DES_CRYPT(3)

NAME
des_read_password, des_string_to_key, des_random_key, des_set_key, des_ecb_encrypt, des_cbc_encrypt, des_pcbc_encrypt, des_cbc_cksum, des_quad_cksum, - (new) DES encryption

SYNOPSIS
#include <des.h>

int des_read_password(key,prompt,verify)
des_cblock *key;
char *prompt;
int verify;

int des_string_to_key(str,key)
char *str;
des_cblock key;

int des_random_key(key)
des_cblock *key;

int des_set_key(key,schedule)
des_cblock *key;
des_key_schedule schedule;

int des_ecb_encrypt(input,output,schedule,encrypt)
des_cblock *input;
des_cblock *output;
des_key_schedule schedule;
int encrypt;

int des_cbc_encrypt(input,output,length,schedule,ivec,encrypt)
des_cblock *input;
des_cblock *output;
long length;
des_key_schedule schedule;
des_cblock *ivec;
int encrypt;

int des_pcbc_encrypt(input,output,length,schedule,ivec,encrypt)
des_cblock *input;
des_cblock *output;
long length;
des_key_schedule schedule;
des_cblock *ivec;
int encrypt;

unsigned long des_cbc_cksum(input,output,length,schedule,ivec)
des_cblock *input;
des_cblock *output;
long length;
des_key_schedule schedule;
des_cblock *ivec;

unsigned long quad_cksum(input,output,length,out_count,seed)
des_cblock *input;
des_cblock *output;
long length;
int out_count;
des_cblock *seed;

DESCRIPTION
This library supports various DES encryption related operations. It differs from the crypt, setkey, and encrypt library routines in that it provides a true DES encryption, without modifying the algorithm, and executes much faster.

For each key that may be simultaneously active, create a des_key_schedule struct, defined in "des.h". Next, create key schedules (from the 8-byte keys) as needed, via des_set_key, prior to using the encryption or checksum routines. Then setup the input and output areas. Make sure to note the restrictions on lengths being multiples of eight bytes. Finally, invoke the encryption/decryption routines, des_ecb_encrypt or des_cbc_encrypt or des_pcbc_encrypt, or, to generate a cryptographic checksum, use quad_cksum (fast) or des_cbc_cksum (slow).

A des_cblock struct is an 8 byte block used as the fundamental unit for DES data and keys, and is defined as:

typedef unsigned char des_cblock[8];

and a des_key_schedule, is defined as:

typedef struct des_ks_struct {des_cblock _;} des_key_schedule[16];

des_read_password writes the string specified by prompt to the standard output, turns off echo (if possible) and reads an input string from standard input until terminated with a newline. If verify is non-zero, it prompts and reads input again, for use in applications such as changing a password; both versions are compared, and the input is requested repeatedly until they match. Then des_read_password converts the input string into a valid DES key, internally using the des_string_to_key routine. The newly created key is copied to the area pointed to by the key argument. des_read_password returns a zero if no errors occurred, or a -1 indicating that an error occurred trying to manipulate the terminal echo.

des_string_to_key converts an arbitrary length null-terminated string to an 8 byte DES key, with odd byte parity, per FIPS specification. A one-way function is used to convert the string to a key, making it very difficult to reconstruct the string from the key. The str argument is a pointer to the string, and key should point to a des_cblock supplied by the caller to receive the generated key. No meaningful value is returned. Void is not used for compatibility with other compilers.

des_random_key generates a random DES encryption key (eight bytes), set to odd parity per FIPS specifications. This routine uses the current time, process id, and a counter as a seed for the random number generator.
The caller must    supply space for the output key, pointed to
by argument key, then after calling des_random_key should call the des_set_key routine when needed. No meaningful value is returned. Void is not used for compatibility with other compilers.

des_set_key calculates a key schedule from all eight bytes of the input key, pointed to by the key argument, and outputs the schedule into the des_key_schedule indicated by the schedule argument. Make sure to pass a valid eight byte key; no padding is done. The key schedule may then be used in subsequent encryption/decryption/checksum operations. Many key schedules may be cached for later use. The user is responsible to clear keys and schedules as soon as no longer needed, to prevent their disclosure. The routine also checks the key parity, and returns a zero if the key parity is correct (odd), a -1 indicating a key parity error, or a -2 indicating use of an illegal weak key. If an error is returned, the key schedule was not created.

des_ecb_encrypt is the basic DES encryption routine that encrypts or decrypts a single 8-byte block in electronic code book mode. It always transforms the input data, pointed to by input, into the output data, pointed to by the output argument.

If the encrypt argument is non-zero, the input (cleartext) is encrypted into the output (ciphertext) using the key_schedule specified by the schedule argument, previously set via des_set_key

If encrypt is zero, the input (now ciphertext) is decrypted into the output (now cleartext).

Input and output may overlap.

No meaningful value is returned. Void is not used for compatibility with other compilers.

des_cbc_encrypt encrypts/decrypts using the cipher-block-chaining mode of DES. If the encrypt argument is non-zero, the routine cipher-block-chain encrypts the cleartext data pointed to by the input argument into the ciphertext pointed to by the output argument, using the key schedule provided by the schedule argument, and initialization vector provided by the ivec argument. If the length argument is not an integral multiple of eight bytes, the last block is copied to a temp and zero filled (highest addresses). The output is ALWAYS an integral multiple of eight bytes.

If encrypt is zero, the routine cipher-block chain decrypts the (now) ciphertext data pointed to by the input argument into (now) cleartext pointed to by the output argument using the key schedule provided by the schedule argument, and initialization vector provided by the ivec argument. Decryption ALWAYS operates on integral multiples of 8 bytes, so it will round the length provided up to the appropriate multiple. Consequently, it will always produce the rounded-up number of bytes of output cleartext. The application must determine if the output cleartext was zero-padded due to original cleartext lengths that were not integral multiples of 8.

No errors or meaningful values are returned. Void is not used for compatibility with other compilers.

A characteristic of cbc mode is that changing a single bit of the cleartext, then encrypting using cbc mode, affects ALL the subsequent ciphertext. This makes cryptanalysis much more difficult. However, modifying a single bit of the ciphertext, then decrypting, only affects the resulting cleartext from the modified block and the succeeding block. Therefore, des_pcbc_encrypt is STRONGLY recommended for applications where indefinite propagation of errors is required in order to detect modifications.

des_pcbc_encrypt encrypts/decrypts using a modified block chaining mode. Its calling sequence is identical to des_cbc_encrypt. It differs in its error propagation characteristics.

des_pcbc_encrypt is highly recommended for most encryption purposes, in that modification of a single bit of the ciphertext will affect ALL the subsequent (decrypted) cleartext. Similarly, modifying a single bit of the cleartext will affect ALL the subsequent (encrypted) ciphertext. "PCBC" mode, on encryption, "xors" both the cleartext of block N and the ciphertext resulting from block N with the cleartext for block N+1 prior to encrypting block N+1.

des_cbc_cksum produces an 8 byte cryptographic checksum by cipher-block-chain encrypting the cleartext data pointed to by the input argument. All of the ciphertext output is discarded, except the last 8-byte ciphertext block, which is written into the area pointed to by the output argument. It uses the key schedule, provided by the schedule argument and initialization vector provided by the ivec argument. If the length argument is not an integral multiple of eight bytes, the last cleartext block is copied to a temp and zero filled (highest addresses). The output is ALWAYS eight bytes.

The routine also returns an unsigned long, which is the last (highest address) half of the 8 byte checksum computed.

quad_cksum produces a checksum by chaining quadratic operations on the cleartext data pointed to by the input argument. The length argument specifies the length of the input -- only exactly that many bytes are included for the checksum, without any padding.

The algorithm may be iterated over the same input data, if the out_count argument is 2, 3 or 4, and the optional output argument is a non-null pointer . The default is one iteration, and it will not run more than 4 times. Multiple iterations run slower, but provide a longer checksum if desired. The seed argument provides an 8-byte seed for the first iteration. If multiple iterations are requested, the results of one iteration are automatically used as the seed for the next iteration.

It returns both an unsigned long checksum value, and if the output argument is not a null pointer, up to 16 bytes of the computed checksum are written into the output.

FILES
/usr/include/des.h
/usr/lib/libdes.a

SEE ALSO

DIAGNOSTICS

BUGS
This software has not yet been compiled or tested on machines other than the VAX and the IBM PC.

AUTHORS
Steve Miller, MIT Project Athena/Digital Equipment Corporation

RESTRICTIONS
COPYRIGHT 1985,1986 Massachusetts Institute of Technology

This software may not be exported outside of the US without a special license from the US Dept of Commerce. It may be replaced by any secret key block cipher with block length and key length of 8 bytes, as long as the interface is the same as described here.