diff --git a/src/core/crypto/ecdsa.cpp b/src/core/crypto/ecdsa.cpp
index f02a951df..eb8c78f19 100644
--- a/src/core/crypto/ecdsa.cpp
+++ b/src/core/crypto/ecdsa.cpp
@@ -82,7 +82,7 @@ otError Ecdsa::Sign(uint8_t *      aOutput,
 
     // Concatenate the two octet sequences in the order R and then S.
     VerifyOrExit(mbedtls_mpi_write_binary(&rMpi, aOutput, mbedtls_mpi_size(&rMpi)) == 0, error = OT_ERROR_FAILED);
-    *aOutputLength = mbedtls_mpi_size(&rMpi);
+    *aOutputLength = static_cast<uint16_t>(mbedtls_mpi_size(&rMpi));
 
     VerifyOrExit(mbedtls_mpi_write_binary(&sMpi, aOutput + *aOutputLength, mbedtls_mpi_size(&sMpi)) == 0,
                  error = OT_ERROR_FAILED);
@@ -99,7 +99,7 @@ exit:
 
 int Ecdsa::FillRandom(void *, unsigned char *aBuffer, size_t aSize)
 {
-    Random::FillBuffer(aBuffer, aSize);
+    Random::FillBuffer(aBuffer, static_cast<uint16_t>(aSize));
 
     return 0;
 }