OPENSSL-VERIFICATION-OPTIONS(1)

NAME
openssl-verification-options - generic X.509 certificate verification options

SYNOPSIS
openssl command [ options ... ] [ parameters ... ]

DESCRIPTION
There are many situations where X.509 public-key certificates are verified within the OpenSSL libraries and in various OpenSSL commands. For checking certificates, the term validation would actually be more appropriate, but for historical reasons the term verification is often used instead.

In the sequel, the certificate to be validated is called target certificate. Since it usually is an end-entity (EE) certificate, it is also known as "leaf" certificate (of a chain or tree), but it may also be a CA certificate. Certificate verification is implemented by X509_verify_cert(3). The validation process involves several steps and depends on multiple options. The most important of them are detailed in the following sections.

In a nutshell, a valid chain of certificates needs to be constructed and checked starting from the target certificate that is to be validated and ending in a trust anchor, which usually is a certificate that due to some policy is trusted. Certificate validation can be performed in the context of a purpose, which is a high-level specification of the intended use of the target certificate, such as "sslserver" for (D)TLS servers, or (by default) for any purpose.

All certificates in the chain except the target ceritificate must be CA certificates. An X.509v3 certificate is considered a CA certificate if it has a basicConstraints extension with the `CA` flag set. If a pathLenConstraint is included then the chain is checked to satisfy it. If a CA certificate has a keyUsage extension then the latter must include the keyCertSign bit, and for proxy certificates used for issuing a certificate it must include also the digitalSignature bit.

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 just a public key (with algorithm and any parameters) and an associated subject name that in a suitable context is considered trusted for some purpose(s) and thus is acceptable as the root of a chain of certificates.

In practice, trust anchors are given in the form of certificates. Commonly they are self-signed root CA certificates. Their most essential fields are the subjectPublicKeyInfo and the subject distinguished name (DN), while the subject key identifier extension is generally optional but highly recommended to be included in CA certificates because they help chain building. In addition to the requirements given in RFC 5280 for trust anchor handling, OpenSSL performs on trust anchor certificates the same checks as on regular CA certificates, in particular those regarding the validity period and X.509v3 extensions like basic constraints and key usage. Depending on the given validation purpose, in part even the extended key usage is checked - for details see the "Extended Key Usage" section below.

The most crucial input to certificate validation is a trust store, which includes a collection of certificates and validation options. This is akin to what is used in the trust stores of Mozilla Firefox, or Apple's and Microsoft's certificate stores, ... By default all apparently self-signed certificates in the trust store are considered to be trust anchors, for all or for specified purposes.

For efficiency reasons, apparently self-signed certificates (based on certificate metadata) are not subjected to cryptographic self-signature check. A certificate is apparently self-signed if and only if all of the following conditions are met:

•   it is self-issued, i.e., its "subject" and "issuer" fields are equal,

•   the algorithm identifiers of its public key and signature match,

•   if it has an authorityKeyIdentifier (AKID) extension containing the "keyIdentifier" field, its value matches the subjectKeyIdentifier (SKID) extension if present,

•   if it has an AKID containing the "authorityCertSerialNumber" field, its value equals the serial number of the certificate, and

•   if it has an AKID containing the "authorityCertIssuer" field and its "GeneralNames" include at least one "directoryName", the first such name equals the "issuer" field of the certificate.

Independent of any extended key usage (EKU) X.509v3 extension included, from the OpenSSL perspective a trust anchor certificate should be augmented with local trust attributes, giving an explicit designation for which uses of a target certificate the certificate may serve as a trust anchor. If this information is missing, it is by default trusted for all uses. In PEM encoding, the presence of trust attributes is indicated by the "TRUSTED CERTIFICATE" string. Such a designation provides a set of positive trust attributes explicitly stating acceptance for the listed uses and/or a set of negative trust attributes explicitly rejecting the listed uses. These uses are encoded using the OID values defined for the extended key usages that may be included in X.509v3 certificate extensions. For the list of EKUs with currently predefined meaning see the "Extended Key Usage" section below. The special EKU anyExtendedKeyUsage enables all uses when trusted or blocks all uses when rejected.

A certificate, which may be CA certificate or an end-entity certificate, is considered a trust anchor for the intended use if and only if all the following conditions hold:

•   It is an element of the trust store.

•   It does not have a negative trust attribute rejecting the EKU associated with the intended purpose.

•   It has a positive trust attribute accepting the EKU associated with the intended purpose or it does not have any positive trust attribute and one of the following compatibility conditions apply: It is apparently 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 goal is that for each certificate in the chain, except for the final one, the subject of the successor is the issuer of the former certificate. Since it often happens that the subject and issuer Distinguished Names are not unique, the subject and issuer key identifier extensions, if present in the certificates, are used to disambiguate conflicts.

The chain is built up iteratively, looking up each certificate in turn that matches as an issuer of the current "subject" certificate as described below. If there is more than one matching certificate, the algorithm uses the first certificate found that has a suitable validity period, otherwise the one with the latest expiration date of all matching certificates. For efficiency, the algorithm does not allow for backtracking, thus any further candidate issuer certificates that would match equally are ignored.

Certificate extensions are available since X.509v3, and though many are optional, some are generally recommended or required. For certificate chain building the subjectKeyIdentifier and authorityKeyIdentifier extensions are very useful. The subject alternative names (SAN) and issuer alternative names given in subjectAltName or issuerAltName extensions are not relevant for chain building.

Once a apparently 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:

•   Its subject Distinguished Name (DN) matches the issuer DN of the subject certificate.

•   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.

•   The certificate signature algorithm used to sign the subject certificate is supported and equals the public key algorithm of the candidate issuer certificate.

    item *

    If the issuer certificate includes a key usage restriction, it must allow for certificate signing (keyCertSign).

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. If one issuer certificate was found in the trust store, the list of untrusted certificates will not be consulted anymore to find further issuer certificates. Therefore, either only the root certificate or an uninterrupted chain to the root certificate must be provided in the trust store for a successful verification, if X509_V_FLAG_PARTIAL_CHAIN is not enabled.

  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 X.509v3 extensions of every certificate for consistency with the intended specific purpose, if any. If no purpose is given explicitly then no such checks are done except for CMS signature checking, where by default "smimesign" is checked, and SSL/(D)TLS connection setup, where by default "sslserver" or "sslclient" are checked. The X.509v3 extensions of the target or "leaf" certificate must be compatible with the specified purpose. All other certificates down the chain are checked to be valid CA certificates, and possibly also further non-standard checks are performed. The precise extensions required are described in detail in the "Certificate Extensions" section below.

The third step is to check the trust settings on the last certificate or trust anchor, which typically is given as a self-signed root CA certificate. If specified, it must be trusted for the given use. For compatibility, an apparently 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 trusted certificate in DER format or potentially several of them in case the input is in PEM 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:

    •   CA certificates must explicitly include the keyUsage extension.

    •   If a pathLenConstraint is given the key usage keyCertSign must be allowed.

    •   The pathLenConstraint must not be given for non-CA certificates.

    •   The issuer name of any certificate must not be empty.

    •   The subject name of CA certs, certs with keyUsage crlSign, and certs without subjectAlternativeName must not be empty.

    •   If a subjectAlternativeName extension is given it must not be empty.

    •   The signatureAlgorithm field and the cert signature must be consistent.

    •   The authorityKeyIdentifier must be given for X.509v3 certs unless they are apparently self-signed.

    •   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 is not self-signed and has no matching positive trust attributes) 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 apparently 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
    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
    Since -trusted_first is 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 apparently 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 or 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
    A high-level specification of the intended use of the target certificate. Currently predefined purposes are "sslclient", "sslserver", "nssslserver", "smimesign", "smimeencrypt", "crlsign", "ocsphelper", "timestampsign", "codesign" and "any". If peer certificate verification is enabled, by default the TLS implementation and thus the commands openssl-s_client(1) and openssl-s_server(1) check for consistency with TLS server ("sslserver") or TLS client use ("sslclient"), respectively. By default, CMS signature validation, which can be done via openssl-cms(1), checks for consistency with S/MIME signing use ("smimesign").

    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.

    For details on the checks performed see the "Certificate Extensions" section.

-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 a set of verification parameters, also known as verification method, identified by name. The currently predefined methods are named "ssl_client", "ssl_server", "smime_sign" with alias "pkcs7", "code_sign", and "default". These mimic the combinations of purpose and trust settings used in SSL/(D)TLS, CMS/PKCS7 (including S/MIME), and code signing.

    The verification parameters include the trust model, various flags that can partly be set also via other command-line options, and the verification purpose, which in turn implies certificate key usage and extended key usage requirements.

    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).

  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 and -verify_name trigger the processing of specific certificate extensions, which determine what certificates can be used for.

The following two subsections on the basicConstraints and keyUsage extensions and on X.509v1 certificates apply to all CA certificates regardless of any required purpose.

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. Further restrictions are checked depending on the required purpose, if any; for details see the list given in the next subsection.

Extended Key Usage

The extKeyUsage (EKU) extension places additional restrictions on certificate use. If this extension is present (whether critical or not) in an end-entity certificate, the key is allowed only for the uses specified, while the special EKU anyExtendedKeyUsage allows for all uses.

Note that according to RFC 5280 section 4.2.1.12, the Extended Key Usage extension will appear only in end-entity certificates, and consequently the standard certification path validation described in its section 6 does not include EKU checks for CA certificates. The CA/Browser Forum requires for TLS server, S/MIME, and code signing use the presence of respective EKUs in subordinate CA certificates (while excluding them for root CA certificates), while taking over from RFC 5280 the certificate validity concept and certificate path validation.

For historic reasons, OpenSSL has its own way of interpreting and checking EKU extensions on CA certificates, which may change in the future. It does not require the presence of EKU extensions in CA certificates, but in case the verification purpose is "sslclient", "nssslserver", "sslserver", "smimesign", or "smimeencrypt", it checks that any present EKU extension (that does not contain anyExtendedKeyUsage) contains the respective EKU as detailed below. Moreover, it does these checks even for trust anchor certificates.

Checks Implied by Specific Predefined Policies

A specific description of each check is given below. The comments about basicConstraints and keyUsage and X.509v1 certificates above apply to all CA certificates.

(D)TLS Client ("sslclient")
    Any given extended key usage extension must allow for "clientAuth" ("TLS WWW client authentication").

    For target certificates, the key usage must allow for "digitalSignature" and/or "keyAgreement". The Netscape certificate type must be absent or have the SSL client bit set.

    For all other certificates the normal CA checks apply. In addition, the Netscape certificate type must be absent or have the SSL CA bit set. This is used as a workaround if the basicConstraints extension is absent.

(D)TLS Server ("sslserver")
    Any given extended key usage extension must allow for "serverAuth" ("TLS WWW server authentication") and/or include one of the SGC OIDs.

    For target certificates, the key usage must allow for "digitalSignature", "keyEncipherment", and/or "keyAgreement". The Netscape certificate type must be absent or have the SSL server bit set.

    For all other certificates the normal CA checks apply. In addition, the Netscape certificate type must be absent or have the SSL CA bit set. This is used as a workaround if the basicConstraints extension is absent.

Netscape SSL Server ("nssslserver")
    In addition to what has been described for sslserver, for a Netscape SSL client to connect to an SSL server, its EE certificate 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 Checks
    Any given extended key usage extension must allow for "emailProtection".

    For target certificates, 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.

    For all other certificates the normal CA checks apply. In addition, the Netscape certificate type must be absent or have the S/MIME CA bit set. This is used as a workaround if the basicConstraints extension is absent.

S/MIME Signing ("smimesign")
    In addition to the common S/MIME checks, for target certificates the key usage must allow for "digitalSignature" and/or nonRepudiation (or its alias name contentCommitment).

S/MIME Encryption ("smimeencrypt")
    In addition to the common S/MIME checks, for target certificates the key usage must allow for "keyEncipherment".

CRL Signing ("crlsign")
    For target certificates, the key usage must allow for "cRLSign".

    For all other certificates the normal CA checks apply. Except in this case the basicConstraints extension must be present.

OCSP Helper ("ocsphelper")
    For target certificates, no checks are performed at this stage, but special checks apply; see OCSP_basic_verify(3).

    For all other certificates the normal CA checks apply.

Timestamp Signing ("timestampsign")
    For target certificates, if the key usage extension is present, it must include "digitalSignature" and/or "nonRepudiation" (or its alias name "contentCommitment") and must not include other bits. The EKU extension must be present and contain "timeStamping" only. Moreover, it must be marked as critical.

    For all other certificates the normal CA checks apply.

Code Signing ("codesign")
    For target certificates, the key usage extension must be present and marked critical and include <digitalSignature>, but must not include "keyCertSign" nor "cRLSign". The EKU extension must be present and contain "codeSign", but must not include "anyExtendedKeyUsage" nor "serverAuth".

    For all other certificates the normal CA checks apply.

BUGS
The issuer checks still suffer from limitations in the underlying X509_LOOKUP API. 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), OCSP_basic_verify(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
Since OpenSSL 1.1.0, the -trusted_first option is always enabled.

The checks enabled by -x509_strict have been extended in OpenSSL 3.0, which has been partially reverted in OpenSSL 4.0.

COPYRIGHT
Copyright 2000-2026 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>.