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 at the cost of being less secure. It allows the
TLS 1.2, and
TLS 1.3 protocols, as well as
IKEv2 and
SSH2.
DSA
is not allowed, while
RSA and
Diffie-Hellman parameters are
accepted if no less than 2048 bits. This policy provides at least 80-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 (no DSA)
•Ciphers: all available >= 112-bit key, >=
128-bit block (excluding 3DES and RC4)
•Key exchange: ECDHE, RSA,
DHE (no DHE-DSS)
•DH params size: >= 2048
•RSA keys size: >= 2048
•TLS protocols: TLS >= 1.2,
DTLS >= 1.2
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
Diffie-Hellman
parameters are accepted if they are at least 2048 bits long. This policy
provides at least 112-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 SHA-224 hash or
better (no DSA)
•TLS Ciphers: >= 128-bit key, >=
128-bit block (AES, ChaCha20, including AES-CBC)
•non-TLS Ciphers: as TLS Ciphers
•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
requirements. This policy is used internally by the
fips-mode-setup(8)
tool which can switch the system into the
FIPS 140 mode. This policy
provides at least 112-bit security.
•MACs: SHA-256 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, excluding 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).
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
•etm:
ANY/DISABLE_ETM/DISABLE_NON_ETM allows both EtM
(Encrypt-then-Mac) and E&M (Encrypt-and-Mac), disables EtM, and disables
E&M respectively. (Currently only implemented for SSH, do not use without
@SSH scope.)
Full policy definition files have suffix .pol, subpolicy files
have suffix .pmod. Subpolicies 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 subpolicies in
/usr/share/crypto-policies/policies/modules.
Locally configured policy files should be placed in
/etc/crypto-policies/policies and subpolicies in
/etc/crypto-policies/policies/modules.
The policy and subpolicy 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 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:
•DH with parameters < 2048 bits
•RSA with key size < 2048 bits
•Camellia
•RC4
•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.
•GnuTLS: PSK key exchanges have to be
explicitly enabled by the applications using them.
•GnuTLS: HMAC-SHA2-256 and HMAC-SHA2-384
MACs are disabled due to concerns over the constant-timedness of the
implementation.
•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.
•Libreswan: The key_exchange
parameter does not affect the generated configuration. The use of regular
DH or ECDH can be limited with appropriate setting of the
group parameter.
•NSS: order of group values is
ignored and built-in order is used instead.
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).
•ssh_etm: Value of 1 if OpenSSH EtM
(encrypt-then-mac) extension is allowed, 0 otherwise. Use etm@SSH
instead.
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 definition files.
/etc/crypto-policies/policies
Custom policy definition files as configured by the
system administrator.
/etc/crypto-policies/policies/modules
Custom subpolicy definition files as configured by the
system administrator.
/usr/share/crypto-policies/<'POLICYNAME'>
Pre-generated back-end configurations for policy
POLICYNAME.
AUTHOR¶
Written by Tomáš Mráz.