(Digital signature: use MD5 to digest, and then send the digest and the original file for re-digest verification
Final version\MD5 message digest.txt
Final Edition\Encryption and Decryption.txt
Final Edition\Encryption and Decryption Modified Edition.txt
Final Edition\Can encrypt files.txt
Final Edition\Digital Signature.txt
Final Edition\Digital Signature Modified Edition.txt
.....too many files .....)