diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index a62d4e1..d8fbd77 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -8942,36 +8942,43 @@
                                                     const int use_context)
 {
     int ret = 0;
-    size_t prf_input_len = use_context ? 64 + 2 + context_len : 64;
     unsigned char *prf_input = NULL;
 
-    if (use_context && context_len >= (1 << 16)) {
-        ret = MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
-        goto exit;
-    }
-
-    prf_input = mbedtls_calloc(prf_input_len, sizeof(unsigned char));
-    if (prf_input == NULL) {
-        ret = MBEDTLS_ERR_SSL_ALLOC_FAILED;
-        goto exit;
-    }
-
     /* The input to the PRF is client_random, then server_random.
      * If a context is provided, this is then followed by the context length
      * as a 16-bit big-endian integer, and then the context itself. */
-    memcpy(prf_input, ssl->transform->randbytes + 32, 32);
-    memcpy(prf_input + 32, ssl->transform->randbytes, 32);
+    const size_t randbytes_len = MBEDTLS_CLIENT_HELLO_RANDOM_LEN + MBEDTLS_SERVER_HELLO_RANDOM_LEN;
+    size_t prf_input_len = randbytes_len;
     if (use_context) {
-        prf_input[64] = (unsigned char) ((context_len >> 8) & 0xff);
-        prf_input[65] = (unsigned char) (context_len & 0xff);
-        memcpy(prf_input + 66, context, context_len);
+        if (context_len > UINT16_MAX) {
+            return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
+        }
+
+        /* This does not overflow a 32-bit size_t because the current value of
+         * prf_input_len is 64 (length of client_random + server_random) and
+         * context_len fits into two bytes (checked above). */
+        prf_input_len += sizeof(uint16_t) + context_len;
     }
-    ret = tls_prf_generic(hash_alg, ssl->session->master, 48,
+
+    prf_input = mbedtls_calloc(prf_input_len, sizeof(unsigned char));
+    if (prf_input == NULL) {
+        return MBEDTLS_ERR_SSL_ALLOC_FAILED;
+    }
+
+    memcpy(prf_input,
+           ssl->transform->randbytes + MBEDTLS_SERVER_HELLO_RANDOM_LEN,
+           MBEDTLS_CLIENT_HELLO_RANDOM_LEN);
+    memcpy(prf_input + MBEDTLS_CLIENT_HELLO_RANDOM_LEN,
+           ssl->transform->randbytes,
+           MBEDTLS_SERVER_HELLO_RANDOM_LEN);
+    if (use_context) {
+        MBEDTLS_PUT_UINT16_BE(context_len, prf_input, randbytes_len);
+        memcpy(prf_input + randbytes_len + sizeof(uint16_t), context, context_len);
+    }
+    ret = tls_prf_generic(hash_alg, ssl->session->master, sizeof(ssl->session->master),
                           label, label_len,
                           prf_input, prf_input_len,
                           out, key_len);
-
-exit:
     mbedtls_free(prf_input);
     return ret;
 }
@@ -8991,7 +8998,11 @@
     const size_t hash_len = PSA_HASH_LENGTH(hash_alg);
     const unsigned char *secret = ssl->session->app_secrets.exporter_master_secret;
 
-    if (key_len > 0xffff || label_len > 250) {
+    /* Check that the label and key_len fit into the HkdfLabel struct as defined
+     * in RFC 8446, Section 7.1. key_len must fit into an uint16 and the label
+     * must be at most 250 bytes long. (The struct allows up to 256 bytes for
+     * the label, but it is prefixed with "tls13 ".) */
+    if (key_len > UINT16_MAX || label_len > 250) {
         return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
     }