Updated 2012-09-10 23:08:27 by RLE

Purpose: to discuss what RSA stands for, where it is documented, and its implications. ---

RSA is

  • a public-key cryptosystem, which can also be used for digital signatures.
  • an acronym for: Rivest Shamir Adleman, the names of the three inventors of this cryptosystem.

Is there a web home for the topic?

The underlying mechanism for this cryptosystem is a generalization of Fermat's Little Theorem, which says that if p is a prime then for all integers a between 0 and p-1 inclusive, it holds that
   a^p = a  (mod p)

(where ^ means exponentiation). For an integer N that is the product of two primes, it similarly holds that there is an exponent m>1 such that
   a^m = a  (mod N)    for all 0 <= a < N.

However in this case (i) m is usually not a prime and (ii) it is rather hard to determine m from N (but rather easy to determine m from the two primes whose product is N).

As is evident from e.g. the fact that 2m-1 is an exponent returning a to itself whenever m is such an exponent, there are for every N infinitely many possible choices of m. Suppose m is composite (i.e., not a prime) and the product of two integers k and l . Then one can use the above to encrypt a number a by computing
   b = a^k  (mod N)

since the decryption procedure consists of computing
   a = b^l  (mod N)

A very useful feature of this cryptosystem is that the information needed to decrypt messages (numbers l and N) is not the same as that needed to encrypt messages (numbers k and N). This means you can make one half of the key public, but keep the other secret, thus allowing anyone to make encrypted messages that only you can read, or conversely produce encrypted messages that anyone can read but which could only have been encrypted by you. The latter can be used to make digital signatures.

The security of the cryptosystem relies on the fact that it is hard to factorize the number N. Should anyone make a major improvement in integer factorization techniques (e.g. so that factorization and primality checking would be of comparable speed) then RSA would probably be rendered useless, but for the moment it seems to be OK. A piece of warning may however be in order: There are some choices of keys for RSA that are easily broken, even if the number N by itself is hard to factorize. Presumably code libraries that generate RSA cryptos have a lot of built-in checks to avoid outputting one of these inferior sets of keys.

2004Sept2 PS Salvatore Sanfilippo has created a Pure-Tcl bignum package (attached to http://wiki.hping.org/108 ). Using this a (naive!) implementation of RSA is rather trivial to do:
    source bignum.tcl

    set P 61
    set Q 53
    set PQ 3233
    set E 17
    set D 2753

    # Turn into bignums:
    set pub [fromstr $E]
    set pri [fromstr $D]
    set pq [fromstr $PQ]
    set text "Salvatore does amaze!"

    proc rsa { c key pq } {
        #puts "RSA: [tostr $c] [tostr $key] [tostr $pq]"
        return [powm $c $key $pq]

    proc encrypt { text key pq } {
        set crypted {}
        foreach char [split $text ""] {
            lappend crypted [tostr [rsa [fromstr [scan $char %c]] $key $pq]]
        return $crypted

    proc decrypt { crypt key pq } {
        set plain ""
        foreach cypher $crypt {
            append plain [format %c [tostr [rsa [fromstr $cypher] $key $pq]]]
        return $plain

    puts "Plain: $text"
    set cypher [encrypt $text $pri $pq]
    puts "Encrypt: $cypher"
    puts "Decrypt: [decrypt $cypher $pub $pq]"

Mind you - this is not how real implementation of RSA would work... Direct encryption of a plain-text with RSA <shudder>. -- Pascal.

20041109 Twylite - See RSA in Tcl for a more complete implementation. Tcllib also has a pure Tcl math::bignum package.