The MD2 Message-Digest Algorithm

Step 1. Append Padding Bytes

The message is "padded" (extended) so that its length (in bytes) is congruent to 0, modulo 16. That is, the message is extended so that it is a multiple of 16 bytes long. Padding is always performed, even if the length of the message is already congruent to 0, modulo 16.

Padding is performed as follows: "i" bytes of value "i" are appended to the message so that the length in bytes of the padded message becomes congruent to 0, modulo 16. At least one byte and at most 16 bytes are appended.

At this point the resulting message (after padding with bytes) has a length that is an exact multiple of 16 bytes. Let M[0 ... N-1] denote the bytes of the resulting message, where N is a multiple of 16.

Step 2. Append Checksum

A 16-byte checksum of the message is appended to the result of the previous step.

This step uses a 256-byte "random" permutation constructed from the digits of pi. Let S[i] denote the i-th element of this table. The table is given in the appendix.

Do the following:

/* Clear checksum. */
For i ← 0 TO 15
    C[i] ← 0
NEXT i
L ← 0.
/* Process each 16-word block. */
block ← N / 16
For i ← 0 To block - 1
    /* Checksum block i. */
    For j ← 0 to 15
        c ← M[i * 16 + j].
        C[j] ← S[c xor L].
        L ← C[j].
    NEXT j
NEXT i

The 16-byte checksum C[0 ... 15] is appended to the message.

Step 3. Initialize MD Buffer

A 48-byte buffer X is used to compute the message digest. The buffer is initialized to zero.

Step 4. Process Message in 16-Byte Blocks

This step uses the same 256-byte permutation S as step 2 does.

Do the following:

/* Process each 16-word block. */
For i ← 0 TO block - 1
    /* Copy block i into X. */
    For j ← 0 TO 15
        X[16 + j] ← M[i * 16 + j].
        X[32 + j] ← (X[16 + j] xor X[j]).
    NEXT j
    t ← 0.
    /* Do 18 rounds. */
    For j ← 0 TO 17
        /* Round j. */
        For k = 0 to 47 do
            t ← X[k] ← (X[k] xor S[t]).
        NEXT k
        t ← (t + j) modulo 256.
    NEXT j
NEXT i

Step 5. Output

The message digest produced as output is X[0 ... 15]. That is, we begin with X[0], and end with X[15].

S-BOX

	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19
0	41	46	67	201	162	216	124	1	61	54	84	161	236	240	6	19	98	167	5	243
20	192	199	115	140	152	147	43	217	188	76	130	202	30	155	87	60	253	212	224	22
40	103	66	111	24	138	23	229	18	190	78	196	214	218	158	222	73	160	251	245	142
60	187	47	238	122	169	104	121	145	21	178	7	63	148	194	16	137	11	34	95	33
80	128	127	93	154	90	144	50	39	53	62	204	231	191	247	151	3	255	25	48	179
100	72	165	181	209	215	94	146	42	172	86	170	198	79	184	56	210	150	164	125	182
120	118	252	107	226	156	116	4	241	69	157	112	89	100	113	135	32	134	91	207	101
140	230	45	168	2	27	96	37	173	174	176	185	246	28	70	97	105	52	64	126	15
160	85	71	163	35	221	81	175	58	195	92	249	206	186	197	234	38	44	83	13	110
180	133	40	132	9	211	223	205	244	65	129	77	82	106	220	55	200	108	193	171	250
200	36	225	123	8	12	189	177	74	120	136	149	139	227	99	232	109	233	203	213	254
220	59	0	29	57	242	239	183	14	102	88	208	228	166	119	114	248	235	117	75	10

	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19
0	41	46	67	201	162	216	124	1	61	54	84	161	236	240	6	19	98	167	5	243
20	192	199	115	140	152	147	43	217	188	76	130	202	30	155	87	60	253	212	224	22
40	103	66	111	24	138	23	229	18	190	78	196	214	218	158	222	73	160	251	245	142
60	187	47	238	122	169	104	121	145	21	178	7	63	148	194	16	137	11	34	95	33
80	128	127	93	154	90	144	50	39	53	62	204	231	191	247	151	3	255	25	48	179
100	72	165	181	209	215	94	146	42	172	86	170	198	79	184	56	210	150	164	125	182
120	118	252	107	226	156	116	4	241	69	157	112	89	100	113	135	32	134	91	207	101
140	230	45	168	2	27	96	37	173	174	176	185	246	28	70	97	105	52	64	126	15
160	85	71	163	35	221	81	175	58	195	92	249	206	186	197	234	38	44	83	13	110
180	133	40	132	9	211	223	205	244	65	129	77	82	106	220	55	200	108	193	171	250
200	36	225	123	8	12	189	177	74	120	136	149	139	227	99	232	109	233	203	213	254
220	59	0	29	57	242	239	183	14	102	88	208	228	166	119	114	248	235	117	75	10

	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19
0	41	46	67	201	162	216	124	1	61	54	84	161	236	240	6	19	98	167	5	243
20	192	199	115	140	152	147	43	217	188	76	130	202	30	155	87	60	253	212	224	22
40	103	66	111	24	138	23	229	18	190	78	196	214	218	158	222	73	160	251	245	142
60	187	47	238	122	169	104	121	145	21	178	7	63	148	194	16	137	11	34	95	33
80	128	127	93	154	90	144	50	39	53	62	204	231	191	247	151	3	255	25	48	179
100	72	165	181	209	215	94	146	42	172	86	170	198	79	184	56	210	150	164	125	182
120	118	252	107	226	156	116	4	241	69	157	112	89	100	113	135	32	134	91	207	101
140	230	45	168	2	27	96	37	173	174	176	185	246	28	70	97	105	52	64	126	15
160	85	71	163	35	221	81	175	58	195	92	249	206	186	197	234	38	44	83	13	110
180	133	40	132	9	211	223	205	244	65	129	77	82	106	220	55	200	108	193	171	250
200	36	225	123	8	12	189	177	74	120	136	149	139	227	99	232	109	233	203	213	254
220	59	0	29	57	242	239	183	14	102	88	208	228	166	119	114	248	235	117	75	10

	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19
0	41	46	67	201	162	216	124	1	61	54	84	161	236	240	6	19	98	167	5	243
20	192	199	115	140	152	147	43	217	188	76	130	202	30	155	87	60	253	212	224	22
40	103	66	111	24	138	23	229	18	190	78	196	214	218	158	222	73	160	251	245	142
60	187	47	238	122	169	104	121	145	21	178	7	63	148	194	16	137	11	34	95	33
80	128	127	93	154	90	144	50	39	53	62	204	231	191	247	151	3	255	25	48	179
100	72	165	181	209	215	94	146	42	172	86	170	198	79	184	56	210	150	164	125	182
120	118	252	107	226	156	116	4	241	69	157	112	89	100	113	135	32	134	91	207	101
140	230	45	168	2	27	96	37	173	174	176	185	246	28	70	97	105	52	64	126	15
160	85	71	163	35	221	81	175	58	195	92	249	206	186	197	234	38	44	83	13	110
180	133	40	132	9	211	223	205	244	65	129	77	82	106	220	55	200	108	193	171	250
200	36	225	123	8	12	189	177	74	120	136	149	139	227	99	232	109	233	203	213	254
220	59	0	29	57	242	239	183	14	102	88	208	228	166	119	114	248	235	117	75	10