Generate SRI Hash with SHA-256

SRI with SHA-256

SHA-256 is a member of the SHA-2 family of cryptographic hash functions. It produces a 256-bit (32-byte) digest, which is Base64-encoded for use in SRI integrity attributes. The resulting attribute value looks like:

integrity="sha256-BpfBw7ivV8q2jLiT13fxDYAe2tJllusRSZ273h2nFSE="

How SHA-256 Works for SRI

The generation process is straightforward:

1. Fetch the complete file content (JavaScript or CSS)
2. Compute the SHA-256 digest of the raw bytes
3. Base64-encode the binary hash
4. Prefix with sha256-

In the browser, the Web Crypto API provides native SHA-256 support:

const data = new TextEncoder().encode(fileContent);
const hashBuffer = await crypto.subtle.digest("SHA-256", data);
const base64 = btoa(String.fromCharCode(...new Uint8Array(hashBuffer)));
const integrity = `sha256-${base64}`;

SHA-256 vs. SHA-384 for SRI

While SHA-256 is cryptographically secure and widely used for file checksums, the SRI specification recommends SHA-384 as the default algorithm. SHA-384 offers a larger digest (384 bits vs. 256 bits), providing a greater margin of security against length-extension attacks — though neither algorithm has been practically broken.

When to Choose SHA-256

• Compatibility: SHA-256 is the most universally supported hash algorithm. Every browser, every tool, and every library supports it.
• Existing workflows: If your build pipeline already generates SHA-256 checksums, reusing them for SRI avoids adding another hash computation step.
• Multiple hash fallback: You can specify multiple hashes — sha256-... and sha384-... — in a single integrity attribute. The browser will use the strongest one it supports.

Performance Considerations

SHA-256 is marginally faster to compute than SHA-384 on 32-bit systems, but the difference is negligible for typical JavaScript and CSS file sizes (usually under 500 KB). On 64-bit systems, SHA-384 is often faster because it is based on SHA-512's internal structure, which uses 64-bit operations.