General Questions

What is an Airgap?

You can’t ever be certain that a network or serial interface between two devices won’t lead to one of those devices corrupting the other. Nor can you be certain that a networked connection is proof from man-in-the-middle attacks. That leads to the need for airgaps, where devices don’t physically connect except “through a gap of air”. In recent years, QR codes have been a prime method of bridging airgaps: they leverage the cameras and displays in the devices to communicate. Airgaps can also be bridged by NFCs, MicroSDs, and other methods that remove the real-time interactivity of a networked connection.

URs are generally important for any type of interoperability between devices, because their self-identifications makes it easy for a receiving device to know what they’re getting. However, they’re particularly important when QRs are being used to communicate through an airgap, because of UR’s support for QRs and particularly for Animated QRs.

Who Supports the Use of Airgaps?

Blockchain Commons’ development of airgap specifications is not just the product of our work, but also cooperation with other Bitcoin wallet companies to create digital formats, specifications, and reference apps that support new ways to protect your digital assets. This discussion happens primarily in the Gordian Developer Community.

What Can Be Encoded in URs?

Any data can be encoded as URs as long as it has a CBOR encoding and a user-defined UR type. The Registry of Uniform Resource types lists data types that Blockchain Commons specifies, maintains, and promotes. You can also define proprietary user-defined types.

To date, the major uses of URs have fallen into three categories:

  • PSBT Signing. URs can transfer PSBTs as they are being signed.
  • Key Transfer. URs can be encode seeds and HD keys.
  • SSKR Shares. URs can encode shards of a key or seed sharded by SSKR.

When data is being transferred between airgapped apps, it is usually done with a Gordian Envelope and may be part of a request-response interaction.

Encoding Questions

Why Not Use Base64?

URs were specifically designed to support more efficient usage in QRs, and that’s one of their prime advantages over base64. When a UR is properly stored as upper-case letters, it can be encoded in a QR using the “alphanumeric” encoding mode, while base64 requires “binary” encoding mode. base64 thus wastes at least two bits per character when used in QRs.

What Tools Can I Use to Understand CBOR?

Obviously, the most important tool is the CBOR reference, with our dCBOR profile trailing close behind.

CBOR has a more human-readable text diagnostic notation you should become familiar with. When you are testing your understanding of how CBOR encoding works or debugging, you can use the CBOR Playground to transform CBOR between hex and diagnostic notation, or if you prefer a command-line implementation, use the CBOR-cli, which can be installed with npm if you have Node.js installed.

Specifications for CBOR structures are written in the Concise Data Definition Language (CDDL).

The bytewords CLI can also be of use, since CBOR is converted to Bytewords for text encoding when constructing URs.

Multipart URs

What is a Multipart UR (MUR)?

A Multipart UR (MUR) is a UR that has been broken into multiple parts and sequenced. Each one includes a sequence number and a sequence length, as described in the UR specification. For example:


How Do MURs Relate to Animated QRs?

MURs are primarily intended for usage in Animated QRs, since QR codes have a low limit on how much data they can contain. But, you don’t want to just keep repeating the same sequence of QRs when you’re displaying an Animated QR because if the recipient misses a single element in the sequence you have to repeat every single one. Instead, Blockchain Commons uses fountain codes, which allow for more efficient reading of the animated QRs. How to do so is described in the Multipart UR (MUR) Implementation Guide.