CODEWITHSUNDEEP
encryptionopenssl
Pepemujica
Pepemujica

Reputation: 123

Decrypt .enc with .pem openssl

I'm trying to decrypt a .enc file with openssl. I have the .pem key available.

This is what I'm doing:

openssl rsautl -decrypt -in file.enc -out dec.txt -inkey private.pem

And returns the following:

data greater than mod len

Is there anything I'm missing?

Upvotes: 1

Views: 2820

Answers (1)

Maarten Bodewes
Maarten Bodewes

Reputation: 93968

We don't know how the file was created or what the contents are. It seems your private key parses correctly, but the modulus has a certain amount of bits, say 2048 bits. That means that raw RSA - modular exponentiation - can only encrypt as many bytes as there are in the modulus.

To be even more precise, the data, when converted to a number using unsigned big endian encoding should be smaller than the modulus as number. If your data is larger in size then the decryption will fail, which means it is either generated differently - it may have additional encoding, for instance - or it has been generated with a different, larger RSA key.

Note that the data that can be encrypted is significantly smaller due to the padding overhead, which is required to make RSA secure. The padding is performed over the input message, before number conversion & modular exponentiation. Also note that RSA is always performed just once, it is not repeated as for instance usual for block ciphers.

So you're not missing something, you're rather having too many bytes in file.enc.

Upvotes: 2

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