Function ed25519ph_sign_prehashed 

pub fn ed25519ph_sign_prehashed(
    sk: &Ed25519SecretKey,
    prehashed: &[u8],
    context: &[u8],
) -> Option<Ed25519Signature>

Variant of ed25519ph_sign that takes a precomputed SHA-512 digest of the message instead of the message itself. Useful when the caller already has the digest from a streaming hash (e.g. a file scanner that produced the digest as it read the file).

The prehashed argument must be exactly 64 bytes (the SHA-512 output length). Returns None for any other length.