This can be accomplished with two commands:

• The tag command can be used to serve as the "key".
• The testfor command can be used to determine if a nearby player has the expected "key".

📝 Required Items

• 1 Command Block
• 1 Redstone Comparator
• 1 Redstone Dust
• 1 Block (any kind that allows placement of redstone dust)
• 2 Buttons

🧰 Configuration

Since you have three spots under the door and one to the side, place 1 command block, 1 redstone comparator, 1 block that allows placement of redstone dust, and 1 sprinkle of redstone dust in the following configuration:

The command block should be given the following command and configuration:

# Command (replace number in the tag with the number of the door)
testfor @p[tag=door_number,r=3]

# Configuration
Repeat
Unconditional
Needs Redstone
50 Ticks

Giving a delay in ticks with a repeating configuration allows the door to automatically close once the player leaves the immediate area. You'll also need to use a button to trigger the command block from both sides of the door (allowing players to open the door):

🔑 Keys

You still need to give players the keys to their rooms, which you can do with the tag command:

tag player_name_here add door_number

_{Note: This answer does NOT meet the requirements of the question, given that a door cannot be placed on a redstone comparator. However, I'm leaving it due to the amount of effort I put into it. The underlying concept may still be useful to future readers. I should really avoid answering in the evenings from now on. 😂}

This can be accomplished with two commands:

• The tag command can be used to serve as the "key".
• The testfor command can be used to determine if a nearby player has the expected "key".

📝 Required Items

• 1 Command Block
• 1 Redstone Comparator
• 1 Redstone Dust
• 1 Block (any kind that allows placement of redstone dust)
• 2 Buttons

🧰 Configuration

Since you have three spots under the door and one to the side, place 1 command block, 1 redstone comparator, 1 block that allows placement of redstone dust, and 1 sprinkle of redstone dust in the following configuration:

The command block should be given the following command and configuration:

# Command (replace number in the tag with the number of the door)
testfor @p[tag=door_number,r=3]

# Configuration
Repeat
Unconditional
Needs Redstone
50 Ticks

Giving a delay in ticks with a repeating configuration allows the door to automatically close once the player leaves the immediate area. You'll also need to use a button to trigger the command block from both sides of the door (allowing players to open the door):

🔑 Keys

You still need to give players the keys to their rooms, which you can do with the tag command:

tag player_name_here add door_number

This can be accomplished with two commands:

• The tag command can be used to serve as the "key".
• The testfor command can be used to determine if a nearby player has the expected "key".

Required Items

• 1 Command Block
• 1 Redstone Comparator
• 1 Redstone Dust
• 1 Block (any kind that allows placement of redstone dust)
• 2 Buttons

Configuration

Since you have three spots under the door and one to the side, place 1 command block, 1 redstone comparator, 1 block that allows placement of redstone dust, and 1 sprinkle of redstone dust in the following configuration:

The command block should be given the following command and configuration:

# Command (replace number in the tag with the number of the door)
testfor @p[tag=door_number,r=3]

# Configuration
Repeat
Unconditional
Needs Redstone
50 Ticks

Giving a delay in ticks with a repeating configuration allows the door to automatically close once the player leaves the immediate area. You'll also need to use a button to trigger the command block from both sides of the door (allowing players to open the door):

Keys

You still need to give players the keys to their rooms, which you can do with the tag command:

tag player_name_here add door_number

This can be accomplished with two commands:

• The tag command can be used to serve as the "key".
• The testfor command can be used to determine if a nearby player has the expected "key".

📝 Required Items

• 1 Command Block
• 1 Redstone Comparator
• 1 Redstone Dust
• 1 Block (any kind that allows placement of redstone dust)
• 2 Buttons

🧰 Configuration

Since you have three spots under the door and one to the side, place 1 command block, 1 redstone comparator, 1 block that allows placement of redstone dust, and 1 sprinkle of redstone dust in the following configuration:

The command block should be given the following command and configuration:

# Command (replace number in the tag with the number of the door)
testfor @p[tag=door_number,r=3]

# Configuration
Repeat
Unconditional
Needs Redstone
50 Ticks

Giving a delay in ticks with a repeating configuration allows the door to automatically close once the player leaves the immediate area. You'll also need to use a button to trigger the command block from both sides of the door (allowing players to open the door):

🔑 Keys

You still need to give players the keys to their rooms, which you can do with the tag command:

tag player_name_here add door_number