Lecture 17: Encryption #2 -- Public Key Systems

Public Key Systems

DES and its derivatives works well, but relies on both parties having a copy of the same secret key. The problem of Key Distribution in such systems can be a very difficult to overcome. In 1976, Diffie and, Hellman[1] proposed an enitrely new system called Public Key Cryptography. The basic ideas are:

The original algorithm proposed by Diffie and Hellman for implementing Public Key Cryptography turned out to be insufficiently strong to implement a secure system. However, the idea of this type of cryptosystem was considered worthy of further work.

[1] This is the standard story on the origins of Public Key Cryptography. It's now apparent that a group of British researchers had come up with the same idea somewhat earlier: see, for example: The Open Secret.


In 1978, Rivest, Shamir and Adleman of MIT proposed a number-theoretic way of implementing a Public Key Cryptosystem. Their method has been widely adopted. The basic technique is:

To use this technique, divide the plaintext (regarded as a bit string) into blocks so that each plaintext message P falls into the interval 0 <= P < n. This can be done by dividing it into blocks of k bits where k is the largest integer for which 2k < n is true.

To encrypt: C = Pe (mod n)
To decrypt: P = Cd (mod n)
The public key, used to encrypt, is thus: (en) and the private key, used to decrypt, is (dn))

RSA Example -- Key Generation

To create the public key, select two large positive prime numbers p and q p = 7, q = 17
Large enough for us!
Compute (p-1) * (q-1) x = 96
Choose an integer E which is relatively prime to x E = 5
Compute n = p * q n = 119
Kp is then n concatenated with E Kp = 119, 5

To create the secret key, compute D such that (D * E) mod x = 1 Ks = 119, 77

RSA Example -- Encryption and Decryption

To compute the ciphertext C of plaintext P, treat P as a numerical value P = 19
C = PE mod n C = 66

To compute the plaintext P from ciphertext C:  
P = CD mod n P = 19

RSA in Practice

RSA works because knowledge of the public key does not reveal the private key. Note that both the public and private keys contain the important number n = p * q. The security of the system relies on the fact that n is hard to factor -- that is, given a large number (even one which is known to have only two prime factors) there is no easy way to discover what they are. This is a well known mathematical fact. If a fast method of factorisation is ever discovered then RSA will cease to be useful.

It is obviously possible to break RSA with a brute force attack -- simply factorise n. To make this difficult, it's usually recommended that p and q be chosen so that n is (in 2003 numbers) at least 1024 bits.

One excellent feature of RSA is that it is symmetrical. We already know that if you encrypt a message with my public key then only I can decrypt that ciphertext, using my secret key. However, it also turns out that a message encrypted with my secret key can only be decrypted with my public key. This has important implications, see later.

The RSA algorithm operates with huge numbers, and involves lots of exponentiation (ie, repeated multiplication) and modulus arithmetic. Such operations are computationally expensive (ie, they take a long time!) and so RSA encryption and decryption are incredibly slow, even on fast computers. Compare this to the operations involved in DES (and other single-key systems) which consist of repeated simple XORs and transpositions. Typical numbers are that DES is 100 times faster than RSA on equivalent hardware. Furthermore, DES can be easily implemented in dedicated hardware (RSA is, generally speaking, a software-only technology) giving a speed improvement of up to 10,000 times.

Public Key Cryptography In Summary

Public Key Authentication

This is the technique by which an entity verifies that her communication partner is who he purports to be and not an imposter. Authentication can be easily achieved if both parties share a common secret key or keys, (eg, typical password authentication) however it's much more nicely done using Public Key cryptography:

Public key cryptosystems can provide authentication if, in addition to:

P = D ( E ( P ) )
we also have:
P = E ( D ( P ) )
This is true for RSA. Now consider Alice, who wishes to authenticate herself to a communications partner Bob. Bob already knows Alice's public key.

Alice convinces Bob that she's who she
    claims to be
Alice announces to Bob that she wishes to communicate. Bob responds by choosing a large, single-use random number R (sometimes called a nonce) which he sends to Alice. Alice encrypts the random number using her private key, da and returns the encrypted value to Bob. Bob applies Alice's public key to the returned value, and if it decrypts to R then he can be certain of the identity of his communication partner. It's obvious that this protocol could be extended to verify the identity of Bob as well.

Digital Signatures

These extend the basic Public Key authentication protocols to documents or messages. The three key objectives are:

  1. The receiver can verify the claimed identity of the sender, because only the sender's public key will decrypt it.

  2. The sender cannot later repudiate the contents of the message, because only the possessor of the specific private key could have generated it.

  3. The receiver cannot possibly have concocted the message himself.

The recipient's public key can (optionally) be used to encrypt the message, so that only the recipient can read it. This step is only necessary if both authentication and secrecy are needed. We can take a plaintext message P, and encode it thus:

Digital signature operation

Message Digests

The digital signature method of the previous slide creates an encrypted message. It's often preferable to leave the message as plaintext, but append a signature which verifies its integrity. A message digest is a one-way hash function which has the following characteristics:

The Internet standard for message digests is the MD5 algorithm, invented by Rivest. Software implementations of this algorithm are widely available. MD5 produces a 128 bit (16 byte) message digest which can be appended to the message. Message digests such as MD5 are often referred to as cryptograhic checksums, because they reveal whether the message has been altered.

Typical usage of message digests combines public key cryptography with the message digest function. In this case, a sender first computes an MD5 digest as above, then encrypts it using her private key, and finally appends the encrypted digest to the message. A recipient can read the message, and can be confident that it originated from the sender.

Digression: The Politics of Encryption

Most of the governments of the world are very interested in encryption. Over the past 20 years, the USA government has proposed and/or mandated a variety of encryption laws to either restrict the use of strong encryption, or to force developers to provide a "back door" to their systems, which would allow officials to read encrypted data. In addition, for many years they have classified all forms of strong encryption as munitions, and restricted export of encryption products. It's only in the past year or so that restrictions on encryption exports have been eased/lifted.

Other governments (eg France, Malaysia, China, etc) have, at various times, simply banned the private use of strong encryption. In most cases, these policies have subsequently been reversed.

The problem for governments is that strong (ie, unbreakable) encryption can be used by criminals (or political enemies?) to communicate securely, so there is a an obvious vested interest in restricting it. On the other hand, strong encryption is absolutely necessary to enable Internet-based commerce (we see some applications later), so a government which restricts its use is limiting its people from participating in the new world of Internet-based business.

Addendum: Avoiding Exponentiation

One of the biggest problems (for beginners) in implementing the RSA algorithm is that of exponentiation -- raising large numbers to the power of other large numbers. In general, this is very difficult without a special-purpose "BigNum" library. However, for our "demonstration" examples we can actually use a very clever algorithm to avoid hitting "integer overflow", and still demonstrate that the numbers are correct. Here's the pseudocode for how we calculate C, the ciphertext, from P, the plaintext using the secret key Kp, defined as n concatenated with E.

C := 1
begin for i := 1 to E do
    C := MOD( C * P, n )
To decrypt, simply replace E with D and P with C in the above algorithm. Exercise: implement this in your favourite programming language, and prove it works...

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Copyright 2003 by Philip Scott, La Trobe University.
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