Sha256 Hashes in Fortran

Our program demonstrates how to compute SHA256 hashes in Fortran. SHA256 hashes are frequently used to compute short identities for binary or text blobs. For example, TLS/SSL certificates use SHA256 to compute a certificate’s signature.

program sha256_hashes
    use iso_fortran_env, only: int8, int32
    use, intrinsic :: iso_c_binding, only: c_char, c_ptr, c_loc, c_size_t
    implicit none

    interface
        subroutine sha256_init() bind(c, name='SHA256_Init')
        end subroutine sha256_init

        subroutine sha256_update(data, len) bind(c, name='SHA256_Update')
            import :: c_ptr, c_size_t
            type(c_ptr), value :: data
            integer(c_size_t), value :: len
        end subroutine sha256_update

        subroutine sha256_final(md) bind(c, name='SHA256_Final')
            import :: c_ptr
            type(c_ptr), value :: md
        end subroutine sha256_final
    end interface

    character(len=:), allocatable :: s
    integer(int8), dimension(32) :: hash
    integer :: i

    s = "sha256 this string"

    ! Initialize the SHA256 context
    call sha256_init()

    ! Update the context with the input string
    call sha256_update(c_loc(s), len(s))

    ! Finalize the hash
    call sha256_final(c_loc(hash))

    ! Print the original string
    print *, s

    ! Print the hash in hexadecimal format
    write(*, '(*(Z2.2))', advance='no') hash
    print *
end program sha256_hashes

In this Fortran implementation:

1. We use the iso_fortran_env and iso_c_binding modules for interoperability with C libraries.

2. We declare interfaces for the C functions SHA256_Init, SHA256_Update, and SHA256_Final from the OpenSSL library.

3. We create a string s that we want to hash.

4. We initialize the SHA256 context using sha256_init().

5. We update the context with our input string using sha256_update().

6. We finalize the hash using sha256_final().

7. Finally, we print the original string and the computed hash in hexadecimal format.

To compile and run this program, you’ll need to link against the OpenSSL library:

$ gfortran -o sha256_hashes sha256_hashes.f90 -lcrypto
$ ./sha256_hashes
sha256 this string
1af1dfa857bf1d8814fe1af8983c18080019922e557f15a8a...

Note that if you need cryptographically secure hashes, you should carefully research hash strength!

Fortran doesn’t have built-in cryptographic functions like some other languages, so we’re using the OpenSSL library here. In a real-world scenario, you might want to use a Fortran-specific cryptography library or write more robust error handling and memory management.