Refactor client into separate files

Add a main loop, move I/O code into io.go, move configuration into config.go.
Use shared.Decode24BitLength instead of manually decoding block lengths.
Fix response block decoding and checksum validation.
Add constants for commonly used byte values and use these in the signer and
the client.

datastructures/signerresponse.go

@@ -17,20 +17,26 @@ type SignerResponse struct {
 }
 
 func SignerResponseFromData(lengthBytes []byte, blockData []byte, checkSum byte) (*SignerResponse, error) {
-	headerLength := decode24BitLength(lengthBytes)
-	headerBytes := blockData[3 : 3+headerLength]
+	if len(blockData) < 3 {
+		return nil, errors.New("begin of structure corrupt")
+	}
 
-	contentBytes := blockData[3+headerLength:]
-	content1Length := decode24BitLength(contentBytes[0:3])
-	content1 := string(contentBytes[3 : 3+content1Length])
+	offset := 0
+	headerLength := Decode24BitLength(blockData[offset : offset+3])
+	offset += 3
+	headerBytes := blockData[offset : offset+headerLength]
+	offset += headerLength
 
-	content2Offset := 3 + content1Length
-	content2Length := decode24BitLength(contentBytes[content2Offset : content2Offset+3])
-	content2 := string(contentBytes[3+content2Offset : 3+content2Offset+content2Length])
-
-	content3Offset := 3 + content2Offset + content2Length
-	content3Length := decode24BitLength(contentBytes[content3Offset : content3Offset+3])
-	content3 := string(contentBytes[3+content3Offset : 3+content3Offset+content3Length])
+	content := make([]string, 3)
+	for offset < len(blockData) {
+		dataLength := Decode24BitLength(blockData[offset : offset+3])
+		if len(blockData)-3 < dataLength {
+			return nil, errors.New("structure cut off")
+		}
+		offset += 3
+		content = append(content, string(blockData[offset:offset+dataLength]))
+		offset += dataLength
+	}
 
 	calculated := CalculateXorCheckSum([][]byte{lengthBytes, blockData})
 	if checkSum != calculated {
@@ -42,9 +48,9 @@ func SignerResponseFromData(lengthBytes []byte, blockData []byte, checkSum byte)
 		Action:    Action(headerBytes[1]),
 		Reserved1: headerBytes[2],
 		Reserved2: headerBytes[3],
-		Content1:  content1,
-		Content2:  content2,
-		Content3:  content3,
+		Content1:  content[0],
+		Content2:  content[1],
+		Content3:  content[2],
 	}, nil
 }