xmlenv: add support for newer RSA OAEP 2009 algorithms (crewjam#600)

This is cherry-picked from crewjam#581 submitted by @wz2b

Author: crewjam

xmlenc/pubkey.go

@@ -125,6 +125,9 @@ func (e RSA) Decrypt(key interface{}, ciphertextEl *etree.Element) ([]byte, erro
 // the block cipher used is AES-256 CBC and the digest method is SHA-256. You can
 // specify other ciphers and digest methods by assigning to BlockCipher or
 // DigestMethod.
+//
+// OAEP implements the older RSA-OAEP (2001 spec) for backward compatibility, you might
+// perfer OAEP_2009_256 over using this method.
 func OAEP() RSA {
 	return RSA{
 		BlockCipher:  AES256CBC,
@@ -139,6 +142,44 @@ func OAEP() RSA {
 	}
 }
 
+// OAEP_SHA256 returns a version of RSA that implements RSA in OAEP mode. By default
+// the block cipher used is AES-256 CBC and the digest method is SHA-256. You can
+// specify other ciphers and digest methods by assigning to BlockCipher or
+// DigestMethod.
+func OAEP_SHA256() RSA { //nolint:revive
+	return RSA{
+		BlockCipher:  AES256CBC,
+		DigestMethod: SHA256,
+		algorithm:    "http://www.w3.org/2009/xmlenc11#rsa-oaep",
+
+		keyEncrypter: func(e RSA, pubKey *rsa.PublicKey, plaintext []byte) ([]byte, error) {
+			return rsa.EncryptOAEP(e.DigestMethod.Hash(), RandReader, pubKey, plaintext, nil)
+		},
+		keyDecrypter: func(e RSA, privKey *rsa.PrivateKey, ciphertext []byte) ([]byte, error) {
+			return rsa.DecryptOAEP(e.DigestMethod.Hash(), RandReader, privKey, ciphertext, nil)
+		},
+	}
+}
+
+// OAEP_SHA512 returns a version of RSA that implements RSA in OAEP mode. By default
+// the block cipher used is AES-256 CBC and the digest method is SHA-512. You can
+// specify other ciphers and digest methods by assigning to BlockCipher or
+// DigestMethod.
+func OAEP_SHA512() RSA { //nolint:revive
+	return RSA{
+		BlockCipher:  AES256CBC,
+		DigestMethod: SHA512,
+		algorithm:    "http://www.w3.org/2009/xmlenc11#rsa-oaep",
+
+		keyEncrypter: func(e RSA, pubKey *rsa.PublicKey, plaintext []byte) ([]byte, error) {
+			return rsa.EncryptOAEP(e.DigestMethod.Hash(), RandReader, pubKey, plaintext, nil)
+		},
+		keyDecrypter: func(e RSA, privKey *rsa.PrivateKey, ciphertext []byte) ([]byte, error) {
+			return rsa.DecryptOAEP(e.DigestMethod.Hash(), RandReader, privKey, ciphertext, nil)
+		},
+	}
+}
+
 // PKCS1v15 returns a version of RSA that implements RSA in PKCS1v15 mode. By default
 // the block cipher used is AES-256 CBC. The DigestMethod field is ignored because PKCS1v15
 // does not use a digest function.