Most people seal the envelope before posting a letter. If asked why, then some immediate responses would probably include comments like 'I don't know really, 'habit', 'why not?' or 'because everyone else does'. More reasoned responses might include 'to stop the letter falling out' or 'to stop people reading it'. Even if the letters do not contain any sensitive or highly personal information, many of us like to think that the contents of our personal correspondence are private and that sealing the envelope protects them from everyone except the intended recipient. If we sent our letters in unsealed envelopes then anyone who gained possession of the envelope would be able to read its contents. Whether or not they would actually do so is a different issue. The point is that there is no denying that they would be able to if they wanted to. Furthermore, if they replaced the letter in the envelope then we would not know they had done so. For many people the use of email is now an alternative to sending letters through the post. It is a fast means of communication but, of course, there are no envelopes to protect the messages. In fact it is often said that sending email messages is like posting a letter without an envelope. Clearly anyone wanting to send confidential, or maybe even just personal, messages via email needs to find some other means of protecting them. One common solution is to use cryptography and to encrypt the message. If an encrypted message falls into the hands of someone other than its intended recipient then it should appear unintelligible. The use of encryption to protect emails is not yet particularly widespread, but it is spreading and this proliferation is likely to continue. Indeed in May 2001 a group of European MPs recommended that computer users across Europe should encrypt all their emails, to 'avoid being spied on by a UK-US eavesdropping network'. Cryptography is a well-established science that has been a significant historical influence for more than 2,000 years. Traditionally its main users were governments and the military, although it is worth noting that The Kama Sutra of Vatsyayana contains a recommendation that women should study 'the art of understanding writing in cypher' (full details of all works cited are given in References and further reading.