From 43f73e62907ab1e6b70f0f087962248c2973e8fa Mon Sep 17 00:00:00 2001
From: lehugueni <huguenindl@gmail.com>
Date: Mon, 11 Nov 2024 12:14:43 +0100
Subject: [PATCH] enforce seed to be nil or 32 bytes

---
 core/rlwe/keygenerator.go |  7 ++++---
 core/rlwe/keys.go         | 36 ++++++++++++++++++++++--------------
 2 files changed, 26 insertions(+), 17 deletions(-)

diff --git a/core/rlwe/keygenerator.go b/core/rlwe/keygenerator.go
index 9fc49c02..0450f87d 100644
--- a/core/rlwe/keygenerator.go
+++ b/core/rlwe/keygenerator.go
@@ -279,12 +279,13 @@ func (kgen KeyGenerator) genEvaluationKey(skIn ring.Poly, skOut ringqp.Poly, evk
 	// For a compressed evaluation key, a seed is created and stored in the EvaluationKey struct
 	// struct while an uncompressed key uses an ephemeral seed.
 	if evk.IsCompressed() {
-		evk.Seed = make([]byte, 32)
-		if n, err := kgen.prng.Read(evk.Seed); n != 32 || err != nil {
+		var seed [32]byte
+		if n, err := kgen.prng.Read(seed[:]); n != 32 || err != nil {
 			panic(fmt.Errorf("unable to sample evaluation key seed"))
 		}
+		evk.Seed = &seed
 
-		sampler, err := sampling.NewKeyedPRNG(evk.Seed)
+		sampler, err := sampling.NewKeyedPRNG(seed[:])
 		if err != nil {
 			panic(fmt.Errorf("sampling.NewKeyedPRNG: %w", err))
 		}
diff --git a/core/rlwe/keys.go b/core/rlwe/keys.go
index e0d10495..ac1f2c6f 100644
--- a/core/rlwe/keys.go
+++ b/core/rlwe/keys.go
@@ -292,7 +292,7 @@ func (p *PublicKey) isEncryptionKey() {}
 //     is used to bring it back to its original key.
 type EvaluationKey struct {
 	GadgetCiphertext
-	Seed []byte
+	Seed *[32]byte // Must be != nil iff EvaluationKey.IsCompressed() = true
 }
 
 type EvaluationKeyParameters struct {
@@ -362,7 +362,11 @@ func (evk EvaluationKey) Expand(params ParameterProvider, buffer *GadgetCipherte
 		return fmt.Errorf("evaluation key is not compressed")
 	}
 
-	prng, err := sampling.NewKeyedPRNG(evk.Seed)
+	if evk.Seed == nil {
+		return fmt.Errorf("seed is missing")
+	}
+
+	prng, err := sampling.NewKeyedPRNG((*evk.Seed)[:])
 	if err != nil {
 		panic(fmt.Errorf("sampling.NewKeyedPRNG: %s", err))
 	}
@@ -419,7 +423,10 @@ func (evk EvaluationKey) Expand(params ParameterProvider, buffer *GadgetCipherte
 
 // BinarySize returns the serialized size of the object in bytes.
 func (evk EvaluationKey) BinarySize() (size int) {
-	return evk.GadgetCiphertext.BinarySize() + len(evk.Seed)
+	if evk.Seed != nil {
+		return evk.GadgetCiphertext.BinarySize() + len(*evk.Seed)
+	}
+	return evk.GadgetCiphertext.BinarySize()
 }
 
 // WriteTo writes the object on an [io.Writer]. It implements the [io.WriterTo]
@@ -446,13 +453,14 @@ func (evk EvaluationKey) WriteTo(w io.Writer) (n int64, err error) {
 		n += inc
 
 		if evk.IsCompressed() {
-			if inc, err = buffer.Write(w, evk.Seed); err != nil {
-				return n + inc, err
+
+			// Sanity check, should not happen unless evk has been manually modified
+			if evk.Seed == nil {
+				return n + inc, fmt.Errorf("writing compressed evaluation key: the seed is nil")
 			}
 
-			// Sanity check, should not happen unless the size of the seed has been modified in the code
-			if inc != 32 {
-				return n + inc, fmt.Errorf("writing compressed evaluation key: the size of the seed=%d != 32", inc)
+			if inc, err = buffer.Write(w, (*evk.Seed)[:]); err != nil {
+				return n + inc, err
 			}
 
 			n += inc
@@ -484,7 +492,6 @@ func (evk *EvaluationKey) ReadFrom(r io.Reader) (n int64, err error) {
 	case buffer.Reader:
 
 		var inc int64
-		var incInt int
 
 		if inc, err = evk.GadgetCiphertext.ReadFrom(r); err != nil {
 			return n + inc, err
@@ -493,13 +500,14 @@ func (evk *EvaluationKey) ReadFrom(r io.Reader) (n int64, err error) {
 		n += inc
 
 		if evk.IsCompressed() {
-			seed := make([]byte, 32)
-			if incInt, err = r.Read(seed); err != nil {
-				return n + int64(incInt), err
+			var seed [32]byte
+			if inc, err = buffer.Read(r, seed[:]); err != nil {
+				return n + inc, err
 			}
-			evk.Seed = seed
 
-			n += int64(incInt)
+			evk.Seed = &seed
+
+			n += inc
 		}
 
 		return