lib: add `RSAPrivate.Sign`

1 files changed, 47 insertions, 0 deletions

diff --git a/lib/rsa.go b/lib/rsa.go
index 9353824..314e8bb 100644
--- a/lib/rsa.go
+++ b/lib/rsa.go
@@ -5,6 +5,7 @@ package lib
 
 import (
 	"crypto/rand"
+	"errors"
 	"math/big"
 )
 
@@ -156,3 +157,49 @@ func (r *RSAPrivate) Decrypt(cipher []byte) []byte {
 
 	return m.Bytes()
 }
+
+// Sign generates a signature for the give data d according to RFC2313
+// sections 8 and 10.
+//
+// https://datatracker.ietf.org/doc/html/rfc2313
+func (r *RSAPrivate) Sign(msg []byte) (ed []byte, err error) {
+	// Hash message using md4.
+	var (
+		dgst Md4    = Md4{}
+		d    []byte // Data to sign.
+	)
+	dgst.Init([]uint32{})
+	dgst.Message(msg)
+	d = dgst.Hash()
+
+	var (
+		kn int = r.n.BitLen() / 8 // Size of N in bytes.
+		dn int = len(d)           // Length of d.
+		pn int = kn - 3 - dn      // Number of padding characters.
+
+		eb []byte   // Encryption block.
+		x  *big.Int // Encryption block as an integer.
+		y  *big.Int // Signature as an integer.
+	)
+	if dn > kn-11 {
+		err = errors.New("rsa: data len > k-11")
+		return
+	}
+
+	// Build encryption block.
+	eb = []byte{0x00, 0x01}
+	for i := 0; i < pn; i++ {
+		eb = append(eb, 0xFF)
+	}
+	eb = append(eb, 0x00)
+	eb = append(eb, d...)
+
+	// Convert to big int.
+	x = new(big.Int).SetBytes(eb)
+
+	// Sign.
+	y = big.NewInt(0).Exp(x, r.d, r.n)
+
+	ed = y.Bytes()
+	return
+}