Master key derivation

Master key derivation in TLS 1.0, denoted CKM_TLS_MASTER_KEY_DERIVE, is a mechanism used to derive one 48-byte generic secret key from another 48-byte generic secret key. It is used to produce the "master_secret" key used in the TLS protocol from the "pre_master" key. This mechanism returns the value of the client version, which is built into the "pre_master" key as well as a handle to the derived "master_secret" key.

It has a parameter, a CK_SSL3_MASTER_KEY_DERIVE_PARAMS structure, which allows for the passing of random data to the token as well as the returning of the protocol version number which is part of the pre-master key. This structure is defined in Section 12.31.

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 the CK_VERSION structure pointed to by the CK_SSL3_MASTER_KEY_DERIVE_PARAMS structure's pVersion field will be modified by the C_DeriveKey call. In particular, when the call returns, this structure will hold the SSL version associated with the supplied pre_master key.

Note that this mechanism is only useable for cipher suites that use a 48-byte "pre_master" secret with an embedded version number. This includes the RSA cipher suites, but excludes the Diffie-Hellman cipher suites.

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