Function ash_build_proof 

pub fn ash_build_proof(
    client_secret: &str,
    timestamp: &str,
    binding: &str,
    body_hash: &str,
) -> Result<String, AshError>

Build cryptographic proof (client-side).

Formula: proof = HMAC-SHA256(clientSecret, timestamp + “|” + binding + “|” + bodyHash)

Arguments

• client_secret - Derived client secret (must not be empty)
• timestamp - Unix timestamp as string (must not be empty)
• binding - Canonical binding (must not be empty)
• body_hash - SHA-256 hash of canonical body (must be 64 hex chars)

Returns

Returns Ok(proof) on success, or Err if any required input is invalid.

Security Note (SEC-012)

All inputs are validated to be non-empty and body_hash is validated for format.