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Use a random IV when encrypting values

Nick Malcolm requested to merge 412174-iv-improvements into master

What does this MR do and why?

The IV itself can be safely transmitted in public, but it should be unpredictable to prevent certain kinds of attacks. You may use #random_iv to create a secure random IV.

https://ruby-doc.org/stdlib-2.4.0/libdoc/openssl/rdoc/OpenSSL/Cipher.html#method-i-iv-3D

Since Create new way of encrypting tokens - phase 1 -... (!58896 - merged) we derive the iv from the hexdigest of the token. Since the tokens are random (e.g. Devise.friendly_token), the IV is random too - that makes this MR a defense-in-depth best practice (not fixing a bugvulnerability).

We should assume that at some point we might have a part of our code base that either can encrypt the same token, or generates a colliding token, and therefore use a random IV.

Before

3.times do
  token = TokenAuthenticatableStrategies::EncryptionHelper.encrypt_token(plaintext)
  puts "Encrypted: #{token}"
  puts "Decrypted: #{TokenAuthenticatableStrategies::EncryptionHelper.decrypt_token(token)}"
end

Encrypted: |sKGWTg2G3hwRKRp3dTVPJPjT0g==2c26b46b68ff
Decrypted: foo
Encrypted: |sKGWTg2G3hwRKRp3dTVPJPjT0g==2c26b46b68ff
Decrypted: foo
Encrypted: |sKGWTg2G3hwRKRp3dTVPJPjT0g==2c26b46b68ff
Decrypted: foo

After

Encrypted: |y8r+WmCQhfECq2oTvenracVSAg==7917cd9ff7f9
Decrypted: foo
Encrypted: |9Eup9OumJO3PTZhQRs9We1IWAA==01b611e4c3d2
Decrypted: foo
Encrypted: |7bqIrs3fI0i55cZoIFamUN5yAA==07360e19bb26
Decrypted: foo

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Before After

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