From: Blanc Weber <blancw@microsoft.com>
Date: Thu, 7 Jul 94 08:47:23 PDT
To: cypherpunks@toad.com
Subject: FW: Windows for Workgroups 3.11
Message-ID: <9407071449.AA11623@netmail2.microsoft.com>
MIME-Version: 1.0
Content-Type: text/plain


Below is the reply which I received internally on the question of 
whether WfW uses RSA:

----------
From: Michael Ginsberg
To: Blanc Weber
Cc: Cypherpunks Mailing List Redistribution
Subject: RE: Windows for Workgroups 3.11
Date: Thursday, July 07, 1994 8:22AM

[The following can be distributed outside of Microsoft, and 
redistributed thereafter, as long as it remains unaltered and is 
reproduced in it's entirety, including this header]

In the Windows for Workgroups 3.11 fax enhancement, we provide the 
ability to send "email format" messages over fax.  These are messages 
that do not lose their original format during faxing, so that they are 
still editable (ie: if you send a Microsoft Word document, the receiver 
gets an actual Microsoft Word document, not just printed pages).  One 
feature of this format is that we allow security enhancements to be 
applied to the message before transmission.  These enhancements are 
encryption (either secret key or public key) and signature (based on 
public key).  I will describe a bit about how we implement these features.

1> Secret key encryption
	This is exposed to the user as password encryption.  The user chooses 
a password (of as long a lenght as the user desires, composed of 
characters with ASCII values 1 through 255) and we hash it down to 64 
bits.  These 64 bits are used to encrypt the message (including all 
attachments, text, properties, etc) using RSA's RC4 algorithm.  A 
header is then placed on the message with sufficient information for 
routing (subject, sender, receiver, time stamps, as well as a marker 
identifying the encryption type).  The receiver enters the password to 
decrypt (the same as the encrypting password), and we verify that the 
password is correct by decrypting the message and seeing if it's valid 
(ie: has the right internal structure to be a mail message).

2> Public key encryption
	We allow the user to generate public/private key pairs.  These key 
pairs are 512-bit RSA key pairs.  The private part is stored encrypted 
(with the user's "fax security" password) on the user's hard drive, and 
the public part is stored in plain (binary form).  The user exchanges 
his public keys with other users in various ways.  If the user sends a 
fax with public key encryption, we first generate a random 64 bit 
number, and encrypt the message with RC4 using that number (similar to 
password encryption above).  We then scatter the 64 bits in a 64 byte 
(randomly initialized) buffer and encrypt that buffer using RSA's RSA 
encryption scheme, using both the sender's private key and the 
recipient's public key.  We then send the encrypted message and header 
containing routing information and the 64 byte block.  The recipient 
performs a similar operation, using his private and the sender's public 
key to obtain the 64 bit "secret key", and then decrypting the message.

3> Digital signature
	While the above encryption schemes are performed on entire messages, 
we provide a method for digitally signing attachments.  Microsoft Mail 
allows the entering of plain text into a message, as well as the 
embedding of entire files.  It is these files which we sign.  Signature 
is accomplished by computing a 128 bit hash of the file using RSA's MD5 
message digest algorithm.  This 128 bit hash is then embedded in a 64 
byte buffer, and it is encrypted using the sender's private key.  The 
buffer (along with some header information) is pre-pended to the 
document, and it's extension is changed to .aws, so that our applet to 
view signatures is called upon double clicking or "running" the signed 
document.  Anyone receiving the document can see the validity of the 
signatures (by hashing themselves, and then decrypting the stored 
hash).  If you sign a signed document, the signatures are cumulative, 
so multiple people can sign the same document.  Signatures are either 
valid (hashes match), invalid (hashes differ), or unverifyable (you 
don't have the sender's public key).

--Michael Ginsberg, Software Design Engineer, Microsoft Corporation.
mikegins@microsoft.com