F5 iRule Example integrating HMAC

Hi Dtwesten,

the mentioned codeshare sample seems to use just commands, that where already available on 10.2 plattforms.

Did you already tried this sample on your box? Any TCL related error messages received?

Cheers, Kai

Nothing yet.. The example didn't really define with comments what the values mean. For example, I believe the message variable is the data I want encrypted and that the token is the output. This could be just an issue of me reading up on HMAC, so just trying to see if someone has a full blown example of how they used it for say... user session authentication or validation.. Make sense? Thanks for replying so quickly

Hi dtwesten,

regarding hmac codeshare example, I remember implementing it based on algorithm found on hmac wiki https://en.wikipedia.org/wiki/Hash-based_message_authentication_code

the hash result in binary is stored in variable "token"

the hash result in hex string is stored in variable "hextoken"

Nat

You'll get the same value with the new(er) CRYPTO::sign command. As an extended example:

when RULE_INIT {
    set message "test"
    set key [sha256 "yyyy123456789012345678901234567890123456789012345678901234567890xxxx"]


     all of this is the manual approach per Nat's example 
    set ipad ""
    set opad ""
    for { set j 0 }{ $j < [string length $key] }{ incr j }{
        binary scan $key @${j}H2 k
        set o [expr 0x$k ^ 0x5c]
        set i [expr 0x$k ^ 0x36]
        append ipad [format %c $i]
        append opad [format %c $o]
    }
    for { }{ $j < $bsize }{ incr j }{
        append ipad 6
        append opad \\
    }
    set token [sha256 $opad[sha256 "${ipad}${message}"]]
    binary scan $token H* hextoken
    log -noname local0. [string toupper "result  = $hextoken"]


     and this is with the CRYPTO::sign command using the same key and message 
    set signed_data [CRYPTO::sign -alg hmac-sha256 -key $key $message]
    binary scan $signed_data H* hexdata
    log -noname local0. [string toupper "signed = $hexdata"]
}

Ugh. You did mention the version earlier. ;)

Well then, you're stuck with the first version using sha256 directly.

Kevin,, I see how to create the HMAC using the 10.2 version of the HMAC iRULE, but I don't see how to sign or verify. That was the piece I was missing. Basically, I am going to set two pieces of information set when HTTP Request as my message and sign them using the HMAC (one piece of info from an application session cookie and the other from X509 Subject field from client presented cert). I'd like to verify the hash each time a request comes in to validate the user and the session. It looks pretty easy in 11.1 (or later), but I don't see many examples using 10.2 code..Thoughts?

By the way.. this was a pretty good read on HMAC.. http://csrc.nist.gov/publications/fips/fips198-1/FIPS-198-1_final.pdf

Kai,, thanks for the reply.. I understand the concept, and I see where in the HMAC irule that the token and hextoken is set. I can use that to send out, and receive back to the F5 as a cookie. Once the cookie comes back, I need to validate the message, so I need to decrypt the cookie. I see the commands in 11.1 with crypto::sign and crypto::verify.. However I don't see how to that in the 10.x version. That is the part I am missing..

Here is the quick coding...

when RULE_INIT {

    set sharedkey "1234"

    set crypto_sign [call crypto_sign "hallo world" $sharedkey]
    log -noname local0. "HMAC Code is = $crypto_sign"
    set crypto_verify [call crypto_verify "hallo world" $sharedkey $crypto_sign]
    log -noname local0. "HMAC Code verified = $crypto_verify"

}

proc crypto_sign { message prekey } {

    set bsize 64
    if { [string length $prekey] > $bsize } {
        set key [sha256 $prekey]
    } else {
        set key $prekey
    }

    set ipad ""
    set opad ""
    for { set j 0 }{ $j < [string length $key] }{ incr j }{
        binary scan $key @${j}H2 k
        set o [expr 0x$k ^ 0x5c]
        set i [expr 0x$k ^ 0x36]
        append ipad [format %c $i]
        append opad [format %c $o]
    }
    for { }{ $j < $bsize }{ incr j }{
        append ipad 6
        append opad \\
    }

    set token [sha256 $opad[sha256 "${ipad}${message}"]]

    binary scan $token H* hextoken
    return $hextoken

}

proc crypto_verify { message prekey hmac } {

    set bsize 64
    if { [string length $prekey] > $bsize } {
        set key [sha256 $prekey]
    } else {
        set key $prekey
    }

    set ipad ""
    set opad ""
    for { set j 0 }{ $j < [string length $key] }{ incr j }{
        binary scan $key @${j}H2 k
        set o [expr 0x$k ^ 0x5c]
        set i [expr 0x$k ^ 0x36]
        append ipad [format %c $i]
        append opad [format %c $o]
    }
    for { }{ $j < $bsize }{ incr j }{
        append ipad 6
        append opad \\
    }

    set token [sha256 $opad[sha256 "${ipad}${message}"]]

    binary scan $token H* hextoken

    if { $hextoken eq $hmac } then {
        return 1
    } else {
        return 0
    }
}

Cheers, Kai

Perfect thanks.. It wasn't making sense to me.. Really appreciate your help.