Forums Bugs & issues zip file encryption Reply To: zip file encryption

#4824 Reply


Hello Anonymous,

While the statement about 7-zip is completely off-topic ;-) I’d still like to correct a misunderstanding here.

You are correct that CBC mode is the foundation for the typical padding oracle attack, it is rarely applicable to file encryption software such as 7-zip or for that matter AxCrypt.

The thing about a padding oracle attack is that you need an oracle. An oracle in this context, is some entity with knowledge that can be questioned. A padding oracle is something we can ask whether or not the padding is correct for a given encrypted message.

For such an oracle to exist, there must be an entity that has knowledge about the decryption key that we can ask whether or not a message is correctly padded.

Such an oracle is typically a server that has the decryption key, and that we can send a message to – and get a response back saying “Invalid padding” or something similar when the padding is incorrect.

For the typical 7-zip or AxCrypt file, no such oracle exists. You just have an encrypted file, and no server or other device you can send a modified file to with the intend of learining whether the padding was correct or not.

So, even if 7-zip and AxCrypt version 1 uses CBC mode, it is a rare situation where a padding oracle attack is possible. You need that oracle.