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Merge pull request #72 from NicolasDorier/fiowgnb

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Derive deterministic keys on IssuerKey
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Peter Rounce 2023-10-20 15:29:43 +01:00 • committed by GitHub
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@ -1,11 +1,11 @@
## Abstract ## Abstract
The NXP NTAG424DNA allows applications to configure five application keys, named K0, K1, K2, K3, and K4. In the Bolt card configuration: 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 `App Master Key`, it 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. * `K1` serves as the `encryption key` for the `PICCData`, represented by the `p=` parameter.
* K2 is the `authentication key` for the PICC Data, represented by the `c=` parameter. * `K2` is the `authentication key` used for calculating the SUN MAC of the `PICCData`, represented by the `c=` parameter.
* K3 and K4 are not used but should be configured as recommended in the application notes. * `K3` and `K4` are not used but should be configured as recommended in the [NTag424 application notes](https://www.nxp.com/docs/en/application-note/AN12196.pdf).
A simplistic approach to issuing Bolt cards would involve randomly generating the five different keys and storing them in a database. A simplistic approach to issuing Bolt cards would involve randomly generating the five different keys and storing them in a database.
@ -17,68 +17,68 @@ In this document, we propose a solution to this issue.
## Key generation ## Key generation
First, it's important to understand that a Bolt Card issuer consists of two distinct services: Assuming the `LNUrl Withdraw Service` generates a random key named (the `IssuerKey`) and has a `batch` of Bolt Cards to configure, it will set the following parameters:
* `Issuing Service`: This agent sets up the cards for lightning payments, which involves specifying a particular `LNUrl Withdraw Service` and generating the application keys.
* `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 = PRF(IssuerKey, '2d003f76' || batchId || UID)`
* `K0 = PRF(IssuerKey, '2d003f76' || UID)` * `K1 = PRF(IssuerKey, '2d003f77' || batchId)`
* `K1 = PRF(IssuerKey, '2d003f77' || batchId)` with `batchId` being 4 bytes identifying the batch of card. (Can be set to `00000000` if uneeded) * `K2 = PRF(IssuerKey, '2d003f78' || batchId || UID)`
* `K2 = PRF(K1, '2d003f78' || UID)` * `K3 = PRF(IssuerKey, '2d003f79' || batchId || UID)`
* `K3 = PRF(K1, '2d003f79' || UID)` * `K4 = PRF(IssuerKey, '2d003f7a' || batchId || UID)`
* `K4 = PRF(K1, '2d003f7a' || UID)`
`batchId`: 4 bytes identifying the batch of card. (Can be set to `00000000` if uneeded)
The Pseudo Random Function `PRF(key, message)` applied during the key generation is the CMAC algorithm described in NIST Special Publication 800-38B. 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 ## 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: If a `LNUrl Withdraw Service` offers a factory reset feature for a user's bolt card, here is the recommended procedure:
1. Read the NDEF lnurlw URL. 1. Read the NDEF lnurlw URL, extract `p=` and `c=`.
2. Brute-force the encryption and authentication of the card using all existing `batchId` values to find `K1`, `K2`, and the `UID`. 2. For each existing `batchId`:
3. Use the `UID` from the `PICCData`, along with `K1` and the `IssuerKey`, to recover `K0`, `K3`, and `K4`. 1. Derive `K1`, decrypts `p=` to get the `PICCData`.
2. If `PICCData[0] != 0xc7`, go to the next `batchId`.
3. Take `UID=PICCData[1..8]`, derive `K2`
4. Calculate the SUN MAC with `K2`, if different from `c=`, go to next `batchId`
3. From the `UID`, the `IssuerKey` and the `batchId` with correct SUN MAC, recover `K0`, `K3`, and `K4`.
5. Execute `AuthenticateEV2First` with `K0` 5. Execute `AuthenticateEV2First` with `K0`
6. Erase the NDEF data file using `WriteData` or `ISOUpdateBinary` 6. Erase the NDEF data file using `WriteData` or `ISOUpdateBinary`
7. Restore the NDEF file settings to default values with `ChangeFileSettings`. 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`. 8. Use `ChangeKey` with the recovered application keys to reset `K4` through `K0` to `00000000000000000000000000000000`.
Rational: Attempting to call `AuthenticateEV2First` without validating the `p=` and `c=` parameters could render the NTag inoperable after a few attempts.
## How to implement a verification ## How to implement a verification
If a `LNUrl Withdraw Service` needs to verify a payment request, follow these steps: If a `LNUrl Withdraw Service` needs to verify a payment request, follow these steps:
1. Read the NDEF lnurlw URL. 1. Read the NDEF lnurlw URL, extract `p=` and `c=`.
2. Brute-force the encryption and authentication of the card using all existing `batchId` values to find `K1`, `K2` and `UID`. 2. For each existing `batchId`:
3. Confirm that the last-seen counter for `ID=PRF(K1, '2d003f7b' || UID)[0..7]` is lower than what is stored in the `PICCData`. 1. Derive `K1`, decrypts `p=` to get the `PICCData`.
2. If `PICCData[0] != 0xc7`, go to the next `batchId`.
3. Take `UID=PICCData[1..8]`, derive `K2`
4. Calculate the SUN MAC with `K2`, if different from `c=`, go to next `batchId`
3. If no correct SUN MAC has been found, returns an error.
3. Confirm that the last-seen counter for `ID=PRF(IssuerKey, '2d003f7b' || batchId || UID)[0..7]` is lower than what is stored in `counter=PICCData[8..11]`.
4. Update the last-seen counter. 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. 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. Mainly, since the `UID` is used to derive keys, it is better to not store it 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 ## Security consideration
Since `K0` and `K1` are shared among multiple Bolt Cards, the security of this scheme is based on the following assumptions: Since `K1` is shared among multiple Bolt Cards, the security of this scheme is based on the following assumptions:
* `K0` and `K1` cannot be extracted from a legitimate NTag424. * `K1` cannot be extracted from a legitimate NTag424.
* Bolt Card setup occurs in a trusted environment. * Bolt Card setup occurs in a trusted environment.
While NXP gives assurance keys can't be extracted, a non genuine NTag424 could potentially expose these keys. While NXP gives assurance keys can't be extracted, a non genuine NTag424 could potentially expose these keys.
Furthermore, because Bolt Card setup uses the well-known initial application keys `00000000000000000000000000000000`, communication between the PCD and the PICC could be intercepted. If the Bolt Card setup doesn't occurs in a trusted environment, `K0` and `K1` could be exposed during the calls to `ChangeKey`. Furthermore, because blank NTag424 uses the well-known initial application keys `00000000000000000000000000000000`, communication between the PCD and the PICC could be intercepted. If the Bolt Card setup doesn't occurs in a trusted environment, `K1` could be exposed during the calls to `ChangeKey`.
Note that verifying the signature returned by `Read_Sig` can only prove NXP issued a card with a specific `UID`. It cannot prove that the current communication channel is established with an authentic NTag424. This is because the signature returned by `Read_Sig` covers only the UID and can therefore be replayed by a non-genuine NTag424. However, if `K1` is compromised, the attacker still cannot produce a valid checksum and can only recover the `UID` for tracking purposes.
Note that verifying the signature returned by `Read_Sig` can only prove NXP issued a card with a specific `UID`. It cannot prove that the current communication channel is established with an authentic NTag424. This is because the signature returned by `Read_Sig` covers only the `UID` and can therefore be replayed by a non-genuine NTag424.
## Test vectors ## Test vectors
@ -92,10 +92,10 @@ Issuer Key: 00000000000000000000000000000001
Expected: Expected:
``` ```
K0: 75da58a68fbb1bef64708e87c7be9ad3 K0: 60ef62b99ed8dc351ef7382b7d9e60f0
K1: aa104a0bef8f751add9f06c5f000837a K1: aa104a0bef8f751add9f06c5f000837a
K2: c98b6607222caffcac227f4f6241bd68 K2: 2ed57c172cf9b2ef8d8bfa6c9175d117
K3: d6e5ce82ec27f9d8c5d91d7c0c3a9f80 K3: b943783b3265f0c9091f716eab470b06
K4: d9352ff7ed7b43a13980a8c78aa4383a K4: 9fdd4ad2e7f2c0030eb84e695b257434
ID: a98da306ba6d90 ID: 3cd713f36fc177
``` ```