aes-256-gcm.rb

require 'openssl'
require 'base64'
require 'digest'

plaintext = "sample"
pass = "pass"

key = Digest::SHA256.digest(pass)

cipher = OpenSSL::Cipher.new('aes-256-gcm')
cipher.encrypt
cipher.key = key

# should randomize
iv = "\x00" * 12
cipher.iv = iv

ciphertext = cipher.update(plaintext) + cipher.final
tag = cipher.auth_tag

encrypted = Base64.strict_encode64(iv + tag + ciphertext)
puts encrypted

decoded = Base64.decode64(encrypted)
extracted_iv = decoded[0, 12]      
extracted_tag = decoded[12, 16]   
extracted_ciphertext = decoded[28..-1] 

decipher = OpenSSL::Cipher.new('aes-256-gcm')
decipher.decrypt
decipher.key = key
decipher.iv = extracted_iv
decipher.auth_tag = extracted_tag

decrypted = decipher.update(extracted_ciphertext) + decipher.final
puts decrypted

example

AAAAAAAAAAAAAAAA+gZt3i+0EOi82KwzS1QaTt6b3FPTlOztsbwvVqCPy5ZPdiQcS/Vaft2cgSJStRD1PAPFVpQ+YKsm57RTkfxGlGFGWbXx26lHCvBJyotoGPugAB58GLCWLSlUp2I2Mzx8xBuOmA==