🔒 Add SASL SCRAM-SHA-* mechanisms

Loosely based on the implementation by @singpolyma at
nevans/net-sasl#5

Co-authored-by: Stephen Paul Weber <singpolyma@singpolyma.net>

lib/net/imap.rb

@@ -1007,6 +1007,17 @@ def starttls(options = {}, verify = true)
     #
     #     Login using clear-text username and password.
     #
+    # +SCRAM-SHA-1+::
+    # +SCRAM-SHA-256+::
+    #     See ScramAuthenticator[rdoc-ref:Net::IMAP::SASL::ScramAuthenticator].
+    #
+    #     Login by username and password.  The password is not sent to the
+    #     server but is used in a salted challenge/response exchange.
+    #     +SCRAM-SHA-1+ and +SCRAM-SHA-256+ are directly supported by
+    #     Net::IMAP::SASL.  New authenticators can easily be added for any other
+    #     <tt>SCRAM-*</tt> mechanism if the digest algorithm is supported by
+    #     OpenSSL::Digest.
+    #
     # +XOAUTH2+::
     #     See XOAuth2Authenticator[rdoc-ref:Net::IMAP::SASL::XOAuth2Authenticator].
     #

lib/net/imap/sasl.rb

@@ -32,6 +32,17 @@ class IMAP
     #
     #     Login using clear-text username and password.
     #
+    # +SCRAM-SHA-1+::
+    # +SCRAM-SHA-256+::
+    #     See ScramAuthenticator.
+    #
+    #     Login by username and password.  The password is not sent to the
+    #     server but is used in a salted challenge/response exchange.
+    #     +SCRAM-SHA-1+ and +SCRAM-SHA-256+ are directly supported by
+    #     Net::IMAP::SASL.  New authenticators can easily be added for any other
+    #     <tt>SCRAM-*</tt> mechanism if the digest algorithm is supported by
+    #     OpenSSL::Digest.
+    #
     # +XOAUTH2+::
     #     See XOAuth2Authenticator.
     #
@@ -69,8 +80,13 @@ module SASL
 
       sasl_dir = File.expand_path("sasl", __dir__)
       autoload :Authenticators,           "#{sasl_dir}/authenticators"
+      autoload :GS2Header,                "#{sasl_dir}/gs2_header"
+      autoload :ScramAlgorithm,           "#{sasl_dir}/scram_algorithm"
+      autoload :ScramAuthenticator,       "#{sasl_dir}/scram_authenticator"
 
       autoload :PlainAuthenticator,       "#{sasl_dir}/plain_authenticator"
+      autoload :ScramSHA1Authenticator,   "#{sasl_dir}/scram_sha1_authenticator"
+      autoload :ScramSHA256Authenticator, "#{sasl_dir}/scram_sha256_authenticator"
       autoload :XOAuth2Authenticator,     "#{sasl_dir}/xoauth2_authenticator"
 
       autoload :CramMD5Authenticator,     "#{sasl_dir}/cram_md5_authenticator"

lib/net/imap/sasl/authenticators.rb

@@ -34,6 +34,8 @@ def initialize(use_defaults: false)
       @authenticators = {}
       if use_defaults
         add_authenticator "Plain"
+        add_authenticator "Scram-SHA-1"
+        add_authenticator "Scram-SHA-256"
         add_authenticator "XOAuth2"
         add_authenticator "Login"      # deprecated
         add_authenticator "Cram-MD5"   # deprecated

lib/net/imap/sasl/gs2_header.rb

@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+module Net
+  class IMAP < Protocol
+    module SASL
+
+      # Originally defined for the GS2 mechanism family in
+      # RFC5801[https://tools.ietf.org/html/rfc5801],
+      # several different mechanisms start with a GS2 header:
+      # * +GS2-*+       --- RFC5801[https://tools.ietf.org/html/rfc5801]
+      # * +SCRAM-*+     --- RFC5802[https://tools.ietf.org/html/rfc5802],
+      #   see ScramAuthenticator.
+      # * +SAML20+      --- RFC6595[https://tools.ietf.org/html/rfc6595]
+      # * +OPENID20+    --- RFC6616[https://tools.ietf.org/html/rfc6616]
+      # * +OAUTH10A+    --- RFC7628[https://tools.ietf.org/html/rfc7628]
+      # * +OAUTHBEARER+ --- RFC7628[https://tools.ietf.org/html/rfc7628]
+      #
+      # Classes that include this module must implement +#authzid+.
+      module GS2Header
+        NO_NULL_CHARS = /\A[^\x00]+\z/u.freeze # :nodoc:
+
+        ##
+        # Matches {RFC5801 §4}[https://www.rfc-editor.org/rfc/rfc5801#section-4]
+        # +saslname+.  The output from gs2_saslname_encode matches this Regexp.
+        RFC5801_SASLNAME = /\A(?:[^,=\x00]|=2C|=3D)+\z/u.freeze
+
+        # The {RFC5801 §4}[https://www.rfc-editor.org/rfc/rfc5801#section-4]
+        # +gs2-header+, which prefixes the #initial_client_response.
+        #
+        # >>>
+        #   <em>Note: the actual GS2 header includes an optional flag to
+        #   indicate that the GSS mechanism is not "standard", but since all of
+        #   the SASL mechanisms using GS2 are "standard", we don't include that
+        #   flag.  A class for a nonstandard GSSAPI mechanism should prefix with
+        #   "+F,+".</em>
+        def gs2_header
+          "#{gs2_cb_flag},#{gs2_authzid},"
+        end
+
+        # The {RFC5801 §4}[https://www.rfc-editor.org/rfc/rfc5801#section-4]
+        # +gs2-cb-flag+:
+        #
+        # "+n+":: The client doesn't support channel binding.
+        # "+y+":: The client does support channel binding
+        #         but thinks the server does not.
+        # "+p+":: The client requires channel binding.
+        #         The selected channel binding follows "+p=+".
+        #
+        # The default always returns "+n+".  A mechanism that supports channel
+        # binding must override this method.
+        #
+        def gs2_cb_flag; "n" end
+
+        # The {RFC5801 §4}[https://www.rfc-editor.org/rfc/rfc5801#section-4]
+        # +gs2-authzid+ header, when +#authzid+ is not empty.
+        #
+        # If +#authzid+ is empty or +nil+, an empty string is returned.
+        def gs2_authzid
+          return "" if authzid.nil? || authzid == ""
+          "a=#{gs2_saslname_encode(authzid)}"
+        end
+
+        module_function
+
+        # Encodes +str+ to match RFC5801_SASLNAME.
+        def gs2_saslname_encode(str)
+          str = str.encode("UTF-8")
+          # Regexp#match raises "invalid byte sequence" for invalid UTF-8
+          NO_NULL_CHARS.match str or
+            raise ArgumentError, "invalid saslname: %p" % [str]
+          str
+            .gsub(?=, "=3D")
+            .gsub(?,, "=2C")
+        end
+
+      end
+    end
+  end
+end

lib/net/imap/sasl/scram_algorithm.rb

@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+module Net
+  class IMAP
+    module SASL
+
+      # For method descriptions,
+      # see {RFC5802 §2.2}[https://www.rfc-editor.org/rfc/rfc5802#section-2.2]
+      # and {RFC5802 §3}[https://www.rfc-editor.org/rfc/rfc5802#section-3].
+      module ScramAlgorithm
+        def Normalize(str) SASL.saslprep(str) end
+
+        def Hi(str, salt, iterations)
+          length = digest.digest_length
+          OpenSSL::KDF.pbkdf2_hmac(
+            str,
+            salt:       salt,
+            iterations: iterations,
+            length: length,
+            hash: digest,
+          )
+        end
+
+        def H(str) digest.digest str end
+
+        def HMAC(key, data) OpenSSL::HMAC.digest(digest, key, data) end
+
+        def XOR(str1, str2)
+          str1.unpack("C*")
+            .zip(str2.unpack("C*"))
+            .map {|a, b| a ^ b }
+            .pack("C*")
+        end
+
+        def auth_message
+          [
+            client_first_message_bare,
+            server_first_message,
+            client_final_message_without_proof,
+          ]
+            .join(",")
+        end
+
+        def salted_password
+          Hi(Normalize(password), salt, iterations)
+        end
+
+        def client_key;       HMAC(salted_password, "Client Key") end
+        def server_key;       HMAC(salted_password, "Server Key") end
+        def stored_key;       H(client_key)                       end
+        def client_signature; HMAC(stored_key, auth_message)      end
+        def server_signature; HMAC(server_key, auth_message)      end
+        def client_proof;     XOR(client_key, client_signature)   end
+      end
+
+    end
+  end
+end