diff --git a/lib/net/imap/sasl/digest_md5_authenticator.rb b/lib/net/imap/sasl/digest_md5_authenticator.rb
index 59080368..2bf08654 100644
--- a/lib/net/imap/sasl/digest_md5_authenticator.rb
+++ b/lib/net/imap/sasl/digest_md5_authenticator.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-# Net::IMAP authenticator for the "`DIGEST-MD5`" SASL mechanism type, specified
+# Net::IMAP authenticator for the +DIGEST-MD5+ SASL mechanism type, specified
 # in RFC-2831[https://tools.ietf.org/html/rfc2831].  See Net::IMAP#authenticate.
 #
 # == Deprecated
@@ -9,11 +9,32 @@
 # RFC-6331[https://tools.ietf.org/html/rfc6331] and should not be relied on for
 # security.  It is included for compatibility with existing servers.
 class Net::IMAP::SASL::DigestMD5Authenticator
+  DataFormatError    = Net::IMAP::DataFormatError
+  ResponseParseError = Net::IMAP::ResponseParseError
+  private_constant :DataFormatError, :ResponseParseError
+
   STAGE_ONE = :stage_one
   STAGE_TWO = :stage_two
   STAGE_DONE = :stage_done
   private_constant :STAGE_ONE, :STAGE_TWO, :STAGE_DONE
 
+  # Directives which must not have multiples.  The RFC states:
+  # >>>
+  #   This directive may appear at most once; if multiple instances are present,
+  #   the client should abort the authentication exchange.
+  NO_MULTIPLES = %w[nonce stale maxbuf charset algorithm].freeze
+
+  # Required directives which must occur exactly once.  The RFC states: >>>
+  #   This directive is required and MUST appear exactly once; if not present,
+  #   or if multiple instances are present, the client should abort the
+  #   authentication exchange.
+  REQUIRED = %w[nonce algorithm].freeze
+
+  # Directives which are composed of one or more comma delimited tokens
+  QUOTED_LISTABLE = %w[qop cipher].freeze
+
+  private_constant :NO_MULTIPLES, :REQUIRED, :QUOTED_LISTABLE
+
   # Authentication identity: the identity that matches the #password.
   #
   # RFC-2831[https://tools.ietf.org/html/rfc2831] uses the term +username+.
@@ -42,6 +63,59 @@ class Net::IMAP::SASL::DigestMD5Authenticator
   #
   attr_reader :authzid
 
+  # A namespace or collection of identities which contains +username+.
+  #
+  # Used by DIGEST-MD5, GSS-API, and NTLM.  This is often a domain name that
+  # contains the name of the host performing the authentication.
+  #
+  # <em>Defaults to the last realm in the server-provided list of
+  # realms.</em>
+  attr_reader :realm
+
+  # Fully qualified canonical DNS host name for the requested service.
+  #
+  # <em>Defaults to #realm.</em>
+  attr_reader :host
+
+  # The service protocol, a
+  # {registered GSSAPI service name}[https://www.iana.org/assignments/gssapi-service-names/gssapi-service-names.xhtml],
+  # e.g. "imap", "ldap", or "xmpp".
+  #
+  # For Net::IMAP, the default is "imap" and should not be overridden.  This
+  # must be set appropriately to use authenticators in other protocols.
+  #
+  # If an IANA-registered name isn't available, GSS-API
+  # (RFC-2743[https://tools.ietf.org/html/rfc2743]) allows the generic name
+  # "host".
+  attr_reader :service
+
+  # The generic server name when the server is replicated.
+  #
+  # +service_name+ will be ignored when it is +nil+ or identical to +host+.
+  #
+  # From RFC-2831[https://tools.ietf.org/html/rfc2831]:
+  # >>>
+  #     The service is considered to be replicated if the client's
+  #     service-location process involves resolution using standard DNS lookup
+  #     operations, and if these operations involve DNS records (such as SRV, or
+  #     MX) which resolve one DNS name into a set of other DNS names.  In this
+  #     case, the initial name used by the client is the "serv-name", and the
+  #     final name is the "host" component.
+  attr_reader :service_name
+
+  # Parameters sent by the server are stored in this hash.
+  attr_reader :sparams
+
+  # The charset sent by the server.  "UTF-8" (case insensitive) is the only
+  # allowed value.  +nil+ should be interpreted as ISO 8859-1.
+  attr_reader :charset
+
+  # nonce sent by the server
+  attr_reader :nonce
+
+  # qop-options sent by the server
+  attr_reader :qop
+
   # :call-seq:
   #   new(username,  password,  authzid = nil, **options) -> authenticator
   #   new(username:, password:, authzid:  nil, **options) -> authenticator
@@ -64,12 +138,23 @@ class Net::IMAP::SASL::DigestMD5Authenticator
   #   When +authzid+ is not set, the server should derive the authorization
   #   identity from the authentication identity.
   #
+  # * _optional_ #realm — A namespace for the #username, e.g. a domain.
+  #   <em>Defaults to the last realm in the server-provided realms list.</em>
+  # * _optional_ #host — FQDN for requested service.
+  #   <em>Defaults to</em> #realm.
+  # * _optional_ #service_name — The generic host name when the server is
+  #   replicated.
+  # * _optional_ #service — the registered service protocol.  E.g. "imap",
+  #   "smtp", "ldap", "xmpp".
+  #   <em>For Net::IMAP, this defaults to "imap".</em>
+  #
   # * _optional_ +warn_deprecation+ — Set to +false+ to silence the warning.
   #
   # Any other keyword arguments are silently ignored.
   def initialize(user = nil, pass = nil, authz = nil,
                  username: nil, password: nil, authzid: nil,
                  authcid: nil, secret: nil,
+                 realm: nil, service: "imap", host: nil, service_name: nil,
                  warn_deprecation: true, **)
     username = authcid || username || user or
       raise ArgumentError, "missing username (authcid)"
@@ -77,14 +162,34 @@ def initialize(user = nil, pass = nil, authz = nil,
     authzid  ||= authz
     if warn_deprecation
       warn "WARNING: DIGEST-MD5 SASL mechanism was deprecated by RFC6331."
-      # TODO: recommend SCRAM instead.
     end
+
     require "digest/md5"
+    require "securerandom"
     require "strscan"
     @username, @password, @authzid = username, password, authzid
+    @realm        = realm
+    @host         = host
+    @service      = service
+    @service_name = service_name
     @nc, @stage = {}, STAGE_ONE
   end
 
+  # From RFC-2831[https://tools.ietf.org/html/rfc2831]:
+  # >>>
+  #     Indicates the principal name of the service with which the client wishes
+  #     to connect, formed from the serv-type, host, and serv-name.  For
+  #     example, the FTP service on "ftp.example.com" would have a "digest-uri"
+  #     value of "ftp/ftp.example.com"; the SMTP server from the example above
+  #     would have a "digest-uri" value of "smtp/mail3.example.com/example.com".
+  def digest_uri
+    if service_name && service_name != host
+      "#{service}/#{host}/#{service_name}"
+    else
+      "#{service}/#{host}"
+    end
+  end
+
   def initial_response?; false end
 
   # Responds to server challenge in two stages.
@@ -92,65 +197,49 @@ def process(challenge)
     case @stage
     when STAGE_ONE
       @stage = STAGE_TWO
-      sparams = {}
-      c = StringScanner.new(challenge)
-      while c.scan(/(?:\s*,)?\s*(\w+)=("(?:[^\\"]|\\.)*"|[^,]+)\s*/)
-        k, v = c[1], c[2]
-        if v =~ /^"(.*)"$/
-          v = $1
-          if v =~ /,/
-            v = v.split(',')
-          end
-        end
-        sparams[k] = v
-      end
+      @sparams = parse_challenge(challenge)
+      @qop     = sparams.key?("qop") ? ["auth"] : sparams["qop"].flatten
+      @nonce   = sparams["nonce"]  &.first
+      @charset = sparams["charset"]&.first
+      @realm ||= sparams["realm"]  &.last
+      @host  ||= realm
 
-      raise Net::IMAP::DataFormatError, "Bad Challenge: '#{challenge}'" unless c.eos? and sparams['qop']
-      raise Net::IMAP::Error, "Server does not support auth (qop = #{sparams['qop'].join(',')})" unless sparams['qop'].include?("auth")
+      if !qop.include?("auth")
+        raise DataFormatError, "Server does not support auth (qop = %p)" % [
+          sparams["qop"]
+        ]
+      elsif (emptykey = REQUIRED.find { sparams[_1].empty? })
+        raise DataFormatError, "Server didn't send %s (%p)" % [emptykey, challenge]
+      elsif (multikey = NO_MULTIPLES.find { sparams[_1].length > 1 })
+        raise DataFormatError, "Server sent multiple %s (%p)" % [multikey, challenge]
+      end
 
       response = {
-        :nonce => sparams['nonce'],
-        :username => @username,
-        :realm => sparams['realm'],
-        :cnonce => Digest::MD5.hexdigest("%.15f:%.15f:%d" % [Time.now.to_f, rand, Process.pid.to_s]),
-        :'digest-uri' => 'imap/' + sparams['realm'],
-        :qop => 'auth',
-        :maxbuf => 65535,
-        :nc => "%08d" % nc(sparams['nonce']),
-        :charset => sparams['charset'],
+        nonce:        nonce,
+        username:     username,
+        realm:        realm,
+        cnonce:       SecureRandom.base64(32),
+        "digest-uri": digest_uri,
+        qop:          "auth",
+        maxbuf:       65535,
+        nc:           "%08d" % nc(nonce),
+        charset:      charset,
       }
 
       response[:authzid] = @authzid unless @authzid.nil?
 
-      # now, the real thing
-      a0 = Digest::MD5.digest( [ response.values_at(:username, :realm), @password ].join(':') )
-
-      a1 = [ a0, response.values_at(:nonce,:cnonce) ].join(':')
-      a1 << ':' + response[:authzid] unless response[:authzid].nil?
-
-      a2 = "AUTHENTICATE:" + response[:'digest-uri']
-      a2 << ":00000000000000000000000000000000" if response[:qop] and response[:qop] =~ /^auth-(?:conf|int)$/
-
-      response[:response] = Digest::MD5.hexdigest(
-        [
-          Digest::MD5.hexdigest(a1),
-          response.values_at(:nonce, :nc, :cnonce, :qop),
-          Digest::MD5.hexdigest(a2)
-        ].join(':')
-      )
-
-      return response.keys.map {|key| qdval(key.to_s, response[key]) }.join(',')
+      response[:response] = response_value(response)
+      format_response(response)
     when STAGE_TWO
       @stage = STAGE_DONE
-      # if at the second stage, return an empty string
-      if challenge =~ /rspauth=/
-        return ''
-      else
-        raise ResponseParseError, challenge
-      end
+      raise ResponseParseError, challenge unless challenge =~ /rspauth=/
+      "" # if at the second stage, return an empty string
     else
       raise ResponseParseError, challenge
     end
+  rescue => error
+    @stage = error
+    raise
   end
 
   def done?; @stage == STAGE_DONE end
@@ -163,7 +252,79 @@ def nc(nonce)
     else
       @nc[nonce] = 1
     end
-    return @nc[nonce]
+  end
+
+  def response_value(response)
+    a1 = compute_a1(response)
+    a2 = compute_a2(response)
+    Digest::MD5.hexdigest(
+      [
+        Digest::MD5.hexdigest(a1),
+        response.values_at(:nonce, :nc, :cnonce, :qop),
+        Digest::MD5.hexdigest(a2)
+      ].join(":")
+    )
+  end
+
+  def compute_a0(response)
+    Digest::MD5.digest(
+      [ response.values_at(:username, :realm), password ].join(":")
+    )
+  end
+
+  def compute_a1(response)
+    a0 = compute_a0(response)
+    a1 = [ a0, response.values_at(:nonce, :cnonce) ].join(":")
+    a1 << ":#{response[:authzid]}" unless response[:authzid].nil?
+    a1
+  end
+
+  def compute_a2(response)
+    a2 = "AUTHENTICATE:#{response[:"digest-uri"]}"
+    if response[:qop] and response[:qop] =~ /^auth-(?:conf|int)$/
+      a2 << ":00000000000000000000000000000000"
+    end
+    a2
+  end
+
+  LWS        = /[\r\n \t]*/n # less strict than RFC, more strict than '\s'
+  TOKEN      = /[^\x00-\x20\x7f()<>@,;:\\"\/\[\]?={}]+/n
+  QUOTED_STR = /"(?: [\t\x20-\x7e&&[^"]] | \\[\x00-\x7f] )*"/nx
+  LIST_DELIM = /(?:#{LWS} , )+ #{LWS}/nx
+  AUTH_PARAM = /
+    (#{TOKEN}) #{LWS} = #{LWS} (#{QUOTED_STR} | #{TOKEN}) #{LIST_DELIM}?
+  /nx
+  private_constant :LWS, :TOKEN, :QUOTED_STR, :LIST_DELIM, :AUTH_PARAM
+
+  def parse_challenge(challenge)
+    sparams = Hash.new {|h, k| h[k] = [] }
+    c = StringScanner.new(challenge)
+    c.skip LIST_DELIM
+    while c.scan AUTH_PARAM
+      k, v = c[1], c[2]
+      k = k.downcase
+      if v =~ /\A"(.*)"\z/mn
+        v = $1.gsub(/\\(.)/mn, '\1')
+        v = split_quoted_list(v, challenge) if QUOTED_LISTABLE.include? k
+      end
+      sparams[k] << v
+    end
+    if !c.eos?
+      raise DataFormatError, "Unparsable challenge: %p" % [challenge]
+    elsif sparams.empty?
+      raise DataFormatError, "Empty challenge: %p" % [challenge]
+    end
+    sparams
+  end
+
+  def split_quoted_list(value, challenge)
+    value.split(LIST_DELIM).reject(&:empty?).tap do
+      _1.any? or raise DataFormatError, "Bad Challenge: %p" % [challenge]
+    end
+  end
+
+  def format_response(response)
+    response.map {|k, v| qdval(k.to_s, v) }.join(",")
   end
 
   # some responses need quoting
diff --git a/test/net/imap/test_imap_authenticators.rb b/test/net/imap/test_imap_authenticators.rb
index e7faf36d..b2c92b4c 100644
--- a/test/net/imap/test_imap_authenticators.rb
+++ b/test/net/imap/test_imap_authenticators.rb
@@ -386,7 +386,7 @@ def test_digest_md5_authenticator_matches_mechanism
 
   def test_digest_md5_authenticator_deprecated
     assert_warn(/DIGEST-MD5.+deprecated.+RFC6331/) do
-      Net::IMAP::SASL.authenticator("DIGEST-MD5", "user", "pass")
+      Net::IMAP.authenticator("DIGEST-MD5", "user", "pass")
     end
   end
 
@@ -422,20 +422,89 @@ def test_digest_md5_authenticator
     )
   end
 
-  def test_digest_md5_authenticator_garbage
+  def test_digest_md5_authenticator_realm_and_digest_uri
+    auth = digest_md5(authcid: "authc",
+                      authzid: "authz",
+                      password: "pass",
+                      realm: "myrealm",
+                      service: "smtp",
+                      host: "mail.example.com",
+                      service_name: "example.com")
+    assert_match(
+      %r{\A
+        nonce="OA6MG9tEQGm2hh",
+        username="authc",
+        realm="myrealm",
+        cnonce="[a-zA-Z0-9+/]{12,}={0,3}",
+        digest-uri="smtp/mail\.example\.com/example\.com",
+        qop="auth",
+        maxbuf=65535,
+        nc=00000001,
+        charset=utf-8,
+        authzid="authz",
+        response=[a-f0-9]+
+      \Z}x,
+      auth.process(
+        %w[
+          realm="somerealm"
+          nonce="OA6MG9tEQGm2hh"
+          qop="auth"
+          charset=utf-8
+          algorithm=md5-sess
+        ].join(",")
+      )
+    )
+  end
+
+  def test_digest_md5_authenticator_empty_challenge
     auth = digest_md5("user", "pass")
     assert_raise(Net::IMAP::DataFormatError) do
+      auth.process("   ")
+    end
+  end
+
+  def test_digest_md5_authenticator_empty_challenge_commas
+    auth = digest_md5("user", "pass")
+    assert_raise_with_message(Net::IMAP::DataFormatError, /empty challenge/i) do
+      auth.process(" ,  ,  ")
+    end
+  end
+
+  def test_digest_md5_authenticator_garbage_no_equal_sign
+    auth = digest_md5("user", "pass")
+    assert_raise_with_message(Net::IMAP::DataFormatError, /unparsable/i) do
+      auth.process('nonce=required,algorithm=md5-sess,foo')
+    end
+  end
+
+  def test_digest_md5_authenticator_qdstr_with_comma
+    auth = digest_md5("user", "pass")
+    assert_raise_with_message(Net::IMAP::DataFormatError, /unparsable/i) do
+      auth.process('nonce=required,algorithm=md5-sess,.')
+    end
+  end
+
+  def test_digest_md5_authenticator_garbage
+    auth = digest_md5("user", "pass")
+    assert_raise_with_message(Net::IMAP::DataFormatError, /unparsable/i) do
       auth.process('.')
     end
   end
 
-  def test_digest_md5_authenticator_no_qop
+  def test_digest_md5_authenticator_empty_qop
     auth = digest_md5("user", "pass")
-    assert_raise(Net::IMAP::DataFormatError) do
+    assert_raise_with_message(Net::IMAP::DataFormatError, /bad challenge/i) do
       auth.process('Qop=""')
     end
   end
 
+  def test_digest_md5_authenticator_missing_nonce
+    auth = digest_md5("user", "pass")
+    assert_raise_with_message(Net::IMAP::DataFormatError, /didn't send nonce/i) do
+      auth.process('Qop="auth"')
+    end
+  end
+
   def test_digest_md5_authenticator_illinear
     pre = ->(n) {'qop="a' + ',x'*n}
     assert_linear_performance([5, 10, 15, 20], pre: pre) do |challenge|