RFR: 8246797: A convenient method to read OPTIONAL element

[Jamil Nimeh]

On Mon, 23 Aug 2021 14:52:46 GMT, Weijun Wang <weijun at openjdk.org> wrote:

> This code change adds new methods to `DerInputStream` to easily and safely read optional fields in a ASN.1 DER-encoded value. It also adds several wrapping methods to `DerValue` to avoid unnecessary memory copying when calling by an internal method.
> 
> The new methods are applied to `OAEPParameters` to show out how they work. A new regression test is added to show that not only the new methods are simper but they are safer and more correct.

src/java.base/share/classes/sun/security/util/DerInputStream.java line 353:

> 351:      */
> 352:     public boolean seeOptionalContextSpecific(int n) throws IOException {
> 353:         return checkNextTag(t -> (t & 0x0c0) == 0x080 && (t & 0x01f) == n);

Minor nit for clarity, the 0x080 could use the static DerValue.TAG_CONTEXT.

On an unrelated note, it's a bummer that we don't have names for the typical masking constants 0x0c0 and 0x01f for getting at the tag class and number values (or static methods maybe).  But those numeric literals are only in wide use in DerValue.java and maybe not worth changing since you don't touch that file.  It might make that code a bit clearer, but it is just a nit.

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PR: https://git.openjdk.java.net/jdk/pull/5221