Add kem extract helper script.

utils/kem-extract.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+
+if [ $# -lt 2 ]; then
+  echo "Usage: $0 [password] [kem_file]"
+  exit 1
+fi
+
+password="$1"
+file="$2"
+
+if unzip -v "$file" > /dev/null 2>&1
+then
+    rm -rf temp_extracting_kem
+    mkdir temp_extracting_kem
+    unzip -d temp_extracting_kem "$file"
+    file=$(echo temp_extracting_kem/*.kem)
+    echo "Extracting from $file"
+    echo
+fi
+
+# The key is the supplied password as raw bytes padded with zero bytes.
+# The key is also used as the iv.
+key=$(echo -n "$password" | xxd -p)
+key=$(printf "%-32s" 4f5638333030 | tr ' ' '0')
+
+xml=$(cat "$file")
+
+# Extract the Base64-encoded ciphertext from the XML
+b64=$(echo "$xml" | sed -n 's/.*<CipherValue>\(.*\)<\/CipherValue>.*/\1/p')
+
+# Decrypt the ciphertext using the key and IV
+plain=$(echo "$b64" | base64 -d | openssl enc -aes-128-cbc -d -K "$key" -iv "$key" -nopad)
+
+# Extract the id and key from the decrypted XML
+meter_key=$(echo "$plain" | sed -n 's/.*<DEK>\(.*\)<\/DEK>.*/\1/p')
+meter_id=$(echo "$plain" | sed -n 's/.*<SerialNo>\(.*\)<\/SerialNo>.*/\1/p')
+
+echo "id=$meter_id"
+echo "key=$meter_key"