How to compute SHA256 hash for bitcoin header

Question

I am trying to understand the proof of work algorithm. I computed a block header (which includes the nonce):

"02000000aaf8ab82362344f49083ee4edef795362cf135293564c4070000000000000000c009bb6222e9bc4cdb8f26b2e8a2f8d163509691a4038fa692abf9a474c9b21476800755c02e17181fe6c1c3"

I have to apply SHA256 to this twice. I tried pack, unpack, hex, etc., but I can't get this output. 

The correct answer is supposed to be:

"00000000000000001354e21fea9c1ec9ac337c8a6c0bda736ec1096663383429"

But i get different answer. Can somebody help me with this?

Answer 1

header_hex = "02000000aaf8ab82362344f49083ee4edef795362cf135293564c4070000000000000000c009bb6222e9bc4cdb8f26b2e8a2f8d163509691a4038fa692abf9a474c9b21476800755c02e17181fe6c1c3"

# Decode header hex into binary string
header = [header_hex].pack("H*")

# Apply SHA256 twice
require "digest"
d1 = Digest::SHA256.digest(header)
d2 = Digest::SHA256.digest(d1)

# Convert to hex
result = d2.unpack("H*").join
# => "293438636609c16e73da0b6c8a7c33acc91e9cea1fe254130000000000000000"

Oops, for some reason the result is somewhat "backwards". Perhaps it is a byte-ordering issue? Let's try that again with the binary data reversed:

result = d2.reverse.unpack("H*").join
# => "00000000000000001354e21fea9c1ec9ac337c8a6c0bda736ec1096663383429"