class Net::IMAP::SASL::ScramAuthenticator

Abstract base class for the “SCRAM-*” family of SASL mechanisms, defined in RFC5802. Use via Net::IMAP#authenticate.

Directly supported:

SCRAM-SHA-1 — ScramSHA1Authenticator
SCRAM-SHA-256 — ScramSHA256Authenticator

New SCRAM-* mechanisms can easily be added for any hash algorithm supported by OpenSSL::Digest. Subclasses need only set an appropriate DIGEST_NAME constant.

SCRAM algorithm¶ ↑

See the documentation and method definitions on ScramAlgorithm for an overview of the algorithm. The different mechanisms differ only by which hash function that is used (or by support for channel binding with -PLUS).

Server messages¶ ↑

As server messages are received, they are validated and loaded into the various attributes, e.g: snonce, salt, iterations, verifier, server_error, etc.

Unlike many other SASL mechanisms, the SCRAM-* family supports mutual authentication and can return server error data in the server messages. If process raises an Error for the server-final-message, then server_error may contain error details.

TLS Channel binding¶ ↑

The SCRAM-*-PLUS mechanisms and channel binding are not supported yet.

Caching SCRAM secrets¶ ↑

Caching of salted_password, client_key, stored_key, and server_key is not supported yet.

Attributes

authcid[R]

Authentication identity: the identity that matches the password.

RFC-2831 uses the term username. “Authentication identity” is the generic term used by RFC-4422. RFC-4616 and many later RFCs abbreviate this to authcid.

authzid[R]

Authorization identity: an identity to act as or on behalf of. The identity form is application protocol specific. If not provided or left blank, the server derives an authorization identity from the authentication identity. For example, an administrator or superuser might take on another role:

imap.authenticate "SCRAM-SHA-256", "root", passwd, authzid: "user"

The server is responsible for verifying the client’s credentials and verifying that the identity it associates with the client’s authentication identity is allowed to act as (or on behalf of) the authorization identity.

cnonce[R]

The client nonce, generated by SecureRandom

iterations[R]

The iteration count for the selected hash function and user

min_iterations[R]

The minimal allowed iteration count. Lower iterations will raise an Error.

password[R]

A password or passphrase that matches the username.

salt[R]

The salt used by the server for this user

secret[R]

A password or passphrase that matches the username.

server_error[R]

An error reported by the server during the SASL exchange.

Does not include errors reported by the protocol, e.g. Net::IMAP::NoResponseError.

server_first_message[R]

Need to store this for auth_message

snonce[R]

The server nonce, which must start with cnonce

username[R]

Authentication identity: the identity that matches the password.

RFC-2831 uses the term username. “Authentication identity” is the generic term used by RFC-4422. RFC-4616 and many later RFCs abbreviate this to authcid.

Public Class Methods

new(username, password, **options) → auth_ctx click to toggle source

new(username:, password:, **options) → auth_ctx

new(authcid:, password:, **options) → auth_ctx

Creates an authenticator for one of the “SCRAM-*” SASL mechanisms. Each subclass defines digest to match a specific mechanism.

Called by Net::IMAP#authenticate and similar methods on other clients.

Parameters¶ ↑

authcid ― Identity whose password is used.

username - An alias for authcid.
password ― Password or passphrase associated with this username.
optional authzid ― Alternate identity to act as or on behalf of.
optional min_iterations - Overrides the default value (4096).

Any other keyword parameters are quietly ignored.

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 80
def initialize(username_arg = nil, password_arg = nil,
               authcid: nil, username: nil,
               authzid: nil,
               password: nil, secret: nil,
               min_iterations: 4096, # see both RFC5802 and RFC7677
               cnonce: nil, # must only be set in tests
               **options)
  @username = username || username_arg || authcid or
    raise ArgumentError, "missing username (authcid)"
  @password = password || secret || password_arg or
    raise ArgumentError, "missing password"
  @authzid = authzid

  @min_iterations = Integer min_iterations
  @min_iterations.positive? or
    raise ArgumentError, "min_iterations must be positive"

  @cnonce = cnonce || SecureRandom.base64(32)
end

Public Instance Methods

digest() click to toggle source

Returns a new OpenSSL::Digest object, set to the appropriate hash function for the chosen mechanism.

The class’s DIGEST_NAME constant must be set to the name of an algorithm supported by OpenSSL::Digest.

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 155
def digest; OpenSSL::Digest.new self.class::DIGEST_NAME end

done?() click to toggle source

Is the authentication exchange complete?

If false, another server continuation is required.

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 185
def done?; @state == :done end

initial_client_response() click to toggle source

See RFC5802 §7 client-first-message.

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 159
def initial_client_response
  "#{gs2_header}#{client_first_message_bare}"
end

process(challenge) click to toggle source

responds to the server’s challenges

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 164
def process(challenge)
  case (@state ||= :initial_client_response)
  when :initial_client_response
    initial_client_response.tap { @state = :server_first_message }
  when :server_first_message
    recv_server_first_message challenge
    final_message_with_proof.tap { @state = :server_final_message }
  when :server_final_message
    recv_server_final_message challenge
    "".tap { @state = :done }
  else
    raise Error, "server sent after complete, %p" % [challenge]
  end
rescue Exception => ex
  @state = ex
  raise
end

Private Instance Methods

client_final_message_without_proof() click to toggle source

See RFC5802 §7 client-final-message-without-proof.

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 240
def client_final_message_without_proof
  @client_final_message_without_proof ||=
    format_message(c: [cbind_input].pack("m0"), # channel-binding
                   r: snonce)                   # nonce
end

client_first_message_bare() click to toggle source

See RFC5802 §7 client-first-message-bare.

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 225
def client_first_message_bare
  @client_first_message_bare ||=
    format_message(n: gs2_saslname_encode(SASL.saslprep(username)),
                   r: cnonce)
end

final_message_with_proof() click to toggle source

See RFC5802 §7 client-final-message.

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 233
def final_message_with_proof
  proof = [client_proof].pack("m0")
  "#{client_final_message_without_proof},p=#{proof}"
end

format_message(hash) click to toggle source

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 192
def format_message(hash) hash.map { _1.join("=") }.join(",") end

parse_challenge(challenge) click to toggle source

RFC5802 specifies “that the order of attributes in client or server messages is fixed, with the exception of extension attributes”, but this parses it simply as a hash, without respect to order. Note that repeated keys (violating the spec) will use the last value.

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 257
def parse_challenge(challenge)
  challenge.split(/,/).to_h {|pair| pair.split(/=/, 2) }
rescue ArgumentError
  raise Error, "unparsable challenge: %p" % [challenge]
end

recv_server_final_message(server_final_message) click to toggle source

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 211
def recv_server_final_message(server_final_message)
  sparams = parse_challenge server_final_message
  @server_error = sparams["e"] and
    raise Error, "server error: %s" % [server_error]
  verifier = sparams["v"].unpack1("m") or
    raise Error, "server did not send verifier"
  verifier == server_signature or
    raise Error, "server verify failed: %p != %p" % [
      server_signature, verifier
    ]
end

recv_server_first_message(server_first_message) click to toggle source

# File net-imap-0.4.4/lib/net/imap/sasl/scram_authenticator.rb, line 194
def recv_server_first_message(server_first_message)
  @server_first_message = server_first_message
  sparams = parse_challenge server_first_message
  @snonce = sparams["r"] or
    raise Error, "server did not send nonce"
  @salt = sparams["s"]&.unpack1("m") or
    raise Error, "server did not send salt"
  @iterations = sparams["i"]&.then {|i| Integer i } or
    raise Error, "server did not send iteration count"
  min_iterations <= iterations or
    raise Error, "too few iterations: %d" % [iterations]
  mext = sparams["m"] and
    raise Error, "mandatory extension: %p" % [mext]
  snonce.start_with? cnonce or
    raise Error, "invalid server nonce"
end