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I am curious about how password recovery works for password protected files. And I want to know the exact flow of the 7-zip encryption mechanism.
7-zip uses AES-256 encryption algorithm in CBC mode, to encrypt files or a folder. The key is generated from user supplied pass-phrase based on SHA-256 hash function. The SHA-256 is executed 219 (524,288) times to increase the cost of exhaustive search. Also, to help reduce the risk of dictionary attacks, salt is appended to original pass-phrase before generating hash.
My First question is how does key_derivation function work to generate a 256-bit key ? What is importance of IV for AES CBC mode and how it is generated?
My second and most important question is how key is verified to decrypt the 7-zip archive ? I mean how does its key_verification function work ?
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You can compress and encrypt archives with 7-Zip with AES-256 encryption.
In the "Archive" field, enter the name of the file or archive you want created. From the "Archive format" field, select zip. Under the "Encryption" section, enter a strong password or passphrase in the "Enter passphrase" field and again in the "Reenter passphrase" field. Ensure the "Encryption method" is AES-256.
zip or . zipx): AES encryption: AES is the Advanced Encryption Standard, which is the result of a three-year competition sponsored by the U.S. Government's National Institute of Standards (NIST). This encryption method, also known as Rijndael, has been adopted by NIST as a Federal Information Processing Standard.
7-Zip is a free file compression program that also allows you to encrypt and password protect the files you compress. You can compress multiple files into a single archive file. For someone to open an encrypted file created with 7-Zip that person will need to have 7-Zip or a compatible program.
The key derivation function is in the source of file 7zAes.cpp, it's called:
void CKeyInfo::CalculateDigest()
and it creates the key using a proprietary (rather dull) mechanism.
I haven't been able to find the key_verification method yet, if it exists. I'll let you know if I do find it.
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