Forbid keys of size 0
Keys of size 0 generally don't make sense: a key is supposed to be
secret. There is one edge case which is "raw data" keys, which are
useful to store non-key objects in the same storage location as keys.
However those are also problematic because they involve a zero-length
buffer. Manipulating zero-length buffers in C requires special cases
with functions like malloc() and memcpy(). Additionally, 0 as a key
size already has a meaning "unspecified", which does not always
overlap seamlessly with the meaning "0".
Therefore, forbid keys of size 0. No implementation may accept them.
diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index c21809c..7c88bd6 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -291,7 +291,8 @@
* \param[out] attributes The attribute structure to write to.
* \param bits The key size in bits.
* If this is 0, the key size in \p attributes
- * becomes unspecified.
+ * becomes unspecified. Keys of size 0 are
+ * not supported.
*/
static void psa_set_key_bits(psa_key_attributes_t *attributes,
size_t bits);
@@ -468,6 +469,13 @@
* and to the documentation of psa_export_key() for the format for
* other key types.
*
+ * The key data determines the key size. The attributes may optionally
+ * specify a key size; in this case it must match the size determined
+ * from the key data. A key size of 0 in \p attributes indicates that
+ * the key size is solely determined by the key data.
+ *
+ * Implementations must reject an attempt to import a key of size 0.
+ *
* This specification supports a single format for each key type.
* Implementations may support other formats as long as the standard
* format is supported. Implementations that support other formats
@@ -3092,6 +3100,8 @@
* - Clean up the key derivation operation object with
* psa_key_derivation_abort().
*
+ * Implementations must reject an attempt to derive a key of size 0.
+ *
* \param[in,out] operation The key derivation operation object
* to set up. It must
* have been initialized but not set up yet.
@@ -3622,6 +3632,8 @@
* The key is generated randomly.
* Its location, usage policy, type and size are taken from \p attributes.
*
+ * Implementations must reject an attempt to generate a key of size 0.
+ *
* The following type-specific considerations apply:
* - For RSA keys (#PSA_KEY_TYPE_RSA_KEY_PAIR),
* the public exponent is 65537.