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.
The individual policy levels (DEFAULT, LEGACY,
FUTURE, and FIPS) are included in the
crypto-policies(7) package. In the future, there will be also a
mechanism for easy creation and deployment of policies defined by the system
administrator or a third party vendor.
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
•GnuTLS TLS library
•OpenJDK runtime environment
•Kerberos 5 library
•Libreswan IPsec and IKE protocol
implementation
•NSS TLS library
•OpenSSH SSH2 protocol implementation
•OpenSSL TLS library
•libssh SSH2 protocol implementation
Applications using the above libraries and tools are covered by
the cryptographic policies unless they are explicitly configured not to be
so.
PROVIDED POLICY LEVELS¶
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. The
level 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 1.0,
TLS 1.1,
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 1023 bits long. The level 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 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: >= 1023
•RSA keys size: >= 2048
•TLS protocols: TLS >= 1.0,
DTLS >= 1.0
NEXT
The
NEXT policy is a policy prepared for the
upcoming release of the operating system so it can be easily tested. 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 level that is believed to
withstand any near-term future attacks. This level does not allow the use of
SHA-1 in signature algorithms. The level 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. The level 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
•non-TLS Ciphers: same as TLS ciphers with
added non AE ciphers and Camellia
•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 level that conforms 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 compliance mode.
The level 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).
The crypto policy definiton 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 can be either decimal integers,
arbitrary strings, or lists of strings without whitespace characters separated
by any number of whitespaces.
The allowed keys are:
•mac: List of allowed MAC algorithms
•group: List of allowed groups or elliptic
curves for key exchanges
•hash: List of allowed cryptographic hash
(message digest) algorithms
•sign: List of allowed signature
algorithms
•tls_cipher: List of allowed symmetric
encryption algorithms (including the modes) for use with the TLS
protocol
•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 and DTLS
protocol versions
•ike_protocol: List of allowed IKE protocol
versions
•min_tls_version: Lowest allowed TLS
protocol version
•min_dtls_version: Lowest allowed DTLS
protocol version
•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.)
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 lists as set in the base (full policy) are modified by the
lists specified in the module files in following way:
•-list-item: The list-item is
removed from the list specified in the base policy.
•+list-item: The list-item is
inserted at the beginning of the list specified in the base policy. The
inserts are done in the order of appearance in the policy module file so the
actual order in the final list will be reversed.
•list-item or list-item+: The
list-item is appended to the end of the list specified in the base
policy.
Non-list key values in the policy module files are simply
overridden.
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 level.
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 level which limits the allowed algorithms and protocols to these
allowed by the FIPS 140-2 requirements.
NOTES¶
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 level
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 level 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 policy
levels
The following ciphersuites and protocols are available but
disabled in all crypto policy levels. They can be enabled only by explicit
configuration of individual applications:
•DH with parameters < 1024 bits
•RSA with key size < 1024 bits
•Camellia
•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
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
The active crypto-policies level 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.
SEE ALSO¶
update-crypto-policies(8), fips-mode-setup(8)
AUTHOR¶
Written by Tomáš Mráz.
