Some of my files have been encrypted by ransomware. I can find some backup of files (unfortunately not all of them). Can I find the password of the 7Z zipped+encrypted files if I also have some of the original files?
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7If the files were encrypted using a pre-2019 version of 7zip, there may be hope. See security.stackexchange.com/questions/100650/… for more info.– mti2935Commented Sep 23, 2021 at 16:24
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3Do you have the ransomware payload by any chance? Maybe submit it to VirusTotal?– Adam KatzCommented Sep 23, 2021 at 21:37
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21) Do you know anything about the password, for example its length and the characters used in it? 2) Some ransomware has been cracked and the decryption is publically available.– Andrew MortonCommented Sep 24, 2021 at 16:22
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1 Answer
7z uses AES-256 with the CBC mode of operation for encryption. Since AES in CBC mode is resistant to known-plaintext attacks ( The CBC mode has greater security than KPA, it has Ind-CPA), having the original files will not assist you in cracking the key/password used for encryption.
mti2935 and Adam Katz point out that 7z used a broken IV generation mechanism up till 2019. While detrimental to secrecy of the encrypted data, as far as I can tell, it only leaks information about the plaintext. I'm not a cryptographer and can't state this authoritatively, but I don't think the weak IV enables attacks against AES that would allow recovery of the key/password faster than brute force. (Edit: Someone with sufficient cryptographic knowledge has commented, confirming that the attack on weak IVs is not a key finding attack.)
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3AES is pretty strong, but 7zip's AES implementation has had at least one vulnerability identified in the past (it was a bad RNG). Commented Sep 23, 2021 at 21:36
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1) (Edited) The link of @AdamKatz is problematic see also ;7zip : Why does encrypting the same file with AES-256 not give the same output?. The bug cleaned, however, one cannot apply a known-plaintext attack to the CBC mode. See the canonical question on Crypto Why is CBC with predictable IV considered insecure against chosen-plaintext attack? what you can do with this.– kelalakaCommented Sep 24, 2021 at 19:41
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42) The attack on weak/predictable IV is not a key finding attack. If the keys are the same for each file then even AES-128 is secure. And in the case of AES-256, the multi-target attack will fail, too. Even for AES-128, multi-target attack is far beyond OP. They should wait until the key is leaked, some attackers leaks after some time.– kelalakaCommented Sep 24, 2021 at 19:41
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1Also, note that AES is a primitive. To talk about KPA,Ind-CPA, Ind-CCA, etc, you need a mode of operation. It is better to say 7Zip uses AES in CBC mode and CBC-mode is CPA-secure– kelalakaCommented Sep 24, 2021 at 19:49