[Cryptography] Does this provide any extra value?

Phillip Hallam-Baker phill at hallambaker.com
Thu May 21 17:03:12 EDT 2020

So I have the following issue:

Imagine that we have a catalog of encrypted items that a user is going to
access by means of a QR code containing a hash of a secret value from which
we derive both a locator key and a decryption key.

So lets say the key is NCCZ-QP4L-2QFS-YFT7-KQAO-RF4F-SSAA

And we can turn this into a URI with some access info that we stuff into
the QR code:

This is expanded to a http: url:


Where MDGC-... = H (NCCZ-...)

And the data is encrypted under the key NCCZ-...

So only a person with the QR code can obtain the locator and fetch the
encrypted data and decrypt the data.

But here is the thing, someone with the locator can still fetch the data,
albeit encrypted. Is this something that should be of concern?

One possibility I am toying with is to add in an authentication public key
into the mix. Generate the key pair deterministically from the shared
secret and specify the public key in the record. Then do an ECDH auth

Question is, is this really necessary or am I just overthinking this?
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