The MD algorithm (or process) is a basic tool in advanced cryptography and security, and can be used to encrypt a password. More importantly it can produce the so-called digital signature in many applications related to the Internet and Web. If you want to sign a document or file just like you sign a paper document so that no one can alter it afterward, you need to use MD. The basic idea is simple and can proceeds as follows:
Run your document or file through the MD or md5() function. This would produce a fixed length MD string similar to the encrypted password representing the signature of the document.
Send your document and the signature to the intended recipient by email or other transmission method.
The recipient ...