Only CA certificates can be self-issued
At the bottom of https://tools.ietf.org/html/rfc5280#page-12 and top of https://tools.ietf.org/html/rfc5280#page-13 (last paragraph of above https://tools.ietf.org/html/rfc5280#section-3.3), we see: This specification covers two classes of certificates: CA certificates and end entity certificates. CA certificates may be further divided into three classes: cross-certificates, self-issued certificates, and self-signed certificates. Cross-certificates are CA certificates in which the issuer and subject are different entities. Cross-certificates describe a trust relationship between the two CAs. Self-issued certificates are CA certificates in which the issuer and subject are the same entity. Self-issued certificates are generated to support changes in policy or operations. Self- signed certificates are self-issued certificates where the digital signature may be verified by the public key bound into the certificate. Self-signed certificates are used to convey a public key for use to begin certification paths. End entity certificates are issued to subjects that are not authorized to issue certificates. that the term "self-issued" is only applicable to CAs, not end-entity certificates. In https://tools.ietf.org/html/rfc5280#section-4.2.1.9 the description of path length constraints says: The pathLenConstraint field is meaningful only if the cA boolean is asserted and the key usage extension, if present, asserts the keyCertSign bit (Section 4.2.1.3). In this case, it gives the maximum number of non-self-issued intermediate certificates that may follow this certificate in a valid certification path. (Note: The last certificate in the certification path is not an intermediate certificate, and is not included in this limit. Usually, the last certificate is an end entity certificate, but it can be a CA certificate.) This makes it clear that exclusion of self-issued certificates from the path length count applies only to some *intermediate* CA certificates. A leaf certificate whether it has identical issuer and subject or whether it is a CA or not is never part of the intermediate certificate count. The handling of all leaf certificates must be the same, in the case of our code to post-increment the path count by 1, so that we ultimately reach a non-self-issued intermediate it will be the first one (not zeroth) in the chain of intermediates. Reviewed-by:Matt Caswell <matt@openssl.org>
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