mGuard

# NAC-ABE Design

### What NAC-ABE Provides

NAC-ABE implements name-based access control using two layers:

1. **Symmetric encryption of data** using Content Key (CK).  

2. **ABE encryption of CK** using AA-issued public parameters and consumer policies.

Data is encrypted with a CK, and the CK itself is encrypted under ABE so that only authorized consumers holding a matching Decryption Key (DKEY) can recover it.

The Attribute Authority (AA) publishes public parameters and issues DKEYs, while NAC-ABE producers generate CKs and encrypted data using these parameters.

This removes the need for any online authorization server during data fetch and enables cryptographically enforced access control at the packet level.

---

### Why mGuard Uses KP-ABE

A KP-ABE policy defines which identities can receive a Decryption Key (DKEY) and which data attributes that DKEY authorizes them to decrypt. Each policy includes:

- A policy-id, which uniquely identifies the policy.

- A list of requester-names, representing the NDN identities that will receive the DKEY generated from this policy.

- One or more attribute-filters, typically expressed as namespace prefixes. These prefixes determine which encrypted Content Keys (CKs) the resulting DKEY can decrypt.

Policies are evaluated using exact prefix matching. If the attribute attached by the producer during CK generation starts with a prefix listed in an allow-filter, a DKEY created from that policy will successfully decrypt the CK.

No role-based or device-level semantics are required; access control is driven entirely by namespace prefixes embedded within attributes.

KP-ABE supports stream-specific semantics and flexible downstream authorization.

In KP-ABE, the producer assigns an attribute string to each Content Key (CK). The consumer receives a DKEY that contains a policy, usually expressed as one or more namespace prefixes. Decryption succeeds when:

- policy_prefix is a prefix of attribute_string

This creates a simple, deterministic Matching Rule:

- The producer attaches attributes to CKs.

- The consumer holds a DKEY containing a prefix-based policy.

- Authorization is validated locally through prefix comparison.

- T he producer never evaluates policies and never needs access to DKEYs. All authorization takes place on the consumer side using the policy embedded in the DKEY.

---

### Certificates

NAC-ABE uses two distinct certificates, each serving a different purpose in the trust and encryption pipeline.

#### Stream Certificate

The stream certificate defines the namespace and identity under which CKs and encrypted data are published. NAC-ABE derives CK names from this certificate:

``` c

/<stream-identity>/CK/<random>/ENC-BY/<attributes>/seg=i

```

Encrypted data packets are also named under the same identity prefix. This ensures that each stream's data and CKs are isolated and validated using the stream's trust chain.

#### Attribute Authority (AA) Certificate

The AA certificate anchors the ABE trust domain. It is used to validate:

- Public Parameters (PUBPARAMS)

- AA KEY packets

- Any ABE-related metadata

- The AA certificate ensures that only authenticated parameters are used for CK generation and that consumers can trust the DKEYs they receive.

## 3. ABE Encryption of CK (Black Box Description)

When the producer encrypts one data chunk:

1. Producer calls:

2. Output:

- a fresh **symmetric CK**  

- CK segments encrypted with KP-ABE (using AA public parameters)  

3. Producer encrypts the data payload with CK.  

4. Producer embeds the **CK name** inside the encrypted data.  

5. Producer publishes:

- CK segments  

- encrypted data segments

The consumer later:

- fetches CK segments,

- decrypts CK using its DKEY,

- decrypts the encrypted data.

The producer treats ABE internals as a complete black box.

---

## 4. Certificates in NAC-ABE (Updated mGuard Behavior)

### 4.1 Stream Identity Certificate

The stream’s certificate is used by NAC-ABE for:

**a) Naming CKs**  

Updated mGuard naming:

/<stream-identity>/CK/<CK-ID>/ENC-BY/<attributes>/seg=i

This scopes CKs to each stream.  

Different streams produce CKs under different prefixes.

**b) Naming encrypted data**

/<stream-identity>/<data-suffix>

This maintains per-stream isolation.

**c) Signing and trust schema validation**

All CK and encrypted data packets are signed according to the stream’s identity chain.

---

### 4.2 AA Certificate

The AA certificate is used as the **trusted root of ABE**.

It enables the producer and consumer to:

1. Validate AA **Public Parameters (PUBPARAMS)**.

2. Validate AA **KEY** packets.

3. Determine **ABE type** from PUBPARAMS (KP-ABE vs CP-ABE).

4. Run `kpContentKeyGen()` (needs verified public params).

The AA certificate defines the mathematical ABE domain; the stream certificate defines the namespace and signing authority for CK and data packets.

---

## 5. End-to-End Flow (Producer → Repo → Consumer)

### 5.1 Producer Path

1. **Per-stream CacheProducer initialization**  

   - loads stream cert  

   - loads AA cert into trust config  

   - fetches AA public parameters

2. **Prepare data for encryption**  

   - data name suffix  

   - attribute list  

   - plaintext bytes

3. **CK caching check**  

   - key = attributes-as-string  

   - if CK exists in cache → reuse  

   - else → generate new CK using `kpContentKeyGen`

4. **CK generation on cache miss**

   - ABE-encrypt CK using AA public parameters  

   - name CK under stream identity  

   - segment CK and store in cache

5. **Data encryption**

   - encrypt data with CK  

   - embed CK name into ciphertext  

   - segment encrypted data

6. **Publishing**

   - publish encrypted data segments  

   - publish CK segments (for new CK)  

---

### 5.2 Consumer Path

1. Fetch encrypted data segments.  

2. Extract CK name from ciphertext.  

3. Fetch CK segments.  

4. Use DKEY to decrypt ABE-encrypted CK.  

5. Use CK to decrypt the data.

All authorization is performed cryptographically using consumer DKEYs and producer-provided attributes.

---

## 6. ABE Key Issuance (DKEY Generation)

1. Consumer sends a request (identity TLV) to AA.  

2. AA verifies consumer identity using trust schema.  

3. AA constructs a **DKEY containing a policy** (e.g., `"role:doctor AND device:dd40c"`).  

4. AA segments and returns DKEY packets.  

5. Consumer installs DKEY and uses it to decrypt CKs.

The AA never needs to be contacted again; DKEY is long-lived until revoked or replaced.

---

## 7. CK Granularity (Temporal Encryption Granularity)

CK granularity determines how often CKs rotate or are changed.

### Common settings:

- **Second-level CKs**  

  Maximum security; high CK overhead.

- **Minute-level CKs**  

  Balanced performance vs granularity.

- **Hour-level CKs**  

  Minimal overhead; coarse access control.

Granularity influences both performance and access separation.

---

## 8. Tradeoffs of CK Reuse

### High CK Reuse (e.g., minute/hour)

**Pros:**  

- fewer CK packets  

- reduced overhead  

- faster encoding and repo insertion

**Cons:**  

- larger window where one CK decrypts many packets  

- coarse-grained access control

---

### Low CK Reuse (e.g., per second)

**Pros:**  

- tight access control window  

- limits impact of any single CK compromise  

- better fine-grain policy enforcement  

**Cons:**  

- more CK segments to generate and publish  

- more consumer fetches  

- higher repo overhead

---

## Summary

- NAC-ABE supplies per-packet cryptographic access control.  

- mGuard uses **KP-ABE** for attribute-driven encryption.  

- **Stream certificates** now define CK and data namespaces.  

- **AA certificate** anchors ABE trust and supplies public parameters.  

- CKs are cached per attribute set, generated on demand, and consumed by any valid DKEY holder.  

- CK granularity and reuse directly impact performance and access control precision.

![](old_nac-abe_in_mguard.png)

Why KP-ABE

Data encryption → CK encryption → CK decryption

ABE encryption of CK (black box)

ABE key issuance (DKEY)

CK granularity (second/minute/hour)

Tradeoffs of CK reuse level

![](att-authority.png)

![](nac-abe-pro-con.png)