Function pss_sign 

pub fn pss_sign<H: Hasher>(
    sk: &RsaSecretKey,
    m_hash: &[u8],
    s_len: usize,
    rng: &mut dyn FnMut(&mut [u8]),
) -> Option<Vec<u8>>

RSASSA-PSS sign of a precomputed digest (RFC 8017 §8.1.1).

Draws a fresh s_len-byte random salt from rng and calls pss_sign_with_salt. Each invocation produces a different signature even for the same (sk, m_hash).

Recommended salt length: s_len = H::OUTPUT_LEN (the same length as the hash). Setting s_len = 0 produces a deterministic signature (“no salt”) – the same (sk, m_hash, H) will always yield the same bytes; this is allowed by the spec but gives up the randomized-signature security property.