Master key derivation for Diffie-Hellman

Master key derivation for Diffie-Hellman in SSL 3.0, denoted CKM_SSL3_MASTER_KEY_DERIVE_DH, is a mechanism used to derive one 48-byte generic secret key from another arbitrary length generic secret key. It is used to produce the "master_secret" key used in the SSL protocol from the "pre_master" key.

It has a parameter, a CK_SSL3_MASTER_KEY_DERIVE_PARAMS structure, which allows for the passing of random data to the token. This structure is defined in Section 12.31. The pVersion field of the structure must be set to NULL_PTR since the version number is not embedded in the "pre_master" key as it is for RSA-like cipher suites.

The mechanism contributes the CKA_CLASS, CKA_KEY_TYPE, and CKA_VALUE attributes to the new key (as well as the CKA_VALUE_LEN attribute, if it is not supplied in the template). Other attributes may be specified in the template, or else are assigned default values.

The template sent along with this mechanism during a C_DeriveKey call may indicate that the object class is CKO_SECRET_KEY, the key type is CKK_GENERIC_SECRET, and the CKA_VALUE_LEN attribute has value 48. However, since these facts are all implicit in the mechanism, there is no need to specify any of them.

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

The CKA_SENSITIVE and CKA_EXTRACTABLE attributes in the template for the new key can both be specified to be either CK_TRUE or CK_FALSE. If omitted, these attributes each take on some default value.
If the base key has its CKA_ALWAYS_SENSITIVE attribute set to CK_FALSE, then the derived key will as well. If the base key has its CKA_ALWAYS_SENSITIVE attribute set to CK_TRUE, then the derived key has its CKA_ALWAYS_SENSITIVE attribute set to the same value as its CKA_SENSITIVE attribute.
Similarly, if the base key has its CKA_NEVER_EXTRACTABLE attribute set to CK_FALSE, then the derived key will, too. If the base key has its CKA_NEVER_EXTRACTABLE attribute set to CK_TRUE, then the derived key has its CKA_NEVER_EXTRACTABLE attribute set to the opposite value from its CKA_EXTRACTABLE attribute. For this mechanism, the ulMinKeySize and ulMaxKeySize fields of the CK_MECHANISM_INFO structure both indicate 48 bytes.

Note that this mechanism is only useable for cipher suites that do not use a fixed length 48-byte "pre_master" secret with an embedded version number. This includes the Diffie-Hellman cipher suites, but excludes the RSA cipher suites.

RSA Security Inc. Public-Key Cryptography Standards - PKCS#11 - v220