Question

please help me with to explain it by your word

logic and algo

Encrypt,Decrypt,solve in Fortran.

Answer 1

Algorithm for Encrypt in Fortran:
--------------------------------------------------

FORTRAN Format--ENCRYPT

CALL <ENCRYPT>(
A status,id,algid,key,keylen,iv,bufcnt,data,d$len,
B outdata,buflen,d$fmt,crpt$m,final,flags)
where <ENCRYPT> is
CI2_ENCRYPT for UCS FORTRAN or
C2FENC for ASCII FORTRAN

Algorithm for Decrypt in Fortran:
--------------------------------------------------

FORTRAN Format--Decrypt
CALL <DECRYPT>(
A status,id,algid,key,keylen,iv,bufcnt,data,d$len,
B outdata,buflen,d$fmt,crpt$m,final,flags)
where <DECRYPT> is
CI2_DECRYPT for UCS FORTRAN or
C2FDEC for ASCII FORTRAN

Here i am providing an example for understanding the logic.

Example :
------------------

implicit none

character(80) :: plaintext = "Beware the Jabberwock, my son! The jaws that bite, the claws that catch!", &
ciphertext = ""
character(14) :: key = "VIGENERECIPHER"


call encrypt(plaintext, ciphertext, key)
write(*,*) plaintext
write(*,*) ciphertext
call decrypt(ciphertext, plaintext, key)
write(*,*) plaintext

contains

subroutine encrypt(intxt, outtxt, k)
character(*), intent(in) :: intxt, k
character(*), intent(out) :: outtxt
integer :: chrn
integer :: cp = 1, kp = 1
integer :: i

outtxt = ""
do i = 1, len(trim(intxt))
select case(intxt(i:i))
case ("A":"Z", "a":"z")
select case(intxt(i:i))
case("a":"z")
chrn = iachar(intxt(i:i)) - 32

case default
chrn = iachar(intxt(i:i))

end select

outtxt(cp:cp) = achar(modulo(chrn + iachar(k(kp:kp)), 26) + 65)
cp = cp + 1
kp = kp + 1
if(kp > len(k)) kp = kp - len(k)

end select
end do
end subroutine

subroutine decrypt(intxt, outtxt, k)
character(*), intent(in) :: intxt, k
character(*), intent(out) :: outtxt
integer :: chrn
integer :: cp = 1, kp = 1
integer :: i

outtxt = ""
do i = 1, len(trim(intxt))
chrn = iachar(intxt(i:i))
outtxt(cp:cp) = achar(modulo(chrn - iachar(k(kp:kp)), 26) + 65)
cp = cp + 1
kp = kp + 1
if(kp > len(k)) kp = kp - len(k)
end do
end subroutine
end program

OUTPUT For the program:

---------------------------------------------

 Beware the Jabberwock, my son! The jaws that bite, the claws that catch!
 WMCEEIKLGRPIFVMEUGXQPWQVIOIAVEYXUEKFKBTALVXTGAFXYEVKPAGY
 BEWARETHEJABBERWOCKMYSONTHEJAWSTHATBITETHECLAWSTHATCATCH