Make Active Storage blob keys lowercase

Accommodate case-insensitive filesystems and database collations.

activestorage/CHANGELOG.md

+*   Use base36 (all lowercase) for all new Blob keys to prevent
+    collisions and undefined behavior with case-insensitive filesystems and
+    database indices.
+
+    *Julik Tarkhanov*
+
 *   It doesn’t include an `X-CSRF-Token` header if a meta tag is not found on
     the page. It previously included one with a value of `undefined`.
 

activestorage/app/models/active_storage/blob.rb

@@ -79,6 +79,15 @@ def create_after_upload!(io:, filename:, content_type: nil, metadata: nil, ident
     def create_before_direct_upload!(filename:, byte_size:, checksum:, content_type: nil, metadata: nil)
       create! filename: filename, byte_size: byte_size, checksum: checksum, content_type: content_type, metadata: metadata
     end
+
+    # To prevent problems with case-insensitive filesystems, especially in combination
+    # with databases which treat indices as case-sensitive, all blob keys generated are going
+    # to only contain the base-36 character alphabet and will therefore be lowercase. To maintain
+    # the same or higher amount of entropy as in the base-58 encoding used by `has_secure_token`
+    # the number of bytes used is increased to 28 from the standard 24
+    def generate_unique_secure_token
+      SecureRandom.base36(28)
+    end
   end
 
   # Returns a signed ID for this blob that's suitable for reference on the client-side without fear of tampering.
@@ -87,9 +96,10 @@ def signed_id
     ActiveStorage.verifier.generate(id, purpose: :blob_id)
   end
 
-  # Returns the key pointing to the file on the service that's associated with this blob. The key is in the
-  # standard secure-token format from Rails. So it'll look like: XTAPjJCJiuDrLk3TmwyJGpUo. This key is not intended
-  # to be revealed directly to the user. Always refer to blobs using the signed_id or a verified form of the key.
+  # Returns the key pointing to the file on the service that's associated with this blob. The key is the
+  # secure-token format from Rails in lower case. So it'll look like: xtapjjcjiudrlk3tmwyjgpuobabd.
+  # This key is not intended to be revealed directly to the user.
+  # Always refer to blobs using the signed_id or a verified form of the key.
   def key
     # We can't wait until the record is first saved to have a key for it
     self[:key] ||= self.class.generate_unique_secure_token

activestorage/test/models/blob_test.rb

@@ -47,6 +47,10 @@ class ActiveStorage::BlobTest < ActiveSupport::TestCase
     assert_equal "text/csv", blob.content_type
   end
 
+  test "create after upload generates a 28-character base36 key" do
+    assert_match(/^[a-z0-9]{28}$/, create_blob.key)
+  end
+
   test "image?" do
     blob = create_file_blob filename: "racecar.jpg"
     assert_predicate blob, :image?

activestorage/test/models/variant_test.rb

@@ -150,7 +150,7 @@ class ActiveStorage::VariantTest < ActiveSupport::TestCase
   test "service_url doesn't grow in length despite long variant options" do
     blob = create_file_blob(filename: "racecar.jpg")
     variant = blob.variant(font: "a" * 10_000).processed
-    assert_operator variant.service_url.length, :<, 726
+    assert_operator variant.service_url.length, :<, 730
   end
 
   test "works for vips processor" do

activesupport/lib/active_support/core_ext/securerandom.rb

@@ -4,17 +4,18 @@
 
 module SecureRandom
   BASE58_ALPHABET = ("0".."9").to_a + ("A".."Z").to_a + ("a".."z").to_a - ["0", "O", "I", "l"]
+  BASE36_ALPHABET = ("0".."9").to_a + ("a".."z").to_a
+
   # SecureRandom.base58 generates a random base58 string.
   #
-  # The argument _n_ specifies the length, of the random string to be generated.
+  # The argument _n_ specifies the length of the random string to be generated.
   #
   # If _n_ is not specified or is +nil+, 16 is assumed. It may be larger in the future.
   #
-  # The result may contain alphanumeric characters except 0, O, I and l
+  # The result may contain alphanumeric characters except 0, O, I and l.
   #
   #   p SecureRandom.base58 # => "4kUgL2pdQMSCQtjE"
   #   p SecureRandom.base58(24) # => "77TMHrHJFvFDwodq8w7Ev2m7"
-  #
   def self.base58(n = 16)
     SecureRandom.random_bytes(n).unpack("C*").map do |byte|
       idx = byte % 64
@@ -22,4 +23,23 @@ def self.base58(n = 16)
       BASE58_ALPHABET[idx]
     end.join
   end
+
+  # SecureRandom.base36 generates a random base36 string in lowercase.
+  #
+  # The argument _n_ specifies the length of the random string to be generated.
+  #
+  # If _n_ is not specified or is +nil+, 16 is assumed. It may be larger in the future.
+  # This method can be used over +base58+ if a deterministic case key is necessary.
+  #
+  # The result will contain alphanumeric characters in lowercase.
+  #
+  #   p SecureRandom.base36 # => "4kugl2pdqmscqtje"
+  #   p SecureRandom.base36(24) # => "77tmhrhjfvfdwodq8w7ev2m7"
+  def self.base36(n = 16)
+    SecureRandom.random_bytes(n).unpack("C*").map do |byte|
+      idx = byte % 64
+      idx = SecureRandom.random_number(36) if idx >= 36
+      BASE36_ALPHABET[idx]
+    end.join
+  end
 end

activesupport/test/core_ext/secure_random_test.rb

@@ -19,4 +19,24 @@ def test_base58_with_length
     assert_not_equal s1, s2
     assert_equal 24, s1.length
   end
+
+  def test_base36_lowercase
+    s1 = SecureRandom.base36
+    s2 = SecureRandom.base36
+
+    assert_not_equal s1, s2
+    assert_equal 16, s1.length
+    assert_match(/^[a-z0-9]+$/, s1)
+    assert_match(/^[a-z0-9]+$/, s2)
+  end
+
+  def test_base36_with_length
+    s1 = SecureRandom.base36(24)
+    s2 = SecureRandom.base36(24)
+
+    assert_not_equal s1, s2
+    assert_equal 24, s1.length
+    assert_match(/^[a-z0-9]+$/, s1)
+    assert_match(/^[a-z0-9]+$/, s2)
+  end
 end