Is there any checksum that results in 2 digit hexadecimal?
I can only find NMEA Checksum...
references:
http://nmeachecksum.eqth.net/
http://en.wikipedia.org/wiki/NMEA_0183
I have some data file that I want to perform reverse engineering to find the kind of checksum.
Thank you in advance,
Two hex digits is one byte. You're looking for a checksum which produces one byte.
Obviously, you've got a simple additive checksum (sum the bytes of the input), and an xor of the input bytes.
It could also be a longer checksum of which only 8 bits have been taken.
It could also be some kind of CRC-8; Wikipedia knows about five kinds of standardised CRC-8.
Related
I'm trying a few days to get the type of the CRC with the following hex stream (sniffed with wireshark):
The Hex data i sniffed:
0000001ec001075465737431323308557365726e616d650850617373776f7264d224
This should be the DATA in HEX:
0000001ec001075465737431323308557365726e616d650850617373776f7264
So the last 4 digits are the checksum, in this case d224
I used many code snippets (PHP, java), and some online checksum calcuation sites:
e.g.:
http://www.scadacore.com/field-applications/programming-calculators/online-checksum-calculator/
But I don't get the correct CRC value.
Thanks!
Update 1
Here are more hex streams with CRC included (the last 4 digits):
0000001dc001045465737409557365726e616d65310950617373776f726431cc96
0000001dc001045465737409557365726e616d65320950617373776f72643289d9
0000001dc001045465737409557365726e616d65330950617373776f726433b51c
0000001dc001045465737409557365726e616d65340950617373776f7264340347
0000001dc001045465737409557365726e616d65350950617373776f7264353f82
It appears to be the ARC CRC, polynomial 0x8005, reflected, zero initial value and no final xor, if I discard the initial 0000001d on each message, and take the CRC at the end to be put in the stream in little-endian order.
I want to send data to a TCP 105 circuit.
The following hex command is OK to send data 123:
7F30001103 313233 45D4
Here, 313233 is hex representation of 123 and 45D4 is the CRC value.
I'm in problem to obtain this 45D4 after calculating CRC. After searching for a long time on the web, I'm getting other CRC values in different standards. But those CRC values are not being accepted by my LED display circuit.
Please help me to know how is it possible to get 45D4 from 7F30001103313233.
Thanks in advance.
The command matches an algorithm called CRC-16/CMS.
$ reveng -w 16 -s 7f30001103313233d445
width=16 poly=0x8005 init=0xffff refin=false refout=false xorout=0x0000 ch
eck=0xaee7 name="CRC-16/CMS"
This is probably the correct algorithm, as you've only given one codeword (and because I've assumed that the CRC has been byte-swapped.)
To generate code that computes this CRC, see Mark Adler's crcany tool, for instance.
A bit stream 11100110 is to be transmitted using CRC method. The generator polynomial is X4+ X3 + 1.
What is the actual bit transmitted ?
Suppose the third bit from the left is inverted during the transmission. How the error is detected.
How the generator polynomial is already known to sender side as well as receiver side, please make this clear.
Solution :
Here, FCS will be 0110 since n = 4.
So actual bit transmitted is >> 11100110 0110
I am confused with the problem 2, 3. please reply my 2, 3 questions.
Thank You!
If you know how to generate the 0110, then invert the bit and generate a new CRC. You will see that it's different. On the other end when you compute the CRC of the eight bits sent, it will not match the four bit CRC sent.
The two sides agree a priori on a protocol, that includes the definition of the CRC to be used.
This is something I have been thinking while reading programming books and in computer science class at school where we learned how to convert decimal values into hexadecimal.
Can someone please tell me what are the advantages of using hexadecimal values and why we use them in programmnig?
Thank you.
In many cases (like e.g. bit masks) you need to use binary, but binary is hard to read because of its length. Since hexadecimal values can be much easier translated to/from binary than decimals, you could look at hex values as kind of shorthand notation for binary values.
It certainly depends on what you're doing.
It comes as an extension of base 2, which you probably are familiar with as essential to computing.
Check this out for a good discussion of
several applications...
https://softwareengineering.stackexchange.com/questions/170440/why-use-other-number-bases-when-programming/
The hexadecimal digit corresponds 1:1 to a given pattern of 4 bits. With experience, you can map them from memory. E.g. 0x8 = 1000, 0xF = 1111, correspondingly, 0x8F = 10001111.
This is a convenient shorthand where the bit patterns do matter, e.g. in bit maps or when working with i/o ports. To visualize the bit pattern for 169d is in comparison more difficult.
A byte consists of 8 binary digits and is the smallest piece of data that computers normally work with. All other variables a computer works with are constructed from bytes. For example; a single character can be stored in a single byte, and a 32bit integer consists of 4 bytes.
As bytes are so fundamental we want a way to write down their value as neatly and efficiently as possible. One option would be to use binary, but then we would need a lot of digits. This takes up a lot of space and can be confusing when many numbers are written in sequence:
200 201 202 == 11001000 11001001 11001010
Using hexadecimal notation, we can write every byte using just two digits:
200 == C8
Also, as 16 is a power of 2, it is easy to convert between hexadecimal and binary representations in your head. This is useful as sometimes we are only interested in a single bit within the byte. As a simple example, if the first digit of a hexadecimal representation is 0 we know that the first four binary digits are 0.
A question in my university homework is why use the one's complement instead of just the sum of bits in a TCP checksum. I can't find it in my book and Google isn't helping. Any chance someone can point me in the right direction?
Thanks,
Mike
Since this is a homework question, here is a hint:
Suppose you calculated a second checksum over the entire packet, including the first checksum? Is there a mathematical expression which would determine the result?
Probably the most important is that it is endian independent.
Little Endian computers store hex numbers with the LSB last (Intel processors for example). Big Endian computers put the LSB first (IBM mainframes for example). When carry is added to the LSB to form the 1's complement sum) it doesn't matter if we add 03 + 01 or 01 + 03: the result is the same.
Other benefits include the easiness of checking the transmission and the checksum calculation plus a variety of ways to speed up the calculation by updating only IP fields that have changed.
Ref: http://www.netfor2.com/checksum.html