Implement NIP-19 bech32-encoded private and public keys

https://github.com/nostr-protocol/nips/blob/master/19.md

lib/nostr/key_pair.rb

@@ -10,7 +10,7 @@ module Nostr
     # @example
     #   keypair.private_key # => '893c4cc8088924796b41dc788f7e2f746734497010b1a9f005c1faad7074b900'
     #
-    # @return [String]
+    # @return [PrivateKey]
     #
     attr_reader :private_key
 
@@ -21,7 +21,7 @@ module Nostr
     # @example
     #   keypair.public_key # => '2d7661527d573cc8e84f665fa971dd969ba51e2526df00c149ff8e40a58f9558'
     #
-    # @return [String]
+    # @return [PublicKey]
     #
     attr_reader :public_key
 
@@ -31,16 +31,40 @@ module Nostr
     #
     # @example
     #   keypair = Nostr::KeyPair.new(
-    #     private_key: '893c4cc8088924796b41dc788f7e2f746734497010b1a9f005c1faad7074b900',
-    #     public_key: '2d7661527d573cc8e84f665fa971dd969ba51e2526df00c149ff8e40a58f9558',
+    #     private_key: Nostr::PrivateKey.new('893c4cc8088924796b41dc788f7e2f746734497010b1a9f005c1faad7074b900'),
+    #     public_key: Nostr::PublicKey.new('2d7661527d573cc8e84f665fa971dd969ba51e2526df00c149ff8e40a58f9558'),
     #   )
     #
-    # @param private_key [String] 32-bytes hex-encoded private key.
-    # @param public_key [String] 32-bytes hex-encoded public key.
+    # @param private_key [PrivateKey] 32-bytes hex-encoded private key.
+    # @param public_key [PublicKey] 32-bytes hex-encoded public key.
+    #
+    # @raise ArgumentError when the private key is not a +PrivateKey+
+    # @raise ArgumentError when the public key is not a +PublicKey+
     #
     def initialize(private_key:, public_key:)
+      validate_keys(private_key, public_key)
+
       @private_key = private_key
       @public_key = public_key
     end
+
+    private
+
+    # Validates the keys
+    #
+    # @api private
+    #
+    # @param private_key [PrivateKey] 32-bytes hex-encoded private key.
+    # @param public_key [PublicKey] 32-bytes hex-encoded public key.
+    #
+    # @raise ArgumentError when the private key is not a +PrivateKey+
+    # @raise ArgumentError when the public key is not a +PublicKey+
+    #
+    # @return [void]
+    #
+    def validate_keys(private_key, public_key)
+      raise ArgumentError, 'private_key is not an instance of PrivateKey' unless private_key.is_a?(Nostr::PrivateKey)
+      raise ArgumentError, 'public_key is not an instance of PublicKey' unless public_key.is_a?(Nostr::PublicKey)
+    end
   end
 end