I've got a public OpenPGP key and need to send an encrypted email using telnet. However, I'm unable to find anything related to this topic.
I have no problem sending an email via telnet, but cannot figure out how to encrypt the mail.
You will have to encrypt the message text before sending.
Old method: Inline PGP
Earlier, PGP-encrypted messages were included directly in the message body. This is simple and allows using programs that lack built-in PGP support, but has its own issues.
New method: PGP/MIME
Most mail clients now create a multipart MIME message that looks like this:
Don't bother with this, let a MUA do it.
I will assume the former.
At the simplest level you can encrypt the body of the message, and, if necessary, convert to a suitable encoding such as base64 before you invoke the mail command.
Update: OK so you are typing SMTP commands by hand into an SMTP server.
Learn how attachments are serialised. Read the RFCs for MIME attachments. Use Thunderbird to view the source of a short message that has a small attachment (Ctrl+u IIRC). Look for the various ways that binary attachments are encoded so that they can pass through a 7-bit ASCII or 8-bit channel.
You can create the attachment as an external file. Even rot13 would demonstrate the process adequately. Then you just have to work out what MIME etc headings to key in and how to paste in the suitable encoded encrypted attachment during your telnet session.
Many email clients, such as Microsoft Outlook, support encryption. Encryption and decryption is the job of the email client, not the transport mechanism.
To clarify, Telnet is a network protocol (http://en.wikipedia.org/wiki/Telnet ) just like TCP. The protocol has nothing to do with encryption. Most mail clients use TCP/IP to send and receive email via the internet. One commonly used protocol used by mail clients to send an email to a server is Simple Mail Transport Protocol (SMTP). When the sending encrypted email, the encryption must take place prior to handing the message off to the protocol.
Which email client are you using? Does it support encryption? If not, perhaps you should try using a different email client that does, many do.
One of the many free clients that support this is Eudora. Here is a description on how to set this up: http://www.ncsa.illinois.edu/people/hkhurana/mithril/Help-SecureEmail.htm
Mozilla Thunderbird is an open source free client that supports this as well.