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

Dropbearkey is a command-line tool used for generating SSH keys in Dropbear format. It’s commonly utilized in lightweight SSH servers and clients, particularly in resource-constrained environments such as embedded systems. The Dropbearkey command facilitates the creation of different types of SSH keys, supporting a variety of cryptographic algorithms to ensure secure communication over networks. Here’s how you can use dropbearkey for various purposes.

Use case 1: Generating an SSH Key of Type Ed25519

Code:

dropbearkey -t ed25519 -f path/to/key_file

Motivation:

Ed25519 is a public-key signature system that is known for its high performance and robust security. It is particularly desirable for embedding environments due to its smaller key size and faster computation. Generating an SSH key of this type is beneficial for users needing secure and efficient authentication in systems with limited resources.

Explanation:

• -t ed25519: This argument specifies the type of key to generate. Ed25519 is chosen for its balance of security and performance.
• -f path/to/key_file: This argument defines the file path where the generated SSH key will be stored. It’s important to choose a secure location to prevent unauthorized access.

Example Output:

Generating 256 bit ed25519 key...
Public key portion is:
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICIo1... user@hostname
Fingerprint: md5 bc:3c:f1:60:00:de:34:78:ab:cd:ef:12:34:56:78:90

Use case 2: Generating an SSH Key of Type ECDSA

Code:

dropbearkey -t ecdsa -f path/to/key_file

Motivation:

ECDSA (Elliptic Curve Digital Signature Algorithm) keys offer strong security with smaller key sizes compared to non-elliptic curve cryptography. This feature makes ECDSA a preferred choice in environments where memory and processing power are limited. It provides an efficient way to secure SSH sessions without compromising on security.

Explanation:

• -t ecdsa: This argument directs the tool to generate an ECDSA type key, leveraging elliptic curve cryptography for strong security.
• -f path/to/key_file: Specifies the destination file path for the newly created SSH key, key management and access restrictions should be considered for this file.

Example Output:

Generating 256 bit ECDSA key...
Public key portion is:
ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTY... user@hostname
Fingerprint: sha256:Lahb2vxwE1iBf9LE0xpQgA+xComprQ/a2Kuqc7OpwMc

Use case 3: Generating an SSH Key of Type RSA with 4096-bit Key Size

Code:

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

Motivation:

RSA is one of the most widely used encryption methods and remains a solid choice for SSH key generation due to its reliability and compatibility across systems. A 4096-bit key size offers a higher level of security, making it ideal for environments where maximum encryption strength is required. It is often used in situations where a long-term key with robust protection against evolving threats is needed.

Explanation:

• -t rsa: This argument specifies RSA as the type, a time-tested encryption method.
• -s 4096: This argument sets the size of the RSA key to 4096 bits, providing enhanced security over smaller key sizes.
• -f path/to/key_file: Determines where the RSA key will be stored, this file path should be protected against unauthorized access.

Example Output:

Generating 4096 bit RSA key...
Public key portion is:
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAACAQC7... user@hostname
Fingerprint: md5 9a:0d:21:4d:21:05:0b:32:1f:2c:8f:c8:78:9b:a4:05

Use case 4: Printing the Fingerprint and Public Key

Code:

dropbearkey -y -f path/to/key_file

Motivation:

Verifying the integrity of the SSH key is crucial for maintaining secure communications. Printing the private key fingerprint and public key allows users to ensure the key has not been tampered with and is correctly formatted. This step is important when distributing public keys or when verifying keys across multiple systems.

Explanation:

• -y: This argument is used to instruct dropbearkey to print the key in the file specified.
• -f path/to/key_file: Specifies which key file to process for extracting the fingerprint and public key.

Example Output:

Fingerprint: md5 47:2d:6f:3a:9c:5d:8d:cd:76:ee:30:ab:cd:78:90:12
Public key portion is:
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAGEA... user@hostname

Conclusion:

The dropbearkey command provides versatile options for generating SSH keys suitable for various security needs and system constraints. Whether selecting Ed25519 for its speed and small size, ECDSA for its efficient elliptic curve cryptography, or RSA for its proven durability and compatibility, users can produce precisely tailored keys. Additionally, being able to print and verify keys ensures continuous, reliable security practices.