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Cover PC-CRYPT Data Encryption and Decryption Program Version 8.0 1 October 1993 Supercedes all prior versions Copyright 1993 by James T. Demberger 9862 Lake Seminole Drive West Seminole, FL 34643 813-397-2930 CompuServe 74425,1642 PC-CRYPT is NOT a public domain program. The program and documentation for PC-CRYPT may be freely copied for archive or working copies as outlined in copyright regulations. PC-CRYPT files as noted in the Miscellaneous Notes section may be made available thru clubs or user groups, program libraries or on remote access data bases or bulletin boards subject to the export restrictions outlined in the EXPORT WARNING section. Disclaimer The PC-CRYPT program and associated documentation is provided on an "as is" basis without warranty of any kind, expressed or implied. Anyone using this software assumes all risks as to the quality and performance of the software. The author disclaims all liability for any special, incidental, consequential, direct or indirect damages due to either proper and improper use of the program. Table of Contents Introduction - - - - - - - - - - - - - - 1 System Requirements - - - - - - - - - - - 1 Vernam Encryption - - - - - - - - - - - - 1 Functional Outline of PC-CRYPT - - - - - 1 Keyword or Phrase Input - - - - - - - 2 Random Number Generator - - - - - - - 2 Keyfile Generation - - - - - - - - - - 2 Encryption and Decryption - - - - - - 3 Using Keyfiles as One-Time-Pads - - - 3 Running PC-CRYPT from Menu - - - - - - - 4 Select Keyword or Phrase - - - - - - - 4 PC-CRYPT Menu - - - - - - - - - - - - 5 Encrypt & Decrypt Test Strings - - - - 5 Processing Disk Files - - - - - - - - 5 Running PC-CRYPT from Command Line - - - 5 Transmitting Encrypt Files - - - - - - - 6 Double Encryption - - - - - - - - - - - - 6 Keyword Security - - - - - - - - - - - - 6 Are PC-CRYPT Encrypt Files Unbreakable? - 7 Customizing PC-CRYPT - - - - - - - - - - 7 Basic Source Code for PC-CRYPT - - - - - 8 Miscellaneous Notes - - - - - - - - - - - 9 EXPORT WARNING - - - - - - - - - - - - - 9 Page 1 Introduction The PC-CRYPT software has four main functions: a: Demonstration of the Vernam encryption and decryption process. b: Outline operation of program and algorithms used by the program. c: Encryption and decryption of disk files using Vernam encryption and decryption combined with use of random or O-T-P keyfiles. d: Provide Basic source code that can be run as is or that can be modified to meet specific user requirements. System Requirements The program requires an IBM computer or compatible running DOS with color or monochrome display and one or more disk drives. File storage space must be available equal to twice the size of the largest file to be encrypt or decrypt. Vernam Encryption In 1917, long before the age of electronic computers, Gilbert S. Vernam developed an encryption process for messages punched in paper tape using Baudot or five channel teletype code. He used the electro-mechanical equivalent of a logical exclusive OR operation (XOR) on each character code in a message tape and a corresponding random character code in a key tape to produce a third tape with the encrypt message. Decryption used the same process except that a tape with the encrypt message and a copy the key tape were XORed to produce the decrypt message. PC-CRYPT uses essentially the same process to encrypt and decrypt disk files. Each character in a clear text file is XORed with the corresponding character in a virtual key file of random eight-bit codes to produce a cipher text file. The program uses the same virtual key file to decrypt the cipher text file and produce a copy of the original clear text file. One problem with the original Vernam process related to the key tapes. For a secure system, the characters in the key tapes had to be in random order and the number of characters in a key tape had to exceed the number of characters in the message to be encrypt. A duplicate of key tapes had to furnished to anyone who needed to decrypt messages. The physical security of the miles of paper tape was another problem. PC-CRYPT can generate thousands of different virtual random keys with lengths of 14,457,349 bytes. There is no need to store these virtual keys as a file since each of the virtual keys can be regenerated when required for decryption of cipher text files. The eXclusive OR function (XOR) performed by Vernam encryption and decryption used by PC-CRYPT is "blind" as to the data in the input file. Any input file is XORed to produce an output file. If a clear text file is used as the input file, a cipher text output file is created. If a cipher text file is used as the input file, a copy of the original clear text file is created. Functional Outline of PC-CRYPT The following four sections outline the functional operation of PC-CRYPT. The Basic source code for the program is in the file PC-CRYPT.BAS. A reasonably proficient programmer, using this text and the Basic source code, should be able to write a program in Basic, Pascal, C or some other high level language that would have the same functionality as PC-CRYPT. Page 2 Keyword or Phrase Input PC-CRYPT uses a keyword input routine to read a keyword or phrase that may be up to 50 characters in length. The number of characters required for the keyword or phrase is relative to number of hexadecimal characters 0 thru 9 and A thru F (either lower or upper case) that are entered. Other alphabetic and special characters are ignored. The keyword input code treats the first eight hexadecimal characters entered as four hexadecimal numeric pairs with this minor exception - no duplication of any of the first three pairs is permitted. An error message will be displayed if a duplicate pair is found and you will be prompted to re-enter the keyword or phrase. The first three pairs of hexadecimal characters are used to set the decimal values for the pointers used for key file generation and the fourth pair is used to set a value for a seed for the random number generator. The MOD function is used to reduce the maximum value for the pointers to 239 for the first pointer, to 241 for the second pointer and to 251 for the third pointer. The value for the seed for the random number generator ranges from 0 to 255. Due to the use of the MOD function, certain hexadecimal characters above EF for one pointer value may result in a duplication of the pointer value for another pointer that has a value from 00 to 0F. This duplication will result in the error message mentioned in the previous paragraph. Since the keyword input code accepts but ignores characters other than hexadecimal characters, use of other than hexadecimal characters for the keyword or phrase can be used to "hide" the characters actually used as pointer values. For example, "SSN 123-45-6789" (= HEX 12 34 56 78) and "The 15 pound fat cat is hungry" (= HEX e1 5d fa ca) are accepted as a valid keywords or phrases. Random Number Generator The random number generator used by PC-CRYPT is a pseudo random number generator that will create an array or string of 256 random numbers. A unique array is created for each of the seed numbers from 0 to 255. The random numbers in each array range in value from 0 to 255 with only one occurrence of each of the numbers in each array. PC-CRYPT uses the 256 numbers in the array in "as generated" order however a production version of the program might reverse the order of the 256 numbers or make some other transformation that will "customize" the encryption pattern. Since this pseudo random generator uses only integer values for input and output, there is no precision problem such as would occur with random number generators that have fractional numbers as output. The compiled 8086/8088 machine code in a PC-CRYPT executable file works the same way and produces the same random array for the same seed number when run on a any computer system using a 80X86/88 processor. Key file Generation PC-CRYPT creates virtual random keyfiles. Each character of a key file is generated as required using three numbers from an array or list of 256 random numbers. Each of the virtual keyfiles may be as long as 14,457,349 bytes. The length is the result of recycling the random numbers in the random number array using three nested loops with counters that are reset to 1 after 239, 241 or 251 cycles. You might note that 14,457,349 is the least common multiple of the three prime numbers 239, 241 and 251. Page 3 The initial counter or pointer settings are determined by three values calculated from the keyword. Key file generation start at the three random numbers pointed to by the initial pointer values. Since each of the initial pointer values in effect defines a different virtual key file, there are almost 14,457,349 potential keyfiles. Almost since there are 239 + 239 hexadecimal values for the first and second pairs that can not be used with the same value for the second or third pairs and 2 values for the second pair that can not be used with the same value for the third pair. A key character is a random number resulting from the XOR operation on the three random numbers in the random number array pointed to by the current value of the three pointers. If we were to assume that the random number in the first position of the random number array was a zero and that initial setting of each of the three array pointers was 1, the XOR value of the key character would be a "natural" zero (as compared to a "calculated" zero that results from 1 XOR 2 XOR 3). Assuming further that there was no other occurrence of a zero in the random number array, the next "natural" zero (as opposed to a "calculated" zero) will not occur until the recycling pointer values are simultaneously reset to a value of 1. Key characters with "calculated" zero value will probably occur at random intervals during the key file generation. Again assuming that the initial values of the three pointers was 1, the pointer values will simultaneously return to 1 when the first pointer value has recycled thru the random number array 60,491 times, the second pointer value has recycled 59,989 times and the third pointer has recycled 57,599 times. This will occur after 14,457,349 key characters have been generated if the length of the clear text file is 14,457,349 bytes or longer in length. Encryption and Decryption Binary file input/output statements are used to read the clear or cipher text input files and to write the cipher or clear text output files. The method used by PC-CRYPT reads sequential 512 byte blocks into a character string array and creates the encrypt or decrypt text in another 512 byte character string array. If the length of the input file is not a multiple of 512, a "short" or partial block is processed as the last block. Each sequential character of an input block is XORed with the next sequential key character from the virtual key file to produce the corresponding sequential character in the output string. Using Keyfiles as One-Time-Pads One-Time-Pad (OTP) encryption systems are considered unbreakable. A virtual key file generated by PC-CRYPT is in effect a OTP so long as the keyword or phrase used to generate the key file is never reused. Changing just one out of the four hex character by incrementing it by one on a daily basis will provide enough different keywords to last for more then eight months. If your only interest is in files of random numbers for use as OTPs, PC-CRYPT can be used to generate these files. A "nothing" file consisting of a character string of the hexadecimal value 00 when encrypt using any keyword or phrase will generate a file of the random characters with decimal codes from 0 to 255. The file NULL.ZIP that is Page 4 included with the other PC-CRYPT files can be unZIPped to the "nothing" file NULL.1MB that is a string of 1,024,000 characters each with a hexadecimal value of 00. If you have the hard drive space, you can concatenate 15 of the million byte files and encrypt the 15 million byte OTP file to verify that the first 239 bytes of the encrypt file are repeated starting at the 14,457,350th position of the of the file. These 239 bytes in the same sequence do not appear anywhere prior to this position however shorter sequences from these 239 bytes may be found before the 14,457,350th position. Running PC-CRYPT from Menu From DOS ready key [dr:][\path\]PC-CRYPT then press the Enter key. A default option is shown for the response to some of the prompts displayed by the program. As an example, a prompt for a yes or no response will display "y/N". Press Enter to take the no default option indicated by the upper case N. Either a lower case y or an upper case Y must be pressed for the yes response. If only lower case options or no option is shown, an entry other than the Enter key must be used. The first screen displayed by the program is the menu for keyword or phrase input. This screen is also displayed as one of the options from the main menu. Select Keyword or Phrase Press Enter for No visible display of Keyword OR Press K for a visible display of Keyword only OR Press D for display of pointers and random numbers Old Keyword is Enter new Keyword or phrase Press any key to continue _ Press the Enter key for the no visible display of the keyword as it is being entered if you don't want someone looking over your shoulder to see the keyword. Press the K key to display both the currently selected keyword and the new keyword being entered. An error message will be displayed if an entered keyword or phrase does not have at least eight hexadecimal characters and there is a duplication among the first three hexadecimal pairs. Press the D key for a visible display of the keywords and a display of the hexadecimal and the decimal values for the loop pointers and random number seed and the numbers in the random number array. The 256 random numbers generated using the seed for the random number generator are displayed on the next 13 lines. Page 5 PC-CRYPT MENU K Select Keyword or Phrase T Turn Timer On/Off S Encrypt & Decrypt Test Strings F Process Disk Files S Shell to DOS X Exit/End Program Enter Option Letter _ Press K for the Select Keyword option if you wish to change the keyword originally entered during the initial Select Keyword display. Press T for a prompt to turn on or off a timer that will total number of characters encrypt or decrypt and the total time required. The total time is for encryption or decryption only and does not include time for reading and writing files. The program uses these totals to compute and display the thruput in characters per second. Timing is accurate to approximately 1/18th of a second or +/- 555 characters if the number of characters per second is around 10,000. Timings are not displayed unless the total time is more than approximately 1/6 of a second. Encrypt & Decrypt Test Strings The Encrypt & Decrypt Test Strings option displays a submenu from which you may select four different types of character strings for encryption and decryption. The clear text, cipher text and decrypt text strings are displayed. This option is primarily used to look at the pattern of characters produced when you encrypt strings of upper case, lower case and numeric characters. Key board input of test records accepts ASCII codes entered with the Alt key and numeric keys. In some cases, a character may be encrypt as the same character. This is not an error; it demonstrates that the XOR function is really working as it should. Encryption may result in the generation of the hexadecimal code 07 (Bell code) in which case you will hear a beep when the encrypt text is displayed. Process Disk Files When you select this option the following prompts and message will be displayed: Enter Input [dr:][\path\]filename.ext Enter Output [dr:][\path\]filename.ext Processing bytes A running total is displayed as each 512 bytes of a file are processed. Total processing time and bytes per second is displayed if the timer option has been turned on. The output file will replace or overlay any file with the same name and extension. Running PC-CRYPT from Command Line If your only need is to encrypt or to decrypt existing files, PC-CRYPT may be run from the DOS command line by entering the keyword, the input filename and the output filename. A command line example follows: Page 6 PC-CRYPT /KW=keyword /FI=dr:\path\fname.ext /FO=dr:\path\fname.ext The command line parameters must be entered in the order shown; /KW=keyword, /FI=filename for the clear text file to be encrypt or the cipher text file to be decrypt, and /FO=filename for the encrypt cipher text file or the decrypt clear text file. Should there be an error in the command line parameters, i.e. /F0 instead of /FO or a file "not found" for the input filename, an error message will be displayed that will permit canceling the run or to resuming using the menu interface. Transmitting Encrypt Files An encrypt data file may be sent to another system for decryption using a communications link or as a disk file. Since encrypt data files may contain character strings corresponding to transmission control codes, encrypt files should be transmitted as eight bit binary files using XMODEM or some equivalent transmission protocol for binary files. File compression utilities will only compress encrypt or cipher text files by a very small percentage. Double Encryption The cipher text produced with PC-CRYPT is probably unbreakable. However there is the possibility that a sufficiently fast and powerful computer using an automated brute force decryption system combined with some technique for clear text recognition might permit the decryption of a file. Double encryption can be implemented by encrypting a cipher text file a second time with a keyword different from that used for the first encryption. The same two keywords must be used to decrypt the double encrypt data. Use of any text recognition method is defeated by double encryption since there is no way to determine which of the many files produced by any attempt at decryption is really the target cipher file rather than a file of computer generated "garbage". As an alternative to using two passes for double encryption, the program could be modified to use two different keywords simultaneously. The XORed character output from the first keyword would be XORed with the key character generated from the second keyword. A much simpler double encryption system is possible thru the use of password protected compressed files. The widely used PKZIP/PKUNZIP compression utility programs can be used to compress or decompress PC-CRYPT encrypt files. The documentation for PKZIP estimates the time to "hack" an eight character (A thru Z) PKZIP password to be 241 days assuming 10,000 tests per second. If the assumption is made that hacking uses some form of text recognition to identify a possible password, hacking will fail since there is nothing other than random characters in the cipher text files produced by PC-CRYPT. Keyword Security The security of keywords or phrases is the most important factor in the use of private key encryption systems. In the case of PC-CRYPT, there is nothing secure about the form of a keyword - it is always four pairs of hexadecimal characters. Since actual keywords can be "hidden" in a list of names and Social Security numbers or as the date that a file was created, there is no need to write a single keyword or list of keywords in eight character hexadecimal format that can be recognized as keywords or phrases. Page 7 There are many different method for distribution of keywords depending on the number of users that require knowledge of the keywords. Lists of "hidden" keywords for use by different users can be distributed by means of an encrypt file; the keyword or phrase for this file may be furnished each user by any secure method. I'll leave how to handle the distribution of the initial keyword up to to those who are going to make use of PC-CRYPT. Are PC-CRYPT Encrypt Files Unbreakable? The algorithm used by the pseudo random number generator and the method used to generate the 14,457,349 byte pseudo random keyfiles in PC-CRYPT are public knowledge. A PC-CRYPT cipher text file is breakable only in the sense that it is known that one of the 3,701,081,344 possible keywords will decrypt the file. Any brute force "try every possible keyword" attempt to break a double encrypt file will produce millions of files of which none can be recognized as the "broken" cipher text file. Neither the array of 256 numbers used by the program nor the 14,457,349 byte keyfiles are truly random since two or more copies of the same compiled PC-CRYPT program will produce the same array of 256 numbers and the same key file when the same keyword is used. A cipher text file produced by the program is truly random so long as the clear text is not known. A clear text file known to have long strings of the space character or repeated strings of text will not produce any discernible pattern of characters in the cipher text file. In those cases where there is a need to encrypt a file longer than 14,457,349 bytes in length, there will be nothing discernible in the cipher text file to indicate the actual point where the key file is recycled. Cipher text files produced with Vernam encryption using a random key file with a length equal to or greater that the clear text to be encrypt are considered to unbreakable unless the key file is known. Cipher text files produced using a random One-Time-Pad key file for encryption are considered to be unbreakable so long as the random One-Time-Pad file is not reused. The cipher text files produced by the PC-CRYPT program meet the conditions required to be considered to be unbreakable for both Vernam and One-Time-Pad encryption. Customizing PC-CRYPT Many users will prefer to use a customized variation of the program and to limit the distribution of the customized program to only "authorized" users. The method PC-CRYPT uses to generate a specific set of random codes and to set the points where encryption and decryption starts using the random codes permits the creation of thousands of different "customized" variations. There are 24 major variations based on the way that PC-CRYPT uses the four hexadecimal pairs to set the initial pointer values and the seed for the random number generator. The first 3 pairs are used for initial pointer values in 1, 2, 3 order while the 4th pair is used for the random number seed. The pairs can be used in any order, e.i. 4, 3, 2, 1. With 24 different sequences available, any single keyword can lead to any of 24 different encryption patterns. Page 8 There are 256 random numbers is the random array used by the program however only the first 239 numbers are used by the first pointer loop, 241 numbers by the second loop and 251 by the third loop. All of the pointers are reset to 1 when the have been incremented to their maximum values. In the case of the first pointer, the maximum value could be set to 256 and reset to 18 while still providing 239 random numbers. The second and third pointers can be set to a maximum value of 256 and reset to 16 and 6 provide 241 and 251 random numbers respectively. There are 18 sets of maximum values and reset values available for the first pointer, 16 for the second and 6 for the third pointer. These sets of values when combined with the 24 sequences of random pair assignments will provide 24*18*16*6 or 41,472 different variations of the PC-CRYPT program. The number of different variations of PC-CRYPT can be double by reversing the order in which the array of 256 random numbers used for key file generation are stored. These random numbers are stored as generated in order from 1 to 256; a change in one statement in the program will store the numbers in reverse order from 256 down to 1. The first use that should be made of any variation of PC-CRYPT that you plan to use for actual encryption of sensitive data files should be the encryption of the Basic source code file and any compiled executable files for that variation. The security of the keyword or phrase used to encrypt the source and compiled code files is even more important than the security of the keywords or phrases that will be used to encrypt other sensitive data files. Basic Code for PC-CRYPT The Basic source code for PC-CRYPT is in the file PC-CRYPT.BAS. This source code file has statement lines for Power BASIC version 3.0 (PB) and for Quick BASIC version 4.5 and QBASIC (QB). The only differences between these two BASIC dialects in so far as the program is concerned are in the statements for the binary file input and output and the fact that Command Line operation is not possible with QBASIC. The PC-CRYPT.EXE file furnished with this software package was compiled with Power BASIC version 3.0c. An EXE file compiled with Quick BASIC version 4.5 may run 1 or 2 percent faster than the Power BASIC EXE file however the PB EXE file is much smaller (44,106 KB) than the QB EXE file (57,460 KB). All statement lines peculiar either to PB or to QB have a REM PB or REM QB comment at character position 60 (+/-). The source code file will run as is under QB since the REM PB statements have a REM at the beginning of the each of the REM PB statements. If you want to run or compile the source code using Power Basic, make a copy of PC-CRYPT.BAS and rename the copy to PB-CRYPT.BAS. Edit the PB copy to add a REM at the beginning of each statement that has a REM QB and delete the REM found at the beginning of each REM PB statement before you run or compile the PB version. The BASIC source code has been re-written over the years starting with interpreted BASIC for the Radio Shack TRS-80 and for the original IBM PC. For the most part, changes to the source were made only when and where required to run or compile the code for newer versions of the IBM, MicroSoft compilers and for Power BASIC. Page 9 The program is written in unstructured "spaghetti" Basic code. The variable names and labels are rather cryptic. No attempt has been made to "prettify" the code. Minimal error checking is used only where required to ensure proper input to the program. Miscellaneous Notes Cipher text files produced with Versions 6.0 and 7.0 of PC-CRYPT use the same random number generator and XOR processing as Version 8.0 of PC-CRYPT however the keyword input processing is entirely different. Any archive type files encrypt with Version 6 or 7 should be decrypt and then re-encrypt with Version 8. No registration or license is required for personal or commercial use of the PC-CRYPT program or for derivative versions of the program. Any comments regarding the PC-CRYPT program and any methods that might be used to "break" encrypt files will be appreciated. The comments can be mailed to the address on the cover page or EMailed thru CompuServe. Files included in both CRYP8USA.ZIP and CRYP8EXP.ZIP file: FILE_ID DIZ --- 10-01-93 8:00a Description CRYP*.ZIP NULL ZIP 1122 10-01-93 8:00a NULL.1MB File PC-CRYPT DOC 31394 10-01-93 8:00a PC-CRYPT V8 Documentation PC-CRYPT EXE 44106 10-01-93 8:00a PC-CRYPT.EXE (PowerBASIC) A license by the Department of State is required for export of the encryption source code. The source code file PC-CRYPT.BAS is included in the CRYP8USA.ZIP file and not in the CRYP8EXP.ZIP file. Do not copy or distribute this source code file in any situation where there is a possibility that export restrictions would apply (see following EXPORT WARNING). PC-CRYPT BAS 14909 10-01-93 8:00a BASIC Source (QuickBASIC) EXPORT WARNING Federal Government regulations require a valid Department of State license for the export of cryptographic source code. These regulations are applicable to the PC-CRYPT.BAS source code file; regulations do not prohibit the export of the executable compiled code in the PC-CRYPT.EXE file. Any individual or activity, such as a bulletin board system or an "information provider" such a CompuServe, etc., that makes the source code available for download to anyone accessing the system from a foriegn country might be considered by the Department of State to be an exporter of the source code.