Unknown bytes in base64 encoded nonce

[xelhark]

I'm testing the Safety Net APIs, and I found out that the base64 encoded nonce in the response also contains several bytes preceding the actual nonce.

What are these?

Here's an example:

I send foobar as nonce, and in the response, once I base 64 decode the nonce value, I find these bytes:

D\xcak\x9bT\xf0\xda\xa1_\xaf-\xc2{\x03)p,\x96\xfaC_?\xc5\xc7foobar

The nonce is there at the end, but what's the rest?