Electronic Mail and Internet Security - Codes: The Guide to Secrecy from Ancient to Modern Times - page 290

users); (2) multipart signed , which is also called clear signing (and this
is viewable by all users).
SignedData MIME : The followingsteps provide the means for con-
structing a SignedData MIME entity. Again, we assume there is only
Alice doing the signing of a given message m .
[i] Select a message digest algorithm, h , such as SHA-1.
[ii] Hash m ,toget h ( m ).
[iii] Encrypt the message digest with Alice's private key to form
d A ( h ( m )).
[iv] Create a message block for Alice, consisting of: an identifier of her
public-key certificate, I e A ; her X.509V3 certificate identifier C ( A ); an
identifier of the hash algorithm being used, I SHA − 1 , say; an identifier
of the algorithm used to encipher h ( m ), I RSA ,say; m itself; and the
encrypted message digest. This produces the SignedData MIME,
SDM =( I e A ,C ( A ) ,I SHA − 1 ,I RSA ,m,d A ( h ( m ))) .
[v] Then SDM is base-64 encoded to produce the SignedData MIME
value.
Upon reception, Bob, strips off the base-64 encoding, then uses e A to get
h ( m ). Then he independently computes h ( m ) from m and compares this
with the deciphered value of h ( m ) to verify Alice's signature.
Clear Signing : This structure allows Alice to communicate with Bob
if he is not an S/MIME-enabled user. The body of the multipart/signed
MIME is comprised of two parts, the first of which can be of any MIME
type, is left to be broadcast in the clear. The second part is actually a
special case of the SignedData MIME type, called a detached signature ,
wherein the plaintext of the message is omitted.
Here are the basic steps in producinga clear signed S/MIME entity:
[A] The message m is signed with Alice's private key to form d A ( m ).
[B] She forms a data block consistingof
CSM =( I e A ,C ( A ) ,I RSA ,d A ( m )) .
[C] Then CSM is base-64 encoded to form CSM 64 and the two-part
message ( m,CSM 64 ) is sent to Bob.
Bob receives the message, strips the base-64 encoding from the second
part of the message, CSM 64 ,toget CSM , uses Alice's public key e A to
recover m and compares it with the message m sent in the clear in the
first part of the message.
Signed-only data in S/MIME provides authentication without secrecy.