Loading…

		

		
		
		
		
		

	
						
		
			

				
				

					
					
Parcel Format

				

				
				

					


	
Structure of Silver Key 4 Parcel



	


		
Parcel Format


		

			The format of Silver Key 4.x parcel is as follows:
		


		
  [Pre-header data]   Optional arbitrary data (decryptor stub, for example)
  [Parcel header]     Always present
  [File area]         A set of encrypted streams containing file data (optional)
  [Script area]       Encrypted stream containing parcel script (always present)
  [Parcel trailer]    Always present
  [Post-parcel data]  Optional arbitrary data
		


		

			Even if the parcel is empty, it contains a header, a script containing
			a null command (marking the end of the script), and a trailer.
		


		

			This structure radically differs from the structure of pre-4.0 parcel,
			which stores commands and data as a single stream. The new Kryptel-like
			format allows one-pass parcel creation and partial decryption (Parcel
			Analyzer is the end-user partial decryption tool).
		



		
Parcel Header


		

			Parcel header contains information about the key material, the components
			used for encryption, and other important data.
		


		

			

				Size
				Description
			


			

				4
				Parcel tag (0x7A95FFEB)
			


			

				2
				Engine version used to create the parcel
			


			

				2
				Extractor version required
			


			

				16
				Engine GUID (11056249-400A-4461-BD5E-FE06113CA001)
			


			

				16
				Parcel GUID
			


			

				4
				Parcel flags
			


			

				16
				Cipher component ID
			


			

				20
				Cipher parameter block
			


			

				16
				Key ID
			


			

				4
				Number of key verification passes
			


			

				HashSize
				Key verification block
			


			

				BlockSize
				Cipher initialization vector (encrypt it to get the actual vector)
			


			

				STR
				Cipher name string
			


			

				STR
				Cipher scheme string
			


			

				STR
				Parcel title
			


			

				 
				--- Description (if SK_FLAG_SHOW_DESCRIPTION) ---
			


			

				4
				Uncompressed description size (number of UTF-16 chars, no terminating zero)
			


			

				4
				Compressed description size (bytes)
			


			

				. . .
				Compressed description (UTF-16, no terminating zero)
			


			

				 
				--- Uninstaller (if SK_FLAG_UNINSTALLER) ---
			


			

				4
				Uncompressed uninstaller size
			


			

				4
				Compressed uninstaller size
			


			

				. . .
				Compressed uninstaller
			


			

				 
				--- Localized strings (if SK_FLAG_LOCALIZED_STRINGS) ---
			


			

				2
				Uncompressed size of string set
			


			

				2
				Compressed size of string set
			


			

				. . .
				Compressed string set
			

		


		

			STR represents a UTF-16 string; first word (2 bytes) contains the string
			length in characters. The string does not contain trailing zero. Each UNICODE
			character occupies 2 bytes.
		


		

			Silver Key engine uses ZIP compressor. SHA-512 is used to produce the key and
			the HMACs so HashSize is always 64.
		


		

			The first 44 bytes of header (from parcel tag to parcel flags, inclusively)
			have the same meaning for all Silver Key 4.0 and later engines.
		



		
Format of string set


		

			

				Size
				Description
			


			

				2
				Language code
			


			

				1
				String 1 identifier
			


			

				STR
				String 1
			


			

				1
				String 2 identifier
			


			

				STR
				String 2
			


			

				. . .
				 
			


			

				1
				String N identifier
			


			

				STR
				String N
			


			

				1
				Zero byte (PARCEL_STRING_TERMINATOR)
			

		


		

			If the extractor can't recognize a string identifier, the string should be quietly
			ignored. The list of the identifiers can be found in SilverKey.h
		



		
Key Usage Notes


		

			Although cipher and HMAC (HMACs are computed for data areas, and are used for
			key verification) use the same key, they use it differently.
		


		

			Cipher uses first no more than KeySize bytes of the key (padded with zeroes if necessary).
		


		

			HMAC reverts and inverts the significant bytes of the key, pads with zeroes if necessary,
			and uses the first bytes of the result.
		


		

			For example, in the default case of 256-bit AES and SHA-512, the cipher will use the first
			256 bits of the SHA-512 password hash, and HMAC will use the whole reversed and inverted
			password hash. In most cases this method ensures that cipher and HMAC use completely different
			or at least weakly related keys.
		


		

			In case of user-defined binary key (also known as 'raw binary key'), HMAC assumes its size is
			512 bits and prepares (reverts and inverts) that portion. It is responsibility
			of the user to pad the key with zeros up to BINARY_KEY_SIZE.
		



		
Key Verification Note


		

			The engine uses the same verification procedure as Kryptel 6 storage handler with
			one difference: it correctly hashes in the value of m_pKeyVerificator.
		



		
Note about parcel GUID


		

			Time has shown that adding this field was a wrong idea. Although a unique parcel ID could be useful
			in some scenarios, a random 128-bit value can be used to store some 'backdoor' information,
			for example, a part of the key. In any case, it is not possible to avoid such a suspicion.
		


		

			As a solution to this problem, the engine should not generate a unique GUID. This field may
			contain any value, but the value must remain the same for all parcels created by the given
			engine version/variation. This way it still can be used for trailer verification, but it is
			impossible to store any backdoor information there.
		



		
Parcel Trailer


		

			

				Size
				Description
			


			

				HashSize
				HMAC of parcel header, starting from parcel tag
			


			

				HashSize
				HMAC of file area (computed on encrypted data)
			


			

				HashSize
				HMAC of script area (computed on encrypted data)
			


			

				4
				Parcel end tag (0xEBFF957A)
			


			

				16
				Parcel GUID
			


			

				8
				Offset to the file area, relative to header start
			


			

				8
				Offset to the script area, relative to header start
			


			

				8
				File position of the parcel ID tag; 0 if no stub
			


			

				16
				MD5 hash of the preceding area, starting from parcel ID tag
			

		


		

			The last 60 bytes of the trailer (starting with parcel end tag) must have the same
			meaning for all Silver Key 4.0 and later engines.
		


		

			Note that the position of the parcel ID tag is just a hint, and may in fact point to a
			wrong place. This pointer is valid for newly created parcels, but after repacking it
			naturally gets out of sync as repacking does not modify the parcel trailer. The purpose
			of this field is to enable quick recognition with IsParcel function. If the parcel gets
			repacked, LocateParcel function is required to locate the parcel inside a file.
		


		

			If the last 16 bytes of the file is not a valid MD5 signature, or if the header position
			offset does not point to a parcel ID tag, then the parcel may contain trailing data. In
			this case the decryptor should scan the file to locate 'parcel start' and 'parcel end' tags.
			After locating the parcel's header and trailer, the decryptor must compare the parcel GUID
			values to ensure that the trailer matches the header.
		



		
Script Area


		

			Silver Key script area is a sequence of commands, encrypted to a standard
			compressed and encrypted stream.
		


		

			The decryptor processes commands in the same order as they appear in the parcel -
			for example, in order to decrypt a file and create a shortcut to it, you must place
			first [COMMAND_FILE], and then [COMMAND_LINK].
		


		

			The last command is always a null command, so the stream cannot be empty. Note
			that the terminating null command may be followed by a block of random obfuscation
			data, so the parser must stop processing and reset the cipher and the compressor
			as soon as it encounters the terminating null command.
		



		
Script Commands



		
Statistics


		

			The primary purpose of this command is setting up a progress bar. It is highly
			recommended to place this command first to ensure that the decryptor's progress
			bar will not get out of sync.
		

		    
			

				

					Size
					Description
				


				

					4
					[COMMAND_PROGRESS]
				


				

					4
					Number of directories
				


				

					4
					Number of files in parcel
				


				

					8
					Total size of all files in the parcel (bytes)
				

			



		
Comment


		

			Shows the encrypted message stored in the file area.
			The stored comment does not include terminating zero.
		

		    
			

				

					Size
					Description
				


				

					4
					[COMMAND_COMMENT]
				


				

					BlockSize
					Cipher initialization vector
				


				

					8
					Comment position (offset from the header start)
				


				

					4
					Comment uncompressed size (number of UTF-16 chars)
				


				

					4
					Comment compressed size (bytes)
				

			



		
Directory


		

			Creates a directory.
		

		    
			

				

					Size
					Description
				


				

					4
					[COMMAND_DIRECTORY]
				


				

					4
					Target (value from TARGETS enumeration)
				


				

					STR
					Target-relative path
				

			



		
File


		

			Decrypts a file stored in the file area.
		

		    
			

				

					Size
					Description
				


				

					4
					[COMMAND_FILE]
				


				

					BlockSize
					Cipher initialization vector
				


				

					8
					File data position (offset from the header start)
				


				

					8
					File original uncompressed size
				


				

					8
					Size of the file's encrypted stream
				


				

					8
					File creation time (time_t value)
				


				

					4
					Target (value from TARGETS enumeration)
				


				

					STR
					Target-relative path
				

			



		
Delete


		

			Deletes a file system object (i.e. file or directory). See the
			Security Considerations chapter in the program's Help for a detailed
			discussion.
		

		    
			

				

					Size
					Description
				


				

					4
					[COMMAND_DELETE]
				


				

					4
					Target (value from TARGETS enumeration)
				


				

					STR
					Target-relative path
				

			



		
Create Link


		

			Creates a shortcut to a decrypted file system object.
		

		    
			

				

					Size
					Description
				


				

					4
					[COMMAND_LINK]
				


				

					4
					Shortcut's target (value from TARGETS enumeration)
				


				

					4
					File's target (value from TARGETS enumeration)
				


				

					4
					Icon's target (value from TARGETS enumeration)
				


				

					STR
					Shortcut target-relative path
				


				

					STR
					File target-relative path
				


				

					STR
					Icon target-relative path
				


				

					STR
					Command-line arguments
				


				

					STR
					Shortcut description string
				

			


			

				The shortcut path and the file path are mandatory arguments and cannot
				be empty. Other strings may be empty; if the icon path is empty, the icon
				target value is ignored.
			



		
Open


		

			Opens a decrypted file system object in the associated application.
		

		    
			

				

					Size
					Description
				


				

					4
					[COMMAND_OPEN]
				


				

					4
					Open mode (value from SK_OPEN enumeration)
				


				

					4
					Target (value from TARGETS enumeration)
				


				

					STR
					Target-relative path
				

			




		
Tag Values


		
  PARCEL_TAG           0x7A95FFEB
  PARCEL_END_TAG       0xEBFF957A

  COMMAND_NULL         0x7A250000
  COMMAND_PROGRESS     0x7A250001
  COMMAND_COMMENT      0x7A250012
  COMMAND_DIRECTORY    0x7A254FAE
  COMMAND_LINK         0x7A25BF00
  COMMAND_FILE         0x7A25BFC0
  COMMAND_DELETE       0x7A25C0BF
  COMMAND_OPEN         0x7A25F005