diff --git a/vlib/crypto/cipher/aes_ctr_test.v b/vlib/crypto/cipher/aes_ctr_test.v
index 31847c0265..e8a0068cfb 100644
--- a/vlib/crypto/cipher/aes_ctr_test.v
+++ b/vlib/crypto/cipher/aes_ctr_test.v
@@ -18,12 +18,12 @@ fn test_aes_ctr() {
 
 fn aes_ctr_en(mut src []u8, key []u8, iv []u8) {
 	block := aes.new_cipher(key)
-	mode := cipher.new_ctr(block, iv)
+	mut mode := cipher.new_ctr(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
 
 fn aes_ctr_de(mut src []u8, key []u8, iv []u8) {
 	block := aes.new_cipher(key)
-	mode := cipher.new_ctr(block, iv)
+	mut mode := cipher.new_ctr(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
diff --git a/vlib/crypto/cipher/cfb.v b/vlib/crypto/cipher/cfb.v
index 0dc0ffc420..b677f5c977 100644
--- a/vlib/crypto/cipher/cfb.v
+++ b/vlib/crypto/cipher/cfb.v
@@ -63,34 +63,34 @@ fn new_cfb(b Block, iv []u8, decrypt bool) Cfb {
 	return x
 }
 
-pub fn (mut x Cfb) xor_key_stream(mut dst_ []u8, src_ []u8) {
+pub fn (mut x Cfb) xor_key_stream(mut dst []u8, src []u8) {
 	unsafe {
-		mut dst := *dst_
-		mut src := src_
-		if dst.len < src.len {
+		mut local_dst := *dst
+		mut local_src := src
+		if local_dst.len < local_src.len {
 			panic('crypto.cipher.xor_key_stream: output smaller than input')
 		}
 
-		if subtle.inexact_overlap(dst[..src.len], src) {
+		if subtle.inexact_overlap(local_dst[..src.len], local_src) {
 			panic('crypto.cipher.xor_key_stream: invalid buffer overlap')
 		}
 
-		for src.len > 0 {
+		for local_src.len > 0 {
 			if x.out_used == x.out.len {
 				x.b.encrypt(mut x.out, x.next)
 				x.out_used = 0
 			}
 
 			if x.decrypt {
-				copy(mut x.next[x.out_used..], src)
+				copy(mut x.next[x.out_used..], local_src)
 			}
 
-			n := xor_bytes(mut dst, src, x.out[x.out_used..])
+			n := xor_bytes(mut local_dst, local_src, x.out[x.out_used..])
 			if !x.decrypt {
-				copy(mut x.next[x.out_used..], dst)
+				copy(mut x.next[x.out_used..], local_dst)
 			}
-			dst = dst[n..]
-			src = src[n..]
+			local_dst = local_dst[n..]
+			local_src = local_src[n..]
 			x.out_used += n
 		}
 	}
diff --git a/vlib/crypto/cipher/cfb_test.v b/vlib/crypto/cipher/cfb_test.v
new file mode 100644
index 0000000000..c896bbfbe3
--- /dev/null
+++ b/vlib/crypto/cipher/cfb_test.v
@@ -0,0 +1,18 @@
+import crypto.cipher
+import crypto.des
+
+struct StreamCipher {
+	cipher cipher.Stream
+}
+
+fn test_cfb_stream_cipher() ! {
+	key := '123456789012345678901234'.bytes()
+	iv := 'abcdegfh'.bytes()
+
+	block := des.new_cipher(key[..8])
+	c := cipher.new_cfb_encrypter(block, iv)
+
+	s := StreamCipher{
+		cipher: c
+	}
+}
diff --git a/vlib/crypto/cipher/cipher.v b/vlib/crypto/cipher/cipher.v
index a364b565e0..086b8f88f5 100644
--- a/vlib/crypto/cipher/cipher.v
+++ b/vlib/crypto/cipher/cipher.v
@@ -16,6 +16,7 @@ pub interface Block {
 
 // A Stream represents a stream cipher.
 pub interface Stream {
+mut:
 	// xor_key_stream XORs each byte in the given slice with a byte from the
 	// cipher's key stream. Dst and src must overlap entirely or not at all.
 	//
diff --git a/vlib/crypto/cipher/ctr.v b/vlib/crypto/cipher/ctr.v
index 48deef0c82..db8b2c4515 100644
--- a/vlib/crypto/cipher/ctr.v
+++ b/vlib/crypto/cipher/ctr.v
@@ -49,25 +49,25 @@ pub fn new_ctr(b Block, iv []u8) Ctr {
 	}
 }
 
-pub fn (x &Ctr) xor_key_stream(mut dst_ []u8, src_ []u8) {
+pub fn (mut x Ctr) xor_key_stream(mut dst []u8, src []u8) {
 	unsafe {
-		mut dst := *dst_
-		mut src := src_
-		if dst.len < src.len {
+		mut local_dst := *dst
+		mut local_src := src
+		if local_dst.len < local_src.len {
 			panic('crypto.cipher.xor_key_stream: output smaller than input')
 		}
 
-		if subtle.inexact_overlap(dst[..src.len], src) {
+		if subtle.inexact_overlap(local_dst[..local_src.len], local_src) {
 			panic('crypto.cipher.xor_key_stream: invalid buffer overlap')
 		}
 
-		for src.len > 0 {
+		for local_src.len > 0 {
 			if x.out_used == x.out.len {
 				x.b.encrypt(mut x.out, x.next)
 				x.out_used = 0
 			}
 
-			n := xor_bytes(mut dst, src, x.out[x.out_used..])
+			n := xor_bytes(mut local_dst, local_src, x.out[x.out_used..])
 
 			// increment counter
 			for i := x.next.len - 1; i >= 0; i-- {
@@ -77,8 +77,8 @@ pub fn (x &Ctr) xor_key_stream(mut dst_ []u8, src_ []u8) {
 				}
 			}
 
-			dst = dst[n..]
-			src = src[n..]
+			local_dst = local_dst[n..]
+			local_src = local_src[n..]
 			x.out_used += n
 		}
 	}
diff --git a/vlib/crypto/cipher/ctr_test.v b/vlib/crypto/cipher/ctr_test.v
new file mode 100644
index 0000000000..b1327ba55b
--- /dev/null
+++ b/vlib/crypto/cipher/ctr_test.v
@@ -0,0 +1,18 @@
+import crypto.cipher
+import crypto.des
+
+struct StreamCipher {
+	cipher cipher.Stream
+}
+
+fn test_ctr_stream_cipher() ! {
+	key := '123456789012345678901234'.bytes()
+	iv := 'abcdegfh'.bytes()
+
+	block := des.new_cipher(key[..8])
+	c := cipher.new_ctr(block, iv)
+
+	s := StreamCipher{
+		cipher: c
+	}
+}
diff --git a/vlib/crypto/cipher/des_ctr_test.v b/vlib/crypto/cipher/des_ctr_test.v
index 923666a73b..73b083fb33 100644
--- a/vlib/crypto/cipher/des_ctr_test.v
+++ b/vlib/crypto/cipher/des_ctr_test.v
@@ -29,24 +29,24 @@ fn test_des_ctr() {
 
 fn des_ctr_en(mut src []u8, key []u8, iv []u8) {
 	block := des.new_cipher(key)
-	mode := cipher.new_ctr(block, iv)
+	mut mode := cipher.new_ctr(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
 
 fn des_ctr_de(mut src []u8, key []u8, iv []u8) {
 	block := des.new_cipher(key)
-	mode := cipher.new_ctr(block, iv)
+	mut mode := cipher.new_ctr(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
 
 fn triple_des_ctr_en(mut src []u8, key []u8, iv []u8) {
 	block := des.new_triple_des_cipher(key)
-	mode := cipher.new_ctr(block, iv)
+	mut mode := cipher.new_ctr(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
 
 fn triple_des_ctr_de(mut src []u8, key []u8, iv []u8) {
 	block := des.new_triple_des_cipher(key)
-	mode := cipher.new_ctr(block, iv)
+	mut mode := cipher.new_ctr(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
diff --git a/vlib/crypto/cipher/ofb.v b/vlib/crypto/cipher/ofb.v
index dbfc3bcad8..4c21f94341 100644
--- a/vlib/crypto/cipher/ofb.v
+++ b/vlib/crypto/cipher/ofb.v
@@ -35,19 +35,19 @@ pub fn new_ofb(b Block, iv []u8) Ofb {
 	return x
 }
 
-pub fn (mut x Ofb) xor_key_stream(mut dst_ []u8, src_ []u8) {
+pub fn (mut x Ofb) xor_key_stream(mut dst []u8, src []u8) {
 	unsafe {
-		mut dst := *dst_
-		mut src := src_
-		if dst.len < src.len {
+		mut local_dst := *dst
+		mut local_src := src
+		if local_dst.len < local_src.len {
 			panic('crypto.cipher.xor_key_stream: output smaller than input')
 		}
 
-		if subtle.inexact_overlap(dst[..src.len], src) {
+		if subtle.inexact_overlap(local_dst[..local_src.len], local_src) {
 			panic('crypto.cipher.xor_key_stream: invalid buffer overlap')
 		}
 
-		for src.len > 0 {
+		for local_src.len > 0 {
 			if x.out_used == x.out.len {
 				x.b.encrypt(mut x.out, x.next)
 				x.out_used = 0
@@ -55,9 +55,9 @@ pub fn (mut x Ofb) xor_key_stream(mut dst_ []u8, src_ []u8) {
 
 			copy(mut x.next, x.out)
 
-			n := xor_bytes(mut dst, src, x.out)
-			dst = dst[n..]
-			src = src[n..]
+			n := xor_bytes(mut local_dst, local_src, x.out)
+			local_dst = local_dst[n..]
+			local_src = local_src[n..]
 			x.out_used += n
 		}
 	}
diff --git a/vlib/crypto/cipher/ofb_test.v b/vlib/crypto/cipher/ofb_test.v
new file mode 100644
index 0000000000..99a2af9c97
--- /dev/null
+++ b/vlib/crypto/cipher/ofb_test.v
@@ -0,0 +1,18 @@
+import crypto.cipher
+import crypto.des
+
+struct StreamCipher {
+	cipher cipher.Stream
+}
+
+fn test_ctr_stream_cipher() ! {
+	key := '123456789012345678901234'.bytes()
+	iv := 'abcdegfh'.bytes()
+
+	block := des.new_cipher(key[..8])
+	c := cipher.new_ofb(block, iv)
+
+	s := StreamCipher{
+		cipher: c
+	}
+}
diff --git a/vlib/crypto/rc4/rc4.v b/vlib/crypto/rc4/rc4.v
index 7046f0ad8c..91240f466d 100644
--- a/vlib/crypto/rc4/rc4.v
+++ b/vlib/crypto/rc4/rc4.v
@@ -65,7 +65,7 @@ pub fn (mut c Cipher) reset() {
 
 // xor_key_stream sets dst to the result of XORing src with the key stream.
 // Dst and src must overlap entirely or not at all.
-pub fn (mut c Cipher) xor_key_stream(mut dst []u8, mut src []u8) {
+pub fn (mut c Cipher) xor_key_stream(mut dst []u8, src []u8) {
 	if src.len == 0 {
 		return
 	}
diff --git a/vlib/crypto/rc4/rc4_test.v b/vlib/crypto/rc4/rc4_test.v
index 3e69a1ed5e..15dc20cb27 100644
--- a/vlib/crypto/rc4/rc4_test.v
+++ b/vlib/crypto/rc4/rc4_test.v
@@ -1,8 +1,22 @@
 // Copyright (c) 2019-2024 Alexander Medvednikov. All rights reserved.
 // Use of this source code is governed by an MIT license
 // that can be found in the LICENSE file.
+import crypto.cipher
 import crypto.rc4
 
+struct StreamCipher {
+	cipher cipher.Stream
+}
+
+fn test_cfb_stream_cipher() ! {
+	key := 'tthisisourrc4key'.bytes()
+	c := rc4.new_cipher(key)!
+
+	s := StreamCipher{
+		cipher: c
+	}
+}
+
 fn test_crypto_rc4() {
 	key := 'tthisisourrc4key'.bytes()
 
@@ -14,7 +28,7 @@ fn test_crypto_rc4() {
 	mut src := 'toencrypt'.bytes()
 
 	// src & dst same, encrypt in place
-	c.xor_key_stream(mut src, mut src) // encrypt data
+	c.xor_key_stream(mut src, src) // encrypt data
 
 	c.reset()
 
diff --git a/vlib/x/crypto/chacha20/chacha_test.v b/vlib/x/crypto/chacha20/chacha_test.v
index 6b464605ab..f74a2edc83 100644
--- a/vlib/x/crypto/chacha20/chacha_test.v
+++ b/vlib/x/crypto/chacha20/chacha_test.v
@@ -1,8 +1,27 @@
 module chacha20
 
+import crypto.cipher
 import rand
 import encoding.hex
 
+struct StreamCipher {
+mut:
+	cipher &cipher.Stream
+}
+
+// Verify chahca20.Cipher implements chiper.Stream correctly.
+fn test_chacha20_stream_cipher() ! {
+	mut key := []u8{len: 32}
+	mut nonce := []u8{len: 12}
+	rand.read(mut key)
+	rand.read(mut nonce)
+
+	mut c := new_cipher(key, nonce)!
+	s := StreamCipher{
+		cipher: c
+	}
+}
+
 fn test_chacha20_cipher_reset() ! {
 	mut key := []u8{len: 32}
 	mut nonce := []u8{len: 12}