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Ok, hier vielleicht der wichtigere Teil
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http://www.pkware.com/business_and_d...ps/appnote.txtCode:Single Password Symmetric Encryption Method: ------------------------------------------- The Single Password Symmetric Encryption Method using strong encryption algorithms operates similarly to the traditional PKWARE encryption defined in this format. Additional data structures are added to support the processing needs of the strong algorithms. The Strong Encryption data structures are: 1. General Purpose Bits - Bits 0 and 6 of the General Purpose bit flag in both local and central header records. Both bits set indicates strong encryption. Bit 13, when set indicates the Central Directory is encrypted and that selected fields in the Local Header are masked to hide their actual value. 2. Extra Field 0x0017 in central header only. Fields to consider in this record are: Format - the data format identifier for this record. The only value allowed at this time is the integer value 2. AlgId - integer identifier of the encryption algorithm from the following range 0x6601 - DES 0x6602 - RC2 (version needed to extract < 5.2) 0x6603 - 3DES 168 0x6609 - 3DES 112 0x660E - AES 128 0x660F - AES 192 0x6610 - AES 256 0x6702 - RC2 (version needed to extract >= 5.2) 0x6801 - RC4 0xFFFF - Unknown algorithm Bitlen - Explicit bit length of key 40 56 64 112 128 168 192 256 Flags - Processing flags needed for decryption 0x0001 - Password is required to decrypt 0x0002 - Certificates only 0x0003 - Password or certificate required to decrypt Values > 0x0003 reserved for certificate processing 3. Decryption header record preceding compressed file data. -Decryption Header: Value Size Description ----- ---- ----------- IVSize 2 bytes Size of initialization vector (IV) IVData IVSize Initialization vector for this file Size 4 bytes Size of remaining decryption header data Format 2 bytes Format definition for this record AlgID 2 bytes Encryption algorithm identifier Bitlen 2 bytes Bit length of encryption key Flags 2 bytes Processing flags ErdSize 2 bytes Size of Encrypted Random Data ErdData ErdSize Encrypted Random Data Reserved1 4 bytes Reserved certificate processing data Reserved2 (var) Reserved for certificate processing data VSize 2 bytes Size of password validation data VData VSize-4 Password validation data VCRC32 4 bytes Standard ZIP CRC32 of password validation data IVData - The size of the IV should match the algorithm block size. The IVData can be completely random data. If the size of the randomly generated data does not match the block size it should be complemented with zero's or truncated as necessary. If IVSize is 0,then IV = CRC32 + Uncompressed File Size (as a 64 bit little-endian, unsigned integer value). Format - the data format identifier for this record. The only value allowed at this time is the integer value 3. AlgId - integer identifier of the encryption algorithm from the following range 0x6601 - DES 0x6602 - RC2 (version needed to extract < 5.2) 0x6603 - 3DES 168 0x6609 - 3DES 112 0x660E - AES 128 0x660F - AES 192 0x6610 - AES 256 0x6702 - RC2 (version needed to extract >= 5.2) 0x6801 - RC4 0xFFFF - Unknown algorithm Bitlen - Explicit bit length of key 40 56 64 112 128 168 192 256 Flags - Processing flags needed for decryption 0x0001 - Password is required to decrypt 0x0002 - Certificates only 0x0003 - Password or certificate required to decrypt Values > 0x0003 reserved for certificate processing ErdData - Encrypted random data is used to generate a file session key for encrypting each file. SHA1 is used to calculate hash data used to derive keys. File session keys are derived from a master session key generated from the user-supplied password. If the Flags field in the decryption header contains the value 0x4000, then the ErdData field must be decrypted using 3DES. Reserved1 - Reserved for certificate processing, if value is zero, then Reserved2 data is absent. See the explanation under the Certificate Processing Method for details on this data structure. Reserved2 - If present, the size of the Reserved2 data structure is located by skipping the first 4 bytes of this field and using the next 2 bytes as the remaining size. See the explanation under the Certificate Processing Method for details on this data structure. VSize - This size value will always include the 4 bytes of the VCRC32 data and will be greater than 4 bytes. VData - Random data for password validation. This data is VSize in length and VSize must be a multiple of the encryption block size. VCRC32 is a checksum value of VData. VData and VCRC32 are stored encrypted and start the stream of encrypted data for a file. 4. Useful Tips Strong Encryption is always applied to a file after compression. The block oriented algorithms all operate in Cypher Block Chaining (CBC) mode. The block size used for AES encryption is 16. All other block algorithms use a block size of 8. Two ID's are defined for RC2 to account for a discrepancy found in the implementation of the RC2 algorithm in the cryptographic library on Windows XP SP1 and all earlier versions of Windows. A pseudo-code representation of the encryption process is as follows: Password = GetUserPassword() RD = Random() ERD = Encrypt(RD,DeriveKey(SHA1(Password))) For Each File IV = Random() VData = Random() FileSessionKey = DeriveKey(SHA1(IV + RD)) Encrypt(VData + VCRC32 + FileData,FileSessionKey) Done The function names and parameter requirements will depend on the choice of the cryptographic toolkit selected. Almost any toolkit supporting the reference implementations for each algorithm can be used. The RSA BSAFE(r), OpenSSL, and Microsoft CryptoAPI libraries are all known to work well.Kommentar
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Teil 2 von 2
http://www.pkware.com/business_and_d...ps/appnote.txtCode:Single Password - Central Directory Encryption: ----------------------------------------------- Central Directory Encryption is achieved within the .ZIP format by encrypting the Central Directory structure. This encapsulates the metadata most often used for processing .ZIP files. Additional metadata is stored for redundancy in the Local Header for each file. The process of concealing metadata by encrypting the Central Directory does not protect the data within the Local Header. To avoid information leakage from the exposed metadata in the Local Header, the fields containing information about a file are masked. Local Header: Masking replaces the true content of the fields for a file in the Local Header with false information. When masked, the Local Header is not suitable for streaming access and the options for data recovery of damaged archives is reduced. Extra Data fields that may contain confidential data should not be stored within the Local Header. The value set into the Version needed to extract field should be the correct value needed to extract the file without regard to Central Directory Encryption. The fields within the Local Header targeted for masking when the Central Directory is encrypted are: Field Name Mask Value ------------------ --------------------------- compression method 0 last mod file time 0 last mod file date 0 crc-32 0 compressed size 0 uncompressed size 0 file name (variable size) Base 16 value from the range 1 - FFFFFFFFFFFFFFFF represented as a string whose size will be set into the file name length field The Base 16 value assigned as a masked file name is simply a sequentially incremented value for each file starting with 1 for the first file. Modifications to a ZIP file may cause different values to be stored for each file. For compatibility, the file name field in the Local Header should never be left blank. As of Version 6.2 of this specification, the Compression Method and Compressed Size fields are not yet masked. Encrypting the Central Directory: Encryption of the Central Directory does not include encryption of the Central Directory Signature data, the ZIP64 End of Central Directory record, the ZIP64 End of Central Directory Locator, or the End of Central Directory record. The ZIP file comment data is never encrypted. Before encrypting the Central Directory, it may optionally be compressed. Compression is not required, but for storage efficiency it is assumed this structure will be compressed before encrypting. Similarly, this specification supports compressing the Central Directory without requiring that it also be encrypted. Early implementations of this feature will assume the encryption method applied to files matches the encryption applied to the Central Directory. Encryption of the Central Directory is done in a manner similar to that of file encryption. The encrypted data is preceded by a decryption header. The decryption header is known as the Archive Decryption Header. The fields of this record are identical to the decryption header preceding each encrypted file. The location of the Archive Decryption Header is determined by the value in the Start of the Central Directory field in the ZIP64 End of Central Directory record. When the Central Directory is encrypted, the ZIP64 End of Central Directory record will always be present. The layout of the ZIP64 End of Central Directory record for all versions starting with 6.2 of this specification will follow the Version 2 format. The Version 2 format is as follows: The first 48 bytes will remain identical to that of Version 1. The record signature for both Version 1 and Version 2 will be 0x06064b50. Immediately following the 48th byte, which identifies the end of the field known as the Offset of Start of Central Directory With Respect to the Starting Disk Number will begin the new fields defining Version 2 of this record. New fields for Version 2: Note: all fields stored in Intel low-byte/high-byte order. Value Size Description ----- ---- ----------- Compression Method 2 bytes Method used to compress the Central Directory Compressed Size 8 bytes Size of the compressed data Original Size 8 bytes Original uncompressed size AlgId 2 bytes Encryption algorithm ID BitLen 2 bytes Encryption key length Flags 2 bytes Encryption flags HashID 2 bytes Hash algorithm identifier Hash Length 2 bytes Length of hash data Hash Data (variable) Hash data The Compression Method accepts the same range of values as the corresponding field in the Central Header. The Compressed Size and Original Size values will not include the data of the Central Directory Signature which is compressed or encrypted. The AlgId, BitLen, and Flags fields accept the same range of values the corresponding fields within the 0x0017 record. Hash ID identifies the algorithm used to hash the Central Directory data. This data does not have to be hashed, in which case the values for both the HashID and Hash Length will be 0. Possible values for HashID are: Value Algorithm ------ --------- 0x0000 none 0x0001 CRC32 0x8003 MD5 0x8004 SHA1 When the Central Directory data is signed, the same hash algorithm used to hash the Central Directory for signing should be used. This is recommended for processing efficiency, however, it is permissible for any of the above algorithms to be used independent of the signing process. The Hash Data will contain the hash data for the Central Directory. The length of this data will vary depending on the algorithm used. The Version Needed to Extract should be set to 62. The value for the Total Number of Entries on the Current Disk will be 0. These records will no longer support random access when encrypting the Central Directory. When the Central Directory is compressed and/or encrypted, the End of Central Directory record will store the value 0xFFFFFFFF as the value for the Total Number of Entries in the Central Directory. The value stored in the Total Number of Entries in the Central Directory on this Disk field will be 0. The actual values will be stored in the equivalent fields of the ZIP64 End of Central Directory record. Decrypting and decompressing the Central Directory is accomplished in the same manner as decrypting and decompressing a file.Kommentar
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