Let’s say you want to transfer a file from one computer to another and it is really important to know that it’s got there intact in one piece erm, you could send it multiple times and then compare them all but what generally gets used is something called a hash algorithm. A hash algorithm is kind of like the check digit in a bar code on a credit card. I think James Grime talked.
About this a long long time ago on numberphile. the last digit in a bar code or on a credit card is determined by all the other digits on it and if you change one of those digits the last one changes as well so as you typed into a computer you can know instantly if you’ve missed a key somewhere so a hash algorithm is kind of like that but for an entire file that might be.
Megabytes or gigabytes in size what it gives you is a code 16 or 32 or 64 characters generally hexadecimal basically just one long number expressed in that way that is a sum up of everything that’s in that file If you crushed it down if you do all these manipulations to it and crush it down crush it down and crush it down and what it comes out with this thing that says this is.
A summary of that file you can never make it work backwards you can’t pull that data back out but it’s like a signature it’s like a confirmation that this file is really who it says it is the simplest hash algorithm I can think of I would just be something like that’s five add up all the digits in the file.
Which is 4, 9, 14, 23 that’s not a good hash algorithm for a few reasons hash algorithms have three main requirements the first one is speed it’s got to be reasonably fast it should be able to churn through a big file in in a second or two at most but it also shouldn’t be too quick if it’s too quick it’s easy to break and I’ll explain that later.
The second requirement is that if you change one byte one bit anywhere in the file of the start of the middle at the end then the whole hash should be completely different this is something called the avalanche effect. If you’re interested in how this is achieved do look up the actual algorithms themselves. It would take me an hour to explain vaguely how they work in a in a friendly way but if it’s your kind of thing do look it up but suffice.
It to say one bit gets flipped anywhere in the message then the whole hash is completely and utterly different the third requirement is that you’ve got to be able to avoid what are called hash collisions this is where you have two documents which have the same hash obviously there is a mathematical principle called the pigeonhole principle you have it if you have 50 pigeons and 25 pigeonholes did you have.
To stuff two pigeons into one of the pigeonholes that’s a terrible analogy when you say it like this but if I could explain it there are incredible numbers of documents out that possible with the hash meanwhile it’s just one fairly long number that will be files out there which naturally have the same hash and that’s okay because the odds against it are so unlikely that we can deal with that it’s never going to happen.
Tutoriel Vrifier le checksum des fichiers tlchargs SHA1 SHA256 MD5 hash HD Franais
Welcome to my tutorial I will show you how to verify the checksum Of files thats you have download Without any softwares. Only the operating system. The software editor gives a sha1 or md5 signature and we verify it, we compare it.
To see if it’s the good file. if it’s not corrupt or integral We begin with Windows In Windows we open the command prompt The command is : Certutil hashfile c:\Users\votrenom\lefichier.exe sha1 (ou md5) Certutil hashfile c:\Users\votrenom\lefichier.exe sha1 (ou md5).
Then we compare the result I look, yes it was the good one. Now in Mac OS X I have download the file NOOBS it’s SHA1 En the terminal I enter :.
Openssl sha1 /users/votrenom/downloads/votre_fichier.zip openssl sha1 /Users/votrenom/Downloads/votre_fichier.zip openssl sha1 /Users/votrenom/Downloads/votre_fichier.zip Enter It’s calculating to find the signature We can compare.
Then, the signature is the same I will verify I already verified before I did this tutorial yes this is the same Now in Linux I downloaded NOOBS Lite.
It’s sha1 In the terminal We go where is the file to check The command is sha1sum (ou md5sum) le_nom_du_fichier.zip sha1sum (ou md5sum) le_nom_du_fichier.zip.
Sha1sum (ou md5sum) le_nom_du_fichier.zip Now I can compare This is the good one Everything is good Now this tutorial is finish I hope it was usefull for your.