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OPENSSL-VERIFICATION-OPTIONS(1SSL) OpenSSL OPENSSL-VERIFICATION-OPTIONS(1SSL)
NAME
openssl-verification-options - generic X.509 certificate verification options
SYNOPSIS
openssl command [ options ... ] [ parameters ... ]
DESCRIPTION
There are many situations where X.509 certificates are verified within the OpenSSL
libraries and in various OpenSSL commands.
Certificate verification is implemented by X509_verify_cert(3). It is a complicated
process consisting of a number of steps and depending on numerous options. The most
important of them are detailed in the following sections.
In a nutshell, a valid chain of certificates needs to be built up and verified starting
from the target certificate that is to be verified and ending in a certificate that due to
some policy is trusted. Verification is done relative to the given purpose, which is the
intended use of the target certificate, such as SSL server, or by default for any purpose.
The details of how each OpenSSL command handles errors are documented on the specific
command page.
DANE support is documented in openssl-s_client(1), SSL_CTX_dane_enable(3),
SSL_set1_host(3), X509_VERIFY_PARAM_set_flags(3), and X509_check_host(3).
Trust Anchors
In general, according to RFC 4158 and RFC 5280, a trust anchor is any public key and
related subject distinguished name (DN) that for some reason is considered trusted and
thus is acceptable as the root of a chain of certificates.
In practice, trust anchors are given in the form of certificates, where their essential
fields are the public key and the subject DN. In addition to the requirements in RFC
5280, OpenSSL checks the validity period of such certificates and makes use of some
further fields. In particular, the subject key identifier extension, if present, is used
for matching trust anchors during chain building.
In the most simple and common case, trust anchors are by default all self-signed "root" CA
certificates that are placed in the trust store, which is a collection of certificates
that are trusted for certain uses. This is akin to what is used in the trust stores of
Mozilla Firefox, or Apple's and Microsoft's certificate stores, ...
From the OpenSSL perspective, a trust anchor is a certificate that should be augmented
with an explicit designation for which uses of a target certificate the certificate may
serve as a trust anchor. In PEM encoding, this is indicated by the "TRUSTED CERTIFICATE"
string. Such a designation provides a set of positive trust attributes explicitly stating
trust for the listed purposes and/or a set of negative trust attributes explicitly
rejecting the use for the listed purposes. The purposes are encoded using the values
defined for the extended key usages (EKUs) that may be given in X.509 extensions of end-
entity certificates. See also the "Extended Key Usage" section below.
The currently recognized uses are clientAuth (SSL client use), serverAuth (SSL server
use), emailProtection (S/MIME email use), codeSigning (object signer use), OCSPSigning
(OCSP responder use), OCSP (OCSP request use), timeStamping (TSA server use), and
anyExtendedKeyUsage. As of OpenSSL 1.1.0, the last of these blocks all uses when rejected
or enables all uses when trusted.
A certificate, which may be CA certificate or an end-entity certificate, is considered a
trust anchor for the given use if and only if all the following conditions hold:
o It is an an element of the trust store.
o It does not have a negative trust attribute rejecting the given use.
o It has a positive trust attribute accepting the given use or (by default) one of the
following compatibilty conditions apply: It is self-signed or the -partial_chain
option is given (which corresponds to the X509_V_FLAG_PARTIAL_CHAIN flag being set).
Certification Path Building
First, a certificate chain is built up starting from the target certificate and ending in
a trust anchor.
The chain is built up iteratively, looking up in turn a certificate with suitable key
usage that matches as an issuer of the current "subject" certificate as described below.
If there is such a certificate, the first one found that is currently valid is taken,
otherwise the one that expired most recently of all such certificates. For efficiency, no
backtracking is performed, thus any further candidate issuer certificates that would match
equally are ignored.
When a self-signed certificate has been added, chain construction stops. In this case it
must fully match a trust anchor, otherwise chain building fails.
A candidate issuer certificate matches a subject certificate if all of the following
conditions hold:
o Its subject name matches the issuer name of the subject certificate.
o If the subject certificate has an authority key identifier extension, each of its sub-
fields equals the corresponding subject key identifier, serial number, and issuer
field of the candidate issuer certificate, as far as the respective fields are present
in both certificates.
o The certificate signature algorithm used to sign the subject certificate is supported
and equals the public key algorithm of the candidate issuer certificate.
The lookup first searches for issuer certificates in the trust store. If it does not find
a match there it consults the list of untrusted ("intermediate" CA) certificates, if
provided.
Certification Path Validation
When the certificate chain building process was successful the chain components and their
links are checked thoroughly.
The first step is to check that each certificate is well-formed. Part of these checks are
enabled only if the -x509_strict option is given.
The second step is to check the extensions of every untrusted certificate for consistency
with the supplied purpose. If the -purpose option is not given then no such checks are
done except for SSL/TLS connection setup, where by default "sslserver" or "sslclient", are
checked. The target or "leaf" certificate, as well as any other untrusted certificates,
must have extensions compatible with the specified purpose. All certificates except the
target or "leaf" must also be valid CA certificates. The precise extensions required are
described in more detail in "CERTIFICATE EXTENSIONS" in openssl-x509(1).
The third step is to check the trust settings on the last certificate (which typically is
a self-signed root CA certificate). It must be trusted for the given use. For
compatibility with previous versions of OpenSSL, a self-signed certificate with no trust
attributes is considered to be valid for all uses.
The fourth, and final, step is to check the validity of the certificate chain. For each
element in the chain, including the root CA certificate, the validity period as specified
by the "notBefore" and "notAfter" fields is checked against the current system time. The
-attime flag may be used to use a reference time other than "now." The certificate
signature is checked as well (except for the signature of the typically self-signed root
CA certificate, which is verified only if the -check_ss_sig option is given). When
verifying a certificate signature the keyUsage extension (if present) of the candidate
issuer certificate is checked to permit digitalSignature for signing proxy certificates or
to permit keyCertSign for signing other certificates, respectively. If all operations
complete successfully then certificate is considered valid. If any operation fails then
the certificate is not valid.
OPTIONS
Trusted Certificate Options
The following options specify how to supply the certificates that can be used as trust
anchors for certain uses. As mentioned, a collection of such certificates is called a
trust store.
Note that OpenSSL does not provide a default set of trust anchors. Many Linux
distributions include a system default and configure OpenSSL to point to that. Mozilla
maintains an influential trust store that can be found at
<https://www.mozilla.org/en-US/about/governance/policies/security-group/certs/>.
The certificates to add to the trust store can be specified using following options.
-CAfile file
Load the specified file which contains a certificate or several of them in case the
input is in PEM or PKCS#12 format. PEM-encoded certificates may also have trust
attributes set.
-no-CAfile
Do not load the default file of trusted certificates.
-CApath dir
Use the specified directory as a collection of trusted certificates, i.e., a trust
store. Files should be named with the hash value of the X.509 SubjectName of each
certificate. This is so that the library can extract the IssuerName, hash it, and
directly lookup the file to get the issuer certificate. See openssl-rehash(1) for
information on creating this type of directory.
-no-CApath
Do not use the default directory of trusted certificates.
-CAstore uri
Use uri as a store of CA certificates. The URI may indicate a single certificate, as
well as a collection of them. With URIs in the "file:" scheme, this acts as -CAfile
or -CApath, depending on if the URI indicates a single file or directory. See
ossl_store-file(7) for more information on the "file:" scheme.
These certificates are also used when building the server certificate chain (for
example with openssl-s_server(1)) or client certificate chain (for example with
openssl-s_time(1)).
-no-CAstore
Do not use the default store of trusted CA certificates.
Verification Options
The certificate verification can be fine-tuned with the following flags.
-verbose
Print extra information about the operations being performed.
-attime timestamp
Perform validation checks using time specified by timestamp and not current system
time. timestamp is the number of seconds since January 1, 1970 (i.e., the Unix Epoch).
-no_check_time
This option suppresses checking the validity period of certificates and CRLs against
the current time. If option -attime is used to specify a verification time, the check
is not suppressed.
-x509_strict
This disables non-compliant workarounds for broken certificates. Thus errors are
thrown on certificates not compliant with RFC 5280.
When this option is set, among others, the following certificate well-formedness
conditions are checked:
o The basicConstraints of CA certificates must be marked critical.
o CA certificates must explicitly include the keyUsage extension.
o If a pathlenConstraint is given the key usage keyCertSign must be allowed.
o The pathlenConstraint must not be given for non-CA certificates.
o The issuer name of any certificate must not be empty.
o The subject name of CA certs, certs with keyUsage crlSign, and certs without
subjectAlternativeName must not be empty.
o If a subjectAlternativeName extension is given it must not be empty.
o The signatureAlgorithm field and the cert signature must be consistent.
o Any given authorityKeyIdentifier and any given subjectKeyIdentifier must not be
marked critical.
o The authorityKeyIdentifier must be given for X.509v3 certs unless they are self-
signed.
o The subjectKeyIdentifier must be given for all X.509v3 CA certs.
-ignore_critical
Normally if an unhandled critical extension is present that is not supported by
OpenSSL the certificate is rejected (as required by RFC5280). If this option is set
critical extensions are ignored.
-issuer_checks
Ignored.
-crl_check
Checks end entity certificate validity by attempting to look up a valid CRL. If a
valid CRL cannot be found an error occurs.
-crl_check_all
Checks the validity of all certificates in the chain by attempting to look up valid
CRLs.
-use_deltas
Enable support for delta CRLs.
-extended_crl
Enable extended CRL features such as indirect CRLs and alternate CRL signing keys.
-suiteB_128_only, -suiteB_128, -suiteB_192
Enable the Suite B mode operation at 128 bit Level of Security, 128 bit or 192 bit, or
only 192 bit Level of Security respectively. See RFC6460 for details. In particular
the supported signature algorithms are reduced to support only ECDSA and SHA256 or
SHA384 and only the elliptic curves P-256 and P-384.
-auth_level level
Set the certificate chain authentication security level to level. The authentication
security level determines the acceptable signature and public key strength when
verifying certificate chains. For a certificate chain to validate, the public keys of
all the certificates must meet the specified security level. The signature algorithm
security level is enforced for all the certificates in the chain except for the
chain's trust anchor, which is either directly trusted or validated by means other
than its signature. See SSL_CTX_set_security_level(3) for the definitions of the
available levels. The default security level is -1, or "not set". At security level
0 or lower all algorithms are acceptable. Security level 1 requires at least
80-bit-equivalent security and is broadly interoperable, though it will, for example,
reject MD5 signatures or RSA keys shorter than 1024 bits.
-partial_chain
Allow verification to succeed if an incomplete chain can be built. That is, a chain
ending in a certificate that normally would not be trusted (because it has no matching
positive trust attributes and is not self-signed) but is an element of the trust
store. This certificate may be self-issued or belong to an intermediate CA.
-check_ss_sig
Verify the signature of the last certificate in a chain if the certificate is
supposedly self-signed. This is prohibited and will result in an error if it is a
non-conforming CA certificate with key usage restrictions not including the
keyCertSign bit. This verification is disabled by default because it doesn't add any
security.
-allow_proxy_certs
Allow the verification of proxy certificates.
-trusted_first
As of OpenSSL 1.1.0 this option is on by default and cannot be disabled.
When constructing the certificate chain, the trusted certificates specified via
-CAfile, -CApath, -CAstore or -trusted are always used before any certificates
specified via -untrusted.
-no_alt_chains
As of OpenSSL 1.1.0, since -trusted_first always on, this option has no effect.
-trusted file
Parse file as a set of one or more certificates. Each of them qualifies as trusted if
has a suitable positive trust attribute or it is self-signed or the -partial_chain
option is specified. This option implies the -no-CAfile, -no-CApath, and -no-CAstore
options and it cannot be used with the -CAfile, -CApath or -CAstore options, so only
certificates specified using the -trusted option are trust anchors. This option may
be used multiple times.
-untrusted file
Parse file as a set of one or more certificates. All certificates (typically of
intermediate CAs) are considered untrusted and may be used to construct a certificate
chain from the target certificate to a trust anchor. This option may be used multiple
times.
-policy arg
Enable policy processing and add arg to the user-initial-policy-set (see RFC5280). The
policy arg can be an object name an OID in numeric form. This argument can appear
more than once.
-explicit_policy
Set policy variable require-explicit-policy (see RFC5280).
-policy_check
Enables certificate policy processing.
-policy_print
Print out diagnostics related to policy processing.
-inhibit_any
Set policy variable inhibit-any-policy (see RFC5280).
-inhibit_map
Set policy variable inhibit-policy-mapping (see RFC5280).
-purpose purpose
The intended use for the certificate. Currently defined purposes are "sslclient",
"sslserver", "nssslserver", "smimesign", "smimeencrypt", "crlsign", "ocsphelper",
"timestampsign", and "any". If peer certificate verification is enabled, by default
the TLS implementation as well as the commands s_client and s_server check for
consistency with TLS server or TLS client use, respectively.
While IETF RFC 5280 says that id-kp-serverAuth and id-kp-clientAuth are only for WWW
use, in practice they are used for all kinds of TLS clients and servers, and this is
what OpenSSL assumes as well.
-verify_depth num
Limit the certificate chain to num intermediate CA certificates. A maximal depth
chain can have up to num+2 certificates, since neither the end-entity certificate nor
the trust-anchor certificate count against the -verify_depth limit.
-verify_email email
Verify if email matches the email address in Subject Alternative Name or the email in
the subject Distinguished Name.
-verify_hostname hostname
Verify if hostname matches DNS name in Subject Alternative Name or Common Name in the
subject certificate.
-verify_ip ip
Verify if ip matches the IP address in Subject Alternative Name of the subject
certificate.
-verify_name name
Use default verification policies like trust model and required certificate policies
identified by name. The trust model determines which auxiliary trust or reject OIDs
are applicable to verifying the given certificate chain. They can be given using the
-addtrust and -addreject options for openssl-x509(1). Supported policy names include:
default, pkcs7, smime_sign, ssl_client, ssl_server. These mimics the combinations of
purpose and trust settings used in SSL, CMS and S/MIME. As of OpenSSL 1.1.0, the
trust model is inferred from the purpose when not specified, so the -verify_name
options are functionally equivalent to the corresponding -purpose settings.
Extended Verification Options
Sometimes there may be more than one certificate chain leading to an end-entity
certificate. This usually happens when a root or intermediate CA signs a certificate for
another a CA in other organization. Another reason is when a CA might have intermediates
that use two different signature formats, such as a SHA-1 and a SHA-256 digest.
The following options can be used to provide data that will allow the OpenSSL command to
generate an alternative chain.
-xkey infile, -xcert infile, -xchain
Specify an extra certificate, private key and certificate chain. These behave in the
same manner as the -cert, -key and -cert_chain options. When specified, the callback
returning the first valid chain will be in use by the client.
-xchain_build
Specify whether the application should build the certificate chain to be provided to
the server for the extra certificates via the -xkey, -xcert, and -xchain options.
-xcertform DER|PEM|P12
The input format for the extra certificate. This option has no effect and is retained
for backward compatibility only.
-xkeyform DER|PEM|P12
The input format for the extra key. This option has no effect and is retained for
backward compatibility only.
Certificate Extensions
Options like -purpose lead to checking the certificate extensions, which determine what
the target certificate and intermediate CA certificates can be used for.
Basic Constraints
The basicConstraints extension CA flag is used to determine whether the certificate can be
used as a CA. If the CA flag is true then it is a CA, if the CA flag is false then it is
not a CA. All CAs should have the CA flag set to true.
If the basicConstraints extension is absent, which includes the case that it is an X.509v1
certificate, then the certificate is considered to be a "possible CA" and other extensions
are checked according to the intended use of the certificate. The treatment of
certificates without basicConstraints as a CA is presently supported, but this could
change in the future.
Key Usage
If the keyUsage extension is present then additional restraints are made on the uses of
the certificate. A CA certificate must have the keyCertSign bit set if the keyUsage
extension is present.
Extended Key Usage
The extKeyUsage (EKU) extension places additional restrictions on the certificate uses. If
this extension is present (whether critical or not) the key can only be used for the
purposes specified.
A complete description of each check is given below. The comments about basicConstraints
and keyUsage and X.509v1 certificates above apply to all CA certificates.
SSL Client
The extended key usage extension must be absent or include the "web client
authentication" OID. The keyUsage extension must be absent or it must have the
digitalSignature bit set. The Netscape certificate type must be absent or it must
have the SSL client bit set.
SSL Client CA
The extended key usage extension must be absent or include the "web client
authentication" OID. The Netscape certificate type must be absent or it must have the
SSL CA bit set. This is used as a work around if the basicConstraints extension is
absent.
SSL Server
The extended key usage extension must be absent or include the "web server
authentication" and/or one of the SGC OIDs. The keyUsage extension must be absent or
it must have the digitalSignature, the keyEncipherment set or both bits set. The
Netscape certificate type must be absent or have the SSL server bit set.
SSL Server CA
The extended key usage extension must be absent or include the "web server
authentication" and/or one of the SGC OIDs. The Netscape certificate type must be
absent or the SSL CA bit must be set. This is used as a work around if the
basicConstraints extension is absent.
Netscape SSL Server
For Netscape SSL clients to connect to an SSL server it must have the keyEncipherment
bit set if the keyUsage extension is present. This isn't always valid because some
cipher suites use the key for digital signing. Otherwise it is the same as a normal
SSL server.
Common S/MIME Client Tests
The extended key usage extension must be absent or include the "email protection" OID.
The Netscape certificate type must be absent or should have the S/MIME bit set. If the
S/MIME bit is not set in the Netscape certificate type then the SSL client bit is
tolerated as an alternative but a warning is shown. This is because some Verisign
certificates don't set the S/MIME bit.
S/MIME Signing
In addition to the common S/MIME client tests the digitalSignature bit or the
nonRepudiation bit must be set if the keyUsage extension is present.
S/MIME Encryption
In addition to the common S/MIME tests the keyEncipherment bit must be set if the
keyUsage extension is present.
S/MIME CA
The extended key usage extension must be absent or include the "email protection" OID.
The Netscape certificate type must be absent or must have the S/MIME CA bit set. This
is used as a work around if the basicConstraints extension is absent.
CRL Signing
The keyUsage extension must be absent or it must have the CRL signing bit set.
CRL Signing CA
The normal CA tests apply. Except in this case the basicConstraints extension must be
present.
BUGS
The issuer checks still suffer from limitations in the underlying X509_LOOKUP API. One
consequence of this is that trusted certificates with matching subject name must appear in
a file (as specified by the -CAfile option), a directory (as specified by -CApath), or a
store (as specified by -CAstore). If there are multiple such matches, possibly in
multiple locations, only the first one (in the mentioned order of locations) is
recognised.
SEE ALSO
X509_verify_cert(3), openssl-verify(1), openssl-ocsp(1), openssl-ts(1),
openssl-s_client(1), openssl-s_server(1), openssl-smime(1), openssl-cmp(1), openssl-cms(1)
HISTORY
The checks enabled by -x509_strict have been extended in OpenSSL 3.0.
COPYRIGHT
Copyright 2000-2021 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use this file except
in compliance with the License. You can obtain a copy in the file LICENSE in the source
distribution or at <https://www.openssl.org/source/license.html>.
3.0.2 2024-08-20 OPENSSL-VERIFICATION-OPTIONS(1SSL)
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