RFC 1186 (rfc1186) - Page 2 of 18
MD4 Message Digest Algorithm
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RFC 1186 MD4 Message Digest Algorithm October 1990 (Note: The document supersedes an earlier draft. The algorithm described here is a slight modification of the one described in the draft.) 2. Terminology and Notation In this note a "word" is a 32-bit quantity and a byte is an 8-bit quantity. A sequence of bits can be interpreted in a natural manner as a sequence of bytes, where each consecutive group of 8 bits is interpreted as a byte with the high-order (most significant) bit of each byte listed first. Similarly, a sequence of bytes can be interpreted as a sequence of 32-bit words, where each consecutive group of 4 bytes is interpreted as a word with the low-order (least significant) byte given first. Let x_i denote "x sub i". If the subscript is an expression, we surround it in braces, as in x_{i+1}. Similarly, we use ^ for superscripts (exponentiation), so that x^i denotes x to the i-th power. Let the symbol "+" denote addition of words (i.e., modulo- 2^32 addition). Let X <<< s denote the 32-bit value obtained by circularly shifting (rotating) X left by s bit positions. Let not(X) denote the bit-wise complement of X, and let X v Y denote the bit-wise OR of X and Y. Let X xor Y denote the bit-wise XOR of X and Y, and let XY denote the bit-wise AND of X and Y. 3. MD4 Algorithm Description We begin by supposing that we have a b-bit message as input, and that we wish to find its message digest. Here b is an arbitrary nonnegative integer; b may be zero, it need not be a multiple of 8, and it may be arbitrarily large. We imagine the bits of the message written down as follows: m_0 m_1 ... m_{b-1} . The following five steps are performed to compute the message digest of the message. Step 1. Append padding bits The message is "padded" (extended) so that its length (in bits) is congruent to 448, modulo 512. That is, the message is extended so that it is just 64 bits shy of being a multiple of 512 bits long. Padding is always performed, even if the length of the message is already congruent to 448, modulo 512 (in which case 512 bits of padding are added). Rivest
