An SSL server and client can be built with the (abstracted) predicate calls from the table below. The tcp_ predicates are provided by library(socket). The predicate ssl_context/3 defines properties of the SSL connection, while ssl_negotiate/5 establishes the SSL connection based on the wire streams created by the TCP predicates and the context.

The library is abstracted to communication over streams, and is not reliant on those streams being directly attached to sockets. The tcp_ calls here are simply the most common way to use the library. Other two-way communication channels such as (named), pipes can just as easily be used.

See also

- library(socket), library(http/http_open), library(crypto)

 ssl_context(+Role, -SSL, :Options) is det

Create an SSL context. The context defines several properties of the SSL connection such as involved keys, preferred encryption, and passwords. After establishing a context, an SSL connection can be negotiated using ssl_negotiate/5, turning two arbitrary plain Prolog streams into encrypted streams. This predicate processes the options below.

host(+HostName)

For the client, the host to which it connects. This option should be specified when Role is client. Otherwise, certificate verification may fail when negotiating a secure connection.

certificate_file(+FileName)

Specify where the certificate file can be found. This can be the same as the key_file(+FileName) option. A server must have at least one certificate before clients can connect. A client must have a certificate only if the server demands the client to identify itself with a client certificate using the peer_cert(true) option. If a certificate is provided, it is necessary to also provide a matching private key via the key_file/1 option. To configure multiple certificates, use the option certificate_key_pairs/1 instead. Alternatively, use ssl_add_certificate_key/4 to add certificates and keys to an existing context.

key_file(+FileName)

Specify where the private key that matches the certificate can be found. If the key is encrypted with a password, this must be supplied using the password(+Text) or pem_password_hook(:Goal) option.

certificate_key_pairs(+Pairs)

Alternative method for specifying certificates and keys. The argument is a list of pairs of the form Certificate-Key, where each component is a string or an atom that holds, respectively, the PEM-encoded certificate and key. To each certificate, further certificates of the chain can be appended. Multiple types of certificates can be present at the same time to enable different ciphers. Using multiple certificate types with completely independent certificate chains requires OpenSSL 1.0.2 or greater.

password(+Text)

Specify the password the private key is protected with (if any). If you do not want to store the password you can also specify an application defined handler to return the password (see next option). Text is either an atom or string. Using a string is preferred as strings are volatile and local resources.

pem_password_hook(:Goal)

In case a password is required to access the private key the supplied predicate will be called to fetch it. The hook is called as call(Goal, +SSL, -Password) and typically unifies Password with a string containing the password.

require_crl(+Boolean)

If true (default is false), then all certificates will be considered invalid unless they can be verified as not being revoked. You can do this explicity by passing a list of CRL filenames via the crl/1 option, or by doing it yourself in the cert_verify_hook. If you specify require_crl(true) and provide neither of these options, verification will necessarily fail

crl(+ListOfFileNames)

Provide a list of filenames of PEM-encoded CRLs that will be given to the context to attempt to establish that a chain of certificates is not revoked. You must also set require_crl(true) if you want CRLs to actually be checked by OpenSSL.

cacert_file(+FileName)

Deprecated. Use cacerts/1 instead. Specify a file containing certificate keys of trusted certificates. The peer is trusted if its certificate is signed (ultimately) by one of the provided certificates. Using the FileName system(root_certificates) uses a list of trusted root certificates as provided by the OS. See system_root_certificates/1 for details.

cacerts(+ListOfCATerms)

Specify a list of sources of trusted certificates. Each element in the list should be one of the following:

• file(Filename): A file containing one or more PEM-encoded certificates
• certificate(Blob): A certificate blob
• system(root_certificates): A special term which refers to the certificates trusted by the host OS.

Additional verification of the peer certificate as well as accepting certificates that are not trusted by the given set can be realised using the hook cert_verify_hook(:Goal).

cert_verify_hook(:Goal)

The predicate ssl_negotiate/5 calls Goal as follows:

call(Goal, +SSL,
     +ProblemCertificate, +AllCertificates, +FirstCertificate,
     +Error)

In case the certificate was verified by one of the provided certifications from the cacert_file option, Error is unified with the atom verified. Otherwise it contains the error string passed from OpenSSL. Access will be granted iff the predicate succeeds. See load_certificate/2 for a description of the certificate terms. See cert_accept_any/5 for a dummy implementation that accepts any certificate.

cipher_list(+Atom)

Specify a cipher preference list (one or more cipher strings separated by colons, commas or spaces). See ssl_secure_ciphers/1.

ecdh_curve(+Atom)

Specify a curve for ECDHE ciphers. If this option is not specified, the OpenSSL default parameters are used. With OpenSSL prior to 1.1.0, prime256v1 is used by default.

peer_cert(+Boolean)

Trigger the request of our peer's certificate while establishing the SSL layer. This option is automatically turned on in a client SSL socket. It can be used in a server to ask the client to identify itself using an SSL certificate.

close_parent(+Boolean)

If true, close the raw streams if the SSL streams are closed. Default is false.

close_notify(+Boolean)

If true (default is false), the server sends TLS close_notify when closing the connection. In addition, this mitigates truncation attacks for both client and server role: If EOF is encountered without having received a TLS shutdown, an exception is raised. Well-designed protocols are self-terminating, and this attack is therefore very rarely a concern.

min_protocol_version(+Atom)

Set the minimum protocol version that can be negotiated. Atom is one of sslv3, tlsv1, tlsv1_1, tlsv1_2 and tlsv1_3. This option is available with OpenSSL 1.1.0 and later, and should be used instead of disable_ssl_methods/1.

max_protocol_version(+Atom)

Set the maximum protocol version that can be negotiated. Atom is one of sslv3, tlsv1, tlsv1_1, tlsv1_2 and tlsv1_3. This option is available with OpenSSL 1.1.0 and later, and should be used instead of disable_ssl_methods/1.

disable_ssl_methods(+List)

A list of methods to disable. Unsupported methods will be ignored. Methods include sslv2, sslv3, sslv23, tlsv1, tlsv1_1 and tlsv1_2. This option is deprecated starting with OpenSSL 1.1.0. Use min_protocol_version/1 and max_protocol_version/1 instead.

ssl_method(+Method)

Specify the explicit Method to use when negotiating. For allowed values, see the list for disable_ssl_methods above. Using this option is discouraged. When using OpenSSL 1.1.0 or later, this option is ignored, and a version-flexible method is used to negotiate the connection. Using version-specific methods is deprecated in recent OpenSSL versions, and this option will become obsolete and ignored in the future.

sni_hook(:Goal)

This option provides Server Name Indication (SNI) for SSL servers. This means that depending on the host to which a client connects, different options (certificates etc.) can be used for the server. This TLS extension allows you to host different domains using the same IP address and physical machine. When a TLS connection is negotiated with a client that has provided a host name via SNI, the hook is called as follows:

call(Goal, +SSL0, +HostName, -SSL)

Given the current context SSL0, and the host name of the client request, the predicate computes SSL which is used as the context for negotiating the connection. The first solution is used. If the predicate fails, the default options are used, which are those of the encompassing ssl_context/3 call. In that case, if no default certificate and key are specified, the client connection is rejected.

alpn_protocols(+ListOfProtoIdentifiers)

Provide a list of acceptable ALPN protocol identifiers as atoms. ALPN support requires OpenSSL 1.0.2 or greater.

alpn_protocol_hook(:Goal)

This options provides a callback for a server context to use to select an ALPN protocol. It will be called as follows:

=== call(Goal, +SSLCtx0, +ListOfClientProtocols, -SSLCtx1, -SelectedProtocol) ===

If this option is unset and the alpn_protocols/1 option is set, then the first common protocol between client & server will be selected.

Arguments:

Role	- is one of server or client and denotes whether the SSL instance will have a server or client role in the established connection.
SSL	- is a SWI-Prolog blob of type ssl_context, i.e., the type-test for an SSL context is blob(SSL, ssl_context).

 ssl_upgrade_legacy_options(+OptionsIn, -Options) is det

Handle deprecated cacert_file(Spec) option and map it to the new cacerts(+List) option.

 ssl_add_certificate_key(+SSL0, +Certificate, +Key, -SSL)

Add an additional certificate/key pair to SSL0, yielding SSL. Certificate and Key are either strings or atoms that hold the PEM-encoded certificate plus certificate chain and private key, respectively. Using strings is preferred for security reasons.

This predicate allows dual-stack RSA and ECDSA servers (for example), and is an alternative for using the certificate_key_pairs/1 option. As of OpenSSL 1.0.2, multiple certificate types with completely independent certificate chains are supported. If a certificate of the same type is added repeatedly to a context, the result is undefined. Currently, up to 12 additional certificates of different types are admissible.

 ssl_set_options(+SSL0, -SSL, +Options)

SSL is the same as SSL0, except for the options specified in Options. The following options are supported: close_notify/1, close_parent/1, host/1, peer_cert/1, ecdh_curve/1, min_protocol_version/1, max_protocol_version/1, disable_ssl_methods/1, sni_hook/1, cert_verify_hook/1, alpn_protocols/1, and alpn_protocol_hook/1. See ssl_context/3 for more information about these options. This predicate allows you to tweak existing SSL contexts, which can be useful in hooks when creating servers with the HTTP infrastructure.

 ssl_property(+SSL, ?Property) is semidet

True when Property is a property of SSL. Defined properties are:

• close_parent(?Bool)

To be done

- This version is a very minimal implementation of the generic property interface. Future versions will add more properties and non-determinism.

 ssl_negotiate(+SSL, +PlainRead, +PlainWrite, -SSLRead, -SSLWrite) is det

Once a connection is established and a read/write stream pair is available, (PlainRead and PlainWrite), this predicate can be called to negotiate an SSL session over the streams. If the negotiation is successful, SSLRead and SSLWrite are returned.

After a successful handshake and finishing the communication the user must close SSLRead and SSLWrite, for example using call_cleanup(close(SSLWrite), close(SSLRead)). If the SSL context (created with ssl_context/3 has the option close_parent(true) (default false), closing SSLRead and SSLWrite also closes the original PlainRead and PlainWrite streams. Otherwise these must be closed explicitly by the user.

Errors

- ssl_error(Code, LibName, FuncName, Reason) is raised if the negotiation fails. The streams PlainRead and PlainWrite are not closed, but an unknown amount of data may have been read and written.

 ssl_peer_certificate(+Stream, -Certificate) is semidet

True if the peer certificate is provided (this is always the case for a client connection) and Certificate unifies with the peer certificate. The example below uses this to obtain the Common Name of the peer after establishing an https client connection:

  http_open(HTTPS_url, In, []),
  ssl_peer_certificate(In, Cert),
  memberchk(subject(Subject), Cert),
  memberchk('CN' = CommonName), Subject)

 ssl_peer_certificate_chain(+Stream, -Certificates) is det

Certificates is the certificate chain provided by the peer, represented as a list of certificates.

 ssl_session(+Stream, -Session) is det

Retrieves (debugging) properties from the SSL context associated with Stream. If Stream is not an SSL stream, the predicate raises a domain error. Session is a list of properties, containing the members described below. Except for Version, all information are byte arrays that are represented as Prolog strings holding characters in the range 0..255.

ssl_version(Version)

The negotiated version of the session as an integer.

cipher(Cipher)

The negotiated cipher for this connection.

session_key(Key)

The key material used in SSLv2 connections (if present).

master_key(Key)

The key material comprising the master secret. This is generated from the server_random, client_random and pre-master key.

client_random(Random)

The random data selected by the client during handshaking.

server_random(Random)

The random data selected by the server during handshaking.

session_id(SessionId)

The SSLv3 session ID. Note that if ECDHE is being used (which is the default for newer versions of OpenSSL), this data will not actually be sent to the server.

alpn_protocol(Protocol)

The negotiated ALPN protocol, if supported. If no protocol was negotiated, this will be an empty string.

 load_certificate(+Stream, -Certificate) is det

Loads a certificate from a PEM- or DER-encoded stream, returning a certificate. The fields of the certificate can be inspected using certificate_field(+Certificate, ?Field).

Note that the OpenSSL CA.pl utility creates certificates that have a human readable textual representation in front of the PEM representation. You can use the following to skip to the certificate if you know it is a PEM certificate:

skip_to_pem_cert(In) :-
      repeat,
      (   peek_char(In, '-')
      ->  !
      ;   skip(In, 0'\n),
          at_end_of_stream(In), !
      ).

 write_certificate(+Stream, +Certificate, +Options) is det

Writes a certificate to the stream Stream. Options is reserved for future use.

 load_crl(+Stream, -CRL) is det

Loads a CRL from a PEM- or DER-encoded stream, returning a term containing terms hash/1, signature/1, issuer_name/1 and revocations/1, which is a list of revoked/2 terms. Each revoked/2 term is of the form revoked(+Serial, DateOfRevocation)

 system_root_certificates(-List) is det

List is a list of trusted root certificates as provided by the OS. This is the list used by ssl_context/3 when using the option system(root_certificates). The list is obtained using an OS specific process. The current implementation is as follows:

• On Windows, CertOpenSystemStore() is used to import the "ROOT" certificates from the OS.
• On MacOSX, the trusted keys are loaded from the SystemRootCertificates key chain. The Apple API for this requires the SSL interface to be compiled with an XCode compiler, i.e., not with native gcc.
• Otherwise, certificates are loaded from a file defined by the Prolog flag system_cacert_filename. The initial value of this flag is operating system dependent. For security reasons, the flag can only be set prior to using the SSL library. For example:

  :- use_module(library(ssl)).
  :- set_prolog_flag(system_cacert_filename,
                     '/home/jan/ssl/ca-bundle.crt').

 load_private_key(+Stream, +Password, -PrivateKey) is det

Load a private key PrivateKey from the given stream Stream, using Password to decrypt the key if it is encrypted. Note that the password is currently only supported for PEM files. DER-encoded keys which are password protected will not load. The key must be an RSA or EC key. DH and DSA keys are not supported, and PrivateKey will be bound to an atom (dh_key or dsa_key) if you try and load such a key. Otherwise PrivateKey will be unified with private_key(KeyTerm) where KeyTerm is an rsa/8 term representing an RSA key, or ec/3 for EC keys.

 load_public_key(+Stream, -PublicKey) is det

Load a public key PublicKey from the given stream Stream. Supports loading both DER- and PEM-encoded keys. The key must be an RSA or EC key. DH and DSA keys are not supported, and PublicKey will be bound to an atom (dh_key or dsa_key) if you try and load such a key. Otherwise PublicKey will be unified with public_key(KeyTerm) where KeyTerm is an rsa/8 term representing an RSA key, or ec/3 for EC keys.

 cert_accept_any(+SSL, +ProblemCertificate, +AllCertificates, +FirstCertificate, +Error) is det

Implementation for the hook `cert_verify_hook(:Hook)` that accepts any certificate. This is intended for http_open/3 if no certificate verification is desired as illustrated below.

  http_open('https:/...', In,
            [ cert_verify_hook(cert_accept_any)
            ])

 same_certificate(+CertificateA, +CertificateB)

True if CertificateA is logically the same as CertificateB, even if they are stored in different blobs

 verify_certificate_issuer(+Certificate, +Issuer)

True if Certificate is a certificate which was issued by the certificate Issuer.

 verify_certificate(+Certificate, +AuxiliaryCertificates, +TrustedCertificates)

True if it is possible to build a chain of trust from Certificate to one of the certificates in TrustedCertificates, optionally using the (untrusted) certificates in AuxiliaryCertificates to complete the chain. To use the system built-in trust store, specify the special term system(root_certificates) for TrustedCertificates.

 certificate_field(+Certificate, ?Field) is nondet

Retrieve the field matching Field from Certificate. May be one of the following:

• subject/1 to retrieve the subject
• issuer/1 to retrieve the issuer's subject
• version/1 to retrieve the version
• serial/1 to retrieve the serial number
• not_before/1 to retrieve the start date
• not_after/1 to retrieve the expiry date
• public_key/1 to retrieve the public key
• crls/1 to retrieve a list of the CRLs
• sans/1 to retrieve a list of the Subject Alternative Names
• signature/1 to retrieve the certificate signature
• signature_algorithm/1 to retrieve the signing algorithm
• hash/1 to retrieve the certificate hash
• to_be_signed/1 to retrieve the data on the certificate which must be signed

 ssl_secure_ciphers(-Ciphers:atom) is det

Ciphers is a secure cipher preference list that can be used in the cipher_list/1 option of ssl_context/3.

Secure ciphers must guarantee forward secrecy, and must mitigate all known critical attacks. As of 2018, using these ciphers allows you to obtain grade A on https://www.ssllabs.com. For A+, you must also enable HTTP Strict Transport Security (HSTS) by sending a suitable header field in replies.

Note that obsolete ciphers must be disabled to reliably prevent protocol downgrade attacks.

The Ciphers list is read from the setting ssl:secure_ciphers and can be controlled using set_setting/2 and other predicates from library(settings).

BEWARE: This list must be changed when attacks on these ciphers become known! Keep an eye on this setting and adapt it as necessary in the future.