📝 Abstract

In the context of security protocol parallel composition, where messages belonging to different protocols can intersect each other, we introduce a new paradigm: term-based composition (i.e. the composition of message components also known as terms). First, we create a protocol specification model by extending the original strand spaces. Then, we provide a term composition algorithm based on which new terms can be constructed. To ensure that security properties are maintained, we introduce the concept of term connections to express the existing connections between terms and encryption contexts. We illustrate the proposed composition process by using two existing protocols.

💡 Analysis

In the context of security protocol parallel composition, where messages belonging to different protocols can intersect each other, we introduce a new paradigm: term-based composition (i.e. the composition of message components also known as terms). First, we create a protocol specification model by extending the original strand spaces. Then, we provide a term composition algorithm based on which new terms can be constructed. To ensure that security properties are maintained, we introduce the concept of term connections to express the existing connections between terms and encryption contexts. We illustrate the proposed composition process by using two existing protocols.

📄 Content

1-4244-2577-8/08/ $20.00 ©2008 IEEE Term-based composition of security protocols

B. Genge1, P. Haller1, R. Ovidiu1, I. Ignat2 1“Petru Maior” University of Targu Mures, Romania, bgenge@upm.ro, phaller@upm.ro, oratoi@engineering.upm.ro 2Technical University of Cluj Napoca, Romania, Iosif.Ignat@cs.utcluj.ro

Abstract-In the context of security protocol parallel compo- sition, where messages belonging to different protocols can intersect each other, we introduce a new paradigm: term- based composition (i.e. the composition of message compo- nents also known as terms). First, we create a protocol speci- fication model by extending the original strand spaces. Then, we provide a term composition algorithm based on which new terms can be constructed. To ensure that security prop- erties are maintained, we introduce the concept of term con- nections to express the existing connections between terms and encryption contexts. We illustrate the proposed composi- tion process by using two existing protocols. I. INTRODUCTION Security protocols are communication protocols in which participants use encryption to send each other en- coded information. With the rapid growth of the Internet and a desperate need to secure communication, in the last few decades the attention of many researchers has been led towards the analysis of security protocols [1], [2], [3], [4], [5], [6]. Recently, there have been several proposals developed to help the process of security protocol design using for- mal methods and tools [7], [8], [9], [10], [11], [12], [13]. Most of the proposed techniques use a modular approach in the design process, where the user is given a set of small protocols from which more complex protocols can be con- structed, process also known as composition [9], [10], [11]. In the existing composition techniques, authors mainly deal with the sequential and parallel composition of secu- rity properties viewed as a set of information transmitted over messages. However, the composition of message components has not been addressed in a proper manner, meaning that users have to solve the problem of creating new messages on their own. Solving this problem, apparently insignificant, can lead to protocols which execute in half the time the original, composed protocols do. In addition, the composition proc- ess can lead to multiple results, which must be carefully analyzed on a message level to increase protocol perform- ance. In this paper, we introduce a novel composition para- digm: term-based composition. The composition problem is addressed at the message level based on syntactical con- structions and analysis. This new paradigm is addressed in the context of parallel composition, where protocol mes- sages intersect each other. The resulting protocol contains not only a set of unified messages but also a unified set of security properties (e.g. secrecy, authentication, integrity). The paper is structured as follows. Section II introduces the concept of k-strands used to model security protocols. Security requirements are addressed in section III. In sec- tion IV we present the problem of generating protocols using parallel composition and term-based composition and we propose a term composition algorithm. We exem- plify the composition process by composing two protocols. II. KNOWLEDGE STRANDS In this section we briefly present the concept of knowl- edge strands (k-strands). For a more detailed presentation, the reader is directed to consult the authors’ previous work [6], [17]. A strand is a sequence of transmission and reception events used to model protocol participants. A collection of strands is called a strand space. The strand space model was introduced by Fabrega, Herzog and Guttman in [15] and extended by the authors with participant knowledge, specialized basic sets and explicit term construction in [5], [6]. The resulting model is called a k-strand space. The rest of this section formally defines the k-strand and k- strand space concepts.
By analyzing the protocol specifications from the SPORE library [20] we can conclude that protocol partici- pants communicate by exchanging terms constructed from elements belonging to the following sets: R, denoting the set of participant names; N, denoting the set of nonces (i.e. “number once used”) and K, denoting the set of crypto- graphic keys. If required, other sets can be easily added without affecting the other components. To denote the encryption type used to create crypto- graphic terms, we define the following function names:

FuncName ::= sk
(secret key) (1)

    | pk  

(public key)

    | pvk 

(private key)

    | h    

(hash).

The above-defined basic sets and function names are used in the definition of terms, where we also introduce constructors for pairing and encryption:

( ) { } ( ) :: .| | | | , | FuncName

T T T T T R N K , (2) where the ‘.’ symbol

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