Create helper functions for parsing public keys

These are useful when verifying and creating PKI signatures.

pkg/pki/BUILD.bazel

@@ -8,6 +8,7 @@ go_library(
         "csr.go",
         "issue.go",
         "privatekey.go",
+        "publickey.go",
         "sshkey.go",
     ],
     importpath = "k8s.io/kops/pkg/pki",

pkg/pki/publickey.go

@@ -0,0 +1,68 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package pki
+
+import (
+	"crypto"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+
+	"k8s.io/klog/v2"
+)
+
+func ParsePEMPublicKey(data []byte) (*PublicKey, error) {
+	k, err := parsePEMPublicKey(data)
+	if err != nil {
+		return nil, err
+	}
+	if k == nil {
+		return nil, nil
+	}
+	return &PublicKey{Key: k}, nil
+}
+
+type PublicKey struct {
+	Key crypto.PublicKey
+}
+
+func parsePEMPublicKey(pemData []byte) (crypto.PublicKey, error) {
+	for {
+		block, rest := pem.Decode(pemData)
+		if block == nil {
+			return nil, fmt.Errorf("could not parse private key")
+		}
+
+		if block.Type == "RSA PUBLIC KEY" {
+			k, err := x509.ParsePKCS1PublicKey(block.Bytes)
+			if err != nil {
+				return nil, err
+			}
+			return k, nil
+		} else if block.Type == "PUBLIC KEY" {
+			k, err := x509.ParsePKIXPublicKey(block.Bytes)
+			if err != nil {
+				return nil, err
+			}
+			return k.(crypto.PublicKey), nil
+		} else {
+			klog.Infof("Ignoring unexpected PEM block: %q", block.Type)
+		}
+
+		pemData = rest
+	}
+}