>
> Let say I have encrypted a large file with a certain key.
> Is it possible to know if the key used for decryption is the good one,
> before trying to decrypt this large file? This would lead to gain time,
> instead of decrypting it and see that the key is not good.
First, how about a checksum of the decrypted data, so you can check if checksum
matches after decryption?
Second, it may be good to use a two level key.
On begin of file is a header. This header has details like length of data
block, encryption key used, some version information of the app which made it
and the metadata and finally a hash of this header. The key you got decrypts
the header and if it's hash matches, the key inside it, decrypts the data.
This way the outer key can be changed without reencrypting all the data.
Sincerely
Christian
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