Viktor Dukhovni c8ce9e50d5 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>
(cherry picked from commit ed422a2d0196ada0f5c1b6e296f4a4e5ed69577f)
2018-10-18 00:40:44 -04:00
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FAQ
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 OpenSSL 1.0.2q-dev

 Copyright (c) 1998-2018 The OpenSSL Project
 Copyright (c) 1995-1998 Eric A. Young, Tim J. Hudson
 All rights reserved.

 DESCRIPTION
 -----------

 The OpenSSL Project is a collaborative effort to develop a robust,
 commercial-grade, fully featured, and Open Source toolkit implementing the
 Secure Sockets Layer (SSLv3) and Transport Layer Security (TLS) protocols as
 well as a full-strength general purpose cryptograpic library. The project is
 managed by a worldwide community of volunteers that use the Internet to
 communicate, plan, and develop the OpenSSL toolkit and its related
 documentation.

 OpenSSL is descended from the SSLeay library developed by Eric A. Young
 and Tim J. Hudson.  The OpenSSL toolkit is licensed under a dual-license (the
 OpenSSL license plus the SSLeay license), which means that you are free to
 get and use it for commercial and non-commercial purposes as long as you
 fulfill the conditions of both licenses.

 OVERVIEW
 --------

 The OpenSSL toolkit includes:

 libssl.a:
     Provides the client and server-side implementations for SSLv3 and TLS.

 libcrypto.a:
     Provides general cryptographic and X.509 support needed by SSL/TLS but
     not logically part of it.

 openssl:
     A command line tool that can be used for:
        Creation of key parameters
        Creation of X.509 certificates, CSRs and CRLs
        Calculation of message digests
        Encryption and decryption
        SSL/TLS client and server tests
        Handling of S/MIME signed or encrypted mail
        And more...

 INSTALLATION
 ------------

 See the appropriate file:
        INSTALL         Linux, Unix, etc.
        INSTALL.DJGPP   DOS platform with DJGPP
        INSTALL.NW      Netware
        INSTALL.OS2     OS/2
        INSTALL.VMS     VMS
        INSTALL.W32     Windows (32bit)
        INSTALL.W64     Windows (64bit)
        INSTALL.WCE     Windows CE

 SUPPORT
 -------

 See the OpenSSL website www.openssl.org for details on how to obtain
 commercial technical support.

 If you have any problems with OpenSSL then please take the following steps
 first:

    - Download the latest version from the repository
      to see if the problem has already been addressed
    - Configure with no-asm
    - Remove compiler optimisation flags

 If you wish to report a bug then please include the following information
 and create an issue on GitHub:

    - On Unix systems:
        Self-test report generated by 'make report'
    - On other systems:
        OpenSSL version: output of 'openssl version -a'
        OS Name, Version, Hardware platform
        Compiler Details (name, version)
    - Application Details (name, version)
    - Problem Description (steps that will reproduce the problem, if known)
    - Stack Traceback (if the application dumps core)

 Just because something doesn't work the way you expect does not mean it
 is necessarily a bug in OpenSSL.

 HOW TO CONTRIBUTE TO OpenSSL
 ----------------------------

 See CONTRIBUTING

 LEGALITIES
 ----------

 A number of nations restrict the use or export of cryptography. If you
 are potentially subject to such restrictions you should seek competent
 professional legal advice before attempting to develop or distribute
 cryptographic code.
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