Systems and methods are described for securing credentials with optical security features formed by quasi-random optical characteristics (QROCs) of credential substrates. A QROC can be a pattern of substrate element locations (SELs) on the substrate that includes some SELs that differ in optical response from surrounding SELs. During manufacturing, a QROC of a substrate can be characterized, hidden by a masking layer, and associated with a substrate identifier. During personalization, personalization data can be converted into an authentication graphic formed on the substrate by de-masking portions of the masking layer according to a de-masking pattern. The graphic formation can result in a representation that manifests a predetermined optical response only when the de-masking pattern is computed with knowledge of the hidden QROC. The authentication graphic and optical response can facilitate simple human authentication of the credential without complex or expensive detection equipment.