A custom file format has a header encrypted with a 2048-bit RSA public key. I need to decrypt it using a 2048-bit RSA private key. The header has 48 bytes, and according to the scheme I got for the task, after decryption I should get a 48 bytes of "plaintext" payload.
However, this scheme does not make any sense to me. It is my understanding that if we encrypt something using a 2048-bit RSA key, we will inevitably get a 256 ciphertext. Am I wrong in my assumption? Is there a way to RSA-decrypt a 48 byte ciphertext with a 2048-bit private key and get a 48 byte plaintext out of it?
I have tried using
-raw mode with
openssl like this, and it produces a 256-byte long plaintext message:
openssl rsautl -decrypt -in ./header.bin -out ./header-out.bin -inkey ./privkey.pem -raw
P. S. After discussion with the client and going through the encryption source code, we've figured out that the documentation had it wrong. In reality, header is not 48 bytes but variable length, and the first 2 bytes of the file contain the header length.