Add Initial GCM Decrypter

go.mod

@@ -1,6 +1,6 @@
 module github.com/amboss-mededu/saml
 
-go 1.13
+go 1.16
 
 require (
 	github.com/beevik/etree v1.1.0

xmlenc/gcm.go

@@ -0,0 +1,91 @@
+package xmlenc
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"fmt"
+	"github.com/beevik/etree"
+	"io"
+)
+
+// struct implements Decrypter and Encrypter for block ciphers in struct mode
+type GCM struct {
+	keySize   int
+	algorithm string
+	cipher    func([]byte) (cipher.Block, error)
+}
+
+// KeySize returns the length of the key required.
+func (e GCM) KeySize() int {
+	return e.keySize
+}
+
+// Algorithm returns the name of the algorithm, as will be found
+// in an xenc:EncryptionMethod element.
+func (e GCM) Algorithm() string {
+	return e.algorithm
+}
+
+func (e GCM) Encrypt(key interface{}, plaintext []byte) (*etree.Element, error) {
+	return nil, nil
+}
+
+// Decrypt decrypts an encrypted element with key. If the ciphertext contains an
+// EncryptedKey element, then the type of `key` is determined by the registered
+// Decryptor for the EncryptedKey element. Otherwise, `key` must be a []byte of
+// length KeySize().
+func (e GCM) Decrypt(key interface{}, ciphertextEl *etree.Element) ([]byte, error) {
+	fmt.Printf("gcm ciphertextEl Type: %T\n", ciphertextEl)
+	fmt.Printf("gcm ciphertextEl Value: %v\n", ciphertextEl)
+	if encryptedKeyEl := ciphertextEl.FindElement("./KeyInfo/EncryptedKey"); encryptedKeyEl != nil {
+		var err error
+		key, err = Decrypt(key, encryptedKeyEl)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	keyBuf, ok := key.([]byte)
+
+	if !ok {
+		return nil, ErrIncorrectKeyType("[]byte")
+	}
+	if len(keyBuf) != e.KeySize() {
+		return nil, ErrIncorrectKeyLength(e.KeySize())
+	}
+
+	block, err := e.cipher(keyBuf)
+	if err != nil {
+		return nil, err
+	}
+
+	aesgcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
+
+	nonce := make([]byte, aesgcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return nil, err
+	}
+
+	plainText, err := aesgcm.Open(nonce, nonce, keyBuf, nil)
+	if err != nil {
+		return nil, err
+	}
+	return plainText, nil
+}
+
+var (
+	// AES128GCM implements AES128-GCM mode for encryption and decryption
+	AES128GCM BlockCipher = GCM{
+		keySize:   16,
+		algorithm: "http://www.w3.org/2009/xmlenc11#aes128-gcm",
+		cipher:    aes.NewCipher,
+	}
+)
+
+func init() {
+	RegisterDecrypter(AES128GCM)
+}