With so much fraud and problems with security on the internet, it’s sometimes hard to know if a file you download on the internet is what you think it is or not.  Fortunately, you have a way of taking a “fingerprint” of any file you wish to post, and likewise a way of verifying any file you download against that same “fingerprint”.  The instructions I give for this tutorial (and every tutorial I’ll do) will be for the Mac, but most everything I show will be able to be done in any *nix OS.  Sorry Windows users.

 

Instructions for hosting a file along with the MD5 digest:

 

Step 1:

Open the terminal (located in /Applications/Utilities), then type:

 

Openssl MD5

 

Then drag the file you wish to “print” into the terminal window.  You should end up with something like this:

 

openssl md5 /Users/myuser/Desktop/Ducks\ HDR.psd

 

Now hit return.  You will now see something similar to this:

 

MD5(/Users/myuser/Desktop/Ducks HDR.psd)= eaa3a55a38acf70efab8259206fd047a

 

The string after the equal sign (=) is your digest.  You can now post this digest along with a file you send/post, and anyone on the receiving end can run the same command to generate the exact same digest.  If the digest is exactly the same, then you know there has been no change to the file.  If the number is different, then something has changed.

 

If you don’t want to use MD5, you can also use some of these other algorithm types.  Just replace md5 with the type you want to use.  These other commands are:

 

md2            md4            md5            rmd160         sha            sha1

 

This should be enough information to get you started.  However, the openssl command is very powerful and useful.  I’m going to include the manpage at the end of the article, glance over it and see if there is anything that you may find useful. 

The openssl manual page:

 

OPENSSL(1)                                         OpenSSL                                        OPENSSL(1)

NAME
       openssl - OpenSSL command line tool

SYNOPSIS
       openssl command [ command_opts ] [ command_args ]

       openssl [ list-standard-commands | list-message-digest-commands | list-cipher-commands ]

       openssl no-XXX [ arbitrary options ]

DESCRIPTION
       OpenSSL is a cryptography toolkit implementing the Secure Sockets Layer (SSL v2/v3) and Transport
       Layer Security (TLS v1) network protocols and related cryptography standards required by them.

       The openssl program is a command line tool for using the various cryptography functions of OpenSSL's
       crypto library from the shell.  It can be used for

        o  Creation of RSA, DH and DSA key parameters
        o  Creation of X.509 certificates, CSRs and CRLs
        o  Calculation of Message Digests
        o  Encryption and Decryption with Ciphers
        o  SSL/TLS Client and Server Tests
        o  Handling of S/MIME signed or encrypted mail

COMMAND SUMMARY
       The openssl program provides a rich variety of commands (command in the SYNOPSIS above), each of
       which often has a wealth of options and arguments (command_opts and command_args in the SYNOPSIS).

       The pseudo-commands list-standard-commands, list-message-digest-commands, and list-cipher-commands
       output a list (one entry per line) of the names of all standard commands, message digest commands, or
       cipher commands, respectively, that are available in the present openssl utility.

       The pseudo-command no-XXX tests whether a command of the specified name is available.  If no command
       named XXX exists, it returns 0 (success) and prints no-XXX; otherwise it returns 1 and prints XXX.
       In both cases, the output goes to stdout and nothing is printed to stderr.  Additional command line
       arguments are always ignored.  Since for each cipher there is a command of the same name, this
       provides an easy way for shell scripts to test for the availability of ciphers in the openssl
       program.  (no-XXX is not able to detect pseudo-commands such as quit, list-...-commands, or no-XXX
       itself.)

       STANDARD COMMANDS

       asn1parse Parse an ASN.1 sequence.

       ca        Certificate Authority (CA) Management.

       ciphers   Cipher Suite Description Determination.

       crl       Certificate Revocation List (CRL) Management.

       crl2pkcs7 CRL to PKCS#7 Conversion.

       dgst      Message Digest Calculation.

       dh        Diffie-Hellman Parameter Management.  Obsoleted by dhparam.

       dsa       DSA Data Management.

       dsaparam  DSA Parameter Generation.

       enc       Encoding with Ciphers.

       errstr    Error Number to Error String Conversion.

       dhparam   Generation and Management of Diffie-Hellman Parameters.

       gendh     Generation of Diffie-Hellman Parameters.  Obsoleted by dhparam.

       gendsa    Generation of DSA Parameters.

       genrsa    Generation of RSA Parameters.

       ocsp      Online Certificate Status Protocol utility.

       passwd    Generation of hashed passwords.

       pkcs12    PKCS#12 Data Management.

       pkcs7     PKCS#7 Data Management.

       rand      Generate pseudo-random bytes.

       req       X.509 Certificate Signing Request (CSR) Management.

       rsa       RSA Data Management.

       rsautl    RSA utility for signing, verification, encryption, and decryption.

       s_client  This implements a generic SSL/TLS client which can establish a transparent connection to a
                 remote server speaking SSL/TLS. It's intended for testing purposes only and provides only
                 rudimentary interface functionality but internally uses mostly all functionality of the
                 OpenSSL ssl library.

       s_server  This implements a generic SSL/TLS server which accepts connections from remote clients
                 speaking SSL/TLS. It's intended for testing purposes only and provides only rudimentary
                 interface functionality but internally uses mostly all functionality of the OpenSSL ssl
                 library.  It provides both an own command line oriented protocol for testing SSL functions
                 and a simple HTTP response facility to emulate an SSL/TLS-aware webserver.

       s_time    SSL Connection Timer.

       sess_id   SSL Session Data Management.

       smime     S/MIME mail processing.

       speed     Algorithm Speed Measurement.

       verify    X.509 Certificate Verification.

       version   OpenSSL Version Information.

       x509      X.509 Certificate Data Management.

       MESSAGE DIGEST COMMANDS

       md2       MD2 Digest

       md5       MD5 Digest

       mdc2      MDC2 Digest

       rmd160    RMD-160 Digest

       sha       SHA Digest

       sha1      SHA-1 Digest

       ENCODING AND CIPHER COMMANDS

       base64    Base64 Encoding

       bf bf-cbc bf-cfb bf-ecb bf-ofb
                 Blowfish Cipher

       cast cast-cbc
                 CAST Cipher

       cast5-cbc cast5-cfb cast5-ecb cast5-ofb
                 CAST5 Cipher

       des des-cbc des-cfb des-ecb des-ede des-ede-cbc des-ede-cfb des-ede-ofb des-ofb
                 DES Cipher

       des3 desx des-ede3 des-ede3-cbc des-ede3-cfb des-ede3-ofb
                 Triple-DES Cipher

       idea idea-cbc idea-cfb idea-ecb idea-ofb
                 IDEA Cipher

       rc2 rc2-cbc rc2-cfb rc2-ecb rc2-ofb
                 RC2 Cipher

       rc4       RC4 Cipher

       rc5 rc5-cbc rc5-cfb rc5-ecb rc5-ofb
                 RC5 Cipher

PASS PHRASE ARGUMENTS
       Several commands accept password arguments, typically using -passin and -passout for input and output
       passwords respectively. These allow the password to be obtained from a variety of sources. Both of
       these options take a single argument whose format is described below. If no password argument is
       given and a password is required then the user is prompted to enter one: this will typically be read
       from the current terminal with echoing turned off.

       pass:password
                 the actual password is password. Since the password is visible to utilities (like 'ps'
                 under Unix) this form should only be used where security is not important.

       env:var   obtain the password from the environment variable var. Since the environment of other
                 processes is visible on certain platforms (e.g. ps under certain Unix OSes) this option
                 should be used with caution.

       file:pathname
                 the first line of pathname is the password. If the same pathname argument is supplied to
                 -passin and -passout arguments then the first line will be used for the input password and
                 the next line for the output password. pathname need not refer to a regular file: it could
                 for example refer to a device or named pipe.

       fd:number read the password from the file descriptor number. This can be used to send the data via a
                 pipe for example.

       stdin     read the password from standard input.

SEE ALSO
       asn1parse(1), ca(1), config(5), crl(1), crl2pkcs7(1), dgst(1), dhparam(1), dsa(1), dsaparam(1),
       enc(1), gendsa(1), genrsa(1), nseq(1), openssl(1), passwd(1), pkcs12(1), pkcs7(1), pkcs8(1), rand(1),
       req(1), rsa(1), rsautl(1), s_client(1), s_server(1), s_time(1), smime(1), spkac(1), verify(1),
       version(1), x509(1), crypto(3), ssl(3)

HISTORY
       The openssl(1) document appeared in OpenSSL 0.9.2.  The list-XXX-commands pseudo-commands were added
       in OpenSSL 0.9.3; the no-XXX pseudo-commands were added in OpenSSL 0.9.5a.  For notes on the
       availability of other commands, see their individual manual pages.

0.9.7l                                           2004-01-04                                       OPENSSL(1)