gpg: Fix corner cases in AEAD encryption.
ebb0fcf6e0bd
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Description

gpg: Fix corner cases in AEAD encryption.

* g10/cipher-aead.c (write_final_chunk): Do not bump up the chunk
index if the previous chunk was empty.
* g10/decrypt-data.c (aead_underflow): Likewise.  Also handle a other
corner cases.  Add more debug output.

This fixes the reported case when the encrypted data is a multiple of
the chunk size. Then the chunk index for the final chunk was wrongly
incremented by 2. The actual fix makes use of the fact that the
current dfx->CHUNKLEN is 0 in this case. There is also some other
reorganizing to help with debugging. The thing seems to work now but
the code is not very clean - should be reworked. Creating test files
can be done with this script:

--8<---------------cut here---------------start------------->8---
csize=6
for len in 0 55 56 57; do

awk </dev/null -v i=$len 'BEGIN{while(i){i--;printf"~"}}' \
  | gpg --no-options -v --rfc4880bis --batch --passphrase "abc" \
        --s2k-count 1025 --s2k-digest-algo sha256 -z0 \
        --force-aead --aead-algo eax --cipher aes -a \
        --chunk-size $csize -c >symenc-aead-eax-c$csize-$len.asc

done
--8<---------------cut here---------------end--------------->8---

A LEN of 56 triggered the bug which can be seen by looking at the
"authdata:" line in the --debug=crypt,filter output.