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

@@ -19,8 +19,9 @@ 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 {
@@ -28,6 +29,10 @@ type stepPayload struct {
 	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 {
@@ -183,6 +188,12 @@ 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,
@@ -190,6 +201,7 @@ func (p *JWK) AuthorizeSign(ctx context.Context, token string) ([]SignOption, er
 		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"
@@ -247,6 +249,9 @@ 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]
 
@@ -254,12 +259,13 @@ func TestJWK_AuthorizeSign(t *testing.T) {
 		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")

authority/provisioner/x5c.go

@@ -21,9 +21,10 @@ import (
 // x5cPayload extends jwt.Claims with step attributes.
 type x5cPayload struct {
 	jose.Claims
-	SANs   []string     `json:"sans,omitempty"`
-	Step   *stepPayload `json:"step,omitempty"`
-	chains [][]*x509.Certificate
+	SANs         []string     `json:"sans,omitempty"`
+	Step         *stepPayload `json:"step,omitempty"`
+	Confirmation *cnfPayload  `json:"cnf,omitempty"`
+	chains       [][]*x509.Certificate
 }
 
 // X5C is the default provisioner, an entity that can sign tokens necessary for
@@ -233,6 +234,12 @@ func (p *X5C) 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,
@@ -243,6 +250,7 @@ func (p *X5C) AuthorizeSign(ctx context.Context, token string) ([]SignOption, er
 			x5cLeaf.NotBefore, x5cLeaf.NotAfter,
 		},
 		// validators
+		fingerprintValidator(fingerprint),
 		commonNameValidator(claims.Subject),
 		newDefaultSANsValidator(ctx, claims.SANs),
 		defaultPublicKeyValidator{},
@@ -285,6 +293,11 @@ func (p *X5C) 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/x5c_test.go

@@ -3,6 +3,7 @@ package provisioner
 import (
 	"context"
 	"crypto/x509"
+	"encoding/base64"
 	"errors"
 	"fmt"
 	"net/http"
@@ -413,11 +414,12 @@ func TestX5C_AuthorizeSign(t *testing.T) {
 	assert.FatalError(t, err)
 
 	type test struct {
-		p     *X5C
-		token string
-		code  int
-		err   error
-		sans  []string
+		p           *X5C
+		token       string
+		code        int
+		err         error
+		sans        []string
+		fingerprint string
 	}
 	tests := map[string]func(*testing.T) test{
 		"fail/invalid-token": func(t *testing.T) test {
@@ -456,13 +458,36 @@ func TestX5C_AuthorizeSign(t *testing.T) {
 				sans:  []string{"127.0.0.1", "foo", "max@smallstep.com"},
 			}
 		},
+		"ok/cnf": func(t *testing.T) test {
+			p, err := generateX5C(nil)
+			assert.FatalError(t, err)
+
+			x5c := make([]string, len(certs))
+			for i, cert := range certs {
+				x5c[i] = base64.StdEncoding.EncodeToString(cert.Raw)
+			}
+			extraHeaders := map[string]any{"x5c": x5c}
+			extraClaims := map[string]any{
+				"sans": []string{"127.0.0.1", "foo", "max@smallstep.com"},
+				"cnf":  map[string]any{"kid": "fingerprint"},
+			}
+
+			tok, err := generateCustomToken("foo", p.GetName(), testAudiences.Sign[0], jwk, extraHeaders, extraClaims)
+			assert.FatalError(t, err)
+			return test{
+				p:           p,
+				token:       tok,
+				sans:        []string{"127.0.0.1", "foo", "max@smallstep.com"},
+				fingerprint: "fingerprint",
+			}
+		},
 	}
 	for name, tt := range tests {
 		t.Run(name, func(t *testing.T) {
 			tc := tt(t)
 			ctx := NewContextWithMethod(context.Background(), SignIdentityMethod)
 			if opts, err := tc.p.AuthorizeSign(ctx, tc.token); err != nil {
-				if assert.NotNil(t, tc.err) {
+				if assert.NotNil(t, tc.err, err.Error()) {
 					var sc render.StatusCodedError
 					if assert.True(t, errors.As(err, &sc), "error does not implement StatusCodedError interface") {
 						assert.Equals(t, sc.StatusCode(), tc.code)
@@ -472,7 +497,7 @@ func TestX5C_AuthorizeSign(t *testing.T) {
 			} else {
 				if assert.Nil(t, tc.err) {
 					if assert.NotNil(t, opts) {
-						assert.Equals(t, 10, len(opts))
+						assert.Equals(t, 11, len(opts))
 						for _, o := range opts {
 							switch v := o.(type) {
 							case *X5C:
@@ -502,6 +527,8 @@ func TestX5C_AuthorizeSign(t *testing.T) {
 								assert.Len(t, 0, v.webhooks)
 								assert.Equals(t, linkedca.Webhook_X509, v.certType)
 								assert.Len(t, 2, v.options)
+							case fingerprintValidator:
+								assert.Equals(t, tc.fingerprint, string(v))
 							default:
 								assert.FatalError(t, fmt.Errorf("unexpected sign option of type %T", v))
 							}
@@ -627,11 +654,13 @@ func TestX5C_AuthorizeSSHSign(t *testing.T) {
 	_, fn := mockNow()
 	defer fn()
 	type test struct {
-		p      *X5C
-		token  string
-		claims *x5cPayload
-		code   int
-		err    error
+		p           *X5C
+		token       string
+		claims      *x5cPayload
+		fingerprint string
+		count       int
+		code        int
+		err         error
 	}
 	tests := map[string]func(*testing.T) test{
 		"fail/sshCA-disabled": func(t *testing.T) test {
@@ -719,7 +748,7 @@ func TestX5C_AuthorizeSSHSign(t *testing.T) {
 					Audience:  []string{testAudiences.SSHSign[0]},
 				},
 				Step: &stepPayload{SSH: &SignSSHOptions{
-					CertType:    SSHHostCert,
+					CertType:    SSHUserCert,
 					KeyID:       "foo",
 					Principals:  []string{"max", "mariano", "alan"},
 					ValidAfter:  TimeDuration{d: 5 * time.Minute},
@@ -732,6 +761,7 @@ func TestX5C_AuthorizeSSHSign(t *testing.T) {
 				p:      p,
 				claims: claims,
 				token:  tok,
+				count:  12,
 			}
 		},
 		"ok/without-claims": func(t *testing.T) test {
@@ -759,6 +789,42 @@ func TestX5C_AuthorizeSSHSign(t *testing.T) {
 				p:      p,
 				claims: claims,
 				token:  tok,
+				count:  10,
+			}
+		},
+		"ok/cnf": func(t *testing.T) test {
+			p, err := generateX5C(nil)
+			assert.FatalError(t, err)
+
+			id, err := randutil.ASCII(64)
+			assert.FatalError(t, err)
+			now := time.Now()
+			claims := &x5cPayload{
+				Claims: jose.Claims{
+					ID:        id,
+					Subject:   "foo",
+					Issuer:    p.GetName(),
+					IssuedAt:  jose.NewNumericDate(now),
+					NotBefore: jose.NewNumericDate(now),
+					Expiry:    jose.NewNumericDate(now.Add(5 * time.Minute)),
+					Audience:  []string{testAudiences.SSHSign[0]},
+				},
+				Step: &stepPayload{SSH: &SignSSHOptions{
+					CertType:   SSHHostCert,
+					Principals: []string{"host.smallstep.com"},
+				}},
+				Confirmation: &cnfPayload{
+					Kid: "fingerprint",
+				},
+			}
+			tok, err := generateX5CSSHToken(x5cJWK, claims, withX5CHdr(x5cCerts))
+			assert.FatalError(t, err)
+			return test{
+				p:           p,
+				claims:      claims,
+				token:       tok,
+				fingerprint: "fingerprint",
+				count:       11,
 			}
 		},
 	}
@@ -808,16 +874,14 @@ func TestX5C_AuthorizeSSHSign(t *testing.T) {
 								assert.Len(t, 0, v.webhooks)
 								assert.Equals(t, linkedca.Webhook_SSH, v.certType)
 								assert.Len(t, 2, v.options)
+							case sshFingerprintValidator:
+								assert.Equals(t, tc.fingerprint, string(v))
 							default:
 								assert.FatalError(t, fmt.Errorf("unexpected sign option of type %T", v))
 							}
 							tot++
 						}
-						if len(tc.claims.Step.SSH.CertType) > 0 {
-							assert.Equals(t, tot, 12)
-						} else {
-							assert.Equals(t, tot, 10)
-						}
+						assert.Equals(t, tc.count, tot)
 					}
 				}
 			}

authority/ssh.go

@@ -148,12 +148,16 @@ func (a *Authority) GetSSHBastion(ctx context.Context, user, hostname string) (*
 // SignSSH creates a signed SSH certificate with the given public key and options.
 func (a *Authority) SignSSH(_ context.Context, key ssh.PublicKey, opts provisioner.SignSSHOptions, signOpts ...provisioner.SignOption) (*ssh.Certificate, error) {
 	var (
-		certOptions []sshutil.Option
-		mods        []provisioner.SSHCertModifier
-		validators  []provisioner.SSHCertValidator
+		certOptions   []sshutil.Option
+		mods          []provisioner.SSHCertModifier
+		validators    []provisioner.SSHCertValidator
+		keyValidators []provisioner.SSHPublicKeyValidator
 	)
 
-	// Validate given options.
+	// Validate given key and options
+	if key == nil {
+		return nil, errs.BadRequest("ssh public key cannot be nil")
+	}
 	if err := opts.Validate(); err != nil {
 		return nil, err
 	}
@@ -177,6 +181,10 @@ func (a *Authority) SignSSH(_ context.Context, key ssh.PublicKey, opts provision
 		case provisioner.SSHCertModifier:
 			mods = append(mods, o)
 
+		// validate the ssh public key
+		case provisioner.SSHPublicKeyValidator:
+			keyValidators = append(keyValidators, o)
+
 		// validate the ssh.Certificate
 		case provisioner.SSHCertValidator:
 			validators = append(validators, o)
@@ -196,6 +204,16 @@ func (a *Authority) SignSSH(_ context.Context, key ssh.PublicKey, opts provision
 		}
 	}
 
+	// Validate public key
+	for _, v := range keyValidators {
+		if err := v.Valid(key); err != nil {
+			return nil, errs.ApplyOptions(
+				errs.ForbiddenErr(err, err.Error()),
+				errs.WithKeyVal("signOptions", signOpts),
+			)
+		}
+	}
+
 	// Simulated certificate request with request options.
 	cr := sshutil.CertificateRequest{
 		Type:       opts.CertType,

authority/tls_test.go

@@ -18,6 +18,7 @@ import (
 	"testing"
 	"time"
 
+	"go.step.sm/crypto/fingerprint"
 	"go.step.sm/crypto/jose"
 	"go.step.sm/crypto/keyutil"
 	"go.step.sm/crypto/minica"
@@ -593,6 +594,43 @@ ZYtQ9Ot36qc=
 				code:     http.StatusForbidden,
 			}
 		},
+		"fail with cnf": func(t *testing.T) *signTest {
+			csr := getCSR(t, priv)
+
+			auth := testAuthority(t)
+			auth.config.AuthorityConfig.Template = a.config.AuthorityConfig.Template
+			auth.db = &db.MockAuthDB{
+				MUseToken: func(id, tok string) (bool, error) {
+					return true, nil
+				},
+				MStoreCertificate: func(crt *x509.Certificate) error {
+					assert.Equals(t, crt.Subject.CommonName, "smallstep test")
+					assert.Equals(t, crt.DNSNames, []string{"test.smallstep.com"})
+					return nil
+				},
+			}
+
+			// Create a token with cnf
+			tok, err := generateCustomToken("smallstep test", "step-cli", testAudiences.Sign[0], key, nil, map[string]any{
+				"sans": []string{"test.smallstep.com"},
+				"cnf":  map[string]any{"kid": "bad-fingerprint"},
+			})
+			assert.FatalError(t, err)
+
+			opts, err := auth.Authorize(ctx, tok)
+			assert.FatalError(t, err)
+
+			return &signTest{
+				auth:      auth,
+				csr:       csr,
+				extraOpts: opts,
+				signOpts:  signOpts,
+				notBefore: signOpts.NotBefore.Time().Truncate(time.Second),
+				notAfter:  signOpts.NotAfter.Time().Truncate(time.Second),
+				err:       errors.New(`certificate request fingerprint does not match "bad-fingerprint"`),
+				code:      http.StatusForbidden,
+			}
+		},
 		"ok": func(t *testing.T) *signTest {
 			csr := getCSR(t, priv)
 			_a := testAuthority(t)
@@ -840,6 +878,44 @@ ZYtQ9Ot36qc=
 				extensionsCount: 6,
 			}
 		},
+		"ok with cnf": func(t *testing.T) *signTest {
+			csr := getCSR(t, priv)
+			fingerprint, err := fingerprint.New(csr.Raw, crypto.SHA256, fingerprint.Base64RawURLFingerprint)
+			assert.FatalError(t, err)
+
+			auth := testAuthority(t)
+			auth.config.AuthorityConfig.Template = a.config.AuthorityConfig.Template
+			auth.db = &db.MockAuthDB{
+				MUseToken: func(id, tok string) (bool, error) {
+					return true, nil
+				},
+				MStoreCertificate: func(crt *x509.Certificate) error {
+					assert.Equals(t, crt.Subject.CommonName, "smallstep test")
+					assert.Equals(t, crt.DNSNames, []string{"test.smallstep.com"})
+					return nil
+				},
+			}
+
+			// Create a token with cnf
+			tok, err := generateCustomToken("smallstep test", "step-cli", testAudiences.Sign[0], key, nil, map[string]any{
+				"sans": []string{"test.smallstep.com"},
+				"cnf":  map[string]any{"kid": fingerprint},
+			})
+			assert.FatalError(t, err)
+
+			opts, err := auth.Authorize(ctx, tok)
+			assert.FatalError(t, err)
+
+			return &signTest{
+				auth:            auth,
+				csr:             csr,
+				extraOpts:       opts,
+				signOpts:        signOpts,
+				notBefore:       signOpts.NotBefore.Time().Truncate(time.Second),
+				notAfter:        signOpts.NotAfter.Time().Truncate(time.Second),
+				extensionsCount: 6,
+			}
+		},
 	}
 
 	for name, genTestCase := range tests {