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crypto-policies(7)
==================
:doctype: manpage
:man source: crypto-policies


NAME
----
crypto-policies - system-wide crypto policies overview


DESCRIPTION
-----------
The security of cryptographic components of the operating system
does not remain constant over time. Algorithms, such as
cryptographic hashing and encryption, typically have a lifetime,
after which they are considered either too risky to use or plain insecure.
That means, we need to phase out such algorithms from the default
settings or completely disable them if they could cause an irreparable problem.

While in the past the algorithms were not disabled in a consistent way
and different applications applied different policies, the system-wide
crypto-policies followed by the crypto core components allow consistently
deprecating and disabling algorithms system-wide.

Several preconfigured policies (*DEFAULT*, *LEGACY*, *FUTURE*, and *FIPS*)
and subpolicies are included in the *crypto-policies(7)* package.
System administrators or third-party vendors can define custom policies.

For rationale, see *RFC 7457* for a list of attacks taking advantage of
legacy crypto algorithms.

COVERED APPLICATIONS
--------------------

Crypto-policies apply to the configuration of the core cryptographic subsystems,
covering *TLS*, *IKE*, *IPSec*, *DNSSec*, and *Kerberos* protocols; i.e., the
supported secure communications protocols on the base operating system.

Once an application runs in the operating system, it follows the default
or selected policy and refuses to fall back to algorithms and protocols not
within the policy, unless the user has explicitly requested the application
to do so. That is, the policy applies to the default behavior of applications
when running with the system-provided configuration but the user can override
it on an application-specific basis.

The policies currently provide settings for these applications and libraries:

* *BIND* DNS name server daemon (scopes: *BIND*, *DNSSec*)
* *GnuTLS* TLS library (scopes: *GnuTLS*, *SSL*, *TLS*)
* *OpenJDK* runtime environment (scopes: *java-tls*, *SSL*, *TLS*)
* *Kerberos 5* library (scopes: *krb5*, *Kerberos*)
* *Libreswan* IPsec and IKE protocol implementation (scopes: *libreswan*, *IPSec*, *IKE*)
* *NSS* TLS library (scopes: *NSS*, *SSL*, *TLS*)
* *OpenSSH* SSH2 protocol implementation (scopes: *OpenSSH*, *SSH*)
* *OpenSSL* TLS library (scopes: *OpenSSL*, *SSL*, *TLS*)
* *libssh* SSH2 protocol implementation (scopes: *libssh*, *SSH*)

Applications using the above libraries and tools are covered by the
cryptographic policies unless they are explicitly configured otherwise.

PROVIDED POLICIES
-----------------

*LEGACY*::
  This policy ensures maximum compatibility with legacy systems; it is
  less secure and it includes support for *TLS 1.0*, *TLS 1.1*, and *SSH2*
  protocols or later. The algorithms *DSA*, *3DES*, and *RC4* are allowed,
  while *RSA* and *Diffie-Hellman* parameters are accepted if larger than 1023 bits.
  This policy provides at least 64-bit security.

  * MACs: all *HMAC* with *SHA-1* or better + all modern MACs (*Poly1305*
    etc.)
  * Curves: all prime >= 255 bits (including Bernstein curves)
  * Signature algorithms: with *SHA1* hash or better (*DSA* allowed)
  * *TLS* Ciphers: all available >= 112-bit key, >= 128-bit block (including
    *RC4* and *3DES*)
  * Non-TLS Ciphers: same as *TLS* ciphers with added *Camellia*
  * Key exchange: *ECDHE*, *RSA*, *DHE*
  * *DH* params size: >= 1023
  * *RSA* keys size: >= 1023
  * *DSA* params size: >= 1023
  * *TLS* protocols: *TLS* >= 1.0, *DTLS* >= 1.0

*DEFAULT*::
  The *DEFAULT* policy is a reasonable default policy for today's standards.
  It allows the *TLS 1.2* and *TLS 1.3* protocols, as well as *IKEv2*
  and *SSH2*. The *RSA* and *Diffie-Hellman* parameters
  are accepted if larger than 2047 bits.
  The level provides at least 112-bit security with the exception of *SHA-1*
  signatures needed for *DNSSec* and other still prevalent legacy use of
  *SHA-1* signatures.

  * MACs: all *HMAC* with *SHA-1* or better + all modern MACs (*Poly1305*
    etc.)
  * Curves: all prime >= 255 bits (including Bernstein curves)
  * Signature algorithms: with *SHA-1* hash or better (no *DSA*)
  * *TLS* Ciphers: >= 128-bit key, >= 128-bit block (*AES*, *ChaCha20*,
    including *AES-CBC*)
  * non-TLS Ciphers: as *TLS* Ciphers with added *Camellia*
  * key exchange: *ECDHE*, *RSA*, *DHE* (no *DHE-DSS*)
  * *DH* params size: >= 2048
  * *RSA* keys size: >= 2048
  * *TLS* protocols: *TLS* >= 1.2, *DTLS* >= 1.2

*FUTURE*::
  A conservative security policy that is believed to withstand any near-term
  future attacks. This policy does not allow the use of *SHA-1* in signature
  algorithms. The policy also provides some (not complete) preparation for
  post-quantum encryption support in form of 256-bit symmetric encryption
  requirement. The *RSA* and *Diffie-Hellman* parameters are accepted if
  larger than 3071 bits. This policy provides at least 128-bit security.

  * MACs: all *HMAC* with *SHA-256* or better + all modern MACs (*Poly1305*
    etc.)
  * Curves: all prime >= 255 bits (including Bernstein curves)
  * Signature algorithms: with *SHA-256* hash or better (no *DSA*)
  * *TLS* Ciphers: >= 256-bit key, >= 128-bit block, only Authenticated
    Encryption (AE) ciphers, no *CBC* ciphers
  * non-TLS Ciphers: same as *TLS* ciphers with added non AE ciphers,
    *CBC* ones enabled only in Kerberos
  * key exchange: *ECDHE*, *DHE* (no *DHE-DSS*, no *RSA*)
  * *DH* params size: >= 3072
  * *RSA* keys size: >= 3072
  * *TLS* protocols: *TLS* >= 1.2, *DTLS* >= 1.2


*FIPS*::
  A policy to aid conformance to the *FIPS 140-2* requirements.
  This policy is used internally by the *fips-mode-setup(8)* tool
  which can switch the system into the *FIPS 140-2* mode.
  This policy provides at least 112-bit security.

  * MACs: all *HMAC* with *SHA1* or better
  * Curves: all prime >= 256 bits
  * Signature algorithms: with *SHA-256* hash or better (no *DSA*)
  * *TLS* Ciphers: >= 128-bit key, >= 128-bit block (*AES*,
    including *AES-CBC*)
  * non-TLS Ciphers: same as *TLS* Ciphers
  * key exchange: *ECDHE*, *DHE* (no *DHE-DSS*, no *RSA*)
  * *DH* params size: >= 2048
  * *RSA* params size: >= 2048
  * *TLS* protocols: *TLS* >= 1.2, *DTLS* >= 1.2

*EMPTY*::
  All cryptographic algorithms are disabled (used for debugging only, do not
  use).

CRYPTO POLICY DEFINITON FORMAT
------------------------------

The crypto policy definition files have a simple syntax following an *INI* file
'key' = 'value' syntax with these particular features:

  * Comments are indicated by '#' character. Everything on the line following the
    character is ignored.
  * Backslash '\' character followed immediately with the end-of-line character
    indicates line continuation. The following line is concatenated to the current
    line after the backslash and end-of-line characters are removed.
  * Value types for integer options can be decimal integers ('option = 1').
  * Multiple-choice options can be specified
    by setting them to a list of values ('option = value1 value2').
    This list can further be altered by prepending/omitting/appending values
    ('option = +prepended -omitted appended+').
    A follow-up reassignment will reset the list.
    The latter syntax cannot be combined with the former one
    in the same directive.
    Setting an option to an empty list is possible with 'option ='.
  * Asterisk sign can be used for wildcard matching as a shortcut
    for specifying multiple values when setting multiple-choice options.
    Note that wildcard matching can lead to future updates
    implicitly enabling algorithms not yet available in the current version.
    If this is a concern, do not use wildcard-matching
    outside of algorithm-omitting directives.
  * In order to limit the scope of the directive
    and make it affect just some of the backends,
    the following extended syntax can be used: 'option@scope = ...',
    'option@{scope1,scope2,...} = ...'.
    Negation of scopes is possible with
    'option@!scope' / 'option@{scope1,scope2,...}.
    Scope selectors are case-insensitive.

The available options are:

  * *mac*: List of allowed MAC algorithms
  * *group*: List of allowed groups or elliptic curves for key exchanges
    for use with other protocols
  * *hash*: List of allowed cryptographic hash (message digest) algorithms
  * *sign*: List of allowed signature algorithms
  * *cipher*: List of allowed symmetric encryption algorithms (including the modes)
    for use with other protocols
  * *key_exchange*: List of allowed key exchange algorithms
  * *protocol*: List of allowed TLS, DTLS and IKE protocol versions;
    mind that some backends do not allow
    selectively disabling protocols versions
    and only use the oldest version as the lower boundary.
  * *min_dh_size*: Integer value of minimum number of bits of parameters
    for *DH* key exchange
  * *min_dsa_size*: Integer value of minimum number of bits for *DSA* keys
  * *min_rsa_size*: Integer value of minimum number of bits for *RSA* keys
  * *sha1_in_certs*: Value of 1 if *SHA1* allowed in certificate signatures,
    0 otherwise (Applies to *GnuTLS* back end only.)
  * *arbitrary_dh_groups*: Value of 1 if arbitrary group in
    *Diffie-Hellman* is allowed, 0 otherwise
  * *ssh_certs*: Value of 1 if *OpenSSH* certificate authentication is
     allowed, 0 otherwise
  * *ssh_etm*: Value of 1 if *OpenSSH* EtM (encrypt-then-mac) extension is
     allowed, 0 otherwise

The full policy definition files have suffix `.pol`, the policy module definition
files have suffix `.pmod`. The policy module files do not have to have values
set for all the keys listed above.

The effective configuration of a policy with subpolicies applied
is the same as a configuration from a single policy
obtained by concatenating the policy and the subpolicies in question.

*Policy file placement and naming:*

The policy files shipped in packages are placed in
`/usr/share/crypto-policies/policies` and the policy modules in
`/usr/share/crypto-policies/policies/modules`.

The locally configured policy files are placed in
`/etc/crypto-policies/policies` and the policy modules in
`/etc/crypto-policies/policies/modules`.

The policy and policy module files must have names in upper-case except
for the `.pol` and `.pmod` suffix as the update-crypto-policies command
always converts the policy name to upper-case before searching for
the policy on the filesystem.

COMMANDS
--------

*update-crypto-policies(8)*::
  This command manages the policies available to the various cryptographic
  back ends and allows the system administrator to change the active
  cryptographic policy.

*fips-mode-setup(8)*::
  This command allows the system administrator to enable, or disable the
  system FIPS mode and also apply the *FIPS* cryptographic policy
  which limits the allowed algorithms and protocols to these allowed by
  the FIPS 140-2 requirements.


NOTES
-----

*Known notable exceptions*

* *Go-language* applications do not yet follow the system-wide policy.
* *GnuPG-2* application does not follow the system-wide policy.

In general only the data-in-transit is currently covered by the system-wide
policy.

If the system administrator changes the system-wide policy with
the *update-crypto-policies(8)* command it is advisable to restart the
system as the individual back-end libraries read the configuration files
usually during their initialization. The changes in the policy
thus take place in most cases only when the applications using the back-end
libraries are restarted.

*Removed cipher suites and protocols*

The following cipher suites and protocols are completely removed from the
core cryptographic libraries listed above:

* *DES*
* All export grade cipher suites
* *MD5* in signatures
* *SSLv2*
* *SSLv3*
* All *ECC* curves smaller than 224 bits
* All binary field *ECC* curves

*Cipher suites and protocols disabled in all predefined policies*

The following ciphersuites and protocols are available but disabled in all
predefined crypto policies. They can be enabled only by explicit configuration
of individual applications:

* *DH* with parameters < 1024 bits
* *RSA* with key size < 1024 bits
* *ARIA*
* *SEED*
* *IDEA*
* Integrity only ciphersuites
* *TLS* *CBC mode* ciphersuites using *SHA-384* HMAC
* *AES-CCM8*
* all *ECC* curves incompatible with *TLS 1.3*, including secp256k1
* *IKEv1*

*Notable irregularities in the individual configuration generators*

* *OpenSSL* and *NSS*: Disabling all TLS and/or all DTLS versions
isn't actually possible.
Trying to do so will result in the library defaults being applied instead.

* *OpenSSL*: The minimum length of the keys and some other parameters
are enforced by the @SECLEVEL value which does not provide a fine
granularity. The list of *TLS* ciphers is not generated as an exact list
but by subtracting from all the supported ciphers for the enabled key
exchange methods. For that reason there is no way to disable a random cipher.
In particular all *AES-128* ciphers are disabled if the *AES-128-GCM* is not
present in the list; all *AES-256* ciphers are disabled if the *AES-256-GCM*
is not present. The *CBC* ciphers are disabled if there isn't *HMAC-SHA1*
in the hmac list and *AES-256-CBC* in the cipher list. To disable the *CCM*
ciphers both *AES-128-CCM* and *AES-256-CCM* must not be present in the cipher
list.

* *GnuTLS*: The minimum length of the keys and some other parameters
are enforced by min-verification-profile setting in the *GnuTLS* configuration
file which does not provide fine granularity.

* *OpenSSH*: *DH* group 1 is always disabled on server even if the policy
allows 1024 bit *DH* groups in general. The OpenSSH configuration option
HostKeyAlgorithms is set only for the *SSH* server as otherwise the
handling of the existing known hosts entries would be broken on client.

HISTORY
-------

The *ECDHE-GSS* and *DHE-GSS* algorithms are newly introduced and must be
specified in the base policy for the SSH GSSAPI key exchange methods
to be enabled. Previously the legacy SSH GSSAPI key exchange methods were
automatically enabled when the *SHA1* hash and *DH* parameters of at least
2048 bits were enabled.

Before the introduction of the *custom crypto policies* support it
was possible to have an completely arbitrary crypto policy created as
a set of arbitrary back-end config files in
`/usr/share/crypto-policies/<POLICYNAME>` directory. With the introduction
of the *custom crypto policies* it is still possible but there must
be an empty (possibly with any comment lines) `<POLICYNAME>.pol` file
in `/usr/share/crypto-policies/policies` so the `update-crypto-policies`
command can recognize the arbitrary custom policy.
No subpolicies must be used with such an arbitrary custom policy.
Modifications from *local.d* will be appended
to the files provided by the policy.

The use of the following historaically available options is discouraged:

  * *min_tls_version*: Lowest allowed TLS protocol version
    (recommended replacement: *protocol@TLS*)
  * *min_dtls_version*: Lowest allowed DTLS protocol version
    (recommended replacement: *protocol@TLS*)

The following options are deprecated, please rewrite your policies:

  * *ike_protocol*: List of allowed IKE protocol versions
    (recommended replacement: *protocol@IKE*,
    mind the relative position to other *protocol* directives).
  * *tls_cipher*: list of allowed symmetric encryption algorithms
    for use with the TLS protocol
    (recommended replacement: *cipher@TLS*,
    mind the relative position to other *cipher* directives).
  * *ssh_cipher*: list of allowed symmetric encryption algorithms
    for use with the SSH protocol
    (recommended replacement: *cipher@SSH*,
    mind the relative position to other *cipher* directives).
  * *ssh_group*: list of allowed groups or elliptic curves for key exchanges
    for use with the SSH protocol
    (recommended replacement: *group@SSH*,
    mind the relative position to other *group* directives).
  * *sha1_in_dnssec*: Allow *SHA1* usage in DNSSec protocol even if it is
    not present in the *hash* and *sign* lists
    (recommended replacements: *hash@DNSSec*, *sign@DNSSec*).

FILES
-----
/etc/crypto-policies/back-ends::
	The individual cryptographical back-end configuration files.
	Usually linked to the configuration shipped in the crypto-policies
	package unless a configuration from `local.d` is added.

/etc/crypto-policies/config::
	A file containing the name of the active crypto-policy
	set on the system.

/etc/crypto-policies/local.d::
	Additional configuration shipped by other packages or created by
	the system administrator. The contents of the `<back-end>-file.config`
	is appended to the configuration from the policy back end as
	shipped in the crypto-policies package.

/usr/share/crypto-policies/policies::
	System policy definition files.

/usr/share/crypto-policies/policies/modules::
	System subpolicy module definition files.

/etc/crypto-policies/policies::
	Custom policy definition files as configured by the system
	administrator.

/etc/crypto-policies/policies/modules::
	Custom subpolicy module definition files as configured by the system
	administrator.

/usr/share/crypto-policies/<'POLICYNAME'>::
	Pre-generated back-end configurations for policy 'POLICYNAME'.

SEE ALSO
--------
update-crypto-policies(8), fips-mode-setup(8)


AUTHOR
------
Written by Tomáš Mráz.
