<p dir="ltr">Modern software relies heavily on third-party dependencies (often distributed via public package registries), making software supply chain attacks a growing threat. Prior work investigated attacks and defenses, but only taxonomized attacks or proposed defensive techniques, did not consistently define software supply chain attacks, and did not provide properties to assess the security of software supply chains. We do not have a unified definition of software supply chain attacks nor a set of properties that a secure software supply chain should follow.</p><p dir="ltr">Guaranteeing authorship in a software supply chain is also a challenge. Package maintainers can guarantee package authorship through software signing. However, it is unclear how common this practice is or if existing signatures are created properly. Prior work provided raw data on registry signing practices, but only measured single platforms, did not consider quality, did not consider time, and did not assess factors that may influence signing. We do not have up-to-date measurements of signing practices nor do we know the quality of existing signatures. Furthermore, we lack a comprehensive understanding of factors that influence signing adoption.</p><p dir="ltr">This thesis addresses these gaps. First, we systematize existing knowledge into: (1) a four-stage supply chain attack pattern; and (2) a set of properties for secure supply chains (transparency, validity, and separation). Next, we measure current signing quantity and quality across three kinds of package registries: traditional software (Maven Central, PyPI), container images (Docker Hub), and machine learning models (Hugging Face). Then, we examine longitudinal trends in signing practices. Finally, we use a quasi-experiment to estimate the effect that various factors had on software signing practices.</p><p dir="ltr">To summarize the findings of our quasi-experiment: (1) mandating signature adoption improves the quantity of signatures; (2) providing dedicated tooling improves the quality of signing; (3) getting started is the hard part — once a maintainer begins to sign, they tend to continue doing so; and (4) although many supply chain attacks are mitigable via signing, signing adoption is primarily affected by registry policy rather than by public knowledge of attacks, new engineering standards, etc. These findings highlight the importance of software package registry managers and signing infrastructure.</p>