how to convert a string from a Pool Byte Array.hex encode() to a readable poolBytearray.

Why are you using hex_encode()? If you want to make a string from utf8 byte data you should use get_string_from_utf8()

What exactly wasn't working with UTF8?

You can do everything with ASCII, seems to work for me:

extends Node2D

onready var aes = get_node("AES")
var copy = "The rabbit-hole went straight on like a tunnel for some way, and then dipped suddenly down, so suddenly that Alice had not a moment to think about stopping herself before she found herself falling down a very deep well."

func _ready():
	randomize()
	var key = aes.rand_bytes(32)
	var cipher_text = aes.encrypt(copy, key)
	print(cipher_text.get_string_from_ascii())
	var plain_text = aes.decrypt(cipher_text, key)
	print(plain_text.get_string_from_ascii())

 P[i5HYCps/^n	m=wQ)EEVJwn\,OK8=v G}A:E@(~[+T{%;(M52y(A}x
The rabbit-hole went straight on like a tunnel for some way, and then dipped suddenly down, so suddenly that Alice had not a moment to think about stopping herself before she found herself falling down a very deep well.

Actually, wait, I see what the problem is. Give me a second.

Yeah, so the issue is that the AES script expects certain size of the input file. You can convert a string to a pool like this:

var encrypted = aes.encrypt(copy, key)
var cipher_text = encrypted.get_string_from_ascii()
print(cipher_text)
var cipher_array = cipher_text.to_utf8()
print(cipher_array)

However, because the AES doesn't like being converted to UTF8 (I had to use ASCII here) then the conversion is messed up and it won't decrypt. There is also something off with the padding, as the AES script expects 16 byte and when you convert manually that is not guaranteed. The strange part is that the AES script does appear to allow sending strings as input (not pools) but you need to ensure that the plain text is multiple of 16 bytes.

Actually, I found that Godot 3.4 has built-in support for AES now. However, I still ran into the same problem with the UTF8. But I found a work around to convert to UTF8 to BASE64 and it seems to work. This example shows two encryption decryption methods. You only need to choose one method.

extends Node2D

onready var aes = AESContext.new()
var data = "The rabbit-hole went straight on like a tunnel for some way, and then dipped suddenly down, so suddenly that Alice had not a moment to think about stopping herself before she found herself falling down a very deep well."

func _ready():
	var key = "My secret key!!!" 
	# Key must be either 16 or 32 bytes.
	# Data size must be multiple of 16 bytes, apply padding if needed.
	data = Marshalls.utf8_to_base64(data)
	while data.length() % 16 != 0:
		data += " "
	# Encrypt ECB
	aes.start(AESContext.MODE_ECB_ENCRYPT, key.to_utf8())
	var encrypted = aes.update(data.to_utf8())
	aes.finish()
	
	var cipher_text = encrypted.get_string_from_ascii()
	print("encrypted = ", cipher_text)
	
	# Decrypt ECB
	aes.start(AESContext.MODE_ECB_DECRYPT, key.to_utf8())
	var decrypted = aes.update(cipher_text.to_ascii())
	aes.finish()
	# Check ECB
	assert(decrypted == data.to_utf8())
	
	print("decrypted = ", Marshalls.base64_to_utf8(decrypted.get_string_from_utf8()))

	var iv = "My secret iv!!!!" # IV must be of exactly 16 bytes.
	# Encrypt CBC
	aes.start(AESContext.MODE_CBC_ENCRYPT, key.to_utf8(), iv.to_utf8())
	encrypted = aes.update(data.to_utf8())
	aes.finish()
	
	cipher_text = encrypted.get_string_from_ascii()
	print("encrypted = ", cipher_text)
	
	# Decrypt CBC
	aes.start(AESContext.MODE_CBC_DECRYPT, key.to_utf8(), iv.to_utf8())
	decrypted = aes.update(cipher_text.to_ascii())
	aes.finish()
	# Check CBC
	assert(decrypted == data.to_utf8())
	
	print("decrypted = ", Marshalls.base64_to_utf8(decrypted.get_string_from_utf8()))

Maybe converting to Base64 before sending to Javascript would help?