[crypto] Add concept of cipher alignment size

The GCM cipher mode of operation (in common with other counter-based
modes of operation) has a notion of blocksize that does not neatly
fall into our current abstraction: it does operate in 16-byte blocks
but allows for an arbitrary overall data length (i.e. the final block
may be incomplete).

Model this by adding a concept of alignment size.  Each call to
encrypt() or decrypt() must begin at a multiple of the alignment size
from the start of the data stream.  This allows us to model GCM by
using a block size of 1 byte and an alignment size of 16 bytes.

As a side benefit, this same concept allows us to neatly model the
fact that raw AES can encrypt only a single 16-byte block, by
specifying an alignment size of zero on this cipher.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/cbc.h

@@ -95,6 +95,7 @@ struct cipher_algorithm _cbc_cipher = {					\
 	.name		= #_cbc_name,					\
 	.ctxsize	= sizeof ( struct _cbc_name ## _context ),	\
 	.blocksize	= _blocksize,					\
+	.alignsize	= _blocksize,					\
 	.authsize	= 0,						\
 	.setkey		= _cbc_name ## _setkey,				\
 	.setiv		= _cbc_name ## _setiv,				\

src/include/ipxe/crypto.h

@@ -51,8 +51,24 @@ struct cipher_algorithm {
 	const char *name;
 	/** Context size */
 	size_t ctxsize;
-	/** Block size */
+	/** Block size
+	 *
+	 * Every call to encrypt() or decrypt() must be for a multiple
+	 * of this size.
+	 */
 	size_t blocksize;
+	/** Alignment size
+	 *
+	 * Every call to encrypt() or decrypt() must begin at a
+	 * multiple of this offset from the start of the stream.
+	 * (Equivalently: all but the last call to encrypt() or
+	 * decrypt() must be for a multiple of this size.)
+	 *
+	 * For ciphers supporting additional data, the main data
+	 * stream and additional data stream are both considered to
+	 * begin at offset zero.
+	 */
+	size_t alignsize;
 	/** Authentication tag size */
 	size_t authsize;
 	/** Set key

src/include/ipxe/ecb.h

@@ -47,6 +47,7 @@ struct cipher_algorithm _ecb_cipher = {					\
 	.name		= #_ecb_name,					\
 	.ctxsize	= sizeof ( _raw_context ),			\
 	.blocksize	= _blocksize,					\
+	.alignsize	= _blocksize,					\
 	.authsize	= 0,						\
 	.setkey		= _ecb_name ## _setkey,				\
 	.setiv		= _ecb_name ## _setiv,				\

src/include/ipxe/gcm.h

@@ -121,6 +121,7 @@ struct cipher_algorithm _gcm_cipher = {					\
 	.name		= #_gcm_name,					\
 	.ctxsize	= sizeof ( struct _gcm_name ## _context ),	\
 	.blocksize	= 1,						\
+	.alignsize	= sizeof ( union gcm_block ),			\
 	.authsize	= sizeof ( union gcm_block ),			\
 	.setkey		= _gcm_name ## _setkey,				\
 	.setiv		= _gcm_name ## _setiv,				\