NDN Certificate Format Version 2.0
==================================

.. contents::

Since signature verification is a common operation in NDN applications, it is
important to define a common certificate format to standardize the public key
authentication procedure.  As every NDN data packet is signed, a data packet
that carries a public key as content is conceptually a certificate.  However,
the specification of a data packet is not sufficient to be the specification of
a common certificate format, as it requires additional components.  For example,
a certificate may follow a specific naming convention and may need to include
validity period, revocation information, etc.  This specification defines
naming and components of the NDN certificates and is complementary to NDN packet
specification.

::

                              Overview of NDN certificate format
                                 +--------------------------+
                                 |           Name           |
                                 +--------------------------+
                                 |         MetaInfo         |
                                 |+------------------------+|
                                 || ContentType:  KEY(2)   ||
                                 |+------------------------+|
                                 +--------------------------+
                                 |          Content         |
                                 |+------------------------+|
                                 ||       Public Key       ||
                                 |+------------------------+|
                                 +--------------------------+
                                 |       SignatureInfo      |
                                 |+------------------------+|
                                 || SignatureType:  ...    ||
                                 || KeyLocator:     ...    ||
                                 || ValidityPeriod: ...    ||
                                 || ...                    ||
                                 |+------------------------+|
                                 +--------------------------+
                                 |       SignatureValue     |
                                 +--------------------------+


Name
----

The name of a certificate consists of five parts as shown below:

::

    /<SubjectName>/[KeyId]/KEY/[IssuerId]/[Version]

A certificate name starts with the subject to which a public key is bound.  The
second part is a single name component, called KeyId, which should uniquely
identify the key under the subject namespace.  The value of KeyId is up to
the owner of the subject namespace (e.g., 8-byte random number, SHA-256 digest
of the public key, timestamp, or numerical identifier).  A special name
component ``KEY`` is appended after KeyId, which indicates that the data is a
certificate.  After ``KEY``, there is an IssuerId name component that
distinguishes different issuers for the same key.  How to specify the IssuerId
is up to the issuer and key owner.  The last component is version number.
For example,

::

    /edu/ucla/cs/yingdi/%03%CD...%F1/KEY/%9F%D3...%B7/%FD%d2...%8E
    \_________________/\___________/    \___________/\___________/
       Subject Name       Key ID          Issuer Id     Version


MetaInfo
--------

The ``ContentType`` of certificate is set to ``KEY`` (2).

The ``FreshnessPeriod`` of certificate must be explicitly specified.  The
recommended value is 1 hour (3,600,000 milliseconds).

Content
-------

By default, the content of a certificate is the public key encoded in
`X509PublicKey <https://tools.ietf.org/html/rfc5280#section-4.1.2.7>`__ format.

SignatureInfo
-------------

Besides, ``SignatureType`` and ``KeyLocator``, the ``SignatureInfo`` field of a
certificate include more optional fields.

::

    SignatureInfo ::= SIGNATURE-INFO-TYPE TLV-LENGTH
                        SignatureType
                        KeyLocator
                        ValidityPeriod?
                        ... (SignatureInfo Extension TLVs)

One optional field is ``ValidityPeriod``, which contains two sub TLV fields:
``NotBefore`` and ``NotAfter``, which are two UTC timestamps in ISO 8601 compact
format (``yyyymmddTHHMMSS``, e.g., "20020131T235959").  NotBefore indicates
when the certificate takes effect while NotAfter indicates when the certificate
expires.

.. note::
    Using ISO style string is the convention of specifying validity period of
    certificate, which has been adopted by many certificate systems, such as
    X.509, PGP, and DNSSEC.

::

    ValidityPeriod ::= VALIDITY-PERIOD-TYPE TLV-LENGTH
                         NotBefore
                         NotAfter

    NotBefore ::= NOT-BEFORE-TYPE TLV-LENGTH
                    BYTE{15}

    NotAfter ::= NOT-AFTER-TYPE TLV-LENGTH
                   BYTE{15}

For each TLV, the TLV-TYPE codes are assigned as below:

+---------------------------------------------+-------------------+----------------+
| TLV-TYPE                                    | Assigned code     | Assigned code  |
|                                             | (decimal)         | (hexadecimal)  |
+=============================================+===================+================+
| ValidityPeriod                              | 253               | 0xFD           |
+---------------------------------------------+-------------------+----------------+
| NotBefore                                   | 254               | 0xFE           |
+---------------------------------------------+-------------------+----------------+
| NotAfter                                    | 255               | 0xFF           |
+---------------------------------------------+-------------------+----------------+

.. note::
    TLV-TYPE code that falls into [253, 65536) is encoded in
    `3-byte <http://named-data.net/doc/ndn-tlv/tlv.html#variable-size-encoding-for-type-t-and-length-l>`__

Extensions
~~~~~~~~~~

A certificate may optionally carry some extensions in SignatureInfo.  An extension
could be either critical or non-critical depends on the TLV-TYPE code convention.  An
critical extension implies that if a validator cannot recognize or cannot parse the
extension, the validator must reject the certificate.  An non-critical extension
implies that if a validator cannot recognize or cannot parse the extension, the
validator may ignore the extension.

The TLV-TYPE code range [256, 512) is reserved for extensions.  The last bit of a
TLV-TYPE code indicates whether the extension is critical or not: ``1`` for critical
while ``0`` for non-critical.  If an extension could be either critical or
non-critical, the extension should be allocated with two TLV-TYPE codes which only
differ at the last bit.  For example, TLV-TYPE codes 256 and 257 are allocated to the
``StatusChecking`` extension, 256 for critical StatusChecking while 257 for
non-critical StatusChecking.


Proposed Extensions
-------------------

We list the proposed extensions here:

+---------------------------------------------+-------------------+----------------+
| TLV-TYPE                                    | Assigned code     | Assigned code  |
|                                             | (decimal)         | (hexadecimal)  |
+=============================================+===================+================+
| StatusChecking (Non-critical)               | 256               | 0x0100         |
+---------------------------------------------+-------------------+----------------+
| StatusChecking (Critical)                   | 257               | 0x0101         |
+---------------------------------------------+-------------------+----------------+
| AdditionalDescription (Non-critical)        | 258               | 0x0102         |
+---------------------------------------------+-------------------+----------------+
| MultipleSignature (Critical)                | 259               | 0x0103         |
+---------------------------------------------+-------------------+----------------+

.. note::
    TLV-TYPE code that falls into [253, 65536) is encoded in
    `3-byte <http://named-data.net/doc/ndn-tlv/tlv.html#variable-size-encoding-for-type-t-and-length-l>`__

Status Checking
~~~~~~~~~~~~~~~

TBD

Multiple Signature
~~~~~~~~~~~~~~~~~~

TBD

AdditionalDescription
~~~~~~~~~~~~~~~~~~~~~

``AdditionalDescription`` is a non-critical extension that provides additional
information about the certificate.  The information is expressed as a set of
key-value pairs.  Both key and value are UTF-8 strings, e.g.,
``("Organization", "UCLA")``. The issuer of a certificate can specify arbitrary
key-value pair to provide additional description about the certificate.

::

    AdditionalDescription ::= ADDITIONAL-DESCRIPTION-TYPE TLV-LENGTH
                                DescriptionEntry+

    DescriptionEntry ::= DESCRIPTION-ENTRY-TYPE TLV-LENGTH
                           DescriptionKey
                           DescriptionValue

    DescriptionKey ::= DESCRIPTION-KEY-TYPE TLV-LENGTH
                         BYTE+

    DescriptionValue ::= DESCRIPTION-VALUE-TYPE TLV-LENGTH
                           BYTE+

+---------------------------------------------+-------------------+----------------+
| TLV-TYPE                                    | Assigned code     | Assigned code  |
|                                             | (decimal)         | (hexadecimal)  |
+=============================================+===================+================+
| DescriptionEntry                            | 512               | 0x0200         |
+---------------------------------------------+-------------------+----------------+
| DescriptionKey                              | 513               | 0x0201         |
+---------------------------------------------+-------------------+----------------+
| DescriptionValue                            | 514               | 0x0202         |
+---------------------------------------------+-------------------+----------------+