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Chapter 4. Configuring applications to use cryptographic hardware through PKCS #11
Separating parts of your secret information about dedicated cryptographic devices, such as smart cards and cryptographic tokens for end-user authentication and hardware security modules (HSM) for server applications, provides an additional layer of security. In RHEL, support for cryptographic hardware through the PKCS #11 API is consistent across different applications, and the isolation of secrets on cryptographic hardware is not a complicated task.
4.1. Cryptographic hardware support through PKCS #11
Public-Key Cryptography Standard (PKCS) #11 defines an application programming interface (API) to cryptographic devices that hold cryptographic information and perform cryptographic functions.
PKCS #11 introduces the cryptographic token, an object that presents each hardware or software device to applications in a unified manner. Therefore, applications view devices such as smart cards, which are typically used by persons, and hardware security modules, which are typically used by computers, as PKCS #11 cryptographic tokens.
A PKCS #11 token can store various object types including a certificate; a data object; and a public, private, or secret key. These objects are uniquely identifiable through the PKCS #11 Uniform Resource Identifier (URI) scheme.
A PKCS #11 URI is a standard way to identify a specific object in a PKCS #11 module according to the object attributes. This enables you to configure all libraries and applications with the same configuration string in the form of a URI.
RHEL provides the OpenSC PKCS #11 driver for smart cards by default. However, hardware tokens and HSMs can have their own PKCS #11 modules that do not have their counterpart in the system. You can register such PKCS #11 modules with the p11-kit tool, which acts as a wrapper over the registered smart-card drivers in the system.
To make your own PKCS #11 module work on the system, add a new text file to the /etc/pkcs11/modules/ directory
You can add your own PKCS #11 module into the system by creating a new text file in the /etc/pkcs11/modules/ directory. For example, the OpenSC configuration file in p11-kit looks as follows:
$ cat /usr/share/p11-kit/modules/opensc.module
module: opensc-pkcs11.soAdditional resources
4.2. Authenticating by SSH keys stored on a smart card
You can create and store ECDSA and RSA keys on a smart card and authenticate by the smart card on an OpenSSH client. Smart-card authentication replaces the default password authentication.
Prerequisites
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On the client side, the
openscpackage is installed and thepcscdservice is running.
Procedure
List all keys provided by the OpenSC PKCS #11 module including their PKCS #11 URIs and save the output to the
keys.pubfile:$ ssh-keygen -D pkcs11: > keys.pubTransfer the public key to the remote server. Use the
ssh-copy-idcommand with thekeys.pubfile created in the previous step:$ ssh-copy-id -f -i keys.pub <username@ssh-server-example.com>Connect to <ssh-server-example.com> by using the ECDSA key. You can use just a subset of the URI, which uniquely references your key, for example:
$ ssh -i "pkcs11:id=%01?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so" <ssh-server-example.com> Enter PIN for 'SSH key': [ssh-server-example.com] $Because OpenSSH uses the
p11-kit-proxywrapper and the OpenSC PKCS #11 module is registered to thep11-kittool, you can simplify the previous command:$ ssh -i "pkcs11:id=%01" <ssh-server-example.com> Enter PIN for 'SSH key': [ssh-server-example.com] $If you skip the
id=part of a PKCS #11 URI, OpenSSH loads all keys that are available in the proxy module. This can reduce the amount of typing required:$ ssh -i pkcs11: <ssh-server-example.com> Enter PIN for 'SSH key': [ssh-server-example.com] $Optional: You can use the same URI string in the
~/.ssh/configfile to make the configuration permanent:$ cat ~/.ssh/config IdentityFile "pkcs11:id=%01?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so" $ ssh <ssh-server-example.com> Enter PIN for 'SSH key': [ssh-server-example.com] $
The
sshclient utility now automatically uses this URI and the key from the smart card.
Additional resources
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p11-kit(8),opensc.conf(5),pcscd(8),ssh(1), andssh-keygen(1)man pages on your system
4.3. Configuring applications for authentication with certificates on smart cards
Authentication by using smart cards in applications may increase security and simplify automation. You can integrate the Public Key Cryptography Standard (PKCS) #11 URIs into your application by using the following methods:
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The
Firefoxweb browser automatically loads thep11-kit-proxyPKCS #11 module. This means that every supported smart card in the system is automatically detected. For using TLS client authentication, no additional setup is required and keys and certificates from a smart card are automatically used when a server requests them. -
If your application uses the
GnuTLSorNSSlibrary, it already supports PKCS #11 URIs. Also, applications that rely on theOpenSSLlibrary can access cryptographic hardware modules, including smart cards, through thepkcs11engine provided by theopenssl-pkcs11package. -
Applications that require working with private keys on smart cards and that do not use
NSS,GnuTLS, norOpenSSLcan use thep11-kitAPI directly to work with cryptographic hardware modules, including smart cards, rather than using the PKCS #11 API of specific PKCS #11 modules. With the the
wgetnetwork downloader, you can specify PKCS #11 URIs instead of paths to locally stored private keys and certificates. This might simplify creation of scripts for tasks that require safely stored private keys and certificates. For example:$ wget --private-key 'pkcs11:token=softhsm;id=%01;type=private?pin-value=111111' --certificate 'pkcs11:token=softhsm;id=%01;type=cert' https://example.com/You can also specify PKCS #11 URI when using the
curltool:$ curl --key 'pkcs11:token=softhsm;id=%01;type=private?pin-value=111111' --cert 'pkcs11:token=softhsm;id=%01;type=cert' https://example.com/NoteBecause a PIN is a security measure that controls access to keys stored on a smart card and the configuration file contains the PIN in the plain-text form, consider additional protection to prevent an attacker from reading the PIN. For example, you can use the
pin-sourceattribute and provide afile:URI for reading the PIN from a file. See RFC 7512: PKCS #11 URI Scheme Query Attribute Semantics for more information. Note that using a command path as a value of thepin-sourceattribute is not supported.
Additional resources
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curl(1),wget(1), andp11-kit(8)man pages on your system
4.4. Using HSMs protecting private keys in Apache
The Apache HTTP server can work with private keys stored on hardware security modules (HSMs), which helps to prevent the keys' disclosure and man-in-the-middle attacks. Note that this usually requires high-performance HSMs for busy servers.
For secure communication in the form of the HTTPS protocol, the Apache HTTP server (httpd) uses the OpenSSL library. OpenSSL does not support PKCS #11 natively. To use HSMs, you have to install the openssl-pkcs11 package, which provides access to PKCS #11 modules through the engine interface. You can use a PKCS #11 URI instead of a regular file name to specify a server key and a certificate in the /etc/httpd/conf.d/ssl.conf configuration file, for example:
SSLCertificateFile "pkcs11:id=%01;token=softhsm;type=cert" SSLCertificateKeyFile "pkcs11:id=%01;token=softhsm;type=private?pin-value=111111"
Install the httpd-manual package to obtain complete documentation for the Apache HTTP Server, including TLS configuration. The directives available in the /etc/httpd/conf.d/ssl.conf configuration file are described in detail in the /usr/share/httpd/manual/mod/mod_ssl.html file.
4.5. Using HSMs protecting private keys in Nginx
The Nginx HTTP server can work with private keys stored on hardware security modules (HSMs), which helps to prevent the keys' disclosure and man-in-the-middle attacks. Note that this usually requires high-performance HSMs for busy servers.
Because Nginx also uses the OpenSSL for cryptographic operations, support for PKCS #11 must go through the openssl-pkcs11 engine. Nginx currently supports only loading private keys from an HSM, and a certificate must be provided separately as a regular file. Modify the ssl_certificate and ssl_certificate_key options in the server section of the /etc/nginx/nginx.conf configuration file:
ssl_certificate /path/to/cert.pem ssl_certificate_key "engine:pkcs11:pkcs11:token=softhsm;id=%01;type=private?pin-value=111111";
Note that the engine:pkcs11: prefix is needed for the PKCS #11 URI in the Nginx configuration file. This is because the other pkcs11 prefix refers to the engine name.
