feat: Util to read private key from file.

pkg/pki/key.go

@@ -6,12 +6,14 @@ import (
 	"crypto/rand"
 	"crypto/x509"
 	"encoding/pem"
+	"errors"
 	"fmt"
+	"os"
 )
 
-// GenerateKey generates a secure ecdsa private key,
+// NewKey generates a secure ecdsa private key,
 // or panics if it can't.
-func GenerateKey() *ecdsa.PrivateKey {
+func NewKey() *ecdsa.PrivateKey {
 	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
 	if err != nil {
 		panic(fmt.Errorf("Generating key: %w", err))
@@ -32,3 +34,29 @@ func MarshalKeyToPEM(key *ecdsa.PrivateKey) []byte {
 		Bytes: der,
 	})
 }
+
+// ParseKeyFromFile retrieves a private key stored in PEM format from a file.
+func ParseKeyFromFile(fn string) (*ecdsa.PrivateKey, error) {
+	pemBytes, err := os.ReadFile(fn)
+	if err != nil {
+		return nil, err
+	}
+
+	keyBlock, _ := pem.Decode(pemBytes)
+	if keyBlock == nil || keyBlock.Type != PEMTypePrivateKey {
+		return nil, fmt.Errorf("file was not PEM or first block wasn't %s", PEMTypePrivateKey)
+	}
+
+	rawKey, err := x509.ParsePKCS8PrivateKey(keyBlock.Bytes)
+	if err != nil {
+		return nil, err
+	}
+
+	var key *ecdsa.PrivateKey
+	var ok bool
+	key, ok = rawKey.(*ecdsa.PrivateKey)
+	if !ok {
+		return nil, errors.New("key was not ecdsa")
+	}
+	return key, nil
+}