[SOLVED] Mismatched Nonce on Server Side Apple App Attest

I am currently trying to learn how to implement Apple's App Attest in my apps and am using Go for my backend server. I found this guide which explains that I need to verify that a specific set of examples are handled correctly by my server-side logic.

I've gotten all of the certificate and key verification steps to yield the correct results, but no matter what I do I can't seem to make clientDataHash match what I'm receiving from the OID object. I've trying hashing the server challenge directly and also hashing the Base64 representation of the challenge. According to some online sources (e.g. W3C and webauthn) suggest that a JSON object is needed for the challenge, but Apple says nothing about this and I couldn't find any information on what origin is supposed to be when the clientDataHash must be known on the client before calling the DeviceCheck SDK.

I have reduced my server-side code down to the minimal working example shown below. Much of the length of the code is Apple's example attestation object, which I decided to retain in this post as it is a public example and to ensure the minimal working example works without any copy-pasting being required. The code I have written that produces the mismatched nonces is as follows:

package main

import (
    "encoding/base64"
    "encoding/asn1"
    "github.com/fxamacker/cbor/v2"
    "crypto/x509"
    "crypto/sha256"
    "fmt"
)

type CertificateChain struct {
    _ struct{}   `cbor:",toarray"`
    Leaf         []byte
    Intermediate []byte
}

type AttestationStatement struct {
    X5C     CertificateChain `cbor:"x5c,toarray"`
    Receipt []byte           `cbor:"receipt"`
}

type AttestationObject struct {
    Format            string               `cbor:"fmt"`
    Statement         AttestationStatement `cbor:"attStmt"`
    AuthorizationData []byte               `cbor:"authData"`
}

func main() {   
    att_b64 := "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"
    att_cbor, _ := base64.StdEncoding.DecodeString(att_b64)
    
    var att AttestationObject
    cbor.Unmarshal(att_cbor, &att)
    
    leaf, _ := x509.ParseCertificate(att.Statement.X5C.Leaf)
    
    serverChallenge := "test_server_challenge"
    challengeHash := sha256.Sum256([]byte(base64.StdEncoding.EncodeToString([]byte(serverChallenge))))
    clientDataHash := append(att.AuthorizationData, challengeHash[:]...)
    computed_nonce := sha256.Sum256(clientDataHash)
    
    oid := asn1.ObjectIdentifier{1, 2, 840, 113635, 100, 8, 2}
    var i []byte
    for _, ext := range leaf.Extensions{
        if ext.Id.Equal(oid){ 
            i = ext.Value
            break
        }
    }
    
    var oct []asn1.RawValue
    asn1.Unmarshal(i, &oct)
    
    var expected_nonce asn1.RawValue
    asn1.Unmarshal(oct[0].Bytes, &expected_nonce)
    
    fmt.Println("Computed Nonce: ", base64.StdEncoding.EncodeToString(computed_nonce[:]))
    fmt.Println("Expected Nonce: ", base64.StdEncoding.EncodeToString(expected_nonce.Bytes))
}

The code is run by calling go run main.go in the working directory (after running go mod init main and go mod tidy). The output of go run main.go is

Computed Nonce:  1fODggLNPe9qk6R9ln8vfRfVPOyuFTfDep0C9kDRuYY=
Expected Nonce:  +20WKnF+yrF3iQBQb6lNZ+4MHcPUWxLN3oG+/Fblt+s=

whereas the expected result was that the nonces matched. This is using go1.24.4 on a Mac with the M2 Max processor.

I apologize if I am missing something obvious here but I have been unable to find any information on whether or not I am computing clientDataHash correctly.