Hexadecimal string representation to hexadecimal

Question

I am currently working with UDP packets and I need to craft and send custom data. Because it is easier for me to read I work with strings representing hexadecimal values. I have something like this :

a = "12"
b = "15"
header = "c56b4040003300" + a + "800401" + b + "90000000"

Now, what I want to do is converting my header variable into hexadecimal (but not with the hexadecimal value of every character in header). It means that if I write my header variable in a file and I open it with a hexadecimal editor, I want to see

c5 6b 40 40 00 33 00 12 80 04 01 15 90 00 00 00

I don't have a good knowledge of ruby and I couldn't find a way to do it so far. The pack function converts characters in hexa but not hexadecimal string representation as hexadecimal value. And doing something like

header = "\xc5\x6b\x40\x40\x00\x33\x00\x" + a + "\x80\x04\x01\x" + b + "\x90\x00\x00\x00"

will throw me an error saying "invalid hex escape" (which make sense). So if you have a solution to this problem please tell me (if possible without using any external library)

Basically what I want is the equivalent of binascii.hexlify and binascii.unhexlify in python — Mtoy
– Mtoy, Commented May 18, 2018 at 14:13

Attersson · Accepted Answer · 2018-05-18 18:46:11Z

2

require 'strscan'
s = StringScanner.new('hexstring here')
s.scan(/../).map { |x| x.hex.chr }.join

edited May 18, 2018 at 18:46

answered May 18, 2018 at 14:15

Attersson

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1 Comment

Mtoy Over a year ago

whow that is something magic but works pretty well ! Thank you

mwp · Accepted Answer · 2018-05-18 18:11:33Z

2

String#to_i takes a base argument that will do what you want:

["c56b4040003300", a, "800401", b, "90000000"].join.to_i(16)

But it may not make sense to represent your data as an large integer. If you just want a blob of binary data, you can concatenate everything together and use Array#pack:

[["c56b4040003300", a, "800401", b, "90000000"].join].pack('H*')

Or you can pack the individual components and concatenate the results:

["c56b4040003300", a, "800401", b, "90000000"].map { |s| [s].pack('H*') }.join

Or you can just work with an array of bytes throughout your program:

bytes = "c56b4040003300".scan(/../)
bytes << a
bytes.concat "800401".scan(/../)
bytes << b
bytes.concat "90000000".scan(/../)
bytes.unpack('H*' * bytes.size)

Hope that helps!

edited May 18, 2018 at 18:11

answered May 18, 2018 at 14:27

mwp

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4 Comments

Mtoy Over a year ago

thx for your answer, it work well ! How would you do the opposite operation ? (starting from the hexa and getting the string ? (tried with unpack but couldn't make it work))

mwp Over a year ago

@Mtoy You're welcome! It should just be result.unpack('H*')

Mtoy Over a year ago

Oh yeah ok. I think I'm not familiar enough with pack and unpack methods. I will take some time to understand it correctly. Thanks a lot for your help

mwp Over a year ago

@Mtoy It's definitely confusing, especially when you get into endianness and word size and all that good stuff. But it is The Way™ to convert to/from binary data in many languages.

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