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CA-based enciphered message transmission system
I. Value and significance of the product
In the environment common network, it is difficult to
guarantee that message transmission can be prevented from
interception and modification. In the case of sending
information by common network, especially for the fields of
national confidential and public security, only enciphering
can be relied on to ensure the transmitted message difficult
for decipherment.
Plenty of exploitation has been made in the aspect of security
information transfer by practicing various technical means
including text encryption, text transform, entrainment,
superimposition of graph and hidden graph. But regarding the
current analysis of the existing international products,
relatively popular means are text encryption (inclusive of
security electronic mail) and entrainment technology
(indicating entrainment of confidential information into
normal file without affecting display and application of the
normal file).
Almost all the details of cipher system find application in
it.
Business
Support algorithm
Remarks
Privacy
Symmetric algorithm (IDEA),
asymmetric algorithm (RSA)
Certification
RSA and MD5
Message hashing in MD5, enciphering with receiver¡¯s RSA
publickey, and sent with the original message
Compression
ZIP
Storage of transferred message with completeness
Fragmentation
Handling maximum message length with automatic fragmentation
Undeniableness
Digital signature, certification of intermediate message
source
II. General description of the system
1.1 Functions of the system
This system is designed for security of message transfer in
common network and has the following main functions:
a. Certification Center (CA) offline registers and confirms
users' identification, registers individual information,
stores users' public keys, issues CA public keys to users for
future verification of CA signatures.
b. If users have the demand of security communication, they
can logon CA through network and obtain receivers' information
and public keys after confirmation of their identifications.
c. All message exchange between CA and users through network
is subject to encryption by public key and symmetric key to
ensure disclosure of both parties' message not to occur. And
enciphered message can be sent by embedding into graphic files
with "info stego" plug-in card. Even if interception
occurs, only a normal graphic message will be seen and it is
very hard to see what's behind it, which makes communication
more secure.
1.2 Flow of work of the system
Model of the system mainly includes two parts: CA terminal and
user terminal as the following figure.
The description of flow of work of the system is as follows:
User submits personal identification certification to CA and
applies for registration.
CA confirms user's identification, assigns ID to user, logon
password, provide CA's publickey to user, and sign to keep
user's publickey. (Both 1 and 2 in offline mode)
User logons CA to apply for confirmation of message receiver's
identification and obtain the receiver's publickey.
CA makes verification of identification of the applicant and
response the request with CA signed receiver's message and
publickey after the verification.
User enciphers message with symmetric algorithm and applies
receiver's publickey in enciphering symmetric key to send to
receiver.
Note:
If user is out of contact with CA, the publickey in cache is
allowed for use. But it cannot be ensured whether the received
privatekey is changed or upgraded. The system supports this
means but does not agree with it. (This is the second mode of
work of the system.)
In order to adapt to the demand of special environment, the
system reserves support to the encipher transmission mechanism
enabling protocol symmetric privatekey mode. There is no
contact for user with CA during the operation. (This is the
third mode of work of the system.)
2. Function modules of the system
2.1 Module of CA terminal
CA terminal is composed of two major parts:
a. User offline registration: user submits personal
identification certification and public key to CA by offline,
while CA receives user's registration and keeps user's private
key after confirmation, and issues CA's public key.
b. User online inquiry: CA receives user's logon and responses
user's inquiry of public key after confirmation of
identification. User's logon password is enciphered with CA's
public key. And CA responses by enciphering with symmetric key
first, and then enciphering this symmetric key with user's
public key, and signs with CA's private key.
2.2 Module of user terminal
User terminal is composed of six major parts:
Application of publickey
User submits user's name and password to CA. The password is
enciphered with publickey provided by CA. After verification
of user's identification, CA provides user registration list
for user's option or directly responses user's request of
inquiry. Thus, logon user can obtain other users' publickeys
through CA for information transmission.
Encipherment and decipherment of publickey
Publickey system means enciphering and deciphering with a pair
of matching keys. It can be proved that encipherment and
decipherment can be completed only by this kind of matching
keys and that it is difficult to find out the other if one of
them is known. Every user can set a dedicated private key (privatekey)
only known by himself/herself for deciphering and signing
purpose, meanwhile set a public key (publickey) and make it
known by himself/herself to the public through CA. This
publickey will be shared by a group of users for deciphering
and signing purpose. When transmitting a privacy file, sender
signs with his/her own privatekey and enciphers data with
receiver's publickey, while receiver deciphers with his/her
own privatekey and verifies signature with sender's publickey,
by which message can be reach destination without any mistake.
Encipherment and decipherment of symmetric key
Both message sender and receiver use identical key to complete
process of encipherment and decipherment respectively. The
process may be the same or different.
Signature verification
Adoption of digital signature can ensure the following two
points:
©~ It is signer who has transmitted the message.
©~ No modification has ever been made from signing and
transmitting to reception.
Thus, digital signature can be applied in preventing
electronic information from fabrication due to easy
modification, from transmitting in the name of others, or from
denying after sending/receiving.
Double enciphering is adopted in digital signature to
implement prevention of fabrication and denial. The process is
as follows:
1) Enciphering of transmitted file to create digital digest.
2) A second enciphering of the digest with sender's own
privatekey, by which digital signature is formed.
3) Transmitting the enciphered digest with original message to
receiver.
4) Receiver deciphering the digest with sender's publickey and
creating another digest by enciphering the received file.
5) Comparison of the deciphered digest with the digest created
by enciphering the received file. If the two digests are
identical, it indicates that no damage or modification has
been made during transmission. Otherwise it is not the case.
Encoding mode of target message
Publickey or symmetric key can be selected in enciphering in
accordance with different demands.
5.1 Encipherment with symmetric key
The advantage of this method is high speed in deciphering and
enciphering, and shorter time in enciphering and deciphering
larger message blocks, while the disadvantage is adoption of
identical key in both enciphering and deciphering, which makes
transfer and privacy of the key more difficult.
5.2 Encipherment with public key
The advantage of this method is that sender can obtain
receiver's encipher key from a public place and only receiver
can decipher the enciphered message without transfer of key,
while the disadvantage is slow speed of encipherment and
decipherment and possible modification of obtained key.
Therefore, we adopt the following mode: encipher message with
symmetric key created by random, and then encipher this
symmetric key with receiver's public key, and transmit with
enciphered message to receiver, who will decipher this
symmetric key with his/her own private key and then decipher
the message with this symmetric key. Thus, not only the
advantage of being high enciphering strength for publickey is
reserved but also the advantage of being high enciphering
speed for symmetric key is considered, and it gives solution
to the disadvantage of being difficult for transfer of
symmetric key. To obtain receiver's public key from CA of the
third organization guarantees correctness of key.
6. Transfer carrier of target message
There are two modes, embedment of graph and non-embedment of
graph, for final creation of target message.
6.1 Non-embedment of graph
In this case, enciphered target message directly forms file to
transmit with higher speed but smaller volume of transmitted
message, by which the enciphered message is liable to be
found.
6.2 Embedment of graph
In this case, enciphered target message is encoded into
graphic file for transmission, by which the carried message is
not liable to be found. Owing to greater volume of message, it
can be reduced through compression.
7. Transmission position of target message
We recommend our dedicated user terminal transmission be
adopted, but also support adoption of the existing mail system
and "smtp pop3" system.
2002-01-29
