As announced on the security-dev mailing list, I'd like to (re)introduce a conditional statement that was first introduced in the context of JDK-8253821 and removed in JDK-8253821:

It used to be this [1]:

if (getMode() != GCM_MODE || outputCapacity < estOutSize) {
    // create temporary output buffer if the estimated size is larger
    // than the user-provided buffer.
    internalOutput = new byte[estOutSize];
    offset = 0;
}

The assumption in the original issue was, that only GCM would benefit from working on the original buffer. However, the temporary internal buffer can be avoided altogether if the output buffer is large enough.

Furthermore, the need to copy the results back from the temporary buffer to the output buffer can be circumvented.

The current implementation will _always_ create a temporary buffer, despite the output buffer being sufficient. For every call to `Cipher.doFinal(ByteBuffer, ByteBuffer)` this causes the allocation of a byte[] with the size depending on the supplied ciphertext.

A small test program for showing these allocations is available on [2].

[1] https://github.com/openjdk/jdk/blame/e546ae27ffc6c19ae078a41ab6e1741a104958c1/src/java.base/share/classes/com/sun/crypto/provider/CipherCore.java#L946-L951

[2] https://gist.github.com/overheadhunter/f3969950c0fdbaecaa77c857b2246cc5