Function message_digits 

pub fn message_digits(
    pk_hash: &Digest,
    nonce: &Nonce,
    message: &[u8],
) -> [u8; 68]

Salted BLAKE3-XOF message mapping; 64 low-bit digits encode 128 digest bits.

BLAKE3-XOF(pk_hash ‖ nonce ‖ message), 64 bytes, then digits_from_xof. Binding pk_hash ties the digest to one key; the secret nonce keeps the digits unpredictable until the signature is out.