Add password based encryption example

examples/password_based_encryption.cr

@@ -0,0 +1,47 @@
+require "../src/sodium"
+
+# Use Password::Create to generate Key params
+# Or hard code them like below
+
+pwkey = Sodium::Password::Key.new
+
+# Uses MINIMUM to speed up this example.  Don't use MINIMUM in real applications.
+# See examples/pw_hash_selector.cr for help on selecting parameters.
+pwkey.mem = Sodium::Password::MEMLIMIT_MIN
+pwkey.ops = Sodium::Password::OPSLIMIT_MIN
+
+pwkey.mode = Sodium::Password::Mode::Argon2id13
+
+# Save opslimit, memlimit, mode and master_key_size somewhere.  They may be hard coded in your application.
+
+# SAVE salt per user.  Every time the password changes also change the salt.
+salt = pwkey.random_salt
+
+password = "1234"
+
+# kdf_derive is a wrapper around following 3 lines.
+# kdf = Sodium::KDF.new master_key
+# master_key_size = 32 # Derive 256 bit key
+# master_key = pwkey.derive_key password, master_key_size, salt: salt
+kdf = pwkey.derive_kdf password, salt: salt
+
+# TODO: verify password
+
+xchacha = kdf.derive_aead_xchacha20poly1305_ietf "newwalle", 0
+# or secretbox = kdf.derive_secretbox "oldcrypt", 0
+# wallet = xchacha.decrypt File.read("blockchain.wallet")
+
+# Returns a Sign::SecretKey
+signkey = kdf.derive_sign "signatur", 0
+
+# Returns a CryptoBox::SecretKey
+secretkey = kdf.derive_cryptobox "pkcrypto", 0
+
+# anonymous_messages.each do |encrypted_message|
+#  decrypted = secretkey.decrypt encrypted_message
+#  inmessage = parse decrypted
+#  signed_outmessage = signkey.sign wallet.transfer(message.recipient, message.amount, "Sure you can have free money")
+#  secretkey.box inmessage.public_key do |box|
+#    mail to: inmessage.email, data: box.encrypt(signed_outmessage)
+#  end
+# end