Add PEM exports to certificates

[vcsjones]

This implements PEM export on X509Certificate2 and X509Certificate2Collection.

Contributes to #51630.

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Issue Details

---

This implements PEM export on X509Certificate2 and X509Certificate2Collection.

Contributes to #51630.

Author:	vcsjones
Assignees:	-
Labels:	area-System.Security, new-api-needs-documentation, community-contribution
Milestone:	-

[bartonjs]

Nit: Mis-aligned

Suggested change

	AssertPemExport(cc, string.Empty);
	AssertPemExport(cc, string.Empty);

[bartonjs]

Should we give this one the full Goldilocks? (One too short (done), one too long (missing), one just right (done)).

[bartonjs]

Do we think "bigger than needed" is an interesting case here?

[vcsjones]

Eh, interesting enough to make you leave a remark, so, let's say yes and add a test case for it.

[bartonjs]

While the repository, collectively, is OK with braceless usings in test code, I'll make one plea to keep the braces :)

[vcsjones]

Your house, your rules 😄.

[bartonjs]

This test is what inspired me to ask the tighter export elsewhere. Seems like we can simplify the code, and increase the amount of stuff that the test covers, if we just take an existing PEM import it, normalize the newlines to LF, and export+Assert.

[Theory]
[InlineData(true)]
[InlineData(false)]
public static void ExportCertificatePem_ContainsOnlyCertPem(bool yoda)
{
    const string Input = ...;

    string output;

    using (X509Certificate2 cert = new X509Certificate2(Encoding.ASCII.GetBytes(Input)))
    {
        if (yoda)
        {
            output = cert.ExportCertificatePem();
        }
        else
        {
            Span<char> destination = stackalloc char[Input.Length + 5];

            Assert.True(cert.TryExportCertificatePem(destination, out int charsWritten));
            Assert.Equal(Input.Length, charsWritten);

            output = destination.Slice(0, charsWritten).ToString();
        }
     }

     Assert.Equal(Input, output);
}

[bartonjs]

Alternatively, we could add the "did I start at the right place?" and "did I end at the right place?" checks to what's already here.

[vcsjones]

Hm, I see. Perhaps "ContainsOnlyCertPem" was misleading in name, the test was ensuring that (if by some miracle) a certificate's private key would not be included, or at least a form of documentation "Yes, the 'pem export' does not include CERTIFICATE nor RSA/EC PRIVATE KEY".

But I see what you are suggesting as well. I suppose by "exact string" match, that would pretty much have the same meaning.

[vcsjones]

I think we can just get rid of these tests if we tighten the others.

[bartonjs]

Code generally looks good, just a couple of comment requests.

Tests generally look good, but since I didn't write the code I have the easy job of just asking for more :)