2 IDEA is a block cipher that operates on 64bit (8 byte) quantities. It
3 uses a 128bit (16 byte) key. It can be used in all the modes that DES can
4 be used. This library implements the ecb, cbc, cfb64 and ofb64 modes.
6 For all calls that have an 'input' and 'output' variables, they can be the
9 This library requires the inclusion of 'idea.h'.
11 All of the encryption functions take what is called an IDEA_KEY_SCHEDULE as an
12 argument. An IDEA_KEY_SCHEDULE is an expanded form of the idea key.
13 For all modes of the IDEA algorithm, the IDEA_KEY_SCHEDULE used for
14 decryption is different to the one used for encryption.
16 The define IDEA_ENCRYPT is passed to specify encryption for the functions
17 that require an encryption/decryption flag. IDEA_DECRYPT is passed to
18 specify decryption. For some mode there is no encryption/decryption
19 flag since this is determined by the IDEA_KEY_SCHEDULE.
21 So to encrypt you would do the following
22 idea_set_encrypt_key(key,encrypt_ks);
23 idea_ecb_encrypt(...,encrypt_ks);
24 idea_cbc_encrypt(....,encrypt_ks,...,IDEA_ENCRYPT);
27 idea_set_encrypt_key(key,encrypt_ks);
28 idea_set_decrypt_key(encrypt_ks,decrypt_ks);
29 idea_ecb_encrypt(...,decrypt_ks);
30 idea_cbc_encrypt(....,decrypt_ks,...,IDEA_DECRYPT);
32 Please note that any of the encryption modes specified in my DES library
33 could be used with IDEA. I have only implemented ecb, cbc, cfb64 and
34 ofb64 for the following reasons.
35 - ecb is the basic IDEA encryption.
36 - cbc is the normal 'chaining' form for block ciphers.
37 - cfb64 can be used to encrypt single characters, therefore input and output
38 do not need to be a multiple of 8.
39 - ofb64 is similar to cfb64 but is more like a stream cipher, not as
40 secure (not cipher feedback) but it does not have an encrypt/decrypt mode.
41 - If you want triple IDEA, thats 384 bits of key and you must be totally
42 obsessed with security. Still, if you want it, it is simple enough to
43 copy the function from the DES library and change the des_encrypt to
44 idea_encrypt; an exercise left for the paranoid reader :-).
46 The functions are as follows:
48 void idea_set_encrypt_key(
50 IDEA_KEY_SCHEDULE *ks);
51 idea_set_encrypt_key converts a 16 byte IDEA key into an
52 IDEA_KEY_SCHEDULE. The IDEA_KEY_SCHEDULE is an expanded form of
53 the key which can be used to perform IDEA encryption.
54 An IDEA_KEY_SCHEDULE is an expanded form of the key which is used to
55 perform actual encryption. It can be regenerated from the IDEA key
56 so it only needs to be kept when encryption is about
57 to occur. Don't save or pass around IDEA_KEY_SCHEDULE's since they
58 are CPU architecture dependent, IDEA keys are not.
60 void idea_set_decrypt_key(
61 IDEA_KEY_SCHEDULE *encrypt_ks,
62 IDEA_KEY_SCHEDULE *decrypt_ks);
63 This functions converts an encryption IDEA_KEY_SCHEDULE into a
64 decryption IDEA_KEY_SCHEDULE. For all decryption, this conversion
65 of the key must be done. In some modes of IDEA, an
66 encryption/decryption flag is also required, this is because these
67 functions involve block chaining and the way this is done changes
68 depending on which of encryption of decryption is being done.
69 Please note that there is no quick way to generate the decryption
70 key schedule other than generating the encryption key schedule and
75 IDEA_KEY_SCHEDULE *ks);
76 This is the IDEA encryption function that gets called by just about
77 every other IDEA routine in the library. You should not use this
78 function except to implement 'modes' of IDEA. I say this because the
79 functions that call this routine do the conversion from 'char *' to
80 long, and this needs to be done to make sure 'non-aligned' memory
82 Data is a pointer to 2 unsigned long's and ks is the
83 IDEA_KEY_SCHEDULE to use. Encryption or decryption depends on the
86 void idea_ecb_encrypt(
88 unsigned char *output,
89 IDEA_KEY_SCHEDULE *ks);
90 This is the basic Electronic Code Book form of IDEA (in DES this
91 mode is called Electronic Code Book so I'm going to use the term
93 Input is encrypted into output using the key represented by
94 ks. Depending on the IDEA_KEY_SCHEDULE, encryption or
95 decryption occurs. Input is 8 bytes long and output is 8 bytes.
97 void idea_cbc_encrypt(
99 unsigned char *output,
101 IDEA_KEY_SCHEDULE *ks,
104 This routine implements IDEA in Cipher Block Chaining mode.
105 Input, which should be a multiple of 8 bytes is encrypted
106 (or decrypted) to output which will also be a multiple of 8 bytes.
107 The number of bytes is in length (and from what I've said above,
108 should be a multiple of 8). If length is not a multiple of 8, bad
109 things will probably happen. ivec is the initialisation vector.
110 This function updates iv after each call so that it can be passed to
111 the next call to idea_cbc_encrypt().
113 void idea_cfb64_encrypt(
121 This is one of the more useful functions in this IDEA library, it
122 implements CFB mode of IDEA with 64bit feedback.
123 This allows you to encrypt an arbitrary number of bytes,
124 you do not require 8 byte padding. Each call to this
125 routine will encrypt the input bytes to output and then update ivec
126 and num. Num contains 'how far' we are though ivec.
127 Enc is used to indicate encryption or decryption.
128 One very important thing to remember is that when decrypting, use
129 the encryption form of the key.
130 CFB64 mode operates by using the cipher to
131 generate a stream of bytes which is used to encrypt the plain text.
132 The cipher text is then encrypted to generate the next 64 bits to
133 be xored (incrementally) with the next 64 bits of plain
134 text. As can be seen from this, to encrypt or decrypt,
135 the same 'cipher stream' needs to be generated but the way the next
136 block of data is gathered for encryption is different for
137 encryption and decryption. What this means is that to encrypt
138 idea_set_encrypt_key(key,ks);
139 idea_cfb64_encrypt(...,ks,..,IDEA_ENCRYPT)
141 idea_set_encrypt_key(key,ks)
142 idea_cfb64_encrypt(...,ks,...,IDEA_DECRYPT)
143 Note: The same IDEA_KEY_SCHEDULE but different encryption flags.
144 For idea_cbc or idea_ecb, idea_set_decrypt_key() would need to be
145 used to generate the IDEA_KEY_SCHEDULE for decryption.
146 The reason I'm stressing this point is that I just wasted 3 hours
147 today trying to decrypt using this mode and the decryption form of
150 void idea_ofb64_encrypt(
157 This functions implements OFB mode of IDEA with 64bit feedback.
158 This allows you to encrypt an arbitrary number of bytes,
159 you do not require 8 byte padding. Each call to this
160 routine will encrypt the input bytes to output and then update ivec
161 and num. Num contains 'how far' we are though ivec.
162 This is in effect a stream cipher, there is no encryption or
163 decryption mode. The same key and iv should be used to
166 For reading passwords, I suggest using des_read_pw_string() from my DES library.
167 To generate a password from a text string, I suggest using MD5 (or MD2) to
168 produce a 16 byte message digest that can then be passed directly to
169 idea_set_encrypt_key().
172 For more information about the specific IDEA modes in this library
173 (ecb, cbc, cfb and ofb), read the section entitled 'Modes of DES' from the
174 documentation on my DES library. What is said about DES is directly