Reusing IV in AES for encrypting file Everyone says that you shouldn't use the same IV and Key.


Is it safe to use the same IV and Key on only one file. EDIT: I will be using CBC or CFB encryption modes so the IV will be updated based on previous block.
Is it safe to use the same IV (EDIT: Same first IV, which will then be updated with each block.) each time I access and add data to the file.  Each time this happens, there will only be one encrypted file so multiple files with same IV and Key will not happen.


Edit: The program using the file will just add data to the end of the file.  From what I could find, this will not change the encryption of the first sections of the file, as the IV is changed based on what was done in the previous block.  Is that correct?

Edit 2: For simplicity, let's say this is a log file that I want encrypted for some reason.  I want to be able to add on to the end of this log while not needing new key/IV.  Since it is the end of a file, I was thinking while decrypting, I will have the next IV for CBC or CFB and use it to begin encrypting the next entry.  As for mutability, I will never have a reason to change anything in the middle of the log/file, only to add on to the end.  I don't know how that lines up with cryptographic mutability/immutability, but that is what I want to do.

Final: I am using CBC.  At the beginning of every line I have a new IV (first 16 bytes are the IV) and the rest of the line is encrypted using that IV.  Thanks for all the pointers.  If anyone sees any major thing wrong with this, please let me know.
You aren't telling us exactly what you plan to do, but plenty enough that we can tell you: no, just no.  The first mistake you're making is that you're talking about "files" in a way that's worrisome in the context of cryptography.  In modern programming parlance, cryptography by default deals with immutable values, not with mutable objects.  But when you talk about "files" in this question it is evident that at least on some occasions you mean a mutable object, one into which different values are written and overwritten at different points in time.  

So the cryptographic requirement that you never use the same key+IV pair to encrypt more than one value then applies not to the file as a unit, but to each operation that encrypts a value to be written into the file.


  
  Is it safe to use the same IV and Key on only one file.
  


Generally, no.  There are special cases where it's safe, if for example you're using files in a strict write-once never-modify manner, where you're only ever writing to a file at the time you create it.  But that's not what you're proposing to do:


  
  Is it safe to use the same IV each time I access and add data to the file. Each time this happens, there will only be one encrypted file so multiple files with same IV and Key will not happen.
  


No.  Each time you wrote to this file you'd be encrypting a different value with the same key and IV, and that violates the IV reuse condition.  Note that the likely practical consequence of this is that your file format will need to be structured to support encryption—for example, you might need to build some notion of "data block" within the file format, so that each data block can be encrypted with a different IV, and the metadata necessary to identify block boundaries and their IVs is present.


  The program using the file will just add data to the end of the file. From what I could find, this will not change the encryption of the first sections of the file, as the IV is changed based on what was done in the previous block. Is that correct?


It sounds like you're looking at some materials that describe CBC mode encryption.  This idea that the IV changes based on what was done in the previous block is likely not something that happens automatically, it'd be something you'd have to understand and implement—which is an error-prone and risky undertaking.  CBC also doesn't provide authenticated encryption, which means that you likely would need to add some sort of message authentication, which makes the whole thing even harder to get right—for example, the recent Efail vulnerability on GPG and email clients hinges on this.

Whatever you're thinking of doing is likely a minefield.