- See also
An SSL server and client can be built with the (abstracted) predicate
calls from the table below. The
tcp_ predicates are
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
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
- 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.
- For the client, the host to which it connects. This option
should be specified when Role is
Otherwise, certificate verification may fail when negotiating a secure
- Specify where the certificate file can be found. This can be the same as
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
to add certificates and keys to an existing context.
- 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
- 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.
- 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.
- 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.
- 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
and provide neither of these options, verification will necessarily fail
- 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.
- 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
uses a list of trusted root certificates as provided by the OS. See
- 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
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).
- The predicate ssl_negotiate/5
calls Goal as follows:
+ProblemCertificate, +AllCertificates, +FirstCertificate,
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.
- Specify a cipher preference list (one or more cipher strings separated
by colons, commas or spaces). See ssl_secure_ciphers/1.
- 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,
is used by default.
- 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.
true, close the raw streams if the SSL
streams are closed. Default is
true (default is
false), the server sends
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.
- Set the minimum protocol version that can be negotiated.
Atom is one of
tlsv1_3. This option is available with OpenSSL 1.1.0 and
later, and should be used instead of disable_ssl_methods/1.
- Set the maximum protocol version that can be negotiated.
Atom is one of
tlsv1_3. This option is available with OpenSSL 1.1.0 and
later, and should be used instead of disable_ssl_methods/1.
- A list of methods to disable. Unsupported methods will be ignored.
option is deprecated starting with OpenSSL 1.1.0. Use min_protocol_version/1
- Specify the explicit Method to use when negotiating. For
allowed values, see the list for
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.
- 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.
- Provide a list of acceptable ALPN protocol identifiers as atoms. ALPN
support requires OpenSSL 1.0.2 or greater.
- 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.
|Role ||is one of |
and denotes whether the
SSL instance will have a server or client role in the
|SSL ||is a SWI-Prolog blob of type |
i.e., the type-test for an SSL context is
- Handle deprecated
cacert_file(Spec) option and map it to
+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 is the same as SSL0, except for the options
Options. The following options are supported: close_notify/1,
close_parent/1, host/1, peer_cert/1, ecdh_curve/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.
- True when Property is a property of SSL. Defined
- To be done
- This version is a very minimal implementation of the generic property
interface. Future versions will add more properties and non-determinism.
+PlainRead, +PlainWrite, -SSLRead, -SSLWrite)
- 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
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
false), closing SSLRead
SSLWrite also closes the original PlainRead and PlainWrite
streams. Otherwise these must be closed explicitly by the user.
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
- 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
http_open(HTTPS_url, In, ),
memberchk('CN' = CommonName), Subject)
- Certificates is the certificate chain provided by the peer,
represented as a list of certificates.
- 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.
- The negotiated version of the session as an integer.
- The negotiated cipher for this connection.
- The key material used in SSLv2 connections (if present).
- The key material comprising the master secret. This is generated from
the server_random, client_random and pre-master key.
- The random data selected by the client during handshaking.
- The random data selected by the server during handshaking.
- 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.
- The negotiated ALPN protocol, if supported. If no protocol was
negotiated, this will be an empty string.
- Loads a certificate from a PEM- or DER-encoded stream, returning a
certificate. The fields of the certificate can be inspected using
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:
( peek_char(In, '-')
; skip(In, 0'\n),
- Writes a certificate to the stream Stream. Options
is reserved for future use.
- Loads a CRL from a PEM- or DER-encoded stream, returning a
term containing terms hash/1, signature/1, issuer_name/1
revocations/1, which is a list of revoked/2
revoked/2 term is of the form
- 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:
- 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
where KeyTerm is an rsa/8 term
representing an RSA key, or ec/3 for EC
- 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
public_key(KeyTerm) where KeyTerm is an rsa/8
term representing an RSA key, or ec/3 for
+ProblemCertificate, +AllCertificates, +FirstCertificate, +Error)
- 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.
- True if CertificateA is logically the same as CertificateB,
even if they are stored in different blobs
- True if Certificate is a certificate which was issued by the
- 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.
- 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
- version/1 to retrieve the version
- serial/1 to retrieve the serial number
- not_before/1 to retrieve the start
- not_after/1 to retrieve the expiry
- public_key/1 to retrieve the public
- 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
- signature_algorithm/1 to retrieve the
- hash/1 to retrieve the certificate
- to_be_signed/1 to retrieve the data on
the certificate which must be signed
- Ciphers is a secure cipher preference list that can be used
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
and can be controlled using set_setting/2
and other predicates from
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.