Help please, Vernam Cipher

[Tarquini Mar]

The goal of the project is to develop a simple application that allows you to encrypt files using a variant of the Vernam cipher, here called bvernan. The proposed algorithm exploits a simple property of the binary operator XOR: (A XOR B) XOR B = A Given a sequence of k-bytes b0 ... bk-1 (called key), the encoding function of a sequence of bytes d0 ... dN, the encoding / decoding function follows the following simple procedure. The sequence d0 ... dN is first divided into N / k blocks (integer division), D0, ..., D [(N / k) -1] each of which consists of exactly k bytes (apart from the 'last sequence which, obviously, may contain a smaller number of bytes). Subsequently each sequence Dj = dj, 0 ... dj, k-1 is transformed into the sequence D'j = d'j, 0 ... d'j, k-1 such that for each i: d'j, i = b (j + i) mod k XOR dj, i That is, the byte in position i of block j is placed in XOR with the byte (j + i) mod k of the key. The output sequence will then be obtained from the juxtaposition of the sequences D'0, ..., D '[(N / k) -1].

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