There is a site called http://www.MD5Decrypter.co.uk where when you give a md5 hash, it gives the original string. How is that possible. As far as I know md5 is an irreversible hash algorithm or is it? Secondly, can salt be used along with md5?
md5 is a hash algorithme so it allows two words to have the same hashcode. I you do not trust me, I can hash a 5-letter word, 10-letter word, 128-letter word with md5 and it will give me 32 characters every time.
The probleme is that md5 is not cryptographicaly secured. One can analyse it and guess what could have been hash. But the technique used by the site you posted is the rainbow table.
It can also be a dictionary, but it is less common with md5.
If you use a salt with your md5, this generator will not find anything until the rainbow table with your salt is filled.
md5 is usefull to sign a file, a cookie, or the name of a cryptography algorithm. It is not secured to store passwords. Some languages advice you to use whirlpool, bloswfish, salsa20 or sha512 instead of md2/5 sha 1/2/256
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The input strings have the following conditions:
Only contain alphanumericals ([a-zA-Z0-9])
The size of a string is always less than 256 bytes
Total number of input strings is less then 1000,000
So what is the conflict probability of md5 digestion if the input strings are all under the above conditions? Can I just assume that there has no conflict?
If the inputs are random the likelihood of a collision in that input set is very low. That being said MD5 is a broken algorithm and a human can easily use software to find a collision. So you probably just shouldn't use MD5, but it depends on what you're using it for. I'm not sure why you would ever want to use MD5 anymore. You should look into the blake2 family or the newer SHAs (SHA256, SHA512, not SHA-1). If these are passwords you should pretty much definitely be using a hash designed for passwords like PBKDF2 or one of the Argons. To be honest I'd recommend just using libsodium's defaults for most things.
If I give you the MD5 checksum of a string, can you generate the SHA checksum?
Or Vice Versa?
If I give you an MD5 checksum and a SHA checksum, can you tell me whether they're generated from the same source string?
(Obviously I'm excluding anything like locating the source string from a rainbow table, etc. etc.)
No. Both are one-way hashes, so the information contained in the original source string is lost in the checksum.
Even if you do have the "original" string as you say, there will be some other data out there that when hashed, results in a collision. This is because MD5 and SHA1 are not perfect hash functions. In the case of MD5, each hash can potentially have an infinite number of collisions. Then there is no guarantee that the generated SHA checksum of the "original" string is actually what you seek.
Disclaimer: I have very little experience with the theoretical side, so you may want to verify with other resources.
I am using md5 to encrypt the password. then, how to get back the original string if i need it. is there any decryption possible?
In a word: no. If you want it to be reversible, this is the wrong approach. MD5 is a hashing algorithm, not an encryption one. Don't use MD5 for passwords.
Use actual encryption, like AES or Blowfish.
You cannot decrypt one-way hashes like MD5 - The best you can do is brute-force them or run a dictionary through and compare the values for matches.
Is it possible to create a MD5 collision based on a known input value?
So for example I have input string abc with MD5 900150983cd24fb0d6963f7d28e17f72.
Now I want to add bytes to string def to get the same MD5 900150983cd24fb0d6963f7d28e17f72.
(I know this is possible by bruteforcing and waiting a long time; I want to know if there is a more efficient way in doing this)
Unitl now no algorithm has been discovered that allows you to find a matching input that will generate a given md5 hash.
What has been proven is that you can create md5 collisions quite easily, for example with what is known as chosen-prefix-collision: you can create two files yielding the same md5 hash by appending different data to a specified file. If you want to know more or get the program to try it, look here.
I want to know if there exists a way to differentiate a md5 hashcode of a sha-1 hashcode?
For example:
d41d8cd98f00b204e9800998ecf8427e
da39a3ee5e6b4b0d3255bfef95601890afd80709
How could I know which are encrypted in md5 and which are not? Is it possible?
I'm not quite sure if this is what you're asking, but MD5 is 128 bits/32 hex digits, while SHA-1 is 160 bits/40 hex digits, so it's fairly easy to tell the difference between them (providing, of course, that you know that your hashes will be either MD5 or SHA-1 and not something else).
(If you're asking whether you can determine if a given MD5 hash is a hash of a hash or a hash of some other data, then I believe the answer is "no".)
MD5 gives a 128-bit hash value.
SHA-1 gives a 160-bit hash value.