Implement NIP-19 bech32-encoded private and public keys

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

spec/nostr/errors/invalid_hrp_error_spec.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Nostr::InvalidHRPError do
+  describe '#initialize' do
+    let(:given_hrp) { 'nwrong' }
+    let(:allowed_hrp) { 'nsec' }
+    let(:error) { described_class.new(given_hrp, allowed_hrp) }
+
+    it 'builds a useful error message' do
+      expect(error.message).to eq("Invalid hrp: nwrong. The allowed hrp value for this kind of entity is 'nsec'.")
+    end
+  end
+end

spec/nostr/errors/invalid_key_format_error_spec.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Nostr::InvalidKeyFormatError do
+  describe '#initialize' do
+    let(:key_kind) { 'private' }
+    let(:error) { described_class.new(key_kind) }
+
+    it 'builds a useful error message' do
+      expect(error.message).to eq('Only lowercase hexadecimal characters are allowed in private keys.')
+    end
+  end
+end

spec/nostr/errors/invalid_key_length_error_spec.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Nostr::InvalidKeyLengthError do
+  describe '#initialize' do
+    let(:key_kind) { 'private' }
+    let(:error) { described_class.new(key_kind) }
+
+    it 'builds a useful error message' do
+      expect(error.message).to eq('Invalid private key length. It should have 64 characters.')
+    end
+  end
+end

spec/nostr/errors/invalid_key_type_error_spec.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Nostr::InvalidKeyTypeError do
+  describe '#initialize' do
+    let(:key_kind) { 'private' }
+    let(:error) { described_class.new(key_kind) }
+
+    it 'builds a useful error message' do
+      expect(error.message).to eq('Invalid private key type')
+    end
+  end
+end