FreeRDP/FreeRDP — pull request #2273 by nfedera (2014-12-12 01:09:12+00:00)
Title: Standard RDP Security Layer Levels/Method Overhaul

[MS-RDPBCGR] Section 5.3 describes the encryption level and method values for standard RDP security.
Looking at the current usage of these values in the FreeRDP code gives me reason to believe that there is a certain lack of understanding of how these values should be handled.

The encryption level is only configured on the server side in the "Encryption Level" setting found in the Remote Desktop Session Host Configuration RDP-Tcp properties dialog.
This value is never transferred from the client to the server over the wire!
The possible options are "None", "Low", "Client Compatible", "High" and "FIPS Compliant".
The client receices this value in the Server Security Data block (TS_UD_SC_SEC1), probably only for informational purposes and maybe to give the client the possibility to verify if the server's decision for the encryption method confirms to the server's encryption level.
The possible encryption methods are "NONE", "40BIT", "56BIT", "128BIT" and "FIPS" and the RDP client advertises the ones it supports to the server in the Client Security Data block (TS_UD_CS_SEC).
The server's configured encryption level value restricts the possible final encryption method.
Something that I was not able to find in the documentation is the priority level of the individual encryption methods based on which the server makes its final method decision if there are several options.
My analysis with Windows Servers reveiled that the order is 128, 56, 40, FIPS.
The server only chooses FIPS if the level is "FIPS Comliant" or if it is the only method advertised by the client.

Bottom line:
* FreeRDP's client side does not need to set settings->EncryptionLevel (which was done quite frequently).
* FreeRDP's server side does not have to set the supported encryption methods list in settings->EncryptionMethods

Changes in this commit:

Removed unnecessary/confusing changes of EncryptionLevel/Methods settings

Refactor settings->DisableEncryption
* This value actually means "Advanced RDP Encryption (NLA/TLS) is NOT used"
* The old name caused lots of confusion among developers
* Renamed it to "UseRdpSecurityLayer" (the compare logic stays untouched)

Any client's setting of settings->EncryptionMethods were annihilated
* All clients "want" to set all supported methods
* Some clients forgot 56bit because 56bit was not supported at the time the code was written
* settings->EncryptionMethods was overwritten anyways in nego_connect()
* Removed all client side settings of settings->EncryptionMethods. The default is "None" (0)
* Changed nego_connect() to advertise all supported methods if settings->EncryptionMethods is 0 (None)
* Added a commandline option /encryption-methods:comma separated list of the values "40", "56", "128", "FIPS". E.g. /encryption-methods:56,128
* Print warning if server chooses non-advertised method

Verify received level and method in client's gcc_read_server_security_data
* Only accept valid/known encryption methods
* Verify encryption level/method combinations according to MS-RDPBCGR 5.3.2

Server implementations can now set settings->EncryptionLevel
* The default for settings->EncryptionLevel is 0 (None)
* nego_send_negotiation_response() changes it to ClientCompatible in that case
* default to ClientCompatible if the server implementation set an invalid level

Fix server's gcc_write_server_security_data
* Verify server encryption level value set by server implementations
* Choose rdp encryption method based on level and supported client methods
* Moved FIPS to the lowest priority (only used if other methods are possible)

Updated sample server
* Support RDP Security (RdpKeyFile was not set)
* Added commented sample code for setting the security level