MD5
תיאור האלגוריתם > קוד לדוגמה

להלן קוד שלם בשפת C++‎ הממחיש את האלגוריתם. הקוד ממוטב חלקית ומיישם מספר אופטימיזציות כדי להשיג ביצועים טובים, ביניהם אפשר למנות; שימוש באופרטורים כמו XOR ו-AND במקום כפל וחיבור, פקודות מאקרו, פתיחת לולאות (loop unrolling) ומיעוט קריאות לפונקציות.
typedef unsigned int uint;
typedef unsigned char byte;

typedef struct {
	uint state[4];
	uint count[2];
	byte buffer[64];
} MD5_CTX;

class MD5
{
	#define ROTATE_LEFT(x, n) (((x) << (n)) | ((x) >> (32-(n))))
	#define F(x, y, z) (((x) & (y)) | ((~x) & (z)))
	#define G(x, y, z) (((x) & (z)) | ((y) & (~z)))
	#define H(x, y, z) ((x) ^ (y) ^ (z))
	#define I(x, y, z) ((y) ^ ((x) | (~z)))

	byte PADDING[64] = {
		0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
	};

	#define FF(a, b, c, d, x, s, ac) { \
		 (a) += F ((b), (c), (d)) + (x) + (uint)(ac); \
		 (a) = ROTATE_LEFT ((a), (s)); \
		 (a) += (b); \
		}
	#define GG(a, b, c, d, x, s, ac) { \
		 (a) += G ((b), (c), (d)) + (x) + (uint)(ac); \
		 (a) = ROTATE_LEFT ((a), (s)); \
		 (a) += (b); \
		 }
	#define HH(a, b, c, d, x, s, ac) { \
		 (a) += H ((b), (c), (d)) + (x) + (uint)(ac); \
		 (a) = ROTATE_LEFT ((a), (s)); \
		 (a) += (b); \
		 }
	#define II(a, b, c, d, x, s, ac) { \
		 (a) += I ((b), (c), (d)) + (x) + (uint)(ac); \
		 (a) = ROTATE_LEFT ((a), (s)); \
		 (a) += (b); \
		 }
	static void MD5Transform(uint state[4], byte block[64])
	{
		uint a = state[0], b = state[1], c = state[2], d = state[3], x[16];
		Decode(x, block, 64);
		/* Round 1 */
		FF(a, b, c, d, x[0], 7, 0xd76aa478); 
		FF(d, a, b, c, x[1], 12, 0xe8c7b756); 
		FF(c, d, a, b, x[2], 17, 0x242070db); 
		FF(b, c, d, a, x[3], 22, 0xc1bdceee); 
		FF(a, b, c, d, x[4], 7, 0xf57c0faf); 
		FF(d, a, b, c, x[5], 12, 0x4787c62a);
		FF(c, d, a, b, x[6], 17, 0xa8304613);
		FF(b, c, d, a, x[7], 22, 0xfd469501); 
		FF(a, b, c, d, x[8], 7, 0x698098d8); 
		FF(d, a, b, c, x[9], 12, 0x8b44f7af);
		FF(c, d, a, b, x[10], 17, 0xffff5bb1); 
		FF(b, c, d, a, x[11], 22, 0x895cd7be);
		FF(a, b, c, d, x[12], 7, 0x6b901122);
		FF(d, a, b, c, x[13], 12, 0xfd987193);
		FF(c, d, a, b, x[14], 17, 0xa679438e);
		FF(b, c, d, a, x[15], 22, 0x49b40821);

	/* Round 2 */
		GG(a, b, c, d, x[1], 5, 0xf61e2562); 
		GG(d, a, b, c, x[6], 9, 0xc040b340); 
		GG(c, d, a, b, x[11], 14, 0x265e5a51);
		GG(b, c, d, a, x[0], 20, 0xe9b6c7aa);
		GG(a, b, c, d, x[5], 5, 0xd62f105d); 
		GG(d, a, b, c, x[10], 9, 0x2441453); 
		GG(c, d, a, b, x[15], 14, 0xd8a1e681); 
		GG(b, c, d, a, x[4], 20, 0xe7d3fbc8);
		GG(a, b, c, d, x[9], 5, 0x21e1cde6); 
		GG(d, a, b, c, x[14], 9, 0xc33707d6); 
		GG(c, d, a, b, x[3], 14, 0xf4d50d87); 
		GG(b, c, d, a, x[8], 20, 0x455a14ed); 
		GG(a, b, c, d, x[13], 5, 0xa9e3e905); 
		GG(d, a, b, c, x[2], 9, 0xfcefa3f8); 
		GG(c, d, a, b, x[7], 14, 0x676f02d9); 
		GG(b, c, d, a, x[12], 20, 0x8d2a4c8a);

	/* Round 3 */
		HH(a, b, c, d, x[5], 4, 0xfffa3942);
		HH(d, a, b, c, x[8], 11, 0x8771f681); 
		HH(c, d, a, b, x[11], 16, 0x6d9d6122); 
		HH(b, c, d, a, x[14], 23, 0xfde5380c);
		HH(a, b, c, d, x[1], 4, 0xa4beea44); 
		HH(d, a, b, c, x[4], 11, 0x4bdecfa9); 
		HH(c, d, a, b, x[7], 16, 0xf6bb4b60); 
		HH(b, c, d, a, x[10], 23, 0xbebfbc70); 
		HH(a, b, c, d, x[13], 4, 0x289b7ec6); 
		HH(d, a, b, c, x[0], 11, 0xeaa127fa); 
		HH(c, d, a, b, x[3], 16, 0xd4ef3085);
		HH(b, c, d, a, x[6], 23, 0x4881d05); 
		HH(a, b, c, d, x[9], 4, 0xd9d4d039); 
		HH(d, a, b, c, x[12], 11, 0xe6db99e5); 
		HH(c, d, a, b, x[15], 16, 0x1fa27cf8); 
		HH(b, c, d, a, x[2], 23, 0xc4ac5665);

	/* Round 4 */
		II(a, b, c, d, x[0], 6, 0xf4292244); 
		II(d, a, b, c, x[7], 10, 0x432aff97); 
		II(c, d, a, b, x[14], 15, 0xab9423a7); 
		II(b, c, d, a, x[5], 21, 0xfc93a039); 
		II(a, b, c, d, x[12], 6, 0x655b59c3); 
		II(d, a, b, c, x[3], 10, 0x8f0ccc92); 
		II(c, d, a, b, x[10], 15, 0xffeff47d);
		II(b, c, d, a, x[1], 21, 0x85845dd1); 
		II(a, b, c, d, x[8], 6, 0x6fa87e4f); 
		II(d, a, b, c, x[15], 10, 0xfe2ce6e0); 
		II(c, d, a, b, x[6], 15, 0xa3014314);
		II(b, c, d, a, x[13], 21, 0x4e0811a1); 
		II(a, b, c, d, x[4], 6, 0xf7537e82);
		II(d, a, b, c, x[11], 10, 0xbd3af235);
		II(c, d, a, b, x[2], 15, 0x2ad7d2bb); 
		II(b, c, d, a, x[9], 21, 0xeb86d391);
		state[0] += a;
		state[1] += b;
		state[2] += c;
		state[3] += d;
		memset((byte *)x, 0, sizeof(x));
	}

	static void Decode(uint *output, byte *input, uint len)
	{
		uint i, j;
		for (i = 0, j = 0; j < len; i++, j += 4)
			output[i] = ((uint)input[j]) | (((uint)input[j + 1]) << 8) |
			(((uint)input[j + 2]) << 16) | (((uint)input[j + 3]) << 24);
	}

	static void Encode(byte *output, uint *input, uint len)
	{
		for (uint i = 0, j = 0; j < len; i++, j += 4) {
			output[j] = (byte)(input[i] & 0xff);
			output[j + 1] = (byte)((input[i] >> 8) & 0xff);
			output[j + 2] = (byte)((input[i] >> 16) & 0xff);
			output[j + 3] = (byte)((input[i] >> 24) & 0xff);
		}
	}

public:
	void Init(MD5_CTX *context)
	{
		context->count[0] = context->count[1] = 0;
		context->state[0] = 0x67452301;
		context->state[1] = 0xefcdab89;
		context->state[2] = 0x98badcfe;
		context->state[3] = 0x10325476;
	}

	void Update(MD5_CTX *context, byte *input, uint inputLen)
	{
		uint i, index = (uint)((context->count[0] >> 3) & 0x3F);
		if ((context->count[0] += ((uint)inputLen << 3)) < ((uint)inputLen << 3))
			context->count[1]++;
		context->count[1] += ((uint)inputLen >> 29);
		uint partLen = 64 - index;
		if (inputLen >= partLen) {
			memcpy((byte *)&context->buffer[index], (byte *)input, partLen);
			MD5Transform(context->state, context->buffer);
			for (i = partLen; i + 63 < inputLen; i += 64)
				MD5Transform(context->state, &input[i]);
			index = 0;
		}
		else i = 0;
		memcpy((byte *)&context->buffer[index], (byte *)&input[i], inputLen - i);
	}

	void Final(byte digest[16], MD5_CTX *context)
	{
		byte bits[8];
		uint index, padLen;
		Encode(bits, context->count, 8);
		index = (uint)((context->count[0] >> 3) & 0x3f);
		padLen = (index < 56) ? (56 - index) : (120 - index);
		Update(context, PADDING, padLen);
		Update(context, bits, 8);
		Encode(digest, context->state, 16);
		memset((byte *)context, 0, sizeof(*context));
	}
};