add ecdsa key support and enable 4,092 RSA

* apcssh: add descriptive error when required file(s) not passed
* create: dont create key+cert file when key isn't supported by NMC2
* config: fix usage messages re: key types
* p15 files: dont generate key+cert when it isn't needed (aka NMC2 doesn't support key)
* pkcs15: pre-calculate envelope when making the p15 struct
* pkcs15: omit key ID 8 & 9 from EC keys
* pkcs15: update key decode logic
* pkcs15: add key type value for easy determination of compatibility
* pkcs15: add ec key support
* pkcs15: separate functions for key and key+cert p15 files
* update README
see: https://github.com/gregtwallace/apc-p15-tool/issues/6

pkg/pkcs15/pem_parse.go

@@ -2,6 +2,8 @@ package pkcs15
 
 import (
 	"crypto"
+	"crypto/ecdsa"
+	"crypto/rsa"
 	"crypto/x509"
 )
 
@@ -10,6 +12,59 @@ import (
 type pkcs15KeyCert struct {
 	key  crypto.PrivateKey
 	cert *x509.Certificate
+	// store the encrypted enveloped Private Key for re-use
+	envelopedPrivateKey []byte
+}
+
+// KeyType is used by consumers to check for compatibility
+type KeyType int
+
+const (
+	KeyTypeRSA1024 KeyType = iota
+	KeyTypeRSA2048
+	KeyTypeRSA3072
+	KeyTypeRSA4096
+
+	KeyTypeECP256
+	KeyTypeECP384
+	KeyTypeECP521
+
+	KeyTypeUnknown
+)
+
+// KeyType returns the private key type
+func (p15 *pkcs15KeyCert) KeyType() KeyType {
+	switch pKey := p15.key.(type) {
+	case *rsa.PrivateKey:
+		switch pKey.N.BitLen() {
+		case 1024:
+			return KeyTypeRSA1024
+		case 2048:
+			return KeyTypeRSA2048
+		case 3072:
+			return KeyTypeRSA3072
+		case 4096:
+			return KeyTypeRSA4096
+
+		default:
+		}
+
+	case *ecdsa.PrivateKey:
+		switch pKey.Curve.Params().Name {
+		case "P-256":
+			return KeyTypeECP256
+		case "P-384":
+			return KeyTypeECP384
+		case "P-521":
+			return KeyTypeECP521
+
+		default:
+		}
+
+	default:
+	}
+
+	return KeyTypeUnknown
 }
 
 // ParsePEMToPKCS15 parses the provide pem files to a pkcs15 struct; it also does some
@@ -27,10 +82,17 @@ func ParsePEMToPKCS15(keyPem, certPem []byte) (*pkcs15KeyCert, error) {
 		return nil, err
 	}
 
+	// create p15 struct
 	p15 := &pkcs15KeyCert{
 		key:  key,
 		cert: cert,
 	}
 
+	// pre-calculate encrypted envelope
+	err = p15.computeEncryptedKeyEnvelope()
+	if err != nil {
+		return nil, err
+	}
+
 	return p15, nil
 }