md2.c

/*
 * md2.c
 *
 * "Naive" [MD2](https://tools.ietf.org/html/rfc1319)) implementation for
 * self-educating purpose, please do *not* use it.
 *
 * License: CC0, see https://creativecommons.org/publicdomain/zero/1.0/
 */
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h>


struct md2_ctx {
	unsigned char	checksum[16];
	unsigned char	state[16];
	unsigned char	chunk[16];
	size_t		clen; /* current chunk data length */
};


void	md2_ctx_init(struct md2_ctx *ctx);
void	md2_ctx_feed(struct md2_ctx *ctx, const unsigned char *buf, size_t buflen);
void	md2_ctx_finalize(struct md2_ctx *ctx, unsigned char *digest);


/**
 * read stdin to feed the md2_ctx, output the digest (in hex) on stdout.
 */
int
main(int argc, char **argv)
{
	struct md2_ctx ctx;
	unsigned char buf[BUFSIZ];
	unsigned char digest[16];
	size_t len;

	md2_ctx_init(&ctx);

	while ((len = fread(buf, 1, sizeof(buf), stdin))) {
		md2_ctx_feed(&ctx, buf, len);
	}

	if (ferror(stdin) != 0)
		return (EXIT_FAILURE);
	md2_ctx_finalize(&ctx, digest);

	for (int i = 0; i < 16; i++)
		(void)printf("%02x", digest[i]);
	(void)printf("\n");

	return (EXIT_SUCCESS);
}


/* Taken from https://en.wikipedia.org/wiki/MD2_(cryptography) */
static const unsigned char SBox[256] = {
	0x29, 0x2E, 0x43, 0xC9, 0xA2, 0xD8, 0x7C, 0x01, 0x3D, 0x36, 0x54, 0xA1, 0xEC, 0xF0, 0x06, 0x13,
	0x62, 0xA7, 0x05, 0xF3, 0xC0, 0xC7, 0x73, 0x8C, 0x98, 0x93, 0x2B, 0xD9, 0xBC, 0x4C, 0x82, 0xCA,
	0x1E, 0x9B, 0x57, 0x3C, 0xFD, 0xD4, 0xE0, 0x16, 0x67, 0x42, 0x6F, 0x18, 0x8A, 0x17, 0xE5, 0x12,
	0xBE, 0x4E, 0xC4, 0xD6, 0xDA, 0x9E, 0xDE, 0x49, 0xA0, 0xFB, 0xF5, 0x8E, 0xBB, 0x2F, 0xEE, 0x7A,
	0xA9, 0x68, 0x79, 0x91, 0x15, 0xB2, 0x07, 0x3F, 0x94, 0xC2, 0x10, 0x89, 0x0B, 0x22, 0x5F, 0x21,
	0x80, 0x7F, 0x5D, 0x9A, 0x5A, 0x90, 0x32, 0x27, 0x35, 0x3E, 0xCC, 0xE7, 0xBF, 0xF7, 0x97, 0x03,
	0xFF, 0x19, 0x30, 0xB3, 0x48, 0xA5, 0xB5, 0xD1, 0xD7, 0x5E, 0x92, 0x2A, 0xAC, 0x56, 0xAA, 0xC6,
	0x4F, 0xB8, 0x38, 0xD2, 0x96, 0xA4, 0x7D, 0xB6, 0x76, 0xFC, 0x6B, 0xE2, 0x9C, 0x74, 0x04, 0xF1,
	0x45, 0x9D, 0x70, 0x59, 0x64, 0x71, 0x87, 0x20, 0x86, 0x5B, 0xCF, 0x65, 0xE6, 0x2D, 0xA8, 0x02,
	0x1B, 0x60, 0x25, 0xAD, 0xAE, 0xB0, 0xB9, 0xF6, 0x1C, 0x46, 0x61, 0x69, 0x34, 0x40, 0x7E, 0x0F,
	0x55, 0x47, 0xA3, 0x23, 0xDD, 0x51, 0xAF, 0x3A, 0xC3, 0x5C, 0xF9, 0xCE, 0xBA, 0xC5, 0xEA, 0x26,
	0x2C, 0x53, 0x0D, 0x6E, 0x85, 0x28, 0x84, 0x09, 0xD3, 0xDF, 0xCD, 0xF4, 0x41, 0x81, 0x4D, 0x52,
	0x6A, 0xDC, 0x37, 0xC8, 0x6C, 0xC1, 0xAB, 0xFA, 0x24, 0xE1, 0x7B, 0x08, 0x0C, 0xBD, 0xB1, 0x4A,
	0x78, 0x88, 0x95, 0x8B, 0xE3, 0x63, 0xE8, 0x6D, 0xE9, 0xCB, 0xD5, 0xFE, 0x3B, 0x00, 0x1D, 0x39,
	0xF2, 0xEF, 0xB7, 0x0E, 0x66, 0x58, 0xD0, 0xE4, 0xA6, 0x77, 0x72, 0xF8, 0xEB, 0x75, 0x4B, 0x0A,
	0x31, 0x44, 0x50, 0xB4, 0x8F, 0xED, 0x1F, 0x1A, 0xDB, 0x99, 0x8D, 0x33, 0x9F, 0x11, 0x83, 0x14,
};


static void	md2_ctx_update(struct md2_ctx *ctx);


/**
 * reset a md2_ctx state
 */
void
md2_ctx_init(struct md2_ctx *ctx)
{
	assert(ctx != NULL);

	bzero(ctx->checksum, sizeof(ctx->checksum));
	bzero(ctx->state, sizeof(ctx->state));
	ctx->clen = 0;
}


/**
 * consume the input data in buf, call md2_ctx_update() for each chunk of 16
 * bytes available.
 */
void
md2_ctx_feed(struct md2_ctx *ctx, const unsigned char *buf, size_t buflen)
{
	assert(ctx != NULL);
	assert(buf != NULL);

	for (const unsigned char *c = buf; c < buf + buflen; ++c) {
		ctx->chunk[ctx->clen++] = *c;
		if (ctx->clen == 16)
			md2_ctx_update(ctx);
	}
}


void
md2_ctx_update(struct md2_ctx *ctx)
{
	assert(ctx != NULL);
	assert(ctx->clen == 16);


	/* checksum update */
	unsigned char prev = ctx->checksum[15];
	for (int i = 0; i < 16; i++) {
		ctx->checksum[i] ^= SBox[prev ^ ctx->chunk[i]];
		prev = ctx->checksum[i];
	}

	/* state update */
	unsigned char x[48];
	/* x is: [ MD intermediate || chunk || MD intermediate XOR chunk ] */
	(void)memcpy(x, ctx->state, 16);
	(void)memcpy(x + 16, ctx->chunk, 16);
	for (int i = 0; i < 16; i++)
		x[32 + i] = ctx->state[i] ^ ctx->chunk[i];
	unsigned char t = 0;
	for (int p = 0; p < 18; p++) { /* 18 passes */
		for (int i = 0; i < 48; i++) {
			t = x[i] = x[i] ^ SBox[t];
		}
		t = (t + p) % 256;
	}
	(void)memcpy(ctx->state, x, 16);
	bzero(x, sizeof(x)); /* XXX: should be explicit_bzero() */

	ctx->clen = 0;
}


/**
 * finalize a md2_ctx after all data have been feeded. Add the padding, add the
 * checksum and update the md. digest length MUST be at least 16.
 */
void
md2_ctx_finalize(struct md2_ctx *ctx, unsigned char *digest)
{
	assert(ctx != NULL);

	/* Append Padding Bytes §3.1 */
	unsigned char padlen = 16 - ctx->clen;
	for (int i = 0; i < padlen; i++)
		ctx->chunk[ctx->clen++] = padlen;
	md2_ctx_update(ctx);

	/* Append Checksum §3.2 */
	(void)memcpy(ctx->chunk, ctx->checksum, 16);
	ctx->clen = 16;
	md2_ctx_update(ctx);

	(void)memcpy(digest, ctx->state, 16);
	bzero(ctx, sizeof(struct md2_ctx)); /* XXX: should be explicit_bzero() */
}