diff --git a/src/crypto/rsa.c b/src/crypto/rsa.c
index 36109280..7ac0bca5 100644
--- a/src/crypto/rsa.c
+++ b/src/crypto/rsa.c
@@ -625,7 +625,7 @@ static int rsa_match ( const void *private_key, size_t private_key_len,
 /** RSA public-key algorithm */
 struct pubkey_algorithm rsa_algorithm = {
 	.name		= "rsa",
-	.ctxsize	= sizeof ( struct rsa_context ),
+	.ctxsize	= RSA_CTX_SIZE,
 	.init		= rsa_init,
 	.max_len	= rsa_max_len,
 	.encrypt	= rsa_encrypt,
diff --git a/src/include/ipxe/rsa.h b/src/include/ipxe/rsa.h
index d947eec7..a1b5e0c0 100644
--- a/src/include/ipxe/rsa.h
+++ b/src/include/ipxe/rsa.h
@@ -77,6 +77,9 @@ struct rsa_context {
 	void *tmp;
 };
 
+/** RSA context size */
+#define RSA_CTX_SIZE sizeof ( struct rsa_context )
+
 extern struct pubkey_algorithm rsa_algorithm;
 
 #endif /* _IPXE_RSA_H */