Amber is a nostr event signer for Android. It allows users to keep their nsec segregated in a single, dedicated app. The goal of Amber is to have your smartphone act as a NIP-46 signing device without any need for servers or additional hardware. "Private keys should be exposed to as few systems as possible as each system adds to the attack surface," as the rationale of said NIP states. In addition to native apps, Amber aims to support all current nostr web applications without requiring any extensions or web servers.

Current Features

Offline
Use nip-46 or make an addendum in nip-46
Improve the ui (currently its showing a text with the raw json of the event)
Check if we can use Amber to sign the events of web applications
Change the sign button to just copy the signature of the event
Use content provider to sign events in background when you checked the remember my choice option on android
Support for multiple accounts

Adding Amber support for your application

Add a package querie in your AndroidManifest.xml

<queries>
    <package android:name="com.greenart7c3.nostrsigner"/>
</queries>

Create the Nostr Event

val event = TextNoteEvent(id, pubKey, TimeUtils.now(), tags, message, signature = "")

Convert the event to json

val json = event.toJson()

Create the intent using the nostrsigner scheme

val intent = Intent(Intent.ACTION_VIEW, Uri.parse("nostrsigner:$json"))

Set the package name of the signer app for the intent

intent.`package` = "com.greenart7c3.nostrsigner"

Set the type eg (sign_event, nip04_encrypt, nip04_decrypt, nip44_encrypt, nip44_decrypt, get_public_key)

val signerType = when (type) {
    SignerType.SIGN_EVENT -> "sign_event"
    SignerType.NIP04_ENCRYPT -> "nip04_encrypt"
    SignerType.NIP04_DECRYPT -> "nip04_decrypt"
    SignerType.NIP44_ENCRYPT -> "nip44_encrypt"
    SignerType.NIP44_DECRYPT -> "nip44_decrypt"
    SignerType.GET_PUBLIC_KEY -> "get_public_key"
    SignerType.DECRYPT_ZAP_EVENT -> "decrypt_zap_event"
}
intent.putExtra("type", signerType)

Set the pubkey used for encryption/decryption

intent.putExtra("pubKey", pubKey)

Create an id or set the id as the id of the event so you can control in your application what response you are receiving

intent.putExtra("id", event.id)

Set the current npub of the logged in user, so amber knows what user it has to sign the transaction (not yet implemented)

intent.putExtra("current_user", account.keyPair.pubKey.toNpub())

Start the signer Activity

context.startActivity(intent)

Sign transactions in background

Create the Nostr Event

val event = TextNoteEvent(id, pubKey, TimeUtils.now(), tags, message, signature = "")

Create the content resolver using the following scheme

"content://com.greenart7c3.nostrsigner.$signerType"

The signer types are

enum class SignerType {
    SIGN_EVENT,
    NIP04_ENCRYPT,
    NIP04_DECRYPT,
    NIP44_ENCRYPT,
    NIP44_DECRYPT,
    GET_PUBLIC_KEY,
    DECRYPT_ZAP_EVENT
}

In the projection parameter of the contentResolver.query you must send a list with the following data
- The event json
- The hex pub key for encryption/decryption
- The current logged in user npub
Example

context.contentResolver.query(
    Uri.parse("content://com.greenart7c3.nostrsigner.$signerType"),
    listOf(event.toJson(), "hex pubKey for encryption/decryption", "logged in user npub"),
    null,
    null,
    null
).use {
    if (it !== null) {
        if (it.moveToFirst()) {
            val index = it.getColumnIndex("signature")
            if (index < 0) {
                Log.d("getDataFromResolver", "column '$columnName' not found")
                return null
            }
            return it.getString(index)
        }
    }
}

If the user did not check the remember my choice option, the npub is not in amber or the signer type is not recognized the contentResolver will return null
For the SIGN_EVENT type amber returns two columns "signature" and "event". The column event is the signed event json
For the other types amber returns the column "signature"

Support for Web Applications

Here's an index.html with an example of how to open amber to sign an event
The S.type= can be sign_event, nip04_encrypt, nip44_encrypt, nip04_decrypt, nip44_decrypt, get_public_key or decrypt_zap_event
You can send a callbackUrl so your user doesn't need to copy the event manually to your application
For Encryption/Decryption you should also send the hex pubkey that will be used to encrypt/decrypt the data eg (S.pubKey=cb8b8d378690f9a4a4f412c4e295051c4b76c3db24dc3941860fd3980f07d21d)
S.returnType can be signature or event
S.compressionType can be none or gzip (used when you using S.returnType == "event") because there are limitations to android intents and browsers urls

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <h1>Test</h1>
       
    <script>
        window.onload = function() {
            var url = new URL(window.location.href);
            var params = url.searchParams;
            if (params) {
                var param1 = params.get("event");
                if (param1) alert(param1)
            }
            let json = {
                kind: 1,
                content: "test"
            }
            let encodedJson = encodeURIComponent(JSON.stringify(json))
            var newAnchor = document.createElement("a");
            newAnchor.href = `intent:${encodedJson}#Intent;scheme=nostrsigner;S.compressionType=none;S.returnType=signature;S.type=sign_event;S.callbackUrl=https://example.com/?event=;end`;
            newAnchor.textContent = "Open amber";
            document.body.appendChild(newAnchor)
        }
    </script>
</body>
</html>

Contributing

Issues and pull requests are very welcome.