Enumerating the Supported Protocols (Diffie-Hellman)

The supported protocols and cipher suites can be determined by calling CryptGetProvParam with PP_ENUMALGS_EX. This works in a manner similar to the use of PP_ENUMALGS, but returns the following structure instead of the more limited one returned when using PP_ENUMALGS.

// protocol flags
#define CRYPT_FLAG_SSL3         0x0004
#define CRYPT_FLAG_TLS1         0x0008

typedef struct _PROV_ENUMALGS_EX 
{
    ALG_ID    aiAlgid;
    DWORD     dwDefaultLen;
    DWORD     dwMinLen;
    DWORD     dwMaxLen;
    DWORD     dwProtocols;
    DWORD     dwNameLen;
    CHAR      szName[20];
    DWORD     dwLongNameLen;
    CHAR      szLongName[40];
} PROV_ENUMALGS_EX;

The following table shows the algorithm list that might be returned by a typical (domestic) PROV_DH_SCHANNEL CSP.

Algid	Min	Max	Protocols	Name
CALG_DH_EPHEM	512	2048	0x000C	"DH_KEYX"
CALG_MD5	128	128	0x000C	"MD5"
CALG_SHA	160	160	0x000C	"SHA"
CALG_RC4	40	128	0x000C	"RC4"
CALG_DES	56	56	0x000C	"DES"

Before sending a ClientHello or ServerHello message, the protocol engine enumerates all of the algorithms and key sizes supported by the CSP and internally build a list of supported cipher suites.