Assertions

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 and the Snap Store 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.

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.

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-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
  • system-user: usually brand authorisation to create local system users on specified devices
  • validation: validates a specific snap revision for a given series

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_544.assert
   snap install gnome-calculator_544.snap
$ cat gnome-calculator_544.assert 
type: account-key
authority-id: canonical
revision: 2
[...]
2 Likes

Can i extract a value from assertion with the key with the snap known ?? command ? For example i’ll use the Unique Id for the device from snap know serial ?serial?

Hi, can you be more specific about the information you are looking for? Also, try using the snap model command, as that will have the serial for your device in it.

Yep actually i’m using:

snap model
brand Generic✓
model generic-classic
serial xxxx-c7da-xxxx-be04-xxxx

And parsing the string to get only the serial id! i’m asking if i can get only the serial id straight with one command to use it in node.js without parsing the returned string !

No, unfortunately using snap model and doing some string parsing is the “least amount of work” you can do today. We have some vague plans to add something like formatted output with JSON that may be closer to what you want, but for now something like snap model --serial or snap model is the closest you can get.

Thanks! Instead i’m using snap model --serial …