Key exchange

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Key exchange (also key establishment) is a method in cryptography by which cryptographic keys are exchanged between two parties, allowing use of a cryptographic algorithm.

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In the Diffie-Hellman key exchange scheme, each party generates a public/private key pair and distributes the public key. After obtaining an authentic copy of each other's public keys, Alice and Bob can compute a shared secret offline. The shared secret can be used, for instance, as the key for a symmetric cipher. Public key shared secret.svg
In the Diffie–Hellman key exchange scheme, each party generates a public/private key pair and distributes the public key. After obtaining an authentic copy of each other's public keys, Alice and Bob can compute a shared secret offline. The shared secret can be used, for instance, as the key for a symmetric cipher.

If the sender and receiver wish to exchange encrypted messages, each must be equipped to encrypt messages to be sent and decrypt messages received. The nature of the equipping they require depends on the encryption technique they might use. If they use a code, both will require a copy of the same codebook. If they use a cipher, they will need appropriate keys. If the cipher is a symmetric key cipher, both will need a copy of the same key. If it is an asymmetric key cipher with the public/private key property, both will need the other's public key.

Channel of exchange

Key exchange is done either in-band or out-of-band. [1]

The key exchange problem

The key exchange problem describes ways to exchange whatever keys or other information are needed for establishing a secure communication channel so that no one else can obtain a copy. Historically, before the invention of public-key cryptography (asymmetrical cryptography), symmetric-key cryptography utilized a single key to encrypt and decrypt messages. For two parties to communicate confidentially, they must first exchange the secret key so that each party is able to encrypt messages before sending, and decrypt received ones. This process is known as the key exchange.

The overarching problem with symmetrical cryptography, or single-key cryptography, is that it requires a secret key to be communicated through trusted couriers, diplomatic bags, or any other secure communication channel. If two parties cannot establish a secure initial key exchange, they won't be able to communicate securely without the risk of messages being intercepted and decrypted by a third party who acquired the key during the initial key exchange.

Public-key cryptography uses a two-key system, consisting of the public and the private keys, where messages are encrypted with one key and decrypted with another. It depends on the selected cryptographic algorithm which key—public or private—is used for encrypting messages, and which for decrypting. For example, in RSA, the private key is used for decrypting messages, while in the Digital Signature Algorithm (DSA), the private key is used for authenticating them. The public key can be sent over non-secure channels or shared in public; the private key is only available to its owner.

Known as the Diffie-Hellman key exchange, the encryption key can be openly communicated as it poses no risk to the confidentiality of encrypted messages. One party exchanges the keys to another party where they can then encrypt messages using the key and send back the cipher text. Only the decryption key—in this case, it's the private key—can decrypt that message. At no time during the Diffie-Hellman key exchange is any sensitive information at risk of compromise, as opposed to symmetrical key exchange.

Identification

In principle, the only remaining problem was to be sure (or at least confident) that a public key actually belonged to its supposed owner. Because it is possible to 'spoof' another's identity in any of several ways, this is not a trivial or easily solved problem, particularly when the two users involved have never met and know nothing about each other.

Diffie–Hellman key exchange

In 1976, Whitfield Diffie and Martin Hellman published a cryptographic protocol called the Diffie–Hellman key exchange (D–H) based on concepts developed by Hellman's PhD student Ralph Merkle. The protocol enables users to securely exchange secret keys even if an opponent is monitoring that communication channel. The D–H key exchange protocol, however, does not by itself address authentication (i.e. the problem of being sure of the actual identity of the person or 'entity' at the other end of the communication channel). Authentication is crucial when an opponent can both monitor and alter messages within the communication channel (AKA man-in-the-middle or MITM attacks) and was addressed in the fourth section of the paper. [2]

Public key infrastructure

Public key infrastructures (PKIs) have been proposed as a workaround for the problem of identity authentication. In their most usual implementation, each user applies to a “certificate authority” (CA), trusted by all parties, for a digital certificate which serves for other users as a non-tamperable authentication of identity. The infrastructure is safe, unless the CA itself is compromised. In case it is, though, many PKIs provide a way to revoke certificates so other users will not trust them. Revoked certificates are usually put in certificate revocation lists which any certificate can be matched against.

Several countries and other jurisdictions have passed legislation or issued regulations encouraging PKIs by giving (more or less) legal effect to these digital certificates (see digital signature). Many commercial firms, as well as a few government departments, have established such certificate authorities.

This does nothing to solve the problem though, as the trustworthiness of the CA itself is still not guaranteed for any particular individual. It is a form of argument from authority fallacy. For actual trustworthiness, personal verification that the certificate belongs to the CA and establishment of trust in the CA are required. This is usually not possible.

There are known cases where authoritarian governments proposed establishing so-called “national CAs” whose certificates would be mandatory to install on citizens’ devices and, once installed and trusted, could be used for monitoring, intercepting, modifying, or blocking the encrypted internet traffic. [3] [4] [5]

For those new to such things, these arrangements are best thought of as electronic notary endorsements that “this public key belongs to this user”. As with notary endorsements, there can be mistakes or misunderstandings in such vouchings. Additionally, the notary itself can be untrusted. There have been several high-profile public failures by assorted certificate authorities. [6] [7]

Web of trust

At the other end of the conceptual range is the web of trust system, which avoids central Certificate Authorities entirely. Each user is responsible for getting a certificate from another user before using that certificate to communicate with the user. PGP and GPG (an implementation of the OpenPGP Internet Standard) employ just such a web of trust mechanism.

Password-authenticated key agreement

Password-authenticated key agreement algorithms can perform a cryptographic key exchange utilizing knowledge of a user's password.

Quantum key exchange

Quantum key distribution exploits certain properties of quantum physics to ensure its security. It relies on the fact that observations (or measurements) of a quantum state introduces perturbations in that state. Over many systems, these perturbations are detectable as noise by the receiver, making it possible to detect man-in-the-middle attacks. Beside the correctness and completeness of quantum mechanics, the protocol assumes the availability of an authenticated channel between Alice and Bob.

See also

Related Research Articles

<span class="mw-page-title-main">Diffie–Hellman key exchange</span> Method of exchanging cryptographic keys

Diffie–Hellman key exchange is a mathematical method of securely exchanging cryptographic keys over a public channel and was one of the first public-key protocols as conceived by Ralph Merkle and named after Whitfield Diffie and Martin Hellman. DH is one of the earliest practical examples of public key exchange implemented within the field of cryptography. Published in 1976 by Diffie and Hellman, this is the earliest publicly known work that proposed the idea of a private key and a corresponding public key.

<span class="mw-page-title-main">Encryption</span> Process of converting plaintext to ciphertext

In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decipher a ciphertext back to plaintext and access the original information. Encryption does not itself prevent interference but denies the intelligible content to a would-be interceptor.

Kerberos is a computer-network authentication protocol that works on the basis of tickets to allow nodes communicating over a non-secure network to prove their identity to one another in a secure manner. Its designers aimed it primarily at a client–server model, and it provides mutual authentication—both the user and the server verify each other's identity. Kerberos protocol messages are protected against eavesdropping and replay attacks.

<span class="mw-page-title-main">Public-key cryptography</span> Cryptographic system with public and private keys

Public-key cryptography, or asymmetric cryptography, is the field of cryptographic systems that use pairs of related keys. Each key pair consists of a public key and a corresponding private key. Key pairs are generated with cryptographic algorithms based on mathematical problems termed one-way functions. Security of public-key cryptography depends on keeping the private key secret; the public key can be openly distributed without compromising security.

A key in cryptography is a piece of information, usually a string of numbers or letters that are stored in a file, which, when processed through a cryptographic algorithm, can encode or decode cryptographic data. Based on the used method, the key can be different sizes and varieties, but in all cases, the strength of the encryption relies on the security of the key being maintained. A key's security strength is dependent on its algorithm, the size of the key, the generation of the key, and the process of key exchange.

<span class="mw-page-title-main">Symmetric-key algorithm</span> Algorithm

Symmetric-key algorithms are algorithms for cryptography that use the same cryptographic keys for both the encryption of plaintext and the decryption of ciphertext. The keys may be identical, or there may be a simple transformation to go between the two keys. The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link. The requirement that both parties have access to the secret key is one of the main drawbacks of symmetric-key encryption, in comparison to public-key encryption. However, symmetric-key encryption algorithms are usually better for bulk encryption. With exception of the one-time pad they have a smaller key size, which means less storage space and faster transmission. Due to this, asymmetric-key encryption is often used to exchange the secret key for symmetric-key encryption.

In cryptography and computer security, a man-in-the-middle (MITM) attack is a cyberattack where the attacker secretly relays and possibly alters the communications between two parties who believe that they are directly communicating with each other, as the attacker has inserted themselves between the two parties.

Transport Layer Security (TLS) is a cryptographic protocol designed to provide communications security over a computer network. The protocol is widely used in applications such as email, instant messaging, and voice over IP, but its use in securing HTTPS remains the most publicly visible.

Articles related to cryptography include:

ID-based encryption, or identity-based encryption (IBE), is an important primitive of ID-based cryptography. As such it is a type of public-key encryption in which the public key of a user is some unique information about the identity of the user. This means that a sender who has access to the public parameters of the system can encrypt a message using e.g. the text-value of the receiver's name or email address as a key. The receiver obtains its decryption key from a central authority, which needs to be trusted as it generates secret keys for every user.

Key/Config-authentication is used to solve the problem of authenticating the keys of the person to some other person is talking to or trying to talk to. In other words, it is the process of assuring that the key of "person A" held by "person B" does in fact belong to "person A" and vice versa.

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In cryptography, a secure channel is a means of data transmission that is resistant to overhearing and tampering. A confidential channel is a means of data transmission that is resistant to overhearing, or eavesdropping, but not necessarily resistant to tampering. An authentic channel is a means of data transmission that is resistant to tampering but not necessarily resistant to overhearing.

Key management refers to management of cryptographic keys in a cryptosystem. This includes dealing with the generation, exchange, storage, use, crypto-shredding (destruction) and replacement of keys. It includes cryptographic protocol design, key servers, user procedures, and other relevant protocols.

CipherSaber is a simple symmetric encryption protocol based on the RC4 stream cipher. Its goals are both technical and political: it gives reasonably strong protection of message confidentiality, yet it's designed to be simple enough that even novice programmers can memorize the algorithm and implement it from scratch. According to the designer, a CipherSaber version in the QBASIC programming language takes just sixteen lines of code. Its political aspect is that because it's so simple, it can be reimplemented anywhere at any time, and so it provides a way for users to communicate privately even if government or other controls make distribution of normal cryptographic software completely impossible.

<span class="mw-page-title-main">Glossary of cryptographic keys</span>

This glossary lists types of keys as the term is used in cryptography, as opposed to door locks. Terms that are primarily used by the U.S. National Security Agency are marked (NSA). For classification of keys according to their usage see cryptographic key types.

In cryptography, the interlock protocol, as described by Ron Rivest and Adi Shamir, is a protocol designed to frustrate eavesdropper attack against two parties that use an anonymous key exchange protocol to secure their conversation. A further paper proposed using it as an authentication protocol, which was subsequently broken.

In cryptography, forward secrecy (FS), also known as perfect forward secrecy (PFS), is a feature of specific key-agreement protocols that gives assurances that session keys will not be compromised even if long-term secrets used in the session key exchange are compromised. For HTTPS, the long-term secret is typically the private key of the server. Forward secrecy protects past sessions against future compromises of keys or passwords. By generating a unique session key for every session a user initiates, the compromise of a single session key will not affect any data other than that exchanged in the specific session protected by that particular key. This by itself is not sufficient for forward secrecy which additionally requires that a long-term secret compromise does not affect the security of past session keys.

Multimedia Internet KEYing (MIKEY) is a key management protocol that is intended for use with real-time applications. It can specifically be used to set up encryption keys for multimedia sessions that are secured using SRTP, the security protocol commonly used for securing real-time communications such as VoIP.

<span class="mw-page-title-main">Cryptography</span> Practice and study of secure communication techniques

Cryptography, or cryptology, is the practice and study of techniques for secure communication in the presence of adversarial behavior. More generally, cryptography is about constructing and analyzing protocols that prevent third parties or the public from reading private messages. Modern cryptography exists at the intersection of the disciplines of mathematics, computer science, information security, electrical engineering, digital signal processing, physics, and others. Core concepts related to information security are also central to cryptography. Practical applications of cryptography include electronic commerce, chip-based payment cards, digital currencies, computer passwords, and military communications.

References

  1. ↑ Emmett Dulaney, Chuck Easttom (October 5, 2017). CompTIA Security+ Study Guide: Exam SY0-501. John Wiley & Sons. ISBN   9781119416906.
  2. ↑ Diffie, Whitfield; Hellman, Martin E. (November 1976). "New Directions in Cryptography" (PDF). IEEE Transactions on Information Theory. IT-22 (6): 644–654. doi:10.1109/TIT.1976.1055638.
  3. ↑ Wolff, Josephine (2015-12-14). "Kazakhstan's Unsettling New Cybersecurity Plan". Slate. Retrieved 2019-01-09.
  4. ↑ Shapovalova, Natalia (2016-01-05). "Security Certificate Of The Republic Of Kazakhstan: The State Will Be Able To Control The Encrypted Internet Traffic Of Users". Mondaq. Retrieved 2019-01-09.
  5. ↑ "The Kremlin reportedly wants to create a state-operated center for issuing SSL certificates". Meduza. 2016-02-15. Retrieved 2019-01-09.
  6. ↑ CA/Symantec Issues
  7. ↑ Symantec caught once again improperly issuing illegitimate HTTPS certificates
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