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This paper analyzes application of Selective Encryption with Automatic Variable Key (AVK) for text based documents in Koyama Public Key Cryptosystem (PKC). In this paper, a new robust and simple PKC based singular cubic curve cryptosystem over the ring Zn, using selective encryption with Automatic Variable Key (AVK), is proposed. In the proposed algorithm, selected parts of text document are encrypted/decrypted using any public key cryptosystem e.g. RSA algorithm based singular cubic curve with A VK approach. The key is configured every time as Automatic Variable Key. Rest of the plain text after selection is either left as it is and can be encrypted/decrypted using a simple DES (Data Encryption Standard) approach. The balance of computational overhead is projected through the technique of selective encryption with A VK, which increases the speed of encryption/decryption and level of security as well. This paper illustrates how the selective encryption with A VK method increases security, robustness and speed of encryption in Koyama scheme. The enhanced algorithm suitably provides for authorization, authentication and confidentiality. The result obtained indicates increase in speed using selective encryption over the conventional technique. It can be established that the selective encryption has an edge over the existing techniques as far as time complexity is concerned.