Add: documentation for NASL cert-functions (#1272)

doc/manual/nasl/built-in-functions/cert-functions/cert_close.md

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+# cert_close
+
+## NAME
+
+**cert_close** - release a certificate object
+
+## SYNOPSIS
+
+*void* **cert_close**(0: *int*);
+
+**cert_close** takes one unnamed argument.
+
+## DESCRIPTION
+
+This function releases a certificate object, which was created by **[cert_open(3)](cert_open.md)** before.
+
+The first unnamed argument is an *int* and contains an identifier to a cert object. This identifier is returned by **[cert_open(3)](cert_open.md)**.
+
+## RETURN VALUE
+
+None
+
+## ERRORS
+
+The first unnamed argument is missing
+
+The given object ID is <0
+
+The given object ID is not in use
+
+## SEE ALSO
+
+**[cert_open(3)](cert_open.md)**

doc/manual/nasl/built-in-functions/cert-functions/cert_open.md

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+# cert_open
+
+## NAME
+
+**cert_open** - create a certificate object
+
+## SYNOPSIS
+
+*int* **cert_open**(0: *string*);
+
+**cert_open** takes one positional argument.
+
+## DESCRIPTION
+
+Takes a string/data as unnamed argument and returns an identifier used with the other cert functions. The data is usually the BER encoded certificate but the function will also try a PEM encoding on failure to parse BER encoded one. An opened certificate object must be closed with **[cert_close(3)](cert_close.md)**.
+
+The first unnamed argument is either *string* or a data object containing the certificate. It is either Binary or PEM encoded.
+
+## RETURN VALUE
+
+Identifier for the certificate object as *int* or *NULL* on error.
+
+## ERRORS
+
+First unnamed argument is missing.
+
+Unable to create certificate object.
+
+## SEE ALSO
+
+**[cert_close(3)](cert_close.md)**

doc/manual/nasl/built-in-functions/cert-functions/cert_query.md

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+# cert_query
+
+## NAME
+
+**cert_query** - query a certificate object
+
+## SYNOPSIS
+
+*any* **cert_query**(0: *int*, 1: *string*, idx: *int*);
+
+**cert_query** takes 2 positional and up to 1 named arguments.
+
+## DESCRIPTION
+
+This function runs a command on a given certificate object.
+
+The first unnamed argument is an *int* containing the certificate ID, given by **[cert_open(3)](cert_open.md)**.
+
+The second unnamed argument is a *string* containing the command to run on the certificate object. Available commands are:
+- *serial* - get the serial number of the certificate as a hex string
+- *issuer* - get the issuer as *string* in the rfc-2253 format
+- *subject* - get the subject of the certificate as *string* in the rfc-2253 format. To query the subjectAltName the named parameter *idx* with values starting at 1 can be used. In this case the format is either a *string* in rfc-2253 format, a rfc2822 mailbox name indicated by the first character being a left angle bracket or a S-expression in advanced format for all other types of subjectAltNames which is indicated by an opening parentheses.
+- *not-before* - get the notBefore time as UTC value in ISO time format (e.g. "20120930T143521")
+- *not-after* - get the notAfter time as UTC value in ISO time format (e.g. "20280929T143520")
+- *all* - get all available information in a human readable format, **NOT YET IMPLEMENTED**
+- *hostnames* - get an *array* containing all hostnames listed in the certificate, i.e. the CN part of the subject and all dns-name type subjectAltNames
+- *fpr-sha-256* - get the SHA-256 fingerprint of the certificate. The fingerprint is, as usual, computed over the entire DER encode certificate
+- *fpr-sha-1* get the SHA-1 fingerprint of the certificate. The fingerprint is, as usual, computed over the entire DER encode certificate.
+- *image* - get the entire certificate as binary data
+- *algorithm-name* - get the algorithm name used to sign the certificate. Get the OID of the digest algorithm and translate to a name from a list of Wireshark.
+- *signature-algorithm-name* - same as *algorithm-name*
+- *public-key-algorithm-name* - get the algorithm name of the public key
+- *modulus* - get the RSA public key's modulus found in the structure of the given cert
+- *exponent* - get the RSA public key's exponent found in the structure of the given cert
+- *key-size* - get the size to hold the parameters size in bits, for RSA the bits returned is the modulus, for DSA the bits returned are of the public exponent
+
+The named parameter *idx* is of type *int*. It used as an index for some of the commands. In general it gives the n-th value of a set of values. If not given 0 is used as default.
+
+## RETURN VALUE
+
+The return value depends of the type of command used. In general it can be a *string*, an *array* of strings or an *int*. In case of an error, the returned value is *NULL*.
+
+## ERRORS
+
+The given certificate is either missing or <0
+
+The given certificate is not in use
+
+No proper command passed
+
+Unable to run given command
+
+## SEE ALSO
+
+**[cert_open(3)](cert_open.md)**

doc/manual/nasl/built-in-functions/cert-functions/index.md

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+# Cert Functions
+
+## GENERAL
+
+Implementation of an API for X.509 certificates.
+
+## TOC