Lecture Notes in Computer Science 5471 - tiera.ru

Merx: Secure and Privacy Preserving Delegated Payments 231government investigations. Customers wishing that level of protection may wantto stick with cash.As we can see, Merx provides high levels of security and privacy against individualactors. The collusion between non-trusted parties leads to minor “controllable”effects. The only severe damages result from collusions involving thebank. However, such collusions are the least likely to occur in everyday life.4.3 Communication and Processing OverheadIn this section we evaluate the additional costs, processing, and communicationoverhead our system requires from all parties. The main observation is that thedelegation mechanism we propose, in its most basic form, requires no additionalhardware. It is a simple software solution using off-the-shelf components thatpiggyback on the existing payment-network infrastructure.For the numerical evaluation, we assume the customer uses a low-capacitymobile device (200MHz processor, 64MB RAM), whereas the merchant and thebank use an average speed server or desktop PC (Intel P4 2.8Ghz, 500MB RAM).We chose the SHA256 algorithm for all hashes, which we compute using thefree OpenSSL library. GnuPG, an open-source implementation of Pretty GoodPrivacy was used for all encryption. We selected GnuPG’s default algorithmsof El Gamal with 2048 bit keys (public key encryption) and AES (symmetricencryption) as these provide sufficient security while being quick to computeon a mobile platform. While we have chosen these particular algorithms, thescheme is flexible, and alternative encryption/hashing technologies could easilybe substituted.The processing time results shown in Fig. 3 are averaged over 10 runs for eachnumber of items. The confidence intervals are very small, therefore omitted fromthe figure.The Customer: To use such a system, the customer must have access to eithera mobile telephone or a computer. These devices do not necessarily need to beowned by the customer. In many cases, she merely needs to have temporaryaccess to them. If the customer owns the device, the she can store her accountnumber in it. If she is borrowing the device from someone else, she will need tofind a way to input her account number. This can be done manually (e.g. byentering a 16+ digit number), or perhaps by reading a QR code printed ontothe back of her bank card with a camera phone.Figure 3 shows the processing time required to create a token, for a givennumber of items in the shopping list. We can observe that in spite of the lowprocessingpower of the customer’s device, the processing delay is still acceptableeven for an exaggerated number of items. For low number of items, the processingdelay remain below 10 s.The total size of an encrypted transaction token for 10 items is approximately1.6 KBytes, with an additional 1 KByte required for the shopping list. For atransaction involving 50 items, the token grows to nearly 3 KBytes while the