Si, y ahora al tener iguales bases, ¿qué hacemos con esto? A: se iguala.

Tag-reader communication is achieved by using a common communications protocol between the tag and the reader. Tag-reader communication protocols are often specified in RFID standards. Prominent international standards include the ISO/IEC 18000 series for item management and the ISO/IEC 14443 and ISO/IEC 15693 standards for contactless smart cards. The most recent EPCglobal Class-1

Generation-2 standard is essentially equivalent to the ISO/IEC 18000-6C standard. A more detailed explanation of RFID standards can be found in Appendix A on RFID Standards and Security Mechanisms.

Tag-reader communication characteristics that affect performance and security include:  How tag-reader communication is initiated,

 How a reader identifies particular tags, and

 How far away a tag or reader’s signal can be reliably detected or interpreted. These are discussed in detail in Sections 2.3.3.1 through 2.3.3.3.

2.3.3.1 Communication Initiation

Tags and readers can initiate RF transactions in two general ways:

 Reader Talks First (RTF). In an RTF transaction, the reader broadcasts a signal that is received by tags in the reader’s vicinity. Those tags may then be commanded to respond to the reader and to continue transactions with the reader.

 Tag Talks First (TTF). In a TTF transaction, a tag communicates its presence to a reader when the tag is within the reader’s RF field. If the tag is passive, then it transmits as soon as it gets power from the reader’s signal to do so. If the tag is active, then it transmits periodically as long as its power supply lasts. This type of transaction might be used when it is necessary to identify objects that pass by a reader, such as objects on a conveyer belt.

Readers and tags in an RFID system typically operate using only RTF or only TTF transactions, not both types. Active tag TTF operation may be easier for an adversary to detect or intercept, because active tags send beaconing signals even when they are not in the presence of a reader. The adversary could eavesdrop on this communication without risking detection because in TTF transactions the adversary never has to send a signal to ascertain the tag’s presence.

SECTION 2:RFIDTECHNOLOGY

2.3.3.2 Singulation

Singulation is the process by which a reader identifies a particular tag. This capability is critical

whenever multiple tags are in close proximity. For instance, when a reader issues a command to modify a tag’s memory, neighboring tags should not accidentally execute the same command. Similarly, when a reader sends a query to a tag, the reader should not receive a response from multiple tags.

In the EPCglobal Class-1 Generation-2 standard, the singulation protocol requires the reader to broadcast commands to all tags within its operating range. By issuing additional commands, the reader may limit interrogation to tags with specific memory contents. Tags respond with a random number. Once the reader acknowledges this number, verifying that no tag collision has occurred, the tag will transmit its unique ID to the reader. The reader may then request another random number that it uses to address the tag in subsequent communication. The random number has significantly fewer bits than the tag’s identifier, which simplifies the processing of later transactions and prevents transmission of the unique identifier by the reader.

Some RFID technologies do not support singulation. For example, ISO 11784/11785 animal tracking tags have no collision detection or avoidance mechanism because multiple tags are not usually read in close proximity for this type of application.

2.3.3.3 Signal Propagation Distance

The communications link between a tag and a reader typically is bi-directional. The reader transmits a signal to a tag over the forward channel. The tag responds on the back channel, which is also called the reverse channel or backscatter channel. When RFID systems use passive tags, signals on the forward channel typically are much more powerful than those on the back channel. Therefore, signals on the forward channel can be detected or properly received over longer distances. This difference has important implications for RFID communications security, including both the vulnerability of RF subsystem traffic and the mechanisms used to protect it. Some relevant operational ranges related to various communications objectives are:15

 Nominal operating range, which is the distance, often specified by standard, over which authorized transactions are expected to occur;

 Back channel eavesdropping range, which is the distance over which a rogue receiver can reliably interpret a tag’s response to a legitimate reader;

 Rogue skimming (or scanning) range, which is the distance over which a rogue reader operating above regulated power limits can reliably communicate with a tag;

 Rogue command range, which is the distance over which a rogue reader can execute a tag command that does not require the reader to successfully receive information from the tag;

 Forward channel eavesdropping range, which is the distance over which a rogue receiver can reliably listen to the transmissions of an authorized reader; and

 Forward channel traffic analysis range, which is the distance over which a rogue receiver can detect the presence of a reader’s signal without having to reliably interpret its content.

15

A. Juels, "RFID security and privacy: a research survey," IEEE Journal on Selected Areas in Communications, vol. 24, no. 2, pp. 381-394, February 2006.

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obscure the content of messages from readers to tags and is described in more detail in Section 5.3.2.1.

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ry ng is called traffic analysis, and it can be performed over much greater distances than eavesdropping.