a.rs

// Cargo.toml:
// [dependencies]
// rsa = "0.9.6"
// rand = "0.8.5"

fn main() -> Result<(), Box<dyn std::error::Error>> {
    use rsa::pkcs8::DecodePrivateKey as _;

    // Ciphertext
    let ciphertext = include_bytes!("signed_message.bin");

    // Unencrypted RSA private key in PEM format
    let rsa_pem = include_str!("sessionprivatekey.pem");

    // Import the RSA private key
    let mut rsa_key = rsa::RsaPrivateKey::from_pkcs8_pem(rsa_pem)?;
    rsa_key.precompute()?;

    // Decrypt the ciphertext
    let mut rng = rand::thread_rng();
    let plaintext = rsa_key.decrypt_blinded(&mut rng, rsa::Pkcs1v15Encrypt, ciphertext)?;
}