[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>

1 files changed, 17 insertions, 1 deletions

diff --git a/src/include/ipxe/crypto.h b/src/include/ipxe/crypto.h
index 842f2f633..ba09c9468 100644
--- a/src/include/ipxe/crypto.h
+++ b/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