Provided by: crypto-policies_20190816git-1_all 
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).
CRYPTO POLICY DEFINITON FORMAT
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.