Local Open Scope logic.
Definition Done (i:Z): int := Int.repr (digest_len*i).
Definition OUTpred PrkCont InfoCont sh z r cont p: mpred:=
data_at sh (tarray tuchar z) (sublist 0 z (map Vubyte (HKDF_expand PrkCont InfoCont cont))) p *
memory_block sh r (offset_val z p).
Definition PREVcont PRK INFO (i: Z): reptype (Tarray tuchar 32 noattr) :=
if zeq i 0 then list_repeat 32 Vundef
else (map Vubyte (Ti (CONT PRK) (CONT INFO) (Z.to_nat i))).

Lemma PREVcont_Sn PRK INFO i p: 0 <= i <= Byte.max_unsigned -> PREVcont PRK INFO i = map Vubyte p ->
PREVcont PRK INFO (i+1) = if zeq i 0 then map Vubyte (HMAC256_functional_prog.HMAC256 (CONT INFO ++ [Byte.one]) (CONT PRK))
else map Vubyte (HMAC256_functional_prog.HMAC256 (p ++ CONT INFO ++ [Byte.add (Byte.repr i) Byte.one]) (CONT PRK)).