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This is how to manually serialize an RSA key for usage in a Minecraft Encryption Request packet for example.
module EncodingPublicKey where
import Data.Maybe
import Data.Semigroup
import Data.Bits
import OpenSSL.RSA
import qualified Data.ByteString as BS
-- Observe the cancer, but don't touch it or you'll contract it.
encodePubKey :: RSAKeyPair -> BS.ByteString
encodePubKey k = asnSequence <> withLength (algIdentifier <> pubKeyBitstring)
where
-- ASN.1 object identifier for RSA keys (decoded from 1.2.840.113549.1.1.1)
-- See section 5.9 for details: http://luca.ntop.org/Teaching/Appunti/asn1.html
asnOIDForRSAKeys = BS.pack [0x2a,0x86,0x48,0x86,0xf7,0x0d,0x01,0x01,0x01]
-- ASN.1 defined tags
-- Usually you use: tag + length + data
nullForGoodLuck = BS.singleton 0x00
asnInt = BS.singleton 0x02
asnBitString = BS.singleton 0x03
asnTag = BS.singleton 0x05
asnObjectId = BS.singleton 0x06
asnSequence = BS.singleton 0x30
-- ASN.1's length format helper functions
-- for short lengths (<128) the length is a single byte
-- for long lengths, the length is a byte with the MSB set and bits 0-7 encode
-- how many more bytes there are. The rest of the bytes are the actual length
--
-- Example: 0b00001100 means Actual Length = 0b00001100 = 12
-- ^ MSB not set
--
-- Example: 0b10000010 0x5e 0x67 means Actual Length = 0x5e 0x67 = 24167
-- MSB is set ^ ^two more bytes
lenOf bs = if BS.length bs < 128
then BS.singleton . fromIntegral . BS.length $ bs
else
let b = (intBytesRaw . fromIntegral . BS.length $ bs)
in (0x80 .|. (fromIntegral . BS.length $ b)) `BS.cons` b
-- A thing and its length
withLength a = lenOf a <> a
algIdentifier = asnSequence <> withLength (algObjectId <> algParams)
-- Use our precalculated magic bytes
algObjectId = asnObjectId <> withLength asnOIDForRSAKeys
-- 0x05 0x00 is NULL in ASN.1, and we don't have any params
algParams = asnTag <> nullForGoodLuck
-- idk why there is a null here
pubKeyBitstring = asnBitString <> withLength (nullForGoodLuck <> pubKeySequence)
pubKeySequence = asnSequence <> withLength (theModulus <> theExponent)
-- The people who made ASN.1 like to put random nulls in for fun
theModulus = asnInt <> withLength (nullForGoodLuck <> bytesOfModulus)
theExponent = asnInt <> withLength bytesOfExponent
bytesOfModulus = intBytesRaw $ rsaN k
bytesOfExponent = intBytesRaw $ rsaE k
-- intBytesRaw gets the variable-length byte encoding of a number
intBytesRaw :: Integer -> BS.ByteString
intBytesRaw = BS.reverse . BS.unfoldr (\i -> if i == 0 then Nothing else Just $ (fromIntegral i, shiftR i 8))
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