diff --git a/crypto/asymmetric_keys/public_key.c b/crypto/asymmetric_keys/public_key.c
index e314fd57e6f88acffad4d4b4d8fa0b1e68071b9b..3474fb34ded9e5a756a11c5d118be4cbff5b9bed 100644
--- a/crypto/asymmetric_keys/public_key.c
+++ b/crypto/asymmetric_keys/public_key.c
@@ -234,6 +234,7 @@ static int software_key_query(const struct kernel_pkey_params *params,
 	info->key_size = len * 8;
 
 	if (strncmp(pkey->pkey_algo, "ecdsa", 5) == 0) {
+		int slen = len;
 		/*
 		 * ECDSA key sizes are much smaller than RSA, and thus could
 		 * operate on (hashed) inputs that are larger than key size.
@@ -247,8 +248,19 @@ static int software_key_query(const struct kernel_pkey_params *params,
 		 * Verify takes ECDSA-Sig (described in RFC 5480) as input,
 		 * which is actually 2 'key_size'-bit integers encoded in
 		 * ASN.1.  Account for the ASN.1 encoding overhead here.
+		 *
+		 * NIST P192/256/384 may prepend a '0' to a coordinate to
+		 * indicate a positive integer. NIST P521 never needs it.
 		 */
-		info->max_sig_size = 2 * (len + 3) + 2;
+		if (strcmp(pkey->pkey_algo, "ecdsa-nist-p521") != 0)
+			slen += 1;
+		/* Length of encoding the x & y coordinates */
+		slen = 2 * (slen + 2);
+		/*
+		 * If coordinate encoding takes at least 128 bytes then an
+		 * additional byte for length encoding is needed.
+		 */
+		info->max_sig_size = 1 + (slen >= 128) + 1 + slen;
 	} else {
 		info->max_data_size = len;
 		info->max_sig_size = len;