A Forward-Secure E-Mail Protocol without Certificated Public Keys

1
A Forward-Secure E-Mail Protocol without
Certificated Public Keys

• Source Information Sciences, article in press
• Author Jeong-Ok Kwon, Ik-Rae Jeong and
  Dong-Hoon Lee
• Speaker Ting-Fang Cheng
• Date 2009/10/08

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Introduction (1/2)
The common e-mail architecture
Mail Servers
Store-and-forward system
Sender (A)
Receiver (B)
M
M
3
Introduction (2/2)
Considerations of an e-mail system for mobile
applications

• Confidentiality and authenticity
• PGP, 1995
• Forward secrecy
• Diffie-Hellman key exchange, 1976
• Sun et al.s e-mail protocol, 2005
• Kim et al.s e-mail protocol, 2006
• Efficiency
• Password-based e-mail protocol

4
Notations

• SA the mail server which A registered
• SB the mail server which B registered
• pwA As password shared with SA
• pwB Bs password shared with SB
• G the finite cyclic group of order q
• p a large prime
• g1, g2 two generators of G
• As one-time private/public key
  pair
• Bs one-time private/public key
  pair
• Ek(.)/Dk(.) the symmetric encryption/decryption
  algorithm with key k
• PEk(.)/PDk(.) the public-key encryption/decryptio
  n algorithm with key k
• Mac(.), F(.), H(.) three pseudorandom functions

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Proposed protocol (1/6)
The scenario

• Assume that A wants to send a message M to B
• A and B have registered at distinct mail servers
  SA and SB respectively
• SA stores (A, )
• SB stores (B, )

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Proposed protocol (2/6)
Login phase of A
SA
A
Selects xA ? Zq Computes
A, XA
Selects yA ? Zq Computes
SA,
Computes
Computes
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Proposed protocol (3/6)
Login phase of A
SA
A
Verifies Computes
Verifies Computes
Key agreement
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Proposed protocol (4/6)
Sending phase of A
SA
SB
A
Establish a short-term key kSS through a secure
AKE protocol
Computes
YB
Decrypts YB Computes
YA
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Proposed protocol (5/6)
Sending phase of A
SA
SB
A
YA
Decrypts YA to get Computes
Z1
Decrypts Z1 to get Z Computes
Z2
Decrypts Z2 Stores (A, B, Z) in database
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Proposed protocol (6/6)
Receiving phase of B
B
SB
B login to SB and produces a key KB shared with SB
Searches database Computes
Computes Chooses new public/private key pair
to replace the old one in its
mobile device
Decrypts the message to get Replaces
with
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Conclusions

• Forward secrecy
• Password-based protocol
• Practicality and efficiency