Public-key cryptography

Public-key cryptography, or asymmetric cryptography, is a cryptographic system that uses pairs of keys: public keys, which may be disseminated widely, and private keys, which are known only to the owner. The generation of such keys depends on cryptographic algorithms based on mathematical problems to produce one-way functions. Effective security only requires keeping the private key private; the public key can be openly distributed without compromising security.
In such a system, any person can encrypt a message using the receiver's public key, but that encrypted message can only be decrypted with the receiver's private key.
Robust authentication is also possible. A sender can combine a message with a private key to create a short digital signature on the message. Anyone with the sender's corresponding public key can combine the same message and the supposed digital signature associated with it to verify whether the signature was valid, i.e. made by the owner of the corresponding private key.
Public key algorithms are fundamental security ingredients in modern cryptosystems, applications and protocols assuring the confidentiality, authenticity and non-repudiability of electronic communications and data storage. They underpin various Internet standards, such as Transport Layer Security, S/MIME, PGP, and GPG. Some public key algorithms provide key distribution and secrecy, some provide digital signatures, and some provide both.


Before the mid-1970s, all cipher systems used symmetric key algorithms, in which the same cryptographic key is used with the underlying algorithm by both the sender and the recipient, who must both keep it secret. Of necessity, the key in every such system had to be exchanged between the communicating parties in some secure way prior to any use of the system – a secure channel. This requirement is never trivial and very rapidly becomes unmanageable as the number of participants increases, or when secure channels aren't available for key exchange, or when,, keys are frequently changed. In particular, if messages are meant to be secure from other users, a separate key is required for each possible pair of users.
By contrast, in a public key system, the public keys can be disseminated widely and openly, and only the private key needs to be kept secure by its owner.
Two of the best-known uses of public key cryptography are:
One important issue is confidence/proof that a particular public key is authentic, i.e. that it is correct and belongs to the person or entity claimed, and has not been tampered with or replaced by a malicious third party. There are several possible approaches, including:
A public key infrastructure, in which one or more third parties – known as certificate authorities – certify ownership of key pairs. TLS relies upon this.
A "web of trust" which decentralizes authentication by using individual endorsements of the link between user and public key. PGP uses this approach, as well as lookup in the domain name system. The DKIM system for digitally signing emails also uses this approach.


The most obvious application of a public key encryption system is in encrypting communication to provide confidentiality – a message that a sender encrypts using the recipient's public key can be decrypted only by the recipient's paired private key.
Another application in public key cryptography is the digital signature. Digital signature schemes can be used for sender authentication.
Non-repudiation systems use digital signatures to ensure that one party cannot successfully dispute its authorship of a document or communication.
Further applications built on this foundation include: digital cash, password-authenticated key agreement, time-stamping services, non-repudiation protocols, etc.
Because asymmetric key algorithms are nearly always much more computationally intensive
than symmetric ones, in many cases it is common to exchange a key using a key-exchange algorithm, then transmit data using that key and a symmetric key algorithm. PGP, SSH, and the SSL/TLS family of schemes use this procedure, and are thus called hybrid cryptosystems.


As with all security-related systems, it is important to identify potential weaknesses.


All public key schemes are in theory susceptible to a "brute-force key search attack". Such attacks are impractical, however, if the amount of computation needed to succeed – termed the "work factor" by Claude Shannon – is out of reach of all potential attackers. In many cases, the work factor can be increased by simply choosing a longer key. But other algorithms may have much lower work factors, making resistance to a brute-force attack irrelevant. Some special and specific algorithms have been developed to aid in attacking some public key encryption algorithms – both RSA and ElGamal encryption have known attacks that are much faster than the brute-force approach.
Major weaknesses have been found for several formerly promising asymmetric key algorithms. The "knapsack packing" algorithm was found to be insecure after the development of a new attack. As with all cryptographic functions, public-key implementations may be vulnerable to side-channel attacks that exploit information leakage to simplify the search for a secret key. Research is underway to both discover, and to protect against, new attacks.

Alteration of public keys

Another potential security vulnerability in using asymmetric keys is the possibility of a "man-in-the-middle" attack, in which the communication of public keys is intercepted by a third party and then modified to provide different public keys instead. Encrypted messages and responses must also be intercepted, decrypted, and re-encrypted by the attacker using the correct public keys for different communication segments, in all instances, so as to avoid suspicion.
A communication is said to be insecure where data is transmitted in a manner that allows for interception. These terms refer to reading the sender's private data in its entirety. A communication is particularly unsafe when interceptions can't be prevented or monitored by the sender.
A man-in-the-middle attack can be difficult to implement due to the complexities of modern security protocols. However, the task becomes simpler when a sender is using insecure mediums such as public networks, the Internet, or wireless communication. In these cases an attacker can compromise the communications infrastructure rather than the data itself. A hypothetical malicious staff member at an Internet Service Provider might find a man-in-the-middle attack relatively straightforward. Capturing the public key would only require searching for the key as it gets sent through the ISP's communications hardware.
In some advanced man-in-the-middle attacks, one side of the communication will see the original data while the other will receive a malicious variant. Asymmetric man-in-the-middle attacks can prevent users from realizing their connection is compromised. This remains true even when one user's data is known to be compromised because the data appears fine to the other user. This can lead to confusing disagreements between users such as "it must be on your end!" when neither user is at fault. Hence, man-in-the-middle attacks are only fully preventable when the communications infrastructure is physically controlled by one or both parties; such as via a wired route inside the sender's own building. In summation, public keys are easier to alter when the communications hardware used by a sender is controlled by an attacker.

Public key infrastructure

One approach to prevent such attacks involves the use of a public key infrastructure ; a set of roles, policies, and procedures needed to create, manage, distribute, use, store and revoke digital certificates and manage public-key encryption. However, this in turn has potential weaknesses.
For example, the certificate authority issuing the certificate must be trusted to have properly checked the identity of the key-holder, must ensure the correctness of the public key when it issues a certificate, must be secure from computer piracy, and must have made arrangements with all participants to check all their certificates before protected communications can begin. Web browsers, for instance, are supplied with a long list of "self-signed identity certificates" from PKI providers – these are used to check the bona fides of the certificate authority and then, in a second step, the certificates of potential communicators. An attacker who could subvert any single one of those certificate authorities into issuing a certificate for a bogus public key could then mount a "man-in-the-middle" attack as easily as if the certificate scheme were not used at all. In an alternate scenario rarely discussed, an attacker who penetrates an authority's servers and obtains its store of certificates and keys would be able to spoof, masquerade, decrypt, and forge transactions without limit.
Despite its theoretical and potential problems, this approach is widely used. Examples include TLS and its predecessor SSL, which are commonly used to provide security for web browser transactions.
Aside from the resistance to attack of a particular key pair, the security of the certification hierarchy must be considered when deploying public key systems. Some certificate authority – usually a purpose-built program running on a server computer – vouches for the identities assigned to specific private keys by producing a digital certificate. Public key digital certificates are typically valid for several years at a time, so the associated private keys must be held securely over that time. When a private key used for certificate creation higher in the PKI server hierarchy is compromised, or accidentally disclosed, then a "man-in-the-middle attack" is possible, making any subordinate certificate wholly insecure.


Examples of well-regarded asymmetric key techniques for varied purposes include:
Examples of asymmetric key algorithms not widely adopted include:
Examples of notable – yet insecure – asymmetric key algorithms include:
Examples of protocols using asymmetric key algorithms include:
During the early history of cryptography, two parties would rely upon a key that they would exchange by means of a secure, but non-cryptographic, method such as a face-to-face meeting or a trusted courier. This key, which both parties kept absolutely secret, could then be used to exchange encrypted messages. A number of significant practical difficulties arise with this approach to distributing keys.


In his 1874 book The Principles of Science, William Stanley Jevons wrote:

Can the reader say what two numbers multiplied together will produce the number 8616460799? I think it unlikely that anyone but myself will ever know.

Here he described the relationship of one-way functions to cryptography, and went on to discuss specifically the factorization problem used to create a trapdoor function. In July 1996, mathematician Solomon W. Golomb said: "Jevons anticipated a key feature of the RSA Algorithm for public key cryptography, although he certainly did not invent the concept of public key cryptography."

Classified discovery

In 1970, James H. Ellis, a British cryptographer at the UK Government Communications Headquarters, conceived of the possibility of "non-secret encryption",, but could see no way to implement it. In 1973, his colleague Clifford Cocks implemented what has become known as the RSA encryption algorithm, giving a practical method of "non-secret encryption", and in 1974, another GCHQ mathematician and cryptographer, Malcolm J. Williamson, developed what is now known as Diffie–Hellman key exchange.
The scheme was also passed to the USA's National Security Agency. With a military focus and low computing power, the power of public key cryptography was unrealised in both organisations:

I judged it most important for military use... if you can share your key rapidly and electronically, you have a major advantage over your opponent. Only at the end of the evolution from Berners-Lee designing an open internet architecture for CERN, its adaptation and adoption for the Arpanet... did public key cryptography realise its full potential.
—Ralph Benjamin

Their discovery was not publicly acknowledged for 27 years, until the research was declassified by the British government in 1997.

Public discovery

In 1976, an asymmetric key cryptosystem was published by Whitfield Diffie and Martin Hellman who, influenced by Ralph Merkle's work on public key distribution, disclosed a method of public key agreement. This method of key exchange, which uses exponentiation in a finite field, came to be known as Diffie–Hellman key exchange. This was the first published practical method for establishing a shared secret-key over an authenticated communications channel without using a prior shared secret. Merkle's "public key-agreement technique" became known as Merkle's Puzzles, and was invented in 1974 and published in 1978.
In 1977, a generalization of Cocks' scheme was independently invented by Ron Rivest, Adi Shamir and Leonard Adleman, all then at MIT. The latter authors published their work in 1978, and the algorithm came to be known as RSA, from their initials. RSA uses exponentiation modulo a product of two very large primes, to encrypt and decrypt, performing both public key encryption and public key digital signature. Its security is connected to the extreme difficulty of factoring large integers, a problem for which there is no known efficient general technique.A description of the algorithm was published in the Mathematical Games column in the August 1977 issue of Scientific American.
Since the 1970s, a large number and variety of encryption, digital signature, key agreement, and other techniques have been developed in the field of public key cryptography, including the Rabin cryptosystem, ElGamal encryption, DSA - and elliptic curve cryptography.