Reference example of AppTest server-side implementation

Question

PhilipTP OP

Created Aug ’21

Replies 4

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Hi,

We have a multi-platform application that requires integrity attestation before the backend will enable supporting services (fairly common scenario).

I've read the documentation for DeviceCheck and AppAttest, as well as SafetyNet on the Android side.

The Android documentation includes lots of examples of use, including server-side (though oddly in C# and Javascript... which I don't see as being server-side languages, but... oh, well).

Anyway, maybe there's a server-side example of using an application attestation on the server when validating a client, as well as validating individual requests with assertions, but I've not been able to find it.

It seems like a relatively important bit of functionality to ensure that apps aren't being compromised, while at the same time requiring a correct implementation... Why not give a reference implementation as a starting point to make sure developers are on the right path?

Can anyone point me at an example as a Gist, etc?

Thanks.

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Answer 1

PhilipTP OP

Sep ’21

Found a few examples on GitHub, but what's worrying is that they don't closely follow the steps outlined in Validating Apps that Connect to Your Server.

In particular, this part:

Create the SHA256 hash of the public key in credCert, and verify that it matches the key identifier from your app.

But this is far from being unambiguous. I'd have thought that this referred to the entire DER of the public key:

% openssl x509 -in k -noout -pubkey | openssl asn1parse -inform pem -dump
    0:d=0  hl=2 l=  89 cons: SEQUENCE          
    2:d=1  hl=2 l=  19 cons: SEQUENCE          
    4:d=2  hl=2 l=   7 prim: OBJECT            :id-ecPublicKey
   13:d=2  hl=2 l=   8 prim: OBJECT            :prime256v1
   23:d=1  hl=2 l=  66 prim: BIT STRING        
      0000 - 00 04 09 1a ae 9f d2 0b-89 e6 6b ab 68 3e 70 e1   ..........k.h>p.
      0010 - 6d 0f b1 2f 8b 4b bd c9-d2 54 ec 15 2c b4 fc 4c   m../.K...T..,..L
      0020 - 8d fb e1 49 0d 90 34 80-10 82 08 6c 49 58 7e 2c   ...I..4....lIX~,
      0030 - 5b 90 2b 80 2d 1f f3 e9-36 59 51 d2 3e 1d d2 f8   [.+.-...6YQ.>...
      0040 - 75 e3                                             u.

But from what I can tell, the checksum is only on the BIT STRING itself from looking at other examples... but even then, extracting just this portion and checksumming only that, I can't match the keyId.

Hmm... Can't seem to format block code...

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Answer 2

PhilipTP OP

Sep ’21

And the answer is, the public key needs to be serialized as an X9.62 Uncompressed Point in order for the digest to match... this is not called out anywhere.

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Answer 3

Superabundant OP

Oct ’23

I'm trying to implement this in TypeScript, and I'm stuck on the step "Decode the sequence and extract the single octet string that it contains. Verify that the string equals nonce."

This is my implementation so far. I'd be grateful for any help anyone can provide.

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Answer 4

srinivas1729 OP

Feb ’24

Hello, I've implemented a Typescript library to verify Attestations and Assertions: https://github.com/srinivas1729/appattest-checker-node

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