The syntax of public key certificates can be found in RFC 5280 section
4.1. The relevant part of the syntax is the following:
TBSCertificate ::= SEQUENCE {
version [0] EXPLICIT Version DEFAULT v1,
serialNumber CertificateSerialNumber,
... remaining fields omitted ...
}
The version field has a default value of v1. RFC 5280 section 4.1.2.1
says the following:
If only basic fields are present, the version SHOULD be 1 (the value
is omitted from the certificate as the default value); however, the
version MAY be 2 or 3.
To help detect if the version field is present or not, the type of the
version field has an explicit tag of [0]. Due to this tag, if the
version field is present, its encoding will have an identifier octet
that is distinct from that of the serialNumber field.
ITU-T X.690 specifies how a value of such a type should be encoded with
DER. There is a PDF of X.690 freely available from ITU-T. X.690 section
8.1.2 specifies the format of identifier octets which is the first
component of every encoded value. Identifier octets encode the tag of a
type. Bits 8 and 7 encode the tag class. Bit 6 will be 0 if the encoding
is primitive and 1 if the encoding is constructed. Bits 5 to 1 encode
the tag number.
X.690 section 8.14 specifies what the identifier octet should be for
explicitly tagged types. Section 8.14.3 says if implicit tagging is not
used, then the encoding shall be constructed. The version field uses
explicit tagging and not implicit tagging, so its encoding will be
constructed. This means bit 6 of the identifier octet should be 1.
X.690 section 8.14 and Annex A provide examples. Note from their
examples that the notation for tags could look like [APPLICATION 2]
where both the tag class and tag number are given. For this example, the
tag class is 1 (application) and the tag number is 2. For notation like
[0] where the tag class is omitted and only the tag number is given, the
tag class will be context-specific.
Putting this all together, the identifier octet for the DER encoding of
the version field should have a tag class of 2 (context-specific), bit 6
as 1 (constructed), and a tag number of 0.
Signed-off-by: Ivan Abrea <ivan@algosolutions.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
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