diff --git a/include/common/tbbr/cot_def.h b/include/common/tbbr/cot_def.h
index 33350a0..c411146 100644
--- a/include/common/tbbr/cot_def.h
+++ b/include/common/tbbr/cot_def.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2015-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -11,4 +11,38 @@
 
 #define COT_MAX_VERIFIED_PARAMS		4
 
+/*
+ * Maximum key and hash sizes (in DER format).
+ *
+ * Both RSA and ECDSA keys may be used at the same time. In this case, the key
+ * buffers must be big enough to hold either. As RSA keys are bigger than ECDSA
+ * ones for all key sizes we support, they impose the minimum size of these
+ * buffers.
+ */
+#if TF_MBEDTLS_USE_RSA
+#if TF_MBEDTLS_KEY_SIZE == 1024
+#define PK_DER_LEN                      162
+#elif TF_MBEDTLS_KEY_SIZE == 2048
+#define PK_DER_LEN                      294
+#elif TF_MBEDTLS_KEY_SIZE == 3072
+#define PK_DER_LEN                      422
+#elif TF_MBEDTLS_KEY_SIZE == 4096
+#define PK_DER_LEN                      550
+#else
+#error "Invalid value for TF_MBEDTLS_KEY_SIZE"
+#endif
+#else /* Only using ECDSA keys. */
+#define PK_DER_LEN                      91
+#endif
+
+#if TF_MBEDTLS_HASH_ALG_ID == TF_MBEDTLS_SHA256
+#define HASH_DER_LEN                    51
+#elif TF_MBEDTLS_HASH_ALG_ID == TF_MBEDTLS_SHA384
+#define HASH_DER_LEN                    67
+#elif TF_MBEDTLS_HASH_ALG_ID == TF_MBEDTLS_SHA512
+#define HASH_DER_LEN                    83
+#else
+#error "Invalid value for TF_MBEDTLS_HASH_ALG_ID"
+#endif
+
 #endif /* COT_DEF_H */