Android jar meta inf

JAR File Specification

Contents

Introduction

In many cases, JAR files are not just simple archives of java classes files and/or resources. They are used as building blocks for applications and extensions. The META-INF directory, if it exists, is used to store package and extension configuration data, including security, versioning, extension and services.

The META-INF directory

Name-Value pairs and Sections

Groups of name-value pairs are known as a «section». Sections are separated from other sections by empty lines.

Binary data of any form is represented as base64. Continuations are required for binary data which causes line length to exceed 72 bytes. Examples of binary data are digests and signatures.

Implementations shall support header values of up to 65535 bytes.

All the specifications in this document use the same grammar in which terminal symbols are shown in fixed width font and non-terminal symbols are shown in italic type face.

Specification:

; Also: To prevent mangling of files sent via straight e-mail, no
; header will start with the four letters «From».

Non-terminal symbols defined in the above specification will be referenced in the following specifications.

JAR Manifest

Overview

The main section contains security and configuration information about the JAR file itself, as well as the application or extension that this JAR file is a part of. It also defines main attributes that apply to every individual manifest entry. No attribute in this section can have its name equal to «Name«. This section is terminated by an empty line.

The individual sections define various attributes for packages or files contained in this JAR file. Not all files in the JAR file need to be listed in the manifest as entries, but all files which are to be signed must be listed. The manifest file itself must not be listed. Each section must start with an attribute with the name as «Name«, and the value must be a relative path to the file, or an absolute URL referencing data outside the archive.

If there are multiple individual sections for the same file entry, the attributes in these sections are merged. If a certain attribute have different values in different sections, the last one is recognized.

Attributes which are not understood are ignored. Such attributes may include implementation specific information used by applications.

Manifest Specification:

In the above specification, attributes that can appear in the main section are referred to as main attributes, whereas attributes that can appear in individual sections are referred to as per-entry attributes. Certain attributes can appear both in the main section and the individual sections, in which case the per-entry attribute value overrides the main attribute value for the specified entry. The two types of attributes are defined as follows.

Main Attributes

Per-Entry Attributes

It means that all the packages archived in a.jar are sealed, except that package foo.bar is not.

The per-entry attributes fall into the following groups:

• attributes defined for file contents:
  • Content-Type: This attribute can be used to specify the MIME type and subtype of data for a specific file entry in the JAR file. The value should be a string in the form of type/subtype. For example «image/bmp» is an image type with a subtype of bmp (representing bitmap). This would indicate the file entry as an image with the data stored as a bitmap. RFC 1521 and 1522 discuss and define the MIME types definition.
• attributes defined for package versioning and sealing information: These are the same set of attributes defined above as main attributes that defines the extension package versioning and sealing information. When used as per-entry attributes, these attributes overwrites the main attributes but only apply to the individual file specified by the manifest entry.
• attribute defined for beans objects:
  • Java-Bean: Defines whether the specific jar file entry is a Java Beans object or not. The value should be either «true» or «false», case is ignored.
• attributes defined for signing: These attributes are used for signing and verifying purposes. More details here.
  • x-Digest-y: The name of this attribute specifies the name of the digest algorithm used to compute the digest value for the corresponding jar file entry. The value of this attribute stores the actual digest value. The prefix ‘x’ specifies the algorithm name and the optional suffix ‘y’ indicates to which language the digest value should be verified against.
  • Magic: This is an optional attribute that can be used by applications to indicate how verifier should compute the digest value contained in the manifest entry. The value of this attribute is a set of comma separated context specific strings. Detailed description is here.

Signed JAR File

Overview

Subsets of a JAR file can be signed by using the java.security API. A signed JAR file is exactly the same as the original JAR file, except that its manifest is updated and two additional files are added to the META-INF directory: a signature file and a signature block file. When jarsigner is not used, the signing program has to construct both the signature file and the signature block file.

For every file entry signed in the signed JAR file, an individual manifest entry is created for it as long as it does not already exist in the manifest. Each manifest entry lists one or more digest attributes and an optional Magic attribute.

Signature File

Paths or URLs appearing in the manifest file but not in the signature file are not used in the calculation.

Signature Validation

Verify the signature over the signature file when the manifest is first parsed. For efficiency, this verification can be remembered. Note that this verification only validates the signature directions themselves, not the actual archive files.

If an x-Digest-Manifest attribute exists in the signature file, verify the value against a digest calculated over the entire manifest. If more than one x-Digest-Manifest attribute exists in the signature file, verify that at least one of them matches the calculated digest value.

If an x-Digest-Manifest attribute does not exist in the signature file or none of the digest values calculated in the previous step match, then a less optimized verification is performed:

If an x-Digest-Manifest-Main-Attributes entry exists in the signature file, verify the value against a digest calculated over the main attributes in the manifest file. If this calculation fails, then JAR file verification fails. This decision can be remembered for efficiency. If an x-Digest-Manifest-Main-Attributes entry does not exist in the signature file, its nonexistence does not affect JAR file verification and the manifest main attributes are not verified.

Verify the digest value in each source file information section in the signature file against a digest value calculated against the corresponding entry in the manifest file. If any of the digest values don’t match, then JAR file verification fails.

One reason the digest value of the manifest file that is stored in the x-Digest-Manifest attribute may not equal the digest value of the current manifest file is that one or more files were added to the JAR file (using the jar tool) after the signature (and thus the signature file) was generated. When the jar tool is used to add files, the manifest file is changed (sections are added to it for the new files), but the signature file is not. A verification is still considered successful if none of the files that were in the JAR file when the signature was generated have been changed since then, which is the case if the digest values in the non-header sections of the signature file equal the digest values of the corresponding sections in the manifest file.

For each entry in the manifest, verify the digest value in the manifest file against a digest calculated over the actual data referenced in the «Name:» attribute, which specifies either a relative file path or URL. If any of the digest values don’t match, then JAR file verification fails.

Example manifest file:

The corresponding signature file would be:

The Magic Attribute

The Magic attribute is optional but it is required that a parser understand the value of an entry’s Magic key if it is verifying that entry’s signature.

The value or values of the Magic attribute are a set of comma-separated context-specific strings. The spaces before and after the commas are ignored. Case is ignored. The exact meaning of the magic attributes is application specific. These values indicate how to compute the hash value contained in the manifest entry, and are therefore crucial to the proper verification of the signature. The keywords may be used for dynamic or embedded content, multiple hashes for multilingual documents, etc.

Here are two examples of the potential use of Magic attribute in the manifest file:

In the first example, these Magic values may indicate that the result of an http query is the script embedded in the document, as opposed to the document itself, and also that the script is generated dynamically. These two pieces of information indicate how to compute the hash value against which to compare the manifest’s digest value, thus comparing a valid signature.

In the second example, the Magic value indicates that the document retrieved may have been content-negotiated for a specific language, and that the digest to verify against is dependent on which language the document retrieved is written in.

Digital Signatures

Digital signature files have the same filenames as the .SF files but different extensions. The extension varies depending on the type of digital signature.

• .RSA (PKCS7 signature, SHA-256 + RSA)
• .DSA (PKCS7 signature, DSA)

Digital signature files for signature algorithms not listed above must reside in the META-INF directory and have the prefix » SIG- «. The corresonding signature file ( .SF file) must also have the same prefix.

For those formats that do not support external signed data, the file shall consist of a signed copy of the .SF file. Thus some data may be duplicated and a verifier should compare the two files.

Formats that support external data either reference the .SF file, or perform calculations on it with implicit reference.

Each .SF file may have multiple digital signatures, but those signatures should be generated by the same legal entity.

File name extensions may be 1 to 3 alphanum characters. Unrecognized extensions are ignored.

Notes on Manifest and Signature Files

JAR Index

Overview

The existing jar tool is enhanced to be able to examine a list of jar files and generate directory information as to which classes and resources reside in which jar file. This directory information is stored in a simple text file named INDEX.LIST in the META-INF directory of the root jar file. When the classloader loads the root jar file, it reads the INDEX.LIST file and uses it to construct a hash table of mappings from file and package names to lists of jar file names. In order to find a class or a resource, the class loader queries the hashtable to find the proper jar file and then downloads it if necessary.

Once the class loader finds a INDEX.LIST file in a particular jar file, it always trusts the information listed in it. If a mapping is found for a particular class, but the class loader fails to find it by following the link, an InvalidJarIndexException is thrown. When this occurs, the application developer should rerun the jar tool on the extension to get the right information into the index file.

To prevent adding too much space overhead to the application and to speed up the construction of the in-memory hash table, the INDEX.LIST file is kept as small as possible. For classes with non-null package names, mappings are recorded at the package level. Normally one package name is mapped to one jar file, but if a particular package spans more than one jar file, then the mapped value of this package will be a list of jar files. For resource files with non-empty directory prefixes, mappings are also recorded at the directory level. Only for classes with null package name, and resource files which reside in the root directory, will the mapping be recorded at the individual file level.

Index File Specification

The UTF-8 encoding is used to support non ASCII characters in file or package names in the index file.

Specification

index file : version-info blankline section*
version-info : JarIndex-Version: version-number
version-number : digit+<.digit+>*
section : body blankline
body : header name*
header : char+.jar newline
name : char+ newline
char : any valid Unicode character except NULL, CR andLF
blankline: newline newline
newline : CR LF | LF | CR (not followed by LF)
digit: <0-9>

The INDEX.LIST file is generated by running jar -i. See the jar man page for more details.

Backward Compatibility

Service Provider

Overview

Files in the META-INF/services directory are service provider configuration files. A service is a well-known set of interfaces and (usually abstract) classes. A service provider is a specific implementation of a service. The classes in a provider typically implement the interfaces and subclass the classes defined in the service itself. Service providers may be installed in an implementation of the Java platform in the form of extensions, that is, jar files placed into any of the usual extension directories. Providers may also be made available by adding them to the applet or application class path or by some other platform-specific means.

A service is represented by an abstract class. A provider of a given service contains one or more concrete classes that extend this service class with data and code specific to the provider. This provider class will typically not be the entire provider itself but rather a proxy that contains enough information to decide whether the provider is able to satisfy a particular request together with code that can create the actual provider on demand. The details of provider classes tend to be highly service-specific; no single class or interface could possibly unify them, so no such class has been defined. The only requirement enforced here is that provider classes must have a zero-argument constructor so that they may be instantiated during lookup.

Provider-Configuration File

A service provider identifies itself by placing a provider-configuration file in the resource directory META-INF/services. The file’s name should consist of the fully-qualified name of the abstract service class. The file should contain a newline-separated list of unique concrete provider-class names. Space and tab characters, as well as blank lines, are ignored. The comment character is ‘#’ (0x23); on each line all characters following the first comment character are ignored. The file must be encoded in UTF-8.

Example

Each method returns an appropriate object or null if it cannot translate the given encoding. Typical CharCodec providers will support more than one encoding.

If sun.io.StandardCodec is a provider of the CharCodec service then its jar file would contain the file META-INF/services/java.io.spi.CharCodec. This file would contain the single line:

To locate an encoder for a given encoding name, the internal I/O code would do something like this:

The provider-lookup mechanism always executes in the security context of the caller. Trusted system code should typically invoke the methods in this class from within a privileged security context.

Источник