Applied Cryptography in E-mail Services and Web Services

Applied Cryptography in E-mail Services and Web Services

Lei Chen (Sam Houston State University, USA), Wen-Chen Hu (University of North Dakota, USA), Ming Yang (Jacksonville State University, USA) and Lei Zhang (Frostburg State University, USA)
DOI: 10.4018/978-1-61520-783-1.ch005
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Abstract

E-mail services are the method of sending and receiving electronic messages over communication networks. Web services on the other hand provide a channel of accessing interlinked hypermeida via the World Wide Web. As these two methods of network communications turn into the most popular services over the Internet, applied cryptography and secure authentication protocols become indispensable in securing confidential data over public networks. In this chapter, we first review a number of cryptographic ciphers widely used in secure communication protocols. We then discuss and compare the popular trust system Web of Trust, the certificate standard X.509, and the standard for public key systems Public Key Infrastructure (PKI). Two secure e-mail standards, OpenPGP and S/MIME, are examined and compared. The de facto standard cryptographic protocol for e-commerce, Secure Socket Layer (SSL) / Transport Layer Security (TLS), and XML Security Standards for secure web services are also discussed.
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Commonly Used Cryptographic Ciphers And Security Protocols

Data Encryption Standard (DES) and Triple-DES

In 1973, National Institute of Standards and Technology (NIST, previously NBS) solicited proposals for a government-wide standard for encryption and decryption. Based on the IBM Lucifer cipher (developed by 1973Feistel and his colleagues in 1973! and 1974), DES was accepted as an official Federal Information Processing Standard (FIPS) for the U.S. in 1976, later widespread internationally. Many later ciphers, including RC5, Blowfish and CAST5, were designed based on DES. DES is basically an iterative symmetric key algorithm that uses a relatively short key with only 56 binary bits in length. In each of its 16 rounds, DES takes a 64-bit data block and a 48-bit sub-key as the inputs and goes through a series of steps including expansion, Substitution Boxes (S-Boxes) and Permutation Boxes (P-Boxes) resulting 64-bit output. Everything except the S-Boxes in DES is linear. Due to short key length of DES, Triple-DES or 3DES was introduced to increase the key length to 112-bit in EDE mode and 168-bit in EEE mode. DES and 3DES had been the most popular symmetric key block ciphers before the emergence of AES.

DES has eight different S-boxes, each of which maps a 6-bit input to a 4-bit output. The first bit and the last bit of the 6-bit input of an S-box form the binary row indexes and the rest 4-bit of the input forms the column indexes of a single S-box conversion table. The table then has the dimension of 4 (00 to 11) rows by 16 (0000 to 1111) columns and the 64 intersections show the possible values of the 4-bit output. Each possible 4-bit output value has 4 occurrences among the intersections. Therefore, a specific 6-bit input value points to a specific intersection and output value. On the other hand, a unique output value does not help find the input value due to the 4 occurrences.

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