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ISBN : 978-2-7460-9712-4
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CentOS 2.1AS







x509 − Certificate display and signing utility


openssl x509 [−inform DER⎪PEM⎪NET] [−outform DER⎪PEM⎪NET] [−keyform DER⎪PEM] [−CAform DER⎪PEM] [−CAkeyform DER⎪PEM] [−in filename] [−out filename] [−serial] [−hash] [−subject] [−issuer] [−nameopt option] [−email] [−startdate] [−enddate] [−purpose] [−dates] [−modulus] [−fingerprint] [−alias] [−noout] [−trustout] [−clrtrust] [−clrreject] [−addtrust arg] [−addreject arg] [−setalias arg] [−days arg] [−signkey filename] [−x509toreq] [−req] [−CA filename] [−CAkey filename] [−CAcreateserial] [−CAserial filename] [−text] [−C] [−md2⎪−md5⎪−sha1⎪−mdc2] [−clrext] [−extfile filename] [−extensions section]


The x509 command is a multi purpose certificate utility. It can be used to display certificate information, convert certificates to various forms, sign certificate requests like a "mini CA" or edit certificate trust settings.

Since there are a large number of options they will split up into various sections.



This specifies the input format normally the command will expect an X509 certificate but this can change if other options such as −req are present. The DER format is the DER encoding of the certificate and PEM is the base64 encoding of the DER encoding with header and footer lines added. The NET option is an obscure Netscape server format that is now obsolete.

−outform DERPEMNET

This specifies the output format, the options have the same meaning as the −inform option.

−in filename

This specifies the input filename to read a certificate from or standard input if this option is not specified.

−out filename

This specifies the output filename to write to or standard output by default.


the digest to use. This affects any signing or display option that uses a message digest, such as the −fingerprint, −signkey and CA options. If not specified then MD5 is used. If the key being used to sign with is a DSA key then this option has no effect: SHA1 is always used with DSA keys.


Note: the −alias and −purpose options are also display options but are described in the TRUST OPTIONS section.

prints out the certificate in text form. Full details are output including the public key, signature algorithms, issuer and subject names, serial number any extensions present and any trust settings.


this option prevents output of the encoded version of the request.


this option prints out the value of the modulus of the public key contained in the certificate.


outputs the certificate serial number.


outputs the "hash" of the certificate subject name. This is used in OpenSSL to form an index to allow certificates in a directory to be looked up by subject name.


outputs the subject name.


outputs the issuer name.

−nameopt option

option which determine how the subject or issuer names are displayed. This option may be used more than once to set multiple options. See the NAME OPTIONS section for more information.


outputs the email address(es) if any.


prints out the start date of the certificate, that is the notBefore date.


prints out the expiry date of the certificate, that is the notAfter date.


prints out the start and expiry dates of a certificate.


prints out the digest of the DER encoded version of the whole certificate.


this outputs the certificate in the form of a C source file.


Please note these options are currently experimental and may well change.

A trusted certificate is an ordinary certificate which has several additional pieces of information attached to it such as the permitted and prohibited uses of the certificate and an "alias".

Normally when a certificate is being verified at least one certificate must be "trusted". By default a trusted certificate must be stored locally and must be a root CA: any certificate chain ending in this CA is then usable for any purpose.

Trust settings currently are only used with a root CA. They allow a finer control over the purposes the root CA can be used for. For example a CA may be trusted for SSL client but not SSL server use.

See the description of the verify utility for more information on the meaning of trust settings.

Future versions of OpenSSL will recognize trust settings on any certificate: not just root CAs.

this causes x509 to output a trusted certificate. An ordinary or trusted certificate can be input but by default an ordinary certificate is output and any trust settings are discarded. With the −trustout option a trusted certificate is output. A trusted certificate is automatically output if any trust settings are modified.

−setalias arg

sets the alias of the certificate. This will allow the certificate to be referred to using a nickname for example "Steve’s Certificate".


outputs the certificate alias, if any.


clears all the permitted or trusted uses of the certificate.


clears all the prohibited or rejected uses of the certificate.

−addtrust arg

adds a trusted certificate use. Any object name can be used here but currently only clientAuth ( SSL client use), serverAuth ( SSL server use) and emailProtection (S/ MIME email) are used. Other OpenSSL applications may define additional uses.

−addreject arg

adds a prohibited use. It accepts the same values as the −addtrust option.


this option performs tests on the certificate extensions and outputs the results. For a more complete description see the CERTIFICATE EXTENSIONS section.


The x509 utility can be used to sign certificates and requests: it can thus behave like a "mini CA".
−signkey filename

this option causes the input file to be self signed using the supplied private key.

If the input file is a certificate it sets the issuer name to the subject name (i.e. makes it self signed) changes the public key to the supplied value and changes the start and end dates. The start date is set to the current time and the end date is set to a value determined by the −days option. Any certificate extensions are retained unless the −clrext option is supplied.

If the input is a certificate request then a self signed certificate is created using the supplied private key using the subject name in the request.


delete any extensions from a certificate. This option is used when a certificate is being created from another certificate (for example with the −signkey or the CA options). Normally all extensions are retained.

−keyform PEMDER

specifies the format ( DER or PEM ) of the private key file used in the −signkey option.

−days arg

specifies the number of days to make a certificate valid for. The default is 30 days.


converts a certificate into a certificate request. The −signkey option is used to pass the required private key.


by default a certificate is expected on input. With this option a certificate request is expected instead.

CA filename

specifies the CA certificate to be used for signing. When this option is present x509 behaves like a "mini CA ". The input file is signed by this CA using this option: that is its issuer name is set to the subject name of the CA and it is digitally signed using the CAs private key.

This option is normally combined with the −req option. Without the −req option the input is a certificate which must be self signed.

−CAkey filename

sets the CA private key to sign a certificate with. If this option is not specified then it is assumed that the CA private key is present in the CA certificate file.

−CAserial filename

sets the CA serial number file to use.

When the CA option is used to sign a certificate it uses a serial number specified in a file. This file consist of one line containing an even number of hex digits with the serial number to use. After each use the serial number is incremented and written out to the file again.

The default filename consists of the CA certificate file base name with ".srl" appended. For example if the CA certificate file is called "mycacert.pem" it expects to find a serial number file called "mycacert.srl".

−CAcreateserial filename

with this option the CA serial number file is created if it does not exist: it will contain the serial number "02" and the certificate being signed will have the 1 as its serial number. Normally if the CA option is specified and the serial number file does not exist it is an error.

−extfile filename

file containing certificate extensions to use. If not specified then no extensions are added to the certificate.

−extensions section

the section to add certificate extensions from. If this option is not specified then the extensions should either be contained in the unnamed (default) section or the default section should contain a variable called "extensions" which contains the section to use.


The nameopt command line switch determines how the subject and issuer names are displayed. If no nameopt switch is present the default "oneline" format is used which is compatible with previous versions of OpenSSL. Each option is described in detail below, all options can be preceded by a to turn the option off. Only the first four will normally be used.

use the old format. This is equivalent to specifying no name options at all.


displays names compatible with RFC2253 equivalent to esc_2253, esc_ctrl, esc_msb, utf8, dump_nostr, dump_unknown, dump_der, sep_comma_plus, dn_rev and sname.


a oneline format which is more readable than RFC2253 . It is equivalent to specifying the esc_2253, esc_ctrl, esc_msb, utf8, dump_nostr, dump_der, use_quote, sep_comma_plus_spc, spc_eq and sname options.


a multiline format. It is equivalent esc_ctrl, esc_msb, sep_multiline, spc_eq and lname.


escape the "special" characters required by RFC2253 in a field That is ,+"<>;. Additionally # is escaped at the beginnging of a string and a space character at the beginning or end of a string.


escape control characters. That is those with ASCII values less than 0x20 (space) and the delete (0x7f) character. They are escaped using the RFC2253 \XX notation (where XX are two hex digits representing the character value).


escape characters with the MSB set, that is with ASCII values larger than 127.


escapes some characters by surrounding the whole string with " characters, without the option all escaping is done with the \ character.


convert all strings to UTF8 format first. This is required by RFC2253 . If you are lucky enough to have a UTF8 compatible terminal then the use of this option (and not setting esc_msb) may result in the correct display of multibyte (international) characters. Is this option is not present then multibyte characters larger than 0xff will be represented using the format \UXXXX for 16 bits and \WXXXXXXXX for 32 bits. Also if this option is off any UTF8Strings will be converted to their character form first.


this option does not attempt to interpret multibyte characters in any way. That is their content octets are merely dumped as though one octet represents each character. This is useful for diagnostic purposes but will result in rather odd looking output.


show the type of the ASN1 character string. The type precedes the field contents. For example " BMPSTRING : Hello World".


when this option is set any fields that need to be hexdumped will be dumped using the DER encoding of the field. Otherwise just the content octets will be displayed. Both options use the RFC2253 # XXXX ... format.


dump non character string types (for example OCTET STRING ) if this option is not set then non character string types will be displayed as though each content octet repesents a single character.


dump all fields. This option when used with dump_der allows the DER encoding of the structure to be unambiguously determined.


dump any field whose OID is not recognised by OpenSSL.

sep_comma_plus, sep_comma_plus_space, sep_semi_plus_space,

these options determine the field separators. The first character is between RDNs and the second between multiple AVAs (multiple AVAs are very rare and their use is discouraged). The options ending in "space" additionally place a space after the separator to make it more readable. The sep_multiline uses a linefeed character for the RDN separator and a spaced + for the AVA separator. It also indents the fields by four characters.


reverse the fields of the DN . This is required by RFC2253 . As a side effect this also reverses the order of multiple AVAs but this is permissible.

nofname, sname, lname, oid

these options alter how the field name is displayed. nofname does not display the field at all. sname uses the "short name" form ( CN for commonName for example). lname uses the long form. oid represents the OID in numerical form and is useful for diagnostic purpose.


places spaces round the = character which follows the field name.


Note: in these examples the ’\’ means the example should be all on one line.

Display the contents of a certificate:

 openssl x509 -in cert.pem -noout -text

Display the certificate serial number:

 openssl x509 -in cert.pem -noout -serial

Display the certificate subject name:

 openssl x509 -in cert.pem -noout -subject

Display the certificate subject name in RFC2253 form:

 openssl x509 -in cert.pem -noout -subject -nameopt RFC2253

Display the certificate subject name in oneline form on a terminal supporting UTF8:

 openssl x509 -in cert.pem -noout -subject -nameopt oneline -nameopt -escmsb

Display the certificate MD5 fingerprint:

 openssl x509 -in cert.pem -noout -fingerprint

Display the certificate SHA1 fingerprint:

 openssl x509 -sha1 -in cert.pem -noout -fingerprint

Convert a certificate from PEM to DER format:

 openssl x509 -in cert.pem -inform PEM -out cert.der -outform DER

Convert a certificate to a certificate request:

 openssl x509 -x509toreq -in cert.pem -out req.pem -signkey key.pem

Convert a certificate request into a self signed certificate using extensions for a CA:

 openssl x509 -req -in careq.pem -extfile openssl.cnf -extensions v3_ca \
        -signkey key.pem -out cacert.pem

Sign a certificate request using the CA certificate above and add user certificate extensions:

 openssl x509 -req -in req.pem -extfile openssl.cnf -extensions v3_usr \
        -CA cacert.pem -CAkey key.pem -CAcreateserial

Set a certificate to be trusted for SSL client use and change set its alias to "Steve’s Class 1 CA"

 openssl x509 -in cert.pem -addtrust sslclient \
        -alias "Steve’s Class 1 CA" -out trust.pem


The PEM format uses the header and footer lines:


it will also handle files containing:

 -----END X509 CERTIFICATE----

Trusted certificates have the lines


The conversion to UTF8 format used with the name options assumes that T61Strings use the ISO8859-1 character set. This is wrong but Netscape and MSIE do this as do many certificates. So although this is incorrect it is more likely to display the majority of certificates correctly.

The −fingerprint option takes the digest of the DER encoded certificate. This is commonly called a "fingerprint". Because of the nature of message digests the fingerprint of a certificate is unique to that certificate and two certificates with the same fingerprint can be considered to be the same.

The Netscape fingerprint uses MD5 whereas MSIE uses SHA1.

The −email option searches the subject name and the subject alternative name extension. Only unique email addresses will be printed out: it will not print the same address more than once.


The −purpose option checks the certificate extensions and determines what the certificate can be used for. The actual checks done are rather complex and include various hacks and workarounds to handle broken certificates and software.

The same code is used when verifying untrusted certificates in chains so this section is useful if a chain is rejected by the verify code.

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 then the certificate is considered to be a "possible CA" other extensions are checked according to the intended use of the certificate. A warning is given in this case because the certificate should really not be regarded as a CA: however it is allowed to be a CA to work around some broken software.

If the certificate is a V1 certificate (and thus has no extensions) and it is self signed it is also assumed to be a CA but a warning is again given: this is to work around the problem of Verisign roots which are V1 self signed certificates.

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.

The extended key usage 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 test is given below. The comments about basicConstraints and keyUsage and V1 certificates above apply to all CA certificates.

The extended key usage extension must be absent or include the "web client authentication" OID . keyUsage must be absent or it must have the digitalSignature bit set. 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 . 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. keyUsage must be absent or it must have the digitalSignature, the keyEncipherment set or both bits set. 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. 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 . Netscape certificate type must be absent or should have the S/ MIME bit set. If the S/ MIME bit is not set in 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 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.


The extended key usage extension must be absent or include the "email protection" OID . 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.


Extensions in certificates are not transferred to certificate requests and vice versa.

It is possible to produce invalid certificates or requests by specifying the wrong private key or using inconsistent options in some cases: these should be checked.

There should be options to explicitly set such things as start and end dates rather than an offset from the current time.

The code to implement the verify behaviour described in the TRUST SETTINGS is currently being developed. It thus describes the intended behaviour rather than the current behaviour. It is hoped that it will represent reality in OpenSSL 0.9.5 and later.


req(1), ca(1), genrsa(1), gendsa(1), verify(1)