CRYPTO-POLICIES(7)

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 1.2 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

    • ssh_group: Optional; list of allowed groups or elliptic curves for key exchanges for use with the SSH protocol. If absent, the value is derived from group.

    • 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

    • tls_cipher: Optional; list of allowed symmetric encryption algorithms (including the modes) for use with the TLS protocol. If absent, the value is derived from cipher.

    • ssh_cipher: Optional; list of allowed symmetric encryption algorithms (including the modes) for use with the SSH protocol. If absent, the value is derived from cipher.

    • 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 (ignored by OpenSSL and NSS back ends)

    • ike_protocol: List of allowed IKE protocol versions

    • min_tls_version: Lowest allowed TLS protocol version (used only by OpenSSL a and NSS back ends)

    • min_dtls_version: Lowest allowed DTLS protocol version (used only by NSS back end)

    • 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 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+: The list-item is appended to the end of the list specified in the base policy.

To completely override a list value in a module file just use list-items without any sign. Combining list-items with and without signs in a single list value assignment is not allowed however an existing list value can be modified in multiple further assignments.

Non-list key values in the policy module files are simply overriden.

The keys marked as Optional can be omitted in the policy definition files. In that case, the values will be derived from the base keys. Note that, this value propagation only applies to the policy definition files. In the policy module files, each key that needs modification must be explicitly specified.

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 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

Notable irregularities in the individual configuration generators

    • 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.

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.

/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.