Simple CAPTCHA?

[PA <petite.abeille@...>]

Hello,

Anyone has a suggestion for a simple (simplistic?) CAPTCHA mechanism? 
By simple, I mean not necessarily requiring heavy duty image generation 
or such...

Any ideas?

Here is a rather feeble attempt which generates a simple mathematical 
question (e.g. 27 + 58), compute its result (85), and generates a HMAC 
of the challenge using its result as the key (e.g. HMAC( '85', '27 + 
58' ) ) for later validation:

local function Challenge()
    local Operator = { '+', '-' }
    local aBuffer  = {}

    math.randomseed( os.time() )
    math.random( 1, os.time() )

    aBuffer[ #aBuffer + 1 ] = tostring( math.random( 1, 100 ) )
    aBuffer[ #aBuffer + 1 ] = Operator[ math.random( 1, #Operator ) ]
    aBuffer[ #aBuffer + 1 ] = tostring( math.random( 1, 100 ) )

    return table.concat( aBuffer, ' ' )
end

local function HMAC( aKey, aValue )
    local md5 = require( 'md5' )
    local bit = require( 'bit' )

    if aKey:len() > 64 then
        aKey = md5.digest( aKey, true )
    end

    if aKey:len() < 64 then
        aKey = aKey .. string.char( 0 ):rep( 64 - aKey:len() )
    end

    local anInnerKey = aKey:gsub( '.', function( aChar ) return 
string.char( bit.bxor( aChar:byte(), 54 ) ) end )
    local anOuterKey = aKey:gsub( '.', function( aChar ) return 
string.char( bit.bxor( aChar:byte(), 92 ) ) end )

    return md5.digest( anOuterKey .. md5.digest( anInnerKey .. aValue, 
true ) )
end

local aChallenge = Challenge()
local aResult = loadstring( 'return ' .. aChallenge )()

print( aChallenge )
print( aResult )
print( HMAC( tostring( aResult ), aChallenge ) )

Thoughts?