diff --git a/core/src/trezor/crypto/der.py b/core/src/trezor/crypto/der.py
index bfe8c6a914..f8525507df 100644
--- a/core/src/trezor/crypto/der.py
+++ b/core/src/trezor/crypto/der.py
@@ -1,4 +1,12 @@
-from trezor.utils import BufferReader
+from micropython import const
+
+from trezor.utils import BufferReader, empty_bytearray
+
+if False:
+    from trezor.utils import Writer
+
+# Maximum length of a DER-encoded secp256k1 or secp256p1 signature.
+MAX_DER_SIGNATURE_LENGTH = const(72)
 
 
 def encode_length(l: int) -> bytes:
@@ -12,7 +20,7 @@ def encode_length(l: int) -> bytes:
         raise ValueError
 
 
-def decode_length(r: BufferReader) -> int:
+def read_length(r: BufferReader) -> int:
     init = r.get()
     if init < 0x80:
         # short form encodes length in initial octet
@@ -32,21 +40,27 @@ def decode_length(r: BufferReader) -> int:
     return n
 
 
-def encode_int(i: bytes) -> bytes:
-    i = i.lstrip(b"\x00")
-    if not i:
-        i = b"\00"
+def write_int(w: Writer, number: bytes) -> None:
+    i = 0
+    while i < len(number) and number[i] == 0:
+        i += 1
 
-    if i[0] >= 0x80:
-        i = b"\x00" + i
-    return b"\x02" + encode_length(len(i)) + i
+    length = len(number) - i
+    w.append(0x02)
+    if length == 0 or number[i] >= 0x80:
+        w.extend(encode_length(length + 1))
+        w.append(0x00)
+    else:
+        w.extend(encode_length(length))
+
+    w.extend(memoryview(number)[i:])
 
 
-def decode_int(r: BufferReader) -> memoryview:
+def read_int(r: BufferReader) -> memoryview:
     if r.get() != 0x02:
         raise ValueError
 
-    n = decode_length(r)
+    n = read_length(r)
     if n == 0:
         raise ValueError
 
@@ -66,10 +80,13 @@ def decode_int(r: BufferReader) -> memoryview:
 
 
 def encode_seq(seq: tuple) -> bytes:
-    res = b""
+    # Preallocate space for a signature, which is all that this function ever encodes.
+    buffer = empty_bytearray(MAX_DER_SIGNATURE_LENGTH)
+    buffer.append(0x30)
     for i in seq:
-        res += encode_int(i)
-    return b"\x30" + encode_length(len(res)) + res
+        write_int(buffer, i)
+    buffer[1:1] = encode_length(len(buffer) - 1)
+    return buffer
 
 
 def decode_seq(data: memoryview) -> list[memoryview]:
@@ -77,12 +94,12 @@ def decode_seq(data: memoryview) -> list[memoryview]:
 
     if r.get() != 0x30:
         raise ValueError
-    n = decode_length(r)
+    n = read_length(r)
 
     seq = []
     end = r.offset + n
     while r.offset < end:
-        i = decode_int(r)
+        i = read_int(r)
         seq.append(i)
 
     if r.offset != end or r.remaining_count():