Fast Decryption of Excel Document Encrypted by RC4 Algorithm

In this paper, we give a fast decryption method of Excel document encrypted by RC4 algorithm. Through a detailed analysis of document storage structure and encryption process, we illustrate the inner principle of key generation and data encryption in the block by block manner. We present an efficient way of recovering the intermediate key by using rainbow table attack, which can be directly applied to decrypt the Excel document. The advantage of our method is that the decryption time of one document is no longer affected by the password length and complexity. In our practical test, it has achieved the decryption of Excel encrypted documents in an average of 3 minutes on a common personal computer, which greatly improves decryption efficiency of encrypted documents compared with the current dictionary and brute force attack methods of recovering document password.