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