Decode a CBOR payload

Goal: Inspect a CBOR payload, either a binary file or a base64 string copied from a log or a token, as JSON. The reverse direction, encoding JSON as CBOR, is covered too.

Prerequisites: The built-in cbor module. Read a binary file with -I cbor. For a base64 string, read it with -I raw and call cbor_parse, which accepts base64 text as well as raw bytes.

Query

A binary file:

$ mq -I cbor -F json '.' payload.cbor

A base64 string:

$ echo 'o2JpZAdkdGFnc4JhYWFiYm9r9Q==' | mq -I raw -F json 'import "cbor" | cbor::cbor_parse'

Input

payload.cbor holds these 19 bytes (shown as hex), and the base64 string above is the same data:

a3 62 69 64 07 64 74 61 67 73 82 61 61 61 62 62 6f 6b f5

Output

Both commands print:

{
  "id": 7,
  "tags": [
    "a",
    "b"
  ],
  "ok": true
}

Pick out one field

$ mq -I cbor 'get("tags")' payload.cbor

["a", "b"]

Encode JSON as base64 CBOR

$ echo '{"id": 7, "tags": ["a", "b"], "ok": true}' | mq -I json 'import "cbor" | cbor::cbor_stringify | base64'

o2JpZPlHAGR0YWdzgmFhYWJib2v1

Notes

cbor_stringify returns bytes. Printed directly they appear as hex (a3626964f94700…), and -o writes that hex text rather than a binary file. Pipe through base64 when you need something you can paste elsewhere.
The base64 above differs from the decode example even though both describe the same data. cbor_stringify writes numbers as CBOR floats, so 7 becomes f94700 instead of the one-byte integer 07. Decoding either form gives 7.
A truncated or invalid payload stops with a Failed to parse CBOR error instead of returning partial data.
Once decoded, the data is an ordinary mq value, so -F can write it in any output format, for example -F yaml or -F toml.

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