Hybrid cryptosystem

Last updated

In cryptography, a hybrid cryptosystem is one which combines the convenience of a public-key cryptosystem with the efficiency of a symmetric-key cryptosystem. [1] Public-key cryptosystems are convenient in that they do not require the sender and receiver to share a common secret in order to communicate securely. [2] However, they often rely on complicated mathematical computations and are thus generally much more inefficient than comparable symmetric-key cryptosystems. In many applications, the high cost of encrypting long messages in a public-key cryptosystem can be prohibitive. This is addressed by hybrid systems by using a combination of both. [3]

Contents

A hybrid cryptosystem can be constructed using any two separate cryptosystems:

The hybrid cryptosystem is itself a public-key system, whose public and private keys are the same as in the key encapsulation scheme. [4]

Note that for very long messages the bulk of the work in encryption/decryption is done by the more efficient symmetric-key scheme, while the inefficient public-key scheme is used only to encrypt/decrypt a short key value. [3]

All practical implementations of public key cryptography today employ the use of a hybrid system. Examples include the TLS protocol [5] and the SSH protocol, [6] that use a public-key mechanism for key exchange (such as Diffie-Hellman) and a symmetric-key mechanism for data encapsulation (such as AES). The OpenPGP [7] file format and the PKCS#7 [8] file format are other examples.

Hybrid Public Key Encryption (HPKE, published as RFC 9180) is a modern standard for generic hybrid encryption. HPKE is used within multiple IETF protocols, including MLS and TLS Encrypted Hello.

Envelope encryption is an example of a usage of hybrid cryptosystems in cloud computing. In a cloud context, hybrid cryptosystems also enable centralized key management. [9] [10]

Example

To encrypt a message addressed to Alice in a hybrid cryptosystem, Bob does the following:

  1. Obtains Alice's public key.
  2. Generates a fresh symmetric key for the data encapsulation scheme.
  3. Encrypts the message under the data encapsulation scheme, using the symmetric key just generated.
  4. Encrypts the symmetric key under the key encapsulation scheme, using Alice's public key.
  5. Sends both of these ciphertexts to Alice.

To decrypt this hybrid ciphertext, Alice does the following:

  1. Uses her private key to decrypt the symmetric key contained in the key encapsulation segment.
  2. Uses this symmetric key to decrypt the message contained in the data encapsulation segment. [11] [1]

Security

If both the key encapsulation and data encapsulation schemes in a hybrid cryptosystem are secure against adaptive chosen ciphertext attacks, then the hybrid scheme inherits that property as well. [4] However, it is possible to construct a hybrid scheme secure against adaptive chosen ciphertext attacks even if the key encapsulation has a slightly weakened security definition (though the security of the data encapsulation must be slightly stronger). [12]

Envelope encryption

Envelope encryption is term used for encrypting with a hybrid cryptosystem used by all major cloud service providers, [9] often as part of a centralized key management system in cloud computing. [13]

Envelope encryption gives names to the keys used in hybrid encryption: Data Encryption Keys (abbreviated DEK, and used to encrypt data) and Key Encryption Keys (abbreviated KEK, and used to encrypt the DEKs). In a cloud environment, encryption with envelope encryption involves generating a DEK locally, encrypting one's data using the DEK, and then issuing a request to wrap (encrypt) the DEK with a KEK stored in a potentially more secure service. Then, this wrapped DEK and encrypted message constitute a ciphertext for the scheme. To decrypt a ciphertext, the wrapped DEK is unwrapped (decrypted) via a call to a service, and then the unwrapped DEK is used to decrypt the encrypted message. [10] In addition to the normal advantages of a hybrid cryptosystem, using asymmetric encryption for the KEK in a cloud context provides easier key management and separation of roles, but can be slower. [13]

In cloud systems, such as Google Cloud Platform and Amazon Web Services, a key management system (KMS) can be available as a service. [13] [10] [14] In some cases, the key management system will store keys in hardware security modules, which are hardware systems that protect keys with hardware features like intrusion resistance. [15] This means that KEKs can also be more secure because they are stored on secure specialized hardware. [13] Envelope encryption makes centralized key management easier because a centralized key management system only needs to store KEKs, which occupy less space, and requests to the KMS only involve sending wrapped and unwrapped DEKs, which use less bandwidth than transmitting entire messages. Since one KEK can be used to encrypt many DEKs, this also allows for less storage space to be used in the KMS. This also allows for centralized auditing and access control at one point of access. [10]

See also

Related Research Articles

<span class="mw-page-title-main">Cipher</span> Algorithm for encrypting and decrypting information

In cryptography, a cipher is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. To encipher or encode is to convert information into cipher or code. In common parlance, "cipher" is synonymous with "code", as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially classical cryptography.

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

In cryptography, encryption is the process of transforming information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Despite its goal, encryption does not itself prevent interference but denies the intelligible content to a would-be interceptor.

Pretty Good Privacy (PGP) is an encryption program that provides cryptographic privacy and authentication for data communication. PGP is used for signing, encrypting, and decrypting texts, e-mails, files, directories, and whole disk partitions and to increase the security of e-mail communications. Phil Zimmermann developed PGP in 1991.

<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.

In cryptography, the ElGamal encryption system is an asymmetric key encryption algorithm for public-key cryptography which is based on the Diffie–Hellman key exchange. It was described by Taher Elgamal in 1985. ElGamal encryption is used in the free GNU Privacy Guard software, recent versions of PGP, and other cryptosystems. The Digital Signature Algorithm (DSA) is a variant of the ElGamal signature scheme, which should not be confused with ElGamal encryption.

A chosen-plaintext attack (CPA) is an attack model for cryptanalysis which presumes that the attacker can obtain the ciphertexts for arbitrary plaintexts. The goal of the attack is to gain information that reduces the security of the encryption scheme.

A chosen-ciphertext attack (CCA) is an attack model for cryptanalysis where the cryptanalyst can gather information by obtaining the decryptions of chosen ciphertexts. From these pieces of information the adversary can attempt to recover the secret key used for decryption.

Identity-based encryption (IBE), is an important primitive of identity-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 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.

In cryptography, a semantically secure cryptosystem is one where only negligible information about the plaintext can be feasibly extracted from the ciphertext. Specifically, any probabilistic, polynomial-time algorithm (PPTA) that is given the ciphertext of a certain message , and the message's length, cannot determine any partial information on the message with probability non-negligibly higher than all other PPTA's that only have access to the message length. This concept is the computational complexity analogue to Shannon's concept of perfect secrecy. Perfect secrecy means that the ciphertext reveals no information at all about the plaintext, whereas semantic security implies that any information revealed cannot be feasibly extracted.

Multiple encryption is the process of encrypting an already encrypted message one or more times, either using the same or a different algorithm. It is also known as cascade encryption, cascade ciphering, multiple encryption, and superencipherment. Superencryption refers to the outer-level encryption of a multiple encryption.

<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.

The Cramer–Shoup system is an asymmetric key encryption algorithm, and was the first efficient scheme proven to be secure against adaptive chosen ciphertext attack using standard cryptographic assumptions. Its security is based on the computational intractability of the Decisional Diffie–Hellman assumption. Developed by Ronald Cramer and Victor Shoup in 1998, it is an extension of the ElGamal cryptosystem. In contrast to ElGamal, which is extremely malleable, Cramer–Shoup adds other elements to ensure non-malleability even against a resourceful attacker. This non-malleability is achieved through the use of a universal one-way hash function and additional computations, resulting in a ciphertext which is twice as large as in ElGamal.

Proxy re-encryption (PRE) schemes are cryptosystems which allow third parties (proxies) to alter a ciphertext which has been encrypted for one party, so that it may be decrypted by another.

Ciphertext indistinguishability is a property of many encryption schemes. Intuitively, if a cryptosystem possesses the property of indistinguishability, then an adversary will be unable to distinguish pairs of ciphertexts based on the message they encrypt. The property of indistinguishability under chosen plaintext attack is considered a basic requirement for most provably secure public key cryptosystems, though some schemes also provide indistinguishability under chosen ciphertext attack and adaptive chosen ciphertext attack. Indistinguishability under chosen plaintext attack is equivalent to the property of semantic security, and many cryptographic proofs use these definitions interchangeably.

A deterministic encryption scheme is a cryptosystem which always produces the same ciphertext for a given plaintext and key, even over separate executions of the encryption algorithm. Examples of deterministic encryption algorithms include RSA cryptosystem, and many block ciphers when used in ECB mode or with a constant initialization vector.

<span class="mw-page-title-main">Key encapsulation mechanism</span>

In cryptography, a key encapsulation mechanism, or KEM, is a public-key cryptosystem that allows a sender to generate a short secret key and transmit it to a receiver securely, in spite of eavesdropping and intercepting adversaries.

<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.

In cryptography, a padding oracle attack is an attack which uses the padding validation of a cryptographic message to decrypt the ciphertext. In cryptography, variable-length plaintext messages often have to be padded (expanded) to be compatible with the underlying cryptographic primitive. The attack relies on having a "padding oracle" who freely responds to queries about whether a message is correctly padded or not. The information could be directly given, or leaked through a side-channel.

Identity-based conditional proxy re-encryption (IBCPRE) is a type of proxy re-encryption (PRE) scheme in the identity-based public key cryptographic setting. An IBCPRE scheme is a natural extension of proxy re-encryption on two aspects. The first aspect is to extend the proxy re-encryption notion to the identity-based public key cryptographic setting. The second aspect is to extend the feature set of proxy re-encryption to support conditional proxy re-encryption. By conditional proxy re-encryption, a proxy can use an IBCPRE scheme to re-encrypt a ciphertext but the ciphertext would only be well-formed for decryption if a condition applied onto the ciphertext together with the re-encryption key is satisfied. This allows fine-grained proxy re-encryption and can be useful for applications such as secure sharing over encrypted cloud data storage.

References

  1. 1 2 Shoukat, Ijaz Ali (2013). "A Generic Hybrid Encryption System (HES)".
  2. ↑ Paar, Christof; Pelzl, Jan; Preneel, Bart (2010). "Chapter 6: Introduction to Public-Key Cryptography". Understanding Cryptography: A Textbook for Students and Practitioners (PDF). Springer. ISBN   978-3-642-04100-6.
  3. 1 2 Deng, Juan; Brooks, Richard (2012). "Chapter 26 - Cyber-Physical Security of Automotive Information Technology". Handbook on Securing Cyber-Physical Critical Infrastructure. Elsevier. pp. 655–676. ISBN   978-0-12-415815-3.
  4. 1 2 Cramer, Ronald; Shoup, Victor (2019). "Design and Analysis of Practical Public-Key Encryption Schemes Secure against Adaptive Chosen Ciphertext Attack" (PDF). SIAM Journal on Computing. 33 (1): 167–226. CiteSeerX   10.1.1.76.8924 . doi:10.1137/S0097539702403773.
  5. ↑ Fox, Pamela. "Transport Layer Security (TLS) (article)". Khan Academy. Retrieved 2022-02-06.
  6. ↑ Ellingwood, Justin. "Understanding the SSH Encryption and Connection Process | DigitalOcean". www.digitalocean.com. Retrieved 2022-02-06.
  7. ↑ "RFC 9580 - OpenPGP". datatracker.ietf.org. Retrieved 2024-08-02.
  8. ↑ "RFC 2315 - PKCS #7: Cryptographic Message Syntax Version 1.5". datatracker.ietf.org. Retrieved 2022-02-06.
  9. 1 2 Albertini, Ange; Duong, Thai; Gueron, Shay; Kölbl, Stefan; Luykx, Atul; Schmieg, Sophie (November 17, 2020). "How to Abuse and Fix Authenticated Encryption Without Key Commitment". USENIX Security 2022– via Cryptology ePrint Archive.
  10. 1 2 3 4 "Envelope encryption | Cloud KMS Documentation". Google Cloud. Retrieved 2021-12-30.
  11. ↑ St Denis, Tom; Johnson, Simon (2006). "9". Cryptography for Developers. Elsevier. ISBN   978-1-59749-104-4.
  12. ↑ Hofheinz, Dennis; Kiltz, Eike (2019). "Secure Hybrid Encryption from Weakened Key Encapsulation" (PDF). Advances in Cryptology – CRYPTO 2007. Springer. pp. 553–571.
  13. 1 2 3 4 "AWS KMS concepts - AWS Key Management Service". docs.aws.amazon.com. Retrieved 2021-12-30.
  14. ↑ "What is envelope encryption? - FAQ| Alibaba Cloud Documentation Center". www.alibabacloud.com. Retrieved 2021-12-30.
  15. ↑ "Hardware Security Module (HSM) - Glossary | CSRC". csrc.nist.gov. Retrieved 2022-01-23.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.