Add authentication to the command line client

Sadly even OpenSSL's newer "openssl mac" subcommand needs the key to be
provided on the command line. Give up and shell out to Perl and
implement HMAC ourselves. Probably a clue this shouldn't be a shell
script. However the domain *is* paste.sh.

paste.sh

@@ -90,10 +90,10 @@ encrypt() {
   (echo -n "${header}${header:+$'\n\n'}"; $cmd "$arg") \
     | 3<<<"$(writekey)" openssl enc -aes-256-cbc -md sha512 -pass fd:3 -iter 1 -base64 > "${file}"
 
-  # Get rid of the temp file once server supports HTTP/1.1 chunked uploads
-  # correctly.
+  et="\"$(etag <"${file}")\""
   (curl -sS -o /dev/fd/3 -H "X-Server-Key: ${serverkey}" \
     -H "Content-type: text/vnd.paste.sh-${VERSION}" \
+    -H "ETag: $et" \
     -T "${file}" "${HOST}/${id}" -b "$pasteauth" -w '%{http_code}' \
     | grep -q 200) 3>&1 || exit $?
 
@@ -116,6 +116,15 @@ decrypt() {
     -D "${headfile}" "${url}.txt" || exit $?
   serverkey=$(head -n1 "${tmpfile}")
   ct="$(grep -i '^content-type:' ${headfile} | cut -d':' -f2)"
+
+  et="$(grep -i '^etag:' ${headfile} | cut -d':' -f2 | tr -d '\015 ')"
+  if [[ -n "$et" ]]; then
+    expected_et="\"$(etag <"${tmpfile}")\""
+    if [[ "$et" != "$expected_et" ]]; then
+      die "Decryption failed (Invalid decryption key or damaged data)" 1
+    fi
+  fi
+
   remove=cat
   ITERS="-iter 1"
   if [[ $ct = *text/plain* ]]; then
@@ -145,6 +154,20 @@ fsize() {
   fi
 }
 
+etag() {
+  # "61757468206b6579" is "auth key"
+  openssl base64 -d | hmac "$(writekey | hmac "61757468206b6579")" "-binary" | openssl base64 -A | sed 's/==$//'
+}
+
+# Implement HMAC on top of OpenSSL's digest command, as openssl's CLI support
+# for generating a HMAC needs the key on the command line.
+hmac() {
+  local key="$1"
+  local opts="$2"
+  key="$key" perl -e'print join("", map { chr hex } $ENV{key} =~ /(..)/g)^"\x36"x128; print join "", <>' | openssl sha512 -binary | \
+    key="$key" perl -e'print join("", map { chr hex } $ENV{key} =~ /(..)/g)^"\x5c"x128; print join "", <>' | openssl sha512 $opts
+}
+
 # Try to use memory if we can (Linux, probably) and the user hasn't set TMPDIR
 if [ -z "$TMPDIR" -a -w /dev/shm ]; then
   export TMPDIR=/dev/shm