Big-Endian Word Packing

SHA-256 operates on 32-bit words, but its input is bytes. The convention is big-endian: the high byte first.

bytes:    61  62  63  64       (ASCII 'a','b','c','d')
big-endian word: 0x61626364   ('abcd' as one 32-bit number)

Most modern x86/ARM CPUs are little-endian, so you must explicitly pack big-endian:

To unpack a 256-bit hash (the final output) back to bytes, also big-endian:

The full output is 8 such words concatenated = 32 bytes = 64 hex chars.

Why big-endian? Network byte order. SHA, MD5, SHA-1, AES, etc. all use big-endian for interop with network protocols.