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The CKA_DERIVE_TEMPLATE attribute is an optional extension to the C_DeriveKey function. This attribute points to an array template which provides additional security by restricting important attributes in the resulting derived key. This derive template, along with the user-supplied application template (called pTemplate in the PKCS#11 specification), determine the attributes of the derived key.
To invoke a derive template, the base key must have the CKA_DERIVE_TEMPLATE attribute set, pointing to a user-supplied derive template. When you specify this attribute on the base key and then attempt to derive a key, the derive operation adds the attributes of the application template to the attributes in the derive template. If there are any mismatches between attribute values specified in the two templates, the derive operation fails. Otherwise, the operation succeeds, producing a derived key with the combined attributes of the two templates.
Any and all attributes which are valid for application template of a particular mechanism are also valid for the derive template. For security, the most effective attributes to restrict are those which might allow the derived key to be misused or expose secret information. Broadly these include but are not limited to encryption/decryption capabilities, extractability, the CKA_SENSITIVE attribute and the CKA_MODIFIABLE attribute. All mechanisms which support key derivation also support derive templates.
The following examples show a successful derivation with a derive template, and a failed derivation.
Here, the base key has the CKA_DERIVE_TEMPLATE attribute pointing to the derive template dTmplt. There are no conflicts between dTmplt and the application template. The application template's extra attributes are added to dTmplt's attributes, and the derivation operation produces a derived key containing the attributes in the two templates.
Here, the base key has the CKA_DERIVE_TEMPLATE attribute pointing to the derive template dTmplt. Notice that dTmplt has the CKA_DECRYPT attribute set to false, where the application template has the CKA_DECRYPT attribute set to true. This conflict causes the derivation operation to fail with the error TEMPLATE_INCONSISTENT.
