An assertion is a digitally signed document that either verifies the validity of a process, as attested by the signer, or carries policy information, as formulated by the signer.

Snapcraft, snapd, the Snap Store and Brand stores all use assertions to handle a variety of functions and processes, including authentication, policy setting, identification and validation.

Assertions are text-based and take a context-dependent format that always includes one or more headers, an optional body, and the encoded signature.

Assertion types

These are the currently used assertion types:

  • account: links an account name to its identifier and other properties
  • account-key: holds the public part of a key belonging to the account
  • model: brand-specified properties for the device, used to drive the building of an Ubuntu Core image
  • serial: binds the device identity to the device’s key by carrying the public part
  • snap-build: the basic properties of a snap at the time it was built by the developer
  • snap-declaration: defines various snap properties, such as snap-id, its name, and the publisher, plus policy related to accessing privileged interfaces
  • snap-revision: store acknowledgement on receipt of a snap build labelled with a revision
  • store: defines the configuration needed to connect a device to a store
  • system-user: usually brand authorisation to create local system users on specified devices
  • validation: validates a specific snap revision for a given series
  • validation-set: a group of snaps that are either installed or permitted to be installed together

Assertion format

The typical format of an assertion, with common headers, is as follows:

type:          <type>       # For example, “account” or “model”
authority-id:  <account id> # On whose authority this assertion is made
<key field 1>: <value>      # Fields identifying the object of the assertion
<key field N>: <value>
<other field>: <value>
revision: <int>             # Assertions can be updated with a higher revision
format: <int>               # Assertion types can have backward incompatible format changes signaled by a higher format
body-length: <int>          # Present if a body is provided with this assertion
sign-key-sha3-384: <key id> # Encoded key id of signing key

<body>                      # Optional type-dependent body of length `body-length` bytes

<signature>                 # Encoded signature
  • every assertion has type, sign-key-sha3-384 and a signature
  • most assertions have authority-id
  • values are scalars (strings, integers, booleans), lists, or maps
  • some headers are used as a unique index to specify the context of an assertion of the given, together they form the so-called primary key of the assertion
  • most assertions also have a revision to enable a particular assertion to be updated by issuing another assertion of the same type and index with a higher revision.

Given a particular type and index, there is only one “latest” valid assertion that properly determines policy for a system - the one with the highest revision. For a given assertion, the index headers must all be defined.

Viewing assertions

The snap known <type> [<header>=<value>...] command can be used to view assertions or a specific type:

$ snap known account account-id=generic
type: account
authority-id: canonical
account-id: generic
display-name: Generic
timestamp: 2017-07-27T00:00:00.0Z
username: generic
validation: certified
sign-key-sha3-384: [...]

Similarly, a snap’s assertions are downloaded alongside the snap using the snap download command:

$ snap download gnome-calculator
Fetching snap "gnome-calculator"
Fetching assertions for "gnome-calculator"
Install the snap with:
   snap ack gnome-calculator_945.assert
   snap install gnome-calculator_945.snap

Encoded sha3-384 differences

Assertions include a snap-sha3-384 hash value to ensure their integrity:

snap-sha3-384: j73cFx0pIMoX4U[...]

However, these hash values will appear different, depending on whether they were either downloaded from the Snap Store, such as with the snap download command, or retrieved with via the snapd REST API.

This difference is because the Store uses hex-encoded byte arrays while the snapd REST API encodes hashes with with base64.

Developers can typically use hash-encoded values directly in their code, whereas base64 values will need to be decoded first, such as with the base64 command.

Last updated 11 months ago.