Theory EMSAPSS

(*  Title:      RSAPSS/EMSAPSS.thy
    Author:     Christina Lindenberg, Kai Wirt, Technische Universität Darmstadt
    Copyright:  2005 - Technische Universität Darmstadt 
*)

section "EMSA-PSS encoding and decoding operation" 

theory EMSAPSS 
imports SHA1 Wordarith
begin

text We define the encoding and decoding operations for the probabilistic
 signature scheme. Finally we show, that encoded messages always can be 
 verified

definition show_rightmost_bits:: "bv  nat  bv" (* extract n rightmost bits of a bit string*)
  where "show_rightmost_bits bvec n = rev (take n (rev bvec))"

definition BC:: "bv" (* 0xbc *)
  where "BC = [One, Zero, One, One, One, One, Zero, Zero]"

definition salt:: "bv"
  where "salt = []"

definition sLen:: "nat" (* length of salt *)
  where "sLen = length salt"

definition generate_M':: "bv  bv  bv" (* generate_M' outputs "M'" a bit string of length "64 + length(sha1 _ ) + sLen" with 64 initial zero bits *)
  where "generate_M' mHash salt_new = bv_prepend 64 𝟬 [] @ mHash @ salt_new"

definition generate_PS:: "nat  nat  bv" (* generate_PS outputs "PS" a bit string consisting of "nat (roundup emBits 8) * 8 - sLen - length (sha1 _ ) - 16" zero bits *)
  where "generate_PS emBits hLen = bv_prepend ((roundup emBits 8)*8 - sLen - hLen - 16) 𝟬 []"

definition generate_DB:: "bv  bv" (* generate_DB outputs the bit string DB="PS#(0x01)#salt" of length "nat (roundup emBits 8) * 8 - length (sha1 _ ) - 8" *)
  where "generate_DB PS = PS @ [Zero, Zero, Zero, Zero, Zero, Zero, Zero, One] @ salt"

definition maskedDB_zero:: "bv  nat  bv" (* sets "roundup emBits 8 * 8 - emBits" bits of maskedDB to zero *)
  where "maskedDB_zero maskedDB emBits = bv_prepend ((roundup emBits 8) * 8 - emBits)  𝟬 (drop ((roundup emBits 8)*8 - emBits) maskedDB)"

definition generate_H:: "bv  nat  nat  bv" (* generate_H returns a bit string of length (sha1 _) *)
  where "generate_H EM emBits hLen = take hLen (drop ((roundup emBits 8)*8 - hLen - 8) EM)"

definition generate_maskedDB:: "bv  nat  nat  bv " (* outputs a bit string of length "nat (roundup emBits 8) * 8 - length (sha1 _) - 8" *)
  where "generate_maskedDB EM emBits hLen = take ((roundup emBits 8)*8 - hLen - 8) EM"

definition generate_salt:: "bv  bv" (* outputs a bit string of length sLen *)
  where "generate_salt DB_zero = show_rightmost_bits DB_zero sLen"

primrec MGF2:: "bv  nat  bv" (* help function for MGF1 *)
where
  "MGF2 Z 0 = sha1 (Z@(nat_to_bv_length 0 32))"
| "MGF2 Z (Suc n) = (MGF2 Z n)@(sha1 (Z@(nat_to_bv_length (Suc n) 32)))"

definition MGF1:: "bv  nat  nat  bv" (* help function for MGF *)
  where "MGF1 Z n l = take l (MGF2 Z n)"

definition MGF:: "bv  nat  bv" (* mask generation function *)
where
  "MGF Z l = (if l = 0  2^32*(length (sha1 Z)) < l
              then []
              else MGF1 Z ( roundup l (length (sha1 Z))  - 1 ) l)"


definition emsapss_encode_help8:: "bv  bv  bv"
  where "emsapss_encode_help8 DBzero H = DBzero @ H @ BC"

definition emsapss_encode_help7:: "bv  bv  nat  bv"
  where "emsapss_encode_help7 maskedDB H emBits =
    emsapss_encode_help8 (maskedDB_zero maskedDB emBits) H"

definition emsapss_encode_help6:: "bv  bv  bv  nat  bv"
  where "emsapss_encode_help6 DB dbMask H emBits =
    (if dbMask = []
     then []
     else emsapss_encode_help7 (bvxor DB dbMask) H emBits)"

definition emsapss_encode_help5:: "bv  bv  nat  bv"
  where "emsapss_encode_help5 DB H emBits =
    emsapss_encode_help6 DB (MGF H (length DB)) H emBits"

definition emsapss_encode_help4:: "bv  bv  nat  bv"
  where "emsapss_encode_help4 PS H emBits =
    emsapss_encode_help5 (generate_DB PS) H emBits"

definition emsapss_encode_help3:: "bv  nat  bv"
  where "emsapss_encode_help3 H emBits =
    emsapss_encode_help4 (generate_PS emBits (length H)) H emBits"

definition emsapss_encode_help2:: "bv  nat  bv"
  where "emsapss_encode_help2 M' emBits = emsapss_encode_help3 (sha1 M') emBits"

definition emsapss_encode_help1:: "bv  nat  bv"
  where "emsapss_encode_help1 mHash emBits =
    (if emBits  < length (mHash) + sLen + 16
     then []
     else emsapss_encode_help2 (generate_M' mHash salt) emBits)"

definition emsapss_encode:: "bv  nat  bv" (* outputs the encoded message of length emBits *)
  where "emsapss_encode M emBits =
    (if (2^64  length M  2^32 * 160 < emBits)
     then []
     else emsapss_encode_help1 (sha1 M) emBits)"


definition emsapss_decode_help11:: "bv  bv  bool"
  where "emsapss_decode_help11 H' H = (if H'  H then False else True)"

definition emsapss_decode_help10:: "bv  bv  bool"
  where "emsapss_decode_help10 M' H = emsapss_decode_help11 (sha1 M') H"

definition emsapss_decode_help9:: "bv  bv  bv  bool"
  where "emsapss_decode_help9 mHash salt_new H =
    emsapss_decode_help10 (generate_M' mHash salt_new) H"

definition emsapss_decode_help8:: "bv  bv  bv  bool"
  where "emsapss_decode_help8 mHash DB_zero H =
    emsapss_decode_help9 mHash (generate_salt DB_zero) H"

definition emsapss_decode_help7:: "bv  bv  bv  nat  bool"
  where "emsapss_decode_help7 mHash DB_zero H emBits =
   (if (take ( (roundup emBits 8)*8 - (length mHash) - sLen - 16) DB_zero  bv_prepend ( (roundup emBits 8)*8 - (length mHash) - sLen - 16) 𝟬 [])  (take 8 ( drop ((roundup emBits 8)*8 - (length mHash) - sLen - 16 ) DB_zero )  [Zero, Zero, Zero, Zero, Zero, Zero, Zero, One])
    then False
    else emsapss_decode_help8 mHash DB_zero H)"

definition emsapss_decode_help6:: "bv  bv  bv  nat  bool"
  where "emsapss_decode_help6 mHash DB H emBits =
    emsapss_decode_help7 mHash (maskedDB_zero DB emBits) H emBits"

definition emsapss_decode_help5:: "bv  bv  bv  bv  nat  bool"
  where "emsapss_decode_help5 mHash maskedDB dbMask H emBits =
    emsapss_decode_help6 mHash (bvxor maskedDB dbMask) H emBits"

definition emsapss_decode_help4:: "bv  bv  bv  nat  bool"
  where "emsapss_decode_help4 mHash maskedDB H emBits =
   (if take ((roundup emBits 8)*8 - emBits) maskedDB  bv_prepend ((roundup emBits 8)*8 - emBits) 𝟬 []
    then False
    else emsapss_decode_help5 mHash maskedDB (MGF H ((roundup emBits 8)*8 - (length mHash) - 8)) H emBits)"

definition emsapss_decode_help3:: "bv  bv  nat  bool"
  where "emsapss_decode_help3 mHash EM emBits =
    emsapss_decode_help4 mHash (generate_maskedDB EM emBits (length mHash)) (generate_H EM emBits (length mHash)) emBits"

definition emsapss_decode_help2:: "bv  bv  nat  bool"
  where "emsapss_decode_help2 mHash EM emBits =
   (if show_rightmost_bits EM 8  BC
    then False
    else emsapss_decode_help3 mHash EM emBits)"

definition emsapss_decode_help1:: "bv  bv  nat  bool"
  where "emsapss_decode_help1 mHash EM emBits =
    (if emBits < length (mHash) + sLen + 16
     then False 
     else emsapss_decode_help2 mHash EM emBits)"

definition emsapss_decode:: "bv  bv  nat  bool" (* outputs "true" if EM is encoding of M *)
  where "emsapss_decode M EM emBits =
    (if (2^64  length M  2^32*160<emBits)
     then False
     else emsapss_decode_help1 (sha1 M) EM emBits)"

lemma roundup_positiv: "0 < emBits  0 < roundup emBits 160"
  by (auto simp add: roundup)

lemma roundup_ge_emBits:" 0 < emBits  0 < x  emBits  (roundup emBits x) * x"
  apply (simp add: roundup mult.commute)
  apply (safe)
  apply (simp)
  apply (simp add: add.commute [of x "x*(emBits div x)" ])
  apply (insert mult_div_mod_eq [of x emBits])
  apply (subgoal_tac "emBits mod x < x")
  apply (arith)
  apply (simp only: mod_less_divisor)
  done

lemma roundup_ge_0: "0 < emBits  0 < x  0  roundup emBits x * x - emBits"
  by (simp add: roundup)

lemma roundup_le_7: "0 < emBits  roundup emBits 8 * 8 - emBits   7"
  by (auto simp add: roundup) arith

lemma roundup_nat_ge_8_help:
  "length (sha1 M) + sLen + 16  emBits  8  roundup emBits 8 * 8 - (length (sha1 M) + 8)"
  apply (insert roundup_ge_emBits [of emBits 8])
  apply (simp add: roundup sha1len sLen_def)
  done

lemma roundup_nat_ge_8:
  "length (sha1 M) + sLen + 16  emBits  8  roundup emBits 8 * 8 - (length (sha1 M) + 8)"
  apply (insert roundup_nat_ge_8_help [of M emBits])
  apply arith
  done

lemma roundup_le_ub:
  " 176 + sLen  emBits; emBits  2^32 * 160  (roundup emBits 8) * 8 - 168  2^32 * 160"
  apply (simp add: roundup)
  apply (safe)
  apply (simp)
  apply (arith)+
  done
 
lemma modify_roundup_ge1: "8  roundup emBits 8 * 8 - 168  176   roundup emBits 8 * 8"
  by arith

lemma modify_roundup_ge2: " 176  roundup emBits 8 * 8  21 < roundup emBits 8"
  by simp

lemma roundup_help1: " 0 < roundup l 160  (roundup l 160 - 1) + 1 = (roundup l 160)"
  by arith

lemma roundup_help1_new: " 0 < l  (roundup l 160 - 1) + 1 = (roundup l 160)"
  apply (drule roundup_positiv [of l])
  apply arith
  done

lemma roundup_help2: "176 + sLen  emBits  roundup emBits 8 * 8 - emBits <=  roundup emBits 8 * 8 - 160 - sLen - 16"
  by (simp add: sLen_def)

lemma bv_prepend_equal: "bv_prepend (Suc n) b l = b#bv_prepend n b l" 
  by (simp add: bv_prepend)

lemma length_bv_prepend: "length (bv_prepend n b l) = n+length l"
  by (induct n) (simp_all add: bv_prepend)

lemma length_bv_prepend_drop: "a <= length xs  length (bv_prepend a b (drop a xs)) = length xs"
  by (simp add:length_bv_prepend)

lemma take_bv_prepend: "take n (bv_prepend n b x) = bv_prepend n b []"
  by (induct n) (simp add: bv_prepend)+

lemma take_bv_prepend2: "take n (bv_prepend n b xs@ys@zs) = bv_prepend n b []"
  by (induct n) (simp add: bv_prepend)+

lemma bv_prepend_append: "bv_prepend a b x = bv_prepend a b [] @ x"
  by (induct a) (simp add: bv_prepend, simp add: bv_prepend_equal)

lemma bv_prepend_append2:
  "x < y  bv_prepend y b xs = (bv_prepend x b [])@(bv_prepend (y-x) b [])@xs"
  by (simp add: bv_prepend replicate_add [symmetric])

lemma drop_bv_prepend_help2: "x < y  drop x (bv_prepend y b []) = bv_prepend (y-x) b []"
  apply (insert bv_prepend_append2 [of "x" "y" b "[]"])
  by (simp add: length_bv_prepend)

lemma drop_bv_prepend_help3: "x = y  drop x (bv_prepend y b []) = bv_prepend (y-x) b []"
  apply (insert length_bv_prepend [of y b "[]"])
  by (simp add: bv_prepend)

lemma drop_bv_prepend_help4: "x  y  drop x (bv_prepend y b []) = bv_prepend (y-x) b []"
  apply (insert drop_bv_prepend_help2 [of x y b] drop_bv_prepend_help3 [of x y b])
  by (arith)

lemma bv_prepend_add: "bv_prepend x b [] @ bv_prepend y b [] = bv_prepend (x + y) b []"
  by (induct x) (simp add: bv_prepend)+

lemma bv_prepend_drop: "x  y  bv_prepend x b (drop x (bv_prepend y b [])) = bv_prepend y b []"
  apply (simp add: drop_bv_prepend_help4 [of x y b])
  by (simp add: bv_prepend_append [of "x" b "(bv_prepend (y - x) b [])"] bv_prepend_add)

lemma bv_prepend_split: "bv_prepend x b (left @ right) = bv_prepend x b left @ right"
  by (induct x) (simp add: bv_prepend)+

lemma length_generate_DB: "length (generate_DB PS) = length PS + 8 + sLen"
  by (simp add: generate_DB_def sLen_def)

lemma length_generate_PS: "length (generate_PS emBits 160) = (roundup emBits 8)*8 - sLen - 160 - 16"
  by (simp add: generate_PS_def length_bv_prepend)

lemma length_bvxor: "length a = length b  length (bvxor a b) = length a"
  by (simp add: bvxor)

lemma length_MGF2: "length (MGF2 Z m) = Suc m * length (sha1 (Z @ nat_to_bv_length m 32))"
  by (induct m) (simp+, simp add: sha1len)

lemma length_MGF1: "l  (Suc n) * 160  length (MGF1 Z n l) = l"
  by (simp add: MGF1_def length_MGF2 sha1len)

lemma length_MGF: "0 < l  l   2^32 * length (sha1 x)  length (MGF x l) = l"
  apply (simp add: MGF_def sha1len)
  apply (insert roundup_help1_new [of l])
  apply (rule length_MGF1)
  apply (simp)
  apply (insert roundup_ge_emBits [of l 160])
  apply (arith)
  done

lemma solve_length_generate_DB:
  " 0 < emBits; length (sha1 M) + sLen + 16  emBits
   length (generate_DB (generate_PS emBits (length (sha1 x)) )) = (roundup emBits 8) * 8 - 168"
  apply (insert roundup_ge_emBits [of emBits 8])
  apply (simp add: length_generate_DB length_generate_PS sha1len)
  done

lemma length_maskedDB_zero:
  " roundup emBits 8 * 8 - emBits  length maskedDB
   length (maskedDB_zero maskedDB emBits) = length maskedDB"
  by (simp add: maskedDB_zero_def length_bv_prepend)

lemma take_equal_bv_prepend:
  " 176 + sLen  emBits; roundup emBits 8 * 8 - emBits  7
   take (roundup emBits 8 * 8 - length (sha1 M) - sLen - 16) (maskedDB_zero (generate_DB (generate_PS emBits 160)) emBits) =
    bv_prepend (roundup emBits 8 * 8 - length (sha1 M) - sLen - 16) 𝟬 []"
  apply (insert roundup_help2 [of emBits] length_generate_PS [of emBits])
  apply (simp add: sha1len maskedDB_zero_def generate_DB_def generate_PS_def
    bv_prepend_split bv_prepend_drop)
  done

lemma lastbits_BC: "BC = show_rightmost_bits (xs @ ys @ BC) 8"
  by (simp add: show_rightmost_bits_def BC_def)

lemma equal_zero:
  "176 + sLen  emBits  roundup emBits 8 * 8 - emBits  roundup emBits 8 * 8 - (176 + sLen)
   0 = roundup emBits 8 * 8 - emBits - (roundup emBits 8 * 8 - (176 + sLen))"
  by arith

lemma get_salt: " 176 + sLen  emBits; roundup emBits 8 * 8 - emBits  7  (generate_salt (maskedDB_zero (generate_DB (generate_PS emBits 160)) emBits)) = salt"
  apply (insert roundup_help2 [of emBits] length_generate_PS [of emBits] equal_zero [of emBits])
  apply (simp add: generate_DB_def generate_PS_def maskedDB_zero_def)
  apply (simp add: bv_prepend_split bv_prepend_drop generate_salt_def
    show_rightmost_bits_def sLen_def)
  done

lemma generate_maskedDB_elim: "roundup emBits 8 * 8 - emBits  length x; ( roundup emBits 8) * 8 - (length (sha1 M)) - 8 = length (maskedDB_zero x emBits)  generate_maskedDB (maskedDB_zero x emBits @ y @ z) emBits (length(sha1 M)) = maskedDB_zero x emBits"
  apply (simp add: maskedDB_zero_def)
  apply (insert length_bv_prepend_drop [of "(roundup emBits 8 * 8 - emBits)" "x"])
  apply (simp add: generate_maskedDB_def)
  done

lemma generate_H_elim: " roundup emBits 8 * 8 - emBits  length x; length (maskedDB_zero x emBits) =  (roundup emBits 8) * 8 - 168; length y = 160  generate_H (maskedDB_zero x emBits @ y @ z) emBits 160 = y"
  apply (simp add: maskedDB_zero_def)
  apply (insert length_bv_prepend_drop [of "roundup emBits 8 * 8 - emBits" "x"])
  apply (simp add: generate_H_def)
  done

lemma length_bv_prepend_drop_special: "[|roundup emBits 8 * 8 - emBits <= roundup emBits 8 * 8 - (176 + sLen); length (generate_PS emBits 160) = roundup emBits 8 * 8 - (176 + sLen)|] ==> length ( bv_prepend (roundup emBits 8 * 8 - emBits) 𝟬 (drop (roundup emBits 8 * 8 - emBits) (generate_PS emBits 160))) = length (generate_PS emBits 160)"
  by (simp add: length_bv_prepend_drop)

lemma x01_elim: "176 + sLen  emBits; roundup emBits 8 * 8 - emBits  7  take 8 (drop (roundup emBits 8 * 8 - (length (sha1 M) + sLen + 16))(maskedDB_zero (generate_DB (generate_PS emBits 160)) emBits)) = [𝟬, 𝟬, 𝟬, 𝟬, 𝟬, 𝟬, 𝟬, 𝟭]"
  apply (insert roundup_help2 [of emBits] length_generate_PS [of emBits] equal_zero [of emBits])
  apply (simp add: sha1len maskedDB_zero_def generate_DB_def generate_PS_def
    bv_prepend_split bv_prepend_drop)
  done

lemma drop_bv_mapzip:
  assumes "n <= length x" "length x = length y"
  shows "drop n (bv_mapzip f x y) = bv_mapzip f (drop n x) (drop n y)"
proof -
  have "x y. n <= length x  length x = length y 
      drop n (bv_mapzip f x y) = bv_mapzip f (drop n x) (drop n y)"
    apply (induct n)
    apply simp
    apply (case_tac x, case_tac[!] y, auto)
    done
  with assms show ?thesis by simp
qed

lemma [simp]:
  assumes "length a = length b"
  shows "bvxor (bvxor a b) b = a"
proof -
  have "b. length a = length b  bvxor (bvxor a b) b = a"
    apply (induct a)
    apply (auto simp add: bvxor)
    apply (case_tac b)
    apply (simp)+
    apply (case_tac a1)
    apply (case_tac a)
    apply (safe)
    apply (simp)+
    apply (case_tac a)
    apply (simp)+
    done
  with assms show ?thesis by simp
qed

lemma bvxorxor_elim_help:
  assumes "x <= length a" and "length a = length b"
  shows "bv_prepend x 𝟬 (drop x (bvxor (bv_prepend x 𝟬 (drop x (bvxor a b))) b)) =
    bv_prepend x 𝟬 (drop x a)"
proof -
  have "drop x (bvxor (bv_prepend x 𝟬 (drop x (bvxor a b))) b) = drop x a"
    apply (unfold bvxor bv_prepend)
    apply (cut_tac assms)
    apply (insert length_replicate [of x 𝟬 ])
    apply (insert length_drop [of x a])
    apply (insert length_drop [of x b])
    apply (insert length_bvxor [of "drop x a" "drop x b"])
    apply (subgoal_tac "length (replicate x 𝟬 @ drop x (bv_mapzip (⊕b) a b)) = length b")
    apply (subgoal_tac "b = (take x b)@(drop x b)")
    apply (insert drop_bv_mapzip [of x "(replicate x 𝟬 @ drop x (bv_mapzip (⊕b) a b))" b "(⊕b)"])
    apply (simp)
    apply (insert drop_bv_mapzip [of x a b "(⊕b)"])
    apply (simp)
    apply (fold bvxor)
    apply (simp_all)
    done
  with assms show ?thesis by simp
qed

lemma bvxorxor_elim: " roundup emBits 8 * 8 - emBits  length a; length a = length b  (maskedDB_zero (bvxor (maskedDB_zero (bvxor a b) emBits)b) emBits) = bv_prepend (roundup emBits 8 * 8 - emBits) 𝟬 (drop (roundup emBits 8 * 8 - emBits) a)"
  by (simp add: maskedDB_zero_def bvxorxor_elim_help)

lemma verify: "(emsapss_encode M emBits)  []; EM=(emsapss_encode M emBits)  emsapss_decode M EM emBits = True"
  apply (simp add: emsapss_decode_def emsapss_encode_def)
  apply (safe, simp+)
  apply (simp add: emsapss_decode_help1_def emsapss_encode_help1_def)
  apply (safe, simp+)
  apply (simp add: emsapss_decode_help2_def emsapss_encode_help2_def)
  apply (safe)
  apply (simp add: emsapss_encode_help3_def emsapss_encode_help4_def
    emsapss_encode_help5_def emsapss_encode_help6_def)
  apply (safe)
  apply (simp add: emsapss_encode_help7_def emsapss_encode_help8_def lastbits_BC [symmetric])+
  apply (simp add: emsapss_decode_help3_def emsapss_encode_help3_def
    emsapss_decode_help4_def emsapss_encode_help4_def)
  apply (safe)
  apply (insert roundup_le_7 [of emBits] roundup_ge_0 [of emBits 8] roundup_nat_ge_8 [of M emBits])
  apply (simp add: generate_maskedDB_def emsapss_encode_help5_def emsapss_encode_help6_def)
  apply (safe)
  apply (simp)
  apply (simp add: emsapss_encode_help7_def)
  apply (simp only: emsapss_encode_help8_def)
  apply (simp only: maskedDB_zero_def)
  apply (simp only: take_bv_prepend2 min.absorb1)
  apply (simp)
  apply (simp add: emsapss_encode_help5_def emsapss_encode_help6_def)
  apply (safe)
  apply (simp)+
  apply (insert solve_length_generate_DB [of emBits M "generate_M' (sha1 M) salt"] roundup_le_ub [of emBits])
  apply (insert length_MGF [of "(roundup emBits 8) * 8 - 168" "(sha1 (generate_M' (sha1 M) salt))"])
  apply (insert modify_roundup_ge1 [of emBits] modify_roundup_ge2 [of emBits])
  apply (simp add: sha1len emsapss_encode_help7_def emsapss_encode_help8_def)
  apply (insert length_bvxor [of "(generate_DB (generate_PS emBits 160))" "(MGF (sha1 (generate_M' (sha1 M) salt)) ((roundup emBits 8) * 8 - 168))"])
  apply (insert generate_maskedDB_elim [of emBits "(bvxor (generate_DB (generate_PS emBits 160))(MGF (sha1 (generate_M' (sha1 M) salt)) ((roundup emBits 8) * 8 - 168)))" M "sha1 (generate_M' (sha1 M) salt)" BC])
  apply (insert length_maskedDB_zero [of emBits "(bvxor (generate_DB (generate_PS emBits 160))(MGF (sha1 (generate_M' (sha1 M) salt)) ((roundup emBits 8) * 8 - 168)))"])
  apply (insert generate_H_elim [of emBits "(bvxor (generate_DB (generate_PS emBits 160))(MGF (sha1 (generate_M' (sha1 M) salt)) (roundup emBits 8 * 8 - 168)))" "sha1 (generate_M' (sha1 M) salt)" BC])
  apply (simp add: sha1len emsapss_decode_help5_def)
  apply (simp only: emsapss_decode_help6_def emsapss_decode_help7_def)
  apply (insert bvxorxor_elim [of emBits "(generate_DB (generate_PS emBits 160))" "(MGF (sha1 (generate_M' (sha1 M) salt)) ((roundup emBits 8) * 8 - 168))"])
  apply (fold maskedDB_zero_def)
  apply (insert take_equal_bv_prepend [of emBits M] x01_elim [of emBits M] get_salt [of emBits])
  apply (simp add: emsapss_decode_help8_def emsapss_decode_help9_def emsapss_decode_help10_def emsapss_decode_help11_def) 
  done

end