Hi all, I have been working lately on use cases where OAuth is used to authorize transactions in the financial sector and electronic signing. What I learned is there is always the need to pass resource ids (e.g. account numbers) or transaction-specific values (e.g. amount or hash to be signed) to the OAuth authorization process to further qualify the scope of the requested access token.
It is obvious a static scope value, such as „payment“or „sign“, won’t do the
job. For example in case of electronic signing, one must bind the
authorization/access token to a particular document, typically represented by
its hash.
I would like to get your feedback on what you consider a good practice to cope
with that challenge. As a starting point for a discussion, I have assembled a
list of patterns I have seen in the wild (feel free to extend).
(1) Parameter is part of the scope value, e.g. „sign:<hash_to_be_signed>“ or
"payments:<payment_resource_id>" - I think this is an obvious way to represent
such parameters in OAuth, as it extends the scope parameter, which is intended
to represent the requested scope of an access token. I used this pattern in the
OAuth SCA mode in Berlin Group's PSD API.
(2) One could also use additional query parameter to add further details re the
requested authorization, e.g.
GET /authorize?
....
&scope=sign
....
&hash_to_be_signed=<hash_to_be_signed>
It seems to be robust (easier to implement?) but means the scope only
represents the static part of the action. The AS needs to look into a further
parameter to fully understand the requested authorization.
(3) Open Banking UK utilizes the (OpenID Connect) „claims“ parameter to carry
additional data.
Example:
"claims": {
"id_token": {
"acr": {
"essential": true,
"value": "..."
},
"hash_to_be_signed": {
"essential": true,
"value": "<hash_to_be_signed>"
}
},
"userinfo": {
"hash_to_be_signed": {
"essential": true,
"value": "<hash_to_be_signed>"
}
}
}
I‘m looking forward for your feedback. Please also indicated whether you think
we should flush out a BCP on that topic.
kind regards,
Torsten.
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