Generation of plaintext-independent private key based on conditional decomposition strategy

•A plaintext-independent private key is generated based on conditional decomposition.•A novel category of structured spiral phase mask (SSPK) is constructed.•A performance comparison is made using either SSPK or random phase key (RPK).

We propose to generate the plaintext-independent private keys in optical asymmetric cryptosystem (OACS) based on the strategy of conditional decomposition (CD). Following this strategy, an OACS is designed with the principle of superposition of two vectorial beams. The proposed cryptosystem can remove the silhouette which is discovered in the two beams interference-based cryptosystem. To relieve the difficulty of key distribution, a structured spiral phase key (SSPK) is utilized instead of the random phase key (RPK). And a comparison on the performance of two kinds of keys in both the encryption and decryption process is made to show the advantage of SSPK over RPK.