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AWScloud~5 mins

Key pairs for SSH access in AWS - Time & Space Complexity

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Time Complexity: Key pairs for SSH access
O(n)
Understanding Time Complexity

When creating key pairs for SSH access in AWS, it is important to understand how the time to create and manage these keys grows as you add more keys.

We want to know how the number of operations changes when we create many key pairs.

Scenario Under Consideration

Analyze the time complexity of the following operation sequence.


// Create multiple key pairs for SSH access
for (let i = 0; i < n; i++) {
  aws ec2 create-key-pair --key-name "my-key-" + i
}

This sequence creates n separate SSH key pairs in AWS, each with a unique name.

Identify Repeating Operations
  • Primary operation: The create-key-pair API call to AWS EC2.
  • How many times: This call is made once for each key pair, so n times.
How Execution Grows With Input

Each new key pair requires one API call, so the total number of calls grows directly with the number of keys.

Input Size (n)Approx. API Calls/Operations
1010
100100
10001000

Pattern observation: The number of API calls increases one-for-one with the number of key pairs created.

Final Time Complexity

Time Complexity: O(n)

This means the time to create key pairs grows linearly as you add more keys.

Common Mistake

[X] Wrong: "Creating multiple key pairs can be done with a single API call, so time stays the same no matter how many keys."

[OK] Correct: Each key pair requires its own API call; AWS does not support batch creation of key pairs in one request.

Interview Connect

Understanding how operations scale with input size helps you design efficient cloud workflows and shows you can think about resource management clearly.

Self-Check

"What if AWS allowed batch creation of key pairs in one API call? How would the time complexity change?"

Practice

(1/5)
1. What is the main purpose of a key pair in AWS for SSH access?
easy
A. To store server data securely
B. To securely connect to a server without using a password
C. To create a backup of the server
D. To monitor server performance

Solution

  1. Step 1: Understand SSH access

    SSH uses keys to allow secure login without passwords.

  2. Step 2: Role of key pairs in AWS

    A key pair provides a private key for the user and a public key for the server to verify identity.

  3. Final Answer:

    To securely connect to a server without using a password -> Option B

  4. Quick Check:

    Key pairs enable passwordless secure login [OK]
Hint: Key pairs replace passwords for secure server login [OK]
Common Mistakes:
  • Thinking key pairs store server data
  • Confusing key pairs with backups
  • Assuming key pairs monitor performance
2. Which AWS CLI command correctly creates a new key pair named MyKey and saves the private key to a file?
easy
A. aws ec2 create-key-pair --key-name MyKey --query 'KeyMaterial' --output text > MyKey.pem
B. aws ec2 create-key-pair MyKey > MyKey.pem
C. aws ec2 generate-key-pair --name MyKey > MyKey.pem
D. aws ec2 new-key --key-name MyKey > MyKey.pem

Solution

  1. Step 1: Identify correct AWS CLI syntax

    The correct command uses create-key-pair with --key-name and outputs the private key material.

  2. Step 2: Confirm output redirection

    The private key is saved by redirecting the output to a file with > MyKey.pem.

  3. Final Answer:

    aws ec2 create-key-pair --key-name MyKey --query 'KeyMaterial' --output text > MyKey.pem -> Option A

  4. Quick Check:

    Correct AWS CLI syntax for key pair creation [OK]
Hint: Use create-key-pair with --query 'KeyMaterial' to save private key [OK]
Common Mistakes:
  • Using wrong command like generate-key-pair
  • Omitting --query to extract key material
  • Not redirecting output to save private key
3. You launched an EC2 instance with key pair MyKey. Which command will you use to connect to it if the instance's public IP is 54.12.34.56 and your private key file is MyKey.pem?
medium
A. ssh ec2-user@54.12.34.56 -i MyKey.pem
B. ssh -key MyKey.pem ec2-user@54.12.34.56
C. ssh -p MyKey.pem ec2-user@54.12.34.56
D. ssh -i MyKey.pem ec2-user@54.12.34.56

Solution

  1. Step 1: Understand SSH command syntax for key usage

    The -i option specifies the private key file for authentication.

  2. Step 2: Confirm correct order of arguments

    The correct syntax is ssh -i private_key user@host. ssh -i MyKey.pem ec2-user@54.12.34.56 matches this exactly.

  3. Final Answer:

    ssh -i MyKey.pem ec2-user@54.12.34.56 -> Option D

  4. Quick Check:

    SSH uses -i to specify private key file [OK]
Hint: Use ssh -i private_key user@ip to connect [OK]
Common Mistakes:
  • Using -key or -p instead of -i
  • Placing -i after user@host
  • Omitting the private key option
4. You tried to connect to your EC2 instance using SSH but got a permission denied error. Which of these is the most likely cause?
medium
A. The private key file has incorrect permissions (too open)
B. The instance is stopped
C. The key pair was deleted from AWS
D. The instance has no public IP

Solution

  1. Step 1: Check SSH private key file permissions

    SSH requires private key files to have strict permissions (e.g., 400). Too open permissions cause denial.

  2. Step 2: Understand other options

    While stopped instances or no public IP prevent connection, the error message differs. Deleted key pairs do not affect existing instances.

  3. Final Answer:

    The private key file has incorrect permissions (too open) -> Option A

  4. Quick Check:

    Private key file permissions cause SSH denial [OK]
Hint: Set private key file permission to 400 or stricter [OK]
Common Mistakes:
  • Ignoring file permission errors
  • Assuming instance state causes permission denied
  • Confusing deleted key pairs with connection errors
5. You lost your private key file for an EC2 instance launched with key pair OldKey. What is the best way to regain SSH access without stopping the instance?
hard
A. Use the AWS console to download the lost private key again
B. Delete the instance and launch a new one with a new key pair
C. Create a new key pair, then update the instance's authorized keys by connecting through Systems Manager or another user
D. Generate a new private key file with the same key name in AWS

Solution

  1. Step 1: Understand private key loss impact

    Private keys cannot be recovered or downloaded again from AWS once lost.

  2. Step 2: Regain access without stopping instance

    Use AWS Systems Manager or another user with access to add a new public key from a new key pair to the instance's authorized keys.

  3. Final Answer:

    Create a new key pair, then update the instance's authorized keys by connecting through Systems Manager or another user -> Option C

  4. Quick Check:

    Lost private key requires new key and authorized keys update [OK]
Hint: Use Systems Manager to add new key without stopping instance [OK]
Common Mistakes:
  • Trying to download lost private key again
  • Assuming new key pair with same name works
  • Deleting instance unnecessarily