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RSA / AES issue : Decryption

cz flag
Aura

I perform the following commands because I need to decrypt a file -encryption key file- (which is longer than module):

cat key.enc | base64 -d > encryptedrawfile.txt

openssl rsautl -decrypt -inkey privkey.pem -in encryptedrawfile.txt -out key.txt

I obtain the following key.txt: ’àÝ g ™`çïrM¡.€:

Now I need to use encryption key decrypted to decrypt a file using this key considering aes-csc-128. The file I need to decrypt with key above decrypted using aes is : QLTCpr0LK9K5Sw8MWX15LRergsaE+jEtCs3e7tTrATU=

osboxes@osboxes:~/$ echo QLTCpr0LK9K5Sw8MWX15LRergsaE+jEtCs3e7tTrATU= | base64 -d | openssl aes-128-cbc -d -nosalt -iv p1mce4lzT7bSsoUPFTFGTg== -k key.txt -out ficheroDescifrado.txt

*** WARNING : deprecated key derivation used. Using -iter or -pbkdf2 would be better. hex string is too short, padding with zero bytes to length non-hex digit invalid hex iv value

If I include -pbkdf2 I receive the following:

osboxes@osboxes:~/RsaCtfTool$ echo QLTCpr0LK9K5Sw8MWX15LRergsaE+jEtCs3e7tTrATU= | base64 -d | openssl aes-128-cbc -d -nosalt -pbkdf2 -iv p1mce4lzT7bSsoUPFTFGTg== -k key.txt -out ficheroDescifrado.txt

hex string is too short, padding with zero bytes to length non-hex digit invalid hex iv value

What is happening?

Thanks.

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Morrolan avatar
ng flag
Morrolan
By OpenSSL's manpage, the `-k` flag is for passing a *passphrase*. Hence what you are doing is passing `key.txt` (and not its content!) as a passphrase to OpenSSL, upon which it then applies some key derivation. What you probably mean to do is to pass the raw key (there's a flag for that which you'll find in the help output) as e.g. a hex-encoded string to `openssl`. A secondary issue is that you are passing the IV as what seems to be a Base64-encoded string, while `openssl`'s `-iv` flag expects a hex-encoded one.
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Morrolan avatar
ng flag
Morrolan
Generally your question is about how to use a specific tool, which is not perfectly on topic on here. But a quick perusal of the man page should solve these issues you are facing.
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