Implement NIP-19 bech32-encoded private and public keys

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

lib/nostr/errors/error.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Base error class
+  class Error < StandardError
+  end
+end

lib/nostr/errors/invalid_hrp_error.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Raised when the human readable part of a Bech32 string is invalid
+  #
+  # @api public
+  #
+  class InvalidHRPError < KeyValidationError
+    # Initializes the error
+    #
+    # @example
+    #   InvalidHRPError.new('example wrong hrp', 'nsec')
+    #
+    # @param given_hrp [String] The given human readable part of the Bech32 string
+    # @param allowed_hrp [String] The allowed human readable part of the Bech32 string
+    #
+    def initialize(given_hrp, allowed_hrp)
+      super("Invalid hrp: #{given_hrp}. The allowed hrp value for this kind of entity is '#{allowed_hrp}'.")
+    end
+  end
+end

lib/nostr/errors/invalid_key_format_error.rb

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+# frozen_string_literal: true
+
+module Nostr
+  # Raised when the private key is in an invalid format
+  #
+  # @api public
+  #
+  class InvalidKeyFormatError < KeyValidationError
+    # Initializes the error
+    #
+    # @example
+    #   InvalidKeyFormatError.new('private'')
+    #
+    # @param [String] key_kind The kind of key that is invalid (public or private)
+    #
+    def initialize(key_kind)
+      super("Only lowercase hexadecimal characters are allowed in #{key_kind} keys.")
+    end
+  end
+end

lib/nostr/errors/invalid_key_length_error.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Raised when the private key's length is not 64 characters
+  #
+  # @api public
+  #
+  class InvalidKeyLengthError < KeyValidationError
+    # Initializes the error
+    #
+    # @example
+    #   InvalidKeyLengthError.new('private'')
+    #
+    # @param [String] key_kind The kind of key that is invalid (public or private)
+    #
+    def initialize(key_kind)
+      super("Invalid #{key_kind} key length. It should have 64 characters.")
+    end
+  end
+end

lib/nostr/errors/invalid_key_type_error.rb

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+# frozen_string_literal: true
+
+module Nostr
+  # Raised when the private key is not a string
+  #
+  # @api public
+  #
+  class InvalidKeyTypeError < KeyValidationError
+    # Initializes the error
+    #
+    # @example
+    #   InvalidKeyTypeError.new('private'')
+    #
+    # @param [String] key_kind The kind of key that is invalid (public or private)
+    #
+    def initialize(key_kind) = super("Invalid #{key_kind} key type")
+  end
+end

lib/nostr/errors/key_validation_error.rb

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+# frozen_string_literal: true
+
+module Nostr
+  # Base class for all key validation errors
+  class KeyValidationError < Error; end
+end