 <p><code>CR</code> represents the regular Viginere ciphertext, <code>CM</code> the modified Viginere ciphertext</p> <pre><code>CR[i] = (P[i] + K[i]) mod 26 CM[i] = (P[i] + K[i] + CM[i-1]) mod 26 = (CR[i] + CM[i-1]) mod 26 </code></pre> <p>Now simply solve for the original ciphertext <code>CR</code></p> <pre><code>CR[i] = (CM[i] - CM[i-1]) mod 26 </code></pre> <p>Once you have the regular ciphertext, break it as usual.</p>