FAB-5029 Document that there is no support for RSA keys

This CR updates documentation on MSP to make clear that in the
current MSP implementation there is no suppor for RSA.

Change-Id: Ib11c8039e0da20c34d38ce9c27c11ac35ed86a9e
Signed-off-by: Elli Androulaki <[email protected]>

Author: elli-androulaki

docs/source/msp.rst

@@ -84,7 +84,8 @@ In addition to verification related parameters, for the MSP to enable
 the node on which it is instantiated to sign or authenticate, one needs to
 specify:
 
-- The signing key used for signing by the node, and
+- The signing key used for signing by the node (currently only ECDSA keys are
+  supported), and
 - The node's X.509 certificate, that is a valid identity under the
   verification parameters of this MSP
 
@@ -96,7 +97,8 @@ How to generate MSP certificates and their signing keys?
 --------------------------------------------------------
 
 To generate X.509 certificates to feed its MSP configuration, the application
-can use `Openssl <https://www.openssl.org/>`_.
+can use `Openssl <https://www.openssl.org/>`_. We emphasise that in Hyperledger
+Fabric there is no support for certificates including RSA keys.
 
 Alternatively one can use ``cryptogen`` tool, whose operation is explained in
 :doc:`getting_started`.
@@ -127,7 +129,8 @@ and a file:
    ``OrganizationalUnitIdentifier`` represents the actual string as
    expected to appear in X.509 certificate OU-field (e.g. "COP")
 5. (optional) a folder ``crls`` to include the considered CRLs
-6. a folder ``keystore`` to include a PEM file with the node's signing key
+6. a folder ``keystore`` to include a PEM file with the node's signing key;
+   we emphasise that currently RSA keys are not supported
 7. a folder ``signcerts`` to include a PEM file with the node's X.509
    certificate
 8. (optional) a folder ``tlscacerts`` to include PEM files each corresponding to a TLS root