Check cnf claim with CSR or SSH public key fingerprint

This commit allows to tie 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/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/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 SSHCertValidator 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/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
+	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/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,