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x509certificatex509pkiasn.1

Why is extnValue in X.509 Extensions always encapsulated in an OCTET_STRING?

I'm curious, and I was not able to find an explanation so far.

In RFC 5280 Extensions define the following:

   Extension  ::=  SEQUENCE  {
        extnID      OBJECT IDENTIFIER,
        critical    BOOLEAN DEFAULT FALSE,
        extnValue   OCTET STRING
                    -- contains the DER encoding of an ASN.1 value
                    -- corresponding to the extension type identified
                    -- by extnID
        }

What is the reason for defining the encapsulating OCTET_STRING for extnValue, instead of directly defining extnValue as the "DER encoding of an ASN.1 value corresponding to the extension type identified by extnID".

Thank you.

Solution

  • Not an authoritative answer, but my thoughts are: this is because extension values may have arbitrary enclosing tags and can be defined in external modules:

    enter image description here

    Most extensions use SEQUENCE, but some are not, like in a given example, Subject Key Identifier is just another OCTET_STRING, Key Usages is a BIT_STRING. And in base type definition you have to use fixed tag to represent variable content (ANY).

    In addition, parsers may not know how to parse particular extension, so they read it as octet string without having to dig deeper if extension type is unknown to parser.

    update 13.02.2023 (based on comments):

    Regarding the type / tag, from my understanding, each different type can be easily identified by the leading tag byte, such as SEQUENCE=0x10, OCTET_STRING=0x04 or BIT_STRING=0x03

    you cannot define the field with variable tag, because you introduce type ambiguity. That is, extnValue ANY field definition is not valid, because its type is indeterminate. When you define a type (in this case, it is Extension type), all fields must have deterministic tag.