Function raw_sign_byupdate 

pub fn raw_sign_byupdate<CtxDigest, F>(
    esk: &ExpandedSecretKey,
    msg_update: F,
    verifying_key: &VerifyingKey,
) -> Result<Signature, SignatureError>
where
    CtxDigest: Digest<OutputSize = U64>,
    F: Fn(&mut CtxDigest) -> Result<(), SignatureError>,

Available on crate feature hazmat only.

Compute an ordinary Ed25519 signature, with the message contents provided incrementally by updating a digest instance.

The msg_update closure provides the message content, updating a hasher argument. It will be called twice. This closure MUST leave its hasher in the same state (i.e., must hash the same values) after both calls. Otherwise it will produce an invalid signature.

CtxDigest is the digest used to calculate the pseudorandomness needed for signing. According to the Ed25519 spec, CtxDigest = Sha512.

⚠️ Cryptographically Unsafe

Do NOT use this function unless you absolutely must. Using the wrong values in ExpandedSecretKey can leak your signing key. See here for more details on this attack.