Digital Signature

Enhanced Definition

A digital signature is a mathematical scheme used to verify the authenticity and integrity of digital messages, documents, or software. In the mainframe and z/OS context, it ensures that data originated from a trusted sender and has not been altered during transmission or storage, providing non-repudiation and data integrity. A digital signature on z/OS is a cryptographic mechanism used to verify the authenticity, integrity, and non-repudiation of digital data or messages. It ensures that data has not been tampered with in transit or storage and confirms the identity of the entity (user, application, or system) that created or sent the data.

Key Characteristics

- Asymmetric Cryptography: Relies on a public/private key pair, where the sender signs data with their private key, and the receiver verifies it using the sender's public key.
- Message Digest (Hash Function): The signature is generated from a fixed-size cryptographic hash (message digest) of the data, not the data itself, ensuring efficiency and sensitivity to any data alteration.
- Non-repudiation: Provides strong evidence of the signer's identity, preventing them from falsely denying having signed the data.
- Data Integrity: Any modification to the original data after signing will cause the signature verification to fail, immediately indicating tampering.
- Authenticity: Confirms the identity of the signer, assuring the recipient that the data comes from the claimed source.
- z/OS Implementation: Often leverages Integrated Cryptographic Service Facility (ICSF) and IBM Crypto Express hardware security modules (HSMs) for secure key management and high-performance cryptographic operations.

Use Cases

- Secure Software Distribution: Verifying the authenticity and integrity of program modules, load libraries, or system updates deployed onto z/OS systems from trusted vendors or internal development teams.
- Secure File Transfers: Ensuring the integrity and origin of critical data files exchanged between z/OS systems or with external platforms using protocols like SFTP or FTPS.
- Digital Certificates: Forming the basis of Public Key Infrastructure (PKI) on z/OS, where Certificate Authorities (CAs) digitally sign certificates to establish trust in identities.
- Transaction Authorization: Providing non-repudiation for high-value or sensitive transactions within financial or enterprise applications running on z/OS.
- API Security: Securing RESTful API interactions (e.g., via z/OS Connect EE) by digitally signing requests or responses to guarantee sender authenticity and data integrity.

Related Concepts

Digital signatures are foundational to Public Key Infrastructure (PKI) on z/OS, which manages the lifecycle of digital certificates and key pairs, often stored in RACF or ICSF key repositories. They are heavily dependent on Integrated Cryptographic Service Facility (ICSF), the z/OS component that provides cryptographic services, including hashing and asymmetric encryption/decryption, frequently offloading these operations to Crypto Express hardware. Digital signatures are also crucial for establishing trust in secure communication protocols like TLS/SSL and SSH within the mainframe environment, where server and client identities are verified via digitally signed certificates.

Best Practices:

Secure Key Management: Ensure private signing keys are securely generated, stored, and managed, ideally within Hardware Security Modules (HSMs) like IBM Crypto Express adapters and managed by ICSF key data sets (CKDS, PKDS).
Robust Certificate Management: Implement clear policies and procedures for the issuance, renewal, and timely revocation of signing certificates to mitigate risks from compromised or expired keys.
Algorithm Strength: Utilize strong, industry-standard hashing and signing algorithms (e.g., SHA-256 or SHA-512 with RSA-2048 or ECC) and regularly review and update them in line with evolving cryptographic best practices.
Policy and Procedure: Establish clear organizational policies defining who is authorized to sign what, under what conditions, and how digital signatures are to be verified and trusted within z/OS applications and system processes.
Auditing and Logging: Implement comprehensive auditing and logging of all digital signing and verification activities, including successes and failures, for compliance, security monitoring, and forensic analysis.