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Do not used a fixed K0 for deterministic keys

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nicolas.dorier 2023-10-10 18:08:40 +09:00
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docs/DETERMINISTIC.md
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@ -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
```