readme.des

DES加密解密源代码

Data Encryption Standard routines in portable C and 386/486 assembler
Phil Karn, KA9Q
September 2, 1994 - initial release
November 4, 1995 - updated for triple DES
November 15, 1995 - updated 3DES for 3 independent keys
December 14, 1995 - pipeline improvements to desgnu.s 
April 29, 1999 - repackaged into separate x86 and portable C versions;
		timings added for some more modern machines

For conventional (single key) DES, there are two user calls, deskey()
for creating a key schedule for encryption or decryption, and des()
that does the actual encryption or decryption.

The triple DES counterparts are des3key() and des3(). These implement
triple DES in the "EDE" mode. Three distinct 56-bit keys are used, and
the key schedule array is three times the size of that used for
conventional DES. When encrypting, the first key is used in the
encryption mode, then the second key is used in the decryption mode,
and then the third key is again used in the encryption
mode. Decryption in triple DES is the same, except that the three
operations become DED (decrypt-encrypt-decrypt).

This convention is followed by many 3DES implementations to facilitate
backward compatibility with conventional DES. Setting all three keys
in 3DES to the same value is equivalent to encrypting once with the
single key.

The functions are declared as follows:

typedef unsigned long DES_KS[16][2];	/* Single-key DES key schedule */
typedef unsigned long DES3_KS[48][2];	/* Triple-DES key schedule */

void
deskey(DES_KS,unsigned char key[8],int decrypt);

void
des3key(DES3_KS,unsigned char key[16],int decrypt);

void
des(DES_KS,unsigned char buf[8]);

void
des3(DES3_KS,unsigned char buf[8]);


The deskey() and des3key() functions initialize a key schedule 'k'
from 'key' for later use by des() or des3(), respectively; space for
these arrays must be allocated by the caller. Be sure to note the
different size requirements.

The des() and des3() functions encrypt or decrypt the user-provided
buffer 'buf' in place under control of the key schedule 'k'. Whether
des() and des3() encrypt or decrypt is controlled by the 'decrypt'
argument to deskey() or des3key() when the key schedule is
created. This is because DES encryption and decryption are identical
except for the key schedule being in reverse order.

The deskey() and des3key() functions are not optimized for speed.  If
you wish to alternate quickly between encryption and decryption using
the same key it is best to allocate two separate key schedules with
the same key, one for encryption and one for decryption.

The deskey() and des3key() routines are in portable C; if you want
great speed (e.g., for key searching or UNIX password cracking) you'll
have to tune it yourself.

The des() and des3() routines in desgnu.s and des3gnu.s, respectively,
are in GNU 'as' format.

The des() and des3() routines refer to an external table containing
the combined S&P boxes. This table is generated as a C source file by
the program gensp.

This is an almost complete rewrite of my old public domain DES code
written circa 1987, which in turn borrowed heavily from Jim Gillogly's
1977 public domain code. I pretty much kept my key scheduling code,
but the actual encrypt/decrypt routines are taken from from Richard
Outerbridge's DES code as printed in Schneier's "Applied
Cryptography."

Some measured execution speeds of des() (single-key DES) in crypts/sec:

Borland Assembler version, Borland C++ 3.1 -O2, DOS 16-bit real mode:
110,254 (90 Mhz Pentium)
72,098 (60 Mhz Pentium)
43,460 (486DX-50)

Gnu assembler version, 32-bit protected mode, BSDI 2.0:
167,476 (Pentium 90)

Gnu assembler version, Linux 2.2.6:
877,193 (Pentium-II 400 MHz)

Portable C version:
109,890 (Sparc 20 Model 61, GCC 2.5.8 -O)
99,300 (Pentium 90, BSDI 2.0, GCC 2.6.3 -O2, 32-bit prot mode)
93,457 (Sparc 1000)
88,495 (Sparc 5/85)
84,034 (Sparc 2000)
83,333 (Sparc 20 Model 51)
80,000 (Sparc 10 Model 51)
71,942 (Sparc 5/70)
66,666 (Sparc 10 Model 41)
66,225 (Sparc 10 Model 30)
45,248 (Sun 4/15)
44,643 (Sun 4/30)
36,101 (Sun 4/690)
35,335 (Sun 4/75)
35,199 (486DX2-66, BSDI 1.1, GCC 1.42 -O, 32-bit prot mode)
11,561 (486DX-50, DOS, Borland C++ 3.1 -O2, 16-bit real mode)

Triple-key DES:
GNU assembler version, Linux 2.2.6
378,000 (Pentium-II 400 MHz)

This code is in the public domain. Bug reports and enhancements are
appreciated.

Phil Karn KA9Q, [e-mail]