Add convenience function to return the size of an encrypted segment

in the datastream the header is from.

model/headers/headers.go

@@ -262,6 +262,29 @@ func NewHeaderPacket(reader io.Reader, readerPrivateKey [chacha20poly1305.KeySiz
 	return &headerPacket, nil
 }
 
+// EncryptedSegmentSize returns the size of an encrypted segment for a given header and reader private key.
+func EncryptedSegmentSize(header []byte, readerPrivateKey [chacha20poly1305.KeySize]byte) (int, error) {
+
+	buffer := bytes.NewBuffer(header)
+	decryptedHeader, err := NewHeader(buffer, readerPrivateKey)
+	if err != nil {
+		return 0, err
+	}
+
+	dataEncryptionParametersHeaderPackets, err := decryptedHeader.GetDataEncryptionParameterHeaderPackets()
+	if err != nil {
+		return 0, err
+	}
+
+	firstDataEncryptionParametersHeader := (*dataEncryptionParametersHeaderPackets)[0]
+	for _, dataEncryptionParametersHeader := range *dataEncryptionParametersHeaderPackets {
+		if dataEncryptionParametersHeader.GetPacketType() != firstDataEncryptionParametersHeader.GetPacketType() {
+			return 0, fmt.Errorf("different data encryption methods are not supported")
+		}
+	}
+	return firstDataEncryptionParametersHeader.EncryptedSegmentSize, nil
+}
+
 // MarshalBinary implements method MarshalBinary.BinaryMarshaler.
 func (hp *HeaderPacket) MarshalBinary() (data []byte, err error) {
 	var encryptedMarshalledEncryptedHeaderPacket []byte

model/headers/headers_test.go

@@ -299,3 +299,37 @@ func TestReEncryptedHeader(t *testing.T) {
 		t.Fail()
 	}
 }
+
+func TestEncryptedSegmentSize(t *testing.T) {
+	inFile, err := os.Open("../../test/sample.txt.enc")
+	if err != nil {
+		t.Errorf("Fileopen failed: %v", err)
+	}
+	readerSecretKey, err := keys.ReadPrivateKey(strings.NewReader(crypt4ghX25519Sec), []byte("password"))
+	if err != nil {
+		t.Errorf("ReadPrivateKey failed: %v", err)
+	}
+
+	header, err := ReadHeader(inFile)
+	if err != nil {
+		t.Errorf("ReadHeader failed: %v", err)
+	}
+
+	size, err := EncryptedSegmentSize(header, readerSecretKey)
+	if err != nil {
+		t.Errorf("EncryptedSegmentSize failed where it should work: %v", err)
+	} else if size != 65564 {
+		t.Errorf("EncryptedSegmentSize returned unexpected size %d (expected 65564)", size)
+	}
+
+	size, err = EncryptedSegmentSize(header, ([32]byte)(make([]byte, 32)))
+	if err == nil {
+		t.Errorf("EncryptedSegmentSize worked where it should fail: %v", err)
+	}
+
+	size, err = EncryptedSegmentSize(make([]byte, 2), readerSecretKey)
+	if err == nil {
+		t.Errorf("EncryptedSegmentSize worked where it should fail: %v", err)
+	}
+
+}