The surname Rivas which is identical to Ribas is derived from a locality in Spain. In fact there are more than 10 places by this name which contain Rivas or Ribas in their name, only one of which is a town, namely Rivas de Jarama nowadays called Rivas Vaciamadrid. In fact, the author Duque de Rivas was not called Rivas as a surname but it was his title, referring to Rivas de Jarama. So he is not related to people who like myself are called Rivas as their surname.
Rivas or Ribas is a medieval Spanish word, derived from the Latin Ripa and related to the French Rive, indicating a "riverbank". Rivas is shorthand, always indicating the riverbanks of a particular river, as in Ribas de Sil (Galicia, Sil being a river) or Rivas de Jarama (Jarama being another river).
This and confusing heraldic information has caused researchers to doubt whether there really is one clan.
However, other data strongly suggest that there really is one clan and that its origins were Jewish. My own theory is that the clan originated in a place called Rivas or Ribas Altas very close to the town of Monforte de Lemos (Galicia, Spain) which was known for its Jewish population. There was a period that Galicia was known to be tolerant towards its Jewish citizens. There has been a Jewish victim of the Inquisition by the name Ribas Altas and a Sephardic Jew in the 17th century called Rivas Altas.
Nowadays, there are Jewish families called Rivas in Mexico, Texas, Colombia, etc. Jewish personal names often run among the non-Jewish branches of the clan, such as Abraham, Moises, Samuel, Rafael, etc. A German victim of the Holocaust was called Ribas, and there have been Sephardic families called Rivas, Rivash and Ribas in the Ottoman Empire.
All of this strongly suggests that there is one big, originally Jewish clan of Rivas/Ribas-families, some of them still Jewish and some converso (anusim).
Titus Rivas, The Netherlands