Credentials Manager
==================================================
A ``Credentials_Manager`` is a way to abstract how the application
stores credentials. The main user is the :doc:`tls` implementation.
.. cpp:class:: Credentials_Manager
.. cpp:function:: std::vector<Certificate_Store*> \
trusted_certificate_authorities( \
const std::string& type, \
const std::string& context)
Return the list of certificate stores, each of which is assumed
to contain (only) trusted certificate authorities. The
``Credentials_Manager`` retains ownership of the
Certificate_Store pointers.
.. note::
It would have been a better API to return a vector of
``shared_ptr`` here. This may change in a future major release.
When *type* is "tls-client", *context* will be the hostname of
the server, or empty if the hostname is not known. This allows
using a different set of certificate stores in different contexts,
for example using the system certificate store unless contacting
one particular server which uses a cert issued by an internal CA.
When *type* is "tls-server", the *context* will again be the
hostname of the server, or empty if the client did not send a
server name indicator. For TLS servers, these CAs are the ones
trusted for signing of client certificates. If you do not want
the TLS server to ask for a client cert,
``trusted_certificate_authorities`` should return an empty list
for *type* "tls-server".
The default implementation returns an empty list.
.. cpp:function:: std::vector<X509_Certificate> find_cert_chain( \
const std::vector<std::string>& cert_key_types, \
const std::vector<X509_DN>& acceptable_CAs, \
const std::string& type, \
const std::string& context)
Return the certificate chain to use to identify ourselves. The
``acceptable_CAs`` parameter gives a list of CAs the peer trusts.
This may be empty.
.. warning::
If this function returns a certificate that is not one of the
types given in ``cert_key_types`` confusing handshake
failures will result.
.. cpp:function:: std::vector<X509_Certificate> cert_chain( \
const std::vector<std::string>& cert_key_types, \
const std::string& type, \
const std::string& context)
Return the certificate chain to use to identify ourselves. Starting in
2.5, prefer ``find_cert_chain`` which additionally provides the CA list.
.. cpp:function:: std::vector<X509_Certificate> cert_chain_single_type( \
const std::string& cert_key_type, \
const std::string& type, \
const std::string& context)
Return the certificate chain to use to identifier ourselves, if
we have one of type *cert_key_type* and we would like to use a
certificate in this *type*/*context*.
.. cpp:function:: Private_Key* private_key_for(const X509_Certificate& cert, \
const std::string& type, \
const std::string& context)
Return the private key for this certificate. The *cert* will be
the leaf cert of a chain returned previously by ``cert_chain``
or ``cert_chain_single_type``.
In versions before 1.11.34, there was an additional function on `Credentials_Manager`
.. cpp::function:: void verify_certificate_chain( \
const std::string& type, \
const std::string& hostname, \
const std::vector<X509_Certificate>& cert_chain)
This function has been replaced by `TLS::Callbacks::tls_verify_cert_chain`.
SRP Authentication
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
``Credentials_Manager`` contains the hooks used by TLS clients and
servers for SRP authentication.
.. note::
Support for TLS-SRP is deprecated, and will be removed in a future
major release. When that occurs these APIs will be removed. Prefer
instead performing a standard TLS handshake, then perform a PAKE
authentication inside of (and cryptographically bound to) the TLS
channel.
.. cpp:function:: bool attempt_srp(const std::string& type, \
const std::string& context)
Returns if we should consider using SRP for authentication
.. cpp:function:: std::string srp_identifier(const std::string& type, \
const std::string& context)
Returns the SRP identifier we'd like to use (used by client)
.. cpp:function:: std::string srp_password(const std::string& type, \
const std::string& context, \
const std::string& identifier)
Returns the password for *identifier* (used by client)
.. cpp:function:: bool srp_verifier(const std::string& type, \
const std::string& context, \
const std::string& identifier, \
std::string& group_name, \
BigInt& verifier, \
std::vector<uint8_t>& salt, \
bool generate_fake_on_unknown)
Returns the SRP verifier information for *identifier* (used by server)
Preshared Keys
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TLS supports the use of pre shared keys for authentication.
.. cpp:function:: SymmetricKey psk(const std::string& type, \
const std::string& context, \
const std::string& identity)
Return a symmetric key for use with *identity*
One important special case for ``psk`` is where *type* is
"tls-server", *context* is "session-ticket" and *identity* is an
empty string. If a key is returned for this case, a TLS server
will offer session tickets to clients who can use them, and the
returned key will be used to encrypt the ticket. The server is
allowed to change the key at any time (though changing the key
means old session tickets can no longer be used for resumption,
forcing a full re-handshake when the client next connects). One
simple approach to add support for session tickets in your server
is to generate a random key the first time ``psk`` is called to
retrieve the session ticket key, cache it for later use in the
``Credentials_Manager``, and simply let it be thrown away when the
process terminates. See :rfc:`4507` for more information about TLS
session tickets.
A similar special case exists for DTLS cookie verification. In
this case *type* will be "tls-server" and *context* is
"dtls-cookie-secret". If no key is returned, then DTLS cookies are
not used. Similar to the session ticket key, the DTLS cookie
secret can be chosen during server startup and rotated at any time
with no ill effect.
.. warning::
If DTLS cookies are not used then the server is prone to be
abused as a DoS amplifier, where the attacker sends a
relatively small client hello in a UDP packet with a forged
return address, and then the server replies to the victim with
several messages that are larger. This not only hides the
attackers address from the victim, but increases their
effective bandwidth. This is not an issue when using DTLS over
SCTP or TCP.
.. cpp:function:: std::string psk_identity_hint(const std::string& type, \
const std::string& context)
Returns an identity hint which may be provided to the client. This
can help a client understand what PSK to use.
.. cpp:function:: std::string psk_identity(const std::string& type, \
const std::string& context, \
const std::string& identity_hint)
Returns the identity we would like to use given this *type* and
*context* and the optional *identity_hint*. Not all servers or
protocols will provide a hint.