ASN.1 Continued 2

ASN.1 protocol

So once you have read the tag and the length; the rest is simple. Read the bytes from the stream according to the length. This will be the value of that tag. Now you can interpret that accordingly. For example if you an element version in your stream

1. First you will read that tag
2. Then you will read the length ( Length is actually the number of the bytes for the in which value of version is stored)
3. Then you will read the number of bytes according to the length and this will be value of the your 'version tag'

Thats it from my side on ASN.1 protocol using BER ( basic encoding rule). If you have any questions, queries please do write me in the comments section

ASN.1 Continued

Continuing on the previous post

Reading Length in ASN.1

private int readLength(DataInputStream inputStream)

{

byte[] bytes = readBytes(inputStream, 1);

BitIterator iterator = new BitIterator(bytes);

String mostSignificant = iterator.getBits(1);

String value = iterator.getBits(7);

int length = Integer.parseInt(value, 2);

if (!mostSignificant.equals("0"))

{

bytes = readBytes(inputStream, length);

value = byteArraytoHex(bytes);

length = Integer.parseInt(value, 16);

}

System.out.println("Length of the record " + length);

return length;

}

ASN.1

A few days earlier I have to work on a system using ASN.1 protocol. I did some research to find a solution written in java for BER so that i can test the files encoded through ASN.1; to my surprise the libraries I found further confuse me. I didn't want to blame any one but people have written more complicated things on the subject thus making more confusing for a beginner to understand what the protocol is and how he should write some code to start. So I decided to write a simple piece of code myself.

I am not writing here about the ASN.1 protocol itself; I am writing a piece of code to start decoding a file in ASN.1 protocol using BER. The structure of ASN.1 is like TLV ( which is tag, length, value). You can read more about ASN.1 and BER at wiki.

Reading tag of ASN.1

private String readTag(DataInputStream inputStream)
{

byte[] bytes = readBytes(inputStream, 1);

Of course we can easily read bytes to a byte array, then covert this byte array to hex like

String hexValue = byteArraytoHex(bytes).trim();


If you need help converting byte array to hex please take a look at previous post here
Rest is simple

int tagClass = (bytes[0] & 0xC0) >> 6;

int tagType = (bytes[0] & 0x20) >> 5;
int tag = bytes[0] & 0x1F;
if (tag == 31)
{
StringBuffer buffer = new StringBuffer(byteArraytoHex(bytes).trim());
bytes = readBytes(inputStream, 1);

buffer.append(byteArraytoHex(bytes).trim());
BitIterator iterator = new BitIterator(bytes);
String mostSignificant = iterator.getBits(1);
while (!mostSignificant.equals("0"))
{
bytes = readBytes(inputStream, 1);

buffer.append(byteArraytoHex(bytes).trim());
iterator = new BitIterator(bytes);
mostSignificant = iterator.getBits(1);
}
hexValue = buffer.toString();
}

// System.out.print("Tag Class = ");
// switch (tagClass)
// {
// case 0:
// System.out.print("Universal");
// break;
// case 1:
// System.out.print("Application");
// break;
// case 2:
// System.out.print("Context-Specific");
// break;
// case 3:
// System.out.print("Private");
// break;
// }
//
// System.out.print(" ------ Type = ");
//
// switch (tagType)
// {
// case 0:
// System.out.print("Primitive");
// break;
// case 1:
// System.out.print("Constructed");
// break;
// }
//
// System.out.println(" ------- tage Value = " + tag);

return hexValue;

}



And you might need BitIterator class too; can be found here