4 files changed, 193 insertions, 0 deletions
diff --git a/plumbing/hash/hash.go b/plumbing/hash/hash.go
new file mode 100644
index 0000000..82d1856
--- /dev/null
+++ b/plumbing/hash/hash.go
@@ -0,0 +1,60 @@
+// package hash provides a way for managing the
+// underlying hash implementations used across go-git.
+package hash
+
+import (
+	"crypto"
+	"fmt"
+	"hash"
+
+	"github.com/pjbgf/sha1cd"
+)
+
+// algos is a map of hash algorithms.
+var algos = map[crypto.Hash]func() hash.Hash{}
+
+func init() {
+	reset()
+}
+
+// reset resets the default algos value. Can be used after running tests
+// that registers new algorithms to avoid side effects.
+func reset() {
+	algos[crypto.SHA1] = sha1cd.New
+	algos[crypto.SHA256] = crypto.SHA256.New
+}
+
+// RegisterHash allows for the hash algorithm used to be overriden.
+// This ensures the hash selection for go-git must be explicit, when
+// overriding the default value.
+func RegisterHash(h crypto.Hash, f func() hash.Hash) error {
+	if f == nil {
+		return fmt.Errorf("cannot register hash: f is nil")
+	}
+
+	switch h {
+	case crypto.SHA1:
+		algos[h] = f
+	case crypto.SHA256:
+		algos[h] = f
+	default:
+		return fmt.Errorf("unsupported hash function: %v", h)
+	}
+	return nil
+}
+
+// Hash is the same as hash.Hash. This allows consumers
+// to not having to import this package alongside "hash".
+type Hash interface {
+	hash.Hash
+}
+
+// New returns a new Hash for the given hash function.
+// It panics if the hash function is not registered.
+func New(h crypto.Hash) Hash {
+	hh, ok := algos[h]
+	if !ok {
+		panic(fmt.Sprintf("hash algorithm not registered: %v", h))
+	}
+	return hh()
+}
diff --git a/plumbing/hash/hash_sha1.go b/plumbing/hash/hash_sha1.go
new file mode 100644
index 0000000..e3cb60f
--- /dev/null
+++ b/plumbing/hash/hash_sha1.go
@@ -0,0 +1,15 @@
+//go:build !sha256
+// +build !sha256
+
+package hash
+
+import "crypto"
+
+const (
+	// CryptoType defines what hash algorithm is being used.
+	CryptoType = crypto.SHA1
+	// Size defines the amount of bytes the hash yields.
+	Size = 20
+	// HexSize defines the strings size of the hash when represented in hexadecimal.
+	HexSize = 40
+)
diff --git a/plumbing/hash/hash_sha256.go b/plumbing/hash/hash_sha256.go
new file mode 100644
index 0000000..1c52b89
--- /dev/null
+++ b/plumbing/hash/hash_sha256.go
@@ -0,0 +1,15 @@
+//go:build sha256
+// +build sha256
+
+package hash
+
+import "crypto"
+
+const (
+	// CryptoType defines what hash algorithm is being used.
+	CryptoType = crypto.SHA256
+	// Size defines the amount of bytes the hash yields.
+	Size = 32
+	// HexSize defines the strings size of the hash when represented in hexadecimal.
+	HexSize = 64
+)
diff --git a/plumbing/hash/hash_test.go b/plumbing/hash/hash_test.go
new file mode 100644
index 0000000..f70ad11
--- /dev/null
+++ b/plumbing/hash/hash_test.go
@@ -0,0 +1,103 @@
+package hash
+
+import (
+	"crypto"
+	"crypto/sha1"
+	"crypto/sha512"
+	"encoding/hex"
+	"hash"
+	"strings"
+	"testing"
+)
+
+func TestRegisterHash(t *testing.T) {
+	// Reset default hash to avoid side effects.
+	defer reset()
+
+	tests := []struct {
+		name    string
+		hash    crypto.Hash
+		new     func() hash.Hash
+		wantErr string
+	}{
+		{
+			name: "sha1",
+			hash: crypto.SHA1,
+			new:  sha1.New,
+		},
+		{
+			name:    "sha1",
+			hash:    crypto.SHA1,
+			wantErr: "cannot register hash: f is nil",
+		},
+		{
+			name:    "sha512",
+			hash:    crypto.SHA512,
+			new:     sha512.New,
+			wantErr: "unsupported hash function",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := RegisterHash(tt.hash, tt.new)
+			if tt.wantErr == "" && err != nil {
+				t.Errorf("unexpected error: %v", err)
+			} else if tt.wantErr != "" && err == nil {
+				t.Errorf("expected error: %v got: nil", tt.wantErr)
+			} else if err != nil && !strings.Contains(err.Error(), tt.wantErr) {
+				t.Errorf("expected error: %v got: %v", tt.wantErr, err)
+			}
+		})
+	}
+}
+
+// Verifies that the SHA1 implementation used is collision-resistant
+// by default.
+func TestSha1Collision(t *testing.T) {
+	defer reset()
+
+	tests := []struct {
+		name    string
+		content string
+		hash    string
+		before  func()
+	}{
+		{
+			name:    "sha-mbles-1: with collision detection",
+			content: "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",
+			hash:    "4f3d9be4a472c4dae83c6314aa6c36a064c1fd14",
+		},
+		{
+			name:    "sha-mbles-1: with default SHA1",
+			content: "99040d047fe81780012000ff4b65792069732070617274206f66206120636f6c6c6973696f6e212049742773206120747261702179c61af0afcc054515d9274e7307624b1dc7fb23988bb8de8b575dba7b9eab31c1674b6d974378a827732ff5851c76a2e60772b5a47ce1eac40bb993c12d8c70e24a4f8d5fcdedc1b32c9cf19e31af2429759d42e4dfdb31719f587623ee552939b6dcdc459fca53553b70f87ede30a247ea3af6c759a2f20b320d760db64ff479084fd3ccb3cdd48362d96a9c430617caff6c36c637e53fde28417f626fec54ed7943a46e5f5730f2bb38fb1df6e0090010d00e24ad78bf92641993608e8d158a789f34c46fe1e6027f35a4cbfb827076c50eca0e8b7cca69bb2c2b790259f9bf9570dd8d4437a3115faff7c3cac09ad25266055c27104755178eaeff825a2caa2acfb5de64ce7641dc59a541a9fc9c756756e2e23dc713c8c24c9790aa6b0e38a7f55f14452a1ca2850ddd9562fd9a18ad42496aa97008f74672f68ef461eb88b09933d626b4f918749cc027fddd6c425fc4216835d0134d15285bab2cb784a4f7cbb4fb514d4bf0f6237cf00a9e9f132b9a066e6fd17f6c42987478586ff651af96747fb426b9872b9a88e4063f59bb334cc00650f83a80c42751b71974d300fc2819a2e8f1e32c1b51cb18e6bfc4db9baef675d4aaf5b1574a047f8f6dd2ec153a93412293974d928f88ced9363cfef97ce2e742bf34c96b8ef3875676fea5cca8e5f7dea0bab2413d4de00ee71ee01f162bdb6d1eafd925e6aebaae6a354ef17cf205a404fbdb12fc454d41fdd95cf2459664a2ad032d1da60a73264075d7f1e0d6c1403ae7a0d861df3fe5707188dd5e07d1589b9f8b6630553f8fc352b3e0c27da80bddba4c64020d",
+			hash:    "8ac60ba76f1999a1ab70223f225aefdc78d4ddc0",
+			before: func() {
+				RegisterHash(crypto.SHA1, sha1.New)
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.before != nil {
+				tt.before()
+			}
+
+			h := New(crypto.SHA1)
+			data, err := hex.DecodeString(tt.content)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			h.Reset()
+			h.Write(data)
+			sum := h.Sum(nil)
+			got := hex.EncodeToString(sum)
+
+			if tt.hash != got {
+				t.Errorf("\n   got: %q\nwanted: %q", got, tt.hash)
+			}
+		})
+	}
+}