“Jarsigner” is a command-line tool provided by Oracle’s Java Development Kit (JDK) that is used to digitally sign and verify Java archive (JAR) files. Digital signatures are used to ensure the authenticity and integrity of JAR files, providing a mechanism for users to verify that the contents of a JAR file have not been tampered with and originate from a trusted source. Here’s a detailed explanation of the “jarsigner” tool and its functionality:
- Digital Signing: The primary function of “jarsigner” is to apply digital signatures to JAR files. Digital signatures are generated using cryptographic algorithms and private keys, and they provide a way to verify the authenticity and integrity of the signed JAR file. When a JAR file is signed with “jarsigner,” a digital signature block is appended to the file, containing information about the signer and a hash of the file’s contents.
- Authentication and Trust: Digital signatures allow users to authenticate the origin of a JAR file and verify that it has not been modified since it was signed. Users can trust signed JAR files if they trust the signer’s certificate and the signing process used. This provides assurance that the contents of the JAR file are trustworthy and have not been altered by malicious actors.
- Verification: “Jarsigner” also provides functionality to verify the digital signatures of signed JAR files. By using the “jarsigner -verify” command, users can check the validity of the signatures on a JAR file and ensure that the file has not been tampered with since it was signed. Verification involves validating the digital signatures against the signer’s public key and checking that the file’s contents match the signed hash values.
- Certificate Management: “Jarsigner” works with X.509 digital certificates, which are used to identify the signer of a JAR file. Users need to have a valid certificate and corresponding private key to sign JAR files. “Jarsigner” supports various certificate formats, including PKCS#12 keystores and JKS (Java KeyStore) keystores, allowing users to manage their signing certificates and keys securely.
- Timestamping: To provide long-term validity and reliability of digital signatures, “jarsigner” supports timestamping services. When signing a JAR file, users can include a timestamp from a trusted time-stamping authority (TSA) to indicate the signing time. This ensures that the signature remains valid even after the signer’s certificate expires, as long as the timestamp remains valid.
- Command-Line Interface: “Jarsigner” is a command-line tool, meaning it is operated via text-based commands in a terminal or command prompt. Users invoke “jarsigner” with various command-line options to sign, verify, and manage the digital signatures of JAR files.
jarsigner Command Examples
1. Sign a JAR file:
# jarsigner [path/to/file.jar] [keystore_alias]
2. Sign a JAR file with a specific algorithm:
# jarsigner -sigalg [algorithm] [path/to/file.jar] [keystore_alias]
3. Verify the signature of a JAR file:
# jarsigner -verify [path/to/file.jar]
Summary
Overall, “jarsigner” is an essential tool for Java developers and distributors who need to ensure the authenticity and integrity of their JAR files. By applying digital signatures and verifying their validity, “jarsigner” helps maintain trust and security in Java applications and libraries distributed over the internet.