Internet Engineering Task Force (IETF) W. Kumari
Request for Comments: 8886 Google
Category: Informational C. Doyle
ISSN: 2070-1721 Juniper Networks
September 2020
Secure Device Install
Abstract
Deploying a new network device in a location where the operator has
no staff of its own often requires that an employee physically travel
to the location to perform the initial install and configuration,
even in shared facilities with "remote-hands" (or similar) support.
In many cases, this could be avoided if there were an easy way to
transfer the initial configuration to a new device while still
maintaining confidentiality of the configuration.
This document extends existing vendor proprietary auto-install to
provide limited confidentiality to initial configuration during
bootstrapping of the device.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8886.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Overview
2.1. Example Scenario
3. Vendor Role
3.1. Device Key Generation
3.2. Directory Server
4. Operator Role
4.1. Administrative
4.2. Technical
4.3. Example Initial Customer Boot
5. Additional Considerations
5.1. Key Storage
5.2. Key Replacement
5.3. Device Reinstall
6. IANA Considerations
7. Security Considerations
8. Informative References
Appendix A. Proof of Concept
A.1. Step 1: Generating the Certificate
A.1.1. Step 1.1: Generate the Private Key
A.1.2. Step 1.2: Generate the Certificate Signing Request
A.1.3. Step 1.3: Generate the (Self-Signed) Certificate Itself
A.2. Step 2: Generating the Encrypted Configuration
A.2.1. Step 2.1: Fetch the Certificate
A.2.2. Step 2.2: Encrypt the Configuration File
A.2.3. Step 2.3: Copy Configuration to the Configuration
Server
A.3. Step 3: Decrypting and Using the Configuration
A.3.1. Step 3.1: Fetch Encrypted Configuration File from
Configuration Server
A.3.2. Step 3.2: Decrypt and Use the Configuration
Acknowledgments
Authors' Addresses
1. Introduction
In a growing, global network, significant amounts of time and money
are spent deploying new devices and "forklift" upgrading existing
devices. In many cases, these devices are in shared facilities (for
example, Internet Exchange Points (IXP) or "carrier-neutral data
centers"), which have staff on hand that can be contracted to perform
tasks including physical installs, device reboots, loading initial
configurations, etc. There are also a number of (often proprietary)
protocols to perform initial device installs and configurations. For
example, many network devices will attempt to use DHCP [RFC2131] or
DHCPv6 [RFC8415] to get an IP address and configuration server and
then fetch and install a configuration when they are first powered
on.
The configurations of network devices contain a significant amount of
security-related and proprietary information (for example, RADIUS
[RFC2865] or TACACS+ [TACACS] secrets). Exposing these to a third
party to load onto a new device (or using an auto-install technique
that fetches an unencrypted configuration file via TFTP [RFC1350]) or
something similar is an unacceptable security risk for many
operators, and so they send employees to remote locations to perform
the initial configuration work; this costs time and money.
There are some workarounds to this, such as asking the vendor to
preconfigure the device before shipping it; asking the remote support
to install a terminal server; providing a minimal, unsecured
configuration and using that to bootstrap to a complete
configuration; etc. However, these are often clumsy and have
security issues. As an example, in the terminal server case, the
console port connection could be easily snooped.
An ideal solution in this space would protect both the
confidentiality of device configuration in transit and the
authenticity (and authorization status) of configuration to be used
by the device. The mechanism described in this document only
addresses the former and makes no effort to do the latter, with the
device accepting any configuration file that comes its way and is
encrypted to the device's key (or not encrypted, as the case may be).
Other solutions (such as Secure Zero Touch Provisioning (SZTP)
[RFC8572], Bootstrapping Remote Secure Key Infrastructures (BRSKI)
[BRSKI], and other voucher-based methods) are more fully featured but
also require more complicated machinery. This document describes
something much simpler, at the cost of only providing limited
protection.
This document layers security onto existing auto-install solutions
(one example of which is [Cisco_AutoInstall]) to provide a method to
initially configure new devices while maintaining (limited)
confidentiality of the initial configuration. It is optimized for
simplicity, for both the implementor and the operator. It is
explicitly not intended to be a fully featured system for managing
installed devices nor is it intended to solve all use cases; rather,
it is a simple targeted solution to solve a common operational issue
where the network device has been delivered, fiber has been laid (as
appropriate), and there is no trusted member of the operator's staff
to perform the initial configuration. This solution is only intended
to increase confidentiality of the information in the configuration
file and does not protect the device itself from loading a malicious
configuration.
This document describes a concept and some example ways of
implementing this concept. As devices have different capabilities
and use different configuration paradigms, one method will not suit
all, and so it is expected that vendors will differ in exactly how
they implement this.
This solution is specifically designed to be a simple method on top
of exiting device functionality. If devices do not support this new
method, they can continue to use the existing functionality. In
addition, operators can choose to use this to protect their
configuration information or can continue to use the existing
functionality.
The issue of securely installing devices is in no way a new issue nor
is it limited to network devices; it occurs when deploying servers,
PCs, Internet of Things (IoT) devices, and in many other situations.
While the solution described in this document is obvious (encrypt the
config, then decrypt it with a device key), this document only
discusses the use for network devices, such as routers and switches.
2. Overview
Most network devices already include some sort of initial
bootstrapping logic (sometimes called 'autoboot' or 'autoinstall').
This generally works by having a newly installed, unconfigured device
obtain an IP address for itself and discover the address of a
configuration server (often called 'next-server', 'siaddr', or 'tftp-
server-name') using DHCP or DHCPv6 (see [RFC2131] and [RFC8415]).
The device then contacts this configuration server to download its
initial configuration, which is often identified using the device's
serial number, Media Access Control (MAC) address, or similar. This
document extends this (vendor-specific) paradigm by allowing the
configuration file to be encrypted.
This document uses the serial number of the device as a unique device
identifier for simplicity; some vendors may not want to implement the
system using the serial number as the identifier for business reasons
(a competitor or similar could enumerate the serial numbers and
determine how many devices have been manufactured). Implementors are
free to choose some other way of generating identifiers (e.g., a
Universally Unique Identifier (UUID) [RFC4122]), but this will likely
make it somewhat harder for operators to use (the serial number is
usually easy to find on a device).
2.1. Example Scenario
Operator_A needs another peering router, and so they order another
router from Vendor_B to be drop-shipped to the facility. Vendor_B
begins assembling the new device and tells Operator_A what the new
device's serial number will be (SN:17894321). When Vendor_B first
installs the firmware on the device and boots it, the device
generates a public-private key pair, and Vendor_B publishes the
public key on its key server (in a public key certificate, for ease
of use).
While the device is being shipped, Operator_A generates the initial
device configuration and fetches the certificate from Vendor_B key
servers by providing the serial number of the new device. Operator_A
then encrypts the device configuration and puts this encrypted
configuration on a (local) TFTP server.
When the device arrives at the Point of Presence (POP), it gets
installed in Operator_A's rack and cabled as instructed. The new
device powers up and discovers that it has not yet been configured.
It enters its autoboot state and begins the DHCP process.
Operator_A's DHCP server provides it with an IP address and the
address of the configuration server. The router uses TFTP to fetch
its configuration file. Note that all of this is existing
functionality. The device attempts to load the configuration file.
As an added step, if the configuration file cannot be parsed, the
device tries to use its private key to decrypt the file and, assuming
it validates, proceeds to install the new, decrypted configuration.
Only the "correct" device will have the required private key and be
able to decrypt and use the configuration file (see Security
Considerations (Section 7)). An attacker would be able to connect to
the network and get an IP address. They would also be able to
retrieve (encrypted) configuration files by guessing serial numbers
(or perhaps the server would allow directory listing), but without
the private keys, an attacker will not be able to decrypt the files.
3. Vendor Role
This section describes the vendor's roles and provides an overview of
what the device needs to do.
3.1. Device Key Generation
Each device requires a public-private key pair and for the public
part to be published and retrievable by the operator. The
cryptographic algorithm and key lengths to be used are out of the
scope of this document. This section illustrates one method, but, as
with much of this document, the exact mechanism may vary by vendor.
Enrollment over Secure Transport [RFC7030] and possibly the Simple
Certificate Enrollment Protocol [RFC8894] are methods that vendors
may want to consider.
During the manufacturing stage, when the device is initially powered
on, it will generate a public-private key pair. It will send its
unique device identifier and the public key to the vendor's directory
server [RFC5280] to be published. The vendor's directory server
should only accept certificates that are from the manufacturing
facility and that match vendor-defined policies (for example,
extended key usage and extensions). Note that some devices may be
constrained and so may send the raw public key and unique device
identifier to the certificate publication server, while more capable
devices may generate and send self-signed certificates. This
communication with the directory server should be integrity protected
and should occur in a controlled environment.
This reference architecture needs a serialization format for the key
material. Due to the prevalence of tooling support for it on network
devices, X.509 certificates are a convenient format to exchange
public keys. However, most of the metadata that would be used for
revocation and aging will not be used and should be ignored by both
the client and server. In such cases, the signature on the
certificate conveys no value, and the consumer of the certificate is
expected to pin the end-entity key fingerprint (versus using a PKI
and signature chain).
3.2. Directory Server
The directory server contains a database of certificates. If newly
manufactured devices upload certificates, the directory server can
simply publish these; if the devices provide the raw public keys and
unique device identifier, the directory server will need to wrap
these in a certificate.
The customers (e.g., Operator_A) query this server with the serial
number (or other provided unique identifier) of a device and retrieve
the associated certificate. It is expected that operators will
receive the unique device identifier (serial number) of devices when
they purchase them and will download and store the certificate. This
means that there is not a hard requirement on the reachability of the
directory server.
+------------+
+------+ | |
|Device| | Directory |
+------+ | Server |
+------------+
+----------------+ +--------------+
| +---------+ | | |
| | Initial | | | |
| | boot? | | | |
| +----+----+ | | |
| | | | |
| +------v-----+ | | |
| | Generate | | | |
| |Self-signed | | | |
| |Certificate | | | |
| +------------+ | | |
| | | | +-------+ |
| +-------|---|-->|Receive| |
| | | +---+---+ |
| | | | |
| | | +---v---+ |
| | | |Publish| |
| | | +-------+ |
| | | |
+----------------+ +--------------+
Figure 1: Initial Certificate Generation and Publication
4. Operator Role
4.1. Administrative
When purchasing a new device, the accounting department will need to
get the unique device identifier (e.g., serial number) of the new
device and communicate it to the operations group.
4.2. Technical
The operator will contact the vendor's publication server and
download the certificate (by providing the unique device identifier
of the device). The operator fetches the certificate using a secure
transport that authenticates the source of the certificate, such as
HTTPS (confidentiality protection can provide some privacy and
metadata-leakage benefit but is not key to the primary mechanism of
this document). The operator will then encrypt the initial
configuration (for example, using S/MIME [RFC8551]) using the key in
the certificate and place it on their configuration server.
See Appendix A for examples.
+------------+
+--------+ | |
|Operator| | Directory |
+--------+ | Server |
+------------+
+----------------+ +----------------+
| +-----------+ | | +-----------+ |
| | Fetch | | | | | |
| | Device |<------>|Certificate| |
| |Certificate| | | | | |
| +-----+-----+ | | +-----------+ |
| | | | |
| +-----v------+ | | |
| | Encrypt | | | |
| | Device | | | |
| | Config | | | |
| +-----+------+ | | |
| | | | |
| +-----v------+ | | |
| | Publish | | | |
| | TFTP | | | |
| | Server | | | |
| +------------+ | | |
| | | |
+----------------+ +----------------+
Figure 2: Fetching the Certificate, Encrypting the Configuration, and
Publishing the Encrypted Configuration
4.3. Example Initial Customer Boot
When the device is first booted by the customer (and on subsequent
boots), if the device does not have a valid configuration, it will
use existing auto-install functionality. As an example, it performs
DHCP Discovery until it gets a DHCP offer including DHCP option 66
(Server-Name) or 150 (TFTP server address), contacts the server
listed in these DHCP options, and downloads its configuration file.
Note that this is existing functionality (for example, Cisco devices
fetch the config file named by the Bootfile-Name DHCP option (67)).
After retrieving the configuration file, the device needs to
determine if it is encrypted or not. If it is not encrypted, the
existing behavior is used. If the configuration is encrypted, the
process continues as described in this document. If the device has
been configured to only support encrypted configuration and
determines that the configuration file is not encrypted, it should
abort. The method used to determine if the configuration is
encrypted or not is implementation dependent; there are a number of
(obvious) options, including having a magic string in the file
header, using a file name extension (e.g., config.enc), or using
specific DHCP options.
If the file is encrypted, the device will attempt to decrypt and
parse the file. If able, it will install the configuration and start
using it. If it cannot decrypt the file or if parsing the
configuration fails, the device will either abort the auto-install
process or repeat this process until it succeeds. When retrying,
care should be taken to not overwhelm the server hosting the
encrypted configuration files. It is suggested that the device retry
every 5 minutes for the first hour and then every hour after that.
As it is expected that devices may be installed well before the
configuration file is ready, a maximum number of retries is not
specified.
Note that the device only needs to be able to download the
configuration file; after the initial power on in the factory, it
never needs to access the Internet, vendor, or directory server. The
device (and only the device) has the private key and so has the
ability to decrypt the configuration file.
+--------+ +--------------+
| Device | |Config server |
+--------+ |(e.g., TFTP) |
+--------------+
+---------------------------+ +------------------+
| +-----------+ | | |
| | | | | |
| | DHCP | | | |
| | | | | |
| +-----+-----+ | | |
| | | | |
| +-----v------+ | | +-----------+ |
| | | | | | Encrypted | |
| |Fetch config|<------------------>| config | |
| | | | | | file | |
| +-----+------+ | | +-----------+ |
| | | | |
| X | | |
| / \ | | |
| / \ N +--------+ | | |
| | Enc?|---->|Install,| | | |
| \ / | Boot | | | |
| \ / +--------+ | | |
| V | | |
| |Y | | |
| | | | |
| +-----v------+ | | |
| |Decrypt with| | | |
| |private key | | | |
| +-----+------+ | | |
| | | | |
| v | | |
| / \ | | |
| / \ Y +--------+ | | |
| |Sane?|---->|Install,| | | |
| \ / | Boot | | | |
| \ / +--------+ | | |
| V | | |
| |N | | |
| | | | |
| +----v---+ | | |
| |Retry or| | | |
| | abort | | | |
| +--------+ | | |
| | | |
+---------------------------+ +------------------+
Figure 3: Device Boot, Fetch, and Install Configuration File
5. Additional Considerations
5.1. Key Storage
Ideally, the key pair would be stored in a Trusted Platform Module
(TPM) on something that is identified as the "router" -- for example,
the chassis/backplane. This is so that a key pair is bound to what
humans think of as the "device" and not, for example, (redundant)
routing engines. Devices that implement IEEE 802.1AR [IEEE802-1AR]
could choose to use the Initial Device Identifier (IDevID) for this
purpose.
5.2. Key Replacement
It is anticipated that some operator may want to replace the (vendor-
provided) keys after installing the device. There are two options
when implementing this: a vendor could allow the operator's key to
completely replace the initial device-generated key (which means
that, if the device is ever sold, the new owner couldn't use this
technique to install the device), or the device could prefer the
operator's installed key. This is an implementation decision left to
the vendor.
5.3. Device Reinstall
Increasingly, operations are moving towards an automated model of
device management, whereby portions of the configuration (or the
entire configuration) are programmatically generated. This means
that operators may want to generate an entire configuration after the
device has been initially installed and ask the device to load and
use this new configuration. It is expected (but not defined in this
document, as it is vendor specific) that vendors will allow the
operator to copy a new, encrypted configuration (or part of a
configuration) onto a device and then request that the device decrypt
and install it (e.g., 'load replace <filename> encrypted'). The
operator could also choose to reset the device to factory defaults
and allow the device to act as though it were the initial boot (see
Section 4.3).
6. IANA Considerations
This document has no IANA actions.
7. Security Considerations
This reference architecture is intended to incrementally improve upon
commonly accepted "auto-install" practices used today that may
transmit configurations unencrypted (e.g., unencrypted configuration
files that can be downloaded connecting to unprotected ports in data
centers, mailing initial configuration files on flash drives, or
emailing configuration files and asking a third party to copy and
paste them over a serial terminal) or allow unrestricted access to
these configurations.
This document describes an object-level security design to provide
confidentiality assurances for the configuration stored at rest on
the configuration server and for configuration while it is in transit
between the configuration server and the unprovisioned device, even
if the underlying transport does not provide this security service.
The architecture provides no assurances about the source of the
encrypted configuration or protect against theft and reuse of
devices.
An attacker (e.g., a malicious data center employee, person in the
supply chain, etc.) who has physical access to the device before it
is connected to the network or who manages to exploit it once
installed may be able to extract the device private key (especially
if it is not stored in a TPM), pretend to be the device when
connecting to the network, and download and extract the (encrypted)
configuration file.
An attacker with access to the configuration server (or the ability
to route traffic to configuration server under their control) and the
device's public key could return a configuration of the attacker's
choosing to the unprovisioned device.
This mechanism does not protect against a malicious vendor. While
the key pair should be generated on the device and the private key
should be securely stored, the mechanism cannot detect or protect
against a vendor who claims to do this but instead generates the key
pair off device and keeps a copy of the private key. It is largely
understood in the operator community that a malicious vendor or
attacker with physical access to the device is largely a "Game Over"
situation.
Even when using a secure bootstrap mechanism, security-conscious
operators may wish to bootstrap devices with a minimal or less-
sensitive configuration and then replace this with a more complete
one after install.
8. Informative References
[BRSKI] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructures (BRSKI)", Work in Progress, Internet-
Draft, draft-ietf-anima-bootstrapping-keyinfra-44, 21
September 2020, <https://tools.ietf.org/html/draft-ietf-
anima-bootstrapping-keyinfra-44>.
[Cisco_AutoInstall]
Cisco Systems, Inc., "Using AutoInstall to Remotely
Configure Cisco Networking Devices", Configuration
Fundamentals Configuration Guide, Cisco IOS Release 15M&T,
January 2018, <https://www.cisco.com/c/en/us/td/docs/ios-
xml/ios/fundamentals/configuration/15mt/fundamentals-15-
mt-book/cf-autoinstall.html>.
[IEEE802-1AR]
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks - Secure Device Identity", IEEE Std 802-1AR, June
2018,
<https://standards.ieee.org/standard/802_1AR-2018.html>.
[RFC1350] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33,
RFC 1350, DOI 10.17487/RFC1350, July 1992,
<https://www.rfc-editor.org/info/rfc1350>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, DOI 10.17487/RFC2865, June 2000,
<https://www.rfc-editor.org/info/rfc2865>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<https://www.rfc-editor.org/info/rfc7030>.
[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <https://www.rfc-editor.org/info/rfc8551>.
[RFC8572] Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
Touch Provisioning (SZTP)", RFC 8572,
DOI 10.17487/RFC8572, April 2019,
<https://www.rfc-editor.org/info/rfc8572>.
[RFC8894] Gutmann, P., "Simple Certificate Enrolment Protocol",
RFC 8894, DOI 10.17487/RFC8894, September 2020,
<https://www.rfc-editor.org/info/rfc8894>.
[TACACS] Dahm, T., Ota, A., Medway Gash, D., Carrel, D., and L.
Grant, "The TACACS+ Protocol", Work in Progress, Internet-
Draft, draft-ietf-opsawg-tacacs-18, 20 March 2020,
<https://tools.ietf.org/html/draft-ietf-opsawg-tacacs-18>.
Appendix A. Proof of Concept
This section contains a proof of concept of the system. It is only
intended for illustration and is not intended to be used in
production.
It uses OpenSSL from the command line. In production, something more
automated would be used. In this example, the unique device
identifier is the serial number of the router, SN19842256.
A.1. Step 1: Generating the Certificate
This step is performed by the router. It generates a key, then a
Certificate Signing Request (CSR), and then a self-signed
certificate.
A.1.1. Step 1.1: Generate the Private Key
$ openssl ecparam -out privatekey.key -name prime256v1 -genkey
$
A.1.2. Step 1.2: Generate the Certificate Signing Request
$ openssl req -new -key key.pem -out SN19842256.csr
Common Name (e.g., server FQDN or YOUR name) []:SN19842256
A.1.3. Step 1.3: Generate the (Self-Signed) Certificate Itself
$ openssl req -x509 -days 36500 -key key.pem -in SN19842256.csr
-out SN19842256.crt
The router then sends the key to the vendor's key server for
publication (not shown).
A.2. Step 2: Generating the Encrypted Configuration
The operator now wants to deploy the new router.
They generate the initial configuration (using whatever magic tool
generates router configs!), fetch the router's certificate, and
encrypt the configuration file to that key. This is done by the
operator.
A.2.1. Step 2.1: Fetch the Certificate
$ wget http://keyserv.example.net/certificates/SN19842256.crt
A.2.2. Step 2.2: Encrypt the Configuration File
S/MIME is used here because it is simple to demonstrate. This is
almost definitely not the best way to do this.
$ openssl smime -encrypt -aes-256-cbc -in SN19842256.cfg\
-out SN19842256.enc -outform PEM SN19842256.crt
$ more SN19842256.enc
-----BEGIN PKCS7-----
MIICigYJKoZIhvcNAQcDoIICezCCAncCAQAxggE+MIIBOgIBADAiMBUxEzARBgNV
BAMMClNOMTk4NDIyNTYCCQDJVuBlaTOb1DANBgkqhkiG9w0BAQEFAASCAQBABvM3
...
LZoq08jqlWhZZWhTKs4XPGHUdmnZRYIP8KXyEtHt
-----END PKCS7-----
A.2.3. Step 2.3: Copy Configuration to the Configuration Server
$ scp SN19842256.enc config.example.com:/tftpboot
A.3. Step 3: Decrypting and Using the Configuration
When the router connects to the operator's network, it will detect
that it does not have a valid configuration file and will start the
"autoboot" process. This is a well-documented process, but the high-
level overview is that it will use DHCP to obtain an IP address and
configuration server. It will then use TFTP to download a
configuration file, based upon its serial number (this document
modifies the solution to fetch an encrypted configuration file
(ending in .enc)). It will then decrypt the configuration file and
install it.
A.3.1. Step 3.1: Fetch Encrypted Configuration File from Configuration
Server
$ tftp 2001:0db8::23 -c get SN19842256.enc
A.3.2. Step 3.2: Decrypt and Use the Configuration
$ openssl smime -decrypt -in SN19842256.enc -inform pkcs7\
-out config.cfg -inkey key.pem
If an attacker does not have the correct key, they will not be able
to decrypt the configuration file:
$ openssl smime -decrypt -in SN19842256.enc -inform pkcs7\
-out config.cfg -inkey wrongkey.pem
Error decrypting PKCS#7 structure
140352450692760:error:06065064:digital envelope
routines:EVP_DecryptFinal_ex:bad decrypt:evp_enc.c:592:
$ echo $?
4
Acknowledgments
The authors wish to thank everyone who contributed, including Benoit
Claise, Francis Dupont, Mirja Kuehlewind, Sam Ribeiro, Michael
Richardson, Sean Turner, and Kent Watsen. Joe Clarke also provided
significant comments and review, and Tom Petch provided significant
editorial contributions to better describe the use cases and clarify
the scope.
Roman Danyliw and Benjamin Kaduk also provided helpful text,
especially around the certificate usage and security considerations.
Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Email: warren@kumari.net
Colin Doyle
Juniper Networks
1133 Innovation Way
Sunnyvale, CA 94089
United States of America
Email: cdoyle@juniper.net