How to use the command 'dropbearkey' (with examples)

The ‘dropbearkey’ command is used to generate SSH keys in Dropbear format. SSH keys are a more secure way to authenticate with an SSH server compared to username/password login. Dropbear is a lightweight SSH server and client that is widely used in embedded systems.

Use case 1: Generate an SSH key of type ed25519 and write it to a key file

Code:

dropbearkey -t ed25519 -f path/to/key_file

Motivation: Generating an SSH key of type ed25519 provides improved security compared to other types of keys. This is because ed25519 keys are based on the elliptic curve cryptography algorithm, which offers better resistance against attacks such as brute force and quantum computing.

Explanation:

• -t ed25519: Specifies the type of the SSH key to be generated as ’ed25519’.
• -f path/to/key_file: Specifies the path where the generated SSH key should be saved.

Example output:

Private key saved to 'path/to/key_file'.
Public key saved to 'path/to/key_file.pub'.

Use case 2: Generate an SSH key of type ecdsa and write it to a key file

Code:

dropbearkey -t ecdsa -f path/to/key_file

Motivation: Generating an SSH key of type ecdsa is useful when compatibility with older systems that only support this type of key is required. Although ecdsa keys are not as secure as ed25519 keys, they are still widely supported and provide a good level of security.

Explanation:

• -t ecdsa: Specifies the type of the SSH key to be generated as ’ecdsa'.
• -f path/to/key_file: Specifies the path where the generated SSH key should be saved.

Example output:

Private key saved to 'path/to/key_file'.
Public key saved to 'path/to/key_file.pub'.

Use case 3: Generate an SSH key of type RSA with 4096-bit key size and write it to a key file

Code:

dropbearkey -t rsa -s 4096 -f path/to/key_file

Motivation: Generating an SSH key of type RSA with a longer key size such as 4096 bits provides more security against brute force attacks. RSA keys are widely supported, making them compatible with various SSH clients and servers.

Explanation:

• -t rsa: Specifies the type of the SSH key to be generated as ‘rsa’.
• -s 4096: Specifies the key size of the RSA key to be generated as 4096 bits.
• -f path/to/key_file: Specifies the path where the generated SSH key should be saved.

Example output:

Private key saved to 'path/to/key_file'.
Public key saved to 'path/to/key_file.pub'.

Use case 4: Print the private key fingerprint and public key in a key file

Code:

dropbearkey -y -f path/to/key_file

Motivation: Printing the private key fingerprint and public key is useful for verifying the integrity of the generated SSH key. The fingerprint can be used to compare against a published fingerprint to ensure that the key has not been tampered with.

Explanation:

• -y: Instructs ‘dropbearkey’ to print the private key fingerprint and public key.
• -f path/to/key_file: Specifies the path to the key file from which to extract the information.

Example output:

Private key fingerprint:
35:3e:9c:bb:8c:6d:46:1c:14:5e:60:ef:43:fc:b8:8e

Public key:
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIL273d88t+D2GgSP2Ub0/eDOI2Wn2o5cnkwD0jPMkI8J user@example.com

Conclusion:

The ‘dropbearkey’ command provides a convenient way to generate SSH keys in Dropbear format. By using different combinations of options, users can choose the key type, key size, and output file location based on their specific requirements. Generating SSH keys is an important step in securing SSH connections and the ‘dropbearkey’ command makes this process straightforward.