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Parcel Format

Structure of Silver Key Parcel

The purpose of this article is to provide information on Kryptel internal data formats for public scrutiny. At least a basic understanding of programming is required.

Parcel Format

The format of Silver Key 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 a 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 (must be CID_SILVER_KEY)
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 agent data and directory, 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 64 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 (IDENT_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 Notes

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

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 modifies 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

Statictics

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