Enable SHA-1 signatures in OpenSSL on Red Hat Enterprise Linux 10

Solution Verified - Updated

Environment

Red Hat Enterprise Linux 10
OpenSSL

Issue

Red Hat Enterprise Linux (RHEL) 10 has disabled the creation and verification of signatures that use the SHA-1 digest in OpenSSL by default. SHA-1 is Content from sha-mbles.github.io is not included.considered insecure (Github).

You can re-enable support for SHA-1 to ensure compatibility with legacy software and hardware that does not support newer digest algorithms.

Resolution

SHA-1 can be used in different contexts. Instructions to re-enable support for SHA-1 differ depending on whether it is required in TLS, or for other protocols that use OpenSSL.

Use outside of TLS (Kerberos, DNSSEC, …)

To re-enable SHA-1 signatures in a non-TLS context, create a new file /etc/pki/tls/openssl-sha1.cnf with the following content:

.include /etc/ssl/openssl.cnf
[evp_properties]
rh-allow-sha1-signatures = yes

Use in applications

Next, set the OPENSSL_CONF environment variable to /etc/pki/tls/openssl-sha1.cnf for the application or service that requires SHA-1 signatures. For applications, you can do this on the command line:

$ OPENSSL_CONF=/etc/pki/tls/openssl-sha1.cnf <your-application>

Use in services

For systemd services, create a systemd override file for the particular service using systemctl edit. For example, for the unbound.service, run systemctl edit unbound.service and in the editor that opens, add

[Service]
Environment=OPENSSL_CONF=/etc/pki/tls/openssl-sha1.cnf

Save and close the file, restart the service using systemctl restart. Then, check that setting the environment variable was successful:

# tr '\0' '\n' </proc/"$(systemctl show --property MainPID --value <servicename>)"/environ | grep OPENSSL_CONF

Allow SHA-1 system-wide

You can allow SHA-1 on the system by using the system-wide cryptographic policies. To do that, create a file /etc/crypto-policies/policies/modules/SHA1.pmod with the following content:

# This subpolicy adds SHA1 hash and signature support

hash = SHA1+
sign = ECDSA-SHA1+ RSA-PSS-SHA1+ RSA-SHA1+
sha1_in_certs = 1

Apply the previous configuration changes by entering the update-crypto-policies --set DEFAULT:SHA1 command. If the update-crypto-policies tool is not available, install the crypto-policies-scripts package.

Note that this lowers the security of the entire operating system.

Use of SHA-1 in TLS

Two major parts of TLS use SHA-1 signatures: X.509 certificates, and TLS signature algorithms.

If your network depends on certificates signed with SHA-1, modernize the certificate chain. The CA/Browser forum, which defines requirements for publicly accepted certificate authorities, Content from cabforum.org is not included.has adopted a ballot (cabforum.org) to phase out certificates signed with SHA-1 by January 1st, 2016. Private certificate authorities that still use SHA-1 should be replaced.

The use of SHA-1 as a TLS signature algorithm is commonly required with TLS versions lower than 1.2, which do not support the negotiation of a signature algorithm and always use a combination of the SHA-1 and MD-5 digests. TLS 1.2 introduced the Content from www.rfc-editor.org is not included.signature_algorithms extension, which allows the negotiation of modern hash algorithms.

If your scenario requires support for a certificate chain that uses SHA-1, TLS versions earlier than 1.2, or TLS 1.2 with a very limited set of supported signature algorithms, create a new file /etc/pki/tls/openssl-sha1-tls.cnf with the following content:

.include /etc/pki/tls/openssl.cnf

[openssl_init]
ssl_conf = ssl_module_sha1

[evp_properties]
rh-allow-sha1-signatures = yes

[ssl_module_sha1]
system_default = crypto_policy_sha1

[crypto_policy_sha1]
# @SECLEVEL=0 is required to allow SHA-1, the rest are copied from the DEFAULT policy but can be adjusted as required
CipherString = @SECLEVEL=0:kEECDH:kEDH:kPSK:kDHEPSK:kECDHEPSK:kRSAPSK:-kRSA:-aDSS:-3DES:!DES:!RC4:!RC2:!IDEA:-SEED:!eNULL:!aNULL:!MD5:-SHA384:-CAMELLIA:-ARIA:-AESCCM8
Ciphersuites = TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256:TLS_AES_128_CCM_SHA256
# TLS < 1.2 can be enabled here, if required
TLS.MinProtocol = TLSv1.2
TLS.MaxProtocol = TLSv1.3
DTLS.MinProtocol = DTLSv1.2
DTLS.MaxProtocol = DTLSv1.2
# Copied from the DEFAULT policy, with ECDSA+SHA1:RSA+SHA1 added at the end
SignatureAlgorithms = ECDSA+SHA256:ECDSA+SHA384:ECDSA+SHA512:ed25519:ed448:rsa_pss_pss_sha256:rsa_pss_pss_sha384:rsa_pss_pss_sha512:rsa_pss_rsae_sha256:rsa_pss_rsae_sha384:rsa_pss_rsae_sha512:RSA+SHA256:RSA+SHA384:RSA+SHA512:ECDSA+SHA224:RSA+SHA224:ECDSA+SHA1:RSA+SHA1
Groups = X25519:secp256r1:X448:secp521r1:secp384r1:ffdhe2048:ffdhe3072:ffdhe4096:ffdhe6144:ffdhe8192

Then, follow the previously described steps in the "Use in applications" or "Use in services" sections to use this file in your application or service.

Note that setting @SECLEVEL=0 in the CipherString option is required to enable SHA-1, but this setting also enables many other insecure algorithms. To lower the attack surface, run services that require such a configuration behind a proxy that supports modern TLS.

Root Cause

Signatures using the SHA-1 hash algorithm have been disabled by default in Red Hat Enterprise Linux 9. The SHA1 system-wide crypto-policies module that existed in Red Hat Enterprise Linux 9 can no longer enable SHA-1 in TLS in OpenSSL because this requires setting @SECLEVEL=0 due to an upstream change, which also allows other insecure algorithms.

Diagnostic Steps

The following error messages might be indicators of this problem:

verify error:num=68:CA signature digest algorithm too weak
verify error:num=7:certificate signature failure
007621A1FFFF0000:error:0A000076:SSL routines:tls12_copy_sigalgs:no suitable signature algorithm:ssl/t1_lib.c:2648:
0026EFB8FFFF0000:error:0A000410:SSL routines:ssl3_read_bytes:ssl/tls alert handshake failure:ssl/record/rec_layer_s3.c:909:SSL alert number 40
0006F986FFFF0000:error:0A000178:SSL routines:tls1_set_server_sigalgs:no shared signature algorithms:ssl/t1_lib.c:2181:
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