Replaced SHA-256 with SHAd-256 to prevent length extension attacks.

api/net/sf/briar/api/crypto/CryptoComponent.java

api/net/sf/briar/api/crypto/MessageDigest.java

api/net/sf/briar/api/serial/CopyingConsumer.java

api/net/sf/briar/api/serial/CountingConsumer.java

api/net/sf/briar/api/serial/DigestingConsumer.java

api/net/sf/briar/api/serial/SigningConsumer.java

components/net/sf/briar/crypto/CryptoComponentImpl.java

components/net/sf/briar/crypto/DoubleDigest.java

components/net/sf/briar/protocol/AckReader.java

components/net/sf/briar/protocol/AuthorFactoryImpl.java

components/net/sf/briar/protocol/AuthorReader.java

components/net/sf/briar/protocol/BatchReader.java

components/net/sf/briar/protocol/GroupFactoryImpl.java

components/net/sf/briar/protocol/GroupReader.java

components/net/sf/briar/protocol/MessageEncoderImpl.java

components/net/sf/briar/protocol/MessageReader.java

components/net/sf/briar/protocol/OfferReader.java

components/net/sf/briar/protocol/RequestReader.java

...nents/net/sf/briar/protocol/SubscriptionUpdateReader.java

components/net/sf/briar/protocol/TransportUpdateReader.java