golang.org/x/crypto v0.27.0: hkdf.Expand

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify golang.org/x/crypto/hkdf.Expand in pkg:golang/golang.org/x/crypto@v0.27.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/crypto@v0.27.0
Demonstrate these symbols/APIs:
  - golang.org/x/crypto/hkdf.Expand

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:05e6a98004fb4c7bd80dab47497f72e59c1faf35d6f748e05adacee7d940d80b","contract":["hkdf.Expand expands pseudorandom key into key stream matching RFC 5869 test vector","hkdf.Expand with distinct info produces distinct key streams","hkdf.Expand returns error when reading beyond 255 times hash size","hkdf.Expand supports sequential reads matching single full read"],"goal":"verify golang.org/x/crypto/hkdf.Expand in pkg:golang/golang.org/x/crypto@v0.27.0","kind":"HOW","packages":["pkg:golang/golang.org/x/crypto@v0.27.0"],"schemaVersion":1,"symbols":["golang.org/x/crypto/hkdf.Expand"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/golang.org/x/crypto@v0.27.0"],"schemaVersion":1,"subject":"pkg:golang/golang.org/x/crypto@v0.27.0","symbols":["golang.org/x/crypto/hkdf.Expand"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.22

require golang.org/x/crypto v0.27.0

golang.org/x/crypto v0.27.0 h1:GXm2NjJrPaiv/h1tb2UH8QfgC/hOf/+z0p6PT8o1w7A=
golang.org/x/crypto v0.27.0/go.mod h1:1Xngt8kV6Dvbssa53Ziq6Eqn0HqbZi5Z6R0ZpwQzt70=

package main

import (
	"crypto/sha256"
	"fmt"
	"io"
	"log"

	"golang.org/x/crypto/hkdf"
)

func main() {
	// Pseudorandom key (PRK) typically obtained from an extraction step or key exchange
	prk := []byte{
		0x07, 0x77, 0x09, 0x36, 0x2c, 0x2e, 0x32, 0xdf,
		0x0d, 0xdc, 0x3f, 0x0d, 0xc4, 0x7b, 0xba, 0x63,
		0x90, 0xb6, 0xc7, 0x3b, 0xb5, 0x0f, 0x9c, 0x31,
		0x22, 0xec, 0x84, 0x4a, 0xd7, 0xc2, 0xb3, 0xe5,
	}
	info := []byte("example.com/sample/v1/aes-key")

	// Expand the PRK into application-specific output keying material (OKM)
	expandReader := hkdf.Expand(sha256.New, prk, info)
	derivedKey := make([]byte, 32) // 256-bit AES key
	if _, err := io.ReadFull(expandReader, derivedKey); err != nil {
		log.Fatalf("Failed to expand key: %v", err)
	}

	fmt.Printf("Derived key: %x\n", derivedKey)
}

{
  "schemaVersion": 1,
  "goal": "verify golang.org/x/crypto/hkdf.Expand in pkg:golang/golang.org/x/crypto@v0.27.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/crypto@v0.27.0"
  ],
  "symbols": [
    "golang.org/x/crypto/hkdf.Expand"
  ]
}

package main

import (
	"bytes"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"io"
	"os"

	"golang.org/x/crypto/hkdf"
)

func main() {
	// RFC 5869 Test Case 1 (SHA-256)
	prk1, _ := hex.DecodeString("077709362c2e32df0ddc3f0dc47bba6390b6c73bb50f9c3122ec844ad7c2b3e5")
	info1, _ := hex.DecodeString("f0f1f2f3f4f5f6f7f8f9")
	expectedOKM1, _ := hex.DecodeString("3cb25f25faacd57a90434f64d0362f2a2d2d0a90cf1a5a4c5db02d56ecc4c5bf34007208d5b887185865")

	// 1. hkdf.Expand expands pseudorandom key into key stream matching RFC 5869 test vector
	expandReader1 := hkdf.Expand(sha256.New, prk1, info1)
	okm1 := make([]byte, len(expectedOKM1))
	if _, err := io.ReadFull(expandReader1, okm1); err != nil {
		fmt.Fprintf(os.Stderr, "hkdf.Expand read failed: %v\n", err)
		os.Exit(1)
	}
	if !bytes.Equal(okm1, expectedOKM1) {
		fmt.Fprintf(os.Stderr, "hkdf.Expand OKM mismatch: got %x, want %x\n", okm1, expectedOKM1)
		os.Exit(1)
	}

	// 2. hkdf.Expand with distinct info produces distinct key streams
	expandReader2A := hkdf.Expand(sha256.New, prk1, info1)
	expandReader2B := hkdf.Expand(sha256.New, prk1, []byte("different info parameter"))
	bufA := make([]byte, 32)
	bufB := make([]byte, 32)
	if _, err := io.ReadFull(expandReader2A, bufA); err != nil {
		fmt.Fprintf(os.Stderr, "expandReader2A read failed: %v\n", err)
		os.Exit(1)
	}
	if _, err := io.ReadFull(expandReader2B, bufB); err != nil {
		fmt.Fprintf(os.Stderr, "expandReader2B read failed: %v\n", err)
		os.Exit(1)
	}
	if bytes.Equal(bufA, bufB) {
		fmt.Fprintf(os.Stderr, "hkdf.Expand produced identical output for different info\n")
		os.Exit(1)
	}

	// 3. hkdf.Expand returns error when reading beyond 255 times hash size
	maxBytes := 255 * sha256.Size // 8160 bytes for SHA-256
	limitReader := hkdf.Expand(sha256.New, prk1, info1)
	maxBuf := make([]byte, maxBytes)
	if _, err := io.ReadFull(limitReader, maxBuf); err != nil {
		fmt.Fprintf(os.Stderr, "Failed to read max allowed HKDF bytes (%d): %v\n", maxBytes, err)
		os.Exit(1)
	}
	extraByte := make([]byte, 1)
	if _, err := limitReader.Read(extraByte); err == nil {
		fmt.Fprintf(os.Stderr, "hkdf.Expand unexpectedly allowed reading beyond 255 * hash.Size\n")
		os.Exit(1)
	}

	// 4. hkdf.Expand supports sequential reads matching single full read
	seqReader := hkdf.Expand(sha256.New, prk1, info1)
	part1 := make([]byte, 10)
	part2 := make([]byte, 16)
	part3 := make([]byte, len(expectedOKM1)-26)
	if _, err := io.ReadFull(seqReader, part1); err != nil {
		fmt.Fprintf(os.Stderr, "seqReader part1 failed: %v\n", err)
		os.Exit(1)
	}
	if _, err := io.ReadFull(seqReader, part2); err != nil {
		fmt.Fprintf(os.Stderr, "seqReader part2 failed: %v\n", err)
		os.Exit(1)
	}
	if _, err := io.ReadFull(seqReader, part3); err != nil {
		fmt.Fprintf(os.Stderr, "seqReader part3 failed: %v\n", err)
		os.Exit(1)
	}
	combined := append(append(part1, part2...), part3...)
	if !bytes.Equal(combined, expectedOKM1) {
		fmt.Fprintf(os.Stderr, "Sequential reads mismatch with single read: got %x, want %x\n", combined, expectedOKM1)
		os.Exit(1)
	}

	fmt.Println("All contract assertions passed.")
}