Check cnf claim with CSR or SSH public key fingerprint

This commit allows tying tokens with the provided  CSR or SSH public
key. Tokens with a confirmation claim kid (cnf.kid) will validate that
the provided fingerprint (kid) matches the CSR or SSH public key.

This check will only be present in JWK and X5C provisioners.

Fixes #1637

authority/authorize_test.go

@@ -88,6 +88,39 @@ func generateToken(sub, iss, aud string, sans []string, iat time.Time, jwk *jose
 	return jose.Signed(sig).Claims(claims).CompactSerialize()
 }
 
+func generateCustomToken(sub, iss, aud string, jwk *jose.JSONWebKey, extraHeaders, extraClaims map[string]any) (string, error) {
+	so := new(jose.SignerOptions)
+	so.WithType("JWT")
+	so.WithHeader("kid", jwk.KeyID)
+
+	for k, v := range extraHeaders {
+		so.WithHeader(jose.HeaderKey(k), v)
+	}
+
+	sig, err := jose.NewSigner(jose.SigningKey{Algorithm: jose.ES256, Key: jwk.Key}, so)
+	if err != nil {
+		return "", err
+	}
+
+	id, err := randutil.ASCII(64)
+	if err != nil {
+		return "", err
+	}
+
+	iat := time.Now()
+	claims := jose.Claims{
+		ID:        id,
+		Subject:   sub,
+		Issuer:    iss,
+		IssuedAt:  jose.NewNumericDate(iat),
+		NotBefore: jose.NewNumericDate(iat),
+		Expiry:    jose.NewNumericDate(iat.Add(5 * time.Minute)),
+		Audience:  []string{aud},
+	}
+
+	return jose.Signed(sig).Claims(claims).Claims(extraClaims).CompactSerialize()
+}
+
 func TestAuthority_authorizeToken(t *testing.T) {
 	a := testAuthority(t)
 
@@ -491,7 +524,7 @@ func TestAuthority_authorizeSign(t *testing.T) {
 				}
 			} else {
 				if assert.Nil(t, tc.err) {
-					assert.Equals(t, 10, len(got)) // number of provisioner.SignOptions returned
+					assert.Equals(t, 11, len(got)) // number of provisioner.SignOptions returned
 				}
 			}
 		})

authority/provisioner/jwk.go

@@ -19,15 +19,20 @@ import (
 // jwtPayload extends jwt.Claims with step attributes.
 type jwtPayload struct {
 	jose.Claims
-	SANs []string     `json:"sans,omitempty"`
-	Step *stepPayload `json:"step,omitempty"`
+	SANs         []string     `json:"sans,omitempty"`
+	Step         *stepPayload `json:"step,omitempty"`
+	Confirmation *cnfPayload  `json:"cnf,omitempty"`
 }
 
 type stepPayload struct {
 	SSH *SignSSHOptions `json:"ssh,omitempty"`
 	RA  *RAInfo         `json:"ra,omitempty"`
 }
 
+type cnfPayload struct {
+	Kid string `json:"kid,omitempty"`
+}
+
 // JWK is the default provisioner, an entity that can sign tokens necessary for
 // signature requests.
 type JWK struct {
@@ -183,13 +188,20 @@ func (p *JWK) AuthorizeSign(ctx context.Context, token string) ([]SignOption, er
 		}
 	}
 
+	// Check the fingerprint of the certificate request if given.
+	var fingerprint string
+	if claims.Confirmation != nil {
+		fingerprint = claims.Confirmation.Kid
+	}
+
 	return []SignOption{
 		self,
 		templateOptions,
 		// modifiers / withOptions
 		newProvisionerExtensionOption(TypeJWK, p.Name, p.Key.KeyID).WithControllerOptions(p.ctl),
 		profileDefaultDuration(p.ctl.Claimer.DefaultTLSCertDuration()),
 		// validators
+		fingerprintValidator(fingerprint),
 		commonNameSliceValidator(append([]string{claims.Subject}, claims.SANs...)),
 		defaultPublicKeyValidator{},
 		newDefaultSANsValidator(ctx, claims.SANs),
@@ -229,6 +241,11 @@ func (p *JWK) AuthorizeSSHSign(_ context.Context, token string) ([]SignOption, e
 		sshCertOptionsValidator(SignSSHOptions{KeyID: claims.Subject}),
 	}
 
+	// Check the fingerprint of the certificate request if given.
+	if claims.Confirmation != nil && claims.Confirmation.Kid != "" {
+		signOptions = append(signOptions, sshFingerprintValidator(claims.Confirmation.Kid))
+	}
+
 	// Default template attributes.
 	certType := sshutil.UserCert
 	keyID := claims.Subject

authority/provisioner/jwk_test.go

@@ -13,7 +13,9 @@ import (
 	"testing"
 	"time"
 
+	"go.step.sm/crypto/fingerprint"
 	"go.step.sm/crypto/jose"
+	"golang.org/x/crypto/ssh"
 
 	"github.com/smallstep/assert"
 	"github.com/smallstep/certificates/api/render"
@@ -247,19 +249,23 @@ func TestJWK_AuthorizeSign(t *testing.T) {
 	t2, err := generateToken("subject", p1.Name, testAudiences.Sign[0], "name@smallstep.com", []string{}, time.Now(), key1)
 	assert.FatalError(t, err)
 
+	t3, err := generateCustomToken("subject", p1.Name, testAudiences.Sign[0], key1, nil, map[string]any{"cnf": map[string]any{"kid": "fingerprint"}})
+	assert.FatalError(t, err)
+
 	// invalid signature
 	failSig := t1[0 : len(t1)-2]
 
 	type args struct {
 		token string
 	}
 	tests := []struct {
-		name string
-		prov *JWK
-		args args
-		code int
-		err  error
-		sans []string
+		name        string
+		prov        *JWK
+		args        args
+		code        int
+		err         error
+		sans        []string
+		fingerprint string
 	}{
 		{
 			name: "fail-signature",
@@ -284,6 +290,15 @@ func TestJWK_AuthorizeSign(t *testing.T) {
 			err:  nil,
 			sans: []string{"subject"},
 		},
+		{
+			name:        "ok-cnf",
+			prov:        p1,
+			args:        args{t3},
+			code:        http.StatusOK,
+			err:         nil,
+			sans:        []string{"subject"},
+			fingerprint: "fingerprint",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -297,7 +312,7 @@ func TestJWK_AuthorizeSign(t *testing.T) {
 				}
 			} else {
 				if assert.NotNil(t, got) {
-					assert.Equals(t, 10, len(got))
+					assert.Equals(t, 11, len(got))
 					for _, o := range got {
 						switch v := o.(type) {
 						case *JWK:
@@ -321,6 +336,8 @@ func TestJWK_AuthorizeSign(t *testing.T) {
 						case *x509NamePolicyValidator:
 							assert.Equals(t, nil, v.policyEngine)
 						case *WebhookController:
+						case fingerprintValidator:
+							assert.Equals(t, tt.fingerprint, string(v))
 						default:
 							assert.FatalError(t, fmt.Errorf("unexpected sign option of type %T", v))
 						}
@@ -393,17 +410,6 @@ func TestJWK_AuthorizeSSHSign(t *testing.T) {
 	jwk, err := decryptJSONWebKey(p1.EncryptedKey)
 	assert.FatalError(t, err)
 
-	iss, aud := p1.Name, testAudiences.SSHSign[0]
-
-	t1, err := generateSimpleSSHUserToken(iss, aud, jwk)
-	assert.FatalError(t, err)
-
-	t2, err := generateSimpleSSHHostToken(iss, aud, jwk)
-	assert.FatalError(t, err)
-
-	// invalid signature
-	failSig := t1[0 : len(t1)-2]
-
 	key, err := generateJSONWebKey()
 	assert.FatalError(t, err)
 
@@ -417,6 +423,39 @@ func TestJWK_AuthorizeSSHSign(t *testing.T) {
 	rsa1024, err := rsa.GenerateKey(rand.Reader, 1024)
 	assert.FatalError(t, err)
 
+	// Calculate fingerprint
+	sshPub, err := ssh.NewPublicKey(pub)
+	assert.FatalError(t, err)
+	fp, err := fingerprint.New(sshPub.Marshal(), crypto.SHA256, fingerprint.Base64RawURLFingerprint)
+	assert.FatalError(t, err)
+
+	iss, aud := p1.Name, testAudiences.SSHSign[0]
+
+	t1, err := generateSimpleSSHUserToken(iss, aud, jwk)
+	assert.FatalError(t, err)
+
+	t2, err := generateSimpleSSHHostToken(iss, aud, jwk)
+	assert.FatalError(t, err)
+
+	t3, err := generateCustomToken("sub", iss, aud, jwk, nil, map[string]any{
+		"step": map[string]any{
+			"ssh": map[string]any{"certType": "host", "principals": []string{"smallstep.com"}},
+		},
+		"cnf": map[string]any{"kid": fp},
+	})
+	assert.FatalError(t, err)
+
+	t4, err := generateCustomToken("sub", iss, aud, jwk, nil, map[string]any{
+		"step": map[string]any{
+			"ssh": map[string]any{"certType": "host", "principals": []string{"smallstep.com"}},
+		},
+		"cnf": map[string]any{"kid": "bad-fingerprint"},
+	})
+	assert.FatalError(t, err)
+
+	// invalid signature
+	failSig := t1[0 : len(t1)-2]
+
 	userDuration := p1.ctl.Claimer.DefaultUserSSHCertDuration()
 	hostDuration := p1.ctl.Claimer.DefaultHostSSHCertDuration()
 	expectedUserOptions := &SignSSHOptions{
@@ -451,9 +490,11 @@ func TestJWK_AuthorizeSSHSign(t *testing.T) {
 		{"host-type", p1, args{t2, SignSSHOptions{CertType: "host"}, pub}, expectedHostOptions, http.StatusOK, false, false},
 		{"host-principals", p1, args{t2, SignSSHOptions{Principals: []string{"smallstep.com"}}, pub}, expectedHostOptions, http.StatusOK, false, false},
 		{"host-options", p1, args{t2, SignSSHOptions{CertType: "host", Principals: []string{"smallstep.com"}}, pub}, expectedHostOptions, http.StatusOK, false, false},
+		{"host-cnf", p1, args{t3, SignSSHOptions{CertType: "host", Principals: []string{"smallstep.com"}}, pub}, expectedHostOptions, http.StatusOK, false, false},
 		{"fail-sshCA-disabled", p2, args{"foo", SignSSHOptions{}, pub}, expectedUserOptions, http.StatusUnauthorized, true, false},
 		{"fail-signature", p1, args{failSig, SignSSHOptions{}, pub}, nil, http.StatusUnauthorized, true, false},
-		{"rail-rsa1024", p1, args{t1, SignSSHOptions{}, rsa1024.Public()}, expectedUserOptions, http.StatusOK, false, true},
+		{"fail-rsa1024", p1, args{t1, SignSSHOptions{}, rsa1024.Public()}, expectedUserOptions, http.StatusOK, false, true},
+		{"fail-cnf", p1, args{t4, SignSSHOptions{CertType: "host", Principals: []string{"smallstep.com"}}, pub}, expectedHostOptions, http.StatusUnauthorized, false, true},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

authority/provisioner/sign_options.go

@@ -5,7 +5,10 @@ import (
 	"crypto/ecdsa"
 	"crypto/ed25519"
 	"crypto/rsa"
+	"crypto/sha256"
+	"crypto/subtle"
 	"crypto/x509"
+	"encoding/base64"
 	"encoding/json"
 	"net"
 	"net/http"
@@ -492,3 +495,21 @@ func (o *provisionerExtensionOption) Modify(cert *x509.Certificate, _ SignOption
 	cert.ExtraExtensions = append(cert.ExtraExtensions, ext)
 	return nil
 }
+
+// fingerprintValidator is a CertificateRequestValidator that checks the
+// fingerprint of the certificate with the provided one.
+type fingerprintValidator string
+
+func (s fingerprintValidator) Valid(cr *x509.CertificateRequest) error {
+	if s != "" {
+		expected, err := base64.RawURLEncoding.DecodeString(string(s))
+		if err != nil {
+			return errs.ForbiddenErr(err, "error decoding fingerprint")
+		}
+		sum := sha256.Sum256(cr.Raw)
+		if subtle.ConstantTimeCompare(expected, sum[:]) != 1 {
+			return errs.Forbidden("certificate request fingerprint does not match %q", s)
+		}
+	}
+	return nil
+}

authority/provisioner/sign_ssh_options.go

@@ -2,6 +2,9 @@ package provisioner
 
 import (
 	"crypto/rsa"
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/base64"
 	"encoding/binary"
 	"encoding/json"
 	"fmt"
@@ -44,6 +47,13 @@ type SSHCertOptionsValidator interface {
 	Valid(got SignSSHOptions) error
 }
 
+// SSHPublicKeyValidator is the interface used to validate the public key of an
+// SSH certificate.
+type SSHPublicKeyValidator interface {
+	SignOption
+	Valid(got ssh.PublicKey) error
+}
+
 // SignSSHOptions contains the options that can be passed to the SignSSH method.
 type SignSSHOptions struct {
 	CertType     string          `json:"certType"`
@@ -419,6 +429,24 @@ func (v *sshNamePolicyValidator) Valid(cert *ssh.Certificate, _ SignSSHOptions)
 	}
 }
 
+// sshFingerprintValidator is a SSHPublicKeyValidator that checks the
+// fingerprint of the public key with the provided one.
+type sshFingerprintValidator string
+
+func (s sshFingerprintValidator) Valid(key ssh.PublicKey) error {
+	if s != "" {
+		expected, err := base64.RawURLEncoding.DecodeString(string(s))
+		if err != nil {
+			return errs.ForbiddenErr(err, "error decoding fingerprint")
+		}
+		sum := sha256.Sum256(key.Marshal())
+		if subtle.ConstantTimeCompare(expected, sum[:]) != 1 {
+			return errs.Forbidden("ssh public key fingerprint does not match %q", s)
+		}
+	}
+	return nil
+}
+
 // sshCertTypeUInt32
 func sshCertTypeUInt32(ct string) uint32 {
 	switch ct {

authority/provisioner/ssh_test.go

@@ -51,6 +51,7 @@ func signSSHCertificate(key crypto.PublicKey, opts SignSSHOptions, signOpts []Si
 	var mods []SSHCertModifier
 	var certOptions []sshutil.Option
 	var validators []SSHCertValidator
+	var keyValidators []SSHPublicKeyValidator
 
 	for _, op := range signOpts {
 		switch o := op.(type) {
@@ -71,11 +72,19 @@ func signSSHCertificate(key crypto.PublicKey, opts SignSSHOptions, signOpts []Si
 			}
 		// call webhooks
 		case *WebhookController:
+		case sshFingerprintValidator:
+			keyValidators = append(keyValidators, o)
 		default:
 			return nil, fmt.Errorf("signSSH: invalid extra option type %T", o)
 		}
 	}
 
+	for _, v := range keyValidators {
+		if err := v.Valid(pub); err != nil {
+			return nil, err
+		}
+	}
+
 	// Simulated certificate request with request options.
 	cr := sshutil.CertificateRequest{
 		Type:       opts.CertType,

authority/provisioner/utils_test.go

@@ -765,6 +765,37 @@ func generateToken(sub, iss, aud, email string, sans []string, iat time.Time, jw
 	return jose.Signed(sig).Claims(claims).CompactSerialize()
 }
 
+func generateCustomToken(sub, iss, aud string, jwk *jose.JSONWebKey, extraHeaders, extraClaims map[string]any) (string, error) {
+	so := new(jose.SignerOptions)
+	so.WithType("JWT")
+	so.WithHeader("kid", jwk.KeyID)
+
+	for k, v := range extraHeaders {
+		so.WithHeader(jose.HeaderKey(k), v)
+	}
+
+	sig, err := jose.NewSigner(jose.SigningKey{Algorithm: jose.ES256, Key: jwk.Key}, so)
+	if err != nil {
+		return "", err
+	}
+
+	id, err := randutil.ASCII(64)
+	if err != nil {
+		return "", err
+	}
+	iat := time.Now()
+	claims := jose.Claims{
+		ID:        id,
+		Subject:   sub,
+		Issuer:    iss,
+		IssuedAt:  jose.NewNumericDate(iat),
+		NotBefore: jose.NewNumericDate(iat),
+		Expiry:    jose.NewNumericDate(iat.Add(5 * time.Minute)),
+		Audience:  []string{aud},
+	}
+	return jose.Signed(sig).Claims(claims).Claims(extraClaims).CompactSerialize()
+}
+
 func generateOIDCToken(sub, iss, aud, email, preferredUsername string, iat time.Time, jwk *jose.JSONWebKey, tokOpts ...tokOption) (string, error) {
 	so := new(jose.SignerOptions)
 	so.WithType("JWT")