What kind of encrypted data is this?

A friend of me ask this, and i was thinking of asking this here too..

"What kind of da开发者_开发技巧ta are this, how are they encrypted, or decrypted?"

My friend told me he got this from facebook.

d9ca6435295fcd89e85bd56c2fd51ccc

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It looks like it could be an md5 hash.

Basically a hash is a one-way function. The idea is that you take some input data and run it through the algorithm to create a value (such as the string above) that has a low probability of collisions (IE, two input values hashing to the same string).

You cannot decrypt a hash because there is not enough information in the resultant string to go back. However, it may be possible for someone to figure out your input values if you use a 'weak' hashing algorithm and do not do proper techniques such as salting a hash, etc.

I don't know how FaceBook uses hashes, but a common use for a hash might be to uniquely identify a page. For example, if you had a private image on a page, you might ask to generate a link to the image that you can email to friends. That link might use a hash as part of the URL since the value can be computed quickly, is reasonably unique, and has a low probability of a third party figuring it out.

This is actually a large topic that I am by no means doing justice to. I suggest googling around for hash, md5, etc to learn more, if you are so inclinded.

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It is a sequence of 128 bits, encoded as a lower-case hex string.

If you are talking about a Facebook API key, there is no deeper meaning to decode from the bits. The keys are created at random by Facebook and assigned to a particular application to identify it. Each application gets a different set of random bits for its API key.

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This appears the be the...
    hexadecimal representation for...
        - ... a 16 bytes encryption block or..
        - ... some 128 bits hash code or even
        - ... just for some plain random / identifying number.

(Hexadecimal? : note how there are only 0 thru 9 digits and a thru f letters.)

While the MD5 Hash guess suggested by others is quite plausible, it could be just about anything...

If it is a hash or a identifying / randomly assigned number, its meaning is external to the code itself.
For example it could be a key to be used to locate records in a database, or a value to be compared with the result of the hash function applied to the user supplied password etc.

If it is an encrypted value, its meaning (decrypted value) is directly found within the code, but it could be just about anything. Also, assuming it is produced with modern encryption algorithm, it could take a phenomenal amount of effort to crack the code (if at all possible).