Double Random Phase Encoding (DRPE), which is a typical optical encryption technique, has been reported to be
vulnerable to Known Plaintext-Attacks (KPAs) using a Phase Retrieval Algorithm (PRA). But the reported case in which
the encryption key is successfully estimated was that the plain image was rather simple such as the image of a character.
In addition, although Phase Only DRPE (PO-DRPE) was proposed to achieve more resistance to the KPA than Complex
DRPE (C-DRPE) in which both amplitude and phase components are used as an encrypted image, no quantitative results
about the relationship between the vulnerability and the plaintext image. In this paper, we show the result of quantitative
analysis on KPA by PRA to C-DRPE and PO-DRPE, for the plaintext images of different characteristics. As a result of
experiment, KPA to C-DRPE succeeded to estimate the correct key while the probability of success became lower when
the number of non-zero pixel increases in the plaintext image. However, KPA to PO-DRPE enabled to estimate only
"singular" keys, which are effective for no more than given plaintext/ciphertext image pair and far different from the
correct encryption key. We also conducted KPA using two plaintext-ciphertext image pairs for KPA. In the case when
two plaintext-ciphertext image pairs were given to KPA, the cryptanalysis succeeded with higher probability than the
case of one. Moreover, the probability of success in the KPA was high even in PO-DRPE.