Key server (cryptographic)

Last updated

In computer security, a key server is a computer that receives and then serves existing cryptographic keys to users or other programs. The users' programs can be running on the same network as the key server or on another networked computer.

Contents

The keys distributed by the key server are almost always provided as part of a cryptographically protected public key certificates containing not only the key but also 'entity' information about the owner of the key. The certificate is usually in a standard format, such as the OpenPGP public key format, the X.509 certificate format, or the PKCS format. Further, the key is almost always a public key for use with an asymmetric key encryption algorithm.

History

Key servers play an important role in public key cryptography. In public key cryptography an individual is able to generate a key pair, where one of the keys is kept private while the other is distributed publicly. Knowledge of the public key does not compromise the security of public key cryptography. An individual holding the public key of a key pair can use that key to carry out cryptographic operations that allow secret communications with strong authentication of the holder of the matching private key. The need to have the public key of a key pair in order to start communication or verify signatures is a bootstrapping problem. Locating keys on the web or writing to the individual asking them to transmit their public keys can be time consuming and unsecure. Key servers act as central repositories to alleviate the need to individually transmit public keys and can act as the root of a chain of trust.

The first web-based PGP keyserver was written for a thesis by Marc Horowitz, [1] while he was studying at MIT. Horowitz's keyserver was called the HKP Keyserver after a web-based OpenPGP HTTP Keyserver Protocol (HKP), [2] used to allow people to interact with the keyserver. Users were able to upload, download, and search keys either through HKP on TCP port 11371, or through web pages which ran CGI scripts. Before the creation of the HKP Keyserver, keyservers relied on email processing scripts for interaction.

Enterprise PGP

A separate key server, known as the PGP Certificate Server, was developed by PGP, Inc. and was used as the software (through version 2.5.x for the server) for the default key server in PGP through version 8.x (for the client software), keyserver.pgp.com. Network Associates was granted a patent co-authored by Jon Callas (United States Patent 6336186) [3] on the key server concept.

To replace the aging Certificate Server, an LDAP-based key server was redesigned at Network Associates in part by Randy Harmon and Len Sassaman, called PGP Keyserver 7. With the release of PGP 6.0, LDAP was the preferred key server interface for Network Associates’ PGP versions. This LDAP and LDAPS key server (which also spoke HKP for backwards compatibility, though the protocol was (arguably correctly) referred to as “HTTP” or “HTTPS”) also formed the basis for the PGP Administration tools for private key servers in corporate settings, along with a schema for Netscape Directory Server.

PGP Keyserver 7 was later replaced by the new PGP Corporation PGP Global Directory of 2011 which allows PGP keys to be published and downloaded using HTTPS or LDAP. [4]

OpenPGP

The OpenPGP world largely used its own development of keyserver software independent from the PGP Corporation suite. The main software used until the 2019 spamming attack was "SKS" (Synchronizing Key Server), written by Yaron Minsky. [5] The public SKS pool (consisting of many interconnected SKS instances) provided access via HKPS (HLP with TLS) and HTTPS. It finally shut down in 2021 following a number of GDPR that it was unable to process effectively. [6]

A number of newer pools using other software has been made available following the shutdown of the SKS pool, see #Keyserver examples.

Public versus private keyservers

Many publicly accessible key servers, located around the world, are computers which store and provide OpenPGP keys over the Internet for users of that cryptosystem. In this instance, the computers can be, and mostly are, run by individuals as a pro bono service, facilitating the web of trust model PGP uses.

Several publicly accessible S/MIME key servers are available to publish or retrieve certificates used with the S/MIME cryptosystem.

There are also multiple proprietary public key infrastructure systems which maintain key servers for their users; those may be private or public, and only the participating users are likely to be aware of those keyservers at all.

Problems with keyservers

Lack of retraction mechanism

The OpenPGP keyservers since their development in 1990s suffered from a few problems. Once a public key has been uploaded, it was purposefully made difficult to remove it as servers auto-synchronize between each other (it was done in order to fight government censorship). Some users stop using their public keys for various reasons, such as when they forget their pass phrase, or if their private key is compromised or lost. In those cases, it was hard to delete a public key from the server, and even if it were deleted, someone else can upload a fresh copy of the same public key to the server. This leads to an accumulation of old fossil public keys that never go away, a form of "keyserver plaque".

The lack of a retraction mechanism also breached the European General Data Protection Regulation, which was cited as a reason for the closure of the SKS pool. [6] Modern PGP keyservers allow deletion of keys. Because only the owner of a key's e-mail address can upload a key (see next section) in such servers, the key stays deleted unless the owner decides otherwise.

Lack of ownership check

The keyserver also had no way to check to see if the key was legitimate (belong to true owner). As consequence anyone can upload a bogus public key to the keyserver, bearing the name of a person who in fact does not own that key, or even worse, use it as vulnerability: the Certificate Spamming Attack. [5] [7] : §2.2 

Modern keyservers, starting with the PGP Global Directory, now use the e-mail address for confirmation. This keyserver sent an email confirmation request to the putative key owner, asking that person to confirm that the key in question is theirs. If they confirm it, the PGP Global Directory accepts the key. The confirmation can be renewed periodically, to prevent the accumulation of keyserver plaque. The result is a higher quality collection of public keys, and each key has been vetted by email with the key's apparent owner. But as consequence, another problem arise: because PGP Global Directory allows key account maintenance and verifies only by email, not cryptographically, anybody having access to the email account could for example delete a key and upload a bogus one.

The last Internet Engineering Task Force draft for HKP also defines a distributed key server network, based on DNS SRV records: to find the key of someone@example.com, one can ask it by requesting example.com's key server.

Leakage of personal relationships

For many individuals, the purpose of using cryptography is to obtain a higher level of privacy in personal interactions and relationships. It has been pointed out that allowing a public key to be uploaded in a key server when using decentralized web of trust based cryptographic systems, like PGP, may reveal a good deal of information that an individual may wish to have kept private. Since PGP relies on signatures on an individual's public key to determine the authenticity of that key, potential relationships can be revealed by analyzing the signers of a given key. In this way, models of entire social networks can be developed. (Mike Perry's 2013 criticism of the Web of Trust mentions the issue as already been "discussed at length".) [8]

A number of modern key servers remove third-party signatures from the uploaded key. Doing so removes all personal connections into the Web of Trust, thus preventing any leakage from happening. The main goal, however, was to minimize the storage space required, as "signature spamming" can easily add megabytes to a key. [9] [7] : §2.1 

Keyserver examples

These are some keyservers that are often used for looking up keys with gpg --recv-keys. [10] These can be queried via https:// (HTTPS) or hkps:// (HKP over TLS) respectively.

See also

Related Research Articles

Active Directory (AD) is a directory service developed by Microsoft for Windows domain networks. Windows Server operating systems include it as a set of processes and services. Originally, only centralized domain management used Active Directory. However, it ultimately became an umbrella title for various directory-based identity-related services.

<span class="mw-page-title-main">HTTPS</span> Extension of the HTTP communications protocol to support TLS encryption

Hypertext Transfer Protocol Secure (HTTPS) is an extension of the Hypertext Transfer Protocol (HTTP). It uses encryption for secure communication over a computer network, and is widely used on the Internet. In HTTPS, the communication protocol is encrypted using Transport Layer Security (TLS) or, formerly, Secure Sockets Layer (SSL). The protocol is therefore also referred to as HTTP over TLS, or HTTP over SSL.

The Lightweight Directory Access Protocol is an open, vendor-neutral, industry standard application protocol for accessing and maintaining distributed directory information services over an Internet Protocol (IP) network. Directory services play an important role in developing intranet and Internet applications by allowing the sharing of information about users, systems, networks, services, and applications throughout the network. As examples, directory services may provide any organized set of records, often with a hierarchical structure, such as a corporate email directory. Similarly, a telephone directory is a list of subscribers with an address and a phone number.

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.

<span class="mw-page-title-main">Email client</span> Computer program used to access and manage a users email

An email client, email reader or, more formally, message user agent (MUA) or mail user agent is a computer program used to access and manage a user's email.

X.500 is a series of computer networking standards covering electronic directory services. The X.500 series was developed by the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T). ITU-T was formerly known as the Consultative Committee for International Telephony and Telegraphy (CCITT). X.500 was first approved in 1988. The directory services were developed to support requirements of X.400 electronic mail exchange and name lookup. The International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC) were partners in developing the standards, incorporating them into the Open Systems Interconnection suite of protocols. ISO/IEC 9594 is the corresponding ISO/IEC identification.

<span class="mw-page-title-main">Public key infrastructure</span> System that can issue, distribute and verify digital certificates

A public key infrastructure (PKI) is a set of roles, policies, hardware, software and procedures needed to create, manage, distribute, use, store and revoke digital certificates and manage public-key encryption. The purpose of a PKI is to facilitate the secure electronic transfer of information for a range of network activities such as e-commerce, internet banking and confidential email. It is required for activities where simple passwords are an inadequate authentication method and more rigorous proof is required to confirm the identity of the parties involved in the communication and to validate the information being transferred.

In cryptography, a public key certificate, also known as a digital certificate or identity certificate, is an electronic document used to prove the validity of a public key. The certificate includes the public key and information about it, information about the identity of its owner, and the digital signature of an entity that has verified the certificate's contents. If the device examining the certificate trusts the issuer and finds the signature to be a valid signature of that issuer, then it can use the included public key to communicate securely with the certificate's subject. In email encryption, code signing, and e-signature systems, a certificate's subject is typically a person or organization. However, in Transport Layer Security (TLS) a certificate's subject is typically a computer or other device, though TLS certificates may identify organizations or individuals in addition to their core role in identifying devices. TLS, sometimes called by its older name Secure Sockets Layer (SSL), is notable for being a part of HTTPS, a protocol for securely browsing the web.

<span class="mw-page-title-main">Web of trust</span> Mechanism for authenticating cryptographic keys

In cryptography, a web of trust is a concept used in PGP, GnuPG, and other OpenPGP-compatible systems to establish the authenticity of the binding between a public key and its owner. Its decentralized trust model is an alternative to the centralized trust model of a public key infrastructure (PKI), which relies exclusively on a certificate authority. As with computer networks, there are many independent webs of trust, and any user can be a part of, and a link between, multiple webs.

In cryptography, a certificate authority or certification authority (CA) is an entity that stores, signs, and issues digital certificates. A digital certificate certifies the ownership of a public key by the named subject of the certificate. This allows others to rely upon signatures or on assertions made about the private key that corresponds to the certified public key. A CA acts as a trusted third party—trusted both by the subject (owner) of the certificate and by the party relying upon the certificate. The format of these certificates is specified by the X.509 or EMV standard.

S/MIME is a standard for public-key encryption and signing of MIME data. S/MIME is on an IETF standards track and defined in a number of documents, most importantly RFC 8551. It was originally developed by RSA Data Security, and the original specification used the IETF MIME specification with the de facto industry standard PKCS #7 secure message format. Change control to S/MIME has since been vested in the IETF, and the specification is now layered on Cryptographic Message Syntax (CMS), an IETF specification that is identical in most respects with PKCS #7. S/MIME functionality is built into the majority of modern email software and interoperates between them. Since it is built on CMS, MIME can also hold an advanced digital signature.

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.

Single sign-on (SSO) is an authentication scheme that allows a user to log in with a single ID to any of several related, yet independent, software systems.

<span class="mw-page-title-main">Zimbra</span> Collaborative software suite

Zimbra Collaboration, formerly known as the Zimbra Collaboration Suite (ZCS) before 2019, is a collaborative software suite that includes an email server and a web client.

Email encryption is encryption of email messages to protect the content from being read by entities other than the intended recipients. Email encryption may also include authentication.

CrushFTP is a proprietary multi-protocol, multi-platform file transfer server originally developed in 1999. CrushFTP is shareware with a tiered pricing model. It is targeted at home users on up to enterprise users.

<span class="mw-page-title-main">OpenPGP card</span> Type of cryptographic smart card

In cryptography, the OpenPGP card is an ISO/IEC 7816-4, -8 compatible smart card that is integrated with many OpenPGP functions. Using this smart card, various cryptographic tasks can be performed. It allows secure storage of secret key material; all versions of the protocol state, "Private keys and passwords cannot be read from the card with any command or function." However, new key pairs may be loaded onto the card at any time, overwriting the existing ones.

Secure messaging is a server-based approach to protect sensitive data when sent beyond the corporate borders, and it provides compliance with industry regulations such as HIPAA, GLBA and SOX. Advantages over classical secure e-mail are that confidential and authenticated exchanges can be started immediately by any internet user worldwide since there is no requirement to install any software nor to obtain or to distribute cryptographic keys beforehand. Secure messages provide non-repudiation as the recipients are personally identified and transactions are logged by the secure email platform.

<span class="mw-page-title-main">Vincent Moscaritolo</span> American computer security expert

Vincent (Vinnie) Moscaritolo is a retired American computer security expert known for his work in encryption applications for mobile devices. After decades in the computer industry, he now volunteers as a search and rescue professional. He holds NREMT, WFR, and Amateur Radio Extra Class and a General Radiotelephone Operator with Ships Radar License.

References

  1. ↑ Horowitz, Marc (1996-11-18). "A PGP Public Key Server" . Retrieved 2018-05-02.
  2. ↑ Shaw, David (March 2003). "The OpenPGP HTTP Keyserver Protocol (HKP)". Internet Engineering Task Force . Retrieved 2018-05-02.
  3. ↑ Cryptographic system and methodology for creating and managing crypto policy on certificate servers
  4. ↑ "PGP Global Directory - Terms and Conditions".
  5. 1 2 "SKS Keyserver Network Under Attack". Gist. Retrieved 2020-09-17.
  6. 1 2 "SKS Keyservers". Archived from the original on 2022-01-19.
  7. 1 2 Gillmor, Daniel Kahn (18 August 2023). "Abuse-Resistant OpenPGP Keystores [draft-dkg-openpgp-abuse-resistant-keystore-06]". Internet Engineering Task Force.
  8. ↑ Perry, Mike (Sep 29, 2013). "[tor-talk] Why the Web of Trust Sucks".
  9. ↑ "keys.openpgp.org FAQ". keys.openpgp.org.
  10. ↑ "recv-keys documentation". GPG Manual. Retrieved 30 June 2020.
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.