NAME

	setupDESstate, des_key_setup, block_cipher, desCBCencrypt, desCBCdecrypt, desECBencrypt, desECBdecrypt, des3CBCencrypt, des3CBCdecrypt, des3ECBencrypt, des3ECBdecrypt, key_setup, des56to64, des64to56, setupDES3state, triple_block_cipher – single and triple digital encryption standard

SYNOPSIS

#include <u.h> #include <libc.h> #include <mp.h> #include <libsec.h> void des_key_setup(uchar key[8], ulong schedule[32]) void block_cipher(ulong *schedule, uchar *data, int decrypting) void setupDESstate(DESstate *s, uchar key[8], uchar *ivec) void desCBCencrypt(uchar *p, int len, DESstate *s) void desCBCdecrypt(uchar *p, int len, DESstate *s) void desECBencrypt(uchar *p, int len, DESstate *s) void desECBdecrypt(uchar *p, int len, DESstate *s) void triple_block_cipher(ulong expanded_key[3][32], uchar text[8], int ende) void setupDES3state(DES3state *s, uchar key[3][8], uchar *ivec) void des3CBCencrypt(uchar *p, int len, DES3state *s) void des3CBCdecrypt(uchar *p, int len, DES3state *s) void des3ECBencrypt(uchar *p, int len, DES3state *s) void des3ECBdecrypt(uchar *p, int len, DES3state *s) void key_setup(uchar[7], ulong[32]) void des56to64(uchar *k56, uchar *k64) void des64to56(uchar *k64, uchar *k56)

DESCRIPTION

The Digital Encryption Standard (DES) is a shared–key or symmetric encryption algorithm using either a 56–bit key for single DES or three 56–bit keys for triple DES. The keys are encoded into 64 bits where every eight bit is parity. The basic DES function, block_cipher, works on a block of 8 bytes, converting them in place. It takes a key schedule, a pointer to the block, and a flag indicating encrypting (0) or decrypting (1). The key schedule is created from the key using des_key_setup. Since it is a bit awkward, block_cipher is rarely called directly. Instead, one normally uses routines that encrypt larger buffers of data and which may chain the encryption state from one buffer to the next. These routines keep track of the state of the encryption using a DESstate structure that contains the key schedule and any chained state. SetupDESstate sets up the DESstate structure using the key and an 8–byte initialization vector. Electronic code book, using desECBencrypt and desECBdecrypt, is the less secure mode. The encryption of each 8 bytes does not depend on the encryption of any other. Hence the encryption is a substitution cipher using 64 bit characters. Cipher block chaining mode, using desCBCencrypt and desCBCdecrypt, is more secure. Every block encrypted depends on the initialization vector and all blocks encrypted before it. For both CBC and ECB modes, a stream of data can be encrypted as multiple buffers. However, all buffers except the last must be a multiple of 8 bytes to ensure successful decryption of the stream. There are equivalent triple–DES (DES3–EDE) functions for each of the DES functions. In the past, Plan 9 used a 56–bit or 7–byte format for DES keys. To be compatible with the rest of the world, we've abandoned this format. There are two functions, des56to64 and des64to56, to convert back and forth between the two formats. Also a key schedule can be set up from the 7–byte format using key_setup.

SOURCE

	/sys/src/libsec

SEE ALSO

	mp(2), aes(2), blowfish(2), dsa(2), elgamal(2), rc4(2), rsa(2), sechash(2), prime(2), rand(2) Breaking DES, Electronic Frontier Foundation, O'Reilly, 1998

BUGS

	Single DES can be realistically broken by brute–force; its 56–bit key is just too short. It should not be used in new code, which should probably use aes(2) instead, or at least triple DES.

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