The IANA registry

Tag numbers are held in the CBOR tags registry of IANA. The library implements the ones below, and registers all of them in the decoder by default. The registry is open ended, and most of what is missing from the table is specific to one application: any other number still decodes, as a tag that keeps its number without interpreting it.

Tag	Meaning	Class	Specification
0	Date-time string, RFC 3339	DatetimeTag	RFC 8949 § 3.4.1
1	Epoch timestamp	TimestampTag	RFC 8949 § 3.4.2
2	Unsigned bignum	UnsignedBigIntegerTag	RFC 8949 § 3.4.3
3	Negative bignum	NegativeBigIntegerTag	RFC 8949 § 3.4.3
4	Decimal fraction	DecimalFractionTag	RFC 8949 § 3.4.4
5	Bigfloat	BigFloatTag	RFC 8949 § 3.4.4
16	COSE_Encrypt0	CoseEncrypt0Tag	RFC 9052
17	COSE_Mac0	CoseMac0Tag	RFC 9052
18	COSE_Sign1	CoseSign1Tag	RFC 9052
21	base64url expected	Base64UrlEncodingTag	RFC 8949 § 3.4.5.2
22	base64 expected	Base64EncodingTag	RFC 8949 § 3.4.5.2
23	base16 expected	Base16EncodingTag	RFC 8949 § 3.4.5.2
24	Encoded CBOR data item	CBOREncodingTag	RFC 8949 § 3.4.5.1
25	String reference	StringReferenceTag	stringref
26	Serialised Perl object	PerlObjectTag	IANA registry
27	Serialised language-independent object	LanguageIndependentObjectTag	IANA registry
28	Shareable value	ShareableTag	value sharing
29	Shared value reference	SharedReferenceTag	value sharing
30	Rational number	RationalNumberTag	IANA registry
32	URI	UriTag	RFC 8949 § 3.4.5.3
33	base64url encoded text	Base64UrlTag	RFC 8949 § 3.4.5.2
34	base64 encoded text	Base64Tag	RFC 8949 § 3.4.5.2
35	Regular expression	RegexpTag	RFC 7049
36	MIME message	MimeTag	RFC 8949 § 3.4.5.3
37	Binary UUID	UuidTag	RFC 4122
38	Language-tagged string	LanguageTaggedStringTag	IANA registry
39	Identifier	IdentifierTag	IANA registry
40	Multi-dimensional array, row-major	RowMajorMultiDimensionalArrayTag	RFC 8746
41	Homogeneous array	HomogeneousArrayTag	RFC 8746
42	IPLD content identifier	IpldContentIdentifierTag	IANA registry
52	IPv4 address or prefix	Ipv4Tag	RFC 9164
54	IPv6 address or prefix	Ipv6Tag	RFC 9164
61	CBOR Web Token	CwtTag	RFC 8392
63	Encoded CBOR sequence	CBORSequenceTag	RFC 8742
64 to 87	Typed arrays	CBOR\Tag\TypedArray\*	RFC 8746
96	COSE_Encrypt	CoseEncryptTag	RFC 9052
97	COSE_Mac	CoseMacTag	RFC 9052
98	COSE_Sign	CoseSignTag	RFC 9052
100	Date, days since the epoch	DateTag	RFC 8943
256	String reference namespace	StringReferenceNamespaceTag	stringref
257	Binary MIME message	BinaryMimeTag	IANA registry
258	Mathematical finite set	SetTag	IANA registry
259	Explicit map	ExplicitMapTag	IANA registry
260	Network address, legacy	NetworkAddressTag	RFC 9164 app. A
261	Network address prefix, legacy	NetworkAddressPrefixTag	RFC 9164 app. A
1001	Extended time	ExtendedTimeTag	RFC 9581
1002	Duration	DurationTag	RFC 9581
1003	Period	PeriodTag	RFC 9581
1004	Date string	DateStringTag	RFC 8943
1040	Multi-dimensional array, column-major	ColumnMajorMultiDimensionalArrayTag	RFC 8746
55799	Self-described CBOR	CBORTag	RFC 8949 § 3.4.6

Every class is in the CBOR\Tag namespace, the typed arrays in CBOR\Tag\TypedArray. The tags of RFC 8949 itself are described in the built-in tags, the others in more tags of the registry.

A tag the library does not know

A tag number no class is registered for is not an error: the decoder returns a GenericTag, which keeps the head and the object the tag wrapped, so a document survives a round trip through code that does not understand all of it. Its normalize() normalizes the wrapped item on its own, since the number carries no meaning the library knows.

GenericTag::getTagId() returns -1, because the class stands for no number in particular. The number itself is in the head of the item: it is the additional information when that is 23 or less, and the big-endian payload getData() returns otherwise.

use CBOR\CBORObject;
use CBOR\Tag\GenericTag;

function tagNumberOf(GenericTag $tag): int
{
    $ai = $tag->getAdditionalInformation();
    if ($ai <= 23) {
        return $ai;
    }

    $data = (string) $tag->getData();

    return match ($ai) {
        CBORObject::LENGTH_1_BYTE => ord($data),
        CBORObject::LENGTH_2_BYTES => unpack('n', $data)[1],
        CBORObject::LENGTH_4_BYTES => unpack('N', $data)[1],
        CBORObject::LENGTH_8_BYTES => unpack('J', $data)[1],
    };
}

$decoded = $decoder->decode(StringStream::create(hex2bin('d904d201')));

$decoded instanceof GenericTag;   // true
tagNumberOf($decoded);            // 1234
$decoded->getValue();             // UnsignedIntegerObject, the tagged item

When a number matters to your application, give it a class of its own — writing a tag of your own — and register that class with the decoder. The decoder then hands you that class instead of a GenericTag, and getTagId() answers properly.