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Security Internet-Draft This document specifies a certificate extension for including logotypes in public key certificates and attribute certificates. This document obsoletes RFC 3709 and RFC 6170.

Introduction This specification supplements , which profiles public-key certificates and certificate revocation lists (CRLs) for use in the Internet, and it supplements which profiles attribute certificates for use in the Internet. This document obsoletes RFC 3709 and RFC 6170 . provides a summary of the changes since the publication of RFC 3709 and RFC 6170. The basic function of a certificate is to bind a public key to the identity of an entity (the subject). From a strictly technical viewpoint, this goal could be achieved by signing the identity of the subject together with its public key. However, the art of Public Key Infrastructure (PKI) has developed certificates far beyond this functionality in order to meet the needs of modern global networks and heterogeneous information and operational technology structures. Certificate users must be able to determine certificate policies, appropriate key usage, assurance level, and name form constraints. Before a relying party can make an informed decision whether a particular certificate is trustworthy and relevant for its intended usage, a certificate may be examined from several different perspectives. Systematic processing is necessary to determine whether a particular certificate meets the predefined prerequisites for an intended usage. Much of the information contained in certificates is appropriate and effective for machine processing; however, this information is not suitable for a corresponding human trust and recognition process. Humans prefer to structure information into categories and symbols. Most humans associate complex structures of reality with easily recognizable logotypes and marks. Humans tend to trust things that they recognize from previous experiences. Humans may examine information to confirm their initial reaction. Very few consumers actually read all terms and conditions they agree to in accepting a service, rather they commonly act on trust derived from previous experience and recognition. A big part of this process is branding. Service providers and product vendors invest a lot of money and resources into creating a strong relation between positive user experiences and easily recognizable trademarks, servicemarks, and logotypes. Branding is also pervasive in identification instruments, including identification cards, passports, driver's licenses, credit cards, gasoline cards, and loyalty cards. Identification instruments are intended to identify the holder as a particular person or as a member of the community. The community may represent the subscribers of a service or any other group. Identification instruments, in physical form, commonly use logotypes and symbols, solely to enhance human recognition and trust in the identification instrument itself. They may also include a registered trademark to allow legal recourse for unauthorized duplication. Since certificates play an equivalent role in electronic exchanges, we examine the inclusion of logotypes in certificates. We consider certificate-based identification and certificate selection.

Certificate-based Identification The need for human recognition depends on the manner in which certificates are used and whether certificates need to be visible to human users. If certificates are to be used in open environments and in applications that bring the user in conscious contact with the result of a certificate-based identification process, then human recognition is highly relevant, and may be a necessity. Examples of such applications include:

• Web server identification where a user identifies the owner of the web site.
• Peer e-mail exchange in business-to-business (B2B), business-to-consumer (B2C), and private communications.
• Exchange of medical records, and system for medical prescriptions.
• Unstructured e-business applications (i.e., non-EDI applications).
• Wireless client authenticating to a service provider.

Most applications provide the human user with an opportunity to view the results of a successful certificate-based identification process. When the user takes the steps necessary to view these results, the user is presented with a view of a certificate. This solution has two major problems. First, the function to view a certificate is often rather hard to find for a non-technical user. Second, the presentation of the certificate is too technical and is not user friendly. It contains no graphic symbols or logotypes to enhance human recognition. Many investigations have shown that users of today's applications do not take the steps necessary to view certificates. This could be due to poor user interfaces. Further, many applications are structured to hide certificates from users. The application designers do not want to expose certificates to users at all.

Selection of Certificates One situation where software applications must expose human users to certificates is when the user must select a single certificate from a portfolio of certificates. In some cases, the software application can use information within the certificates to filter the list for suitability; however, the user must be queried if more than one certificate is suitable. The human user must select one of them. This situation is comparable to a person selecting a suitable plastic card from his wallet. In this situation, substantial assistance is provided by card color, location, and branding. In order to provide similar support for certificate selection, the users need tools to easily recognize and distinguish certificates. Introduction of logotypes into certificates provides the necessary graphic.

Combination of Verification Techniques The use of logotypes will, in many cases, affect the users decision to trust and use a certificate. It is therefore important that there be a distinct and clear architectural and functional distinction between the processes and objectives of the automated certificate verification and human recognition. Since logotypes are only aimed for human interpretation and contain data that is inappropriate for computer based verification schemes, the logotype extension MUST NOT be an active component in automated certification path validation as specified in . Automated certification path verification determines whether the end-entity certificate can be verified according to defined policy. The algorithm for this verification is specified in . The automated processing provides assurance that the certificate is valid. It does not indicate whether the subject is entitled to any particular information, or whether the subject ought to be trusted to perform a particular service. These are authorization decisions. Automatic processing will make some authorization decisions, but others, depending on the application context, involve the human user. In some situations, where automated procedures have failed to establish the suitability of the certificate to the task, the human user is the final arbitrator of the post certificate verification authorization decisions. In the end, the human will decide whether or not to accept an executable email attachment, to release personal information, or follow the instructions displayed by a web browser. This decision will often be based on recognition and previous experience. The distinction between systematic processing and human processing is rather straightforward. They can be complementary. While the systematic process is focused on certification path construction and verification, the human acceptance process is focused on recognition and related previous experience. There are some situations where systematic processing and human processing interfere with each other. These issues are discussed in the .

Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Different Types of Logotypes in Certificates This specification defines the inclusion of three standard logotype types:

• Community logotype
• Issuer organization logotype
• Subject organization logotype

The community logotype is the general mark for a community. It identifies a service concept for entity identification and certificate issuance. Many issuers may use a community logotype to co-brand with a global community in order to gain global recognition of its local service provision. This type of community branding is very common in the credit card business, where local independent card issuers include a globally recognized brand (such as VISA and MasterCard). Certificate issuers may include more than one community logotype to indicate participation in more than one global community. Issuer organization logotype is a logotype representing the organization identified as part of the issuer name in the certificate. Subject organization logotype is a logotype representing the organization identified in the subject name in the certificate. In addition to the standard logotype types, this specification accommodates inclusion of other logotype types where each class of logotype is defined by an object identifier. The object identifier can be either locally defined or an identifier defined in of this document.

Logotype Data This specification defines two types of logotype data: image data and audio data. Implementations MUST support image data; however, support for audio data is OPTIONAL. Image and audio data for logotypes can be provided by reference by including a URI that identifies the location to the logotype data and a one-way hash of the referenced data in the certificate. The privacy-related properties for remote logotype data depend on four parties: the certificate relying parties that use the information in the certificate extension to fetch the logotype data, the certificate issuers that populate the certificate extension, certificate subscribers that request certificates that include the certificate extension, and server operators that provide the logotype data. Alternatively, embedding the logotype data in the certificate with direct addressing (as defined in ) provides improved privacy properties and depends upon fewer parties. However, this approach can significantly increase the size of the certificate. Several image objects, representing the same visual content in different formats, sizes, and color palates, may represent each logotype image. At least one of the image objects representing a logotype SHOULD contain an image with a width between 60 pixels and 200 pixels and a height between 45 pixels and 150 pixels. Several instances of audio data may further represent the same audio sequence in different formats, resolutions, and languages. At least one of the audio objects representing a logotype SHOULD provide text-based audio data suitable for processing by text-to-speech software. A typical use of text based audio data is inclusion in web applications where the audio text is placed as the "alt" atttribute value of an html image (img) element and the language value obtained from LogotypeAudioInfo is included as the "lang" attribute of that image. If a logotype of a certain type (as defined in ) is represented by more than one image object, then each image object MUST contain variants of roughly the same visual content. Likewise, if a logotype of a certain type is represented by more than one audio object, then the audio objects MUST contain variants of the same audio information. A spoken message in different languages is considered a variation of the same audio information. When more than one image object or more than one audio object for the same logotype type is included in the certificate, the certificate issuer is responsible for ensuring that the objects contain roughly the same content. Compliant applications MUST NOT display more than one of the image objects and MUST NOT play more than one of the audio objects for any logotype type (see ) at the same time. A client MAY simultaneously display multiple logotypes of different logotype types. For example, it may display one subject organization logotype while also displaying a community logotype, but it MUST NOT display multiple image variants of the same community logotype. Each logotype present in a certificate MUST be represented by at least one image data object. Client applications SHOULD enhance processing and off-line functionality by caching logotype data.

Logotype Extension This section specifies the syntax and semantics of the logotype certificate extension.

Extension Format The logotype extension MAY be included in public key certificates or attribute certificates . The logotype extension MUST be identified by the following object identifier: This extension MUST NOT be marked critical. Logotype data may be referenced through either direct or indirect addressing. Client applications SHOULD support both direct and indirect addressing. Certificate issuing applications MUST support direct addressing, and certificate issuing applications SHOULD support indirect addressing. The direct addressing includes information about each logotype in the certificate, and URIs point to the image and audio data object. Multiple URIs MAY be included for locations for obtaining the same logotype object. Multiple hash values MAY be included, each computed with a different one-way hash function. Direct addressing supports cases where just one or a few alternative images and audio objects are referenced. The indirect addressing includes one or more references to an external hashed data structure that contains information on the type, content, and location of each image and audio object. Indirect addressing supports cases where each logotype is represented by many alternative audio or image objects. Both direct and indirect addressing accommodate alternative URIs to obtain exactly the same logotype data. This opportunity for replication is intended to improve availability. Therefore, if a client is unable to fetch the item from one URI, the client SHOULD try another URI in the sequence. All direct addressing URIs SHOULD use the HTTPS scheme (https://...) or the HTTP scheme (http://...) or the DATA scheme (data://...) . However, the "data" URI scheme MUST NOT be used with the indirect addressing. Clients MUST support retrieval of referenced LogoTypeData with the HTTP and the HTTP with TLS , or subsequent versions of these protocols. Client applications SHOULD also support the "data" URI scheme for direct addressing with embedded logotype data within the extension. Note that the HTTPS scheme (https://...) requires the validation of other certificates to establish a secure connection. For this reason, the HTTP scheme (http://...) may be easier for a client to handle. Also, the hash of the logotype data provides data integrity. The logotype extension MUST have the following syntax: When using indirect addressing, the URI (refStructURI) pointing to the external data structure MUST point to a resource that contains the DER-encoded data with the syntax LogotypeData. At least one of the optional elements in the LogotypeExtn structure MUST be present. When using direct addressing, at least one of the optional elements in the LogotypeData structure MUST be present. The LogotypeReference and LogotypeDetails structures explicitly identify one or more one-way hash functions employed to authenticate referenced image or audio objects. CAs MUST include a hash value for each referenced object, calculated on the whole object. CAs MUST use the one-way hash function that is associated with the certificate signature to compute one hash value, and CAs MAY include other hash values. Clients MUST compute a one-way hash value using one of the identified functions, and clients MUST discard the logotype data if the computed hash value does not match the hash value in the certificate extension. A MIME type is used to specify the format of the image or audio object containing the logotype data. The mediaType field MUST contain a string that is constructed according to the ABNF provided in Section 4.2 of . MIME types MAY include parameters. Image format requirements are specified in , and audio format requirements are specified in . When language is specified, the language tag MUST use the syntax. Logotype types defined in this specification are:

• Community Logotype: If communityLogos is present, the logotypes MUST represent one or more communities with which the certificate issuer is affiliated. The communityLogos MAY be present in an end entity certificate, a CA certificate, or an attribute certificate. The communityLogos contains a sequence of Community Logotypes, each representing a different community. If more than one Community logotype is present, they MUST be placed in order of preferred appearance. Some clients MAY choose to display a subset of the present community logos; therefore the placement within the sequence aids the client selection. The most preferred logotype MUST be first in the sequence, and the least preferred logotype MUST be last in the sequence.

• Issuer Organization Logotype: If issuerLogo is present, the logotype MUST represent the issuer's organization. The logotype MUST be consistent with, and require the presence of, an organization name stored in the organization attribute in the issuer field (for either a public key certificate or attribute certificate). The issuerLogo MAY be present in an end entity certificate, a CA certificate, or an attribute certificate.

• Subject Organization Logotype: If subjectLogo is present, the logotype MUST represent the subject's organization. The logotype MUST be consistent with, and require the presence of, an organization name stored in the organization attribute in the subject field (for either a public key certificate or attribute certificate). The subjectLogo MAY be present in an end entity certificate, a CA certificate, or an attribute certificate.

The relationship between the subject organization and the subject organization logotype, and the relationship between the issuer and either the issuer organization logotype or the community logotype, are relationships asserted by the issuer. The policies and practices employed by the issuer to check subject organization logotypes or claims its issuer and community logotypes is outside the scope of this document.

Conventions for LogotypeImageInfo When the optional LogotypeImageInfo is included with a logotype image, the parameters MUST be used with the following semantics and restrictions. The xSize and ySize fields represent the recommended display size for the logotype image. When a value of 0 (zero) is present, no recommended display size is specified. When non-zero values are present and these values differ from corresponding size values in the referenced image object, then the referenced image SHOULD be scaled to fit within the size parameters of LogotypeImageInfo, while preserving the x and y ratio. The resolution field is redundant for all logotype image formats listed in . The optional resolution field SHOULD be omitted when the image format already contains this information.

Embedded Images If the logotype image is provided through direct addressing, then the image MAY be stored within the logotype certificate extension using the "data" scheme . The syntax of the "data" URI scheme defined is included here for convenience: When including the image data in the logotype extension using the "data" URI scheme, the following conventions apply:

• The value of mediaType in LogotypeDetails MUST be identical to the media type value in the "data" URL.
• The hash of the image MUST be included in logotypeHash and MUST be calculated over the same data as it would have been, had the image been referenced through a link to an external resource.

NOTE: As the "data" URI scheme is processed as a data source rather than as a URL, the image data is typically not limited by any URL length limit settings that otherwise apply to URLs in general. NOTE: Implementations need to be cautious about the size of images included in a certificate in order to ensure that the size of the certificate does not prevent the certificate from being used as intended.

Other Logotypes Logotypes identified by otherLogos (as defined in ) can be used to enhance the display of logotypes and marks that represent partners, products, services, or any other characteristic associated with the certificate or its intended application environment when the standard logotype types are insufficient. The conditions and contexts of the intended use of these logotypes are defined at the discretion of the local client application. Three other logotype types are defined in the follow subsections.

Loyalty Logotype When a loyalty logotype appears in the otherLogos, it MUST be identified by the id-logo-loyalty object identifier. A loyalty logotype, if present, MUST contain a logotype associated with a loyalty program related to the certificate or its use. The relation between the certificate and the identified loyalty program is beyond the scope of this document. The logotype extension MAY contain more than one Loyalty logotype. If more than one loyalty logotype is present, they MUST be placed in order of preferred appearance. Some clients MAY choose to display a subset of the present loyalty logotype data; therefore the placement within the sequence aids the client selection. The most preferred loyalty logotype data MUST be first in the sequence, and the least preferred loyalty logotype data MUST be last in the sequence.

Certificate Background Logotype When a certificate background logotype appears in the otherLogos, it MUST be identified by the id-logo-background object identifier. The certificate background logotype, if present, MUST contain a graphical image intended as a background image for the certificate, and/or a general audio sequence for the certificate. The background image MUST allow black text to be clearly read when placed on top of the background image. The logotype extension MUST NOT contain more than one certificate background logotype.

Certificate Image Logotype When a certificate image logotype appears in the otherLogos, it MUST be identified by the id-logo-certImage object identifier. The certificate image logotype, if present, aids human interpretation of a certificate by providing meaningful visual information to the user interface (UI). The logotype extension MUST NOT contain more than one certificate image logotype. Typical situations when a human needs to examine the visual representation of a certificate are:

• A person establishes a secured channel with an authenticated service. The person needs to determine the identity of the service based on the authenticated credentials.
• A person validates the signature on critical information, such as signed executable code, and needs to determine the identity of the signer based on the signer's certificate.
• A person is required to select an appropriate certificate to be used when authenticating to a service or Identity Management infrastructure. The person needs to see the available certificates in order to distinguish between them in the selection process.

The display of certificate information to humans is challenging due to lack of well-defined semantics for critical identity attributes. Unless the application has out-of-band knowledge about a particular certificate, the application will not know the exact nature of the data stored in common identification attributes such as serialNumber, organizationName, country, etc. Consequently, the application can display the actual data, but faces the problem of labeling that data in the UI and informing the human about the exact nature (semantics) of that data. It is also challenging for the application to determine which identification attributes are important to display and how to organize them in a logical order. When present, the certificate image MUST be a complete visual representation of the certificate. This means that the display of this certificate image represents all information about the certificate that the issuer subjectively defines as relevant to show to a typical human user within the typical intended use of the certificate, giving adequate information about at least the following three aspects of the certificate:

• Certificate Context
• Certificate Issuer
• Certificate Subject

Certificate Context information is visual marks and/or textual information that helps the typical user to understand the typical usage and/or purpose of the certificate. It is up to the issuer to decide what information -- in the form of text, graphical symbols, and elements -- represents a complete visual representation of the certificate. However, the visual representation of Certificate Subject and Certificate Issuer information from the certificate MUST have the same meaning as the textual representation of that information in the certificate itself. Applications providing a Graphical User Interface (GUI) to the certificate user MAY present a certificate image as the only visual representation of a certificate; however, the certificate user SHOULD be able to easily obtain the details of the certificate content.

Type of Certificates Logotypes MAY be included in public key certificates and attribute certificates at the discretion of the certificate issuer; however, the relying party MUST NOT use the logotypes as part of certification path validation or automated trust decision. The sole purpose of logotypes is to enhance the display of a particular certificate, regardless of its position in a certification path.

Use in Clients All PKI implementations require relying party software to have some mechanism to determine whether a trusted CA issues a particular certificate. This is an issue for certification path validation, including consistent policy and name checking. After a certification path is successfully validated, the replying party trusts the information that the CA includes in the certificate, including any certificate extensions. The client software can choose to make use of such information, or the client software can ignore it. If the client is unable to support a provided logotype, the client MUST NOT report an error, rather the client MUST behave as though no logotype extension was included in the certificate. Current standards do not provide any mechanism for cross-certifying CAs to constrain subordinate CAs from including private extensions (see ). Consequently, if relying party software accepts a CA, then it should be prepared to (unquestioningly) display the associated logotypes to its human user, given that it is configured to do so. Information about the logotypes is provided so that the replying party software can select the one that will best meet the needs of the human user. This choice depends on the abilities of the human user, as well as the capabilities of the platform on which the replaying party software is running. If none of the provided logotypes meets the needs of the human user or matches the capabilities of the platform, then the logotypes can be ignored. A client MAY, subject to local policy, choose to display none, one, or any number of the logotypes in the logotype extension. In many cases, a client will be used in an environment with a good network connection and also used in an environment with little or no network connectivity. For example, a laptop computer can be docked with a high-speed LAN connection, or it can be disconnected from the network altogether. In recognition of this situation, the client MUST include the ability to disable the fetching of logotypes. However, locally cached logotypes can still be displayed when the user disables the fetching of additional logotypes. A client MAY, subject to local policy, choose any combination of audio and image presentation for each logotype. That is, the client MAY display an image with or without playing a sound, and it MAY play a sound with or without displaying an image. A client MUST NOT play more than one logotype audio sequence at the same time. The logotype is to be displayed in conjunction with other identity information contained in the certificate. The logotype is not a replacement for this identity information. Care is needed when designing replying party software to ensure that an appropriate context of logotype information is provided. This is especially difficult with audio logotypes. It is important that the human user be able to recognize the context of the logotype, even if other audio streams are being played. If the relying party software is unable to successfully validate a particular certificate, then it MUST NOT display any logotype data associated with that certificate.

Image Formats Animated images SHOULD NOT be used. The following table lists many common image formats and their corresponding MIME type. The table also indicates the support requirements for these image formats. The filename extensions commonly used for each of these formats is also provided. Implementations MAY support other image formats. Image Formats

Format	MIME Type	Extension	References	Implement?
JPEG	image/jpeg	.jpg .jpeg		MUST support
GIF	image/gif	.gif		MUST support
SVG	image/svg+xml	.svg		SHOULD support
SVG + GZIP	image/svg+xml+gzip	.svgz .svg.gz		MUST support
PNG	image/png	.png		SHOULD support
PDF	application/pdf	.pdf		MAY support

NOTE: The image/svg+xml-compressed media type is widely implemented, but it has not yet been registered with IANA. When a Scalable Vector Graphics (SVG) image is used, whether the image is compressed or not, the SVG Tiny profile MUST be followed, with these additional restrictions:

• The SVG image MUST NOT contain any Internationalized Resource Identifier (IRI) references to information stored outside of the SVG image of type B, C, or D, according to Section 14.1.4 of .
• The SVG image MUST NOT contain any 'script' element, according to Section 15.2 of .
• The XML structure in the SVG file MUST use linefeed (0x0A) as the end-of-line (EOL) character when calculating a hash over the SVG image.

When a GZIP-compressed SVG image is fetched with HTTP, the client will receive a response that includes these headers: In this case, the octet stream of type image/svg+xml is compressed with GZIP as specified in . When a uncompressed SVG image is fetched with HTTP, the client will receive a response with the same Content-Type header, but no Content-Encoding header. Whether the SVG image is GZIP-compressed or uncompressed, the hash value for the SVG image is calculated over the uncompressed SVG content with canonicalized EOL characters as specified above. When a SVG image is embedded in the certificate extension using the "data" URL scheme, the SVG image data MUST be provided in GZIP-compressed form, and the XML structure, prior to compression, SHOULD use linefeed (0x0A) as the end-of-line (EOL) character. When a bitmapped image is used, the PNG format SHOULD be used. When a Portable Document Format (PDF) document according to is used, it MUST also be formatted according to the profile PDF/A .

Audio Formats Implementations that support audio MUST support the MP3 audio format with a MIME type of "audio/mpeg" . Implementations SHOULD support text-based audio data with a MIME type of "text/plain;charset=UTF-8". Implementations MAY support other audio formats. Text-based audio data using the MIME type of "text/plain;charset=UTF-8" is intended to be used by text-to-speech software. When this audio type is used, the following requirements apply:

• LogotypeAudioInfo MUST be present and specify the language of the text.
• The fileSize, playTime, and channels elements of LogotypeAudioInfo MUST have the value of 0.
• The sampleRate element of LogotypeAudioInfo MUST be absent.

Security Considerations Implementations that simultaneously display multiple logotype types (subject organization, issuer, community, or other), MUST ensure that there is no ambiguity as to the binding between the image and the type of logotype that the image represents. "Logotype type" is defined in , and it refers to the type of entity or affiliation represented by the logotype, not the of binary format of the image or audio. Logotypes are very difficult to securely and accurately define. Names are also difficult in this regard, but logotypes are even worse. It is quite difficult to specify what is, and what is not, a legitimate logotype of an organization. There is an entire legal structure around this issue, and it will not be repeated here. However, issuers should be aware of the implications of including images associated with a trademark or servicemark before doing so. As logotypes can be difficult (and sometimes expensive) to verify, the possibility of errors related to assigning wrong logotypes to organizations is increased. This is not a new issue for electronic identification instruments. It is already dealt with in a number of similar situations in the physical world, including physical employee identification cards. In addition, there are situations where identification of logotypes is rather simple and straightforward, such as logotypes for well-known industries and institutes. These issues should not stop those service providers who want to issue logotypes from doing so, where relevant. It is impossible to prevent fraudulent creation of certificates by dishonest or badly performing issuers, containing names and logotypes that the issuer has no claim to or has failed to check correctly. Such certificates could be created in an attempt to socially engineer a user into accepting a certificate. The premise used for the logotype work is thus that logotype graphics in a certificate are trusted only if the certificate is successfully validated within a valid path. It is thus imperative that the representation of any certificate that fails to validate is not enhanced in any way by using the logotype data. This underlines the necessity for CAs to provide reliable services, and the relying party's responsibility and need to carefully select which CAs are trusted to provide public key certificates. This also underlines the general necessity for relying parties to use up-to-date software libraries to render or dereference data from external sources, including logotype data in certificates, to minimize risks related to processing potentially malicious data before it has been adequately verified and validated. Implementers should review the guidance in . Referenced image objects are hashed in order to bind the image to the signature of the certificate. Some image types, such as SVG, allow part of the image to be collected from an external source by incorporating a reference to an external file that contains the image. If this feature were used within a logotype image, the hash of the image would only cover the URI reference to the external image file, but not the referenced image data. Clients SHOULD verify that SVG images meet all requirements listed in and reject images that contain references to external data. CAs issuing certificates with embedded logotype images should be cautious when accepting graphics from the certificate requestor for inclusion in the certificate if the hash algorithm used to sign the certificate is vulnerable to collision attacks such as . In such a case, the accepted image may contain data that could help an attacker to obtain colliding certificates with identical certificate signatures. Certification paths may also impose name constraints that are systematically checked during certification path processing, which, in theory, may be circumvented by logotypes. Certificate path processing as defined in does not constrain the inclusion of logotype data in certificates. A parent CA can constrain certification path validation such that subordinate CAs cannot issue valid certificates to end-entities outside a limited name space or outside specific certificate polices. A malicious CA can comply with these name and policy requirements and still include inappropriate logotypes in the certificates that it issues. These certificates will pass the certification path validation algorithm, which means the client will trust the logotypes in the certificates. Since there is no technical mechanism to prevent or control subordinate CAs from including the logotype extension or its contents, where appropriate, a parent CA could employ a legal agreement to impose a suitable restriction on the subordinate CA. This situation is not unique to the logotype extension. When a relying party fetches remote logotype data, a mismatch between the media type provided in the mediaType field of the LogotypeDetails and the Content-Type HTTP header of the retrieved object MUST be treated as a failure and the fetched logotype data should not be presented to the user. However, if more than one location for the remote logotype data is provided in the certificate extension, the relying party MAY try to fetch the remote logotype data from an alternate location to resolve the failure. When a subscriber requests the inclusion of remote logotype data in a certificate, the CA cannot be sure that any logotype data will be available at the provided URI for the entire validity period of the certificate. To mitigate this concern, the CA may provide the logotype data from a server under its control, rather than a subscriber-controlled server. The controls available to a parent CA to protect itself from rogue subordinate CAs are non-technical. They include:

• Contractual agreements of suitable behavior, including terms of liability in case of material breach.
• Control mechanisms and procedures to monitor and follow the behavior of subordinate CAs, including Certificate Transparency .
• Use of certificate policies to declare an assurance level of logotype data, as well as to guide applications on how to treat and display logotypes.
• Use of revocation functions to revoke any misbehaving CA.

There is not a simple, straightforward, and absolute technical solution. Rather, involved parties must settle some aspects of PKI outside the scope of technical controls. As such, issuers need to clearly identify and communicate the associated risks.

Privacy Considerations Certificates are commonly public objects, so the inclusion of privacy-sensitive information in certificates should be avoided. The more information that is included in a certificate, the greater the likelihood that the certificate will reveal privacy-sensitive information. The inclusion of logotype data needs to be considered in this context. Logotype data might be fetched from a server when it is needed. By watching activity on the network, an observer can determine which clients are making use of certificates that contain particular logotype data. Since clients are expected to locally cache logotype data, network traffic to the server containing the logotype data will not be generated every time the certificate is used. Further, when logotype data is not cached, activity on the network would reveal certificate usage frequency. Even when logotype data is cached, regardless of whether direct or indirect addressing is employed, network traffic monitoring could reveal when logotype data is fetched for the first time. Implementations MAY encrypt fetches of logotype data using HTTPS and pad them to a common size to reduce visibility into the data that is being fetched. Likewise, servers MAY reduce visibility into the data that is being returned by encrypting with HTTPS and padding to a few common sizes. Similarly, when fetching logotype data from a server, the server operator can determine which clients are making use of certificates that contain particular logotype data. As above, locally caching logotype data will eliminate the need to fetch the logotype data each time the certificate is used, and lack of caching would reveal usage frequency. Even when implementations cache logotype data, regardless of whether direct or indirect addressing is employed, the server operator could observe when logotype data is fetched for the first time. In addition, the use of an encrypted DNS mechanism, such as DoT or DoH , hides the name resolution traffic associated fetching remote logotype objects. When the "data" URI scheme is used with direct addressing, there is no network traffic to fetch logotype data, which avoids the observations of network traffic or server operations described above. To obtain this benefit, the certificate will be larger than one that contains a URL. Due to the improved privacy posture, the "data" URI scheme with direct addressing will be the only one that is supported by some CAs. Privacy-aware certificate subscribers MAY wish to insist on their logotype data being embedded in the certificate with the "data" URI scheme with direct addressing. In cases where logotype data is cached by the relying party, the cache index should include the hash values of the associated logotype data with the goal of fetching the logotype data only once, even when it is referenced by multiple URIs. The index should include hash values for all supported hash algorithms. The cached data should include the media type as well as the logotype data. Implementations should give preference to logotype data that is already in the cache when multiple alternatives are offered in the LogotypeExtn certificate extension. When the "data" URI scheme is used, the relying party MAY add the embedded logotype data to the local cache, which could avoid the need to fetch the logotype data if it is referenced by a URL in another certificate. When fetching remote logotype data, relying parties should use the most privacy-preserving options that are available to minimize the opportunities for servers to "fingerprint" clients. For example, avoid cookies, e-tags, and client certificates. When a relying party encounters a new certificate, the lack of network traffic to fetch logotype data might indicate that a certificate with references to the same logotype data has been previously processed and cached. TLS 1.3 includes the ability to encrypt the server's certificate in the TLS handshake, which helps hide the server's identity from anyone that is watching activity on the network. If the server's certificate includes remote logotype data, the client fetching that data might disclose the otherwise protected server identity.

IANA Considerations For the new ASN.1 Module in , IANA is requested to assign an object identifier (OID) for the module identifier. The OID for the module should be allocated in the "SMI Security for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0).

Acknowledgments

Acknowledgments from RFC 3709 This document is the result of contributions from many professionals. The authors appreciate contributions from all members of the IETF PKIX Working Group. We extend a special thanks to Al Arsenault, David Cross, Tim Polk, Russel Weiser, Terry Hayes, Alex Deacon, Andrew Hoag, Randy Sabett, Denis Pinkas, Magnus Nystrom, Ryan Hurst, and Phil Griffin for their efforts and support. Russ Housley thanks the management at RSA Laboratories, especially Burt Kaliski, who supported the development of this specification. The vast majority of the work on this specification was done while Russ was employed at RSA Laboratories.

Acknowledgments from RFC 6170 The authors recognize valuable contributions from members of the PKIX working group, the CA Browser Forum, and James Manger, for their review and sample data.

Additional Acknowledgments Combining RFC 3709 and RFC 6170 has produced an improved specification. The authors appreciate contributions from all members of the IETF LAMPS Working Group. We extend a special thanks to Alexey Melnikov for his guidance on media types. We extend a special thanks to Tim Geiser for his careful checking of the new examples in Appendix B.4 and B.5. We extend a special thanks to Corey Bonnell, Daniel Kahn Gillmor, Roman Danyliw, Paul Kyzivat, Shuping Peng, Sheng Jiang, Rob Wilton, Eric Vyncke, and Donald Eastlake for their careful review and helpful comments.

References Normative References This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This specification defines a profile for the use of X.509 Attribute Certificates in Internet Protocols. Attribute certificates may be used in a wide range of applications and environments covering a broad spectrum of interoperability goals and a broader spectrum of operational and assurance requirements. The goal of this document is to establish a common baseline for generic applications requiring broad interoperability as well as limited special purpose requirements. The profile places emphasis on attribute certificate support for Internet electronic mail, IPsec, and WWW security applications. This document obsoletes RFC 3281. [STANDARDS-TRACK] A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] A new URL scheme, "data", is defined. It allows inclusion of small data items as "immediate" data, as if it had been included externally. [STANDARDS-TRACK] This second document defines the general structure of the MIME media typing system and defines an initial set of media types. [STANDARDS-TRACK] The audio layers of the MPEG-1 and MPEG-2 standards are in frequent use on the internet, but there is no uniform Multipurpose Internet Mail Extension (MIME) type for these files. The intention of this document is to define the media type audio/mpeg to refer to this kind of contents. [STANDARDS-TRACK] This document describes the structure, content, construction, and semantics of language tags for use in cases where it is desirable to indicate the language used in an information object. It also describes how to register values for use in language tags and the creation of user-defined extensions for private interchange. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document defines procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice. Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] This specification defines a lossless compressed data format that is compatible with the widely used GZIP utility. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. ITU-T World Wide Web Consortium ISO/IEC ITU-T CompuServe Incorporated ISO/IEC In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References The Public Key Infrastructure using X.509 (PKIX) certificate format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates those ASN.1 modules to conform to the 2002 version of ASN.1. There are no bits-on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes. This document updates the security considerations for the MD5 message digest algorithm. It also updates the security considerations for HMAC-MD5. This document is not an Internet Standards Track specification; it is published for informational purposes. The Cryptographic Message Syntax (CMS) format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates some auxiliary ASN.1 modules to conform to the 2008 version of ASN.1; the 1988 ASN.1 modules remain the normative version. There are no bits- on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes. The Portable Document Format (PDF) is an ISO standard (ISO 32000-1:2008) defining a final-form document representation language in use for document exchange, including on the Internet, since 1993. This document provides an overview of the PDF format and updates the media type registration of "application/pdf". It obsoletes RFC 3778. This document specifies a certificate extension for including logotypes in public key certificates and attribute certificates. [STANDARDS-TRACK] This document specifies a method to bind a visual representation of a certificate in the form of a certificate image to a public key certificate as defined in RFC 5280, by defining a new "otherLogos" image type according to RFC 3709. [STANDARDS-TRACK] This document describes the use of Transport Layer Security (TLS) to provide privacy for DNS. Encryption provided by TLS eliminates opportunities for eavesdropping and on-path tampering with DNS queries in the network, such as discussed in RFC 7626. In addition, this document specifies two usage profiles for DNS over TLS and provides advice on performance considerations to minimize overhead from using TCP and TLS with DNS. This document focuses on securing stub-to-recursive traffic, as per the charter of the DPRIVE Working Group. It does not prevent future applications of the protocol to recursive-to-authoritative traffic. This document describes version 2.0 of the Certificate Transparency (CT) protocol for publicly logging the existence of Transport Layer Security (TLS) server certificates as they are issued or observed, in a manner that allows anyone to audit certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. This document obsoletes RFC 6962. It also specifies a new TLS extension that is used to send various CT log artifacts. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. The S/MIME development community benefits from sharing samples of signed or encrypted data. This document facilitates such collaboration by defining a small set of X.509v3 certificates and keys for use when generating such samples. This document describes a protocol that allows clients to hide their IP addresses from DNS resolvers via proxying encrypted DNS over HTTPS (DoH) messages. This improves privacy of DNS operations by not allowing any one server entity to be aware of both the client IP address and the content of DNS queries and answers. This experimental protocol has been developed outside the IETF and is published here to guide implementation, ensure interoperability among implementations, and enable wide-scale experimentation. CCITT CCITT Recommendation X.208 ISO ISO World Wide Web Consortium World Wide Web Consortium

ASN.1 Modules

ASN.1 Modules with 1988 Syntax This appendix contains two ASN.1 modules, both using the old syntax . The first ASN.1 module provides the syntax for the Logotype certificate extension. Only comments have changed in the module from RFC 3709, and the IMPORTS now come from . The second ASN.1 module provides the Certificate Image object identifier. The module is unchanged from RFC 6170.

ASN.1 Module with 2002 Syntax Some developers like to use the latest version of ASN.1 standards. This appendix provides an ASN.1 module to assist in that goal. It uses the ASN.1 syntax defined in , and it follows the conventions established in and . This ASN.1 module incorporates the module from RFC 3709 and the module from RFC 6170. Note that was published in 2021, and all of the features used in this module are backward compatible with the specification that was published in 2002.

Examples

Example from RFC 3709 The following example displays a logotype extension containing one Issuer logotype using direct addressing. The issuer logotype image is of the type image/gif. The logotype image is referenced through one URI and the image is hashed with SHA-1. This example is unchanged from RFC 3709, except that shallow indenting is used to keep the example within traditional margins. The use of SHA-1 was reasonable at the time that RFC 3709 was published, but many better choices are available today. The values on the left are the ASN.1 tag (in hexadecimal) and the length (in decimal).

Issuer Logotype Example The following example displays a logotype extension containing one Issuer logotype using direct addressing. The issuer logotype image is of the type image/jpeg. The logotype image is referenced through one URI and the image is hashed with SHA-256. The values on the left are the ASN.1 tag (in hexadecimal) and the length (in decimal).

Embedded Image Example The following example displays a logotype extension containing one Subject logotype using direct addressing. The subject logotype image uses image/svg+xml-compressed. The logotype image is embedded in the certificate extension with a "data:" URI and the image is hashed by SHA-256. This technique produces a large certificate extension, but offers reduced latency and improved privacy. The values on the left are the ASN.1 tag (in hexadecimal) and the length (in decimal).

Embedded Certificate Image Example The following example displays a logotype extension containing one Certificate Image logotype using direct addressing. The Certificate Image logotype uses image/svg+xml-compressed. The logotype image is embedded in the certificate extension with a "data:" URI and the image is hashed by SHA-256. This example contains the image from Appendix B of RFC 6170, however, the media type used here is explicit about the use of GZIP compression . The values on the left are the ASN.1 tag (in hexadecimal) and the length (in decimal).

Full Certificate Example The following example contains a certificate for Alice; it is essentially a renewal of the certificate that appears in . Of course, the serial number and issue dates are different. In addition, Alice's certificate now has a logotype extension. The extension contains URLs for two community logotype images, both at fictional URLs. The extension also contains URLs for two subject logotype images, both at fictional URLs. An implementation would display at most three of these images, both of the community logotype images and one of the subject logotype images. Direct addressing is used for all of the images, and the images are hashed by SHA-256. The following displays the logotype extension from Alice's certificate. The values on the left are the ASN.1 tag (in hexadecimal) and the length (in decimal).

Changes Since RFC 3709 and RFC 6170 This appendix summarizes the changes since RFC 3709. The changes are:

• Combine RFC 3709 and RFC 6170 into one document, and encourage implementers to support the "data" URI scheme (data:...) that was originally specified in RFC 6170. Merging RFC 3709 and RFC 6170 lead to many editoral changes throughout the document.
• Drop SHA-1 as the mandatory-to-implement hash algorithm, and encourage use of the one-way hash function that is employed by the certificate signature algorithm.
• RFC 3709 required client applications to support both direct and indirect addressing. This requirement is changed to SHOULD support both direct and indirect addressing to allow implementations to be more privacy preserving.
• Update the reference for language tags to be RFC 5646 instead of the now obsolete RFC 3066.
• Update the reference for the URI Generic Syntax to be RFC 3986 instead of the now obsolete RFC 2396.
• Update the reference for the application/pdf media type to be RFC 8118 instead of the now obsolete RFC 3778.
• No longer require support for the FTP scheme (ftp://...) URI.
• Require support for the HTTP scheme (http://...) URI and the HTTPS scheme (https://...) URI.
• Require support for the compressed SVG image format with the image/svg+xml+gzip media type.
• Media types MUST follow the ABNF that is provided in Section 4.2 of . This change resolves Errata ID 2679.
• Remove the requirement that the LogotypeData file name have a file extension of ".LTD". This change resolves Errata ID 2325.
• Encourage, instead of requiring, each logotype to be represented by at least one image.
• Encourage the inclusion of text-based audio data suitable for processing by a text-to-speech software using the MIME type of "text/plain;charset=UTF-8".
• Require that the logotype extension not contain more than one certificate image logotype.
• Privacy-related topics that were previously discussed in the Security Considerations section are now covered in a separate Privacy Considerations section. Additional topics are covered in both sections.
• Provide ASN.1 modules for both the older syntax and the most recent ASN.1 syntax .
• Provide additional references.
• Provide additional examples.
• Several editorial changes to improve clarity.