🔒 Add SASL SCRAM-SHA-* mechanisms

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

New authenticators for any digest algorithms supported can be added by
subclassing ScramAuthenticator and adding a DIGEST_NAME constant (and
then registering with an Authenticators registry).

Co-authored-by: Stephen Paul Weber <[email protected]>

Author: nevans

lib/net/imap.rb

@@ -1160,6 +1160,17 @@ def starttls(**options)
     #
     #     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

@@ -51,6 +51,17 @@ 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.
     #
@@ -89,11 +100,15 @@ 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 :AnonymousAuthenticator,   "#{sasl_dir}/anonymous_authenticator"
       autoload :ExternalAuthenticator,    "#{sasl_dir}/external_authenticator"
       autoload :OAuthBearerAuthenticator, "#{sasl_dir}/oauthbearer_authenticator"
       autoload :PlainAuthenticator,       "#{sasl_dir}/plain_authenticator"
+      autoload :ScramAuthenticator,       "#{sasl_dir}/scram_authenticator"
+      autoload :ScramSHA1Authenticator,   "#{sasl_dir}/scram_authenticator"
+      autoload :ScramSHA256Authenticator, "#{sasl_dir}/scram_authenticator"
       autoload :XOAuth2Authenticator,     "#{sasl_dir}/xoauth2_authenticator"
 
       autoload :CramMD5Authenticator,     "#{sasl_dir}/cram_md5_authenticator"

lib/net/imap/sasl/authenticators.rb

@@ -37,6 +37,8 @@ def initialize(use_defaults: false)
         add_authenticator "External"
         add_authenticator "OAuthBearer"
         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

@@ -9,6 +9,7 @@ module SASL
       # several different mechanisms start with a GS2 header:
       # * +GS2-*+       --- RFC5801[https://tools.ietf.org/html/rfc5801]
       # * +SCRAM-*+     --- RFC5802[https://tools.ietf.org/html/rfc5802]
+      #   (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]

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