Overview of the Crypto Kit updates in Citrix Receiver 4.12 and above

• Receiver for Windows 4.12 and later provide support to DTLS v1.2 for connections to the VDA.
• The latest Crypto Kit has deprecated all TLS_RSA_* cipher suites. But, to support backward compatibility with older versions of VDA (before ver.7.18), we provide an option in the GPO to enable/disable these cipher suites. We give the following toggle options on the GPO:
  • Enable/Disable all (flag 1) TLS_RSA_ cipher suites (Totally 8 cipher suites)
  • Enable/Disable (flag 2) TLS_RSA_WITH_RC4_128_MD5 cipher suite
  • Enable/Disable (flag 3) TLS_RSA_WITH_RC4_128_SHA cipher suite

Steps to Disable/Re-Enable Deprecated Ciphers

1. Add the Receiver GPO template if it is not added to the local GPO. Refer to the Product Documentation for detailed instructions. In case of an upgrade, the existing settings are retained when the latest files are imported.
2. Open the Citrix Receiver GPO administrative template by running gpedit.msc
3. Under the Computer Configuration node, go to Administrative Template > Citrix Component > Citrix Receiver > Network Routing > Deprecated Cipher Suites
4. Use the toggle options to Enable/Disable the ciphers
   

Note:

• Enabling TLS_RSA_ flag (flag 1) alone enables 6 cipher suites.
• Although RC4-128-MD5 (flag 2) and RC4-128-SHA (flag 3) are subsets of TLS_RSA_, additional flags are given for the two ciphers.
• This means, if it is intended to enable RC4-128-MD5 cipher suite, both flags 1 and 2 should be enabled. Similarly, to enable RC4-128-SHA cipher suite, both flags 1 and 3 should be enabled.
• To enable or disable DTLS, you can configure the HDX Adaptive Transport policy on the DDC. For more information, you can refer https://docs.citrix.com/en-us/xenapp-and-xendesktop/7-13/hdx/adaptive-transport.html

Please find below the cipher matrix for a deeper understanding of the ciphers supported by the latest SSL SDK.

Ciphersuite (in order of priority)	Native Crypto Kit mode and cipher set
	OPEN			FIPS			SP800-52
	OPEN ALL	OPEN COM	OPEN GOV	FIPS ALL	FIPS COM	FIPS GOV	SP800-52 ALL	SP800-52 COM	SP800-52 GOV	TLS 1.2 PRF uses	TLS 1.2 "Finished" uses
TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA3841)	X		X	X		X	X		X	SHA384	SHA384
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA3841)	X		X	X		X	X		X	SHA384	SHA384
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA	X	X		X	X		X	X		SHA256	SHA1
TLS_RSA_WITH_AES_256_GCM_SHA3841)2)	Δ									SHA384	SHA384
TLS_RSA_WITH_AES_128_GCM_SHA2561)2)	Δ	Δ								SHA256	SHA256
TLS_RSA_WITH_AES_256_CBC_SHA2561)2)	Δ									SHA256	SHA1
TLS_RSA_WITH_AES_256_CBC_SHA2)	Δ									SHA256	SHA1
TLS_RSA_WITH_AES_128_CBC_SHA2)	Δ	Δ								SHA256	SHA1
TLS_RSA_WITH_RC4_128_SHA2)3)	Δ	Δ								SHA256	SHA1
TLS_RSA_WITH_RC4_128_MD52)3)	Δ	Δ								SHA256	SHA1
TLS_RSA_WITH_3DES_EDE_CBC_SHA2)	Δ									SHA256	SHA1
TLS_EMPTY_RENEGOTIATION_INFO_SCSV*	X	X	X	X	X	X	X	X	X	-	-

Expected failure scenarios and corner cases

In the case you disable deprecated cipher suites, make sure to avoid the following failure scenarios:

• TCP
  • OPEN mode: Session launch not supported when Client configures cipher set to GOV and VDA has cipher set configured to COM (due to lack of common cipher suite).
  • FIPS/NIST(SP800-52) compliance mode: Session launch is not supported in the scenarios when Cipher set is configured as COM on the server with any of the cipher sets(COM/GOV/ANY) on the client and vice-versa due to lack of common cipher suites.
• DTLS v1.0 supports 5 of the cipher suites and DTLS v1.2 supports 10 of the cipher suites.

  DTLS v1.0	DTLS v1.2
  TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA	TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
  TLS_RSA_WITH_AES_256_CBC_SHA	TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384
  TLS_RSA_WITH_AES_128_CBC_SHA	TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
  TLS_RSA_WITH_3DES_EDE_CBC_SHA	TLS_RSA_WITH_AES_256_GCM_SHA384
  TLS_EMPTY_RENEGOTIATION_INFO_SCSV	TLS_RSA_WITH_AES_128_GCM_SHA256
  	TLS_RSA_WITH_AES_256_CBC_SHA256
  	TLS_RSA_WITH_AES_256_CBC_SHA
  	TLS_RSA_WITH_AES_128_CBC_SHA
  	TLS_RSA_WITH_3DES_EDE_CBC_SHA
  	TLS_EMPTY_RENEGOTIATION_INFO_SCSV

Hence, session launch is unsupported from GOV on client to COM on VDA here as well, and the fallback to TCP is also unsupported as mentioned above. In the case where DTLS v1.0 is used, session launch is not supported with GOV mode on client, owing to absence of cipher suites to offer.

• ​DTLS does not support FIPS/NIST compliance modes.
• DTLS v1.2 is supported by OSs Win10(1607 or higher) AND Win2k16 VDAs. Please refer article https://support.citrix.com/article/CTX230010
• DTLS v1.2 is not supported yet by NSG. Hence, this scenario can only be tested with DTLS v1.0.

Please find below the matrix containing the details:
1) Matrix for internal network connections (without NSG in picture)
2) Matrix for external network connections (with NSG in picture)

 

This article presents the overview of the new crypto kit introduced in Receiver for Windows 4.12 and provide expected failure scenarios and corner cases in order for users to avoid unwanted situations.