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1.6.4 Ensure system wide crypto policy disables macs less than 128 bits

Information

Message Authentication Code (MAC) algorithm is a family of cryptographic functions that is parameterized by a symmetric key. Each of the functions can act on input data (called a 'message') of variable length to produce an output value of a specified length. The output value is called the MAC of the input message.

A MAC algorithm can be used to provide data-origin authentication and data-integrity protection

Rationale:

Weak algorithms continue to have a great deal of attention as a weak spot that can be exploited with expanded computing power. An attacker that breaks the algorithm could take advantage of a MiTM position to decrypt the tunnel and capture credentials and information.

A MAC algorithm must be computationally infeasible to determine the MAC of a message without knowledge of the key, even if one has already seen the results of using that key to compute the MAC's of other messages.

Solution

Note:

The commands below are written for the included DEFAULT system-wide crypto policy. If another policy is in use and follows local site policy, replace DEFAULT with the name of your system-wide crypto policy.

Multiple subpolicies may be assigned to a policy as a colon separated list. e.g. DEFAULT:NO-SHA1:NO-SSHCBC:NO-WEAKMAC

Any subpolicy not included in the update-crypto-policies --set command will not be applied to the system wide crypto policy.

Subpolicies must exist before they can be applied to the system wide crypto policy.

Create or edit a file in /etc/crypto-policies/policies/modules/ ending in .pmod and add or modify one of the following lines:

mac = -*-64* # Disables weak macs

Example:

# echo -e '# This is a subpolicy to disable weak macs
mac = -*-64' > /etc/crypto-policies/policies/modules/NO-WEAKMAC.pmod

Run the following command to update the system-wide cryptographic policy

# update-crypto-policies --set <CRYPTO_POLICY>:<CRYPTO_SUBPOLICY1>:<CRYPTO_SUBPOLICY2>:<SUBPOLICY3>

Example:

update-crypto-policies --set DEFAULT:NO-SHA1:NO-SSHCBC:NO-WEAKMAC

Run the following command to reboot the system to make your cryptographic settings effective for already running services and applications:

# reboot

See Also

https://workbench.cisecurity.org/benchmarks/15287

Item Details

Category: ACCESS CONTROL, IDENTIFICATION AND AUTHENTICATION, SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|AC-17(2), 800-53|IA-5, 800-53|IA-5(1), 800-53|SC-8, 800-53|SC-8(1), CSCv7|14.4

Plugin: Unix

Control ID: bda522785fe1ae3d28918a4e963c5ef605b8fc926debcfec2fd958d9a0393f8b