document key pair generation (#16)

README.md

@@ -5,7 +5,9 @@ A rubygem for creating and verifying [Minisign](http://jedisct1.github.io/minisi
 - [Installation \& Usage](#installation--usage)
   - [Read a public key](#read-a-public-key)
   - [Verify a signature](#verify-a-signature)
+  - [Read a private key](#read-a-private-key)
   - [Create a signature](#create-a-signature)
+  - [Generate a key pair](#generate-a-key-pair)
 - [Local Development](#local-development)
 - [Documentation](#documentation)
 
@@ -27,31 +29,36 @@ public_key = Minisign::PublicKey.new(File.read("test/minisign.pub"))
 ### Verify a signature
 
 ```rb
-require 'minisign'
-public_key = Minisign::PublicKey.new('RWSmKaOrT6m3TGwjwBovgOmlhSbyBUw3hyhnSOYruHXbJa36xHr8rq2M')
 message = File.read("test/example.txt")
 signature = Minisign::Signature.new(File.read("test/example.txt.minisig"))
 public_key.verify(signature, message)
 ```
 
-The above is equivalent to:
+### Read a private key
 
-```
-minisign -Vm test/example.txt -P RWSmKaOrT6m3TGwjwBovgOmlhSbyBUw3hyhnSOYruHXbJa36xHr8rq2M
+```rb
+password = "password" # optional, if the key is not encrypted
+private_key = Minisign::PrivateKey.new(File.read("minisign.key"), password)
 ```
 
 ### Create a signature
 
 ```rb
-require 'minisign'
 file_path = "example.txt"
 password = "password"
-private_key = Minisign::PrivateKey.new(File.read("minisign.key"), password)
 signature = private_key.sign(file_path, File.read(file_path))
-
 File.write("#{file_path}.minisig", signature.to_s)
 ```
 
+### Generate a key pair
+
+```rb
+password = "password" # or nil, to generate a private key without encryption
+keypair = Minisign::KeyPair.new(password)
+keypair.private_key # Minisign::PrivateKey
+keypair.public_key # Minisign::PublicKey
+```
+
 ## Local Development
 
 ```

lib/minisign/key_pair.rb

@@ -7,7 +7,8 @@ class KeyPair
 
     def initialize(password = nil)
       @password = password
-
+      @key_id = SecureRandom.bytes(8)
+      @signing_key = Ed25519::SigningKey.generate
       kd = key_data
 
       @checksum = blake2b256("Ed#{kd}")
@@ -18,6 +19,7 @@ def initialize(password = nil)
       @kdf_algorithm = password.nil? ? [0, 0].pack('U*') : 'Sc'
     end
 
+    # @return [Minisign::PrivateKey]
     def private_key
       @kdf_opslimit = kdf_opslimit_bytes.pack('C*')
       @kdf_memlimit = kdf_memlimit_bytes.pack('C*')
@@ -28,6 +30,12 @@ def private_key
       )
     end
 
+    # @return [Minisign::PublicKey]
+    def public_key
+      data = Base64.strict_encode64("Ed#{@key_id}#{@signing_key.verify_key.to_bytes}")
+      Minisign::PublicKey.new(data)
+    end
+
     private
 
     def kdf_output
@@ -40,9 +48,7 @@ def kdf_output
     end
 
     def key_data
-      key_id = SecureRandom.bytes(8)
-      signing_key = Ed25519::SigningKey.generate
-      "#{key_id}#{signing_key.to_bytes}#{signing_key.verify_key.to_bytes}"
+      "#{@key_id}#{@signing_key.to_bytes}#{@signing_key.verify_key.to_bytes}"
     end
 
     # 🤷

lib/minisign/private_key.rb

@@ -42,6 +42,7 @@ def initialize(str, password = nil)
     # rubocop:enable Metrics/AbcSize
     # rubocop:enable Metrics/MethodLength
 
+    # @raise [RuntimeError] if the extracted public key does not match the derived public key
     def assert_keypair_match!
       raise 'Wrong password for that key' if @public_key != ed25519_signing_key.verify_key.to_bytes.bytes
     end
@@ -74,6 +75,7 @@ def sign(filename, message, comment = nil)
       ].join("\n"))
     end
 
+    # @return [String] A string in the minisign.pub format
     def to_s
       kdf_salt = @kdf_salt.pack('C*')
       kdf_opslimit = [@kdf_opslimit, 0].pack('L*')