DES Documentation.DES can be decrypted using the password: aztech.
DES is an implementation of the United States National Bureau of Standards data encryption algorithm. A good article describing the theory of this encryption method is presented in BYTE, March 1979, "The Standard Data Encryption Algorithm, Part 1: An Overview," pp. 66-74, by Robert V. Meushaw. Without going into details, the DES algorithm encrypts data into a file of seemingly random bytes with values roughly uniform over the range 0..255. (Any required data compression should be performed on the file before it is encrypted because attempts to compress a file after encryp- tion would be stymied by the resulting randomness.) This application is implementated in C.
Macintosh DES differs from the NBS definition in several ways… (1) A header is encrypted in the file: data fork length, password, resource fork length, creator, file type, and original file name. This information is used to reconstruct the original file exactly. The password is encrypted merely to provide checking during decryption so that the password supplied by the user is correct; the password cannot be deduced from the encrypted file. (2) A random number (0..255) is inserted every eighth byte to disguise runs of the same character. Plain DES encrypts data in eight-byte blocks which come out the same for the same input of eight bytes. A run of spaces (in a program listing or table perhaps) would encrypt to the same eight bytes repeating over and over. The added security of the random bytes unfortunately results in the encrypted file being 1/8 longer. (Inter- estingly, the same file encrypted twice with the same password may result in two completely different output files depending on the initial value of the random number seed.) The random bytes are discarded during decryption.
Version 2.0 offers several improvements the most important of which is an approximate 40% speed improvement over version 1.0. The DES algorithm entails computing sixteen subkeys from the password input by the user. Version 1.0 recomputes the subkeys for every block of data encrypted; version 2.0 improves on this by computing the subkeys only once when the password is entered.
The new version prefills the "save" file name field by appending ".DES" to the original file name for encrypting a file or with the original file name for decrypting a file. The original file name is encrypted in the header so it is used to prefill the "save" file name field for decryption. Version 1.0 doesn't put the original file name in the header so files encrypted by version 1.0 are not compatible with version 2.0. Such files should be decrypted with version 1.0 then re-encrypted with version 2.0.
The new version allows file encryption or decryption to be cancelled with the Command-period abort sequence. This permits a long conversion to be terminated if you grow impatient. If encryption or decryption is interrupted the newly created output file is deleted.
This application is distributed under the ShareWare concept. If this program is useful to you, please send $15 (no price increase, same as DES 1.0) payable in U.S. funds to,
John P. Powers
2406 Richwood Drive
Garland, Texas 75042 U. S. A.
Suggestions for improvements can be sent to the same address or to CompuServe ID 75776,3247.
