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This is supposed to be useful when generating credentials for immutable initrd environments, where it is is relevant to support credentials even on systems lacking a TPM2 chip. With this, if `systemd-creds encrypt --with-key=auto-initrd` is used a credential will be encrypted/signed with the TPM2 if it is available and recognized by the firmware. Otherwise it will be encrypted/signed with the fixed empty key, thus providing no confidentiality or authenticity. The idea is that distributions use this mode to generically create credentials that are as locked down as possible on the specific platform.
388 lines
22 KiB
XML
388 lines
22 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd-creds"
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>systemd-creds</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-creds</refentrytitle>
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<manvolnum>1</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-creds</refname>
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<refpurpose>Lists, shows, encrypts and decrypts service credentials</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<cmdsynopsis>
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<command>systemd-creds</command>
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<arg choice="opt" rep="repeat">OPTIONS</arg>
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</cmdsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><command>systemd-creds</command> is a tool for listing, showing, encrypting and decrypting unit
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credentials. Credentials are limited-size binary or textual objects that may be passed to unit
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processes. They are primarily used for passing cryptographic keys (both public and private) or
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certificates, user account information or identity information from the host to services.</para>
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<para>Credentials are configured in unit files via the <varname>LoadCredential=</varname>,
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<varname>SetCredential=</varname>, <varname>LoadCredentialEncrypted=</varname> and
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<varname>SetCredentialEncrypted=</varname> settings, see
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
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details.</para>
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</refsect1>
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<refsect1>
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<title>Commands</title>
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<para>The following commands are understood:</para>
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<variablelist>
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<varlistentry>
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<term><command>list</command></term>
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<listitem><para>Show a list of credentials passed into the current execution context. This command
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shows the files in the directory referenced by the <varname>$CREDENTIALS_DIRECTORY</varname>
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environment variable, and is intended to be executed from within service context.</para>
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<para>Along with each credential name, the size and security state is shown. The latter is one of
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<literal>secure</literal> (in case the credential is backed by unswappable memory,
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i.e. <literal>ramfs</literal>), <literal>weak</literal> (in case it is backed by any other type of
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memory), or <literal>insecure</literal> (if having any access mode that is not 0400, i.e. if readable
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by anyone but the owner).</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><command>cat</command> <replaceable>credential...</replaceable></term>
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<listitem><para>Show contents of specified credentials passed into the current execution
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context. Takes one or more credential names, whose contents shall be written to standard
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output.</para>
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<para>When combined with <option>--json=</option> or <option>--transcode=</option> the output is
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transcoded in simple ways before outputting.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><command>setup</command></term>
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<listitem><para>Generates a host encryption key for credentials, if one has not been generated
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already. This ensures the <filename>/var/lib/systemd/credential.secret</filename> file is initialized
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with a random secret key if it doesn't exist yet. This secret key is used when encrypting/decrypting
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credentials with <command>encrypt</command> or <command>decrypt</command>, and is only accessible to
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the root user. Note that there's typically no need to invoke this command explicitly as it is
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implicitly called when <command>encrypt</command> is invoked, and credential host key encryption
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selected.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><command>encrypt</command> <replaceable>input|-</replaceable> <replaceable>output|-</replaceable></term>
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<listitem><para>Loads the specified (unencrypted plaintext) input credential file, encrypts it and
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writes the (encrypted ciphertext) version to the specified output credential file. The resulting file
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may be referenced in the <varname>LoadCredentialEncrypted=</varname> setting in unit files, or its
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contents used literally in <varname>SetCredentialEncrypted=</varname> settings.</para>
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<para>Takes two file system paths. The file name part of the output path is embedded as name in the
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encrypted credential, to ensure encrypted credentials cannot be renamed and reused for different
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purposes without this being noticed. The credential name to embed may be overridden with the
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<option>--name=</option> setting. The input or output paths may be specified as <literal>-</literal>,
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in which case the credential data is read from/written to standard input and standard output. If the
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output path is specified as <literal>-</literal> the credential name cannot be derived from the file
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system path, and thus should be specified explicitly via the <option>--name=</option> switch.</para>
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<para>The credential data is encrypted symmetrically with one of the following encryption
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keys:</para>
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<orderedlist>
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<listitem><para>A secret key automatically derived from the system's TPM2 chip. This encryption key
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is not stored on the host system and thus decryption is only possible with access to the original
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TPM2 chip. Or in other words, the credential secured in this way can only be decrypted again by the
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local machine.</para></listitem>
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<listitem><para>A secret key stored in the <filename>/var/lib/systemd/credential.secret</filename>
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file which is only accessible to the root user. This "host" encryption key is stored on the host
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file system, and thus decryption is possible with access to the host file system and sufficient
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privileges. The key is automatically generated when needed, but can also be created explicitly with
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the <command>setup</command> command, see above.</para></listitem>
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<listitem><para>A combination of the above: an encryption key derived from both the TPM2 chip and
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the host file system. This means decryption requires both access to the original TPM2 chip and the
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OS installation. This is the default mode of operation if a TPM2 chip is available and
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<filename>/var/lib/systemd/</filename> resides on persistent media.</para></listitem>
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</orderedlist>
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<para>Which of the three keys shall be used for encryption may be configured with the
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<option>--with-key=</option> switch. Depending on the use-case for the encrypted credential the key to
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use may differ. For example, for credentials that shall be accessible from the initial RAM disk
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(initrd) of the system encryption with the host key is not appropriate since access to the host key
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is typically not available from the initrd. Thus, for such credentials only the TPM2 key should be
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used.</para>
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<para>Encrypted credentials are always encoded in Base64.</para>
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<para>Use <command>decrypt</command> (see below) to undo the encryption operation, and acquire the
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decrypted plaintext credential from the encrypted ciphertext credential.</para>
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<para>The credential data is encrypted using AES256-GCM, i.e. providing both confidentiality and
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integrity, keyed by a SHA256 hash of one or both of the secret keys described above.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><command>decrypt</command> <replaceable>input|-</replaceable>
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<optional><replaceable>output|-</replaceable></optional></term>
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<listitem><para>Undoes the effect of the <command>encrypt</command> operation: loads the specified
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(encrypted ciphertext) input credential file, decrypts it and writes the (decrypted plaintext)
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version to the specified output credential file.</para>
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<para>Takes one or two file system paths. The file name part of the input path is compared with the
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credential name embedded in the encrypted file. If it does not match decryption fails. This is done
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in order to ensure that encrypted credentials are not re-purposed without this being detected. The
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credential name to compare with the embedded credential name may also be overridden with the
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<option>--name=</option> switch. If the input path is specified as <literal>-</literal>, the
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encrypted credential is read from standard input. If only one path is specified or the output path
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specified as <literal>-</literal>, the decrypted credential is written to standard output. In this
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mode, the expected name embedded in the credential cannot be derived from the path and should be
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specified explicitly with <option>--name=</option>.</para>
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<para>Decrypting credentials requires access to the original TPM2 chip and/or credentials host key,
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see above. Information about which keys are required is embedded in the encrypted credential data,
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and thus decryption is entirely automatic.</para></listitem>
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</varlistentry>
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<xi:include href="standard-options.xml" xpointer="help" />
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<xi:include href="standard-options.xml" xpointer="version" />
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Options</title>
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<variablelist>
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<varlistentry>
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<term><option>--system</option></term>
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<listitem><para>When specified with the <command>list</command> and <command>cat</command> commands
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operates on the credentials passed to system as a whole instead of on those passed to the current
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execution context. This is useful in container environments where credentials may be passed in from
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the container manager.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--transcode=</option></term>
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<listitem><para>When specified with the <command>cat</command> or <command>decrypt</command>
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commands, transcodes the output before showing it. Takes one of <literal>base64</literal>,
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<literal>unbase64</literal>, <literal>hex</literal> or <literal>unhex</literal> as argument, in order
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to encode/decode the credential data with Base64 or as series of hexadecimal values.</para>
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<para>Note that this has no effect on the <command>encrypt</command> command, as encrypted
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credentials are unconditionally encoded in Base64.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--newline=</option></term>
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<listitem><para>When specified with <command>cat</command> or <command>decrypt</command> controls
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whether to add a trailing newline character to the end of the output if it doesn't end in one,
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anyway. Takes one of <literal>auto</literal>, <literal>yes</literal> or <literal>no</literal>. The
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default mode of <literal>auto</literal> will suffix the output with a single newline character only
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when writing credential data to a TTY.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--pretty</option></term>
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<term><option>-p</option></term>
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<listitem><para>When specified with <command>encrypt</command> controls whether to show the encrypted
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credential as <varname>SetCredentialEncrypted=</varname> setting that may be pasted directly into a
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unit file.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--name=</option><replaceable>name</replaceable></term>
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<listitem><para>When specified with the <command>encrypt</command> command controls the credential
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name to embed in the encrypted credential data. If not specified the name is chosen automatically
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from the filename component of the specified output path. If specified as empty string no
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credential name is embedded in the encrypted credential, and no verification of credential name is
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done when the credential is decrypted.</para>
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<para>When specified with the <command>decrypt</command> command control the credential name to
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validate the credential name embedded in the encrypted credential with. If not specified the name is
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chosen automatically from the filename component of the specified input path. If no credential name
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is embedded in the encrypted credential file (i.e. the <option>--name=</option> with an empty string
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was used when encrypted) the specified name has no effect as no credential name validation is
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done.</para>
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<para>Embedding the credential name in the encrypted credential is done in order to protect against
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reuse of credentials for purposes they weren't originally intended for, under the assumption the
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credential name is chosen carefully to encode its intended purpose.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--timestamp=</option><replaceable>timestamp</replaceable></term>
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<listitem><para>When specified with the <command>encrypt</command> command controls the timestamp to
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embed into the encrypted credential. Defaults to the current time. Takes a timestamp specification in
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the format described in
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<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
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<para>When specified with the <command>decrypt</command> command controls the timestamp to use to
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validate the "not-after" timestamp that was configured with <option>--not-after=</option> during
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encryption. If not specified defaults to the current system time.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--not-after=</option><replaceable>timestamp</replaceable></term>
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<listitem><para>When specified with the <command>encrypt</command> command controls the time when the
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credential shall not be used anymore. This embeds the specified timestamp in the encrypted
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credential. During decryption the timestamp is checked against the current system clock, and if the
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timestamp is in the past the decryption will fail. By default no such timestamp is set. Takes a
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timestamp specification in the format described in
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<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--with-key=</option></term>
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<term><option>-H</option></term>
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<term><option>-T</option></term>
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<listitem><para>When specified with the <command>encrypt</command> command controls the
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encryption/signature key to use. Takes one of <literal>host</literal>, <literal>tpm2</literal>,
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<literal>host+tpm2</literal>, <literal>tpm2-absent</literal>, <literal>auto</literal>,
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<literal>auto-initrd</literal>. See above for details on the three key types. If set to
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<literal>auto</literal> (which is the default) the TPM2 key is used if a TPM2 device is found and not
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running in a container. The host key is used if <filename>/var/lib/systemd/</filename> is on
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persistent media. This means on typical systems the encryption is by default bound to both the TPM2
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chip and the OS installation, and both need to be available to decrypt the credential again. If
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<literal>auto</literal> is selected but neither TPM2 is available (or running in container) nor
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<filename>/var/lib/systemd/</filename> is on persistent media, encryption will fail. If set to
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<literal>tpm2-absent</literal> a fixed zero length key is used (thus, in this mode no confidentiality
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nor authenticity are provided!). This logic is useful to cover for systems that lack a TPM2 chip but
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where credentials shall be generated. Note that decryption of such credentials is refused on systems
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that have a TPM2 chip and where UEFI SecureBoot is enabled (this is done so that such a locked down
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system cannot be tricked into loading a credential generated this way that lacks authentication
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information). If set to <literal>auto-initrd</literal> a TPM2 key is used if a TPM2 is found. If not
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a fixed zero length key is used, equivalent to <literal>tpm2-absent</literal> mode. This option is
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particularly useful to generate credentials files that are encrypted/authenticated against TPM2 where
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available but still work on systems lacking support for this.</para>
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<para>The <option>-H</option> switch is a shortcut for <option>--with-key=host</option>. Similar,
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<option>-T</option> is a shortcut for <option>--with-key=tpm2</option>.</para>
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<para>When encrypting credentials that shall be used in the initial RAM disk (initrd) where
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<filename>/var/lib/systemd/</filename> is typically not available make sure to use
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<option>--with-key=auto-initrd</option> mode, to disable binding against the host secret.</para>
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<para>This switch has no effect on the <command>decrypt</command> command, as information on which
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key to use for decryption is included in the encrypted credential already.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--tpm2-device=</option><replaceable>PATH</replaceable></term>
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<listitem><para>Controls the TPM2 device to use. Expects a device node path referring to the TPM2
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chip (e.g. <filename>/dev/tpmrm0</filename>). Alternatively the special value <literal>auto</literal>
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may be specified, in order to automatically determine the device node of a suitable TPM2 device (of
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which there must be exactly one). The special value <literal>list</literal> may be used to enumerate
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all suitable TPM2 devices currently discovered.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--tpm2-pcrs=</option><arg rep="repeat">PCR</arg></term>
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<listitem><para>Configures the TPM2 PCRs (Platform Configuration Registers) to bind the encryption
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key to. Takes a <literal>+</literal> separated list of numeric PCR indexes in the range 0…23. If not
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used, defaults to PCR 7 only. If an empty string is specified, binds the encryption key to no PCRs at
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all. For details about the PCRs available, see the documentation of the switch of the same name for
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<citerefentry><refentrytitle>systemd-cryptenroll</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para></listitem>
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</varlistentry>
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<xi:include href="standard-options.xml" xpointer="no-pager" />
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<xi:include href="standard-options.xml" xpointer="no-legend" />
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<xi:include href="standard-options.xml" xpointer="json" />
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Exit status</title>
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<para>On success, 0 is returned.</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<example>
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<title>Encrypt a password for use as credential</title>
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<para>The following command line encrypts the specified password <literal>hunter2</literal>, writing the result
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to a file <filename>password.cred</filename>.</para>
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<programlisting># echo -n hunter2 | systemd-creds encrypt - password.cred</programlisting>
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<para>This decrypts the file <filename>password.cred</filename> again, revealing the literal password:</para>
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<programlisting># systemd-creds decrypt password.cred
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hunter2</programlisting>
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</example>
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<example>
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<title>Encrypt a password and include it in a unit file</title>
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<para>The following command line prompts the user for a password and generates a
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<varname>SetCredentialEncrypted=</varname> line from it for a credential named
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<literal>mysql-password</literal>, suitable for inclusion in a unit file.</para>
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<programlisting># systemd-ask-password -n | systemd-creds encrypt --name=mysql-password -p - -
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🔐 Password: ****
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SetCredentialEncrypted=mysql-password: \
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k6iUCUh0RJCQyvL8k8q1UyAAAAABAAAADAAAABAAAAASfFsBoPLIm/dlDoGAAAAAAAAAA \
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NAAAAAgAAAAAH4AILIOZ3w6rTzYsBy9G7liaCAd4i+Kpvs8mAgArzwuKxd0ABDjgSeO5k \
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mKQc58zM94ZffyRmuNeX1lVHE+9e2YD87KfRFNoDLS7F3YmCb347gCiSk2an9egZ7Y0Xs \
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700Kr6heqQswQEemNEc62k9RJnEl2q7SbcEYguegnPQUATgAIAAsAAAASACA/B90W7E+6 \
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yAR9NgiIJvxr9bpElztwzB5lUJAxtMBHIgAQACCaSV9DradOZz4EvO/LSaRyRSq2Hj0ym \
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gVJk/dVzE8Uxj8H3RbsT7rIBH02CIgm/Gv1ukSXO3DMHmVQkDG0wEciyageTfrVEer8z5 \
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9cUQfM5ynSaV2UjeUWEHuz4fwDsXGLB9eELXLztzUU9nsAyLvs3ZRR+eEK/A==</programlisting>
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<para>The generated line can be pasted 1:1 into a unit file, and will ensure the acquired password will
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be made available in the <varname>$CREDENTIALS_DIRECTORY</varname><filename>/mysql-password</filename>
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credential file for the started service.</para>
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<para>Utilizing the unit file drop-in logic this can be used to securely pass a password credential to
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a unit. A similar, more comprehensive set of commands to insert a password into a service
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<filename>xyz.service</filename>:</para>
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<programlisting># mkdir -p /etc/systemd/system/xyz.service.d
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# systemd-ask-password -n | systemd-creds encrypt --name=mysql-password -p - - > /etc/systemd/system/xyz.service.d/50-password.conf
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# systemctl daemon-reload
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# systemctl restart xyz.service</programlisting>
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</example>
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</refsect1>
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<refsect1>
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|
<title>See Also</title>
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|
<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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</para>
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|
</refsect1>
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|
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</refentry>
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