Proving a Soundness Property for the Joint Design of ASN.1 and the Basic Encoding Rules

Abstract

The Abstract Syntax Notation One (ASN.1) can be used to model types of values carried by signals in SDL or MSC but is also directly used by network protocol implementors. In the last few years, the press has reported several alleged vulnerabilities of ASN.1 and the Basic Encoding Rules (BER) related to network protocols like SNMP and, more recently, OpenSSL. In reality it has been shown that the security issues (theoretically denial of service attacks) were due to low-quality and poorly-tested compiler implementations. We use some formal methods to go further. We review formally the design of the BER themselves and prove that, under some assumptions, it is flawless whatever the network protocol is and whatever the values to be transmitted are. More precisely, we start with a formal modeling of the BER which abstracts away low-level details but captures the design principles. Then we define a soundness property stating that the composition of encoding and decoding yields a value which is equivalent to the original. Finally we prove that this property holds for all values specified with ASN.1.