Implement NIP-04: Encrypted Direct Messages

lib/nostr/crypto.rb

@@ -3,6 +3,72 @@
 module Nostr
   # Performs cryptographic operations on a +Nostr::Event+.
   class Crypto
+    # Numeric base of the OpenSSL big number used in an event content's encryption.
+    #
+    # @return [Integer]
+    #
+    BN_BASE = 16
+
+    # Name of the cipher curve used in an event content's encryption.
+    #
+    # @return [String]
+    #
+    CIPHER_CURVE = 'secp256k1'
+
+    # Name of the cipher algorithm used in an event content's encryption.
+    #
+    # @return [String]
+    #
+    CIPHER_ALGORITHM = 'aes-256-cbc'
+
+    # Encrypts a piece of text
+    #
+    # @api public
+    #
+    # @example Encrypting an event's content
+    #   crypto = Nostr::Crypto.new
+    #   encrypted = crypto.encrypt_text(sender_private_key, recipient_public_key, 'Feedback appreciated. Now pay $8')
+    #   encrypted # => "wrYQaHDfpOEvyJELSCg1vzsywmlJTz8NqH03eFW44s8iQs869jtSb26Lr4s23gmY?iv=v38vAJ3LlJAGZxbmWU4qAg=="
+    #
+    # @param sender_private_key [String] 32-bytes hex-encoded private key of the creator.
+    # @param recipient_public_key [String] 32-bytes hex-encoded public key of the recipient.
+    # @param plain_text [String] The text to be encrypted
+    #
+    # @return [String] Encrypted text.
+    #
+    def encrypt_text(sender_private_key, recipient_public_key, plain_text)
+      cipher = OpenSSL::Cipher.new(CIPHER_ALGORITHM).encrypt
+      cipher.iv = iv = cipher.random_iv
+      cipher.key = compute_shared_key(sender_private_key, recipient_public_key)
+      encrypted_text = cipher.update(plain_text) + cipher.final
+      encrypted_text = "#{Base64.encode64(encrypted_text)}?iv=#{Base64.encode64(iv)}"
+      encrypted_text.gsub("\n", '')
+    end
+
+    # Decrypts a piece of text
+    #
+    # @api public
+    #
+    # @example Encrypting an event's content
+    #   crypto = Nostr::Crypto.new
+    #   encrypted # => "wrYQaHDfpOEvyJELSCg1vzsywmlJTz8NqH03eFW44s8iQs869jtSb26Lr4s23gmY?iv=v38vAJ3LlJAGZxbmWU4qAg=="
+    #   decrypted = crypto.decrypt_text(recipient_private_key, sender_public_key, encrypted)
+    #
+    # @param sender_public_key [String] 32-bytes hex-encoded public key of the message creator.
+    # @param recipient_private_key [String] 32-bytes hex-encoded public key of the recipient.
+    # @param encrypted_text [String] The text to be decrypted
+    #
+    # @return [String] Decrypted text.
+    #
+    def decrypt_text(recipient_private_key, sender_public_key, encrypted_text)
+      base64_encoded_text, iv = encrypted_text.split('?iv=')
+      cipher = OpenSSL::Cipher.new(CIPHER_ALGORITHM).decrypt
+      cipher.iv = Base64.decode64(iv)
+      cipher.key = compute_shared_key(recipient_private_key, sender_public_key)
+      plain_text = cipher.update(Base64.decode64(base64_encoded_text)) + cipher.final
+      plain_text.force_encoding('UTF-8')
+    end
+
     # Uses the private key to generate an event id and sign the event
     #
     # @api public
@@ -33,6 +99,35 @@ module Nostr
 
     private
 
+    # Finds a shared key between two keys
+    #
+    # @api private
+    #
+    # @param private_key [String] 32-bytes hex-encoded private key.
+    # @param public_key [String] 32-bytes hex-encoded public key.
+    #
+    # @return [String] A shared key used in the event's content encryption and decryption.
+    #
+    def compute_shared_key(private_key, public_key)
+      group = OpenSSL::PKey::EC::Group.new(CIPHER_CURVE)
+
+      private_key_bn   = OpenSSL::BN.new(private_key, BN_BASE)
+      public_key_bn    = OpenSSL::BN.new("02#{public_key}", BN_BASE)
+      public_key_point = OpenSSL::PKey::EC::Point.new(group, public_key_bn)
+
+      asn1 = OpenSSL::ASN1::Sequence(
+        [
+          OpenSSL::ASN1::Integer.new(1),
+          OpenSSL::ASN1::OctetString(private_key_bn.to_s(2)),
+          OpenSSL::ASN1::ObjectId(CIPHER_CURVE, 0, :EXPLICIT),
+          OpenSSL::ASN1::BitString(public_key_point.to_octet_string(:uncompressed), 1, :EXPLICIT)
+        ]
+      )
+
+      pkey = OpenSSL::PKey::EC.new(asn1.to_der)
+      pkey.dh_compute_key(public_key_point)
+    end
+
     # Generates a SHA256 hash of a +Nostr::Event+
     #
     # @api private