Allow the verification of signatures and events

Added the methods:
- Event#verify_signature
- Crypto#check_sig!
- Crypto#valid_sig?
- Crypto#sign_message

Fixed a primitive obsession by introducing a Signature class to ensure that signatures are valid Nostr signatures.

lib/nostr/crypto.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Nostr
-  # Performs cryptographic operations on a +Nostr::Event+.
+  # Performs cryptographic operations.
   class Crypto
     # Numeric base of the OpenSSL big number used in an event content's encryption.
     #
@@ -90,17 +90,93 @@ module Nostr
     #
     def sign_event(event, private_key)
       event_digest = hash_event(event)
-
-      hex_private_key = Array(private_key).pack('H*')
-      hex_message     = Array(event_digest).pack('H*')
-      event_signature = Schnorr.sign(hex_message, hex_private_key).encode.unpack1('H*')
+      signature = sign_message(event_digest, private_key)
 
       event.id = event_digest
-      event.sig = event_signature
+      event.sig = signature
 
       event
     end
 
+    # Signs a message using the Schnorr signature algorithm
+    #
+    # @api public
+    #
+    # @example Signing a message
+    #   crypto = Nostr::Crypto.new
+    #   message = 'Viva la libertad carajo'
+    #   private_key = Nostr::PrivateKey.new('7d1e4219a5e7d8342654c675085bfbdee143f0eb0f0921f5541ef1705a2b407d')
+    #   signature = crypto.sign_message(message, private_key)
+    #   signature # => 'b2115694a576f5bdcebf8c0951a3c7adcfbdb17b11cb9e6d6b7017691138bc6' \
+    #     '38fee642a7bd26f71b313a7057181294198900a9770d1435e43f182acf3d34c26'
+    #
+    # @param [String] message The message to be signed
+    # @param [PrivateKey] private_key The private key used for signing
+    #
+    # @return [Signature] A signature object containing the signature as a 64-byte hexadecimal string.
+    #
+    def sign_message(message, private_key)
+      hex_private_key = Array(private_key).pack('H*')
+      hex_message     = Array(message).pack('H*')
+      hex_signature   = Schnorr.sign(hex_message, hex_private_key).encode.unpack1('H*')
+
+      Signature.new(hex_signature.to_s)
+    end
+
+    # Verifies the given {Signature} and returns true if it is valid
+    #
+    # @api public
+    #
+    # @example Checking a signature
+    #   public_key = Nostr::PublicKey.new('15678d8fbc126fa326fac536acd5a6dcb5ef64b3d939abe31d6830cba6cd26d6')
+    #   private_key = Nostr::PrivateKey.new('7d1e4219a5e7d8342654c675085bfbdee143f0eb0f0921f5541ef1705a2b407d')
+    #   message = 'Viva la libertad carajo'
+    #   crypto = Nostr::Crypto.new
+    #   signature = crypto.sign_message(message, private_key)
+    #   valid = crypto.valid_sig?(message, public_key, signature)
+    #   valid # => true
+    #
+    # @see #check_sig!
+    #
+    # @param [String] message A message to be signed with binary format.
+    # @param [PublicKey] public_key The public key with binary format.
+    # @param [Signature] signature The signature with binary format.
+    #
+    # @return [Boolean] whether signature is valid.
+    #
+    def valid_sig?(message, public_key, signature)
+      signature = Schnorr::Signature.decode([signature].pack('H*'))
+      Schnorr.valid_sig?([message].pack('H*'), [public_key].pack('H*'), signature.encode)
+    end
+
+    # Verifies the given {Signature} and raises an +Schnorr::InvalidSignatureError+ if it is invalid
+    #
+    # @api public
+    #
+    # @example Checking a signature
+    #   public_key = Nostr::PublicKey.new('15678d8fbc126fa326fac536acd5a6dcb5ef64b3d939abe31d6830cba6cd26d6')
+    #   private_key = Nostr::PrivateKey.new('7d1e4219a5e7d8342654c675085bfbdee143f0eb0f0921f5541ef1705a2b407d')
+    #   message = 'Viva la libertad carajo'
+    #   crypto = Nostr::Crypto.new
+    #   signature = crypto.sign_message(message, private_key)
+    #   valid = crypto.valid_sig?(message, public_key, signature)
+    #   valid # => true
+    #
+    # @see #valid_sig?
+    #
+    # @param [String] message A message to be signed with binary format.
+    # @param [PublicKey] public_key The public key with binary format.
+    # @param [Signature] signature The signature with binary format.
+    #
+    # @raise [Schnorr::InvalidSignatureError] if the signature is invalid.
+    #
+    # @return [Boolean] whether signature is valid.
+    #
+    def check_sig!(message, public_key, signature)
+      signature = Schnorr::Signature.decode([signature].pack('H*'))
+      Schnorr.check_sig!([message].pack('H*'), [public_key].pack('H*'), signature.encode)
+    end
+
     private
 
     # Finds a shared key between two keys

lib/nostr/errors.rb

@@ -6,3 +6,7 @@ require_relative 'errors/invalid_hrp_error'
 require_relative 'errors/invalid_key_type_error'
 require_relative 'errors/invalid_key_length_error'
 require_relative 'errors/invalid_key_format_error'
+require_relative 'errors/signature_validation_error'
+require_relative 'errors/invalid_signature_type_error'
+require_relative 'errors/invalid_signature_length_error'
+require_relative 'errors/invalid_signature_format_error'

lib/nostr/errors/invalid_signature_format_error.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Raised when the signature is in an invalid format
+  #
+  # @api public
+  #
+  class InvalidSignatureFormatError < SignatureValidationError
+    # Initializes the error
+    #
+    # @example
+    #   InvalidSignatureFormatError.new
+    #
+    def initialize
+      super('Only lowercase hexadecimal characters are allowed in signatures.')
+    end
+  end
+end

lib/nostr/errors/invalid_signature_length_error.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Raised when the signature's length is not 128 characters
+  #
+  # @api public
+  #
+  class InvalidSignatureLengthError < SignatureValidationError
+    # Initializes the error
+    #
+    # @example
+    #   InvalidSignatureLengthError.new
+    #
+    def initialize
+      super('Invalid signature length. It should have 128 characters.')
+    end
+  end
+end

lib/nostr/errors/invalid_signature_type_error.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Raised when the signature is not a string
+  #
+  # @api public
+  #
+  class InvalidSignatureTypeError < SignatureValidationError
+    # Initializes the error
+    #
+    # @example
+    #   InvalidSignatureTypeError.new
+    #
+    def initialize = super('Invalid signature type')
+  end
+end

lib/nostr/errors/signature_validation_error.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Base class for all signature validation errors
+  class SignatureValidationError < Error; end
+end

lib/nostr/event.rb

@@ -181,6 +181,34 @@ module Nostr
       crypto.sign_event(self, private_key)
     end
 
+    # Verifies if the signature of the event is valid. A valid signature means that the event was signed by the owner
+    #
+    # @api public
+    #
+    # @example Verifying the signature of an event
+    #   event = Nostr::Event.new(
+    #     id: '90b75b78daf883ae57fbcc414d43faa028560b3211ee58e4ea82bf395bb82042',
+    #     pubkey: Nostr::PublicKey.new('2d7661527d573cc8e84f665fa971dd969ba51e2526df00c149ff8e40a58f9558'),
+    #     created_at: 1667422587,
+    #     kind: Nostr::EventKind::TEXT_NOTE,
+    #     content: 'Your feedback is appreciated, now pay $8',
+    #     sig: '32f18adebe942e19b171c1c7d2fb27ce794dfea4155e289dca7952b43ed1ec39' \
+    #       '1d3dc198ba2761bc6d40c737a6eaf4edcc8963acabd3bfcebd04f16637025bdc'
+    #   )
+    #
+    #   event.verify_signature # => true
+    #
+    # @return [Boolean] Whether the signature is valid or not.
+    #
+    def verify_signature
+      crypto = Crypto.new
+
+      return false if id.nil? || pubkey.nil?
+      return false if sig.nil? # FIXME: See https://github.com/soutaro/steep/issues/1079
+
+      crypto.valid_sig?(id, pubkey, sig)
+    end
+
     # Serializes the event, to obtain a SHA256 digest of it
     #
     # @api public

lib/nostr/signature.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+module Nostr
+  # 64-bytes lowercase hex of the signature of the sha256 hash of the serialized event data,
+  # which is the same as the "id" field
+  class Signature < String
+    # The regular expression for hexadecimal lowercase characters
+    #
+    # @return [Regexp] The regular expression for hexadecimal lowercase characters
+    #
+    FORMAT = /^[a-f0-9]+$/
+
+    # The length of the signature in hex format
+    #
+    # @return [Integer] The length of the signature in hex format
+    #
+    LENGTH = 128
+
+    # Instantiates a new Signature
+    #
+    # @api public
+    #
+    # @example Instantiating a new signature
+    #  Nostr::Signature.new(
+    #    'f418c97b50cc68227e82f4f3a79d79eb2b7a0fa517859c86e1a8fa91e3741b7f' \
+    #    '06e070c44129227b83fcbe93cecb02a346804a4080ce47685ecad60ab4f5f128'
+    #  )
+    #
+    # @param [String] hex_value Hex-encoded value of the signature
+    #
+    # @raise [SignatureValidationError]
+    #
+    def initialize(hex_value)
+      validate(hex_value)
+
+      super(hex_value)
+    end
+
+    private
+
+    # Hex-encoded value of the signature
+    #
+    # @api private
+    #
+    # @return [String] hex_value Hex-encoded value of the signature
+    #
+    attr_reader :hex_value
+
+    # Validates the hex value of the signature
+    #
+    # @api private
+    #
+    # @param [String] hex_value The signature in hex format
+    #
+    # @raise InvalidSignatureTypeError when the signature is not a string
+    # @raise InvalidSignatureLengthError when the signature's length is not 128 characters
+    # @raise InvalidSignatureFormatError when the signature is in an invalid format
+    #
+    # @return [void]
+    #
+    def validate(hex_value)
+      raise InvalidSignatureTypeError   unless hex_value.is_a?(String)
+      raise InvalidSignatureLengthError unless hex_value.size == LENGTH
+      raise InvalidSignatureFormatError unless hex_value.match(FORMAT)
+    end
+  end
+end