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89 lines
4.1 KiB
Markdown
89 lines
4.1 KiB
Markdown
# Passwords
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In Java based applications, passing passwords as `String` objects has the
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[disadvantage](https://stackoverflow.com/a/8881376/11150851) that you have to rely on garbage collection to clean up
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once they are no longer used.
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For that reason, `char[]` is the preferred method for dealing with passwords.
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Once a password is no longer used, the character array can simply be overwritten to remove the sensitive data from
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memory.
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## Passphrase
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PGPainless uses a wrapper class `Passphrase`, which takes care for the wiping of unused passwords:
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```java
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Passphrase passphrase = new Passphrase(new char[] {'h', 'e', 'l', 'l', 'o'});
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assertTrue(passphrase.isValid());
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assertArrayEquals(new char[] {'h', 'e', 'l', 'l', 'o'}, passphrase.getChars()):
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// Once we are done, we can clean the data
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passphrase.clear();
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assertFalse(passphrase.isValid());
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assertNull(passphrase.getChars());
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```
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Furthermore, `Passphrase` can also wrap empty passphrases, which increases null-safety of the API:
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```java
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Passphrase empty = Passphrase.emptyPassphrase();
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assertTrue(empty.isValid());
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assertTrue(empty.isEmpty());
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assertNull(empty.getChars());
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empty.clear();
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assertFalse(empty.isValid());
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```
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## SecretKeyRingProtector
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There are certain operations that require you to provide the passphrase for a key.
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Examples are decryption of messages, or creating signatures / certifications.
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The primary way of telling PGPainless, which password to use for a certain key is the `SecretKeyRingProtector`
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interface which maps `Passphrases` to (sub-)keys.
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There are multiple implementations of this interface, which may or may not suite your needs:
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```java
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// If your key is not password protected, this implementation is for you:
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SecretKeyRingProtector unprotected = SecretKeyRingProtector
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.unprotectedKeys();
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// If you use a single passphrase for all (sub-) keys, take this:
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SecretKeyRingProtector singlePassphrase = SecretKeyRingProtector
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.unlockAnyKeyWith(passphrase);
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// If you want to be flexible, use this:
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CachingSecretKeyRingProtector flexible = SecretKeyRingProtector
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.defaultSecretKeyRingProtector(passphraseCallback);
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```
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`SecretKeyRingProtector.unprotectedKeys()` will return an empty passphrase for any key.
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It is best used when dealing with unencrypted secret keys.
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`SecretKeyRingProtector.unlockAnyKeyWith(passphrase)` will return the same exact passphrase for any given key.
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You should use this if you have a single key with a static passphrase.
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The last example shows how to instantiate the `CachingSecretKeyRingProtector` with a `SecretKeyPassphraseProvider`
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as argument.
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As the name suggests, the `CachingSecretKeyRingProtector` caches passphrases it knows about in a map.
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That way, you only have to provide the passphrase for a certain key only once, after which it will be remembered.
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If you try to unlock a protected secret key for which no passphrase is cached, the `getPassphraseFor()` method of
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the `SecretKeyPassphraseProvider` callback will be called to interactively ask for the missing passphrase.
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Afterwards, the acquired passphrase will be cached for future use.
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:::{note}
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While especially the `CachingSecretKeyRingProtector` can handle multiple keys without problems, it is advised
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to use individual `SecretKeyRingProtector` objects per key.
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The reason for this is, that internally the 64bit key-id is used to resolve `Passphrase` objects and collisions are not
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unlikely in this key-space.
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Furthermore, multiple OpenPGP keys could contain the same subkey, but with different passphrases set.
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If the same `SecretKeyRingProtector` is used for two OpenPGP keys with the same subkey, but different passwords,
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the key-id collision will cause the password to be overwritten for one of the keys, which might result in issues.
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See `FLO-04-004 WP2` of the [2021 security audit](https://cure53.de/pentest-report_pgpainless.pdf) for more details.
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:::
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Most `SecretKeyRingProtector` implementations can be instantiated with custom `KeyRingProtectionSettings`.
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By default, most implementations use `KeyRingProtectionSettings.secureDefaultSettings()` which corresponds to iterated
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and salted S2K using AES256 and SHA256 with an iteration count of 65536.
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