Add configuration option for specifying the minimum public key length

Instead of using the defaultPublicKeyValidator a new validator called
publicKeyMinimumLengthValidator has been implemented that uses a
configurable minimum length for public keys in CSRs.

It's also an option to alter the defaultPublicKeyValidator to also
take a parameter, but that would touch quite some lines of code. This
might be a viable option after merging SCEP support.

authority/provisioner/scep.go

@@ -2,6 +2,7 @@ package provisioner
 
 import (
 	"context"
+	"fmt"
 	"time"
 
 	"github.com/pkg/errors"
@@ -17,6 +18,8 @@ type SCEP struct {
 	ForceCN           bool     `json:"forceCN,omitempty"`
 	ChallengePassword string   `json:"challenge,omitempty"`
 	Capabilities      []string `json:"capabilities,omitempty"`
+	// MinimumPublicKeyLength is the minimum length for public keys in CSRs
+	MinimumPublicKeyLength int      `json:"minimumPublicKeyLength,omitempty"`
 	Options                *Options `json:"options,omitempty"`
 	Claims                 *Claims  `json:"claims,omitempty"`
 	claimer                *Claimer
@@ -79,6 +82,15 @@ func (s *SCEP) Init(config Config) (err error) {
 	s.secretChallengePassword = s.ChallengePassword
 	s.ChallengePassword = "*** redacted ***"
 
+	// Default to 2048 bits minimum public key length (for CSRs) if not set
+	if s.MinimumPublicKeyLength == 0 {
+		s.MinimumPublicKeyLength = 2048
+	}
+
+	if s.MinimumPublicKeyLength%8 != 0 {
+		return fmt.Errorf("only minimum public keys exactly divisible by 8 are supported; %d is not exactly divisibly by 8", s.MinimumPublicKeyLength)
+	}
+
 	// TODO: add other, SCEP specific, options?
 
 	return err
@@ -94,7 +106,7 @@ func (s *SCEP) AuthorizeSign(ctx context.Context, token string) ([]SignOption, e
 		newForceCNOption(s.ForceCN),
 		profileDefaultDuration(s.claimer.DefaultTLSCertDuration()),
 		// validators
-		defaultPublicKeyValidator{},
+		newPublicKeyMinimumLengthValidator(s.MinimumPublicKeyLength),
 		newValidityValidator(s.claimer.MinTLSCertDuration(), s.claimer.MaxTLSCertDuration()),
 	}, nil
 }

authority/provisioner/sign_options.go

@@ -8,6 +8,7 @@ import (
 	"crypto/x509/pkix"
 	"encoding/asn1"
 	"encoding/json"
+	"fmt"
 	"net"
 	"net/url"
 	"reflect"
@@ -117,6 +118,36 @@ func (v defaultPublicKeyValidator) Valid(req *x509.CertificateRequest) error {
 	return nil
 }
 
+// publicKeyMinimumLengthValidator validates the length (in bits) of the public key
+// of a certificate request is at least a certain length
+type publicKeyMinimumLengthValidator struct {
+	length int
+}
+
+// newPublicKeyMinimumLengthValidator creates a new publicKeyMinimumLengthValidator
+// with the given length as its minimum value
+// TODO: change the defaultPublicKeyValidator to have a configurable length instead?
+func newPublicKeyMinimumLengthValidator(length int) publicKeyMinimumLengthValidator {
+	return publicKeyMinimumLengthValidator{
+		length: length,
+	}
+}
+
+// Valid checks that certificate request common name matches the one configured.
+func (v publicKeyMinimumLengthValidator) Valid(req *x509.CertificateRequest) error {
+	switch k := req.PublicKey.(type) {
+	case *rsa.PublicKey:
+		minimumLengthInBytes := v.length / 8
+		if k.Size() < minimumLengthInBytes {
+			return fmt.Errorf("rsa key in CSR must be at least %d bits (%d bytes)", v.length, minimumLengthInBytes)
+		}
+	case *ecdsa.PublicKey, ed25519.PublicKey:
+	default:
+		return errors.Errorf("unrecognized public key of type '%T' in CSR", k)
+	}
+	return nil
+}
+
 // commonNameValidator validates the common name of a certificate request.
 type commonNameValidator string