Abstract: To protect classified& important data digital images are used. Providing authentication, veracity and data safety to these digital images seems to be difficult. In current practices of giving security and authentication, conventional watermarking schemes are in use. Cryptography is taken up in these techniques which prove to be of no use in granting protection from hackers tampering the information. This paper puts forth a new proficient authentication method for grayscale document images making use of the Portable Network Graphics (PNG) image and data repair capability. Each block of a grayscale document image generates an authentication signal in this method and then, by using Shamir secret sharing scheme, authentication signal and binarized block content is pooled and altered into multiple numbers of shares after which they come together and combine into an alpha channel plane becoming the PNG image. This resulting layer is then formed into a stego image. For authentication purposes this stego image is sent to the receiver. If the received grayscale image is tampered, reverse Shamir's secret method is applied to repair the tampered image.

Keywords: Data repair capability, grayscale document image, Cryptography, Portable Network Graphics (PNG) image, Shamir secret sharing scheme, Alpha layer, stego image.