Add signed key constraints to SSH CA

builtin/logical/ssh/backend_test.go

@@ -5,12 +5,14 @@ import (
 	"fmt"
 	"os/user"
 	"reflect"
+	"strconv"
 	"testing"
 	"time"
 
 	"golang.org/x/crypto/ssh"
 
 	"encoding/base64"
+	"encoding/json"
 	"errors"
 	"strings"
 
@@ -691,6 +693,58 @@ func TestBackend_OptionsOverrideDefaults(t *testing.T) {
 	logicaltest.Test(t, testCase)
 }
 
+func TestBackend_SignedKeyConstraints(t *testing.T) {
+	config := logical.TestBackendConfig()
+
+	b, err := Factory(context.Background(), config)
+	if err != nil {
+		t.Fatalf("Cannot create backend: %s", err)
+	}
+	testCase := logicaltest.TestCase{
+		Backend: b,
+		Steps: []logicaltest.TestStep{
+			configCaStep(),
+			createRoleStep("weakkey", map[string]interface{}{
+				"key_type":                "ca",
+				"allow_user_certificates": true,
+				"signed_key_constraints": map[string]interface{}{
+					"rsa": json.Number(strconv.FormatInt(4096, 10)),
+				},
+			}),
+			logicaltest.TestStep{
+				Operation: logical.UpdateOperation,
+				Path:      "sign/weakkey",
+				Data: map[string]interface{}{
+					"public_key": publicKey,
+				},
+				ErrorOk: true,
+				Check: func(resp *logical.Response) error {
+					if resp.Data["error"] != "public_key failed to meet the key requirements: your key is of an invalid size: 2048" {
+						return errors.New("a smaller key (2048) was allowed, when the minimum was set for 4096")
+					}
+					return nil
+				},
+			},
+			createRoleStep("stdkey", map[string]interface{}{
+				"key_type":                "ca",
+				"allow_user_certificates": true,
+				"signed_key_constraints": map[string]interface{}{
+					"rsa": json.Number(strconv.FormatInt(2048, 10)),
+				},
+			}),
+			logicaltest.TestStep{
+				Operation: logical.UpdateOperation,
+				Path:      "sign/stdkey",
+				Data: map[string]interface{}{
+					"public_key": publicKey,
+				},
+			},
+		},
+	}
+
+	logicaltest.Test(t, testCase)
+}
+
 func TestBackend_CustomKeyIDFormat(t *testing.T) {
 	config := logical.TestBackendConfig()
 

builtin/logical/ssh/path_roles.go

@@ -48,6 +48,7 @@ type sshRole struct {
 	AllowSubdomains        bool              `mapstructure:"allow_subdomains" json:"allow_subdomains"`
 	AllowUserKeyIDs        bool              `mapstructure:"allow_user_key_ids" json:"allow_user_key_ids"`
 	KeyIDFormat            string            `mapstructure:"key_id_format" json:"key_id_format"`
+	SignedKeyConstraints   map[string]int    `mapstructure:"signed_key_constraints" json:"signed_key_constraints"`
 }
 
 func pathListRoles(b *backend) *framework.Path {
@@ -279,6 +280,13 @@ func pathRoles(b *backend) *framework.Path {
 				'{{public_key_hash}}' - A SHA256 checksum of the public key that is being signed.
 				`,
 			},
+			"signed_key_constraints": &framework.FieldSchema{
+				Type: framework.TypeMap,
+				Description: `
+				[Not applicable for Dynamic type] [Not applicable for OTP type] [Optional for CA type]
+				If set, allows the enforcement of key types and minimum key sizes to be signed.
+				`,
+			},
 		},
 
 		Callbacks: map[logical.Operation]framework.OperationFunc{
@@ -458,6 +466,7 @@ func (b *backend) createCARole(allowedUsers, defaultUser string, data *framework
 
 	defaultCriticalOptions := convertMapToStringValue(data.Get("default_critical_options").(map[string]interface{}))
 	defaultExtensions := convertMapToStringValue(data.Get("default_extensions").(map[string]interface{}))
+	signedKeyConstraints := convertMapToIntValue(data.Get("signed_key_constraints").(map[string]interface{}))
 
 	if ttl != 0 && maxTTL != 0 && ttl > maxTTL {
 		return nil, logical.ErrorResponse(
@@ -469,6 +478,7 @@ func (b *backend) createCARole(allowedUsers, defaultUser string, data *framework
 	role.MaxTTL = maxTTL.String()
 	role.DefaultCriticalOptions = defaultCriticalOptions
 	role.DefaultExtensions = defaultExtensions
+	role.SignedKeyConstraints = signedKeyConstraints
 
 	return role, nil
 }
@@ -534,6 +544,7 @@ func (b *backend) parseRole(role *sshRole) (map[string]interface{}, error) {
 			"key_bits":                 role.KeyBits,
 			"default_critical_options": role.DefaultCriticalOptions,
 			"default_extensions":       role.DefaultExtensions,
+			"signed_key_constraints":   role.SignedKeyConstraints,
 		}
 	case KeyTypeDynamic:
 		result = map[string]interface{}{

builtin/logical/ssh/path_sign.go

@@ -2,7 +2,10 @@ package ssh
 
 import (
 	"context"
+	"crypto/dsa"
+	"crypto/ecdsa"
 	"crypto/rand"
+	"crypto/rsa"
 	"crypto/sha256"
 	"errors"
 	"fmt"
@@ -16,6 +19,7 @@ import (
 	"github.com/hashicorp/vault/helper/strutil"
 	"github.com/hashicorp/vault/logical"
 	"github.com/hashicorp/vault/logical/framework"
+	"golang.org/x/crypto/ed25519"
 	"golang.org/x/crypto/ssh"
 )
 
@@ -110,6 +114,11 @@ func (b *backend) pathSignCertificate(ctx context.Context, req *logical.Request,
 		return logical.ErrorResponse(fmt.Sprintf("failed to parse public_key as SSH key: %s", err)), nil
 	}
 
+	err = b.validateSignedKeyRequirements(userPublicKey, role)
+	if err != nil {
+		return logical.ErrorResponse(fmt.Sprintf("public_key failed to meet the key requirements: %s", err)), nil
+	}
+
 	// Note that these various functions always return "user errors" so we pass
 	// them as 4xx values
 	keyId, err := b.calculateKeyId(data, req, role, userPublicKey)
@@ -384,6 +393,65 @@ func (b *backend) calculateTTL(data *framework.FieldData, role *sshRole) (time.D
 	return ttl, nil
 }
 
+func (b *backend) validateSignedKeyRequirements(publickey ssh.PublicKey, role *sshRole) error {
+	if len(role.SignedKeyConstraints) != 0 {
+		var kstr string
+		var kbits int
+
+		switch k := publickey.(type) {
+		case ssh.CryptoPublicKey:
+			ff := k.CryptoPublicKey()
+			switch k := ff.(type) {
+			case *rsa.PublicKey:
+				kstr = "rsa"
+				kbits = k.N.BitLen()
+			case *dsa.PublicKey:
+				kstr = "dsa"
+				kbits = k.Parameters.P.BitLen()
+			case *ecdsa.PublicKey:
+				kstr = "ecdsa"
+				kbits = k.Curve.Params().BitSize
+			case ed25519.PublicKey:
+				kstr = "ed25519"
+			default:
+				return fmt.Errorf("your public key type of %s is not allowed", kstr)
+			}
+		default:
+			return fmt.Errorf("pubkey not suitable for crypto (expected ssh.CryptoPublicKey but found %T)", k)
+		}
+
+		if value, ok := role.SignedKeyConstraints[kstr]; ok {
+			var pass bool
+			switch kstr {
+			case "rsa":
+				if kbits >= value {
+					pass = true
+				}
+			case "dsa":
+				if kbits >= value {
+					pass = true
+				}
+			case "ecdsa":
+				if kbits >= value {
+					pass = true
+				}
+			case "ed25519":
+				// ed25519 public keys are always 256 bits in length,
+				// so there is no need to inspect their value
+				pass = true
+			}
+
+			if !pass {
+				return fmt.Errorf("your key is of an invalid size: %v", kbits)
+			}
+
+		} else {
+			return fmt.Errorf("your key type of %s is not allowed", kstr)
+		}
+	}
+	return nil
+}
+
 func (b *creationBundle) sign() (retCert *ssh.Certificate, retErr error) {
 	defer func() {
 		if r := recover(); r != nil {

builtin/logical/ssh/util.go

@@ -7,6 +7,7 @@ import (
 	"crypto/rsa"
 	"crypto/x509"
 	"encoding/base64"
+	"encoding/json"
 	"encoding/pem"
 	"fmt"
 	"net"
@@ -215,6 +216,15 @@ func convertMapToStringValue(initial map[string]interface{}) map[string]string {
 	return result
 }
 
+func convertMapToIntValue(initial map[string]interface{}) map[string]int {
+	result := map[string]int{}
+	for key, value := range initial {
+		i, _ := value.(json.Number).Int64()
+		result[key] = int(i)
+	}
+	return result
+}
+
 // Serve a template processor for custom format inputs
 func substQuery(tpl string, data map[string]string) string {
 	for k, v := range data {

website/source/api/secret/ssh/index.html.md

@@ -201,7 +201,10 @@ This endpoint creates or updates a named role.
   available for use: '{{token_display_name}}' - The display name of the token used
   to make the request. '{{role_name}}' - The name of the role signing the request.
   '{{public_key_hash}}' - A SHA256 checksum of the public key that is being signed.
-  e.g. "custom-keyid-{{token_display_name}}",
+  e.g. "custom-keyid-{{token_display_name}}"
+
+- `signed_key_constraints` `(map<string|int>: "")` – Specifies a map of ssh key types
+  and their minimum sizes which are allowed to be signed by the CA type.
 
 ### Sample Payload