Cryptographic Token Interface Standard

PKCS#11


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).

  1. 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, labelled as b0, b1, ..., b31.

  2. 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.

  3. 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 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:


RSA Security Inc. Public-Key Cryptography Standards - PKCS#11 - v210