AWS CloudHSM
User Guide

exSymKey

The exSymKey command in the key_mgmt_util tool exports a plaintext copy of a symmetric key from the HSM and saves it in a file on disk. To export an encrypted (wrapped) copy of a key, use wrapKey. To import a plaintext key, like the ones that exSymKey exports, use imSymKey.

During the export process, exSymKey uses an AES key that you specify (the wrapping key) to wrap (encrypt) and then unwrap (decrypt) the key to be exported. However, the result of the export operation is a plaintext (unwrapped) key on disk.

Only the owner of a key, that is, the CU user who created the key, can export it. Users who share the key can use it in cryptographic operations, but they cannot export it.

The exSymKey operation copies the key material to a file that you specify, but it does not remove the key from the HSM, change its key attributes, or prevent you from using the key in cryptographic operations. You can export the same key multiple times.

exSymKey exports only symmetric keys. To export public keys, use exPubKey. To export private keys, use exportPrivateKey.

Before you run any key_mgmt_util command, you must start key_mgmt_util and login to the HSM as a crypto user (CU).

Syntax

exSymKey -h exSymKey -k <key-to-export> -w <wrapping-key> -out <key-file> [-m 4] [-wk <unwrapping-key-file> ]

Examples

These examples show how to use exSymKey to export symmetric keys that you own from your HSMs.

Example : Export a 3DES Symmetric Key

This command exports a Triple DES (3DES) symmetric key (key handle 7). It uses an existing AES key (key handle 6) in the HSM as the wrapping key. Then it writes the plaintext of the 3DES key to the 3DES.key file.

The output shows that key 7 (the 3DES key) was successfully wrapped and unwrapped, and then written to the 3DES.key file.

Warning

Although the output says that a "Wrapped Symmetric Key" was written to the output file, the output file contains a plaintext (unwrapped) key.

Command: exSymKey -k 7 -w 6 -out 3DES.key Cfm3WrapKey returned: 0x00 : HSM Return: SUCCESS Cfm3UnWrapHostKey returned: 0x00 : HSM Return: SUCCESS Wrapped Symmetric Key written to file "3DES.key"

Example : Exporting with Session-Only Wrapping Key

This example shows how to use a key that exists only in the session as the wrapping key. Because the key to be exported is wrapped, immediately unwrapped, and delivered as plaintext, there is no need to retain the wrapping key.

This series of commands exports an AES key with key handle 8 from the HSM. It uses an AES session key created especially for the purpose.

The first command uses genSymKey to create a 256-bit AES key. It uses the -sess parameter to create a key that exists only in the current session.

The output shows that the HSM creates key 262168.

Command: genSymKey -t 31 -s 32 -l AES-wrapping-key -sess Cfm3GenerateSymmetricKey returned: 0x00 : HSM Return: SUCCESS Symmetric Key Created. Key Handle: 262168 Cluster Error Status Node id 1 and err state 0x00000000 : HSM Return: SUCCESS

Next, the example verifies that key 8, the key to be exported, is a symmetric key that is extractable. It also verifies that the wrapping key, key 262168, is an AES key that exists only in the session. You can use the findKey command, but this example exports the attributes of both keys to files and then uses grep to find the relevant attribute values in the file.

These commands use getAttribute with an -a value of 512 (all) to get all attributes for keys 8 and 262168. For information about the key attributes, see the Key Attribute Reference.

getAttribute -o 8 -a 512 -out attributes/attr_8 getAttribute -o 262168 -a 512 -out attributes/attr_262168

These commands use grep to verify the attributes of the key to be exported (key 8) and the session-only wrapping key (key 262168).

// Verify that the key to be exported is a symmetric key. $ grep -A 1 "OBJ_ATTR_CLASS" attributes/attr_8 OBJ_ATTR_CLASS 0x04 // Verify that the key to be exported is extractable. $ grep -A 1 "OBJ_ATTR_KEY_TYPE" attributes/attr_8 OBJ_ATTR_EXTRACTABLE 0x00000001 // Verify that the wrapping key is an AES key $ grep -A 1 "OBJ_ATTR_KEY_TYPE" attributes/attr_262168 OBJ_ATTR_KEY_TYPE 0x1f // Verify that the wrapping key is a session key $ grep -A 1 "OBJ_ATTR_TOKEN" attributes/attr_262168 OBJ_ATTR_TOKEN 0x00 // Verify that the wrapping key can be used for wrapping $ grep -A 1 "OBJ_ATTR_WRAP" attributes/attr_262168 OBJ_ATTR_WRAP 0x00000001

Finally, we use an exSymKey command to export key 8 using the session key (key 262168) as the wrapping key.

When the session ends, key 262168 no longer exists.

Command: exSymKey -k 8 -w 262168 -out aes256_H8.key Cfm3WrapKey returned: 0x00 : HSM Return: SUCCESS Cfm3UnWrapHostKey returned: 0x00 : HSM Return: SUCCESS Wrapped Symmetric Key written to file "aes256_H8.key"

Example : Use an External Unwrapping Key

This example shows how to use an external unwrapping key to export a key from the HSM.

When you export a key from the HSM, you specify an AES key on the HSM to be the wrapping key. By default, that wrapping key is used to wrap and unwrap the key to be exported. However, you can use the -wk parameter to tell exSymKey to use an external key in a file on disk for unwrapping. When you do, the key specified by the -w parameter wraps the target key, and the key in the file specified by the -wk parameter unwraps the key.

Because the wrapping key must be an AES key, which is symmetric, the wrapping key in the HSM and unwrapping key on disk must be have the same key material. To do this, you must import the wrapping key to the HSM or export the wrapping key from the HSM before the export operation.

This example creates a key outside of the HSM and imports it into the HSM. It uses the internal copy of the key to wrap a symmetric key that is being exported, and the copy of key in the file to unwrap it.

The first command uses OpenSSL to generate a 256-bit AES key. It saves the key to the aes256-forImport.key file. The OpenSSL command does not return any output, but you can use several commands to confirm its success. This example uses the wc (wordcount) tool, which confirms that the file that contains 32 bytes of data.

$ openssl rand -out keys/aes256-forImport.key 32 $ wc keys/aes256-forImport.key 0 2 32 keys/aes256-forImport.key

This command uses the imSymKey command to import the AES key from the aes256-forImport.key file to the HSM. When the command completes, the key exists in the HSM with key handle 262167 and in the aes256-forImport.key file.

Command: imSymKey -f keys/aes256-forImport.key -t 31 -l aes256-imported -w 6 Cfm3WrapHostKey returned: 0x00 : HSM Return: SUCCESS Cfm3CreateUnwrapTemplate returned: 0x00 : HSM Return: SUCCESS Cfm3UnWrapKey returned: 0x00 : HSM Return: SUCCESS Symmetric Key Unwrapped. Key Handle: 262167 Cluster Error Status Node id 1 and err state 0x00000000 : HSM Return: SUCCESS Node id 0 and err state 0x00000000 : HSM Return: SUCCESS

This command uses the key in an export operation. The command uses exSymKey to export key 21, a 192-bit AES key. To wrap the key, it uses key 262167, which is the copy that was imported into the HSM. To unwrap the key, it uses the same key material in the aes256-forImport.key file. When the command completes, key 21 is exported to the aes192_h21.key file.

Command: exSymKey -k 21 -w 262167 -out aes192_H21.key -wk aes256-forImport.key Cfm3WrapKey returned: 0x00 : HSM Return: SUCCESS Wrapped Symmetric Key written to file "aes192_H21.key"

Parameters

-h

Displays help for the command.

Required: Yes

-k

Specifies the key handle of the key to export. This parameter is required. Enter the key handle of a symmetric key that you own. This parameter is required. To find key handles, use the findKey command.

To verify that a key can be exported, use the getAttribute command to get the value of the OBJ_ATTR_EXTRACTABLE attribute, which is represented by constant 354. Also, you can export only keys that you own. To find the owner of a key, use the getKeyInfo command.

Required: Yes

-w

Specifies the key handle of the wrapping key. This parameter is required. To find key handles, use the findKey command.

A wrapping key is a key in the HSM that is used to encrypt (wrap) and then decrypt (unwrap) the key to be exported. Only AES keys can be used as wrapping keys.

You can use any AES key (of any size) as a wrapping key. Because the wrapping key wraps, and then immediately unwraps, the target key, you can use as session-only AES key as a wrapping key. To determine whether a key can be used as a wrapping key, use getAttribute to get the value of the OBJ_ATTR_WRAP attribute, which is represented by the constant 262. To create a wrapping key, use genSymKey to create an AES key (type 31).

If you use the -wk parameter to specify an external unwrapping key, the -w wrapping key is used to wrap, but not to unwrap, the key during export.

Note

Key 4 represents an unsupported internal key. We recommend that you use an AES key that you create and manage as the wrapping key.

Required: Yes

-out

Specifies the path and name of the output file. When the command succeeds, this file contains the exported key in plaintext. If the file already exists, the command overwrites it without warning.

Required: Yes

-m

Specifies the wrapping mechanism. The only valid value is 4, which represents the NIST_AES_WRAP mechanism.

Required: No

Default: 4

-wk

Use the AES key in the specified file to unwrap the key that is being exported. Enter the path and name of a file that contains a plaintext AES key.

When you include this parameter. exSymKey uses the key in the HSM that is specified by the -w parameter to wrap the key that is being exported and it uses the key in the -wk file to unwrap it. The -w and -wk parameter values must resolve to the same plaintext key.

Required: No

Default: Use the wrapping key on the HSM to unwrap.

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