Find IV and the key ? AES with CFB or OFB I have a question about the AES-128 and AES-256 encryption method. I need to encrypt 64-bit size data in and out. I want to encrypt them with AES.
So I turned to AES-OFB or AES-CFB because these methods do not require padding. I have at my disposal a number of 256 bits which are shared by the source and the receiver (this number is created internally by both, this number is totally unknown to the rest of the world). I would like to use half of the number for the IV and the other half for the key. The IV and the key would be fixed.
In my specifications, it doesn't matter if someone can decipher the messages. However, someone should not be able to send encrypted messages that would be recognized by the recipient.
So my question is this:

Can someone find the IV and the key from the encrypted messages?
If so, can they then generate the encrypted messages to take control of the system?
In my specifications, it doesn't matter if someone can decipher the messages. However, someone should not be able to send encrypted messages that would be recognized by the recipient.

If that's the case, then encryption is the wrong solution; you are looking for a Message Authentication Code (MAC), such as CMAC.
With CMAC, you hand the message and the key to the CMAC code, which generates a 'tag'.  You send the message and the tag to the recipient, which takes the message (and their own code of the key) and calls CMAC to generate their own tag.  If the tag they computed agrees with the tag you sent, they accept the message; otherwise, they reject it.
CMAC (and any other MAC) was designed with the security property that, if you don't know the key, you cannot find a message/tag pair that validates (even if you were given a large number of valid message/tag pairs).  This is exactly what you want; someone cannot generate a tag that would be accepted (except for the message/tag pairs they have already seen).
Of course, if replay attacks are potentially an issue (that is, if an adversary takes a message/tag pair verbatim and resubmits it, would that be an issue?); if it is, you can add further protection (e.g. include a sequence number with the message; this sequence number always increases; the recipient verifies that it is higher than the previously seen valid messages, and the sequence number is included in the CMAC computations).
Obviously, someone listening in can find the message (it's in the clear); however you said you didn't care about it.
I suggested CMAC in particular because you appeared to have AES in mind (and CMAC based on AES is not a bad option); other good message authentication codes are HMAC and KMAC.