertificate: challServer.getCertificate,
+	}
+
+	return challServer
+}
+
+func (c *challTLSServer) Shutdown() error {
+	log.Printf("challTLSServer: shutting down")
+	ctx, cancel := context.WithTimeout(context.Background(), 10)
+	defer cancel()
+	return c.Server.Shutdown(ctx)
+}
+
+func (c *challTLSServer) Run() {
+	go func() {
+		// Note: certFile and keyFile are empty because our config uses a
+		// GetCertificate callback.
+		if err := c.Server.ListenAndServeTLS("", ""); err != nil {
+			if !errors.Is(err, http.ErrServerClosed) {
+				log.Printf("challTLSServer error: %v", err)
+			}
+		}
+	}()
+}
+
+func (c *challTLSServer) Supported(chal *acme.Challenge) bool {
+	return chal.Type == "tls-alpn-01"
+}
+
+func (c *challTLSServer) Provision(client *acme.Client, ident acme.AuthzID, chal *acme.Challenge) error {
+	respCert, err := client.TLSALPN01ChallengeCert(chal.Token, ident.Value)
+	if err != nil {
+		return fmt.Errorf("challTLSServer: failed to generate challlenge response cert for %s: %w",
+			ident.Value, err)
+	}
+
+	log.Printf("challTLSServer: setting challenge response certificate for %s", ident.Value)
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.challCerts[ident.Value] = &respCert
+
+	return nil
+}
+
+func (c *challTLSServer) getCertificate(clientHello *tls.ClientHelloInfo) (*tls.Certificate, error) {
+	// Verify the request looks like a TLS-ALPN-01 challenge request.
+	if len(clientHello.SupportedProtos) != 1 || clientHello.SupportedProtos[0] != acmeTLSAlpnProtocol {
+		return nil, fmt.Errorf(
+			"challTLSServer: non-TLS-ALPN-01 challenge request received with SupportedProtos: %s",
+			clientHello.SupportedProtos)
+	}
+
+	serverName := clientHello.ServerName
+
+	// TLS-ALPN-01 challenge requests for IP addresses are encoded in the SNI
+	// using the reverse-DNS notation. See RFC 8738 Section 6:
+	//   https://www.rfc-editor.org/rfc/rfc8738.html#section-6
+	if strings.HasSuffix(serverName, ".in-addr.arpa") {
+		serverName = strings.TrimSuffix(serverName, ".in-addr.arpa")
+		parts := strings.Split(serverName, ".")
+		for i, j := 0, len(parts)-1; i < j; i, j = i+1, j-1 {
+			parts[i], parts[j] = parts[j], parts[i]
+		}
+		serverName = strings.Join(parts, ".")
+	}
+
+	log.Printf("challTLSServer: selecting certificate for request from %s for %s",
+		clientHello.Conn.RemoteAddr(), serverName)
+
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+	cert := c.challCerts[serverName]
+	if cert == nil {
+		return nil, fmt.Errorf("challTLSServer: no challenge response certificate configured for %s", serverName)
+	}
+
+	return cert, nil
+}
+
+// challHTTPServer is a simple challenge response server that listens for HTTP
+// connections on a specific port and if they are HTTP-01 challenge requests,
+// serves the challenge response key authorization.
+type challHTTPServer struct {
+	*http.Server
+	// mu protects challMap
+	mu sync.RWMutex
+	// challMap is a mapping from request path to response body.
+	challMap map[string]string
+}
+
+func newChallHTTPServer(address string) *challHTTPServer {
+	challServer := &challHTTPServer{
+		Server: &http.Server{
+			Addr:         address,
+			ReadTimeout:  5 * time.Second,
+			WriteTimeout: 5 * time.Second,
+		},
+		challMap: make(map[string]string),
+	}
+
+	challServer.Server.Handler = challServer
+
+	return challServer
+}
+
+func (c *challHTTPServer) Supported(chal *acme.Challenge) bool {
+	return chal.Type == "http-01"
+}
+
+func (c *challHTTPServer) Provision(client *acme.Client, ident acme.AuthzID, chall *acme.Challenge) error {
+	path := client.HTTP01ChallengePath(chall.Token)
+	body, err := client.HTTP01ChallengeResponse(chall.Token)
+	if err != nil {
+		return fmt.Errorf("failed to generate HTTP-01 challenge response for %v challenge %s token %s: %w",
+			ident, chall.URI, chall.Token, err)
+	}
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	log.Printf("challHTTPServer: setting challenge response for %s", path)
+	c.challMap[path] = body
+
+	return nil
+}
+
+func (c *challHTTPServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	log.Printf("challHTTPServer: handling %s to %s from %s", r.Method, r.URL.Path, r.RemoteAddr)
+	if r.Method != http.MethodGet {
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+	response, exists := c.challMap[r.URL.Path]
+
+	if !exists {
+		http.NotFound(w, r)
+		return
+	}
+
+	w.Header().Set("Content-Type", "text/plain")
+	w.Write([]byte(response))
+}
+
+func (c *challHTTPServer) Shutdown() error {
+	log.Printf("challHTTPServer: shutting down")
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+	return c.Server.Shutdown(ctx)
+}
+
+func (c *challHTTPServer) Run() {
+	go func() {
+		if err := c.Server.ListenAndServe(); err != nil {
+			if !errors.Is(err, http.ErrServerClosed) {
+				log.Printf("challHTTPServer error: %v", err)
+			}
+		}
+	}()
+}
+
+func testIssuance(t *testing.T, env *environment, challSrv challengeServer) {
+	t.Helper()
+
+	// Bound the total issuance process by a timeout of 60 seconds.
+	ctx, cancel := context.WithTimeout(context.Background(), 60*time.Second)
+	defer cancel()
+
+	// Create a new ACME account.
+	client := env.client
+	acct, err := client.Register(ctx, &acme.Account{}, acme.AcceptTOS)
+	if err != nil {
+		t.Fatalf("failed to register account: %v", err)
+	}
+	if acct.Status != acme.StatusValid {
+		t.Fatalf("expected new account status to be valid, got %v", acct.Status)
+	}
+	log.Printf("registered account: %s", acct.URI)
+
+	// Create a new order for some example identifiers
+	identifiers := []acme.AuthzID{
+		{
+			Type:  "dns",
+			Value: "example.com",
+		},
+		{
+			Type:  "dns",
+			Value: "www.example.com",
+		},
+		{
+			Type:  "ip",
+			Value: "127.0.0.1",
+		},
+	}
+	order, err := client.AuthorizeOrder(ctx, identifiers)
+	if err != nil {
+		t.Fatalf("failed to create order for %v: %v", identifiers, err)
+	}
+	if order.Status != acme.StatusPending {
+		t.Fatalf("expected new order status to be pending, got %v", order.Status)
+	}
+	orderURL := order.URI
+	log.Printf("created order: %v", orderURL)
+
+	// For each pending authz provision a supported challenge type's response
+	// with the test challenge server, and tell the ACME server to verify it.
+	for _, authzURL := range order.AuthzURLs {
+		authz, err := client.GetAuthorization(ctx, authzURL)
+		if err != nil {
+			t.Fatalf("failed to get order %s authorization %s: %v",
+				orderURL, authzURL, err)
+		}
+
+		if authz.Status != acme.StatusPending {
+			continue
+		}
+
+		for _, challenge := range authz.Challenges {
+			if challenge.Status != acme.StatusPending || !challSrv.Supported(challenge) {
+				continue
+			}
+
+			if err := challSrv.Provision(client, authz.Identifier, challenge); err != nil {
+				t.Fatalf("failed to provision challenge %s: %v", challenge.URI, err)
+			}
+
+			_, err = client.Accept(ctx, challenge)
+			if err != nil {
+				t.Fatalf("failed to accept order %s challenge %s: %v",
+					orderURL, challenge.URI, err)
+			}
+		}
+	}
+
+	// Wait for the order to become ready for finalization.
+	order, err = client.WaitOrder(ctx, order.URI)
+	if err != nil {
+		t.Fatalf("failed to wait for order %s: %s", orderURL, err)
+	}
+	if order.Status != acme.StatusReady {
+		t.Fatalf("expected order %s status to be ready, got %v",
+			orderURL, order.Status)
+	}
+
+	// Generate a certificate keypair and a CSR for the order identifiers.
+	certKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatalf("failed to generate certificate key: %v", err)
+	}
+	var dnsNames []string
+	var ipAddresses []net.IP
+	for _, ident := range identifiers {
+		switch ident.Type {
+		case "dns":
+			dnsNames = append(dnsNames, ident.Value)
+		case "ip":
+			ipAddresses = append(ipAddresses, net.ParseIP(ident.Value))
+		default:
+			t.Fatalf("unsupported identifier type: %s", ident.Type)
+		}
+	}
+	csrDer, err := x509.CreateCertificateRequest(rand.Reader, &x509.CertificateRequest{
+		DNSNames:    dnsNames,
+		IPAddresses: ipAddresses,
+	}, certKey)
+	if err != nil {
+		t.Fatalf("failed to create CSR: %v", err)
+	}
+
+	// Finalize the order by creating a certificate with our CSR.
+	chain, _, err := client.CreateOrderCert(ctx, order.FinalizeURL, csrDer, true)
+	if err != nil {
+		t.Fatalf("failed to finalize order %s with finalize URL %s: %v",
+			orderURL, order.FinalizeURL, err)
+	}
+
+	// Split the chain into the leaf and any intermediates.
+	leaf := chain[0]
+	intermediatesDER := chain[1:]
+	leafCert, err := x509.ParseCertificate(leaf)
+	if err != nil {
+		t.Fatalf("failed to parse order %s leaf certificate: %v", orderURL, err)
+	}
+	intermediates := x509.NewCertPool()
+	for i, intermediateDER := range intermediatesDER {
+		intermediate, err := x509.ParseCertificate(intermediateDER)
+		if err != nil {
+			t.Fatalf("failed to parse intermediate %d: %v", i, err)
+		}
+		intermediates.AddCert(intermediate)
+	}
+
+	// Verify there is a valid path from the leaf certificate to Pebble's
+	// issuing root using the provided intermediate certificates.
+	roots, err := env.RootCert()
+	if err != nil {
+		t.Fatalf("failed to get Pebble issuer root certs: %v", err)
+	}
+	paths, err := leafCert.Verify(x509.VerifyOptions{
+		Intermediates: intermediates,
+		Roots:         roots,
+	})
+	if err != nil {
+		t.Fatalf("failed to verify order %s leaf certificate: %v", orderURL, err)
+	}
+	log.Printf("verified %d path(s) from issued leaf certificate to Pebble root CA", len(paths))
+
+	// Also verify that the leaf cert is valid for each of the DNS names
+	// and IP addresses from our order's identifiers.
+	for _, name := range dnsNames {
+		if err := leafCert.VerifyHostname(name); err != nil {
+			t.Fatalf("failed to verify order %s leaf certificate for order DNS name %s: %v",
+				orderURL, name, err)
+		}
+	}
+	for _, ip := range ipAddresses {
+		if err := leafCert.VerifyHostname(ip.String()); err != nil {
+			t.Fatalf("failed to verify order %s leaf certificate for order IP address %s: %v",
+				orderURL, ip, err)
+		}
+	}
+}
+
+type environment struct {
+	config *environmentConfig
+	client *acme.Client
+}
+
+// RootCert returns the Pebble CA's primary issuing hierarchy root certificate.
+// This is generated randomly at each startup and can be used to verify
+// certificate chains issued by Pebble's ACME interface. Note that this
+// is separate from the static root certificate used by the Pebble ACME
+// HTTPS interface.
+func (e *environment) RootCert() (*x509.CertPool, error) {
+	// NOTE: in the future we may want to consider the alternative chains
+	// 		 returned as Link alternative headers.
+	rootURL := fmt.Sprintf("https://%s/roots/0", e.config.pebbleConfig.ManagementListenAddress)
+	resp, err := e.client.HTTPClient.Get(rootURL)
+	if err != nil || resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("failed to GET Pebble root CA from %s: %v", rootURL, err)
+	}
+
+	roots := x509.NewCertPool()
+	rootPEM, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse Pebble root CA PEM: %v", err)
+	}
+	rootDERBlock, _ := pem.Decode(rootPEM)
+	rootCA, err := x509.ParseCertificate(rootDERBlock.Bytes)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse Pebble root CA DER: %v", err)
+	}
+	roots.AddCert(rootCA)
+
+	return roots, nil
+}
+
+// environmentConfig describes the Pebble configuration, and configuration
+// shared between pebble and pebble-challtestsrv.
+type environmentConfig struct {
+	pebbleConfig
+	dnsPort uint32
+}
+
+// defaultConfig returns an environmentConfig populated with default values.
+// The provided pebbleDir is used to specify certificate/private key paths
+// for the HTTPS ACME interface.
+func defaultConfig(pebbleDir string) environmentConfig {
+	return environmentConfig{
+		pebbleConfig: pebbleConfig{
+			ListenAddress:                  fmt.Sprintf("127.0.0.1:%d", takeNextPort()),
+			ManagementListenAddress:        fmt.Sprintf("127.0.0.1:%d", takeNextPort()),
+			HTTPPort:                       takeNextPort(),
+			TLSPort:                        takeNextPort(),
+			Certificate:                    fmt.Sprintf("%s/test/certs/localhost/cert.pem", pebbleDir),
+			PrivateKey:                     fmt.Sprintf("%s/test/certs/localhost/key.pem", pebbleDir),
+			OCSPResponderURL:               "",
+			ExternalAccountBindingRequired: false,
+			ExternalAccountMACKeys:         make(map[string]string),
+			DomainBlocklist:                []string{"blocked-domain.example"},
+			Profiles: map[string]struct {
+				Description    string
+				ValidityPeriod uint64
+			}{
+				"default": {
+					Description:    "default profile",
+					ValidityPeriod: 3600,
+				},
+			},
+			RetryAfter: struct {
+				Authz int
+				Order int
+			}{
+				3,
+				5,
+			},
+		},
+		dnsPort: takeNextPort(),
+	}
+}
+
+// pebbleConfig matches the JSON structure of the Pebble configuration file.
+type pebbleConfig struct {
+	ListenAddress                  string
+	ManagementListenAddress        string
+	HTTPPort                       uint32
+	TLSPort                        uint32
+	Certificate                    string
+	PrivateKey                     string
+	OCSPResponderURL               string
+	ExternalAccountBindingRequired bool
+	ExternalAccountMACKeys         map[string]string
+	DomainBlocklist                []string
+	Profiles                       map[string]struct {
+		Description    string
+		ValidityPeriod uint64
+	}
+	RetryAfter struct {
+		Authz int
+		Order int
+	}
+}
+
+func takeNextPort() uint32 {
+	return nextPort.Add(1) - 1
+}
+
+// startPebbleEnvironment is a test helper that spawns Pebble and Pebble
+// challenge test server processes based on the provided environmentConfig. The
+// processes will be torn down when the test ends.
+func startPebbleEnvironment(t *testing.T, config *environmentConfig) environment {
+	t.Helper()
+
+	var pebbleDir string
+	if *pebbleLocalDir != "" {
+		pebbleDir = *pebbleLocalDir
+	} else {
+		pebbleDir = fetchModule(t, "github.com/letsencrypt/pebble/v2", pebbleModVersion)
+	}
+
+	binDir := prepareBinaries(t, pebbleDir)
+
+	if config == nil {
+		cfg := defaultConfig(pebbleDir)
+		config = &cfg
+	}
+
+	marshalConfig := struct {
+		Pebble pebbleConfig
+	}{
+		Pebble: config.pebbleConfig,
+	}
+
+	configData, err := json.Marshal(marshalConfig)
+	if err != nil {
+		t.Fatalf("failed to marshal config: %v", err)
+	}
+
+	configFile, err := os.CreateTemp("", "pebble-config-*.json")
+	if err != nil {
+		t.Fatalf("failed to create temp config file: %v", err)
+	}
+	t.Cleanup(func() { os.Remove(configFile.Name()) })
+
+	if _, err := configFile.Write(configData); err != nil {
+		t.Fatalf("failed to write config file: %v", err)
+	}
+	configFile.Close()
+
+	log.Printf("pebble dir: %s", pebbleDir)
+	log.Printf("config file: %s", configFile.Name())
+
+	// Spawn the Pebble CA server. It answers ACME requests and performs
+	// outbound validations. We configure it to use a mock DNS server that
+	// always answers 127.0.0.1 for all A queries so that validation
+	// requests for any domain name will resolve to our local challenge
+	// server instances.
+	spawnServerProcess(t, binDir, "pebble", "-config", configFile.Name(),
+		"-dnsserver", fmt.Sprintf("127.0.0.1:%d", config.dnsPort),
+		"-strict")
+
+	// Spawn the Pebble challenge test server. We'll use it to mock DNS
+	// responses but disable all the other interfaces. We want to stand
+	// up our own challenge response servers for TLS-ALPN-01,
+	// etc.
+	// Note: we specify -defaultIPv6 "" so that no AAAA records are served.
+	// The LUCI CI runners have issues with IPv6 connectivity on localhost.
+	spawnServerProcess(t, binDir, "pebble-challtestsrv",
+		"-dns01", fmt.Sprintf(":%d", config.dnsPort),
+		"-defaultIPv6", "",
+		"-management", fmt.Sprintf(":%d", takeNextPort()),
+		"-doh", "",
+		"-http01", "",
+		"-tlsalpn01", "",
+		"-https01", "")
+
+	waitForServer(t, config.pebbleConfig.ListenAddress)
+	waitForServer(t, fmt.Sprintf("127.0.0.1:%d", config.dnsPort))
+
+	log.Printf("pebble environment ready")
+
+	// Construct a cert pool that contains the CA certificate used by the ACME
+	// interface's certificate chain. This is separate from the issuing
+	// hierarchy and is used for the ACME client to interact with the ACME
+	// interface without cert verification error.
+	caCertPath := filepath.Join(pebbleDir, "test/certs/pebble.minica.pem")
+	caCert, err := os.ReadFile(caCertPath)
+	if err != nil {
+		t.Fatalf("failed to read CA certificate %s: %v", caCertPath, err)
+	}
+	caCertPool := x509.NewCertPool()
+	if !caCertPool.AppendCertsFromPEM(caCert) {
+		t.Fatalf("failed to parse CA certificate %s", caCertPath)
+	}
+	httpClient := &http.Client{
+		Transport: &http.Transport{
+			TLSClientConfig: &tls.Config{
+				RootCAs: caCertPool,
+			},
+		},
+	}
+
+	// Create an ACME account keypair/client and verify it can discover
+	// the Pebble server's ACME directory without error.
+	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatalf("failed to generate account key: %v", err)
+	}
+	client := &acme.Client{
+		Key:          key,
+		HTTPClient:   httpClient,
+		DirectoryURL: fmt.Sprintf("https://%s/dir", config.ListenAddress),
+	}
+	_, err = client.Discover(context.TODO())
+	if err != nil {
+		t.Fatalf("failed to discover ACME directory: %v", err)
+	}
+
+	return environment{
+		config: config,
+		client: client,
+	}
+}
+
+func waitForServer(t *testing.T, addr string) {
+	t.Helper()
+
+	for i := 0; i < 10; i++ {
+		if conn, err := net.Dial("tcp", addr); err == nil {
+			conn.Close()
+			return
+		}
+		time.Sleep(time.Duration(i*100) * time.Millisecond)
+	}
+	t.Fatalf("failed to connect to %q after 10 tries", addr)
+}
+
+// fetchModule fetches the module at the given version and returns the directory
+// containing its source tree. It skips the test if fetching modules is not
+// possible in this environment.
+//
+// Copied from the stdlib cryptotest.FetchModule and adapted to not rely on the
+// stdlib internal testenv package.
+func fetchModule(t *testing.T, module, version string) string {
+	// If the default GOMODCACHE doesn't exist, use a temporary directory
+	// instead. (For example, run.bash sets GOPATH=/nonexist-gopath.)
+	out, err := exec.Command("go", "env", "GOMODCACHE").Output()
+	if err != nil {
+		t.Errorf("go env GOMODCACHE: %v\n%s", err, out)
+		if ee, ok := err.(*exec.ExitError); ok {
+			t.Logf("%s", ee.Stderr)
+		}
+		t.FailNow()
+	}
+	modcacheOk := false
+	if gomodcache := string(bytes.TrimSpace(out)); gomodcache != "" {
+		if _, err := os.Stat(gomodcache); err == nil {
+			modcacheOk = true
+		}
+	}
+	if !modcacheOk {
+		t.Setenv("GOMODCACHE", t.TempDir())
+		// Allow t.TempDir() to clean up subdirectories.
+		t.Setenv("GOFLAGS", os.Getenv("GOFLAGS")+" -modcacherw")
+	}
+
+	t.Logf("fetching %s@%s\n", module, version)
+
+	output, err := exec.Command("go", "mod", "download", "-json", module+"@"+version).CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to download %s@%s: %s\n%s\n", module, version, err, output)
+	}
+	var j struct {
+		Dir string
+	}
+	if err := json.Unmarshal(output, &j); err != nil {
+		t.Fatalf("failed to parse 'go mod download': %s\n%s\n", err, output)
+	}
+
+	return j.Dir
+}
+
+func prepareBinaries(t *testing.T, pebbleDir string) string {
+	t.Helper()
+
+	// We don't want to build in the module cache dir, which might not be
+	// writable or to pollute the user's clone with binaries if pebbleLocalDir
+	//is used.
+	binDir := t.TempDir()
+
+	build := func(cmd string) {
+		log.Printf("building %s", cmd)
+		buildCmd := exec.Command(
+			"go",
+			"build", "-o", filepath.Join(binDir, cmd), "-mod", "mod", "./cmd/"+cmd)
+		buildCmd.Dir = pebbleDir
+		output, err := buildCmd.CombinedOutput()
+		if err != nil {
+			t.Fatalf("failed to build %s: %s\n%s\n", cmd, err, output)
+		}
+	}
+
+	build("pebble")
+	build("pebble-challtestsrv")
+
+	return binDir
+}
+
+func spawnServerProcess(t *testing.T, dir string, cmd string, args ...string) {
+	t.Helper()
+
+	cmdInstance := exec.Command("./"+cmd, args...)
+	cmdInstance.Dir = dir
+	cmdInstance.Stdout = os.Stdout
+	cmdInstance.Stderr = os.Stderr
+	if err := cmdInstance.Start(); err != nil {
+		t.Fatalf("failed to start %s: %v", cmd, err)
+	}
+	t.Cleanup(func() {
+		cmdInstance.Process.Kill()
+	})
+}
diff --git a/acme/types.go b/acme/types.go
index 640223cb7d..c466645ca1 100644
--- a/acme/types.go
+++ b/acme/types.go
@@ -619,7 +619,7 @@ func (*certOptKey) privateCertOpt() {}
 //
 // In TLS ChallengeCert methods, the template is also used as parent,
 // resulting in a self-signed certificate.
-// The DNSNames field of t is always overwritten for tls-sni challenge certs.
+// The DNSNames or IPAddresses fields of t are always overwritten for tls-alpn challenge certs.
 func WithTemplate(t *x509.Certificate) CertOption {
 	return (*certOptTemplate)(t)
 }
diff --git a/blake2b/blake2x.go b/blake2b/blake2x.go
index 52c414db0e..7692bb3460 100644
--- a/blake2b/blake2x.go
+++ b/blake2b/blake2x.go
@@ -12,6 +12,8 @@ import (
 
 // XOF defines the interface to hash functions that
 // support arbitrary-length output.
+//
+// New callers should prefer the standard library [hash.XOF].
 type XOF interface {
 	// Write absorbs more data into the hash's state. It panics if called
 	// after Read.
@@ -47,6 +49,8 @@ const maxOutputLength = (1 << 32) * 64
 //
 // A non-nil key turns the hash into a MAC. The key must between
 // zero and 32 bytes long.
+//
+// The result can be safely interface-upgraded to [hash.XOF].
 func NewXOF(size uint32, key []byte) (XOF, error) {
 	if len(key) > Size {
 		return nil, errKeySize
@@ -93,6 +97,10 @@ func (x *xof) Clone() XOF {
 	return &clone
 }
 
+func (x *xof) BlockSize() int {
+	return x.d.BlockSize()
+}
+
 func (x *xof) Reset() {
 	x.cfg[0] = byte(Size)
 	binary.LittleEndian.PutUint32(x.cfg[4:], uint32(Size)) // leaf length
diff --git a/blake2b/go125.go b/blake2b/go125.go
new file mode 100644
index 0000000000..67e990b7e1
--- /dev/null
+++ b/blake2b/go125.go
@@ -0,0 +1,11 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.25
+
+package blake2b
+
+import "hash"
+
+var _ hash.XOF = (*xof)(nil)
diff --git a/go.mod b/go.mod
index 90256c0493..5cd0c6410d 100644
--- a/go.mod
+++ b/go.mod
@@ -3,9 +3,9 @@ module golang.org/x/crypto
 go 1.23.0
 
 require (
-	golang.org/x/net v0.21.0 // tagx:ignore
-	golang.org/x/sys v0.33.0
-	golang.org/x/term v0.32.0
+	golang.org/x/net v0.41.0 // tagx:ignore
+	golang.org/x/sys v0.34.0
+	golang.org/x/term v0.33.0
 )
 
-require golang.org/x/text v0.26.0 // indirect
+require golang.org/x/text v0.27.0 // indirect
diff --git a/go.sum b/go.sum
index 0eb4cba189..93d2855a8c 100644
--- a/go.sum
+++ b/go.sum
@@ -1,8 +1,8 @@
-golang.org/x/net v0.21.0 h1:AQyQV4dYCvJ7vGmJyKki9+PBdyvhkSd8EIx/qb0AYv4=
-golang.org/x/net v0.21.0/go.mod h1:bIjVDfnllIU7BJ2DNgfnXvpSvtn8VRwhlsaeUTyUS44=
-golang.org/x/sys v0.33.0 h1:q3i8TbbEz+JRD9ywIRlyRAQbM0qF7hu24q3teo2hbuw=
-golang.org/x/sys v0.33.0/go.mod h1:BJP2sWEmIv4KK5OTEluFJCKSidICx8ciO85XgH3Ak8k=
-golang.org/x/term v0.32.0 h1:DR4lr0TjUs3epypdhTOkMmuF5CDFJ/8pOnbzMZPQ7bg=
-golang.org/x/term v0.32.0/go.mod h1:uZG1FhGx848Sqfsq4/DlJr3xGGsYMu/L5GW4abiaEPQ=
-golang.org/x/text v0.26.0 h1:P42AVeLghgTYr4+xUnTRKDMqpar+PtX7KWuNQL21L8M=
-golang.org/x/text v0.26.0/go.mod h1:QK15LZJUUQVJxhz7wXgxSy/CJaTFjd0G+YLonydOVQA=
+golang.org/x/net v0.41.0 h1:vBTly1HeNPEn3wtREYfy4GZ/NECgw2Cnl+nK6Nz3uvw=
+golang.org/x/net v0.41.0/go.mod h1:B/K4NNqkfmg07DQYrbwvSluqCJOOXwUjeb/5lOisjbA=
+golang.org/x/sys v0.34.0 h1:H5Y5sJ2L2JRdyv7ROF1he/lPdvFsd0mJHFw2ThKHxLA=
+golang.org/x/sys v0.34.0/go.mod h1:BJP2sWEmIv4KK5OTEluFJCKSidICx8ciO85XgH3Ak8k=
+golang.org/x/term v0.33.0 h1:NuFncQrRcaRvVmgRkvM3j/F00gWIAlcmlB8ACEKmGIg=
+golang.org/x/term v0.33.0/go.mod h1:s18+ql9tYWp1IfpV9DmCtQDDSRBUjKaw9M1eAv5UeF0=
+golang.org/x/text v0.27.0 h1:4fGWRpyh641NLlecmyl4LOe6yDdfaYNrGb2zdfo4JV4=
+golang.org/x/text v0.27.0/go.mod h1:1D28KMCvyooCX9hBiosv5Tz/+YLxj0j7XhWjpSUF7CU=
diff --git a/ssh/certs.go b/ssh/certs.go
index a3dc629c62..139fa31e1b 100644
--- a/ssh/certs.go
+++ b/ssh/certs.go
@@ -233,7 +233,11 @@ func parseCert(in []byte, privAlgo string) (*Certificate, error) {
 	if err != nil {
 		return nil, err
 	}
-
+	// The Type() function is intended to return only certificate key types, but
+	// we use certKeyAlgoNames anyway for safety, to match [Certificate.Type].
+	if _, ok := certKeyAlgoNames[k.Type()]; ok {
+		return nil, fmt.Errorf("ssh: the signature key type %q is invalid for certificates", k.Type())
+	}
 	c.SignatureKey = k
 	c.Signature, rest, ok = parseSignatureBody(g.Signature)
 	if !ok || len(rest) > 0 {
@@ -301,16 +305,13 @@ type CertChecker struct {
 	SupportedCriticalOptions []string
 
 	// IsUserAuthority should return true if the key is recognized as an
-	// authority for the given user certificate. This allows for
-	// certificates to be signed by other certificates. This must be set
-	// if this CertChecker will be checking user certificates.
+	// authority for user certificate. This must be set if this CertChecker
+	// will be checking user certificates.
 	IsUserAuthority func(auth PublicKey) bool
 
 	// IsHostAuthority should report whether the key is recognized as
-	// an authority for this host. This allows for certificates to be
-	// signed by other keys, and for those other keys to only be valid
-	// signers for particular hostnames. This must be set if this
-	// CertChecker will be checking host certificates.
+	// an authority for this host. This must be set if this CertChecker
+	// will be checking host certificates.
 	IsHostAuthority func(auth PublicKey, address string) bool
 
 	// Clock is used for verifying time stamps. If nil, time.Now
@@ -447,12 +448,19 @@ func (c *CertChecker) CheckCert(principal string, cert *Certificate) error {
 // SignCert signs the certificate with an authority, setting the Nonce,
 // SignatureKey, and Signature fields. If the authority implements the
 // MultiAlgorithmSigner interface the first algorithm in the list is used. This
-// is useful if you want to sign with a specific algorithm.
+// is useful if you want to sign with a specific algorithm. As specified in
+// [SSH-CERTS], Section 2.1.1, authority can't be a [Certificate].
 func (c *Certificate) SignCert(rand io.Reader, authority Signer) error {
 	c.Nonce = make([]byte, 32)
 	if _, err := io.ReadFull(rand, c.Nonce); err != nil {
 		return err
 	}
+	// The Type() function is intended to return only certificate key types, but
+	// we use certKeyAlgoNames anyway for safety, to match [Certificate.Type].
+	if _, ok := certKeyAlgoNames[authority.PublicKey().Type()]; ok {
+		return fmt.Errorf("ssh: certificates cannot be used as authority (public key type %q)",
+			authority.PublicKey().Type())
+	}
 	c.SignatureKey = authority.PublicKey()
 
 	if v, ok := authority.(MultiAlgorithmSigner); ok {
diff --git a/ssh/certs_test.go b/ssh/certs_test.go
index 1a4b49990a..e2a6fedc8b 100644
--- a/ssh/certs_test.go
+++ b/ssh/certs_test.go
@@ -404,3 +404,32 @@ func TestCertSignWithMultiAlgorithmSigner(t *testing.T) {
 		})
 	}
 }
+
+func TestCertSignWithCertificate(t *testing.T) {
+	cert := &Certificate{
+		Key:         testPublicKeys["rsa"],
+		ValidBefore: CertTimeInfinity,
+		CertType:    UserCert,
+	}
+	if err := cert.SignCert(rand.Reader, testSigners["ecdsa"]); err != nil {
+		t.Fatalf("SignCert: %v", err)
+	}
+	signer, err := NewSignerWithAlgorithms(testSigners["rsa"].(AlgorithmSigner), []string{KeyAlgoRSASHA256})
+	if err != nil {
+		t.Fatal(err)
+	}
+	certSigner, err := NewCertSigner(cert, signer)
+	if err != nil {
+		t.Fatalf("NewCertSigner: %v", err)
+	}
+
+	cert1 := &Certificate{
+		Key:         testPublicKeys["ecdsa"],
+		ValidBefore: CertTimeInfinity,
+		CertType:    UserCert,
+	}
+
+	if err := cert1.SignCert(rand.Reader, certSigner); err == nil {
+		t.Fatal("successfully signed a certificate using another certificate, it is expected to fail")
+	}
+}
diff --git a/ssh/client_auth.go b/ssh/client_auth.go
index b86dde151d..c12818fdc5 100644
--- a/ssh/client_auth.go
+++ b/ssh/client_auth.go
@@ -289,7 +289,7 @@ func pickSignatureAlgorithm(signer Signer, extensions map[string][]byte) (MultiA
 		}
 	}
 
-	algo, err := findCommon("public key signature algorithm", keyAlgos, serverAlgos)
+	algo, err := findCommon("public key signature algorithm", keyAlgos, serverAlgos, true)
 	if err != nil {
 		// If there is no overlap, return the fallback algorithm to support
 		// servers that fail to list all supported algorithms.
diff --git a/ssh/common.go b/ssh/common.go
index 0415d33968..f2ec0896c2 100644
--- a/ssh/common.go
+++ b/ssh/common.go
@@ -336,7 +336,7 @@ func parseError(tag uint8) error {
 	return fmt.Errorf("ssh: parse error in message type %d", tag)
 }
 
-func findCommon(what string, client []string, server []string) (common string, err error) {
+func findCommon(what string, client []string, server []string, isClient bool) (string, error) {
 	for _, c := range client {
 		for _, s := range server {
 			if c == s {
@@ -344,7 +344,32 @@ func findCommon(what string, client []string, server []string) (common string, e
 			}
 		}
 	}
-	return "", fmt.Errorf("ssh: no common algorithm for %s; client offered: %v, server offered: %v", what, client, server)
+	err := &AlgorithmNegotiationError{
+		What: what,
+	}
+	if isClient {
+		err.SupportedAlgorithms = client
+		err.RequestedAlgorithms = server
+	} else {
+		err.SupportedAlgorithms = server
+		err.RequestedAlgorithms = client
+	}
+	return "", err
+}
+
+// AlgorithmNegotiationError defines the error returned if the client and the
+// server cannot agree on an algorithm for key exchange, host key, cipher, MAC.
+type AlgorithmNegotiationError struct {
+	What string
+	// RequestedAlgorithms lists the algorithms supported by the peer.
+	RequestedAlgorithms []string
+	// SupportedAlgorithms lists the algorithms supported on our side.
+	SupportedAlgorithms []string
+}
+
+func (a *AlgorithmNegotiationError) Error() string {
+	return fmt.Sprintf("ssh: no common algorithm for %s; we offered: %v, peer offered: %v",
+		a.What, a.SupportedAlgorithms, a.RequestedAlgorithms)
 }
 
 // DirectionAlgorithms defines the algorithms negotiated in one direction
@@ -379,12 +404,12 @@ var aeadCiphers = map[string]bool{
 func findAgreedAlgorithms(isClient bool, clientKexInit, serverKexInit *kexInitMsg) (algs *NegotiatedAlgorithms, err error) {
 	result := &NegotiatedAlgorithms{}
 
-	result.KeyExchange, err = findCommon("key exchange", clientKexInit.KexAlgos, serverKexInit.KexAlgos)
+	result.KeyExchange, err = findCommon("key exchange", clientKexInit.KexAlgos, serverKexInit.KexAlgos, isClient)
 	if err != nil {
 		return
 	}
 
-	result.HostKey, err = findCommon("host key", clientKexInit.ServerHostKeyAlgos, serverKexInit.ServerHostKeyAlgos)
+	result.HostKey, err = findCommon("host key", clientKexInit.ServerHostKeyAlgos, serverKexInit.ServerHostKeyAlgos, isClient)
 	if err != nil {
 		return
 	}
@@ -394,36 +419,36 @@ func findAgreedAlgorithms(isClient bool, clientKexInit, serverKexInit *kexInitMs
 		ctos, stoc = stoc, ctos
 	}
 
-	ctos.Cipher, err = findCommon("client to server cipher", clientKexInit.CiphersClientServer, serverKexInit.CiphersClientServer)
+	ctos.Cipher, err = findCommon("client to server cipher", clientKexInit.CiphersClientServer, serverKexInit.CiphersClientServer, isClient)
 	if err != nil {
 		return
 	}
 
-	stoc.Cipher, err = findCommon("server to client cipher", clientKexInit.CiphersServerClient, serverKexInit.CiphersServerClient)
+	stoc.Cipher, err = findCommon("server to client cipher", clientKexInit.CiphersServerClient, serverKexInit.CiphersServerClient, isClient)
 	if err != nil {
 		return
 	}
 
 	if !aeadCiphers[ctos.Cipher] {
-		ctos.MAC, err = findCommon("client to server MAC", clientKexInit.MACsClientServer, serverKexInit.MACsClientServer)
+		ctos.MAC, err = findCommon("client to server MAC", clientKexInit.MACsClientServer, serverKexInit.MACsClientServer, isClient)
 		if err != nil {
 			return
 		}
 	}
 
 	if !aeadCiphers[stoc.Cipher] {
-		stoc.MAC, err = findCommon("server to client MAC", clientKexInit.MACsServerClient, serverKexInit.MACsServerClient)
+		stoc.MAC, err = findCommon("server to client MAC", clientKexInit.MACsServerClient, serverKexInit.MACsServerClient, isClient)
 		if err != nil {
 			return
 		}
 	}
 
-	ctos.compression, err = findCommon("client to server compression", clientKexInit.CompressionClientServer, serverKexInit.CompressionClientServer)
+	ctos.compression, err = findCommon("client to server compression", clientKexInit.CompressionClientServer, serverKexInit.CompressionClientServer, isClient)
 	if err != nil {
 		return
 	}
 
-	stoc.compression, err = findCommon("server to client compression", clientKexInit.CompressionServerClient, serverKexInit.CompressionServerClient)
+	stoc.compression, err = findCommon("server to client compression", clientKexInit.CompressionServerClient, serverKexInit.CompressionServerClient, isClient)
 	if err != nil {
 		return
 	}
diff --git a/ssh/doc.go b/ssh/doc.go
index f5d352fe3a..04ccce3461 100644
--- a/ssh/doc.go
+++ b/ssh/doc.go
@@ -16,6 +16,7 @@ References:
 	[PROTOCOL]: https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL?rev=HEAD
 	[PROTOCOL.certkeys]: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys?rev=HEAD
 	[SSH-PARAMETERS]:    http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xml#ssh-parameters-1
+	[SSH-CERTS]:	https://datatracker.ietf.org/doc/html/draft-miller-ssh-cert-01
 
 This package does not fall under the stability promise of the Go language itself,
 so its API may be changed when pressing needs arise.
diff --git a/ssh/handshake_test.go b/ssh/handshake_test.go
index 7f0ecef05f..61f978491f 100644
--- a/ssh/handshake_test.go
+++ b/ssh/handshake_test.go
@@ -1294,3 +1294,73 @@ func TestNegotiatedAlgorithms(t *testing.T) {
 		}
 	}
 }
+
+func TestAlgorithmNegotiationError(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	serverConf := &ServerConfig{
+		Config: Config{
+			Ciphers: []string{CipherAES128CTR, CipherAES256CTR},
+		},
+		PasswordCallback: func(conn ConnMetadata, password []byte) (*Permissions, error) {
+			return &Permissions{}, nil
+		},
+	}
+	serverConf.AddHostKey(testSigners["rsa"])
+
+	srvErrCh := make(chan error, 1)
+	go func() {
+		_, _, _, err := NewServerConn(c1, serverConf)
+		srvErrCh <- err
+	}()
+
+	clientConf := &ClientConfig{
+		Config: Config{
+			Ciphers: []string{CipherAES128GCM, CipherAES256GCM},
+		},
+		User:            "test",
+		Auth:            []AuthMethod{Password("testpw")},
+		HostKeyCallback: FixedHostKey(testSigners["rsa"].PublicKey()),
+	}
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err == nil {
+		t.Fatal("client connection succeded expected algorithm negotiation error")
+	}
+	var negotiationError *AlgorithmNegotiationError
+	if !errors.As(err, &negotiationError) {
+		t.Fatalf("expected algorithm negotiation error, got %v", err)
+	}
+	expectedErrorString := fmt.Sprintf("ssh: handshake failed: ssh: no common algorithm for client to server cipher; we offered: %v, peer offered: %v",
+		clientConf.Ciphers, serverConf.Ciphers)
+	if err.Error() != expectedErrorString {
+		t.Fatalf("expected error string %q, got %q", expectedErrorString, err.Error())
+	}
+	if !reflect.DeepEqual(negotiationError.RequestedAlgorithms, serverConf.Ciphers) {
+		t.Fatalf("expected requested algorithms %v, got %v", serverConf.Ciphers, negotiationError.RequestedAlgorithms)
+	}
+	if !reflect.DeepEqual(negotiationError.SupportedAlgorithms, clientConf.Ciphers) {
+		t.Fatalf("expected supported algorithms %v, got %v", clientConf.Ciphers, negotiationError.SupportedAlgorithms)
+	}
+	err = <-srvErrCh
+	negotiationError = nil
+	if !errors.As(err, &negotiationError) {
+		t.Fatalf("expected algorithm negotiation error, got %v", err)
+	}
+	expectedErrorString = fmt.Sprintf("ssh: no common algorithm for client to server cipher; we offered: %v, peer offered: %v",
+		serverConf.Ciphers, clientConf.Ciphers)
+	if err.Error() != expectedErrorString {
+		t.Fatalf("expected error string %q, got %q", expectedErrorString, err.Error())
+	}
+	if !reflect.DeepEqual(negotiationError.RequestedAlgorithms, clientConf.Ciphers) {
+		t.Fatalf("expected requested algorithms %v, got %v", clientConf.Ciphers, negotiationError.RequestedAlgorithms)
+	}
+	if !reflect.DeepEqual(negotiationError.SupportedAlgorithms, serverConf.Ciphers) {
+		t.Fatalf("expected supported algorithms %v, got %v", serverConf.Ciphers, negotiationError.SupportedAlgorithms)
+	}
+}
diff --git a/ssh/keys.go b/ssh/keys.go
index 566e09d5a1..a28c0de503 100644
--- a/ssh/keys.go
+++ b/ssh/keys.go
@@ -273,7 +273,7 @@ func ParseAuthorizedKey(in []byte) (out PublicKey, comment string, options []str
 	return nil, "", nil, nil, errors.New("ssh: no key found")
 }
 
-// ParsePublicKey parses an SSH public key formatted for use in
+// ParsePublicKey parses an SSH public key or certificate formatted for use in
 // the SSH wire protocol according to RFC 4253, section 6.6.
 func ParsePublicKey(in []byte) (out PublicKey, err error) {
 	algo, in, ok := parseString(in)
diff --git a/ssh/keys_test.go b/ssh/keys_test.go
index 7d5b86ff0d..f3eb223a9c 100644
--- a/ssh/keys_test.go
+++ b/ssh/keys_test.go
@@ -810,3 +810,30 @@ func TestCryptoPublicKey(t *testing.T) {
 		}
 	}
 }
+
+func TestParseCertWithCertSignatureKey(t *testing.T) {
+	certBytes := []byte(`-----BEGIN SSH CERTIFICATE-----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+-----END SSH CERTIFICATE-----
+	`)
+	block, _ := pem.Decode(certBytes)
+	if block == nil {
+		t.Fatal("invalid test certificate")
+	}
+
+	if _, err := ParsePublicKey(block.Bytes); err == nil {
+		t.Fatal("parsing an SSH certificate using another certificate as signature key succeeded; expected failure")
+	}
+}
diff --git a/x509roots/fallback/bundle.go b/x509roots/fallback/bundle.go
index 81ad6e8b5a..19a168c4bb 100644
--- a/x509roots/fallback/bundle.go
+++ b/x509roots/fallback/bundle.go
@@ -4148,44 +4148,6 @@ Jd1hJyMctTEHBDa0GpC9oHRxUIltvBTjD4au8as+x6AJzKNI0eDbZOeStc+vckNw
 i/nDhDwTqn6Sm1dTk/pwwpEOMfmbZ13pljheX7NzTogVZ96edhBiIL5VaZVDADlN
 9u6wWk5JRFRYX0KD
 -----END CERTIFICATE-----
-`,
-	},
-	{
-		cn:         "OU=ePKI Root Certification Authority,O=Chunghwa Telecom Co.\\, Ltd.,C=TW",
-		sha256Hash: "c0a6f4dc63a24bfdcf54ef2a6a082a0a72de35803e2ff5ff527ae5d87206dfd5",
-		pem: `-----BEGIN CERTIFICATE-----
-MIIFsDCCA5igAwIBAgIQFci9ZUdcr7iXAF7kBtK8nTANBgkqhkiG9w0BAQUFADBe
-MQswCQYDVQQGEwJUVzEjMCEGA1UECgwaQ2h1bmdod2EgVGVsZWNvbSBDby4sIEx0
-ZC4xKjAoBgNVBAsMIWVQS0kgUm9vdCBDZXJ0aWZpY2F0aW9uIEF1dGhvcml0eTAe
-Fw0wNDEyMjAwMjMxMjdaFw0zNDEyMjAwMjMxMjdaMF4xCzAJBgNVBAYTAlRXMSMw
-IQYDVQQKDBpDaHVuZ2h3YSBUZWxlY29tIENvLiwgTHRkLjEqMCgGA1UECwwhZVBL
-SSBSb290IENlcnRpZmljYXRpb24gQXV0aG9yaXR5MIICIjANBgkqhkiG9w0BAQEF
-AAOCAg8AMIICCgKCAgEA4SUP7o3biDN1Z82tH306Tm2d0y8U82N0ywEhajfqhFAH
-SyZbCUNsIZ5qyNUD9WBpj8zwIuQf5/dqIjG3LBXy4P4AakP/h2XGtRrBp0xtInAh
-ijHyl3SJCRImHJ7K2RKilTza6We/CKBk49ZCt0Xvl/T29de1ShUCWH2YWEtgvM3X
-DZoTM1PRYfl61dd4s5oz9wCGzh1NlDivqOx4UXCKXBCDUSH3ET00hl7lSM2XgYI1
-TBnsZfZrxQWh7kcT1rMhJ5QQCtkkO7q+RBNGMD+XPNjX12ruOzjjK9SXDrkb5wdJ
-fzcq+Xd4z1TtW0ado4AOkUPB1ltfFLqfpo0kR0BZv3I4sjZsN/+Z0V0OWQqraffA
-sgRFelQArr5T9rXn4fg8ozHSqf4hUmTFpmfwdQcGlBSBVcYn5AGPF8Fqcde+S/uU
-WH1+ETOxQvdibBjWzwloPn9s9h6PYq2lY9sJpx8iQkEeb5mKPtf5P0B6ebClAZLS
-nT0IFaUQAS2zMnaolQ2zepr7BxB4EW/hj8e6DyUadCrlHJhBmd8hh+iVBmoKs2pH
-dmX2Os+PYhcZewoozRrSgx4hxyy/vv9haLdnG7t4TY3OZ+XkwY63I2binZB1NJip
-NiuKmpS5nezMirH4JYlcWrYvjB9teSSnUmjDhDXiZo1jDiVN1Rmy5nk3pyKdVDEC
-AwEAAaNqMGgwHQYDVR0OBBYEFB4M97Zn8uGSJglFwFU5Lnc/QkqiMAwGA1UdEwQF
-MAMBAf8wOQYEZyoHAAQxMC8wLQIBADAJBgUrDgMCGgUAMAcGBWcqAwAABBRFsMLH
-ClZ87lt4DJX5GFPBphzYEDANBgkqhkiG9w0BAQUFAAOCAgEACbODU1kBPpVJufGB
-uvl2ICO1J2B01GqZNF5sAFPZn/KmsSQHRGoqxqWOeBLoR9lYGxMqXnmbnwoqZ6Yl
-PwZpVnPDimZI+ymBV3QGypzqKOg4ZyYr8dW1P2WT+DZdjo2NQCCHGervJ8A9tDkP
-JXtoUHRVnAxZfVo9QZQlUgjgRywVMRnVvwdVxrsStZf0X4OFunHB2WyBEXYKCrC/
-gpf36j36+uwtqSiUO1bd0lEursC9CBWMd1I0ltabrNMdjmEPNXubrjlpC2JgQCA2
-j6/7Nu4tCEoduL+bXPjqpRugc6bY+G7gMwRfaKonh+3ZwZCc7b3jajWvY9+rGNm6
-5ulK6lCKD2GTHuItGeIwlDWSXQ62B68ZgI9HkFFLLk3dheLSClIKF5r8GrBQAuUB
-o2M3IUxExJtRmREOc5wGj1QupyheRDmHVi03vYVElOEMSyycw5KFNGHLD7ibSkNS
-/jQ6fbjpKdx2qcgw+BRxgMYeNkh0IkFch4LoGHGLQYlE535YW6i4jRPpp2zDR+2z
-Gp1iro2C6pSe3VkQw63d4k3jMdXH7OjysP6SHhYKGvzZ8/gntsm+HbRsZJB/9OTE
-W9c3rkIO3aQab3yIVMUWbuF6aC74Or8NpDyJO3inTmODBCEIZ43ygknQW/2xzQ+D
-hNQ+IIX3Sj0rnP0qCglN6oH4EZw=
------END CERTIFICATE-----
 `,
 	},
 	{