cryptutil: move to pkg dir, add token generator (#1029)

* cryptutil: move to pkg dir, add token generator

* add gitignored files

* add tests

pkg/cryptutil/sign.go

@@ -0,0 +1,50 @@
+package cryptutil
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha256"
+	"math/big"
+)
+
+// NewSigningKey generates a random P-256 ECDSA private key.
+// Go's P-256 is constant-time (which prevents certain types of attacks)
+// while its P-384 and P-521 are not.
+func NewSigningKey() (*ecdsa.PrivateKey, error) {
+	return ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+}
+
+// Sign signs arbitrary data using ECDSA.
+func Sign(data []byte, privkey *ecdsa.PrivateKey) ([]byte, error) {
+	// hash message
+	digest := sha256.Sum256(data)
+
+	// sign the hash
+	r, s, err := ecdsa.Sign(rand.Reader, privkey, digest[:])
+	if err != nil {
+		return nil, err
+	}
+
+	// encode the signature {R, S}
+	// big.Int.Bytes() will need padding in the case of leading zero bytes
+	params := privkey.Curve.Params()
+	curveOrderByteSize := params.P.BitLen() / 8
+	rBytes, sBytes := r.Bytes(), s.Bytes()
+	signature := make([]byte, curveOrderByteSize*2)
+	copy(signature[curveOrderByteSize-len(rBytes):], rBytes)
+	copy(signature[curveOrderByteSize*2-len(sBytes):], sBytes)
+	return signature, nil
+}
+
+// Verify checks a raw ECDSA signature.
+// Returns true if it's valid and false if not.
+func Verify(data, signature []byte, pubkey *ecdsa.PublicKey) bool {
+	// hash message
+	digest := sha256.Sum256(data)
+	curveOrderByteSize := pubkey.Curve.Params().P.BitLen() / 8
+	r, s := new(big.Int), new(big.Int)
+	r.SetBytes(signature[:curveOrderByteSize])
+	s.SetBytes(signature[curveOrderByteSize:])
+	return ecdsa.Verify(pubkey, digest[:], r, s)
+}