BCryptDeriveKey function (bcrypt.h)

The BCryptDeriveKey function derives a key from a secret agreement value.

For key derivation from a given secret, see BCryptKeyDerivation.

Syntax

NTSTATUS BCryptDeriveKey(
  [in]            BCRYPT_SECRET_HANDLE hSharedSecret,
  [in]            LPCWSTR              pwszKDF,
  [in, optional]  BCryptBufferDesc     *pParameterList,
  [out, optional] PUCHAR               pbDerivedKey,
  [in]            ULONG                cbDerivedKey,
  [out]           ULONG                *pcbResult,
  [in]            ULONG                dwFlags
);

Parameters

[in] hSharedSecret

The secret agreement handle to create the key from. This handle is obtained from the BCryptSecretAgreement function.

[in] pwszKDF

A pointer to a null-terminated Unicode string that identifies the key derivation function (KDF) to use to derive the key. This can be one of the following strings.

BCRYPT_KDF_HASH (L"HASH")

Use the hash key derivation function.

If the cbDerivedKey parameter is less than the size of the derived key, this function will only copy the specified number of bytes to the pbDerivedKey buffer. If the cbDerivedKey parameter is greater than the size of the derived key, this function will copy the key to the pbDerivedKey buffer and set the variable pointed to by the pcbResult to the actual number of bytes copied.

The parameters identified by the pParameterList parameter either can or must contain the following parameters, as indicated by the Required or optional column.

The call to the KDF is made as shown in the following pseudocode.

KDF-Prepend = KDF_SECRET_PREPEND[0] + 
    KDF_SECRET_PREPEND[1] + 
    ... +
    KDF_SECRET_PREPEND[n]

KDF-Append = KDF_SECRET_APPEND[0] + 
    KDF_SECRET_APPEND[1] + 
    ... + 
    KDF_SECRET_APPEND[n]

KDF-Output = Hash(
    KDF-Prepend + 
    hSharedSecret + 
    KDF-Append)

BCRYPT_KDF_HMAC (L"HMAC")

Use the Hash-Based Message Authentication Code (HMAC) key derivation function.

If the cbDerivedKey parameter is less than the size of the derived key, this function will only copy the specified number of bytes to the pbDerivedKey buffer. If the cbDerivedKey parameter is greater than the size of the derived key, this function will copy the key to the pbDerivedKey buffer and set the variable pointed to by the pcbResult to the actual number of bytes copied.

The parameters identified by the pParameterList parameter either can or must contain the following parameters, as indicated by the Required or optional column.

The call to the KDF is made as shown in the following pseudocode.

KDF-Prepend = KDF_SECRET_PREPEND[0] + 
    KDF_SECRET_PREPEND[1] + 
    ... +
    KDF_SECRET_PREPEND[n]

KDF-Append = KDF_SECRET_APPEND[0] + 
    KDF_SECRET_APPEND[1] + 
    ... + 
    KDF_SECRET_APPEND[n]

KDF-Output = HMAC-Hash(
    KDF_HMAC_KEY,
    KDF-Prepend + 
    hSharedSecret + 
    KDF-Append)

BCRYPT_KDF_TLS_PRF (L"TLS_PRF")

Use the transport layer security (TLS) pseudo-random function (PRF) key derivation function. The size of the derived key is always 48 bytes, so the cbDerivedKey parameter must be 48.

The parameters identified by the pParameterList parameter either can or must contain the following parameters, as indicated by the Required or optional column.

The call to the KDF is made as shown in the following pseudocode.

KDF-Output = PRF(
    hSharedSecret, 
    KDF_TLS_PRF_LABEL, 
    KDF_TLS_PRF_SEED)

BCRYPT_KDF_SP80056A_CONCAT (L"SP800_56A_CONCAT")

Use the SP800-56A key derivation function.

The parameters identified by the pParameterList parameter either can or must contain the following parameters, as indicated by the Required or optional column. All parameter values are treated as opaque byte arrays.

The call to the KDF is made as shown in the following pseudocode.

KDF-Output = SP_800-56A_KDF(
	   hSharedSecret,
	   KDF_ALGORITHMID,
	   KDF_PARTYUINFO,
	   KDF_PARTYVINFO,
	   KDF_SUPPPUBINFO,
	   KDF_SUPPPRIVINFO)

Windows Server 2008, Windows Vista, Windows Server 2003 and Windows XP:  This value is not supported.

BCRYPT_KDF_RAW_SECRET (L"TRUNCATE")

Returns the little-endian representation of the raw secret without any modification.

If the cbDerivedKey parameter is less than the size of the derived key, this function will only copy the specified number of bytes to the pbDerivedKey buffer. If the cbDerivedKey parameter is greater than the size of the derived key, this function will copy the key to the pbDerivedKey buffer and set the variable pointed to by the pcbResult to the actual number of bytes copied.

Windows 8, Windows Server 2008, Windows Vista, Windows Server 2003 and Windows XP:  This value is not supported.

[in, optional] pParameterList

The address of a BCryptBufferDesc structure that contains the KDF parameters. This parameter is optional and can be NULL if it is not needed.

[out, optional] pbDerivedKey

The address of a buffer that receives the key. The cbDerivedKey parameter contains the size of this buffer. If this parameter is NULL, this function will place the required size, in bytes, in the ULONG pointed to by the pcbResult parameter.

[in] cbDerivedKey

The size, in bytes, of the pbDerivedKey buffer.

[out] pcbResult

A pointer to a ULONG that receives the number of bytes that were copied to the pbDerivedKey buffer. If the pbDerivedKey parameter is NULL, this function will place the required size, in bytes, in the ULONG pointed to by this parameter.

[in] dwFlags

A set of flags that modify the behavior of this function. This can be zero or the following value.

Return value

Returns a status code that indicates the success or failure of the function.

Possible return codes include, but are not limited to, the following.

Remarks

The BCryptBufferDesc structure in the pParameterList parameter can contain more than one of the KDF_SECRET_PREPEND and KDF_SECRET_APPEND parameters. If more than one of these parameters is specified, the parameter values are concatenated in the order in which they are contained in the array before the KDF is called. For example, assume the following parameter values are specified.

BYTE pbValue0[1] = {0x01};
BYTE pbValue1[2] = {0x04, 0x05};
BYTE pbValue2[3] = {0x10, 0x11, 0x12};
BYTE pbValue3[4] = {0x20, 0x21, 0x22, 0x23};

Parameter[0].type = KDF_SECRET_APPEND
Parameter[0].value = pbValue0;
Parameter[0].length = sizeof  (pbValue0);
Parameter[1].type = KDF_SECRET_PREPEND
Parameter[1].value = pbValue1;
Parameter[1].length = sizeof (pbValue1);
Parameter[2].type = KDF_SECRET_APPEND
Parameter[2].value = pbValue2;
Parameter[2].length = sizeof (pbValue2);
Parameter[3].type = KDF_SECRET_PREPEND
Parameter[3].value = pbValue3;
Parameter[3].length = sizeof (pbValue3);

If the above parameter values are specified, the concatenated values to the actual KDF are as follows.

Type: KDF_SECRET_PREPEND
Value: {0x04, 0x05, 0x20, 0x21, 0x22, 0x23}, length 6

Type: KDF_SECRET_APPEND
Value: {0x01, 0x10, 0x11, 0x12}, length 4

If the pwszKDF parameter is set to BCRYPT_KDF_RAW_SECRET, The returned secret (unlike the other pwszKDF values) will be encoded in little-endian format. It is important to take note of this when using the raw secret in any other CNG functions, as most of them take in big-endian encoded inputs.

Depending on what processor modes a provider supports, BCryptDeriveKey can be called either from user mode or kernel mode. Kernel mode callers can execute either at PASSIVE_LEVEL IRQL or DISPATCH_LEVEL IRQL. If the current IRQL level is DISPATCH_LEVEL, the handle provided in the hSharedSecret parameter must be located in nonpaged (or locked) memory and must be derived from an algorithm handle returned by a provider that was opened by using the BCRYPT_PROV_DISPATCH flag.

To call this function in kernel mode, use Cng.lib, which is part of the Driver Development Kit (DDK). Windows Server 2008 and Windows Vista:  To call this function in kernel mode, use Ksecdd.lib.

Requirements

See also

BCryptSecretAgreement