Specifying Schannel Ciphers and Cipher Strengths
For client/server information exchanges, the default behavior of Schannel is to negotiate the best cipher available based on those enabled in the registry. Applications may limit the ciphers and cipher strengths allowed for a connection by using members of the SCHANNEL_CRED structure as follows:
- Set the palgSupportedAlgs member to an array of ALG_ID containing the allowable ciphers. For more information, see Cipher IDs.
- Set the dwMinimumCipherStrength and/or dwMaximumCipherStrength members to the allowable minimum and maximum strengths. For more information, see Cipher Strength Values.
- Pass the SCHANNEL_CRED structure (by means of the pAuthData parameter) in a call to the AcquireCredentialsHandle function. This function returns a credentials handle.
- Specify the credentials handle in a call to the client-side InitializeSecurityContext (General) function or the server-side AcceptSecurityContext (General) function.
The default behavior of Schannel is to request the best cipher available based on Schannel entries in the system registry. Do not change the system registry; the settings it contains for Schannel are used globally and will affect other applications. For the list of valid constants, see ALG_ID.
This Schannel feature is typically used to limit a connection to either domestic or export strength ciphers. Domestic strengths include 56, and 128 bits, while export strength is limited to 56 bits. If you set the minimum and maximum values to zero Schannel will use all available cipher strengths.
Using TLS or SSL 3.0, set the dwMinimumCipherStrength member to -1 (negative one) to enable the "Null Cipher" cipher suites, which provide signatures but no encryption. If dwMaximumCipherStrength is also set to -1 then only the "Null Cipher" suites will be enabled. This setting is intended for development only and should not be used in production systems.
To retrieve the cipher strength settings for a credential, call the QueryCredentialsAttributes function and specify SECPKG_ATTR_CIPHER_STRENGTHS as the ulAttribute parameter.
To retrieve the list of supported algorithms for a credential, call the QueryCredentialsAttributes with SECPKG_ATTR_SUPPORTED_ALGS as the ulAttribute parameter.