From: Sergey Matveev <stargrave@stargrave.org>
Date: Sun, 30 Nov 2025 08:11:45 +0000 (+0300)
Subject: /kem/*/auth instead of zero-filled /kem/*/from
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;ds=sidebyside;p=keks.git

/kem/*/auth instead of zero-filled /kem/*/from
---

diff --git a/go/cm/cmd/cmenctool/main.go b/go/cm/cmd/cmenctool/main.go
index ebde3ea..f3fadc0 100644
--- a/go/cm/cmd/cmenctool/main.go
+++ b/go/cm/cmd/cmenctool/main.go
@@ -332,8 +332,8 @@ func main() {
 					log.Fatalln("invalid private keys len")
 				}
 				var from *sign.PubData
-				if kem.From != nil {
-					if bytes.Equal(kem.From, bytes.Repeat([]byte{0}, 32)) {
+				if kem.Auth != nil && *kem.Auth {
+					if kem.From == nil {
 						kem.From = assumeFrom
 					}
 					var signed *sign.Signed
@@ -481,8 +481,8 @@ func main() {
 					log.Fatalln("invalid private keys len")
 				}
 				var from *sign.PubData
-				if kem.From != nil {
-					if bytes.Equal(kem.From, bytes.Repeat([]byte{0}, 32)) {
+				if kem.Auth != nil && *kem.Auth {
+					if kem.From == nil {
 						kem.From = assumeFrom
 					}
 					var signed *sign.Signed
@@ -794,9 +794,11 @@ func main() {
 						if err != nil {
 							log.Fatal(err)
 						}
-						if *noFrom {
-							kem.From = bytes.Repeat([]byte{0}, 32)
-						} else {
+						{
+							t := true
+							kem.Auth = &t
+						}
+						if !*noFrom {
 							kem.From = fromId
 						}
 					}
@@ -923,9 +925,11 @@ func main() {
 						if err != nil {
 							log.Fatal(err)
 						}
-						if *noFrom {
-							kem.From = bytes.Repeat([]byte{0}, 32)
-						} else {
+						{
+							t := true
+							kem.Auth = &t
+						}
+						if !*noFrom {
 							kem.From = fromId
 						}
 					}
diff --git a/go/cm/enc/kem.go b/go/cm/enc/kem.go
index 1a42145..ee7d37e 100644
--- a/go/cm/enc/kem.go
+++ b/go/cm/enc/kem.go
@@ -17,6 +17,7 @@ type KEM struct {
 	A    string `keks:"a"`
 	CEK  []byte `keks:"cek"`
 	To   []byte `keks:"to,omitempty"`
+	Auth *bool  `keks:"auth,omitempty"`
 	From []byte `keks:"from,omitempty"`
 
 	// balloon-blake2b-hkdf related
diff --git a/spec/cm/encrypted/authcrypt b/spec/cm/encrypted/authcrypt
index b7d7ceb..e10ff4b 100644
--- a/spec/cm/encrypted/authcrypt
+++ b/spec/cm/encrypted/authcrypt
@@ -1,10 +1,9 @@
-Public-key based [cm/kem/]s provides sender authentication
-*only* if "/kem/*/from" field is specified. It should contain public
-key's "/data/id", but may be equal to 256-bit zeros, to explicitly
-specify that sender's public key is used, but it is anonymous and
-hidden. It is not specified how recipient should find corresponding
-sender's key that way -- implementation/protocol specific.
+Public-key based [cm/kem/]s provides sender authentication *only* if
+"/kem/*/auth" field is set. "/kem/*/from" field may contain public
+key's "/data/id", otherwise sender is hidden. It is not specified
+how recipient should find corresponding sender's key that way --
+implementation/protocol specific.
 
 Optional "/pubs" is a list public keys, which may be used to supply
-sender's public key(s). Public keys may be encrypted, to hide the actual
-deanonymisation contents.
+sender's public key(s). Public keys may be encrypted, to hide the
+actual deanonymisation contents.
diff --git a/tcl/schemas/kem-gost3410-hkdf.tcl b/tcl/schemas/kem-gost3410-hkdf.tcl
index 59a6e3d..91e352c 100644
--- a/tcl/schemas/kem-gost3410-hkdf.tcl
+++ b/tcl/schemas/kem-gost3410-hkdf.tcl
@@ -5,5 +5,6 @@ kem-gost3410-hkdf {
     {field ukm {bin} len=16} {# additional keying material}
     {field pub {bin} >0} {# sender's ephemeral public key}
     {field to {with fpr} optional} {# recipient's public key}
+    {field auth {bool} optional}
     {field from {with fpr} optional} {# sender's public key}
 }
diff --git a/tcl/schemas/kem-with-encap.tcl b/tcl/schemas/kem-with-encap.tcl
index 0db2b68..48c8750 100644
--- a/tcl/schemas/kem-with-encap.tcl
+++ b/tcl/schemas/kem-with-encap.tcl
@@ -5,5 +5,6 @@ kem-with-encap {
     {field cek {bin} >0} {# wrapped CEK}
     {field encap {bin} >0}
     {field to {with fpr} optional} {# recipient's public key}
+    {field auth {bool} optional}
     {field from {with fpr} optional} {# sender's public key}
 }