1. SDLS Protocol Padding concept Ignacio Aguilar Sanchez (ESA) CCSDS Spring Meeting 2010 | Portsmouth, VA

2. Padding description –External Padding is required for some encryption and authentication algorithms that operate with input data blocks of 128-bit whenever input data length is not a block multiple. –Encryption example: Cipher Block Chaining (CBC); –Authentication example: CBC-MAC; –1 flag in security header (padding signalling flag) to signal presence or absence of padding –limit padding to cases where it is needed; –CCSDS recommended cryptographic algorithms do not require external padding. –Padding signalling flag is authenticated –as part of security header, which is always authenticated. –If padding is present security header includes 1 octet, placed at the end of the security header, to indicate the number of padding bits (0-127) –Padding pattern does not need to be transmitted; –Padding convention: 1, 10, 100, 1000, 10000,…known as ISO/IEC 9797-1 Padding Method 2. –Padding is granular at bit level.

3. Open points (1) –Padding signalling flag management alternatives: –Per frame –SDLS protocol determines need of padding and length for every frame and sets signalling flag accordingly; –Security header length can vary between frames; variation to be considered and accommodated by SDLS to determine SDLS output data length. –Per mission (preferred) –SDLS indicates presence of padding for all frames; –Padding length can include ‘0’ value (i.e., no padding); –Security header length is equal for all frames; simpler SDLS prediction of output data length.

4. Open points (2) –Security: –Suitability of proposed padding pattern has been studied for CBC and CBC-MAC; assessment required for other algorithms. –Definition of padding pattern and corresponding security can be left to implementer if algorithms other than above are used; recall that CCSDS recommended algorithms do not require padding; text to be added in BB and GB accordingly.