crypto: aead - improve documentation for scatterlist layout

Properly document the scatterlist layout for AEAD ciphers.
Reported-by: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/aead.h

@@ -43,27 +43,33 @@
  *
  * Memory Structure:
  *
- * To support the needs of the most prominent user of AEAD ciphers, namely
- * IPSEC, the AEAD ciphers have a special memory layout the caller must adhere
- * to.
- *
- * The scatter list pointing to the input data must contain:
- *
- * * for RFC4106 ciphers, the concatenation of
- *   associated authentication data || IV || plaintext or ciphertext. Note, the
- *   same IV (buffer) is also set with the aead_request_set_crypt call. Note,
- *   the API call of aead_request_set_ad must provide the length of the AAD and
- *   the IV. The API call of aead_request_set_crypt only points to the size of
- *   the input plaintext or ciphertext.
- *
- * * for "normal" AEAD ciphers, the concatenation of
- *   associated authentication data || plaintext or ciphertext.
- *
- * It is important to note that if multiple scatter gather list entries form
- * the input data mentioned above, the first entry must not point to a NULL
- * buffer. If there is any potential where the AAD buffer can be NULL, the
- * calling code must contain a precaution to ensure that this does not result
- * in the first scatter gather list entry pointing to a NULL buffer.
+ * The source scatterlist must contain the concatenation of
+ * associated data || plaintext or ciphertext.
+ *
+ * The destination scatterlist has the same layout, except that the plaintext
+ * (resp. ciphertext) will grow (resp. shrink) by the authentication tag size
+ * during encryption (resp. decryption).
+ *
+ * In-place encryption/decryption is enabled by using the same scatterlist
+ * pointer for both the source and destination.
+ *
+ * Even in the out-of-place case, space must be reserved in the destination for
+ * the associated data, even though it won't be written to.  This makes the
+ * in-place and out-of-place cases more consistent.  It is permissible for the
+ * "destination" associated data to alias the "source" associated data.
+ *
+ * As with the other scatterlist crypto APIs, zero-length scatterlist elements
+ * are not allowed in the used part of the scatterlist.  Thus, if there is no
+ * associated data, the first element must point to the plaintext/ciphertext.
+ *
+ * To meet the needs of IPsec, a special quirk applies to rfc4106, rfc4309,
+ * rfc4543, and rfc7539esp ciphers.  For these ciphers, the final 'ivsize' bytes
+ * of the associated data buffer must contain a second copy of the IV.  This is
+ * in addition to the copy passed to aead_request_set_crypt().  These two IV
+ * copies must not differ; different implementations of the same algorithm may
+ * behave differently in that case.  Note that the algorithm might not actually
+ * treat the IV as associated data; nevertheless the length passed to
+ * aead_request_set_ad() must include it.
  */
 
 struct crypto_aead;