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Key Block

				

				
				

					


	
Unified Key Block



	


		
Base Key, Cipher Key, HMAC Key


		

            Any key material is converted to a base key,
            i.e. a hash value of the key material. The base key is then used to produce
            the corresponding encryption key, and the corresponding HMAC key.
		


		

            The encryption key is produced as follows:
		


		

            If the cipher key size is greater than the hash size, the base key is
            extended with zeros. If less, the base key is truncated to cipher's key size.
		


		

            The HMAC key is computed by reverting the base key and inverting its bytes.
		



		
Key Block Format


		

            Key block includes a 64-bit random salt and exactly one key record
            (which may, however, represent a key group).
		


		

			

				Size
				Description
			


			

				8
				Random salt
			


			

				. . .
				Key record
			

		



		
Atomic Key Records


		

			There are three types of atomic keys: password, binary key, and Yubikey.
		



		
Password


		

			

				Size
				Description
			


			

				1
				Key type (0x01)
			


			

				1
				Flags
			


			

				HashSize
				Verificator
			

		


		

            The 64-bit salt and the UTF-16 password (without terminating zero) are
            concatenated and the resulting byte string is hashed.
		



		
Binary Key


		

			

				Size
				Description
			


			

				1
				Key type (0x05)
			


			

				1
				Flags
			


			

				HashSize
				Verificator
			

		


		

            The 64-bit salt and the 512-byte long binary key are concatenated and the
            resulting byte string is hashed.
		



		
Yubikey


		

			

				Size
				Description
			


			

				1
				Key type (0x08)
			


			

				1
				Flags
			


			

				HashSize
				Verificator
			

		


		

            The 64-bit salt and the 20-byte long Yubikey response are concatenated and the
            resulting byte string is hashed.
		



		
Complex Key Records


		

            There are two types of complex keys - composite key and key group.
		


		

            Composite uses several atomic keys to produce the base key. In order to
            decrypt data the user must provide all the composite's atomic keys.
		


		

            Key group also contains several simpler keys, but unlike composite key (which
            also may be a part of the group), the user needs to produce any of the group
            subkeys in order to decrypt.
		



		
Composite Key


		

			

				Size
				Description
			


			

				1
				Key type (0x6F)
			


			

				1
				Flags
			


			

				1
				Number of keys in the composite
			


			

				. . .
				Key records
			

		


		

            Only atomic keys are allowed to be parts of a composite.
		


		

            The base key is XOR of the composite's atomic keys, which are computed
            the usual way.
		



		
Key Group


		

			

				Size
				Description
			


			

				1
				Key type (0x05)
			


			

				1
				Flags
			


			

				1
				Number of subkeys in the group
			


			

				. . .
				Subkeys
			

		


		

            Every subkey record consists of a key record (which may be either an atomic
            key or a composite key), and an encrypted session key:
		


		
Subkey


		

			

				Size
				Description
			


			

				. . .
				Key record
			


			

				n * BlockSize
				Encrypted session key
			

		



		
Session Key


		

            Session key is used with key group only. It is produced exactly as composite one,
            i.e. as a XOR of all the computed subkeys.
		


		

            The same individual subkeys are also used to produce the corresponding encrypted
            session key.
		


		

            The size of the encrypted session key field is the minimal number of cipher blocks
            which is greater or equal to the hash size. For example, if the hash size is 48 bytes
            (384 bits), and the cipher block is 32 bytes (256 bits), then the size of the key
            field will be 64 (2 * 32) bytes. The session key is stored
            in the beginning of the field, the rest (if present) is not used and must be filled with
            random data. After that the field is encrypted in CBC mode using the corresponding subkeys.
		



		
Verificator


		

            Key verificator is used to determine whether the user specified the correct key
            material. It is value is a hash value computed 257 times as follows:
		


		

            The initial value V[0] computed exactly as the corrsponding base key, i.e.
		

		
		
		    V[0] = HASH(salt + password)
		

		

            or
		

		
		
		    V[0] = HASH(salt + 512-byte binary key)
		

		

            or
		

		
		
		    V[0] = HASH(salt + 20-byte Yubikey response)
		

		

            Then 256 iterations are performed:
		

		
		
            V[1] = HASH( 0 + V[0] )

            V[2] = HASH( 0 + 1 + V[1] )

            . . .

            V[256] = HASH( 0 + 1 + ... + 255 + V[255] )

            

            Verificator = V[256]
		

		

            The '+' sign here means byte concatenation. Values 0, 1, ..., 255 are bytes.
		



		
Constants and Structures



		
Key Types


		

            Key type is a byte value. Its upper 3 bits is the key level, and the lower 5 bits
            is the unique key ID.
		


		

            Atomic keys have key level 0. Keys with key level >0 are complex ones. A complex
            key may include other keys as its components provided that their key levels are
            less than the key level of the complex key. For example, a key group (level 5)
            may include a composite key (level 3) but not another key group. Likewise, a
            composite key may not include another composite key.
		

		
		
            0x01	- Password

            0x05	- Binary key

            0x08	- Yubikey

            0x6F	- Composite (level 3, ID 0x0F)

            0xBC	- Key group (level 5, ID 0x1C)
		


		
Flags

		
		
            0x0001  - Can be used for container creation

            0x0002  - Can be used for container modification

            0x0004  - Can be used for data decryption

            0x0400  - Password is accessible in the text form

            0x0800  - Is a master key
		


		
Component Descriptor


		

            Key block is usually accompanied by a component descriptor, which specifies what
            components should be used for key hashing (and ciphering if a session key is used).
		


		

			

				Size
				Description
			


			

				16
				Hash function CID
			


			

				4
				Hash size
			


			

				4
				Hashing passes
			


			

				4
				Hashing scheme
			


			

				16
				Cipher CID
			


			

				4
				Key size
			


			

				4
				Block size
			


			

				4
				Rounds
			


			

				4
				Scheme
			

		


		

            Cipher chaining mode is always ECB (and so init vector is not used).