From d8206b8bc40ea114c7eeb54879f9797d4dec13eb Mon Sep 17 00:00:00 2001
From: deadcafe <deadcaf3@pm.me>
Date: Thu, 22 Oct 2026 00:04:00 +0530
Subject: [PATCH] Fix TPM2_ASN_TrimZeros skipping bytes

The loop advanced both idx and the in pointer, so each check after the
first read two bytes past the intended one. An ECDSA R or S starting
00 <0x80 00 <0x80 lost a significant byte, and wolfTPM2_PK_EccSign then
DER-encoded a wrong signature that the TLS peer rejected (-330).

Only advance idx and return in + idx. Add a unit test using the S bytes
from the failing CI run.
---
 src/tpm2_asn.c     |  3 +--
 tests/unit_tests.c | 31 +++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/src/tpm2_asn.c b/src/tpm2_asn.c
index dff4df3d1..7b1053e60 100644
--- a/src/tpm2_asn.c
+++ b/src/tpm2_asn.c
@@ -34,10 +34,9 @@ byte* TPM2_ASN_TrimZeros(byte* in, word32* len)
     word32 idx = 0;
     while (idx+1 < *len && in[idx] == 0 && (in[idx+1] & 0x80) == 0) {
         idx++;
-        in++;
     }
     *len -= idx;
-    return in;
+    return in + idx;
 }
 
 int TPM2_ASN_GetLength_ex(const uint8_t* input, word32* inOutIdx, int* len,
diff --git a/tests/unit_tests.c b/tests/unit_tests.c
index 9347c5d0e..e15a18ca2 100644
--- a/tests/unit_tests.c
+++ b/tests/unit_tests.c
@@ -8840,6 +8840,36 @@ static void test_TPM2_ASN_RsaUnpadPkcsv15(void)
 #endif
 }
 
+static void test_TPM2_ASN_TrimZeros(void)
+{
+#if !defined(WOLFTPM2_NO_WRAPPER) && !defined(WOLFTPM2_NO_ASN)
+    /* ECDSA S from a failed CI run: only the first zero may go */
+    byte s[] = { 0x00, 0x71, 0x00, 0x4F, 0x95 };
+    byte hi[] = { 0x00, 0x80, 0x01 };
+    byte two[] = { 0x00, 0x00, 0x05 };
+    byte* p;
+    word32 len;
+
+    len = (word32)sizeof(s);
+    p = TPM2_ASN_TrimZeros(s, &len);
+    AssertIntEQ(len, 4);
+    AssertTrue(p == &s[1]);
+
+    /* A zero before a high bit byte is kept */
+    len = (word32)sizeof(hi);
+    p = TPM2_ASN_TrimZeros(hi, &len);
+    AssertIntEQ(len, 3);
+    AssertTrue(p == &hi[0]);
+
+    len = (word32)sizeof(two);
+    p = TPM2_ASN_TrimZeros(two, &len);
+    AssertIntEQ(len, 1);
+    AssertTrue(p == &two[2]);
+
+    printf("Test TPM Wrapper: %-40s Passed\n", "ASN TrimZeros:");
+#endif
+}
+
 #if !defined(WOLFTPM2_NO_WRAPPER) && !defined(WOLFTPM2_NO_ASN)
 #include <examples/endorsement/trusted_certs_der.h>
 #endif
@@ -11371,6 +11401,7 @@ int unit_tests(int argc, char *argv[])
     test_wolfTPM2_CryptoDevCb_MlDsaKeyId();
     test_TPM2_ASN_DecodeX509Cert_Errors();
     test_TPM2_ASN_RsaUnpadPkcsv15();
+    test_TPM2_ASN_TrimZeros();
     test_TPM2_ASN_DecodeX509Cert_Valid();
     test_TPM2_ASN_DecodeTag_Errors();
     #if !defined(WOLFTPM2_NO_WOLFCRYPT) && defined(WOLFTPM2_PEM_DECODE) && \