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Set crl_check to best_effort #38

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Set crl_check to best_effort #38

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8 changes: 5 additions & 3 deletions docs/secure_coding_and_deployment_hardening/ssl.md
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Original file line number Diff line number Diff line change
Expand Up @@ -60,7 +60,7 @@ Make sure to test the selected options against test endpoints, such as those pro

## Revocation check

One scenario that’s not handled by the above examples is certificate revocation: no revocation check is performed, and therefore a revoked but otherwise valid certificate would be accepted. It is possible to check certificates against the CA’s Certificate Revocation List (CRL) by setting the `crl_check` option to true. This also requires the `crl_cache` to be configured:
One scenario that’s not handled by the above examples is certificate revocation: no revocation check is performed, and therefore a revoked but otherwise valid certificate would be accepted. It is possible to check certificates against the CA’s Certificate Revocation List (CRL) by setting the `crl_check` option to `best_effort`. This also requires the `crl_cache` to be configured:

```erlang
%% Erlang
Expand All @@ -71,7 +71,7 @@ ssl:connect("revoked.badssl.com", 443, [
{customize_hostname_check, [
{match_fun, public_key:pkix_verify_hostname_match_fun(https)}
]},
{crl_check, true},
{crl_check, best_effort},
{crl_cache, {ssl_crl_cache, {internal, [{http, 1000}]}}}
]).
```
Expand All @@ -85,11 +85,13 @@ ssl:connect("revoked.badssl.com", 443, [
customize_hostname_check: [
match_fun: :public_key.pkix_verify_hostname_match_fun(:https)
],
crl_check: true,
crl_check: :best_effort,
crl_cache: {:ssl_crl_cache, {:internal, [http: 1000]}}
)
```

The stricter `true` can be used instead of `best_effort`: in this case validation will fail if CRL is missing, which can happen if the certificate has no CRL or exclusively uses OCSP.

However, please note that the `ssl_crl_cache` module does not actually cache the CRL contents, so each handshake will trigger a new CRL lookup, which impacts the performance and reliability of TLS connections. In applications that require revocation checks as well as high throughput a custom CRL cache implementation will be needed.

## Selecting protocol versions and ciphers
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