thread not safe error: OpenSSL::Cipher::CipherError: Cipher not initialized

[merlixelastic]

Products/Versions (include any installed plugins):
Logstash 6.7 (also reproduce on Logstash 6.5.1)
logstash-filter-prune 3.0.3
logstash-filter-mutate 3.5.0
logstash-filter-cipher 4.0.0

Operating system:
Linux Ubuntu

Description:
The following use of filters is triggering errors on the cipher plugin:

                 prune {
			blacklist_values => [ "message", "-"]
		}
		mutate {
			lowercase => ["message"]
		}
		cipher {
		    algorithm => "aes-256-cbc"
			cipher_padding => 1
			iv_random_length => 16
			key => "12345678901234567890123456789012"
			key_size => 32
			mode => "encrypt"
			source => "message"
			target => "encrypted_message"
			base64 => true
			max_cipher_reuse => 1
		}

A number of errror are seen such as:

[2019-06-19T14:48:52,234][WARN ][logstash.filters.cipher  ] Exception catch on cipher filter {:event=>#<LogStash::Event:0x57f70d58>, :error=>#<OpenSSL::Cipher::CipherError: Cipher not initialized>}
[2019-06-19T14:48:52,234][WARN ][logstash.filters.cipher  ] Exception catch on cipher filter {:event=>#<LogStash::Event:0x452f4cbd>, :error=>#<OpenSSL::Cipher::CipherError: No message available>}
[2019-06-19T14:48:52,284][WARN ][logstash.filters.cipher  ] Exception catch on cipher filter {:event=>#<LogStash::Event:0x7c5477d9>, :error=>#<NoMethodError: undefined method `update' for nil:NilClass>}

Important note: it is noticed the error is not seen is the number of worker is set to 1.
The test was done by setting the following value in logstash.yml file:

pipeline.workers:1

Hence the issue appears to be related to the fact we are not calling OpenSSL method in a thread safe way like the github issue linked above.

Steps to reproduce:
Setup a pipeline to parse logs as per tutorial below:
https://www.elastic.co/guide/en/logstash/current/advanced-pipeline.html
Then edit the conf file to use filters described above.
Run the pipeline.