How to restrict the use of certain cryptographic algorithms and protocols in Schannel.dll

The following cryptographic service providers (CSPs) that are included with Windows NT 4.0 Service Pack 6 were awarded the certificates for FIPS-140-1 crypto validation:

• Microsoft Base Cryptographic Provider (Rsabase.dll)
• Microsoft Enhanced Cryptographic Provider (Rsaenh.dll) (non-export version)

Microsoft TLS/SSL Security Provider, the Schannel.dll file, uses the CSPs that are listed here to conduct secure communications over SSL or TLS in its support for Internet Explorer and Internet Information Services (IIS).

You can change the Schannel.dll file to support Cipher Suite 1 and 2. However, the program must also support Cipher Suite 1 and 2. Cipher Suite 1 and 2 are not supported in IIS 4.0 and 5.0.

This article contains the necessary information to configure the TLS/SSL Security Provider for Windows NT 4.0 Service Pack 6 and later versions. You can use the Windows registry to control the use of specific SSL 3.0 or TLS 1.0 cipher suites with respect to the cryptographic algorithms that are supported by the Base Cryptographic Provider or the Enhanced Cryptographic Provider.

Note In Windows NT 4.0 Service Pack 6, the Schannel.dll file does not use the Microsoft Base DSS Cryptographic Provider (Dssbase.dll) or the Microsoft DS/Diffie-Hellman Enhanced Cryptographic Provider (Dssenh.dll).

Cipher suites

Both SSL 3.0 (http://www.mozilla.org/projects/security/pki/nss/ssl/draft302.txt) and TLS 1.0 (RFC2246) with INTERNET-DRAFT "56-bit Export Cipher Suites For TLS draft-ietf-tls-56-bit-ciphersuites-00.txt" provide options to use different cipher suites. Each cipher suite determines the key exchange, authentication, encryption, and MAC algorithms that are used in an SSL/TLS session. Notice that when you use RSA as both key exchange and authentication algorithms, the term RSA appears only one time in the corresponding cipher suite definitions.

The Windows NT 4.0 Service Pack 6 Microsoft TLS/SSL Security Provider supports the following SSL 3.0-defined "CipherSuite" when you use the Base Cryptographic Provider or the Enhanced Cryptographic Provider:

Note Neither SSL_RSA_EXPORT1024_WITH_DES_CBC_SHA nor SSL_RSA_EXPORT1024_WITH_RC4_56_SHA is defined in SSL 3.0 text. However, several SSL 3.0 vendors support them. This includes Microsoft.

Windows NT 4.0 Service Pack 6 Microsoft TLS/SSL Security Provider also supports the following TLS 1.0-defined "CipherSuite" when you use the Base Cryptographic Provider or Enhanced Cryptographic Provider:

Note A cipher suite that is defined by using the first byte "0x00" is non-private and is used for open interoperable communications. Therefore, the Windows NT 4.0 Service Pack 6 Microsoft TLS/SSL Security Provider follows the procedures for using these cipher suites as specified in SSL 3.0 and TLS 1.0 to make sure of interoperability.

Schannel-specific registry keys

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base: Note Any changes to the contents of the CIPHERS key or the HASHES key take effect immediately, without a system restart.

SCHANNEL key

Start Registry Editor (Regedt32.exe), and then locate the following registry key:

SCHANNEL\Protocols subkey

To enable the system to use the protocols that will not be negotiated by default (such as TLS 1.1 and TLS 1.2), change the DWORD value data of the DisabledByDefault value to 0x0 in the following registry keys under the Protocols key:

• SCHANNEL\Protocols\TLS 1.1\Client
• SCHANNEL\Protocols\TLS 1.1\Server
• SCHANNEL\Protocols\TLS 1.2\Client
• SCHANNEL\Protocols\TLS 1.2\Server

Warning The DisabledByDefault value in the registry keys under the Protocols key does not take precedence over the grbitEnabledProtocols value that is defined in the SCHANNEL_CRED structure that contains the data for an Schannel credential.

SCHANNEL\Ciphers subkey

The Ciphers registry key under the SCHANNEL key is used to control the use of symmetric algorithms such as DES and RC4. The following are valid registry keys under the Ciphers key.

SCHANNEL\Ciphers\RC4 128/128 subkey

RC4 128/128


This subkey refers to 128-bit RC4.


To allow this cipher algorithm, change the DWORD value data of the Enabled value to 0xffffffff. Or, change the DWORD value data to 0x0. If you do not configure the Enabled value, the default is enabled. This registry key does not apply to an exportable server that does not have an SGC certificate.

Disabling this algorithm effectively disallows the following:

• SSL_RSA_WITH_RC4_128_MD5
• SSL_RSA_WITH_RC4_128_SHA
• TLS_RSA_WITH_RC4_128_MD5
• TLS_RSA_WITH_RC4_128_SHA

SCHANNEL\Ciphers\Triple DES 168

Triple DES 168

This registry key refers to 168-bit Triple DES as specified in ANSI X9.52 and Draft FIPS 46-3. This registry key does not apply to the export version.

To allow this cipher algorithm, change the DWORD value data of the Enabled value to 0xffffffff. Or, change the DWORD data to 0x0. If you do not configure the Enabled value, the default is enabled.

Disabling this algorithm effectively disallows the following:

• SSL_RSA_WITH_3DES_EDE_CBC_SHA
• SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA
• TLS_RSA_WITH_3DES_EDE_CBC_SHA
• TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA

Note For the versions of Windows that releases before Windows Vista, the key should be Triple DES 168/168.

SCHANNEL\Ciphers\RC2 128/128 subkey

RC2 128/128

This registry key refers to 128-bit RC2. It does not apply to the export version.

To allow this cipher algorithm, change the DWORD value data of the Enabled value to 0xffffffff. Otherwise, change the DWORD value data to 0x0. If you do not configure the Enabled value, the default is enabled.

SCHANNEL\Ciphers\RC4 64/128 subkey

RC4 64/128

This registry key refers to 64-bit RC4. It does not apply to the export version (but is used in Microsoft Money).

To allow this cipher algorithm, change the DWORD value data of the Enabled value to 0xffffffff. Otherwise, change the DWORD value data to 0x0. If you do not configure the Enabled value, the default is enabled.

SCHANNEL\Ciphers\RC4 56/128 subkey

RC4 56/128

This registry key refers to 56-bit RC4.

To allow this cipher algorithm, change the DWORD value data of the Enabled value to 0xffffffff. Otherwise, change the DWORD value data to 0x0. If you do not configure the Enabled value, the default is enabled.

Disabling this algorithm effectively disallows the following:

• TLS_RSA_EXPORT1024_WITH_RC4_56_SHA

SCHANNEL\Ciphers\RC2 56/128 subkey

RC2 56/128

This registry key refers to 56-bit RC2.

To allow this cipher algorithm, change the DWORD value data of the Enabled value to 0xffffffff. Otherwise, change the DWORD value data to 0x0. If you do not configure the Enabled value, the default is enabled.

SCHANNEL\Ciphers\RC2 56/56 subkey

DES 56

This registry key refers to 56-bit DES as specified in FIPS 46-2. Its implementation in the Rsabase.dll and Rsaenh.dll files is validated under the FIPS 140-1 Cryptographic Module Validation Program.

To allow this cipher algorithm, change the DWORD value data of the Enabled value to 0xffffffff. Otherwise, change the DWORD value data to 0x0. If you do not configure the Enabled value, the default is enabled.

Disabling this algorithm effectively disallows the following:

• SSL_RSA_WITH_DES_CBC_SHA
• TLS_RSA_WITH_DES_CBC_SHA

SCHANNEL\Ciphers\RC4 40/128 subkey

RC4 40/128

This refers to 40-bit RC4.

To allow this cipher algorithm, change the DWORD value data of the Enabled value to 0xffffffff. Otherwise, change the DWORD value data to 0x0. If you do not configure the Enabled value, the default is enabled.

Disabling this algorithm effectively disallows the following:

• SSL_RSA_EXPORT_WITH_RC4_40_MD5
• TLS_RSA_EXPORT_WITH_RC4_40_MD5

SCHANNEL\Ciphers\RC2 40/128 subkey

RC2 40/128

This registry key refers to 40-bit RC2.

To allow this cipher algorithm, change the DWORD value data of the Enabled value to 0xffffffff. Otherwise, change the DWORD value data to 0x0. If you do not configure the Enabled value, the default is enabled.

Disabling this algorithm effectively disallows the following:

• SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5
• TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5

SCHANNEL\Ciphers\NULL subkey

NULL

This registry key means no encryption. By default, it is turned off.

To turn off encryption (disallow all cipher algorithms), change the DWORD value data of the Enabled value to 0xffffffff. Otherwise, change the DWORD value data to 0x0.

SCHANNEL/Hashes subkey

The Hashes registry key under the SCHANNEL key is used to control the use of hashing algorithms such as SHA-1 and MD5. The following are valid registry keys under the Hashes key.

SCHANNEL\Hashes\MD5 subkey

MD5

To allow this hashing algorithm, change the DWORD value data of the Enabled value to the default value 0xffffffff. Otherwise, change the DWORD value data to 0x0.

Disabling this algorithm effectively disallows the following:

• SSL_RSA_EXPORT_WITH_RC4_40_MD5
• SSL_RSA_WITH_RC4_128_MD5
• SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5
• TLS_RSA_EXPORT_WITH_RC4_40_MD5
• TLS_RSA_WITH_RC4_128_MD5
• TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5

SCHANNEL\Hashes\SHA subkey

SHA

This registry key refers to Secure Hash Algorithm (SHA-1), as specified in FIPS 180-1. Its implementation in the Rsabase.dll and Rsaenh.dll files is validated under the FIPS 140-1 Cryptographic Module Validation Program.

To allow this hashing algorithm, change the DWORD value data of the Enabled value to the default value 0xffffffff. Otherwise, change the DWORD value data to 0x0.

Disabling this algorithm effectively disallows the following:

• SSL_RSA_WITH_RC4_128_SHA
• SSL_RSA_WITH_DES_CBC_SHA
• SSL_RSA_WITH_3DES_EDE_CBC_SHA
• SSL_RSA_EXPORT1024_WITH_DES_CBC_SHA
• SSL_RSA_EXPORT1024_WITH_RC4_56_SHA
• TLS_RSA_WITH_RC4_128_SHA
• TLS_RSA_WITH_DES_CBC_SHA
• TLS_RSA_WITH_3DES_EDE_CBC_SHA
• TLS_RSA_EXPORT1024_WITH_DES_CBC_SHA
• TLS_RSA_EXPORT1024_WITH_RC4_56_SHA

SCHANNEL/KeyExchangeAlgorithms subkey

The KeyExchangeAlgorithms registry key under the SCHANNEL key is used to control the use of key exchange algorithms such as RSA. The following are valid registry keys under the KeyExchangeAlgorithms key.

SCHANNEL\KeyExchangeAlgorithms\PKCS Subkey

PKCS

This registry key refers to the RSA as the key exchange and authentication algorithms.

To allow RSA, change the DWORD value data of the Enabled value to the default value 0xffffffff. Otherwise, change the DWORD data to 0x0.

Disabling RSA effectively disallows all RSA-based SSL and TLS cipher suites supported by the Windows NT4 SP6 Microsoft TLS/SSL Security Provider.

FIPS 140-1 cipher suites

You may want to use only those SSL 3.0 or TLS 1.0 cipher suites that correspond to FIPS 46-3 or FIPS 46-2 and FIPS 180-1 algorithms provided by the Microsoft Base or Enhanced Cryptographic Provider.

In this article, we refer to them as FIPS 140-1 cipher suites. Specifically, they are as follows:

• SSL_RSA_WITH_DES_CBC_SHA
• SSL_RSA_WITH_3DES_EDE_CBC_SHA
• SSL_RSA_EXPORT1024_WITH_DES_CBC_SHA
• TLS_RSA_WITH_DES_CBC_SHA
• TLS_RSA_WITH_3DES_EDE_CBC_SHA
• TLS_RSA_EXPORT1024_WITH_DES_CBC_SHA

To use only FIPS 140-1 cipher suites as defined here and supported by Windows NT 4.0 Service Pack 6 Microsoft TLS/SSL Security Provider with the Base Cryptographic Provider or the Enhanced Cryptographic Provider, configure the DWORD value data of the Enabled value in the following registry keys to 0x0:

• SCHANNEL\Ciphers\RC4 128/128
• SCHANNEL\Ciphers\RC2 128/128
• SCHANNEL\Ciphers\RC4 64/128
• SCHANNEL\Ciphers\RC4 56/128
• SCHANNEL\Ciphers\RC2 56/128
• SCHANNEL\Ciphers\RC4 40/128
• SCHANNEL\Ciphers\RC2 40/128
• SCHANNEL\Ciphers\NULL
• SCHANNEL\Hashes\MD5

And configure the DWORD value data of the Enabled value in the following registry keys to 0xffffffff:

• SCHANNEL\Ciphers\DES 56/56
• SCHANNEL\Ciphers\Triple DES 168/168" [not applicable in export version]
• SCHANNEL\Hashes\SHA
• SCHANNEL\KeyExchangeAlgorithms\PKCS

Master secret computation by using FIPS 140-1 cipher suites

The procedures for using the FIPS 140-1 cipher suites in SSL 3.0 differ from the procedures for using the FIPS 140-1 cipher suites in TLS 1.0.

In SSL 3.0, the following is the definition master_secret computation:

In TLS 1.0, the following is the definition master_secret computation:

where:

Selecting the option to use only FIPS 140-1 cipher suites in TLS 1.0:

Because of this difference, customers may want to prohibit the use of SSL 3.0 even though the allowed set of cipher suites is limited to only the subset of FIPS 140-1 cipher suites. In that case, change the DWORD value data of the Enabled value to 0x0 in the following registry keys under the Protocols key:

• SCHANNEL\Protocols\SSL 3.0\Client
• SCHANNEL\Protocols\SSL 3.0\Server

Warning The Enabled value data in these registry keys under the Protocols key takes precedence over the grbitEnabledProtocols value that is defined in the SCHANNEL_CRED structure that contains the data for a Schannel credential. The default Enabled value data is 0xffffffff.

Example registry files

Two examples of registry file content for configuration are provided in this section of the article. They are Export.reg and Non-export.reg.

In a computer that is running Windows NT 4.0 Service Pack 6 with the exportable Rasbase.dll and Schannel.dll files, run Export.reg to make sure that only TLS 1.0 FIPS cipher suites are used by the computer.

In a computer that is running Windows NT 4.0 Service Pack 6 that includes the non-exportable Rasenh.dll and Schannel.dll files, run Non-export.reg to make sure that only TLS 1.0 FIPS cipher suites are used by the computer.

For the Schannel.dll file to recognize any changes under the SCHANNEL registry key, you must restart the computer.

To return the registry settings to default, delete the SCHANNEL registry key and everything under it. If these registry keys are not present, the Schannel.dll rebuilds the keys when you restart the computer.