How to Use the Command `keyctl` (with examples)

The keyctl command is a versatile tool used to manipulate the Linux kernel keyrings. This utility is critical for managing and manipulating encryption keys in scenarios where secure communication and operations rely heavily on these cryptographic elements. The keyctl command interfaces with the Linux kernel’s key management facility, allowing users to perform tasks such as listing keys in keyrings, adding new keys, setting timeouts, reading keys, and revoking access. Below are examples illustrating the diverse use cases of the keyctl command.

Use case 1: List keys in a specific keyring

Code:

keyctl list target_keyring

Motivation:

In environments where secure key management is paramount, being able to list the keys in a specific keyring helps administrators audit which keys are currently available and ensure that unauthorized keys are not present. Listing keys is an essential operation to verify the contents of a keyring and maintain security compliance.

Explanation:

• keyctl: The core command that allows you to manage keyrings.
• list: The action to be performed, which in this case, is listing the keys in the specified keyring.
• target_keyring: The specific keyring whose keys you want to list. This is often a keyring identifier or special keyring such as @u or @s.

Example Output:

Keyring
  258733440 --alswrv    0     0  user: my_key
  258733441 --alswrv    0     0   user: your_key

Use case 2: List current keys in the user default session

Code:

keyctl list @us

Motivation:

By listing the current keys in the user default session, you gain visibility into what keys are currently in play for that session. This is particularly useful for users troubleshooting application access issues where a key might have been unexpectedly removed or not loaded.

Explanation:

• keyctl: Execute the command managing the keyrings.
• list: Specifies the action to perform—listing keys.
• @us: This argument refers to the user’s default session keyring, making it specific to the current user’s session.

Example Output:

Keyring
  782346522 --alswrv   0  1180  key: session_key1
  782346523 --alswrv   0  1180  key: session_key2

Use case 3: Store a key in a specific keyring

Code:

keyctl add type_keyring key_name key_value target_keyring

Motivation:

Storing a key in a specific keyring is a fundamental operation when setting up secure systems that require keys for cryptographic functions such as encryption, decryption, or signing. This operation is key to ensuring that specific keyrings are populated with the necessary keys for operational tasks.

Explanation:

• keyctl: Execute the command managing the keyrings.
• add: The operation to perform; adding a new key.
• type_keyring: Denotes the type of keyring (e.g., “user”).
• key_name: The name identifier for the key being added.
• key_value: The actual content/value of the key.
• target_keyring: Specifies the keyring into which the key should be added.

Example Output:

Key added: 758992543

Use case 4: Store a key with its value from stdin

Code:

echo -n key_value | keyctl padd type_keyring key_name target_keyring

Motivation:

This is particularly useful when the key value is generated dynamically by a script or needs to be protected from exposure on the command line. By feeding the key value via stdin, you can prevent sensitive data from being exposed in shell histories or process lists.

Explanation:

• echo -n: Outputs the key value without a trailing newline.
• key_value: The raw key value being added.
• |: Pipes the output of the echo command to the keyctl command.
• keyctl: The command managing keyrings.
• padd: Short for “paste add,” indicates adding a key value read from standard input.
• type_keyring: Specifies the key ring type.
• key_name: Designates the name for the key.
• target_keyring: The destination keyring for storing the key.

Example Output:

Key added: 789543321

Use case 5: Put a timeout on a key

Code:

keyctl timeout key_name timeout_in_seconds

Motivation:

Placing a timeout on a key ensures that the key is automatically invalidated after a specified time, hence reducing the risk of unauthorized access if a key is inadvertently exposed. It’s particularly important in contexts where keys should not persist longer than a specific session.

Explanation:

• keyctl: Executes the key management command.
• timeout: Specifies the operation to set a timeout on a key.
• key_name: Identifies the key on which to set the timeout.
• timeout_in_seconds: Defines how long, in seconds, the key will remain valid.

Example Output:

Timeout set on key: 452671332 to 300 seconds

Use case 6: Read a key and format it as a hex-dump if not printable

Code:

keyctl read key_name

Motivation:

Reading a key in a hex-dump format is essential when dealing with non-printable or binary data. It allows you to verify or debug the contents of a key without requiring specific applications to interpret the data correctly.

Explanation:

• keyctl: Executes the key management command.
• read: Operation to read data from a key.
• key_name: Identifies the key whose value you wish to read.

Example Output:

f1 b2 c3 d4 ...

Use case 7: Read a key and format as-is

Code:

keyctl pipe key_name

Motivation:

By reading the key as-is, you can obtain the exact value stored in the keyring, which is useful when dealing with plain text keys in operations or scripting environments that interact directly with such cryptographic primitives.

Explanation:

• keyctl: Executes the key management command.
• pipe: Operation to read and output the key value directly as-is.
• key_name: Identifies the key whose value is output directly.

Example Output:

my_plaintext_key_value

Use case 8: Revoke a key and prevent any further action on it

Code:

keyctl revoke key_name

Motivation:

Revoking a key is a critical security function, ensuring that a compromised key cannot be used further. It effectively renders the key unusable, ensuring secure practices are maintained even when key exposure is suspected.

Explanation:

• keyctl: Executes the key management command.
• revoke: The operation to invalidate the key permanently.
• key_name: Identifies the key to be revoked.

Example Output:

Key revoked: 673291842

Conclusion

Using keyctl opens up a suite of operations for managing keyrings and keys in a Linux environment, from listing and adding keys to setting timeouts, reading, and revoking them. Each command and argument is integral to maintaining a secure system where cryptographic keys are pivotal. Understanding these use cases allows administrators and users to effectively manage their key security, protecting sensitive data and maintaining operational integrity.