MD5 , inputs larger than 512 bits

tiredpotato

12/1/23, 3:28 PM

So I get how it works for inputs shorter than $512$, but as for larger inputs "In the case where the remaining input data is exactly $448$ bits long, an entire extra block would need to be added for the padding." what do they mean by the 'remaining' input data is exactly $448$ bits long? if input was like, lets say $500$ bits, $1024-64=960$, $960 - 500= 460$, so $500 + 1 + 459$ zero's, followed by the $64$-bit message length. Is this correct?

117

0 + 4

fgrieu

12/1/23, 3:47 PM

We have [many questions and answers around MD5 padding](https://crypto.stackexchange.com/search?tab=relevance&q=%5bmd5%5d%20padding), I'm not sure we need another. If the input data is $b$-bit long, then there are $\lfloor(b+64)/512\rfloor+1=\lceil(b+65)/512\rceil$ block(s). Yes for a 500-bit message, there are 1+459+64 bits after the data bits. If the input is 447, or 959, or 1471-bit, there are 1+64 (the min). If the input is 448, or 960, or 1472-bit, there are 1+447+64 (the max). Beware that MD5 is little-endian everywhere except for bits in a byte.

tiredpotato

12/1/23, 3:58 PM

sorry it's my first time on this site T_T and thank u for answering. So, if the input is 448 bits, I'll be needing 2 blocks? so, 448 + 1 + 63 zeros and the 2nd block 448 zeros + 64? did I get it right?

fgrieu

12/1/23, 5:03 PM

Yes, that's exactly it. Again, beware of endianness: the first 32-bit word of the last 64 bits of the second block are the __low__-order 32 bits of the length in bits, but the first bit of the first 32-bit word after the 56 bytes (14 words) of data is the __high__-order bit of the low-order byte of that word, thus that word has value 128 (0x00000080, that is 0x80 0x00 0x00 0x00).

Maarten Bodewes

12/2/23, 11:41 AM

The stupid thing is that the padding should be the same regardless of the endianness as the order of bits **in a byte** is always highest to lowest, i.e. big endian. I've seen `0x01` used instead of `0x80` which is ridiculous. That's one reason why little endian is so confusing; in hexadecimals it reverses order every byte.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: MD5 , inputs larger than 512 bits

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