Do not used a fixed K0 for deterministic keys

docs/DETERMINISTIC.md

@@ -2,7 +2,7 @@
 
 The NXP NTAG424DNA allows applications to configure five application keys, named K0, K1, K2, K3, and K4. In the Bolt card configuration:
 
-* K0 is the only key permitted to change the application keys.
+* K0 is the `App Master Key`, it is the only key permitted to change the application keys.
 * K1 serves as the `encryption key` for the PICC Data, represented by the `p=` parameter.
 * K2 is the `authentication key` for the PICC Data, represented by the `c=` parameter.
 * K3 and K4 are not used but should be configured as recommended in the application notes.
@@ -22,18 +22,51 @@ First, it's important to understand that a Bolt Card issuer consists of two dist
 * `LNUrl Withdraw Service`: This service authenticates the card and completes the payment.
 
 Assuming the `Issuing Service` generates a random key named (the `Issuer Key`) and has a batch of Bolt Cards to configure, it will set the following parameters:
-* `K0 = IssuerKey`.
-* `K1 = PRF(K0, '2d003f77' || batchId)` with `batchId` being 4 bytes identifying the batch of card. (Can be set to `00000000` if uneeded)
+* `K0 = PRF(IssuerKey, '2d003f76' || UID)`
+* `K1 = PRF(IssuerKey, '2d003f77' || batchId)` with `batchId` being 4 bytes identifying the batch of card. (Can be set to `00000000` if uneeded)
 * `K2 = PRF(K1, '2d003f78' || UID)`
 * `K3 = PRF(K1, '2d003f79' || UID)`
 * `K4 = PRF(K1, '2d003f7a' || UID)`
 
+The Pseudo Random Function `PRF(key, message)` applied during the key generation is the CMAC algorithm described in NIST Special Publication 800-38B.
+
 Under this proposed solution:
 * With a card and the `Issuer Key`, the `Issuing Service` can recover all five application keys for that card.
 * With a card and the `Encryption Key`, the `LNUrl Withdraw Service` can recover all application keys except for the `Issuer Key` (`K0`).
 * The `Issuing Service` can reset any Bolt Card using only the `Issuer Key`.
 * The `LNUrl Withdraw Service` might still need to brute-force encryption keys if there are multiple batches of Bolt Cards and no information in the lnurlw specifies to which batch a card belongs. However, this would require brute-forcing only one encryption key per batch, rather than one per card.
 
+## How the to implement a Reset feature
+
+If an `Issuing Service` offers a factory reset feature for a user's bolt card, here is the recommended procedure:
+
+1. Read the NDEF lnurlw URL.
+2. Brute-force the encryption and authentication of the card using all existing `batchId` values to find `K1`, `K2`, and the `UID`.
+3. Use the `UID` from the `PICCData`, along with `K1` and the `IssuerKey`, to recover `K0`, `K3`, and `K4`.
+5. Execute `AuthenticateEV2First` with `K0`
+6. Erase the NDEF data file using `WriteData` or `ISOUpdateBinary`
+7. Restore the NDEF file settings to default values with `ChangeFileSettings`.
+8. Use `ChangeKey` with the recovered application keys to reset `K4` through `K0` to `00000000000000000000000000000000`.
+
+## How to implement a verification
+
+If a `LNUrl Withdraw Service` needs to verify a payment request, follow these steps:
+
+1. Read the NDEF lnurlw URL.
+2. Brute-force the encryption and authentication of the card using all existing `batchId` values to find `K1`, `K2` and `UID`.
+3. Confirm that the last-seen counter for `ID=PRF(K1, '2d003f7b' || UID)[0..7]` is lower than what is stored in the `PICCData`.
+4. Update the last-seen counter.
+
+Note that `LNUrl Withdraw Service` can't derive `App Master Key` (`K0`), and thus is unable to change the keys of the bolt card.
+
+The specific method for calculating `ID` is not crucial; the recommendation is to avoid using `UID` directly. This approach offers both privacy and security benefits.
+
+Firstly, since the `UID` is used to derive keys, it should not be stored outside the NTag.
+
+Secondly, this allows a user to re-flash the same NTag with a different `batchId` or through a different `Issuing Service`, letting the user to obtain a different `ID` for the same NTag.
+
+Third, this prevent tracking of the NTag across different `Issuing Service`.
+
 ## Security consideration
 
 Since `K0` and `K1` are shared among multiple Bolt Cards, the security of this scheme is based on the following assumptions:
@@ -53,14 +86,16 @@ Input:
 ```
 UID: 04a39493cc8680
 Batch: 01000000
-K0: 00000000000000000000000000000001
+Issuer Key: 00000000000000000000000000000001
 ```
 
 Expected:
 
 ```
+K0: 75da58a68fbb1bef64708e87c7be9ad3
 K1: aa104a0bef8f751add9f06c5f000837a
 K2: c98b6607222caffcac227f4f6241bd68
 K3: d6e5ce82ec27f9d8c5d91d7c0c3a9f80
 K4: d9352ff7ed7b43a13980a8c78aa4383a
+ID: a98da306ba6d90
 ```