Modem key management

The modem key management library provides functions to manage the credentials stored in the nRF9160 LTE modem. The library uses AT commands (Credential storage management %CMNG) to add, update, and delete credentials.

Each set of keys and certificates that is stored in the modem is identified by a security tag (sec_tag). You specify this tag when adding the credentials and use it when you update or delete them. All related credentials share the same security tag. You can use the library to check if a specific security tag exists.

To establish a connection, pass the security tag to the Modem library when creating a secure socket.

See Security tags for more information about how security tags are used in the Modem library.

Important

Security credentials usually exceed the default AT command response length. Therefore, you must set CONFIG_AT_CMD_RESPONSE_MAX_LEN to a sufficiently high value when using this library.

Certificates

You can download a certificate for a given server using your web browser. Alternatively, you can obtain it from a dedicated website like SSL Labs.

Certificates come in different formats. To provision the certificate to the nRF9160 DK, it must be in PEM format. The PEM format looks like this:

"-----BEGIN CERTIFICATE-----\n"
"MIIDujCCAqKgAwIBAgILBAAAAAABD4Ym5g0wDQYJKoZIhvcNAQEFBQAwTDEgMB4G\n"
"A1UECxMXR2xvYmFsU2lnbiBSb290IENBIC0gUjIxEzARBgNVBAoTCkdsb2JhbFNp\n"
"Z24xEzARBgNVBAMTCkdsb2JhbFNpZ24wHhcNMDYxMjE1MDgwMDAwWhcNMjExMjE1\n"
"MDgwMDAwWjBMMSAwHgYDVQQLExdHbG9iYWxTaWduIFJvb3QgQ0EgLSBSMjETMBEG\n"
"A1UEChMKR2xvYmFsU2lnbjETMBEGA1UEAxMKR2xvYmFsU2lnbjCCASIwDQYJKoZI\n"
"hvcNAQEBBQADggEPADCCAQoCggEBAKbPJA6+Lm8omUVCxKs+IVSbC9N/hHD6ErPL\n"
"v4dfxn+G07IwXNb9rfF73OX4YJYJkhD10FPe+3t+c4isUoh7SqbKSaZeqKeMWhG8\n"
"eoLrvozps6yWJQeXSpkqBy+0Hne/ig+1AnwblrjFuTosvNYSuetZfeLQBoZfXklq\n"
"tTleiDTsvHgMCJiEbKjNS7SgfQx5TfC4LcshytVsW33hoCmEofnTlEnLJGKRILzd\n"
"C9XZzPnqJworc5HGnRusyMvo4KD0L5CLTfuwNhv2GXqF4G3yYROIXJ/gkwpRl4pa\n"
"zq+r1feqCapgvdzZX99yqWATXgAByUr6P6TqBwMhAo6CygPCm48CAwEAAaOBnDCB\n"
"mTAOBgNVHQ8BAf8EBAMCAQYwDwYDVR0TAQH/BAUwAwEB/zAdBgNVHQ4EFgQUm+IH\n"
"V2ccHsBqBt5ZtJot39wZhi4wNgYDVR0fBC8wLTAroCmgJ4YlaHR0cDovL2NybC5n\n"
"bG9iYWxzaWduLm5ldC9yb290LXIyLmNybDAfBgNVHSMEGDAWgBSb4gdXZxwewGoG\n"
"3lm0mi3f3BmGLjANBgkqhkiG9w0BAQUFAAOCAQEAmYFThxxol4aR7OBKuEQLq4Gs\n"
"J0/WwbgcQ3izDJr86iw8bmEbTUsp9Z8FHSbBuOmDAGJFtqkIk7mpM0sYmsL4h4hO\n"
"291xNBrBVNpGP+DTKqttVCL1OmLNIG+6KYnX3ZHu01yiPqFbQfXf5WRDLenVOavS\n"
"ot+3i9DAgBkcRcAtjOj4LaR0VknFBbVPFd5uRHg5h6h+u/N5GJG79G+dwfCMNYxd\n"
"AfvDbbnvRG15RjF+Cv6pgsH/76tuIMRQyV+dTZsXjAzlAcmgQWpzU/qlULRuJQ/7\n"
"TBj0/VLZjmmx6BEP3ojY+x1J96relc8geMJgEtslQIxq/H5COEBkEveegeGTLg==\n"
"-----END CERTIFICATE-----\n"

Note the \n at the end of each line.

See the comprehensive tutorial on SSL.com for instructions on how to convert between different certificate formats and encodings.

API documentation

Header file: include/modem/modem_key_mgmt.h
Source files: lib/modem_key_mgmt/
group modem_key_mgmt

Enums

enum modem_key_mgmt_cred_type

Credential types.

Values:

enumerator MODEM_KEY_MGMT_CRED_TYPE_CA_CHAIN
enumerator MODEM_KEY_MGMT_CRED_TYPE_PUBLIC_CERT
enumerator MODEM_KEY_MGMT_CRED_TYPE_PRIVATE_CERT
enumerator MODEM_KEY_MGMT_CRED_TYPE_PSK
enumerator MODEM_KEY_MGMT_CRED_TYPE_IDENTITY

Functions

int modem_key_mgmt_write(nrf_sec_tag_t sec_tag, enum modem_key_mgmt_cred_type cred_type, const void *buf, size_t len)

Write or update a credential in persistent storage.

Store the credential and associate it with the given security tag, which can later be used to access it.

Note

If used when the LTE link is active, the function will return an error and the key will not be written.

Parameters

  • sec_tag – [in] Security tag to associate with this credential.

  • cred_type – [in] The credential type.

  • buf – [in] Buffer containing the credential data.

  • len – [in] Length of the buffer.

Returns 0

On success.

Returns -EINVAL

Invalid parameters.

Returns -ENOBUFS

Internal buffer is too small.

Returns -ENOMEM

Not enough memory to store the credential.

Returns -EACCES

Th operation failed because the LTE link is active.

Returns -ENOENT

The security tag could not be written.

Returns -EPERM

Insufficient permissions.

int modem_key_mgmt_read(nrf_sec_tag_t sec_tag, enum modem_key_mgmt_cred_type cred_type, void *buf, size_t *len)

Read a credential from persistent storage.

Parameters

  • sec_tag – [in] The security tag of the credential.

  • cred_type – [in] The credential type.

  • buf – [out] Buffer to read the credential into.

  • len – [inout] Length of the buffer, length of the credential.

Returns 0

On success.

Returns -ENOBUFS

Internal buffer is too small.

Returns -ENOMEM

If provided buffer is too small for result data.

Returns -ENOENT

No credential associated with the given sec_tag and cred_type.

Returns -EPERM

Insufficient permissions.

int modem_key_mgmt_cmp(nrf_sec_tag_t sec_tag, enum modem_key_mgmt_cred_type cred_type, const void *buf, size_t len)

Compare a credential with a credential in persistent storage.

Parameters

  • sec_tag – [in] The security tag of the credential.

  • cred_type – [in] The credential type.

  • buf – [in] Buffer to compare the credential to.

  • len – [in] Length of the buffer.

Returns 0

If the credentials match.

Returns 1

If the credentials do not match.

Returns -ENOBUFS

Internal buffer is too small.

Returns -ENOENT

No credential associated with the given sec_tag and cred_type.

Returns -EPERM

Insufficient permissions.

int modem_key_mgmt_delete(nrf_sec_tag_t sec_tag, enum modem_key_mgmt_cred_type cred_type)

Delete a credential from persistent storage.

Note

If used when the LTE link is active, the function will return an error and the key will not be written.

Parameters

  • sec_tag – [in] The security tag of the credential.

  • cred_type – [in] The credential type.

Returns 0

On success.

Returns -ENOBUFS

Internal buffer is too small.

Returns -EACCES

The operation failed because the LTE link is active.

Returns -ENOENT

No credential associated with the given sec_tag and cred_type.

Returns -EPERM

Insufficient permissions.

int modem_key_mgmt_permission_set(nrf_sec_tag_t sec_tag, enum modem_key_mgmt_cred_type cred_type, uint8_t perm_flags)

Set permissions for a credential in persistent storage.

Parameters

  • sec_tag – [in] The security tag of the credential.

  • cred_type – [in] The credential type.

  • perm_flags – [in] Permission flags. Not yet implemented.

Returns 0

On success.

Returns -ENOBUFS

Internal buffer is too small.

Returns -ENOENT

No credential associated with the given sec_tag and cred_type.

Returns -EPERM

Insufficient permissions.

int modem_key_mgmt_exists(nrf_sec_tag_t sec_tag, enum modem_key_mgmt_cred_type cred_type, bool *exists, uint8_t *perm_flags)

Check if a credential exists in persistent storage.

Parameters

  • sec_tag – [in] The security tag to search for.

  • cred_type – [in] The credential type.

  • exists – [out] Whether the credential exists. Only valid if the operation is successful.

  • perm_flags – [out] The permission flags of the credential. Not yet implemented.

Returns 0

On success.

Returns -ENOBUFS

Internal buffer is too small.

Returns -EPERM

Insufficient permissions.