GH-455: ensure BaseCipher.update() fulfills the contract #463
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The org.apache.sshd.common.cipher.Cipher interface specifies for update(byte[] buffer, int offset, int length) that length bytes are encrypted or decrypted in-place in the given buffer, starting at the given offset. The BaseCipher implementation just called javax.crypto.Cipher.update(). That, however, may buffer blocks and not update all data right away. (For instance, AES pipelined implementations may behave that way.) Buffered blocks may be returned/updated in subsequent update() calls. To ensure that really all bytes given are updated, one needs to call doFinal(), which always returns/updates such buffered blocks. But javax.crypto.Cipher.doFinal() resets the cipher to its initial state. For use in SSH, this is not appropriate: the cipher must be reset not to the initial state but to the final state. This is done for CTR ciphers by adding the number of processed blocks to the initial IV and then using that IV for re-initialization. For CBC ciphers, the re-initialization IV must be the last encrypted block processed. Note that in CTR mode, we cannot check for IV re-use. This is not a problem in practice because in the SSH protocol key exchanges happen long before an IV can wrap around.
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The org.apache.sshd.common.cipher.Cipher interface specifies for update(byte[] buffer, int offset, int length) that length bytes are encrypted or decrypted in-place in the given buffer, starting at the given offset.
The BaseCipher implementation just called javax.crypto.Cipher.update(). That, however, may buffer blocks and not update all data right away. (For instance, AES pipelined implementations may behave that way.) Buffered blocks may be returned/updated in subsequent update() calls. To ensure that really all bytes given are updated, one needs to call doFinal(), which always returns/updates such buffered blocks.
But javax.crypto.Cipher.doFinal() resets the cipher to its initial state. For use in SSH, this is not appropriate: the cipher must be reset not to the initial state but to the final state. This is done for CTR ciphers by adding the number of processed blocks to the initial IV and then using that IV for re-initialization. For CBC ciphers, the re-initialization IV must be the last encrypted block processed.
Note that in CTR mode, we cannot check for IV re-use. This is not a problem in practice because in the SSH protocol key exchanges happen long before an IV can wrap around.
Fixes #455.