Function sign 

fn sign<R: CryptoRngCore + ?Sized>(
    rng: Option<&mut R>,
    priv_key: &RsaPrivateKey,
    prefix: &[u8],
    hashed: &[u8],
) -> Result<Vec<u8>>

Calculates the signature of hashed using RSASSA-PKCS1-V1_5-SIGN from RSA PKCS#1 v1.5. Note that hashed must be the result of hashing the input message using the given hash function. If hash is None, hashed is signed directly. This isn’t advisable except for interoperability.

If rng is not None then RSA blinding will be used to avoid timing side-channel attacks.

This function is deterministic. Thus, if the set of possible messages is small, an attacker may be able to build a map from messages to signatures and identify the signed messages. As ever, signatures provide authenticity, not confidentiality.