Sword Art Online: MD - Decryption Keys Drops and Farming Locations

Here's a list of farming locations where you can farm your Lvl 10, 20, 30, 40, 50, 60, 70, or 80 Decryption Keys while playing Sword Art Online: Memory Defrag. Note that all highlighted quests below are under Normal Mode (Story Mode).

Decryption Keys are used to unlock higher attributes, but only if your character has reached the required Lvl. You may also unlock the highest Lvl available using multiple keys to speed things up a little.
Sword Art Online: MD - Decryption Keys Drop and Farming Locations

Farming Locations For Decryption Keys

*Note that all highlighted quests below are under Normal Mode (Story Mode).
Lvl 10 Decryption Key
   - Quest 2-3 (x1), Quest 2-7 (x2)

Lvl 20 Decryption Key
   - Quest 3-9 (x2)

Lvl 30 Decryption Key
   - Quest 4-7 (x1), Quest 4-9 (x1), Quest 5-8 (x1)

Lvl 40 Decryption Key
   - Quest 6-5 (x1)

Lvl 50 Decryption Key
   - Quest 7-9 (x1)

Lvl 60 Decryption Key
   - Quest 9-5 (x2)

Lvl 10 to Lvl 60 Decryption Keys
   - Quest 13-8 (x1 each)
   - The chance drops are pretty high here!
   - Retry Trick!: If you didn't get the key you wanted after clearing the 1st wave of Ruin Kobold Sentinels, just hit "Main Menu" and select "Retry".
   - Note that the Lvl 60 Description Key can only be obtained from the 2nd wave of Ruin Kobold Sentinels.

Lvl 70 and Lvl 80 Decryption Keys

   - Note that all highlighted quests below are under Normal Mode (Story Mode).
   - Both keys can be obtained from Chapter Quest 12 onwards as after battle treasure chest reward. It's random so you might need to do multiple re-runs just to get these keys. Here are some stages that I would recommend:

SAO Quests

   - Quest 13-1 is easy to complete but if you have a nice party, go clear the Boss stage Quest 13-3.
   - Quest 13-8 if you want to farm other keys too (check info above).
   - or any other Chapter or Quest stages that you might prefer.

ALO Quests

   - Quest 16-2 can be completed for just around 5 seconds, plus you can farm ATK shards and crystals too!
   - or any other Chapter or Quest stages that you might prefer.

Other Sources for Decryption Keys:

Sunday Exclusive Col Daily Quest

Sunday Exclusive (Col) Daily Quest

   - Quest 1 (Beginner): Drops Lvl 10 to Lvl 30 Decryption Keys.
   - Quest 2 (Intermediate): Drops Lvl 20 to Lvl 40 Decryption Keys.
   - Quest 3 (Expert): Drops Lvl 50 or Lvl 60 Decryption Keys.
   - Quest 4 (EX): Drops Lvl 10 to Lvl 70 Decryption Keys. (Nice Drop Rate for Lvl 70 Keys!)

Event Rewards

   - Events that would let you earn Event Points will sometimes give you Decryption Keys once you have reached a certain milestone.
   - This is your best source for Lvl 80 Keys.

Mission Rewards

   - Login for a number of days to receive your 1-time rewards.

Special "Event Only" Decryption Keys

   - If you can't unlock the attributes of your character even though you have a key with the required Lvl on it, then the reason probably is because you have a character that needs special keys.
   - Example: Asuna [Captive Princess] requires Fairy Princess Keys to unlock her attributes. These keys can only be obtained from a specific limited event (Grand Quest of the Top-Ranked event).

Related Sword Art Online: MD Tips & Guides:
Sword Art Online: Memory Defrag - FAQs, Tips and Re-Roll Guide
Sword Art Online MD: Shards, Crystals and Holy Farming Locations