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Add more docs for Policy related configuration
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@ -325,4 +325,166 @@ verificationStream.close(); // finish verification
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MessageMetadata result = verificationStream.getMetadata(); // get metadata of signed message
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assertTrue(result.isVerifiedSignedBy(certificate)); // check if message was in fact signed
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```
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### Legacy Compatibility
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Out of the box, PGPainless is configured to use secure defaults and perform checks for recommended
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security features. This means that for example messages generated using older OpenPGP
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implementations which do not follow those best practices might fail to decrypt/verify.
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It is however possible to circumvent certain security checks to allow processing of such messages.
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:::{note}
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It is not recommended to disable security checks, as that might enable certain attacks on the OpenPGP protocol.
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:::
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#### Missing / broken MDC (modification detection code)
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RFC4880 has two different types of encrypted data packets. The *Symmetrically Encrypted Data* packet (SED) and the *Symmetrically Encrypted Integrity-Protected Data* packet.
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The latter has an added MDC packet which prevents modifications to the ciphertext.
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While implementations are highly encouraged to only use the latter package type, some older implementations still generate
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encrypted data packets which are not integrity protected.
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To allow PGPainless to decrypt such messages, you need to set a flag in the `ConsumerOptions` object:
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```java
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ConsumerOptions options = ConsumerOptions.get()
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.setIgnoreMDCErrors(true) // <-
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.setDecryptionKey(secretKey)
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...
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DecryptionStream decryptionStream = PGPainless.decryptAndOrVerify()
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.onInputStream(ciphertextIn)
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.withOptions(options);
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...
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```
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:::{note}
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It is highly advised to only set this flag if you know what you are doing.
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It might also be a good idea to try decrypting a message without the flag set first and only re-try
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decryption with the flag set in case of a `MessageNotIntegrityProtectedException` (don't forget to rewind the ciphertextInputStream).
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:::
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#### Weak keys and broken algorithms
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Some users might cling on to older keys using weak algorithms / small key sizes.
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PGPainless refuses to encrypt to weak certificates and sign with weak keys.
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By default, PGPainless follows the recommendations for acceptable key sizes of [the German BSI in 2021](https://www.bsi.bund.de/SharedDocs/Downloads/EN/BSI/Publications/TechGuidelines/TG02102/BSI-TR-02102-1.pdf).
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It can however be configured to accept older key material / algorithms too.
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Minimal key lengths can be configured by changing PGPainless' policy:
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```java
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Map<PublicKeyAlgorithm, Integer> algorithms = new HashMap<>();
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// put all acceptable algorithms and their minimal key length
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algorithms.put(PublicKeyAlgorithm.RSA_GENERAL, 1024);
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algorithms.put(PublicKeyAlgorithm.ECDSA, 100);
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...
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Policy.PublicKeyAlgorithmPolicy pkPolicy =
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new Policy.PublicKeyAlgorithmPolicy(algorithms);
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// set the custom algorithm policy
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PGPainless.getPolicy().setPublicKeyAlgorithmPolicy();
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```
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Since OpenPGP uses a hybrid encryption scheme of asymmetric and symmetric encryption algorithms,
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it also comes with a policy for symmetric encryption algorithms.
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This list can be modified to allow for weaker algorithms like follows:
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```java
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// default fallback algorithm for message encryption
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SymmetricKeyAlgorithm fallbackAlgorithm = SymmetricKeyAlgorithm.AES_256;
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// acceptable algorithms
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List<SymmetricKeyAlgorithm> algorithms = new ArrayList<>();
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algorithms.add(SymmetricKeyAlgorithm.AES_256);
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algorithms.add(SymmetricKeyAlgorithm.AES_192);
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algorithms.add(SymmetricKeyAlgorithm.AES_128);
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algorithms.add(SymmetricKeyAlgorithm.TWOFISH);
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algorithms.add(SymmetricKeyAlgorithm.BLOWFISH);
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...
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Policy.SymmetricKeyAlgorithmPolicy skPolicy =
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new SymmtricKeyAlgorithmPolicy(fallbackAlgorithm, algorithms);
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// set the custom algorithm policy
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// algorithm policy applicable when decrypting messages created by legacy senders:
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PGPainless.getPolicy()
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.setSymmetricKeyDecryptionAlgorithmPolicy(skPolicy);
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// algorithm policy applicable when generating messages for legacy recipients:
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PGPainless.getPolicy()
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.setSymmetricKeyEncryptionAlgorithmPolicy(skPolicy);
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```
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Hash algorithms are used in OpenPGP to create signatures.
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Since signature verification is an integral part of the OpenPGP protocol, PGPainless comes
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with multiple policies for acceptable hash algorithms, depending on the use-case.
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Revocation signatures are critical, so you might want to handle revocation signatures differently from normal signatures.
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By default, PGPainless uses a smart hash algorithm policy for both use-cases, which takes into consideration
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not only the hash algorithm itself, but also the creation date of the signature.
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That way, signatures using SHA-1 are acceptable if they were created before February 2013, but are rejected if their
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creation date is after that point in time.
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A custom hash algorithm policy can be set like this:
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```java
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HashAlgorithm fallbackAlgorithm = HashAlgorithm.SHA512;
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Map<HashAlgorithm, Date> algorithms = new HashMap<>();
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// Accept MD5 on signatures made before 1997-02-01
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algorithms.put(HashAlgorithm.MD5,
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DateUtil.parseUTCDate("1997-02-01 00:00:00 UTC"));
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// Accept SHA-1, regardless of signature creation time
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algorithms.put(HashAlgorithm.SHA1, null);
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...
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Policy.HashAlgorithmPolicy hPolicy =
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new Policy.HashAlgorithmPolicy(fallbackAlgorithm, algorithms);
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// set policy for revocation signatures
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PGPainless.getPolicy()
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.setRevocationSignatureHashAlgorithmPolicy(hPolicy);
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// set policy for normal signatures (certifications and document signatures)
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PGPainless.getPolicy()
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.setSignatureHashAlgorithmPolicy(hPolicy);
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```
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Lastly, PGPainless comes with a policy on acceptable compression algorithms, which currently accepts any
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compression algorithm.
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A custom compression algorithm policy can be set in a similar way:
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```java
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CompressionAlgorithm fallback = CompressionAlgorithm.ZIP;
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List<CompressionAlgorithm> algorithms = new ArrayList<>();
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algorithms.add(CompressionAlgorith.ZIP);
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algorithms.add(CompressionAlgorithm.BZIP2);
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...
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Policy.CompressionAlgorithmPolicy cPolicy =
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new Policy.CompressionAlgorithmPolicy(fallback, algorithms);
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PGPainless.getPolicy()
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.setCompressionAlgorithmPolicy(cPolicy);
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```
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To prevent a class of attacks described in the [paper](https://www.kopenpgp.com/#paper)
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"Victory by KO: Attacking OpenPGP Using Key Overwriting",
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PGPainless offers the option to validate private key material each time before using it,
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to make sure that an attacker didn't tamper with the corresponding public key parameters.
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These checks are disabled by default, but they can be enabled as follows:
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```java
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PGPainless.getPolicy()
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.setEnableKeyParameterValidation(true);
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```
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:::{note}
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Validation checks against KOpenPGP attacks are disabled by default, since they are very costly
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and only make sense in certain scenarios.
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Please read and understand the paper to decide, if enabling the checks makes sense for your use-case.
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:::
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### Known Notations
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In OpenPGP, signatures can contain [notation subpackets](https://www.rfc-editor.org/rfc/rfc4880#section-5.2.3.16).
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A notation can give meaning to a signature, or add additional contextual information.
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Signature subpackets can be marked as critical, meaning an implementation that does not know about
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a certain subpacket MUST reject the signature.
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The same is true for critical notations.
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For that reason, PGPainless comes with a `NotationRegistry` class which can be used to register known notations,
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such that a signature containing a critical notation of a certain value is not rejected.
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To register a known notation, you can do the following:
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```java
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NotationRegistry registry = PGPainless.getPolicy()
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.getNotationRegistry();
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registry.addKnownNotation("sample@example.com");
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```
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