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Cover PC-CRYPT Data Encryption and Decryption Program Version 7.0 20 August 1993 Supercedes all prior versions Copyright 1993 by James T. Demberger 9862 Lake Seminole Drive West Seminole, FL 34643 813-397-2930 PC-CRYPT is a user supported program. You are encouraged to copy and share this program with other users so long as the program is not distributed in modified form and this notice is not bypassed or removed. PC-CRYPT is NOT a public domain program. The program and documentation for PC-CRYPT may be freely copied for archive or working copies for personal non-profit use as outlined in copyright regulations. PC-CRYPT may be made available thru clubs or user groups, program libraries or on remote access data bases or bulletin boards. 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 - - - - - 2 Keyword Input - - - - - - - - - - - - 2 Random Number Generator - - - - - - - 2 Keyfile Generation - - - - - - - - - - 3 Encryption and Decryption - - - - - - 3 Using Keyfiles as One-Time-Pads - - - 3 Running PC-CRYPT from Menu - - - - - - - 4 Select Keyword - - - - - - - - - - - - 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 - - - - - - - - - - - - 7 Are PC-CRYPT Encrypt Files Unbreakable? - 7 Commercial User License - - - - - - - - - 7 Pseudo Basic Code for PC-CRYPT - - - - - 8 Miscellaneous Notes - - - - - - - - - - - 9 Page 1 Introduction The program has three 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 keyfiles. 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 file of random eight-bit codes to produce a cipher text file. The program uses the same virtual 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 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. Page 2 Functional Outline of PC-CRYPT The following four sections outline the functional operation of PC-CRYPT. The functional operation of the program in the form of pseudo Basic source code is shown on Pages 8 and 9. A reasonably proficient programmer, using this text and the pseudo source code, should be able to write a program in Basic or some other high level language that would have the same functionality as PC-CRYPT. Keyword Input PC-CRYPT uses a keyword input routine to read an eight character keyword or password. More than eight characters can be enter for the keyword, the excess characters are ignored. The keyword input code treats the first eight character entered as four hexadecimal characters. The first three pairs of characters are used to set values for the pointers used for keyfile generation and the fourth pair 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. The keyword input code accepts characters other than the normal hex characters 0 thru 9 and A thru F however these other characters do not enter into the conversion to decimal values for the pairs of hex characters. The use of other than normal hex characters for the keyword can be used to "hide" the characters actually used as pointer values. For example, the Social Security Number 214-35-1234 converts to pointer numbers 34, 5, 54 and 0. ZIP code + four numbers might also be used to conceal the pointer numbers. The program has a display capability that can be used to analyze the conversion process for the hexadecimal codes to decimal values. 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. The random numbers in each array range in value from 0 to 255 with only one occurrence of each random number in each array. One unique array is created for each of the seed numbers from 0 to 255. 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 the 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 processor. Page 3 Keyfile Generation PC-CRYPT creates virtual random keyfiles. Each character of a keyfile is generated as required using three numbers from an array 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. The initial counter or pointer settings are determined by three values calculated from the keyword. Keyfile 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 keyfile, there are 14,457,349 potential keyfiles. 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 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 pointer values all have a value of 1. Key characters with "calculated" zero value will probably occur at random intervals during the keyfile 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 Any file input/output method can be 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 keyfile generated by PC-CRYPT is in effect a OTP so long as the keyword used to generate the keyfile is never reused. In theory, Page 4 someone with a copy of the program could try all of the keywords that might have been used and by looking at the decrypt output, eventually "find" the keyword that was used to encrypt the clear text. In practice, this "brute force" decryption would probably be totally impractical since the number of possible keywords is 3,701,081,344 (239*241*251*256). 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 most 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 two screens display information about the program. These displays are followed by the keyword entry display: SELECT KEYWORD 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 K Old Keyword is Enter new 8 character (hex) Keyword AaBbCcDd 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 less than eight characters were entered before the Enter key was pressed. 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. If both of the character in a pair are the characters 0 thru 9 and A thru F (either upper or lower case), the equivalent decimal value for the hex pair is displayed. If neither of the characters is a hex character, the conversion results in a decimal zero. Combinations of one hex character and one non-hex character may convert to a zero or some value from 0 to 15. 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 T Turn Timer On/Off S Encrypt & Decrypt Test Strings F Process Disk Files 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. 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. 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 period (.) is printed as each 512 bytes of a file is processed. Total processing time and bytes per second is displayed if the timer option has been turned on. 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: PC-CRYPT /KW=keyword /FI=dr:\path\fname.ext /FO=dr:\path\fname.ext Page 6 The command line parameters must be entered in the order shown; /KW=keyword, /FI=filename for the file to be encrypt or the file to be decrypt, and /FO=filename for the encrypt file or the decrypt file. There is no output shown on the display nor is any other input required other than the parameters entered on the command line. 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 probably not compress encrypt files or only compress these 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 prepare password protected files that are probably as secure as the files produced by PC-CRYPT. A second layer of encryption using PC-CRYPT is just additional insurance that the encryption is unbreakable. The short string of characters for the filename found in compressed files is not long enough to permit brute force decryption of an encrypt compressed file since the remainder of the compressed file looks like a string of random characters with no discernible pattern. Page 7 Keyword Security The security of keywords or passwords 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 - security comes from the fact that the four hexadecimal numbers represent 3,701,081,344 usable keywords. 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 eight months or more. There are many different method for distribution of keywords depending on the number of users that require knowledge of the keywords. Lists of keywords for use by different users can be distributed by means of an encrypt file for which each user has been furnished the initial keyword. 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. Had the "clear text" for a file been a cipher text file (double encryption), any brute force "try every possible keyword" attempt at decryption 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 copies of the PC-CRYPT program will produce the same array of 256 numbers and the same keyfile 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 keyfile is recycled. Cipher text files produced with Vernam encryption using a random keyfile with a length equal to or greater that the clear text to be encrypt are considered to unbreakable unless the keyfile is known. Cipher text files produced using a random One-Time-Pad keyfile 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. Commercial User License No registration or license is required for personal non-profit use of the PC-CRYPT program. A $25.00 license fee for commercial users has been set just to find out if there are honest commercial users who want to use the program instead of having house programmers write a functionally equivalent program. Page 8 Pseudo Basic Code for PC-CRYPT The following pseudo Basic source code outlines keyword input processing, random number generation and XOR encryption of disk files. These are the three essential routines used by the PC-CRYPT to produce cipher text files and to decrypt cipher text files. I am not releasing the Basic source code for a variety of reasons, several of which are: a) The source code was written using PowerBASIC and it probably will require considerable modification before it will run using other more common dialects of Basic. b) The program contains a lot of testing and debugging code that is not relevant to the normal operation of the program. c) The program is written in unstructured "spaghetti" Basic code and I don't have the time to "clean it up" to the point where it would not invite negative comments regarding my profeciency as a programmer. d) Knowledge of the internal operation of the program is not required in order to evaluate the validity of the methods used file encryption and decryption. Users can write their own programs incorporating the following pseudo code with the assurance that the encryption methods used will produce unbreakable cipher text. rem Keyword Input rem Input is 8 hex(?) characters in variable Key.word$ P.W1$ = MID$(Key.word$,1,2): P.W1$ = "&h" + P.W1$ P.W2$ = MID$(Key.word$,3,2): P.W2$ = "&h" + P.W2$ P.W3$ = MID$(Key.word$,5,2): P.W3$ = "&h" + P.W3$ P.W4$ = MID$(Key.word$,7,2): P.W4$ = "&h" + P.W4$ Point.1 = VAL (P.W1$) MOD 239 + 1 Point.2 = VAL (P.W2$) MOD 241 + 1 Point.3 = VAL (P.W3$) MOD 251 + 1 Random.No.Pointer = VAL (P.W4$) MOD 256 rem output is Point.1 = (1,2,....,238,239) rem Point.2 = (1,2,....,240,241) rem Point.3 = (1,2,....,250,251) rem Random.No.Pointer = (0,1,....,254,255) rem Random Number Generator rem start of loop to generate 256 pseudo random numbers SEED = Random.No.Pointer FOR INDEX = 1 TO 256 rem next three statements generate pseudo random numbers rem that range in value from 0 to 255 SEED = (SEED * 997) + 32771 SEED = SEED - (INT (SEED / 32768) * 32768) SEED = FIX (SEED) MOD 256 rem this line puts the random numbers in a 256 byte array Random.Nos Random.Nos (INDEX) = SEED rem end of loop to generate 256 pseudo random numbers NEXT INDEX Page 9 rem Key File Generation and File Encryption and Decryption rem use LOF to determine number character in file to be processed Length.Input = LOF(INPUT.FILE) Main.Loop = 1 Do While Main.Loop <= Length.Input rem code to read input characters from INPUT.FILE Input.Char = INPUT.FILE (Main.Loop) rem values for (Point.x) from Keyword Input rem Random.Nos array from Random Number Generator rem Key.Character created by XOR operation on three numbers from rem Random.Nos array rem next line continues (_) to second line following Key.Character = _ Random.Nos(Point.1) XOR Random.Nos(Point.2) XOR Random.Nos(Point.3) rem increment counters and test for reset Point.1 = Point.1 + 1: IF Point.1 > 239 THEN Point.1 = 1 Point.2 = Point.2 + 1: IF Point.2 > 241 THEN Point.2 = 1 Point.3 = Point.3 + 1: IF Point.3 > 251 THEN Point.3 = 1 rem Code to XOR input character with Key.Character goes here Output.Char = Input.Char XOR Key.Character rem Code to write output character to OUTPUT.FILE OUTPUT.FILE (Main.Loop) = Output.Char rem increment main loop counter Main.Loop = Main.Loop + 1 DO END Miscellaneous Notes Cipher text files produced with Version 6 of PC-CRYPT use the same random number generator and XOR processing as Version 7 of PC-CRYPT however the keyword input processing is entirely different. Any archive type files encrypt with Version 6 should be decrypt and then reencrypt with Version 7. There are Federal Government regulations restricting the export of "encryption devices". I do not know if, under these regulations, the PC-CRYPT program would be classified as an "encryption device". The PC-CRYPT program uses the four hexidecimal values in the keyword to assign values to the three pointer and the random number seed in 1234 order. Changing the order in which the four hexidecimal values are used to assign values to the three pointers and the random number seed can be used to create 23 additional variations of the program.