Structures inside an OCTET STRING

Some formats, such as X.509 extensions, store an encoded structure inside an OCTET STRING. Set primitiveSchema on the OctetString schema to validate that inner structure and read its named blocks.

import * as asn1js from "asn1js";

// Some formats put an encoded structure inside an OCTET STRING.
// For example, an X.509 extension value:
// Extension ::= SEQUENCE {
//   extnID     OBJECT IDENTIFIER,
//   critical   BOOLEAN DEFAULT FALSE,
//   extnValue  OCTET STRING } -- contains a DER-encoded BasicConstraints
// BasicConstraints ::= SEQUENCE { cA BOOLEAN DEFAULT FALSE, ... }
const basicConstraints = new asn1js.Sequence({
  value: [new asn1js.Boolean({ value: true })],
});
const extension = new asn1js.Sequence({
  value: [
    new asn1js.ObjectIdentifier({ value: "2.5.29.19" }),
    new asn1js.Boolean({ value: true }),
    new asn1js.OctetString({ valueHex: basicConstraints.toBER() }),
  ],
});

// `primitiveSchema` describes the structure inside the OCTET STRING,
// and its named blocks are returned together with the outer ones
const schema = new asn1js.Sequence({
  value: [
    new asn1js.ObjectIdentifier({ name: "extnID" }),
    new asn1js.Boolean({ name: "critical", optional: true }),
    new asn1js.OctetString({
      name: "extnValue",
      primitiveSchema: new asn1js.Sequence({
        value: [new asn1js.Boolean({ name: "cA", optional: true })],
      }),
    }),
  ],
});

const { verified, result } = asn1js.verifySchema(extension.toBER(), schema);
console.log("verified:", verified);
if (verified) {
  console.log("extnID:", result.extnID.getValue());
  console.log("critical:", result.critical.getValue());
  console.log("cA:", result.cA.getValue());
}

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API: OctetString, verifySchema()