signature: Small signature optimization

Only calculate the signature when necessary. When sigsize is smaller
than the expected signature length, preemptively exit the function
instead of doing that after having calculated the signature.

This saves some computation power.

Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>

Author: tgonzalezorlandoarm

parsec-openssl-provider/src/signature/mod.rs

@@ -165,6 +165,14 @@ unsafe extern "C" fn parsec_provider_signature_sign(
             return Ok(OPENSSL_SUCCESS);
         }
 
+        if (sigsize as usize) < siglength {
+            return Err(format!(
+                "Signature length is bigger than sigsize. Signature length: {}",
+                siglength
+            )
+            .into());
+        }
+
         if tbs.is_null() {
             return Err("Received unexpected NULL pointer as an argument.".into());
         }
@@ -186,16 +194,12 @@ unsafe extern "C" fn parsec_provider_signature_sign(
             .psa_sign_hash(key_name, tbs_slice, sign_algorithm)
             .map_err(|e| format!("Parsec Client failed to sign: {:?}", e))?;
 
-        if sigsize >= sign_res.len() as u32 {
+        if siglength != sign_res.len() {
+            Err(format!("Unexpected signature length: {}", sign_res.len()).into())
+        } else {
             std::ptr::copy(sign_res.as_ptr(), sig, sign_res.len());
             *siglen = sign_res.len() as u32;
             Ok(OPENSSL_SUCCESS)
-        } else {
-            Err(format!(
-                "Signature length is bigger than sigsize. Signature length: {}",
-                sign_res.len()
-            )
-            .into())
         }
     });