Extraction of one key from another key

Extraction of one key from another key, denoted CKM_EXTRACT_KEY_FROM_KEY, is a mechanism which provides the capability of creating one secret key from the bits of another secret key.

This mechanism has a parameter, a CK_EXTRACT_PARAMS, which specifies which bit of the original key should be used as the first bit of the newly-derived key.

We give an example of how this mechanism works. Suppose a token has a secret key with the 4-byte value 0x329F84A9. We will derive a 2-byte secret key from this key, starting at bit position 21 (i.e., the value of the parameter to the CKM_EXTRACT_KEY_FROM_KEY mechanism is 21).

We write the key's value in binary: 0011 0010 1001 1111 1000 0100 1010 1001. We regard this binary string as holding the 32 bits of the key, labeled as b0, b1, ..., b31.
We then extract 16 consecutive bits (i.e., 2 bytes) from this binary string, starting at bit b21. We obtain the binary string 1001 0101 0010 0110.
The value of the new key is thus 0x9526.

Note that when constructing the value of the derived key, it is permissible to wrap around the end of the binary string representing the original key's value.

If the original key used in this process is sensitive, then the derived key must also be sensitive for the derivation to succeed.

If no length or key type is provided in the template, then an error will be returned.
If no key type is provided in the template, but a length is, then the key produced by this mechanism will be a generic secret key of the specified length.
If no length is provided in the template, but a key type is, then that key type must have a well-defined length. If it does, then the key produced by this mechanism will be of the type specified in the template. If it doesn't, an error will be returned.
If both a key type and a length are provided in the template, the length must be compatible with that key type. The key produced by this mechanism will be of the specified type and length. If a DES, DES2, DES3, or CDMF key is derived with this mechanism, the parity bits of the key will be set properly.

If the requested type of key requires more bytes than the original key has, an error is generated.

This mechanism has the following rules about key sensitivity and extractability:

If the base key has its CKA_SENSITIVE attribute set to CK_TRUE, so does the derived key. If not, then the derived key's CKA_SENSITIVE attribute is set either from the supplied template or from a default value.
Similarly, if the base key has its CKA_EXTRACTABLE attribute set to CK_FALSE, so does the derived key. If not, then the derived key's CKA_EXTRACTABLE attribute is set either from the supplied template or from a default value.
The derived key's CKA_ALWAYS_SENSITIVE attribute is set to CK_TRUE if and only if the base key has its CKA_ALWAYS_SENSITIVE attribute set to CK_TRUE.
Similarly, the derived key's CKA_NEVER_EXTRACTABLE attribute is set to CK_TRUE if and only if the base key has its CKA_NEVER_EXTRACTABLE attribute set to CK_TRUE.

RSA Security Inc. Public-Key Cryptography Standards - PKCS#11 - v230