crypto/x509: reject CA w/ empty subject

[cpu]

RFC 5280 §4.1.2.6 when specifying Subject says:

If the subject is a CA (e.g., the basic constraints extension, as
discussed in Section 4.2.1.9, is present and the value of cA is
TRUE), then the subject field MUST be populated with a non-empty
distinguished name matching the contents of the issuer field (Section
4.1.2.4) in all certificates issued by the subject CA.

This is tested by x509-limbo's rfc5280::ca-empty-subject testcase.

Separately it may be worth considering if x509.CreateCertificate should also error if asked to create a CA certificate without a subject.

Relates to #65085 & https://go-review.googlesource.com/c/go/+/783980

[gabyhelp]

Related Issues

• crypto/x509: tighten up SAN criticality handling #79741
• crypto/x509: reject misencoded empty EKU #79743
• crypto/x509: reject end-entity certs w/ name constraint ext. #79742
• crypto/x509: don't apply name constraints to non-terminal self-issued certs #79746
• crypto/x509: don't count self-signed certs in pathlen budget #79745
• crypto/x509: CreateCertificate should mark SubjectAltName as critical iff Subject is empty #22249 (closed)
• crypto/x509: always emit a critical SAN extension if the Subject is empty in a CSR #46810
• crypto/x509: Verify should reject certificates with empty issuers #71832

Related Code Changes

• crypto/x509: improve Certificate validation avoiding empty issuers
• crypto/x509: check the private key passed to CreateCertificate

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[mrkfrmn]

cc @golang/security