+.TP
+.B aead
+A transform based on an all-in-one `authenticated encryption with
+additional data' scheme. The scheme is named in the
+.B cipher
+attribute; the default is
+.BR rijndael-ocb3 .
+If the
+.B mac
+attribute is given, it must be either
+.B aead
+or
+.BR aead/ \c
+.IR tagsz ,
+where
+.I tagsz
+is the desired tag length in bits; alternatively, the tag length can be
+set in the
+.B tagsz
+attribute. The chosen AEAD scheme must accept at least a 64-bit nonce
+(this rules out OCB3 and CCM with 64-bit blockciphers); it mustn't
+require an absurdly large nonce size (none of the schemes implemented in
+Catacomb present a problem here, but it bears mentioning); it must
+actually support additional header data (which rules out the
+.B naclbox
+schemes, but see the
+.B naclbox
+transform below); and it must produce an empty ciphertext when
+encrypting an empty message (again, all of Catacomb's schemes meet this
+requirement).
+.TP
+.B naclbox
+A transform based on the NaCl
+.B crypto_secretbox
+transformation.
+The main difference is that NaCl uses XSalsa20,
+while TrIPE uses plain Salsa20 or ChaCha,
+because it doesn't need the larger nonce space.
+You can set the
+.B cipher
+key attribute to one of
+.BR salsa20 ,
+.BR salsa20/12 ,
+.BR salsa20/8 ,
+.BR chacha20 ,
+.BR chacha12 ,
+or
+.B chacha8
+to select the main cipher.
+You can set the
+.B mac
+key attribute to
+.B poly1305
+or
+.B poly1305/128
+but these are the default and no other choice is permitted.
+(This is for forward compatibility,
+in case other MACs and/or tag sizes are allowed later.)
+.SS "Other key attributes"
+The following attributes can also be set on keys.
+.TP
+.B serialization
+Selects group-element serialization formats.
+The recommended setting is
+.BR constlen ,
+which selects a constant-length encoding when hashing group elements.
+The default,
+for backwards compatibility, is
+.BR v0 ;
+but this is deprecated.
+(The old format uses a variable length format for hashing,
+which can leak information through timing.)