33.4 DES Encryption

The Data Encryption Standard is described in the US Government Federal Information Processing Standards (FIPS) 46-3 published by the National Institute of Standards and Technology. The DES has been very thoroughly analyzed since it was developed in the late 1970s, and no new significant flaws have been found.

However, the DES uses only a 56-bit key (plus 8 parity bits), and a machine has been built in 1998 which can search through all possible keys in about 6 days, which cost about US$200000; faster searches would be possible with more money. This makes simple DES insecure for most purposes, and NIST no longer permits new US government systems to use simple DES.

For serious encryption functionality, it is recommended that one of the many free encryption libraries be used instead of these routines.

The DES is a reversible operation which takes a 64-bit block and a 64-bit key, and produces another 64-bit block. Usually the bits are numbered so that the most-significant bit, the first bit, of each block is numbered 1.

Under that numbering, every 8th bit of the key (the 8th, 16th, and so on) is not used by the encryption algorithm itself. But the key must have odd parity; that is, out of bits 1 through 8, and 9 through 16, and so on, there must be an odd number of ‘1’ bits, and this completely specifies the unused bits.

Function: void setkey (const char *key)

Preliminary: | MT-Unsafe race:crypt | AS-Unsafe corrupt lock | AC-Unsafe lock | See POSIX Safety Concepts.

The setkey function sets an internal data structure to be an expanded form of key. key is specified as an array of 64 bits each stored in a char, the first bit is key[0] and the 64th bit is key[63]. The key should have the correct parity.

Function: void encrypt (char *block, int edflag)

Preliminary: | MT-Unsafe race:crypt | AS-Unsafe corrupt lock | AC-Unsafe lock | See POSIX Safety Concepts.

The encrypt function encrypts block if edflag is 0, otherwise it decrypts block, using a key previously set by setkey. The result is placed in block.

Like setkey, block is specified as an array of 64 bits each stored in a char, but there are no parity bits in block.

Function: void setkey_r (const char *key, struct crypt_data * data)

Function: void encrypt_r (char *block, int edflag, struct crypt_data * data)

Preliminary: | MT-Safe | AS-Unsafe corrupt lock | AC-Unsafe lock | See POSIX Safety Concepts.

These are reentrant versions of setkey and encrypt. The only difference is the extra parameter, which stores the expanded version of key. Before calling setkey_r the first time, data->initialized must be cleared to zero.

The setkey_r and encrypt_r functions are GNU extensions. setkey, encrypt, setkey_r, and encrypt_r are defined in crypt.h.

Function: int ecb_crypt (char *key, char *blocks, unsigned len, unsigned mode)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The function ecb_crypt encrypts or decrypts one or more blocks using DES. Each block is encrypted independently.

The blocks and the key are stored packed in 8-bit bytes, so that the first bit of the key is the most-significant bit of key[0] and the 63rd bit of the key is stored as the least-significant bit of key[7]. The key should have the correct parity.

len is the number of bytes in blocks. It should be a multiple of 8 (so that there is a whole number of blocks to encrypt). len is limited to a maximum of DES_MAXDATA bytes.

The result of the encryption replaces the input in blocks.

The mode parameter is the bitwise OR of two of the following:

DES_ENCRYPT

This constant, used in the mode parameter, specifies that blocks is to be encrypted.

DES_DECRYPT

This constant, used in the mode parameter, specifies that blocks is to be decrypted.

DES_HW

This constant, used in the mode parameter, asks to use a hardware device. If no hardware device is available, encryption happens anyway, but in software.

DES_SW

This constant, used in the mode parameter, specifies that no hardware device is to be used.

The result of the function will be one of these values:

DESERR_NONE

The encryption succeeded.

DESERR_NOHWDEVICE

The encryption succeeded, but there was no hardware device available.

DESERR_HWERROR

The encryption failed because of a hardware problem.

DESERR_BADPARAM

The encryption failed because of a bad parameter, for instance len is not a multiple of 8 or len is larger than DES_MAXDATA.

Function: int DES_FAILED (int err)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This macro returns 1 if err is a ‘success’ result code from ecb_crypt or cbc_crypt, and 0 otherwise.

Function: int cbc_crypt (char *key, char *blocks, unsigned len, unsigned mode, char *ivec)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The function cbc_crypt encrypts or decrypts one or more blocks using DES in Cipher Block Chaining mode.

For encryption in CBC mode, each block is exclusive-ored with ivec before being encrypted, then ivec is replaced with the result of the encryption, then the next block is processed. Decryption is the reverse of this process.

This has the advantage that blocks which are the same before being encrypted are very unlikely to be the same after being encrypted, making it much harder to detect patterns in the data.

Usually, ivec is set to 8 random bytes before encryption starts. Then the 8 random bytes are transmitted along with the encrypted data (without themselves being encrypted), and passed back in as ivec for decryption. Another possibility is to set ivec to 8 zeroes initially, and have the first the block encrypted consist of 8 random bytes.

Otherwise, all the parameters are similar to those for ecb_crypt.

Function: void des_setparity (char *key)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The function des_setparity changes the 64-bit key, stored packed in 8-bit bytes, to have odd parity by altering the low bits of each byte.

The ecb_crypt, cbc_crypt, and des_setparity functions and their accompanying macros are all defined in the header rpc/des_crypt.h.