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DESCRIPTION

There are two phases to the use of DES encryption. The first is the generation of a DES_key_schedule from a key, and the second is the actual encryption. A DES key is of type DES_cblock. This type consists of 8 bytes with odd parity. The least significant bit in each byte is the parity bit. The key schedule is an expanded form of the key; it is used to speed the encryption process.

DES_random_key() generates a random key in odd parity.

Before a DES key can be used, it must be converted into the architecture dependent DES_key_schedule via the DES_set_key_checked() or DES_set_key_unchecked() function.

DES_set_key_checked() will check that the key passed is of odd parity and is not a weak or semi-weak key. If the parity is wrong, then -1 is returned. If the key is a weak key, then -2 is returned. If an error is returned, the key schedule is not generated.

DES_set_key() works like DES_set_key_checked() if the DES_check_key flag is non-zero, otherwise like DES_set_key_unchecked(). These functions are available for compatibility; it is recommended to use a function that does not depend on a global variable.

DES_set_odd_parity() sets the parity of the passed key to odd.

The following routines mostly operate on an input and output stream of DES_cblocks.

DES_ecb_encrypt() is the basic DES encryption routine that encrypts or decrypts a single 8-byte DES_cblock in electronic code book (ECB) mode. It always transforms the input data, pointed to by input, into the output data, pointed to by the output argument. If the enc argument is non-zero (DES_ENCRYPT), 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 enc is zero (DES_DECRYPT), the input (now ciphertext) is decrypted into the output (now cleartext). Input and output may overlap. DES_ecb_encrypt() does not return a value.

DES_ecb3_encrypt() encrypts/decrypts the input block by using three-key Triple-DES encryption in ECB mode. This involves encrypting the input with ks1, decrypting with the key schedule ks2, and then encrypting with ks3. This routine greatly reduces the chances of brute force breaking of DES and has the advantage of if ks1, ks2, and ks3 are the same, it is equivalent to just encryption using ECB mode and ks1 as the key.

The macro DES_ecb2_encrypt() is provided to perform two-key Triple-DES encryption by using ks1 for the final encryption.

DES_ncbc_encrypt() encrypts/decrypts using the cipher-block-chaining (CBC) mode of DES. If the enc 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 temporary area and zero filled. The output is always an integral multiple of eight bytes.

DES_xcbc_encrypt() is RSA's DESX mode of DES. It uses inw and outw to "whiten" the encryption. inw and outw are secret (unlike the iv) and are as such, part of the key. So the key is sort of 24 bytes. This is much better than CBC DES.

DES_ede3_cbc_encrypt() implements outer triple CBC DES encryption with three keys. This means that each DES operation inside the CBC mode is "C=E(ks3,D(ks2,E(ks1,M)))". This mode is used by SSL.

The DES_ede2_cbc_encrypt() macro implements two-key Triple-DES by reusing ks1 for the final encryption. "C=E(ks1,D(ks2,E(ks1,M)))". This form of Triple-DES is used by the RSAREF library.

DES_pcbc_encrypt() encrypts/decrypts using the propagating cipher block chaining mode used by Kerberos v4. Its parameters are the same as DES_ncbc_encrypt().

DES_cfb_encrypt() encrypts/decrypts using cipher feedback mode. This method takes an array of characters as input and outputs an array of characters. It does not require any padding to 8 character groups. Note: the ivec variable is changed and the new changed value needs to be passed to the next call to this function. Since this function runs a complete DES ECB encryption per numbits, this function is only suggested for use when sending a small number of characters.

DES_cfb64_encrypt() implements CFB mode of DES with 64-bit feedback. Why is this useful you ask? Because this routine will allow you to encrypt an arbitrary number of bytes, without 8 byte padding. Each call to this routine will encrypt the input bytes to output and then update ivec and num. num contains "how far" we are though ivec. If this does not make much sense, read more about CFB mode of DES.

DES_ede3_cfb64_encrypt() and DES_ede2_cfb64_encrypt() is the same as DES_cfb64_encrypt() except that Triple-DES is used.

DES_ofb_encrypt() encrypts using output feedback mode. This method takes an array of characters as input and outputs an array of characters. It does not require any padding to 8 character groups. Note: the ivec variable is changed and the new changed value needs to be passed to the next call to this function. Since this function runs a complete DES ECB encryption per numbits, this function is only suggested for use when sending a small number of characters.

DES_ofb64_encrypt() is the same as DES_cfb64_encrypt() using Output Feed Back mode.

DES_ede3_ofb64_encrypt() and DES_ede2_ofb64_encrypt() is the same as DES_ofb64_encrypt(), using Triple-DES.

The following functions are included in the DES library for compatibility with the MIT Kerberos library.

DES_cbc_cksum() produces an 8-byte checksum based on the input stream (via CBC encryption). The last 4 bytes of the checksum are returned and the complete 8 bytes are placed in output. This function is used by Kerberos v4. Other applications should use EVP_DigestInit(3) etc. instead.

DES_quad_cksum() is a Kerberos v4 function. It returns a 4-byte checksum from the input bytes. The algorithm can be iterated over the input, depending on out_count, 1, 2, 3 or 4 times. If output is non-NULL, the 8 bytes generated by each pass are written into output.

The following are DES-based transformations:

DES_fcrypt() is a fast version of the Unix crypt(3) function. The salt must be two ASCII characters. This version is different from the normal crypt in that the third parameter is the buffer that the return value is written into. It needs to be at least 14 bytes long. The fourteenth byte is set to NUL. This version takes only a small amount of space relative to other fast crypt implementations. It is thread safe, unlike the normal crypt.

DES_crypt() is a faster replacement for the normal system crypt(3). This function calls DES_fcrypt() with a static array passed as the third parameter. This emulates the normal non-thread safe semantics of crypt(3).

DES_enc_write() writes len bytes to file descriptor fd from buffer buf. The data is encrypted via pcbc_encrypt (default) using sched for the key and iv as a starting vector. The actual data send down fd consists of 4 bytes (in network byte order) containing the length of the following encrypted data. The encrypted data then follows, padded with random data out to a multiple of 8 bytes.

DES_enc_read() is used to read len bytes from file descriptor fd into buffer buf. The data being read from fd is assumed to have come from DES_enc_write() and is decrypted using sched for the key schedule and iv for the initial vector.

Warning: The data format used by DES_enc_write() and DES_enc_read() has a cryptographic weakness: when asked to write more than MAXWRITE bytes, DES_enc_write() will split the data into several chunks that are all encrypted using the same IV. So don't use these functions unless you are sure you know what you do (in which case you might not want to use them anyway). They cannot handle non-blocking sockets. DES_enc_read() uses an internal state and thus cannot be used on multiple files.

DES_rw_mode is used to specify the encryption mode to use with DES_enc_read(). If set to DES_PCBC_MODE (the default), DES_pcbc_encrypt is used. If set to DES_CBC_MODE DES_cbc_encrypt is used.