Allow OpenSSH-style key type identifiers (#14143)

* Allow OpenSSH-style key type identifiers

To bring better parity with the changes of #14008, wherein we allowed
OpenSSH-style key identifiers during generation. When specifying a list
of allowed keys, validate against both OpenSSH-style key identifiers
and the usual simplified names as well ("rsa" or "ecdsa"). Notably, the
PKI secrets engine prefers "ec" over "ecdsa", so we permit both as well.

Signed-off-by: Alexander Scheel <[email protected]>

* Fix missing quote in docs

builtin/logical/ssh/backend_test.go

@@ -119,6 +119,9 @@ SjOQL/GkH1nkRcDS9++aAAAAAmNhAQID
 -----END OPENSSH PRIVATE KEY-----
 `
 
+	publicKeyECDSA256 = `ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBJsfOouYIjJNI23QJqaDsFTGukm21fRAMeGvKZDB59i5jnX1EubMH1AEjjzz4fgySUlyWKo+TS31rxU8kX3DDM4= [email protected]`
+	publicKeyECDSA521 = `ecdsa-sha2-nistp521 AAAAE2VjZHNhLXNoYTItbmlzdHA1MjEAAAAIbmlzdHA1MjEAAACFBAEg73ORD4J3FV2CrL01gLSKREO2EHrZPlJCOeDL5OKD3M1GCHv3q8O452RW49Aw+8zFFFU5u6d1Ys3Qsj05zdaQwQDt/D3ceWLGVkWiKyLPQStfn0GGOZh3YFKEw5XmeW9jh6xudEHlKs4Pfv2FrroaUKZvM2SlxR/feOK0tCQyq3MN/g== [email protected]`
+
 	// testPublicKeyInstall is the public key that is installed in the
 	// admin account's authorized_keys
 	testPublicKeyInstall = "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC9i+hFxZHGo6KblVme4zrAcJstR6I0PTJozW286X4WyvPnkMYDQ5mnhEYC7UWCvjoTWbPEXPX7NjhRtwQTGD67bV+lrxgfyzK1JZbUXK4PwgKJvQD+XyyWYMzDgGSQY61KUSqCxymSm/9NZkPU3ElaQ9xQuTzPpztM4ROfb8f2Yv6/ZESZsTo0MTAkp8Pcy+WkioI/uJ1H7zqs0EA4OMY4aDJRu0UtP4rTVeYNEAuRXdX+eH4aW3KMvhzpFTjMbaJHJXlEeUm2SaX5TNQyTOvghCeQILfYIL/Ca2ij8iwCmulwdV6eQGfd4VDu40PvSnmfoaE38o6HaPnX0kUcnKiT"
@@ -1307,6 +1310,60 @@ func TestBackend_AllowedUserKeyLengths(t *testing.T) {
 					return nil
 				},
 			},
+			// Fail with ECDSA key
+			{
+				Operation: logical.UpdateOperation,
+				Path:      "sign/multikey",
+				Data: map[string]interface{}{
+					"public_key": publicKeyECDSA256,
+				},
+				ErrorOk: true,
+				Check: func(resp *logical.Response) error {
+					if resp.Data["error"] != "public_key failed to meet the key requirements: key of type ecdsa is not allowed" {
+						return errors.New("an ECDSA key was allowed under RSA-only policy")
+					}
+					return nil
+				},
+			},
+			createRoleStep("ectypes", map[string]interface{}{
+				"key_type":                "ca",
+				"allow_user_certificates": true,
+				"allowed_user_key_lengths": map[string]interface{}{
+					"ec":                  []int{256},
+					"ecdsa-sha2-nistp521": 0,
+				},
+			}),
+			// Pass with ECDSA P-256
+			{
+				Operation: logical.UpdateOperation,
+				Path:      "sign/ectypes",
+				Data: map[string]interface{}{
+					"public_key": publicKeyECDSA256,
+				},
+			},
+			// Pass with ECDSA P-521
+			{
+				Operation: logical.UpdateOperation,
+				Path:      "sign/ectypes",
+				Data: map[string]interface{}{
+					"public_key": publicKeyECDSA521,
+				},
+			},
+			// Fail with RSA key
+			{
+				Operation: logical.UpdateOperation,
+				Path:      "sign/ectypes",
+				Data: map[string]interface{}{
+					"public_key": publicKey3072,
+				},
+				ErrorOk: true,
+				Check: func(resp *logical.Response) error {
+					if resp.Data["error"] != "public_key failed to meet the key requirements: key of type rsa is not allowed" {
+						return errors.New("an RSA key was allowed under ECDSA-only policy")
+					}
+					return nil
+				},
+			},
 		},
 	}
 

builtin/logical/ssh/path_sign.go

@@ -448,63 +448,100 @@ func (b *backend) calculateTTL(data *framework.FieldData, role *sshRole) (time.D
 
 func (b *backend) validateSignedKeyRequirements(publickey ssh.PublicKey, role *sshRole) error {
 	if len(role.AllowedUserKeyTypesLengths) != 0 {
-		var kstr string
-		var kbits int
+		var keyType string
+		var keyBits int
 
 		switch k := publickey.(type) {
 		case ssh.CryptoPublicKey:
 			ff := k.CryptoPublicKey()
 			switch k := ff.(type) {
 			case *rsa.PublicKey:
-				kstr = "rsa"
-				kbits = k.N.BitLen()
+				keyType = "rsa"
+				keyBits = k.N.BitLen()
 			case *dsa.PublicKey:
-				kstr = "dsa"
-				kbits = k.Parameters.P.BitLen()
+				keyType = "dsa"
+				keyBits = k.Parameters.P.BitLen()
 			case *ecdsa.PublicKey:
-				kstr = "ecdsa"
-				kbits = k.Curve.Params().BitSize
+				keyType = "ecdsa"
+				keyBits = k.Curve.Params().BitSize
 			case ed25519.PublicKey:
-				kstr = "ed25519"
+				keyType = "ed25519"
 			default:
-				return fmt.Errorf("public key type of %s is not allowed", kstr)
+				return fmt.Errorf("public key type of %s is not allowed", keyType)
 			}
 		default:
 			return fmt.Errorf("pubkey not suitable for crypto (expected ssh.CryptoPublicKey but found %T)", k)
 		}
 
-		if allowed_values, ok := role.AllowedUserKeyTypesLengths[kstr]; ok {
-			var pass bool
+		keyTypeToMapKey := map[string][]string{
+			"rsa":     {"rsa", ssh.KeyAlgoRSA},
+			"dsa":     {"dsa", ssh.KeyAlgoDSA},
+			"ecdsa":   {"ecdsa", "ec"},
+			"ed25519": {"ed25519", ssh.KeyAlgoED25519},
+		}
+
+		if keyType == "ecdsa" {
+			ecCurveBitsToAlgoName := map[int]string{
+				256: ssh.KeyAlgoECDSA256,
+				384: ssh.KeyAlgoECDSA384,
+				521: ssh.KeyAlgoECDSA521,
+			}
+
+			if algo, ok := ecCurveBitsToAlgoName[keyBits]; ok {
+				keyTypeToMapKey[keyType] = append(keyTypeToMapKey[keyType], algo)
+			}
+
+			// If the algorithm is not found, it could be that we have a curve
+			// that we haven't added a constant for yet. But they could allow it
+			// (assuming x/crypto/ssh can parse it) via setting a ec: <keyBits>
+			// mapping rather than using a named SSH key type, so erring out here
+			// isn't advisable.
+		}
+
+		var present bool
+		var pass bool
+		for _, kstr := range keyTypeToMapKey[keyType] {
+			allowed_values, ok := role.AllowedUserKeyTypesLengths[kstr]
+			if !ok {
+				continue
+			}
+
+			present = true
+
 			for _, value := range allowed_values {
-				switch kstr {
-				case "rsa":
-					if kbits == value {
+				if keyType == "rsa" || keyType == "dsa" {
+					// Regardless of map naming, we always need to validate the
+					// bit length of RSA and DSA keys. Use the keyType flag to
+					if keyBits == value {
 						pass = true
-						break
 					}
-				case "dsa":
-					if kbits == value {
+				} else if kstr == "ec" || kstr == "ecdsa" {
+					// If the map string is "ecdsa", we have to validate the keyBits
+					// are a match for an allowed value, meaning that our curve
+					// is allowed. This isn't necessary when a named curve (e.g.
+					// ssh.KeyAlgoECDSA256) is allowed (and hence kstr is that),
+					// because keyBits is already specified in the kstr. Thus,
+					// we have conditioned around kstr and not keyType (like with
+					// rsa or dsa).
+					if keyBits == value {
 						pass = true
-						break
 					}
-				case "ecdsa":
-					if kbits == value {
-						pass = true
-						break
-					}
-				case "ed25519":
-					// ed25519 public keys are always 256 bits in length,
-					// so there is no need to inspect their value
+				} else {
+					// We get here in two cases: we have a algo-named EC key
+					// matching a format specifier in the key map (e.g., a P-256
+					// key with a KeyAlgoECDSA256 entry in the map) or we have a
+					// ed25519 key (which is always allowed).
 					pass = true
-					break
 				}
 			}
+		}
 
-			if !pass {
-				return fmt.Errorf("key is of an invalid size: %v", kbits)
-			}
-		} else {
-			return fmt.Errorf("key type of %s is not allowed", kstr)
+		if !present {
+			return fmt.Errorf("key of type %s is not allowed", keyType)
+		}
+
+		if !pass {
+			return fmt.Errorf("key is of an invalid size: %v", keyBits)
 		}
 	}
 	return nil

website/content/api-docs/secret/ssh.mdx

@@ -213,7 +213,21 @@ This endpoint creates or updates a named role.
 - `allowed_user_key_lengths` `(map<string|(int|[]int|string)>: "")` – Specifies a
   map of ssh key types and their expected sizes which are allowed to be signed by
   the CA type. To specify multiple sizes, either use a comma-separated list or an
-  array of allowed key widths.
+  array of allowed key widths. We support both OpenSSH-style key identifiers and
+  short names (`rsa`, `ecdsa`, `dsa`, or `ed25519`) as keys. For example, a valid
+  policy to allow common RSA and ECDSA key lengths might be:
+
+  ```
+  {
+    "rsa": [2048, 3072, 4096],
+    "ec": 256,
+    "ecdsa-sha2-nistp521": 0
+  }
+  ```
+
+  Note that when an algorithm identifier uniquely specifies a key length (such as
+  with `ecdsa-sha2-nistp256` or `ed25519`), the value of the length is ignored (and
+  can be zero).
 
 - `algorithm_signer` `(string: "default")` - Algorithm to sign keys with. Valid
   values are `ssh-rsa`, `rsa-sha2-256`, `rsa-sha2-512`, or `default`. This