150 lines
3.8 KiB
Plaintext
150 lines
3.8 KiB
Plaintext
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.TH DES 2
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.SH NAME
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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
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.SH SYNOPSIS
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.B #include <u.h>
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.br
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.B #include <libc.h>
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.br
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.B #include <mp.h>
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.br
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.B #include <libsec.h>
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.PP
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.B
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void des_key_setup(uchar key[8], ulong schedule[32])
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.PP
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.B
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void block_cipher(ulong *schedule, uchar *data, int decrypting)
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.PP
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.B
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void setupDESstate(DESstate *s, uchar key[8], uchar *ivec)
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.PP
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.B
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void desCBCencrypt(uchar *p, int len, DESstate *s)
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.PP
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.B
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void desCBCdecrypt(uchar *p, int len, DESstate *s)
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.PP
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.B
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void desECBencrypt(uchar *p, int len, DESstate *s)
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.PP
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.B
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void desECBdecrypt(uchar *p, int len, DESstate *s)
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.PP
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.in +0.5i
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.ti -0.5i
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.B
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void triple_block_cipher(ulong expanded_key[3][32], uchar text[8], int ende)
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.PP
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.B
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void setupDES3state(DES3state *s, uchar key[3][8], uchar *ivec)
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.PP
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.B
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void des3CBCencrypt(uchar *p, int len, DES3state *s)
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.PP
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.B
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void des3CBCdecrypt(uchar *p, int len, DES3state *s)
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.PP
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.B
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void des3ECBencrypt(uchar *p, int len, DES3state *s)
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.PP
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.B
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void des3ECBdecrypt(uchar *p, int len, DES3state *s)
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.PP
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.B
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void key_setup(uchar[7], ulong[32])
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.PP
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.B
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void des56to64(uchar *k56, uchar *k64)
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.PP
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.B
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void des64to56(uchar *k64, uchar *k56)
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.SH DESCRIPTION
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The Digital Encryption Standard (DES)
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is a shared-key or symmetric encryption algorithm using either
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a 56-bit key for single DES or three 56-bit keys for triple DES.
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The keys are encoded into 64 bits where every eight bit
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is parity.
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.PP
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The basic DES function,
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.IR block_cipher ,
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works on a block of 8 bytes, converting them in place.
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It takes a key schedule, a pointer to the block, and
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a flag indicating encrypting (0) or decrypting (1).
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The key schedule is created from the key using
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.IR des_key_setup .
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.PP
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Since it is a bit awkward,
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.I block_cipher
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is rarely called directly. Instead, one normally uses
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routines that encrypt larger buffers of data and
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which may chain the encryption state from one buffer
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to the next.
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These routines keep track of the state of the
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encryption using a
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.B DESstate
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structure that contains the key schedule and any chained
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state.
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.I SetupDESstate
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sets up the
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.B DESstate
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structure using the key and an 8-byte initialization vector.
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.PP
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Electronic code book, using
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.I desECBencrypt
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and
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.IR desECBdecrypt ,
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is the less secure mode. The encryption of each 8 bytes
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does not depend on the encryption of any other.
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Hence the encryption is a substitution
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cipher using 64 bit characters.
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.PP
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Cipher block chaining mode, using
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.I desCBCencrypt
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and
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.IR desCBCdecrypt ,
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is more secure. Every block encrypted depends on the initialization
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vector and all blocks encrypted before it.
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.PP
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For both CBC and ECB modes, a stream of data can be encrypted as
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multiple buffers. However, all buffers except the last must
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be a multiple of 8 bytes to ensure successful decryption of
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the stream.
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.PP
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There are equivalent triple-DES (DES3-EDE) functions for each of the
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DES functions.
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.PP
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In the past, Plan 9 used a 56-bit or 7-byte
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format for DES keys. To be compatible with the rest
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of the world, we've abandoned this format.
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There are two functions,
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.I des56to64
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and
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.IR des64to56 ,
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to convert back and forth between the two formats.
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Also a key schedule can be set up from the 7-byte format using
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.IR key_setup .
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.SH SOURCE
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.B /sys/src/libsec
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.SH SEE ALSO
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.IR mp (2),
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.IR aes (2),
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.IR blowfish (2),
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.IR dsa (2),
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.IR elgamal (2),
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.IR rc4 (2),
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.IR rsa (2),
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.IR sechash (2),
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.IR prime (2),
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.IR rand (2)
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.br
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.IR "Breaking DES" ,
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Electronic Frontier Foundation,
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O'Reilly, 1998
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.SH BUGS
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Single DES can be realistically broken by brute-force;
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its 56-bit key is just too short.
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It should not be used in new code, which should probably use
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.IR aes (2)
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instead, or at least triple DES.
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